EDN 1

Exclusive EDN e-Book

Santa Clara, Ca

Papers & Presentations from the Technical Seminar and Workshop

March 17, 2010

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Welcome to the e-book version of EDN’s Designing with LEDs seminar and workshop

EDN’s third Designing with LEDs event, held in Santa Clara, California on March 17th, 2010, addressed high-brightness (HB) LED design challenges from the view¬point of the hardware engineer. Topics included power control, thermal management, and optics—all of which affect the cost, efficiency, and lifespan of LEDs. Also discussed was the impact of lighting control and communication on system cost and usability. The event featured keynotes, papers, and workshops by some of the electronics industry’s foremost experts on LED technology.

In order to broaden the reach of the event and make its content available to those unable to attend, we have published three technical papers and two slideshowpresentations from the October 2009 workshop in this e-book. We will also be featuring an on-demand Webinar complete with technical papers and the proceedings of the LED Manufacturers’ Panel Discussion at www.edn.com/leds.

For a pictorial version of the introductory remarks from the workshop, please visit EDN’s PowerSource blog post at: www.edn.com/ledblog1

Best Regards,

Margery ConnerTechnical Editor, Power and Components EDN magazine mconner@reedbusiness.com

High-Brightness LEDs save power, reduce space, and shape light into unlimited colors and places. New HB LED devices, packages, control electronics and thermal devices combine to revolutionize lighting for consumer and medical devices, automotive, architectural, and signage applications.

Designing with LEDs Workshop has expanded into four new technical paper tracks:

Power management y

Thermal management y

Optics and light measurement y

NEW: LEDs and solar power y

PLUS: Panel presentations by major LED manufacturers Cree, Phillips yLumileds, and OSRAM discussing LED life, reliability, and future products.

Workshop booths with the latest in HB LEDs, power ymanagement, thermal management, and packaging, and connectors.

Join the staff of EDN and Design News magazines, circuit designers, system architects, LED light designers, device packaging engineers, lighting component designers, students, professors and other industry movers and shakers as we explore the world of HB LEDs and learn new design techniques.

5White Paper: Design Challenges for

Solar-powered HBLED Lighting Heather Roberts, Avnet

8White Paper: Understanding Efficient Heat

Removal in HB LED Applications Barry Dagan, Cool Innovations

11White Paper: Practically Speaking: LED Light

Measurement Wolfgang Daehn & Bob Angelo, Gigahertz-Optik

16White Paper: What Mechanical Engineers

Should Know About LEDs Richard Zarr, National Semiconductor

17White Paper: Integrated Solar Powered

Lighting Solutions Luca Difalco, STMicroelectronics

21White Paper: On the Useful Lifetime of LED

Lighting Systems Geof Potter, Texas Instruments

Table of Contents

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Sponsor descriptions (ebook):

Allied Electronics is a small order, high service level distributor of electronic components and electromechanical products with 55 sales branches across the United States and in Canada.

ASIC Advantage, Inc. designs and manufactures mixed-signal ICs for leading automotive, aerospace, communications, industrial, and medical companies. AAI has developed a strong expertise in mixed-signal solutions and has specialized in the development and production of products within Power Management, LED Lighting, Portable Electronics, Sensors and Instrumentation, Digital Interface, and Motor Controls. ASIC Advantage, Inc.’s analog and digital mixed-signal expertise includes high-voltage and radiation-tolerant applications. As a fabless manufacturer, we have maximum flexibility in deciding which tools and processes to use, allowing us to deliver optimized designs and cost effective solutions.

Avnet LightSpeed offers access to a national team of illumination-focused engineers we call Illumineers. Illumineers are experienced in LED technology, thermal management, power driver stage and secondary optics. Whether you are considering a new application or are interested in re-visiting an exist¬ing design, a complete illumination solution integrates a number of carefully chosen elements. As a unit of Avnet Electronics Marketing, LightSpeed brings together the world’s foremost LED, high-performance analog and optical/electromechanical manu¬facturers, along with best-in-class technical expertise and supply chain management services.

Bergquist Thermal Clad is an insulated metal substrate circuit board providing complete thermal management systems for surface mount and High Power LED applications. Available in standard and custom configurations, Bergquist Thermal Clad solutions provide better thermal management with lower die temperatures, extended LED lifetimes, and increased light output. The Bergquist Company designs and manufactures high performance thermal management materials used to dissipate heat and keep electronic components cool. With some of the best-known brands in the business, including Sil-Pad, Gap Pad, Gap Fillers, Bond-Ply, and Hi-Flow phase change grease replacement materials, Bergquist is your total thermal management supplier.

California Micro Devices Corporation is a leading supplier of protection devices for the mobile handset, high brightness LED (HBLED), digital consumer electronics and personal computer markets. Detailed corporate and product information may be accessed at www.cmd.com.

Coilcraft: See how Coilcraft’s LED Design Center makes it easy to pick the perfect inductor for your LED driver circuit. Start with a specific IC, a driver topology, or inductor specs. In seconds, you’ll get a list of options with performance data, pricing, even detailed loss calculations.

Digi-Key Corporation is a leading provider of lighting solutions, offering products from more than 60 manufacturers of some of the world’s most innovative lighting products. With a wealth of information and helpful tools on its Lighting Technology Zone at Digikey.com and 24/7 technical support, Digi-Key is committed to assisting design engineers as they develop, design, and bring their next-generation lighting products to market. With a focus on providing its customers with the best possible service, Digi-Key Corporation serves a global customer base from its 600,000 square foot headquarters and Product Distribution Center in Thief River Falls, Minnesota.

Everlight Electronics is a leading global Optoelectronics manufacturer of low and high power LEDs, Lamps, Digital Displays, Infrared, Optical Sensors, Fiber Optic and Optocoupler components. We provide solutions for various applications in the lighting, consumer, computing, automotive, telecommunication and industrial market segments. Everlight, continuously advancing their technology, recently introduced AC LEDs to complement their portfolio of MR, PAR, Office Lighting and Street Lighting solutions. Founded in 1983, Everlight is a $400 million company headquartered in Taipei, Taiwan, with over 5,300 employees and operations in Asia, Americas and Europe.

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International Rectifier (NYSE:IRF) is a world leader in power management technology. IR’s analog and mixed-signal ICs, advanced circuit devices, integrated power systems and components enable high performance computing and reduce energy waste from motors, the world’s single largest consumer of electricity. Leading manufacturers of computers, energy efficient appliances, lighting, automobiles, satellites, aircraft and defense systems rely on IR’s power management benchmarks to power their next generation products.

Jameco has been supplying electronic components to design engineers, product developers, educators and hobbyists for over 35 years. Known for its personalized service, Jameco offers a wide range of name brand and house brand pricing options, a low-price guarantee and the highest quality catalog in the industry. Get your free catalog and start saving today.

Marvell: Founded in 1995, Marvell Technology Group Ltd. has operations worldwide and approximately 5,000 employees. Marvell’s U.S. operating subsidiary is based in Santa Clara, California and Marvell has international design centers located in the U.S., Europe, Israel, Singapore and China. A leading global fabless semiconductor company, Marvell’s expertise in microprocessor architecture and digital signal processing, drives multiple platforms including high volume storage solutions, mobile and wireless, networking, consumer and green products. The company ships over one billion chips a year. World class engineering and mixed-signal design expertise helps Marvell deliver critical building blocks to its customers, giving them the competitive edge to succeed in today’s dynamic market

National Semiconductor’s PowerWise energy-efficient LED drivers provide constant current to arrays of LEDs, enabling color and brightness matching over a wide temperature range. Through dimming, thermal management, and fault protection, National’s drivers improve performance in a variety of applications. National’s WEBENCH LED Designer online tool lets designers quickly and easily configure a complete LED circuit with a few keystrokes.

NEC Electronics America is a leading provider of semiconductor products for the broadband and communications markets; system solutions for the mobile, PC, automotive and digital consumer markets; and multi-market solutions for a wide range of consumer applications. NEC Electronics America offers local manufacturing in Roseville, California.

NXP Semiconductors provides High Performance Mixed Signal and Standard Product solutions that leverage its leading RF, Analog, Power, Digital Processing and manufacturing expertise. NXP is the number one supplier of lighting ICs with a strong portfolio of high voltage solutions emphasizing superior control of deep dimming and highly efficient current control.

Seoul Semiconductor is a world leader in LEDs for all types of illumination and innovator of the AC LED family, Ariche.

Logo TK Philips Lumileds delivers reliable, illumination-grade power LEDs and outstanding world-wide service and the design support that customers need to rapidly develop LED lighting solutions. As the industry leader Philips Lumileds’ LUXEON LEDs are used widely in retail, entertainment, outdoor, automotive, and many other applications. Philips Lumileds is a technological pioneer in LED development, delivering the most lumens per watt, per package and per dollar with the widest operating range, reliability and lumen maintenance. LUXEON LEDs are enabling lighting solutions that are more environmentally friendly, help reduce CO2 emissions and reduce the need for power plant expansion.

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Solar powered high brightness LED (HBLED) lighting systems are a smart energy choice, and one which will continue to see increasing adoption. Powered by a clean energy source—the sun, they also take advantage of an extremely efficient form of lighting in HBLEDs. In this whitepaper, we will discuss the major components of a solar powered HBLED lighting system, as well as the design challenges in implementing such a system.

Figure 1 Solar Powered HBLED Lighting System

In typical HBLED lighting applications, power conversion is an important factor—and even more so in solar power applications. Not only must power be maximized from the solar panel, but it must also be conditioned for maximum energy storage in the battery array, and converted for use powering the HBLEDs.

Fundamentals of Driving HBLEDsAs a diode, LEDs exhibit a steep voltage-current (V-I) curve, which means that even a small change in voltage will result in a large change in current and brightness. It follows then, that the most convenient method to regulate their output is to control current and not voltage. When multiple HBLEDs are used in a current-controlled configuration, they are often wired in series to ensure uniformity of brightness.

Based upon application, there are a large number of high efficiency, highly integrated HBLED driver solutions available. Common power conversion topologies for HBLED constant current drivers are: boost, buck, sepic, and flyback.

HBLED drivers typically look like a well behaved load on the battery—in most applications, there will be no need to support a high current surge during start up due to the fact that LEDs are not a capacitive load, and most integrated HBLED driver circuits have soft startup protection.

Estimating Current LoadWhen designing a system, the size of batteries and solar panels must be established. To do this, an estimate of current consumption is required, usually measured in Amp-hours. Current consumption in a design consists of standby power and active power. In this case, standby power would refer to times when the HBLEDs are off, such as in the daytime. In standby mode, current use would be minimized by design, with only a small amount of sense/control circuitry “awake”, or shut off completely with only leakage currents

remaining. Active power refers to power consumption when the system is operating, in this case when the HBLEDs are on. Designs may have only one active/standby mode, or multiple active/standby modes. Active modes usually have a well understood duty cycle-- a percentage of time in a 24hr day in which the system is active.

