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Selection of Switch Mode Power Supply (SMPS) for Reliability By Ron Wunderlich, PhD

DfR Solutions Open House March 16, 2015


History

o The following is a history of Switch Mode Power Supplies:

o SMPS were developed back in the late 70’s and early 80’s

o This technology helped to significantly reduce the size of

power supplies by switching at high frequency compared to

line frequency

o Previous designs were linear regulators, ferro-resonant

transformers and even motor-generators with phase control

(IBM Mainframes)


History

o Back in the late 80’s there was a shift in the industry

o SMPS were becoming a commodity no different than a

resistor or capacitor that you buy

o So, costs and profit margins were driven down over the

years

o Profit margins for a power supply is less than a few percent

while IC chips are 30% to 70%

o Almost all power supply design and manufacturing is now

overseas

o However, SMPS are still required

o They have become a “Necessary Evil”


Complexity

o Even though SMPS are treated like a commodity, they are

in some cases more complex than the final product they go

in

o Let’s look at some of the technology that is involved in a

SMPS

o Industry has shifted to making SMT parts but many power

components have to be PTH so there is a need to deal with a

mix of SMT/PTH components

o Need high voltage components like MOV’s to handle up to

4KV surges

o Heavy copper boards (2oz or higher) to handle high current


Complexity

o Need large amount of capacitance but with high ripple

current or else you can dry out aluminum electrolytics

o Need fuses that can open easily under faults yet survive

shock/vibe

o Inductors and transformer’s themselves are difficult

handmade sub-assemblies with ferrite material that have

complex behavior

o Semiconductors that must handle both high voltage and high

current and yet act as ideal as possible

o To cool devices need thermal cooling like heatsinks

o Need to operate at high temperature


Complexity

o Then there are regulations that need to be met

o For AC/DC’s there’s UL 60950 and others for safety specs

o To meet safety you need safety approved components

and the required spacing's between primary-secondary

o If there are safety issues in the field, this will result in a

recall

o Worse if someone gets shocked


Complexity

o Conducted and Radiated EMC

o Used to be just conducted noise was an issue

o Now with faster switching converters, radiated noise has

become an issue

o DfR has seen cases where the switching power supply

generated more noise than the high end processor and

FPGAs

o Layout and switching speeds are critical

o Require ferrite beads, Toshiba spike killers, common

mode chokes


Complexity

o Then there are standards (for AC/DC’s)

o IEC61000-4-2 ESD

o IEC61000-4-3 RF Immunity

o IEC61000-4-4 Electrical Fast Transient

o IEC61000-4-5 Surge

o IEC61000-4-6 Conducted Immunity

o IEC61000-4-8 Power Frequency Magnetic Field Immunity

o IEC61000-4-11 Dips and Sags


Complexity

o We are just scratching the surface


Selection

o Unless you do a full detail analysis, you won’t know

exactly how good the power supply is

o However, there are some guidelines that can help you in

your selection process


Selection

o Load current

o You know exactly what voltage you need but the load

current is not as well identified

o The common practice is to add up all the worst case currents

from all the devices in the circuit

o This will give you the worst case load current

o Do you select a power supply that is rated for the worst

case load current or 1.5X or even 2X ?


Selection

o My suggestion is to pick a power supply that is designed for

2X the worst case load

o Just like components, power supplies must be de-rated

o Why choose 2X ?

o In the OEM power supply market there is a dirty little secret

o They assume that customers typically run the power supply

at ½ load so some OEM’s will base their reliability on 50%

load


Selection

o Standards

o As discussed before, there are standards (for AC/DC’s) and

each standard has a level

o The higher the level, the more robust the product will be

o IEC61000-4-2 ESD

o IEC61000-4-3 RF Immunity

o IEC61000-4-4 Electrical Fast Transient

o IEC61000-4-5 Surge

o IEC61000-4-6 Conducted Immunity

o IEC61000-4-8 Power Frequency Magnetic Field Immunity

o IEC61000-4-11 Dips and Sags


Selection

o For example, IEC61000-4-5 Surge

o A product that meets Level/Class 4 will be more robust than

a product that meets Level/Class 3


Selection

o Fans for cooling

o For high power converters, a fan is required for cooling

o Fans are mechanical systems that wear out due to the

bearings

o They are limited by the number of revolutions

o There are many bearing types but all have their pluses and

minuses


Selection

o There are tricks for increasing fan life

o Use a large fan but run it at a lower speed (less

revolutions)