=*24+(**24) (1)IL=Current load (Amp Hours)IST=Standby current (Amps)IAM=Active mode current (Amps)FDutyCycle=Duty Cycle Factor (Percentage)

Sizing & Choosing Solar PanelsThere are a number of technologies available for solar panels, and each has its own unique characteristics. Table 1 is an overview of technologies and their notable features. For the majority of solar lighting applications, crystalline silicon panels are used because of their general availability and high efficiencies. Most HBLED lighting systems require panels less than 100W. More so than any of the other technologies, crystalline silicon panels are available in the form factors and smaller sizes suitable for industrial applications—ranging from 5W to 100W. In an example of an interesting fit for a different technology, flexible amorphous silicon has been used to wrap a light pole with solar materials, forming the solar panel around the light pole itself. See Margery Connor’s article on off-grid lighting on the EDN website for more information.

Table 1 Solar Panel Technology Summary

To size the solar panels, a number of factors need to be considered. Optimum performance for solar panels is achieved (assuming no active tracking) when the panels are pointed south, with a tilt angle approximately equal to the latitude of the installation + 20 degrees. Further optimization can be achieved by adjusting the tilt angle of the panels 2 or 4 times per year to track seasonal changes in the path of the sun. If the solar panels cannot be placed with optimum tilt angle, or facing south, they will not be able to generate maximum power. As an example, some systems require panels to be placed parallel to the ground, with no tilt angle at all, or, they may be placed subject to significant shading. An orientation/shading multiplication factor for these situations must be figured into the panel sizing equation. In addition, it is prudent to factor degradation into the size of the panels as well. Over a 20 year period, solar panels may lose as much as 20% of their rated output.

It is important to understand the amount and power of sunlight that is received in a given geographic location in order to plan a solar powered lighting system. Insolation is the term used to describe the amount of solar irradiation received on a given surface area in a given time, usually measured in KW-hrs/m2 per day. The

Design Challenges for Solar-Powered HBLED LightingBy Heather Roberts, Avnet

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batteries, it is important to include a design factor to account for the impact of ambient temperature, as lead acid battery capacity is reduced in cold temperatures. Table 3 shows design factors for lead acid batteries.

Table 3 Lead Acid Battery Temperature Design Factor

A simple equation for battery capacity is shown below:

(3)Cbattery=Battery CapacityIL=Current LoadTsolar=Number of Solar Autonomy DaysFTemperature=Temperature Design FactorFDoD=Max Battery Discharge %

This equation is a good estimate for lead acid batteries where the rated discharge rate is similar to the expected discharge rate. If the discharge rate is higher or lower than the specified discharge rate of the battery, this estimate must be adjusted accordingly. Peukert’s Law may be used to calculate effects due to discharge rate. Lithium ion batteries generally do not have a significant sensitivity to discharge rate.

Maximum Power Point TrackingSolar panels have a characteristic I-V curve which varies depending upon irradiance and temperature. As can be seen in Figure 3 below, there is a point on the IV curve where the panel will be generating maximum power. In many solar applications, the system is designed to operate the panel at this point, generating maximum power. This is called MPPT (maximum power point tracking). Many MPPT algorithms exist, but the goal of all of them is the same—to operate the panel as close as possible to the characteristic peak power point of the power curve.

Figure 3 Solar Panel Voltage/Current and Power/Voltage Characteristics

National Renewable Energy Lab (NREL) has a variety of solar insolation data available. When sizing solar panels for a system, another fact that must be considered is the number of days that the system must be able to run without any sun at all; this is known as solar autonomy days. Estimates for general and conservative systems are shown in Figure 2 below, based on the worst case winter solar insolation for the regions where the system will be installed.

Figure 2 Solar Autonomy Days

Wpanel (2)Wpanel=Panel SizeILoad=Current LoadForientation =Orientation/Shading Factor/DegradationVbattery=Nominal Battery VoltageWinter Peak Insolation=Number of Winter Sun Hours (NREL data typically used)

Sizing & Choosing BatteriesAnother aspect of a system is energy storage. While sealed lead acid (SLA) batteries are the most common battery type used in off-grid solar powered HBLED lighting applications, other options may be feasible, as shown in Table 2 below. SLA batteries can be charged below freezing, which is desirable in solar powered applications. While other technologies may be able to operate below freezing, charging may be an issue at low temperature.

Table 2 Battery Technology Summary

Lead acid battery performance is impacted significantly by temperature, depth of discharge, and rate of discharge. These batteries are often labeled by not only capacity (AH), but also by rate of discharge. The rate of discharge impacts the capacity of a battery—the faster the rate of discharge, the lower the capacity. The converse is also true—the slower the rate of discharge, the higher the capacity of the battery.

Depth of discharge (DoD) also needs to be carefully considered. DoD is the amount of energy (expressed as a percent of total capacity) that will be discharged from the battery. The greater the DoD, the fewer cycles that the battery will be able to support—there is a direct relationship between DoD and battery life. 50% of capacity is the generally recommended limit for deep-cycle lead acid batteries. Lower DoDs will extend battery life.

In choosing the capacity of a battery, especially lead acid

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Figure 4 Cypress PowerPSOC Reference Design

The Cypress architecture uses a current controlled buck regulator for MPPT and battery charging. The MPPT and battery charging algorithm embedded in the PowerPSoC® uses voltage and current feedback from the panel and operates the panel at its peak power by controlling the switches in the buck regulator. The switches in the synchronous buck regulator circuit are also operated in a way to ensure that the current delivered to the battery is per requirement of the current charge state of the battery.

Figure 5 MPPT/Charge Control Implementation Block Diagram

The solution features two LED drivers: one floating load buck driver for LED loads up to 8V and 2A where the forward voltage is less than the battery voltage and the other is a boost driver for LED loads up to 40V and 2A where the forward voltage is more than the battery voltage.

ST Microelectronics is developing a highly integrated solar MPPT charger/HBLED driver. The fully integrated solution features a lead acid battery charger with MPPT optimization, and integrated HBLED drivers. This high level of integration reduces cost, improves reliability, and simplifies design. Releasing in late 2010, this product is ideal for HBLED street lighting applications.

Additional Resources For additional information on solutions in this whitepaper, as well as industry solar and HBLED related articles, reference designs and solutions, please visit Avnet’s solar and HBLED web pages:

http://www.em.avnet.com/solarhttp://www.em.avnet.com/Lightspeed

Heather RobertsonTechnology Director – SolarAvnet Electronics Marketing

PowerPSoC® is a registered trademark of Cypress Semiconductor Corporation

Implementations of MPPT can be fully analog, or mixed signal, and often include a microcontroller or state machine. In designing a system, a cost benefit analysis should be performed to determine if adding MPPT functionality increases energy capture enough to offset the cost of implementation.

Charge ControlCharge controllers are used to charge batteries in a safe, efficient manner. Depending on the application, charge controllers can be bought off the shelf, or designed for a specific application; often with the MPPT and charge controller circuits combined. As mentioned in a previous section one of the most common battery types for HBLED solar powered lighting applications is lead acid batteries. Efficient charging of lead acid batteries requires a variety of charging modes, including bulk charge, absorption, float and equalize. Each state requires charging with different current and voltage characteristics, making sensing/feedback and control an important element in the controller.

A common architecture for an off-grid MPPT charge controller implementation is the use of a boost, buck, or buck/boost switching regulator, and a microcontroller with analog inputs for sensing current and voltages from both the solar panel and the batteries, and PWM outputs to control switches in the regulator.

Control and CommunicationHBLED lighting can be networked, or stand alone. Networked lighting enables energy saving control and dimming, as well as communication of environmental activity such as movement, traffic, etc., as well as battery and fault status. Both wired and wireless networks are common. Standards based wireless protocols (such as Zigbee, etc.) and proprietary wireless protocols running over the ISM bands of 902-928MHz and 2.4GHz are often used. For wired networking, power line modems (PLM) are often used, communicating over the grid.

While it may seem contradictory to have a grid connected solar powered HBLED luminaire, the grid would primarily be used for networked communication, as well as for an optional power source for battery charging. A potential application using both wired and wireless communication would be a wireless link connecting a subnet of lights, with each subnet controlled by a node connected to the main control center through PLM.

System ExamplesElectronics for solar lighting applications lend themselves well to integration. On the market today are integrated solutions specifically designed for solar lighting, as well as products currently in development.

Cypress Semiconductor has a complete solar charger HBLED reference board designed around their PowerPSOC® processor. Developed to be powered by a 12V solar panel, and to charge 12V lead acid batteries, the reference design includes MPPT optimization, a battery charger and both buck and boost HBLED driver circuits.

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You don’t have to be in the lighting business to know that green technologies are in very high demand and that at the forefront of sustainable lighting are LED lights. LEDs have very high luminous to watt efficiencies, very long lifetime, and are not made of toxic materials. These factors result in reduced down time, reduced maintenance and reduced costs. From architectural illumination to lit street lights/signboards to ordinary home/office lighting, LEDs are rapidly becoming ubiquitous in our reduced carbon consumption world.

However, there is one significant limiting factor with LED lighting. That is heat, or more specifically, temperature. Like all semiconductors, LEDs are very sensitive to high temperatures. To get more brightness, LEDs have to run at higher power, meaning increased heat, and therefore higher temperatures. Running an LED hot is unfavorable for two reasons. The first is that as temperature increases, LEDs actually become dimmer and lose lighting efficiency. Second and perhaps more serious, operating too hot will cut down the lifetime of an LED, wiping out one of the most beneficial features of the technology.

Through the wise selection of appropriate heat sink technology, life-sapping heat can be conducted away from the LED where it is least desirable to the surrounding air. To understand how to effectively ‘heat sink’ an LED application, it is important to know some of the physics behind heat transfer.

Understanding Natural ConvectionMost High Brightness (HB) LED lighting will be designed with natural convection cooling, that is no forced air circulation by fan or other inducement, for a variety of reasons. Fans generate noise, would typically need to be replaced over the lifetime of the LED and add to the complexity and cost of the lighting appliance. As a result, natural convection is preferred in most scenarios.

The motivating force behind natural convection is buoyancy. As the air in direct contact with the heat sink surface heats up, like any gas, it expands and becomes less dense. The less dense pocket of hot air will float upwards like a hot air balloon, drawing cooler denser air into the heat sink. Assuming the rising hot air can be exhausted and a source of cooler air is present, this process will cycle as long as the heat sink remains hot.

These natural convection air currents unfortunately are very weak, typically one to two orders of magnitude less than any forced convection. As a result, natural convection can easily be hindered by friction along the surfaces of the heat sink as well as friction between the individual air molecules themselves.

Maximizing Natural Convection PerformanceWhat heat sinks do fundamentally is multiply the surface area of a heat producing device, such that the device can come into contact with more air simultaneously. The more surface area a heat sink possesses, the more heat it can transfer to the air. The flip side though is that the more surface area a heat sink has, the more friction there will be between the moving air and the heat sink. If there is too much friction, the air will not move at all, rendering the heat sink equivalent to a block of solid metal.

For good heat sink design, it is especially important in natural convection to find the right balance of trying to maximize surface area but not chocking off the air flow.

Heat sink Geometry ConsiderationsThere are a lot of geometric considerations when selecting a heat sink. These include choosing between fins or pins, the density of the fins or pins, the size of the heat sink, the foot print of the heat sink, the material of the heat sink, etc.

Fin density refers to the amount of fins or pins per square inch of the heat sink footprint. The optimum fin density should be determined by the available air flow, in the case of natural convection, low density is required due to low airflow. Having a low fin density results in less surface area, but natural convection air currents will be less encumbered resulting in better performance. If the fin density is not sufficient for the required cooling, then the use of a fan or other non-passive forms of cooling may be required.

Another important consideration is whether to choose a continuous finned heat sink like most extrusions or a pin finned heat sink. In a finned heat sink, the air can travel along the length of the fins and to a limited extent up and down the fins perpendicular to the base. When used in natural convection, a finned heat sink works best when the fins are aligned parallel to gravity so that the rising air can travel along the fins. When the base is horizontal, air can only be drawn in from the two open sides of the heat sink, some air can fall into the heat sink from above but it will be fighting the rising hot air currents. This tends to lead to poorer performance for finned heat sinks when oriented horizontally; they perform worst when the base is vertical but the fins have been oriented perpendicular to gravity.

With respect to natural convection cooling, forged pin fin heat sinks are a more advanced heat sink technology. Since the heat sink is open on all sides, air can be drawn in and exhausted from all sides of the heat sink except the base. This means that the heat sink will perform well in any given orientation. Another not very obvious benefit of pin fins is that since each pin is an individual element, it is very easy to modify pin finned heat sinks into special shapes by trimming or removing only specific pins without damaging the rest of the heat sink. This makes it very easy to add installation features or fit oddly shaped cavities. Pin finned heat sinks can also be forged into round or other shape bases.

Traditional pin finned heat sinks are surpassed in performance only by advanced flared pin finned design heat sinks. A flared pin fin heat sink differs from a traditional vertical pin fin heat sink in that all the pins expand outwards in a radial fashion from the base, resembling a hedgehog.

Flared pin fin designs can significantly improve natural convection performance by increasing the spacing between each pin row while maintaining the same amount of total surface area. The additional space serves to reduce air flow restriction, making it easier for hot air to escape and fresh cold air to enter the pin matrix.

Another important consideration is the effect of pinned and finned surfaces with regards to the aerodynamic flow of air. Air travelling along a straight fin will develop what is called a boundary layer

Understanding Efficient Heat Removal in HB LED ApplicationsBy Barry Dagan, Cool Innovations

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rate, then the ends will be much cooler than the source, meaning less heat will be transferred to the air, thereby reducing efficiency.

In most natural convection scenarios, however, the limiting factor of the performance is not the ability to conduct the heat along the heat sink itself but the ability to convect the hot air out of the heat sink. Thusly, while the conduction of heat in copper may be twice as high as aluminum, in most natural convection situations, performance between aluminum and copper will be practically identical.

Heat sinks in Practical ApplicationsAs can be expected, most lighting fixtures are ceiling mounted with lights aimed downwards. For LEDs, this means a horizontal LED PCB orientation meaning that the heat sink base is also horizontal. Less common but also recurring scenarios occur when the light orientation is unknown or is allowed to move. In this case omni-directional heat sinks are necessary to ensure continuous, efficient operation.

To establish how well various heat sinks performed against each other in possible LED lighting scenarios, three heat sinks of different geometries were tested and compared. The heat sinks were all of identical footprint and overall heights, each to be tested at 60W power and each to be tested in three orientations. The three orientations are vertical heat sink base (parallel with respect to gravity), horizontal heat sink base (perpendicular to gravity) and at a 45° incline.

The three heat sinks tested were a typical extrusion suited for natural convection, a forged pin fin heat sink with normal straight pin fins and a forged flared pin fin heat sink with pin fins that expand outwards from the heat source. See table for details

SEE TABLE 1In the next table, we have the results of the thermal tests.

SEE TABLE 2The test results show that in the most common scenario, downward aimed lighting, the flared heat sink outperformed the extrusion by 34% and the straight pin heat sink by 19%. In the vertical orientation, the extrusion was the strongest performer being 8% more powerful than the flared pin fin heat sink. The 45° orientation predictably had a result in between the vertical and horizontal orientations with the flared pin fin performing best.

From these test results, we can see that the flared pin fin heat sink is the best fit for horizontal (with respect to gravity) orientation lighting scenarios. The performance is significantly better than

along the fin wall. A boundary layer is a region of disrupted airflow where air right against the heat sink surface is slowed by friction against the heat sink; air adjacent to this slow air region is then also slowed. At the beginning of the fin, the boundary layer is very thin but as you travel down the length of the fin, the boundary layer expands until a theoretical maximum or when it is disrupted by other aerodynamic effects such as another boundary layer. These boundary layers insulate the heat sink surfaces further down the fin length reducing cooling efficiency.

In a pin finned heat sink, these boundary layers are broken because each pin fin is a distinct element and there is no wall for the boundary layer to follow. Without a continuous surface, the boundary layer disperses. With reduced boundary layer effects, pockets of warm air are less likely to occur, and so with a more even air temperature, each individual heat sink pin performs better.

Square pins, a hybrid of straight finned heat sinks, do help disrupt the formation of boundary layers. However, their sharp corners add to aerodynamic friction comparatively to round pin fins.

Heat sink MaterialsAnother important consideration when choosing a heat sink is selecting the right material for the heat sink. Primarily two metals are used for heat sinking, namely aluminum and copper. Aluminum is the most commonly used heat sink material for many reasons. First, aluminum is cheaper than copper, not just in material costs but also in fabrication. Copper is much more difficult to extrude, forge and machine than aluminum. Aluminum also weighs less and naturally forms a thin oxide layer that seals the surface preventing further oxidization. Copper on the other hand is three times as heavy and will patina over time.

Copper does have one advantage over aluminum, namely that thermal conduction inside the metal is nearly twice that of aluminum. Thermal conduction is the transport of heat along the metal itself, thus improved thermal conduction means that the heat will travel faster from the source to the farthest corners of the heat sink.

This makes copper suitable for applications where the heat source is very small relative to the heat sink and it is necessary to get the heat away from a focused area fast. Copper is also suitable when there is a very high air speed flushing through the heat sink, as the high air speed will quickly strip heat away from the heat sink surfaces. Heat sinks operate efficiently when the entire structure is of a uniform temperature (the heat source will always be at least slightly hotter than any other part of the heat sink). If the heat is being stripped away from the ends of the heat sink fins at a high

These figures illustrate the effect of air flow around various fin structures. The blue areas represent cool air while the red areas represent hot, slowed air.

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Natural Convection Extrusion Heat sink

Flared Pin Fin Heat sink

Table 1Physical Characteristics of the Tested Heat sinks

Extrusion Vertical Pin Flared Pin

Length (in) 5.0 5.0 5.0

Width (in) 5.0 5.0 5.0

Height (in) 2.5 2.5 2.5

Number of fins/pins

13 360 360

Pitch (in) 0.38 0.25 0.25

Surface area (in2) 332 358 358

Weight (lbs) 2.11 1.84 1.84

Power (Watts) 60 60 60

Table 2 Thermal Performance Results of the Tested Heat sinks

Extrusion Vertical Pin Fin Flared Pin Fin

Horizontal position

0.83(°C/W) 0.77(°C/W) 0.62(°C/W)

H-V45 Deg. position

0.85(°C/W) 1.07(°C/W) 0.78(°C/W)

Vertical position

0.84(°C/W) 1.26(°C/W) 0.88(°C/W)

any of the other heat sinks allowing for very high powered LEDs to be cooled without the use of forced air convection. The flared pin fin heat sink is also suitable for lights that can be installed in any orientation as it exhibits very strong performance characteristics in all orientations.

The natural convection finned heat sink performed best when the light source was oriented vertically. The finned heat sink is sensitive to orientation as the fins must be parallel to gravity. Whether mounted in the horizontal or vertical orientation, the air must be able to travel up along the fin walls in order to develop natural convection.

The straight pin finned heat sink is omni-directional and can handle high power levels. It performs better than the extrusion in the typical horizontal position. The straight pin finned heat sink is suitable for LED fixtures with tight space constraints.

Looking ForwardWhen looking at the expectations of light output from LEDs and their associated power levels, it is obvious that heat is going to be a significant factor in LED designs. Good High Brightness LED design will require the incorporation of more efficient and more compact thermal solutions.

Natural convection is going to be the dominant method of cooling LEDs as it requires no power, no maintenance and has no obvious failure mechanisms. In order to maximize natural convection cooling, it is important to design a system with air flow in mind: this means having an unlimited supply of fresh cool air, having an exhaust for hot air and choosing a heat sink with low aerodynamic resistance to natural convection airflow yet a high cooling capacity. The heat sink must also be able to operate in the given orientation(s) of the application and require minimal space.

Traditional round based light fixtures will have to be replaced with more efficient square and rectangular fixtures in order to maximize heat sink surface area. Rectangular fixtures can also be more cost effective when designing for LED applications that don’t have production volumes to justify a dedicated heat sink die.

Further down the road, very high powered LEDs may necessitate the development of no-noise, extremely long life air movers. Other possibilities include developing conduction cooling mechanisms that use pre-existing structural elements. These may be uncommon solutions, but successful high brightness LED lighting will only be brought about by experimentation, persistence and innovation.

Images and Tables

Straight Pin Finned Heat sink

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Practically Speaking: LED Light MeasurementBy Wolfgang Daehn & Bob Angelo, Gigahertz-Optik

Gigahertz-Optik is a world class manufacturer of innovative UV-VIS-NIR optical radiation and color measurement instrumentation for specification critical industrial, medical and research applications.

Today’s lighting industries continue to be transformed by new light source technologies. Traditional incandescent lamps are being replaced by discharge lamps, physical (LED) and organic (OLED) Light Emitting Diodes. These sources are characterized predominantly by narrow wavelength band emission spectra. New alternative light meter technologies are required that can offer accurate and traceable light measurements in absolute units independent of the light source emission spectrum. Spectral measurement technology is recommended for this new type light source due to high measurement uncertainties of typical filter photometers and color meters. Moving grating monochromator spectroradiometers representing the high end in light measurement instrumentation are too expensive and complex for regular everyday industrial applications. Diode array spectrometers currently being promoted as the preferred measurement device to the standard light and color meter have limitations in absolute scale light measurements. Due to these limitations Gigahertz-Optik has developed a new highly accurate and cost effective spectrolightmeter based on its new BTS256P Bi-Technology Sensor that combines a photodiode and diode array for mutual improvement of each technology. The hand-held instrument allows qualification of luminous flux, spectral flux distribution and color data of single LEDs already assembled to a printed circuit board. In combination with a larger integrating sphere LED arrays or complete luminaires can be measured. Although compact in size the instrument offers all of the latest recommended measurement features like auxiliary lamps for integrating sphere based measurements and integrating time related offset compensation for the diode array. The full function test device is useful in small or large LED processing companies, in incoming inspection, production control and final product qualification. Other beam type light sources such as endoscopes, cool light fiber bundle sources and narrow beam lensed sources can also be measured.

Light measurement plays a significant role in science and industrial research as well as in production and quality control. But it is not as well known a science as say voltage or current measurement. To help inform and support readers in classifying their application this article includes basic information on light and light measurement.

Basics of LED Light & Color Measurement Light, or the visible part of the electromagnetic radiation spectrum, is the medium through which human beings a major portion of environmental. Evolution has optimized the human eye into a highly sophisticated sensor for electromagnetic radiation. Joint performance between the human eye and visual cortex, a large part of the human brain, dwarfs recent technical and scientific developments in image processing and pattern recognition. In fact a major part of the information flow from external stimuli to our brain is transferred visually.