o Run the fan only when needed for cooling

o Variable speed fan based on cooling requirements

o N+1 fan cooling system

o Sometimes how the cooling system works will be specified in

the datasheet or else you have to ask


Selection

o Aluminum Electrolytics

o These type of caps have a liquid which can dry up over time

o The life of an aluminum electrolytic is determine by

o Lr = rated lifetime of the capacitor at rated ripple

current

o Tr = rated temperature of the capacitor

o TE = actual temperature of the electrolyte

o This formula is based on the evaporation of a liquid

102Er TT

rLL


Selection

o Here’s some interesting things

o The life decreases exponentially with temperature

o The rated life is performed at rated ripple current

o For SMT caps and some PTH, the rated ripple current is 0A

o That means as soon as you apply any ripple current, the life

decreases

o If the caps have rated ripple current > 0A and if you run

them at a lower ripple current, the life increases

o Capacitors with 85C rated temperature are usually made

by 3rd Tier vendors

o First Tier vendors have outsourced these caps but still have

their names on them


Selection

o The larger the capacitor, the higher the rated lifetime is

o Watch for local hotspots

o The transformer below is heating up the capacitor

o DfR has seen in one product a resistor running at 200C next

to the cap

o


Selection

o When looking over a power supply, you would like to have

a 105C/1000Hr minimum capacitor rating

o If possible, ask the vendor for the temperature profile of the

power supply and the ripple current

o This way you can calculate the life of the capacitors your

self

o Instead of life testing to see when the capacitor fails, DfR

has developed a method of using weight loss due to

evaporation as a method of determining life time


Selection

o Parts Position on the PCB

o If the parts are too close to the board edge, they can crack

because of the depanelization process

o If they are placed along the same axis as the bend in the

card, they can also crack


Selection

o Below shows the preferred axis of bending

o There are some components that are close to the edge but

the orientation is such that it minimizes the cracking


Selection

o In this case, parts marked in red are in wrong orientation

o Bend the board and they can crack


Selection

o Board Cleanliness

o You can tell a lot about the company from the construction of

the power supply

o A clean board shows the company is meticulous and has

good processes in place

o A dirty board can put you at risk with abnormal circuit

behavior and arcing


Selection

o This is a nice clean board

o No flux


Selection

o Hand solder flux left on board


Selection

o No clean solder flux left on board

o This is a problem with high impedance analog circuits


Selection

o More solder flux left on the board


Selection

o Solder Workmanship

o Because of mixed technologies (SMT and PTH), certain

components have to be hand soldered

o If you are going to hand solder, make sure its soldered

properly


Selection

o Poor solder workmanship


Selection

o Glue

o For high density converters glue will be required

o Its used to hold components together for shock/vibe

o It's also used as an insulating material to meet primary-

secondary isolation

o However, this needs to be put in by hand which can result in

mistakes

o Safety spacing is most critical


Selection

o Here’s an example with minimum amount of glue and not

used for critical safety areas

o This is a good design


Selection

o Poor glue operation

o Not only is this sloppy but they had to coat high voltage

wires to meet safety requirements

o This cannot be done consistently


Selection

o Tape

o Another method for decreasing the safety spacing's for high

density power supplies is to use tape

o Again, like glue, tape has to be put on manually and

mistakes can happen


Selection

o Here’s an example where only the transformer has tape for

safety requirements

o This is a good design


Selection

o In this example there is a significant amount of tape used

o Hopefully they don’t make a mistake

C01

Heatsink

C01

Transformer


Selection

o Assembly

o Overall look at how the power supply is constructed

o Ask yourself questions

o Are components being force fit ?

o Can you get a screw driver to mount a device to a

heatsink without slipping and causing other damage ?

o If there are problems in the assembly, most likely you will

see issues later on


Selection

o C4, Q6 and Q2 are pressed against each other

o The parts can crack over time with thermal cycling

C4

Q6

Q2


Final Word

o Final Word

o This presentation will give you enough information to choose

a power supply

o Also give an overview on checking the quality of the power

supply

o At DfR Solutions, we take it one step further where a

detailed analysis consisting of a paper/hardware review of

the circuit is performed next

o Also if there are issues with the power supply, we can help

diagnose the problem and suggest corrective actions


o The electrical review process entails the following

o Review of specifications

o This gives an understanding of the product but also if

there is sufficient information and are levels and ranges

correct

o Such as

o Is the operating temperature range sufficient

o Are there appropriate standards and high enough levels

Review Process


o The electrical review process entails the following (cont)

o Review of the circuit

o Understand and review the functionality of the circuit

o Make sure the circuit functions as is

o Sometimes suggest improvements in functionality or cost

o Check the robustness of the circuit

o Look at EMC and ESD aspect of the circuit

o Check the de-rating of components

o Power dissipation, voltage rating (even on resistors)

o dV/dt rating of FETs for LLC

o VGS breakdown of FETs

o Temperature rating

Review Process



o Stability

o Circuits with feedback loops like power supplies

o Even LDOs (like using low ESR ceramics where high ESR aluminum electrolytics are needed to be stable)

o Life of aluminum electrolytics

o Half of all design reviews run into issues

o Use temperature variation over seasons to be as accurate as possible

o Magnetics

o Transformer documentation and strcture

o Saturation current of magnetics

o Core loss

Review Process



o Component Risk

o Flag components that may have known issues or possibly be

discontinued

o Layout

o Placement of components

o Power flow

o Check for sufficient spacing's

o Software

o Averaging routines for noise signals

Review Process


Thanks

Any Questions? Contact Ron Wunderlich

[email protected]

The End
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selection of switch mode power supply (smps) for reliability slides...power supply (smps) for...

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