Photometry deals with the measurement of visible energy. The human eye perceives light with different wavelengths as different colors, as long as the variation of wavelength is limited to the range between 400 and 800 nm. Outside this range, the human eye is insensitive to electromagnetic radiation and thus we have no perception of ultraviolet (UV, below 400 nm) and infrared (IR, above 800 nm) radiation. The sensitivity of the human eye to light of certain intensity varies strongly over the wavelength range between 380 and 800 nm. Under daylight conditions, the average normal sighted human eye is most sensitive at a wavelength of 555 nm, resulting in the fact that green light at this wavelength produces the impression of highest “brightness” when compared to light at other wavelengths. The spectral sensitivity function of the average human eye under daylight conditions (photopic vision) is defined by the CIE and DIN spectral luminous efficiency function V(λ).

Light measurement when correlated to human vision perception is called photometry. The goal of photometric measurements is to quantify human impressions, e.g. brightness, brilliance, brightness contrast, darkness. CIE and DIN specify light measurement quantities for the quantification of light sources and lighting conditions in numbers directly relating to the perception of the human eye. Photometric light measurement quantities are distinguished from radiometric quantities by the index “v” for “visual”.

Light-Meters for photometric measurements must offer a light spectral sensitivity correlating to that of the human eye, typically the day-light adapted eye response V(λ). The quality of the V(λ)

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Luminance Lv describes the measurable photometric brightness yof a certain location on a reflecting or emitting surface when viewed from a certain direction. It describes the luminous flux emitted or reflected from a certain location on an emitting or reflecting surface in a particular direction (the CIE definition of luminance is more general. In detail, the (differential) luminous flux dΦv emitted by a (differential) surface element dA in the direction of the (differential) solid angle element dΩ is given by dΦv = Lv cos(θ) ∙ dA ∙ dΩ with θ denoting the angle between the direction of the solid angle element dΩ and the normal of the emitting or reflecting surface element dA. The unit of luminance is 1 lm m-2 sr-1 = 1 cd m-2

Illuminance Ev describes the luminous flux per area impinging yupon a certain location of an irradiated surface. In detail, the (differential) luminous flux dΦv upon the (differential) surface element dA is given by dΦv = Ev ∙ dA. Generally, the surface element can be oriented at any angle towards the direction of the beam. Similar to the respective relation for irradiance, illuminance Ev upon a surface with arbitrary orientation is related to illuminance Ev,normal upon a surface perpendicular to the beam by Ev = Ev,normal cos(J) with J denoting the angle between the beam and the surface’s normal.

The unit of illuminance is lux (lx) and also foot-candle.

1lx = 0.0929 fc (lm/ft²)

Beside brightness, sensitivity to color sensations are human sensory perceptions and light measurement technology must express them in descriptive and comprehensible quantities. In light measurement applications luminous color of incident light and light sources is of main interest. According to the tristimulus theory, every color which can be perceived by the normal sighted human eye can be described by three numbers which quantify the stimulation of red, green and blue cones. If two color stimuli result in the same values for these three numbers, they produce the same color perception even when their spectral distributions are different. Around 1930, Wright and Guild performed experiments during which observers had to combine light at 435.8 nm, 546.1 nm and

spectral match to that specified by CIE and DIN is one of the key parameters for photometer specifications. Spectral mismatch error is the key source for measurement uncertainty with light sources other than tungsten lamps.

The most common photopic measurements quantities are:

Luminous flux y Φv is the basic photometric quantity and describes the total amount of electromagnetic radiation emitted by a source, spectrally weighted with the human eye’s spectral luminous efficiency function V(λ). Luminous flux is the photometric counterpart to radiant power. The unit of luminous flux is lumen (lm), and at 555 nm, where the human eye has its maximum sensitivity, a radiant power of 1 W corresponds to a luminous flux of 683 lm.

Luminous intensity Iv quantifies the luminous flux emitted by a ysource in a certain direction. In detail, the source’s (differential) luminous flux dΦv emitted in the direction of the (differential) solid angle element dW is given by

dΦv = Iv ∙ dΩ and thus

The unit of luminous intensity is lumen per steradian (lm / sr), which is abbreviated with the expression “candela” (cd).

1 cd = 1 lm / sr and also foot-Lambert (1 cd/m² = 0.2919 fL)

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temperature should not be used if the chromaticity differs more than Δuv=5x10-2 from the Planckian radiator.

Color rendering is the effect of a light on the color appearance of objects. Sources that include light of all spectral colors, e.g. sun light, effect natural color sensations from illuminated objects. Here the color rendering is good. Light sources with irregular spectral color distribution effect unnatural color sensations. Here the color rendering is poor. If for example the color of the object is not included in the source spectrum the color rendering is gray.

The Color Rendering Index CRI specifies the quality of the color rendering of illuminants. The CRI is calculated by comparing the color rendering of a sample source to that of a reference source. For example a black body radiator with CCT below 5000K as compared to day light source like D65 with CCT higher that 5000K. A selection of reflective test color samples (TCS), specified by the CIE are used to calculate the CRI of a test lamp. The first eight samples with relative low saturation are used to calculate the general CRI Ra of a light source. The other seven samples provide supplementary information. Four are with high saturation the others represent well known objects.

Light Emitting Diodes (LED) are semiconductor device incoherent light sources with high electrical power to light power conversion efficacy. As with any semiconductor device, operating temperature effects changes in light output and color performance. This is referred to as a device’s temperature coefficient. Thermal management is therefore of primary importance in the successful implementation of LEDs. Due to thermal drift LEDs are often operated in pulsed mode. High peak intensities can be generated in this mode with reduced average electrical power and therefore reduced junction temperature. Sorting or grading of individual LEDs by color differences caused by tolerances in the semiconductor process is a common

700 nm in such a way that the resulting color perception matched the color perception produced by monochromatic light at a certain wavelength of the visible spectrum. Evaluation of these experiments resulted in the definition of the standardize RGB color matching functions which have been transformed into the CIE 1931 XYZ color matching functions.

These color matching functions define the CIE 1931 standard colorimetric observer and are valid for an observer’s field of view of 2°. Practically, this observer can be used for any field of view smaller than 4°. Although the XYZ tristimulus values define a three-dimensional color space representing all possible color perceptions, for most applications the representation of color in a two-dimensional plane is sufficient. One possibility for a two-dimensional representation is the CIE 1931 (x, y) chromaticity diagram with its coordinates x and y calculated from a projection of the X, Y and Z values.

Color Temperature (CT) is a specification for visible light and used to specify lighting conditions in lighting, photography, film recording, publishing, and other applications. The color temperature of a light source is determined by comparing its chromaticity with that of an ideal black body source. The color temperature describes the emission spectrum of a black body sources or sources which match the color temperature of a black body source. Most artificial light sources such as fluorescent or discharge lamps and LEDs are only nearly-Planckian black body sources. They can be judged by their correlated color temperature (CCT). The CCT can be calculated for any chromaticity coordinate but the result is meaningful only if the light sources are nearly white. The CIE recommends that the correlated color

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One large source of measurement uncertainty inherent with integrating sphere use is the absorption effect. During calibration of the sphere photometer some of the light irradiated into the sphere will exit the sphere through the measurement port and be absorbed in the dark room. But during actual use, the measurement port of the integrating sphere will be fully or partially covered by the device under test DUT. So light leaving the sphere through the measurement port will be reflected back into the sphere adding erroneously to the DUT light signal. Depending on the spectral reflectivity and color of the DUT the re-reflected light will vary in intensity and color and affect an unknown measurement error. Auxiliary lamps are used to compensate this absorption error by measuring the signal of the auxiliary lamp with and without the DUT at the measurement port of the integrating sphere. The difference in intensity is used as a correction factor for subsequent measurements of the same kind of DUT.

Along with light intensity and color data, spectral intensity distribution is another important test property in LED analysis. Spectral based light meters are used for this type of measurement. Filter type light meters employing photometric or tristimulus (RGB) detectors are restricted to comparative or relative measurements, e.g. LED sorting and binning against gold-standards. However spectrometers offer different levels of quality levels, especially diode array type spectrometers which are often limited by intensity linearity and stray light characteristics.

An alternative method is to mate a photodiode with a diode array like Gigahertz-Optik’s BTS256P Bi-Technology Sensor. It’s photodiode with a precise photometric response provides a highly linear ratio between light input and signal output over a very wide dynamic range for very accurate luminous flux measurements. The photodiode offers a fast response time mostly independent from the light intensity so that the measurement signal of the

practice offered by most semiconductor manufacturers. But due to differences in LED manufacturer’s sorting processes and environmental conditions, the LED lighting industry is forced to do in-house qualification measurements.

The most common light measurement quantity used in LED testing is luminous flux measured in lumen. This quantity corresponds to LED efficacy by correlation of the total light output to the electrical power. Measurement of the total light output in lm instead of luminous intensity in cd produces much better reproducibility because it is independent of spatial light distribution which may be influenced by temperature, humidity, distance, different viewing angles, misalignment and other experimental error. In research and industry the most commonly used measurement devices for luminous flux are light meters with an integrating sphere. The integrating sphere acts as a light integrator for spatially emitted light. The light source may be mounted inside or outside the sphere. The integration effect is the result of multiple diffuse reflections on the diffuse reflecting surface of the hollow sphere which results in a uniform light distribution at the sphere surface. The illuminance measured at any position on the integrating sphere surface is therefore an indicator of the total flux generated by a light source inside or outside of the sphere.

The size of the integrating sphere should be much larger than the size of the test sample to keep measurement uncertainty low and independent of the spatial light emission characteristics of the test sample. If a smaller integrating sphere is used this must be accounted for in the calibration procedure of the measurement device.

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ResourcesCIE 127 Measurement of LEDs y

Colorimetry and Color Quality of Solid-State Light Sources, yWendy Davis, Optical Technology Division, NIST

IES LM-79-08 IES Approved Method for the Electrical and yPhotometric Measurements of Solid-State Lighting Products

IES LM-80-08 Approved Method for Measuring Lumen yMaintenance of LED Light Sources

ANSI/IESNA RP-16-05 Nomenclature and Definitions for yIlluminating Engineering

Works in Progress by IESNA, CIE, IEC, ANSI, DOE y

photodiode can be used for fast data logging and pulse synchronized measurement applications.

Spectral distribution data is provided by a separate diode array sensor. The spectral data enables the measurement device to calculate color data e.g. xy and u’v’ color coordinates, color temperature CT, correlated color temperature CCT, color rendering index CRI, peak wavelength λpeak and dominant wavelength. The sensitivity of photodiode array is controlled by integration time so the lower the light level the longer the measurement time. This makes diode arrays unsuitable for fast measurements. Longer integration times effect increases in both real signal and dark signal. To improve the signal to noise ratio offset compensation becomes an important rule for diode array sensors. Best offset compensation is done with a dark signal measurement using the same integration time as the signal measurement. A remote controlled shutter is built into the BTS256-LED tester to support synchronized integration time on-line offset compensation. Low light detection with CMOS photodiode array technology can therefore be achieved by employing offset compensation in combination with very long integration times.

As shown, accurate light and color measurement requires a good basic understanding of radiometric, photometric and colorimetric principles and quantities.

With this knowledge you will be better prepared to make decisions regarding objectives, instrumentation and compromise solutions.

A general plan of action would be to:

1. Determine the goals and purposes for the measurements

2. Estimate the acceptable levels of accuracy required

3. Decide which instrumentation best suits 1 &2 and if it meets budgetary requirements

4. Juggle all decisions for best fit – trade-offs may be necessary

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AbstractWith more lighting companies embracing LEDs as a move to improve efficiency, mechanical engineers are being asked to redesign fixtures to accommodate this technology. However, LEDs have different requirements from incandescent bulbs and florescent tubes which mechanical engineers need to take into account. This paper addresses several of these issues in making the move to LEDs.

IntroductionAs lighting fixture manufacturers begin to move away from incandescent and fluorescent light sources in favor of Light Emitting Diodes (LEDs), engineers need to adapt their designs to accommodate the changes solid-state lighting technology requires. Most of the focus on these transitions has been on the electrical changes to provide the correct currents to high-brightness LEDs. What is often overlooked are the mechanical considerations and how they affect the life of the emitters as well as the physical implementation.

There are several key differences between LEDs and other light sources. Solid-state lighting is more closely related to fluorescent tubes since both require more complex electrical drive, are efficient and emit little IR. However, there are other considerations plus new found advantages that can be exploited to provide new and exciting lighting fixture designs with LEDs.

LED BasicsLight Emitting Diodes, commonly referred to as LEDs, create light through a process called electroluminescence. During this process electrons give up energy in the form of photons when traversing the LED diode junction. The materials selected to make the junction (i.e. gallium, arsenic, phosphorous, indium, etc.) provide very specific properties. On one side of the junction the materials will have an abundance of charge (the “n-type” with electrons) and the other will have a deficit of charge (the “p-type” with electron “holes”). Between them is a zone where the electrons cannot exist (much like the shells of an atom). This zone is called the energy “band gap” or forbidden zone and the materials used in LEDs form band-gaps that have specific energies that provide photon emission. These are called “direct” band-gaps.

As positive charge enters the “p” side and negative charge enters the “n” side, the electrons combine with the “holes” and fall to a lower energy level emitting a photon. This is analogous to water flowing over a dam driving hydroelectric generators. The water is losing energy as it falls which turns the turbines and creates electricity. As the electron crosses the junction, the wavelength of the photon created is determined by the “band-gap” energy. By varying the band-gap, engineers can create LEDs that emit anywhere from the infrared, through the visible spectrum and all the way to the ultraviolet.

Modern high-brightness LEDs go further and use quantum containment to increase the efficiency of photon generation within the LED and some even provide optical wave-guides formed in the device to allow trapped photons to reach the surface. Some incorporate secondary phosphors which absorb some or all of the LED’s emission and through a phenomenon called Stokes Shift are able to create white light. Anyway you look at it these little devices would have impressed even Edison in their efficiency in converting electricity to light. But they have characteristics that fixture designers must consider when including LEDs in a product.

LED Advantages and DisadvantagesLight Emitting Diodes are current devices—that is, they require a constant current (not voltage) to operate correctly. Also, because of the construction of many white LEDs, you cannot simply vary the current and get smooth dimming. LEDs incorporating phosphors will shift blue if the current gets too low. There are special drive circuits such as the LM3445 made by National Semiconductor that will allow fixtures to be used in retrofit applications with existing TRIAC dimmers—this is quite difficult to accomplish due to the nature of LEDs. The LM3445 driver simplifies this by “reading” the TRIAC signal from standard dimmers and correctly controlling pulses of constant current to “dim” the LEDs providing a smooth, linear range familiar to everyone. Figure 1 shows a typical drive circuit.

LEDs for general lighting are very energy efficient since the emission is tailored to be visible to humans. Unlike incandescent bulbs, general lighting LEDs emit very little infrared energy. One big advantage of LEDs, especially in task lighting for kitchens, is they do not warm food or other items sitting below them—a common problem with incandescent task lighting. This can prematurely cause food products to spoil or melt. A fun little test is to place an unwrapped chocolate bar 18 inches from a 60 watt light bulb… it will melt quickly. This is not due to the visible light, but the large amount of infrared energy that is emitted and invisible to humans, but absorbed easily by the chocolate. An equivalent LED fixture would not cause this to happen.

LEDs do have a dark side. The biggest advantage—the lack of IR emission—is also a problem. Incandescent light bulbs shed most of their waste energy through the emission of infrared. LEDs must conduct their waste heat away through thermal management. The LEDs may not melt the food below them in a kitchen, but they might bake the cabinets above them (and anything in them). Also, if the LEDs are allowed to get hot, their lifespan will decrease. LEDs rarely die a sudden death (as incandescent filament bulbs do). They will lose intensity over time. An LED is said to need replacing where the emission has fallen to either the 70% or 50% point from the original intensity. These are called the L70 or L50 points. Figure 2 shows a typical L50 point based on the temperature of the LED.

Many new high-brightness LEDs include thermistors or RTDs to help protect the device from damage in extreme conditions. The thermal device can be used to fold back the current or energy being supplied to the LED which in turn will reduce the temperature of the device. The LM3424 is an example of a special LED driver designed to incorporate thermal fold-back to protect the LED. This type of control can greatly increase the lifespan of LEDs where temperatures my rise and cause damage such as in emergency lighting, out door lighting, or high intensity applications.

ConclusionsLEDs have great advantages over other light sources. They are very efficient in producing light, are tiny point emitters which provide designers with an array of options for fixture designs and come in every imaginable color (or colors that can be mixed dynamically). They also have very long life-spans when driven correctly—on the order of 20,000 to 50,000 hours or longer. The major downside is managing the waste heat of the current devices. Heat sinks and other passive methods are best suited for low cost fixtures, and active cooling such as that produced by Nuventix can be applied to commercial high-intensity applications. With a bit of thermal management and mechanical tricks, LED fixtures can be engineered for a long life and novel appearance as well as saving significant energy.

What Mechanical Engineers Should Know About LEDsBy Richard Zarr, National Semiconductor

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1. IntroductionThe need for light has become one of the highest priority problems to deal with for the electronic world. And when this need links very stringent geographical and environmental conditions with life saving situations, the priority immediately becomes an urgency—everyone in the electronics world is called to duty to find a solution.

In the engineering world, the best way to solve a very complex problem is to reduce it to a combination of smaller, simpler problems so as to address each of the resulting ones with a quicker, state-of-the-art solution.

2. The Problem IdentificationIn order to comply with this approach, we can split the main problems into two categories:

Need for light y

Lack of a power network y

The need for light issue reflects several potential situations, which can be grouped into four main areas:

Entertainment: Outdoor night time activities such as beach ygames, tournaments, and camping, often require bright lighting sources for safety and accuracy.

Research and Monitoring: Special weather monitoring and ygeological monitoring systems reside in stations very far from civilization and definitely as far as possible from any electromagnetically noisy environments. Roads leading to these remote locations are typically ‘off the grid’ and dangerous, and overnight expeditions require adequate lighting for safety and observation.

Standard Life/Educational: in non-developed areas, there can ybe a complete lack of light during night disruptions and night time emergencies; lighting is also a must for schools with both day and evening classes in remote places where the daylight hours are used for work and family survival.

Life-threatening situations: emergency camps or temporary ymovable hospitals are normally situated in non-serviced areas where bright, stationary light is crucial for medical support and for even the simplest life saving operations.

The need of a power network in non-developed areas seems obvious: remote villages need lighting for schools, hospitals, and jobs. And although the very lack of power networks in natural oases like parks and paradise landscapes is part of their appeal, there are unfortunately cases where the lack of power suddenly becomes a top priority problem for life improvement situations and for more critical life saving situations as well.

Many of the most unfortunate situations happen in cases where there is a pre-existence of a power network but due either to natural disasters (tornados, volcano eruptions, etc.) or to war conditions, the network is inaccessible or very dangerous due to uncapped connections and open wires with high voltage floating around. In these horrible situations, the above-mentioned need for light for emergency camps and any other life saving activities comes into play.

3. Solar Powered LED solutionsThe needs described above require a set of solutions that combined together will result in a single system that can address all of the issues triggered by difficult environments.

Requirements for our solutions:

Use the Sun y

Make it Last y

Make it Bright y

Lighting in rural areas and in developing countries is generally provided by wax candles or combustion lamps (e.g. kerosene), while battery-powered flashlights are typically only used as occasional, portable lights for intermittent use.

Combustible sources are cheaper than any form of electricity. On the other hand the low efficiency, the poor quality of the light, and the intrinsic fire risk advocate the use of electronic lighting in off-grid locations. The unavailability of the power grid implies that the electrical energy must be produced locally. Among the methods by which energy can be produced, photovoltaic systems (solar cells) are by far the most universally applicable.

The general principle is to convert the sunlight, in particular energy carried by photons, into electrical energy.

The use of photovoltaic systems brings some advantages:

Solar energy can be produced locally; hence the solar panels ycan be installed everywhere (also in areas of difficult access) without the need for infrastructures. This also minimizes transmission/distribution losses.

Renewable energy source; it does not impact the environment ywith pollution.

Facilities can operate with little maintenance or intervention yafter initial setup; this contributes to reduce the energy cost.

Low voltage generation; this simplifies the downstream yconversion of the power.

However, energy production by photovoltaic systems must take into account the intrinsic periodicity of the solar source (e.g. during the night the energy source is absent). Electrical storage into batteries assures a continuous availability of energy.

Once the electrical energy has been produced and stored, the next step is generating light in a wise way, in terms of energy saving and environmental respect.

Towards this end, the use of white LEDs is moving to the forefront for several reasons:

High luminous efficiency (more than 100 lumen/watt), which yimplies less wasted power in comparison with other light sources (e.g. incandescent bulbs).

Hazardous materials free (mercury or toxic gases), which makes yLEDs the cleanest light in ecological terms.

Low driving voltages, making LEDs particularly suitable for yphotovoltaic systems.

Integrated Solar Powered Lighting SolutionsBy Luca Difalco, STMicroelectronics

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In the interest of maximizing the power transfer, a dynamic system called a Maximum Power Point Tracker (MPPT) should be implemented between the solar panels and the load (battery).

This circuit samples the voltage and the current at the output of the photovoltaic panel. Then, controlling the downstream DC-DC converter (which can be a step-down or step-up depending on the panel size and the load), the output current of the photovoltaic panel is regulated in order to maximize the power transfer (see Figure 3).

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Figure 3. Block diagram of a photovoltaic system

Without the use of a MPPT dynamic system, the power delivered by the PV panel would change a lot not only with the illumination and the temperature, but also with the battery charge level.

This can further reduce the PV panel efficiency by 30% or more.

(1) Testing conditions to measure PV cells nominal output power that supposes an irradiance of 1kW/m2, solar spectrum of air mass 1.5 and module temperature 25°C.

In order to maintain the battery life as long as possible, a dedicated charging profile has to be performed. At this power level this usually involves a microcontroller-like implementation to achieve what is shown in Figure 4 below.

Figure 4 MPPT and battery charger profile

5. ST LED driving solutionsNow that the energy has been converted from the sun and stored in a battery, the other big portion of the solution would be to transform this energy into bright light with low power consumption. In fact, the efficiency of the conversion of the electric energy into light is always a key point, but it gains further importance in this specific application with the target of minimizing the size and cost of the system (battery and PV panel included).

With their efficiency in power, LEDs make a huge contribution to luminance conversion. Among the all lighting sources and techniques available in the market, LEDs provide the best performance in terms of light efficiency and pave a perfect path to the future as shown in Figure 5.

Lately, significant steps, especially in terms of power capability, have been achieved in LEDs technology. The availability of LEDs up to and above 5W, combined with their high luminous efficiency, contributes to their diffusion in the lighting market as replacements of traditional, and often less efficient, light sources.

The solar LED lighting system described in basic outline above is represented in figure 1.

+ -+ -

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PV cells

batteries

LED lamp

+ -+ -+ -+ -

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PV cells

batteries

LED lamp

Figure 1 Solar LED lighting system.

4. PV battery charger systemsThe operating principle of photovoltaic (PV) systems is quite simple: the sunlight strikes the solar panel, manufactured from semiconductor materials; the energy carried by the photons causes the generation of electron-hole pairs in the semiconductor that, in turn, generate a current that flows into a load connected to the panel.

PV panels are realized through different technologies: monocrystalline silicon, polycrystalline silicon, and thin film.

However, the first two technologies are the most widely employed since they offer the highest efficiency (in the range of 20%), where the efficiency is defined as the ratio between the power produced by the panel and the luminous power that strikes the panel.

Thus the peak power achievable with a PV panel at STC (Standard Test Conditions(1)) can go to almost 200W/m2.

Commercially available PV panels are offered in different sizes, proportionally delivering different power: around 200W for the 1-1.2 m2 modules (typically 72 solar cells) used in home applications, up to 80W for the modules for street-lighting, from 12W to 24W panels for solar lanterns.

In a PV panel, all the cells are generally connected in series, and the typical voltage of each cell, when delivering the maximum power, is around 0.5V (see Figure 2).

Figure 2. Solar cell V-I characteristic.

This voltage, however, depends on the temperature and the illumination level.

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Taking into account less than ideal illumination conditions, the actual power delivered by the panel can be halved; thus considering on average ten hours of sunlight, the energy produced by the panel during the day can be estimated as around 200Wh.

This implies that the energy produced by the panel would be enough to supply the lamp for about 16 hours, fully covering nightly lighting needs in any season.

When the application requires a boost conversion, a solution could be provided, for example, by the LED7707. This is an appropriate solution for all the typical input voltages (6V, 12V and 24V batteries), whenever the output voltage (fixed by the number of LEDs in series) is higher than the input voltage.

Each one of the integrated six current generators can provide from 20mA up to 85mA, and the current can be simply adjusted by a resistor (see Figure 7).

In solar LED lanterns, a typical battery voltage is 6V (ranging from 5.5V up to 7V).

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Figure 7. 3W LED string driven by LED7707

Typically, a 15W solar panel is appropriate for solar LED lantern solutions. Under the same hypothesis of sunlight duration and illumination conditions mentioned in the previous section, the energy produced by a 15W panel is 75Wh.

Therefore, the energy produced by the 15W panel is by far sufficient, assuring more than 20 hours of autonomy.

6. ST Solutions and further integrationThe below block schematic (see Figure 8) shows how the ST system solution approaches the all above requirements in one single system from the MPPT algorithm to the battery charger related profile, the LED driving, and every kind of system protection towards the Panel, the battery and even the lamp, making the entire architecture safe.

Figure 8 Block Schematic of the ST Solar Battery Charger LED Street Light Solution

Figure 5 Evolution of Lighting and LED positioning

However, the method of driving LEDs should also be optimized to improve the overall efficiency.

Because they are current controlled devices, the main requirement to drive LEDs is to control the current, which, in turn, determines the brightness of the light emitted.

And any LED driving solution must consider the application conditions:

Input voltage range—according to the use, different battery ytypes can be chosen: from 6V batteries in the case of solar lanterns, to 12V batteries for home lighting applications, up to 24V batteries mainly dedicated to streetlight solutions.

Number of LEDs and how they are connected—LEDs can be yconnected in series in a single string or arranged in multiple parallel strings. The number of LEDs in series defines the output voltage of the conversion (typical voltage drop across a white LED is around 3.5V), whereas the number of parallel strings indicates the total current to provide.

LED current—high brightness LEDs are supplied by currents of yhundreds of milliamperes (up to more than 1A).

Depending on the battery voltage and the number of LEDs connected in series in one string, a buck or a boost conversion can be the most suitable solution.

STMicroelectronics offers, among a wide range of products for LED driving, both a buck converter (L6902D, [2]) and a boost converter (LED7707, [3]) dedicated to LED driving. Both devices feature techniques to control the current and a high efficiency conversion.

The L6902D is a step-down switching regulator mainly used for home and street lighting.

6W LED string

L6902

FB

CS+CS-

IN VOUT

sense resistor24V

350mA

100mV

6W LED string

L6902

FB

CS+CS-

IN VOUT

sense resistor24V

350mA

100mV

6W LED string

L6902

FB

CS+CS-

IN VOUT

sense resistor24V

350mA

100mV

Figure 6. 6W LED string driven by L6902D

Figure 6 shows a 6W LED string driven by L6902D.

An example of the L6902D used as an LED driver is in street lighting. A 40W solar panel is suitable for this application, and a 12W lamp, considering the high efficiency of LEDs, could be more than enough for illumination of local roads in rural areas.

In a typical application, a 12W lamp can be realized by two strings of 6 LEDs (connecting in series two 3W LED modules), each one driven by one L6902D.

DEsigNiNg with LEDs EDN 20

to further improve the digital function integration, delivering an optimum battery charger profile on top of the already achieved functions with the previous integration levels.

The resulting device main features and characteristics are listed in Table 1.

Table 1 Integrated Solar Powered Battery Charger

Solar lanterns Streetlights

PV panel

Peak power 24W 50-100W

Open circuit 17V 34V

@ MPPT 12V 24V

Lead acid battery

Charge level 6V 13.8V

Energy storage 4.5Ah 20Ah

LED

Power 5-10W 20-40W

Below is a list of the resulting IC features:

Power switch and synchronous rectifier integrated inside IC y

Operating voltage range 3V to 38V y

Perturbation and observation method for MPPT y

Constant current for bulk battery charge and constant voltage yfor floating charge

Battery status indication and automatic day/night detection via ysolar panel

Input pin for battery temperature feedback y

Protection for solar cell short circuit, battery over voltage and yover temperature

7. Conclusions The challenge of providing light in rural areas can be achieved by producing energy locally. Photovoltaic systems make it possible to exploit a readily available energy source while at the same time respecting the environment.

The storage of the energy into batteries overcomes the intrinsic discontinuity of the solar energy.

Light emitting diodes, ever more present in lighting solutions, seem the most appropriate choice for energy saving thanks to their high luminous efficiency.

Moreover, the availability of different LED drivers, with buck or boost configuration, provides flexibility in lighting systems design and high efficient power conversion solutions.

ST’s technologies enable a ready-to-go discrete implementation of all the above features as well as an integration path towards a one or two chip implementation, achieving miniaturization and the best efficiency targets.

Bibliography:[1] “Imaging India. Ideas for the new century”, pages 258, 454, 458 —Nandan Nilekani, Penguin Books India.

[2] Low Voltage LED Driver Using L6920D, L4971 and L6902D—Application Note AN1941, STMicroelectronics.

[3] 6 rows—85 mA LEDs driver with boost converter for LCD panels backlight—Application Note AN2810, STMicroelectronics.

A real implementation of the above block schematic has also been realized to prove the technology and system performance (see below Figure 9). This reference design uses state of the art microcontroller technology as well as best in class analog drivers and discrete power components—all from the ST portfolio of technologies and products.

Figure 9 Real Implementation of the ST Solar Battery Charger Led Street Light Solution

For this demanding industrial application, the challenges of innovation and miniaturization that a leading semiconductor company has to achieve are even more stringent than usual requirements for the power conversion segment. To remain at the leading edge of technology and solutions, ST is engaged in an integration path whose roadmap is seen in Figure 10.

Figure 10. Solar Roadmap Integration Path

Since most of the challenges described above were related to the MPPT and Battery charging profile setting, the first and most important activity in the integration path has engaged research and development resources in first integrating the panel needs and then the battery charging needs, leaving the LED driving as a separate function. This will give the final system designer the flexibility to decide on configurations and other system parameters that could not be made as flexible in an integrated design.

The first and most simple step taken on the integration path has been towards efficiency improvement right out of the panel, making the widely used Bypass schottky diode, an intelligent one integrating best-in-class analog IC technology with ultra low Rdson MOSFETs, to reduce by over one third the losses at that portion of the conversion.

The next and most important integration level was implemented when achieving the MPPT function together with a high power DCDC boost converter into a single silicon delivering over 80W with 92% efficiency. The last portion of the integration has been

DEsigNiNg with LEDs EDN 21

On the Useful Lifetime of LED Lighting SystemsBy Geof Potter, Power Technologist, Texas Instruments, November 2009

AbstractAlthough they are relatively new players in industrial and residential lighting, high-brightness light-emitting diodes (HBLED) are actively being characterized for field reliability prediction by many of the major manufacturers of lighting products. That process requires collection of meaningful long- term experience data upon which credible estimates of expected operating life under actual field conditions can be published. Without credible estimates, field acceptance of LED luminaires will suffer.

This paper reviews what is known about the lifetime of LED’s in the field and discusses how to estimate the operating life of LED-based lighting systems by including reliability factors associated with LED drivers (sometimes called ballasts) that provide controlled voltage or current to power the devices. Coordination of LED and driver life expectancy is necessary for commercial lighting products if they are to fulfill the “long-life” promise of solid-state lighting.

Users of LED lighting products are legitimately concerned about useful life, because the price of HBLED based luminaires has not yet seen the cost benefits of sustained mass production on the scale of incandescent or florescent lighting devices. Reasonably accurate prediction of time-to-failure is only practical where substantial information about the design, construction and environment for actual applications of LED-based products is available. Norms have not yet been established in the minds of the public by high volume manufacturers based on consistent field performance. Consequently, this paper approaches LED lighting longevity by discussing several key factors that influence useful life in order to identify ranges of probable lifetimes based on estimation of application conditions and the general nature of the product design.

It is important to distinguish lifetime from reliability as applied to luminaires. Lifetime is the amount of useful operating time available from the vast majority of lighting product units, under prescribed conditions, exclusive of random failures. Reliability is a term used to describe how often such products fail randomly (i.e. exclusive of infant mortality or wear-out conditions). This document aims to present factors influencing lifetime. Poor reliability resulting from design or construction of luminaires can adversely affect the product lifetime by distorting the impact of premature failures and the apparent onset of wear-out.

A hypothetical LED LuminaireFor reference, in Fig. 1, below, is a simple diagram of an assembly consisting of a light diffuser, a box (chassis), an LED mounting / cooling plate and a circuit board containing components of a constant current driver that provides regulated power to the LED array. Of course this is a representation of some basic elements of a luminaire, not a real device, and is intended to emphasize the point that such a product has more parts to consider from a lifetime viewpoint than just the LED devices themselves.

CHASSIS

( SIDE VIEW )

DBC substrate

COVER

LEDMOUNTED

&CONNECTED

DRIVER CIRCUIT BOARD

FRONT VIEW(14 LEDs )

DRIVER OUTPUT ABOUT 20 WATTS

ALUMINUM ELECTROLYTIC

CAPACITORS (e-caps)

DIAGRAM OF HYPOTHETICAL LUMINARIE PRODUCT SHOWING THE ENTIRE ASSEMBY INCLUDING LEDs, DRIVER BOARD, CHASSIS AND DIFFUSER

Diffuser

Diffuser

Fig. 1

HBLED LifetimeOne advantage that LED devices have over incandescent lamps is their lack of “hard” failures. LEDs display a wear-out mechanism rather that a propensity for catastrophic failure. In fact, the lighting industry has standardized on a definition of failure that declares “end-of-life” to be the point at which LED output (Lumens) has diminished by 30%. The so-called L70 point, is the point at which most people can perceive a loss in intensity and is often used as the measurement standard for failure.

LED output degradation occurs, in part, as a result of reduced transmission of light through the LED package, that is, a lens and the encapsulant that reduce internal reflections within the assembly. There is also a reduction in actual light generation within the die due to an accumulation of defects in the lattice occurring with use. In devices that have external phosphor coatings to adjust color, there is also a loss in phosphorescence that occurs with use.

Fig. 2 Side-view (cut-away) of mounted HBLED with associated structure [source: CREE ]

DEsigNiNg with LEDs EDN 22

Most often, LED devices for lighting applications are mounted onto some sort of thermally conducting structure which is then exposed to ambient air for heat dispersion. Aluminum panels with insulated copper conductors (DBC technology) are particularly useful for large numbers of devices where 100 watts or more is to be dissipated, although panels for OEM luminaire applications are available with fewer than 10 LED units mounted.

Average LED lifetime, from the chart above, is typically 50,000 hours provided forward current, ambient temperature and junction temperature are maintained as specified, meaning temperatures below 105 ºC in typical environments. Junction temperature limits are typically set just above 100ºC to maintain compatibility with substrate materials and packaging as well as to improve device lifetime.

LED DriversHaving briefly described factors influencing the life of LED devices, it is now appropriate to examine a similar aspect of power supplies (drivers) that power them to gain a more complete picture of overall lighting product life.

LED drivers that provide controlled current for LED luminaires share several things in common. Regardless of electronic topology, drivers use soldered interconnections, solid-state switching devices and integrated circuits for control and monitoring. There are also passive components (fuses, resistors, capacitors, connectors, wire, etc.) involved in driver construction. All of these components have failure rates that contribute to the overall reliability picture. In particular, those with significant wear-out mechanisms will contribute disproportionately, and those without contribute almost nothing. There are a few elements used in most drivers that display wear-out mechanisms that are significant relative to the lifetime of LED’s themselves.

Non-Solid Electrolytic capacitors are infamous (not always justifiably) as lifetime limiters in many types of electronic apparatus, and are often cited as the limiting factor in LED driver designs. There are, however, other sources of failure that can be significant, sometimes more significant than “e-caps”.

Soldered interconnections can also display early wear-out, particularly in thermally dynamic situations. Because solder joints are ubiquitous in electronic subsystems and because they are manufactured under a wide variety of quality and process controls ranging from excellent to non-existent, it is both important to deal with interconnect reliability and difficult to predict it.

Optical Coupling Devices (Optocouplers) often used in current (or voltage) regulation feedback paths, have a wear-out mechanism similar to high brightness LED’s. Because they contain an LED light source they are subject to light intensity degradation over time resulting in a reduction in Ctr, or current transfer ratio. If sufficient margins have not been provided in the driver design to allow for a gain reduction due to Ctr drift, loop gain can be compromised resulting in operational failure.

Generally, other active and passive components mentioned above have field failure rates significantly lower than e-caps, solder joints and opto-couplers. Connectors, for example have long-term oxidation issues, especially when used in low voltage circuits (which is not generally the case in LED drivers), some types of resistors can have very long-term drift characteristics, and so forth.

The net effect of both degradation modes is presented as a set of life curves vs operating hours. often for each of several ambient temperatures and drive levels. The curves, published by LED manufacturers, apply only to the diodes themselves, and so, do not provide a complete estimate of the useful life of a complete luminaire product.

Of course, LED life curves are not meant to represent a single device, but rather, the center point of a bell curve that represents lifetime in a large LED population. They predict the point in time by which 50% of the devices have “failed”, or drifted out of specification, (i.e. the “average” lifetime as identified by the term L70).

Operating Hours

Num

ber o

f fai

lure

s

Point Estimate of Failure Rate

50% Failed

Typical maximum operating point target

Fig. 3 Example of HBLED Lifetime curve.

(This is where the commonly used 50K hour life estimate comes from)

In the chart above, the CREE XLamp XR-E average lifetime (the bell curve peak) is plotted as a function of junction temperature. It is apparent from the chart that device junction temperature is a major determining factor in LED longevity. It is also apparent from the separation of forward current curves that drive level (forward current) is another important factor. At lower current levels, defects in the crystal lattice develop more slowly, so the fraction of current that generates photons is higher compared to the fraction that represents non-radiation current.

Building reliable luminaires is very much about controlling the temperature of LED devices and maximizing the longevity of their drivers. Generally, LED temperature control involves heat management through passive dispersion techniques. Heat generated by the diodes must be conducted to a location where it can be effectively coupled to ambient air. Active coupling, by artificially increasing airflow past a coupling medium (heat sink) typically by means of a fan, is more effective than purely passive techniques, but air movers add considerable cost to the lighting product and, more importantly, with their own failure rate they are likely to degrade system reliability more than they improve it. Of course, fans are also problematic for noise and power reasons.

DEsigNiNg with LEDs EDN 23

processes. Life doubles for each 10ºK decrease in process (capacitor core) temperature.

A key point to note is that the expected life of standard grade e-caps, operating at temperatures that are not atypical for passively cooled commercial and consumer products (65ºC), are in the order of 8000 hours (about 11 months ). Of course, many factors affect actual lifetime (on-off cycles, ripple current, etc.) of LED drivers, but it is clear from this chart that an LED based lighting product using a driver that contains standard grade e-caps is much more likely to be limited in lifetime by those e-caps than by LEDs.

An empirically derived formula, based on extensive measurements over long periods of time, used by Cornell Dubilier and other capacitor manufacturers, includes operating DC voltage in addition to temperature to provide a more precise estimate of e-cap life.

L=Lb·Mv·2((Tm-Tc)/10)

L=Life in hoursLb=Life at rated voltage and temperatureMv=4.3 - 3.3(Va/Vr) Va=applied voltage Vr=rated voltageTm=Rated temperatureTc=Operating core temperature

Values for Lb, Tc and Vr are published by capacitor manufacturers and identify various grades of e-caps such as 85°C, 105°C, etc. Notice that 10°C increments in core temperature rise will adjust life by a factor of 2.

Core temperature, one critical factor in capacitor life, is determined by a combination of internal power loss I2R (from ripple current and ESR), external ambient temperature and airflow that tends to lower the effective thermal resistance of the capacitor body. When e-caps are used in low ripple applications, ambient temperature often provides a close approximation to core temperature.

Considering the concept of failure rate (random failures occurring during useful life of the capacitor) could also be used when estimating e-cap life. Capacitor reliability can be compromised by stressful operating conditions leading to premature failures not

Non-solid Electrolytic CapacitorsBecause they are required to operate at low “line” frequencies of 50 or 60 Hz, drivers must have stored energy to fill-in gaps between cycles where the instantaneous input voltage is too low to operate the converter. Capacitors with relatively large CV product (capacitance x voltage rating) to provide cycle-by-cycle ride-through are necessary, resulting in little choice for designers other than to use aluminum electrolytic capacitors, either on the input side or on the isolated output side of the driver.

Non-solid e-caps contain a liquid or gel electrolyte that permits migration of electric current across an internal insulator placed between electrodes. These devices are carefully sealed to prevent loss of electrolyte through evaporation, but in smaller sizes, a “dry-out” phenomenon can occur with time and elevated temperature. Larger sized e-caps seldom actually “dry-out” but electrochemical degradation leads to deterioration of important electrical parameters such as capacitance and electrical leakage, resulting in gradual failure. Heating due to I2R loss (ripple current2 x ESR) contributes to internal temperature rise, as does conduction into the capacitor from a high external ambient, but radiation from a nearby high temperature object (a transformer, or even other e-caps with high ripple current) can also contribute to core temperature rise and should be considered when designing driver layout. Thermal insulation placed between the hot object and any nearby electrolytic capacitors will improve longevity by lowering core temperature.

Below is a graphical representation, provided by VISHAY, showing principal trends of parameters associated with electrolytic capacitors at constant temperature. It is helpful because it shows concisely the relative expected lifetime of several grades of e-cap products: Standard, Long Life, Extra Long Life and Solid Aluminum types. Note that the curves are calibrated at relatively high temperatures and have three scales to show the effect of lower temperature operation for each capacitor type. Useful life increases as temperatures are reduced according to the famous Arrhenius relationship between temperature and the rate of chemical

Fig. 4 Lifetime curves for various grades of non-solid electrolytic capacitors (Source: Vishay)

DEsigNiNg with LEDs EDN 24

built by a wide range of companies with widely varying quality levels associated with their assembly and testing procedures. A user purchasing an LED luminaire would expect the product to perform normally over a long period of time, especially if that user is familiar with lifetime numbers published by LED manufacturers (commonly 50,000 hours). In fact, the luminaire lifetime is much more likely to be limited by the life of the driver and may actually be substantially less than 50,000 hours. This is particularly true if the particular driver circuit design stresses components (e-caps, for example) significantly or the manufacturing process used to build it is not equipped to guarantee the physical integrity of its soldered assemblies.

Traditional failure rate based analysis of drivers accomplished by counting components and assigning failure rates from established tables (MIL HBK 217-x) is not an effective solution to the need for lifetime information, mainly because of the difficulty extrapolating all the rate information from published conditions to actual conditions. Also, the published failure rates assume that design deficiencies are not involved. That is, failures are assumed to be random and constant, i.e. the flat bottom of the famous “bathtub curve”.

Another method for predicting expected lifetime of luminaires (including drivers) is more meaningful, but much more time consuming and expensive. One can build large numbers of the products and subject them to accelerated stress testing to derive a so-called “activation energy” exponent for the entire assembly and use that in a failure rate calculation. This usually takes 6 months or longer, depending on the number of samples, and would be useful only for the tested configuration or close facsimiles.

Possibly the most attractive scheme for predicting the life of LED systems is to apply a “physics of failure” approach such as that proposed (and practiced) by CALCE at the University of Maryland. This approach is applied in three phases and is aimed at answering the question “how much life is left in this device / product / assembly / etc. When performed on a new product design, it can estimate useful lifetime. The process begins with an analysis of degradation in the assembly resulting from storage and / or use. Physics-of-failure models are then applied to the degradation modes to define test conditions that produce similar degradation over a much shorter time. Accelerated tests are then performed (on a smaller number of units) and the results are analyzed to determine if the test item(s) have enough life left to meet expectations. Failure models apply to all aspects of a product design including electrical stress. With well designed drivers, electrical stress is managed to the point where it is secondary to other causes of wear-out. Non-solid electrolytic capacitor wear-out has already been discussed.

Often, when a physics-of-failure approach is used on electronic assemblies, it identifies soldered interconnections as a key source of degradation. Connectors are also frequently identified as failure prone, but typical LED drivers have few, if any, connectors so for the purposes of this discussion they are ignored.

Fig. 5, below, shows a graphical depiction of relative instantaneous failure rates for solder joints compared to typical electronic components. These curves represent the right hand side of those familiar bath tub curves that reliability estimators commonly use. It is assumed that the left hand side, the so-called “infant mortality” characteristic, has been eliminated by any of several manufacturing means prior to shipment of the lighting product. Of course, for any given electronic assembly, the curves will vary considerably due to environmental conditions, operational duty cycle and the quality of assembly processes involved. Note that individual solder joints are thought to have lower failure rates than typical electronic

associated with wear-out. Again, Cornell Dubilier estimates e-cap failure rates using a formula that considers temperature, voltage and capacitor physical size (represented by capacitance). This analysis is useful when a bank of capacitors is used to increase energy storage. Fundamentally, reliability decreases with increasing energy storage.

To estimate random failure rate:

λ=4·105·N·Va3·C0.5·2(Tc-Tm)/10/Lb·Vr2λ=Failure rate in FITN=Number of capacitors C=Capacitance in farads

Failure rate, λ, defines the # of units failing per unit time. One FIT equates to one random failure in 109 unit hours.

For example: assume 2 x 100uf / 450 Vdc 2000 hour standard 85°C e-caps, running in a 70°C environment, with 80% of their rated voltage applied and full rated ripple current…..

λexample 1= 325 FIT or 3.25 x 10-7 failures/hour which equates to roughly one random failure due to e-caps in 350 years.

If larger capacitors were used in the design and their voltage and current stress was higher:

assume 2 x 1000 uf/450 volt 2000 hour standard 85°C e-caps, running in an 80°C environment at rated voltage and ripple current…..

λexample 2= 4025 FIT or 4.02 x 10-6 failures/hour or roughly one random failure in 28 years.

It appears from these two examples of e-caps are unlikely to cause driver failure as long as operating conditions remain within the e-cap ratings. However, the projected lifetime due to wear-out for the same examples yields significantly different answers:

Lexample 1=Lb∙Mv∙2((Tm-Tc)/10)=9320 hours or about 13 months.

Lexample 2=2828 hours or about 4 months.

Example 1 describes a design scenario for low power luminaires that might be found in the field. Voltage and temperature de-rating has been applied, but the wrong capacitor type was chosen, Example 2 is much less reasonable due to design stress. Changing the parameters of example 1 by stipulating that 8000 hour, 105°C capacitors be used would improve the life result to 150,000 hours, eliminating impact by e-caps on the total product life.

LED lighting products present a wide variety of operational conditions including high density configurations (“Edison” style bulb replacements, for example) where 60°C operating temperatures are not atypical. There are also examples of much more benign situations where a driver assembly is located in a separate compartment and is not subject to direct heating by the LED array being powered. While there are many luminaire designs that do not stress e-caps enough to significantly reduce their lifetime, and there are e-caps that are much more suitable for high temperature applications than the one cited in the example above, the message is that if longevity is important (as in street lights, for example), careful attention to e-cap operating temperature and voltage ratings is critical, or the LED driver can substantially reduce the effective life of the system.

Soldered InterconnectionsAnother area of potential field failure in typical LED driver assemblies, for the purposes of this discussion, is a broad spectrum of failure mechanisms that are labeled simply as Soldered Interconnection failures. They are especially significant in assessment of LED driver reliability because LED drivers are

DEsigNiNg with LEDs EDN 25

There have been advancements in material technologies associated with optical isolators and couplers in recent years that make newer couplers more stable and reliable, but they also have increased cost making improved parts less attractive for low-cost lighting products.

KHrs

Diode Current (ma)

4N32 devices

Life defined as 50% loss in Ctr

50 KHrs

Fig. 6 Example Life Curve for common low cost Opto Coupler

Other Components in LED Driver assembliesSemiconductor components found in typical drivers are not subject to wear-out within the lifetime range of non-solid electrolytic capacitors, variable quality soldered interconnections and low-cost opto-couplers. When solid-state devices are operated with reasonable thermal, voltage and current margins (relative to their specified working limits) their lifetimes far exceed the devices discussed above. The same is true of passive components such as small capacitors, resistors, wound inductors, and miscellaneous mechanical elements. Published FIT rates for these “other” components in a driver would be multiplied by the number of pieces per driver and added to yield a composite picture of their random failure rate, λother.

Typical FIT rates for semiconductors are ~ 5, resistors, small ceramic capacitors, signal diodes etc. are <1, so that λother would add up to perhaps 20 FIT for a small single stage LED driver assuming good design practice has been followed throughout.

Summary and RecommendationsWhen users buy LED lighting devices, they naturally ask the question “how long will this product last?” This question is difficult to answer precisely, so the best response is: “it depends!”

In fact, the lifetime of LED luminaires does depend, substantially, on several important factors:

1. Operating temperature of the LED’s, themselves

2. Operating temperature of any power supply (driver) that provides current to the LEDs

3. Types of components used in the driver design

4. Number of components and soldered interconnections in the driver and LED array

5. Power cycle rate

6. Environmental conditions including temperature, and temperature variations

components during normal life but begin to “wear-out” earlier due to structural breakdown induced by normal operational fatigue. In the case of solder joints that are subjected to high thermal fatigue (repeated heating/cooling cycles), wear-out begins significantly earlier. Also note that large numbers of solder joints raise the net failure curve and should be minimized for driver longevity sake.

Failures-in-Time (FITs) are the reciprocal of instantaneous failure rate and represent the number of failures per 109 unit operating hours. For example: if there are 100 joints in the driver assembly, operating under what are here called “High Stress” conditions, Fig. 5 shows that at the point where the LEDs are reaching end-of-life (50KHrs) the system solder joints are already in wear out mode with a failure every 2 weeks.

FIT rate=2 x 106 failures in 109 hours (from Fig. 5 below) or a Mean-Time-To Failure of 500 hours=about 3 weeks!!

The significant point is the range of wear out time involved. Fig. 5 also shows a substantially longer time between failures (38 Years) for those same joints if the stress regime is more normal.

FIG. 5 Comparative Failure Rates for Solder Joints

Optical CouplersOpto-couplers used in isolated converters in output feedback paths have wear-out mechanisms which are time and temperature dependent. Degradation occurs in the device transfer coefficient (Ctr), the amount of current flowing in the optical transistor compared to the LED drive current on the opposite side of the isolation barrier. Driver designs will consider sufficient margin for Ctr to degrade before problems with feedback loop gain become a source of failure. Typical curves for the lifetime of low cost opto couplers are shown in Fig. 6 below.

A continuous decrease in quantum efficiency of the diode half of common opto-couplers is the main cause of transfer coefficient degradation over time. Junction temperature and forward current density influence diode output degradation rates. There is also degradation in light transmission through compounds used to couple diodes and phototransistor devices resulting in further loss of Ctr as well as transistor detector degradation. The latter are usually minor factors. It is possible to encounter coupler degradation by moisture ingress through plastic molding compounds, but that is considered a design issue and not an inherent lifetime determining factor in typical lighting systems.

By running opto coupler circuits with low forward current in the LED half of the device, useful life can be extended enough to remove these parts as potential constraints on luminaire driver lifetime.

DEsigNiNg with LEDs EDN 26

Clearly this luminaire has the potential to last a very long time from the standpoint of random failures. Examining the life based on wear-out we see a limited life impact from the LED driver.

Life estimate for LED array 50,000 hrLife estimate for e-caps 425,000 hrLife estimate for solder joints 33,000 hr (based on 3000 FIT, 10 %

failures because solder joints are in wear-out period)

For discussion purposes, the above analysis shows that our hypothetical luminaire will likely come close to meeting the 50,000 (6 year) lifetime generally advertised by HB LED manufacturers. There may be a small reduction in life resulting from solder joint wear-out before the 6 year target is reached, but that estimate would vary widely depending on joint quality and environmental conditions. Our driver designer used “safer” e-caps for this application causing them to have virtually no impact on luminaire life.

References[1] S.G. Parler, Jr. “Deriving Life Multipliers for Aluminum Electrolytic

Capacitors” Cornell Dubilier Corp. IEEE Power Electronics Society Newsletter, Vol 16. No.1. Feb 2004, pp 11–12

[2] S.G. Parler, Jr. “Reliability of CDE Aluminum Electrolytic Capacitors” Cornell Dublier Corp. Whitepaper

[3] Nichicon, “General Description of Aluminum Electrolytic Capacitors” Nichicon Technical Notes Cat. .8101E

[4] J. Ben Hadj Slama, H. Helali, A.Lahyani, K.Louati, P.Venet and G.Rojat “Study and Modelling of Optocoupers Ageing” J. Automation & Systems Engineering, 2008

Assuming there are no special conditions to consider such as a corrosive atmosphere or unusual moisture situations, the lifetime of a particular luminaire design can be estimated by studying how it might fail and undertaking targeted tests to accelerate the likely types of failure. From results of accelerated testing, an “activation energy” coefficient can be determined and mean-time-to failure calculated from a classic formula.

A more practical approach, for a typical user dealing with a smaller number of luminaires, would be to consider:

1. Is the product going to be used in an elevated ambient temperature or in a confined space where internal heating may cause a substantial temperature rise?

2. If the driver contains electrolytic capacitors, are they rated for high temperature (105°C). (Low cost capacitors rated at 85°C can reduce the useful life of the product significantly).

3. Do e-caps have reasonable operating voltage de-rating under expected conditions? 80% is considered reasonable, lower is better !!

4. Does the driver contain a large number of components? A typical 15 watt single output, driver might contain 30 to 40 miscellaneous components yielding a failure rate on the order of 20–30. (Control IC’s raise the FIT rate much more than resistors, etc.)

5. Higher component count means more solder joints to contribute to FIT rate and reduced wear-out time.

6. Is the luminaire subject to significant thermal excursion such as natural cooling outdoor operation, frequent power cycling, hot or cold air blast, etc.?

7. Does the regulation feedback path contain an optical coupler? If so is it rated for the application conditions? (Low cost opto’s can be a problem.)

Returning to our hypothetical luminaire diagrammed in Fig. 1, we can make some assumptions about it’s design and application in order to evaluate it’s potential useful lifetime.

Operating environment 50°C. y

14 HBLED devices on an aluminum heat spreader, 700 ma. y100°C junction temp.

2 “long life” (105°C rated) electrolytic capacitors operating at y80% of rated voltage

130 PCB solder joints in the driver assembly and LED array y

1 opto-coupler in current control feedback path. Low FIT rate yas applied.

65 assorted passive components and an integrated power ycontrol chip.

Applying typical FIT rates for random failures:

LED 14 x 1.1=15.4e-cap 2 x 3.5 =7Solder Joints 130 x 0.2=26Optocoupler 1 x nil =0Control IC 1 x 5=5Misc comp 65 x 0.3=20

FIT rate 73.4 failures in 109 unit hoursMTBF >13 million hoursTime to 1% failures ~ 136,000 hours