LED Fundamentals

Basics of Heat T fTransfer

08-16-2011

Cooling of LEDsHeat Path through a systemg y

Heat Sink Housing AmbientPCB

Heat tracks out to the ambient

Component Heat Sink Housing AmbientPCB

LED Fundamentals | Basics of Heat Transfer | Page 2

Basics of heat transferHeat transfer occurs when two surfaces have different temperatures, thus causing heat energy eat t a s e occu s e t o su aces a e d e e t te pe atu es, t us caus g eat e e gyto transfer from the hotter surface to the colder surface.

In the simplest of terms, the discipline of heat transfer is concerned with two things:H t Fl Heat Flowrepresents the movement of thermal energy from one place to another

Temperaturerepresents the amount of thermal energy availablep gy

[J]HeatTotal amount of energy transferred

Q

[W] = [Js-1]Heat Flow (Heat Transfer Rate)Heat Transfer per unit time

dQQdt

[Wm-2]Heat FluxHeat Transfer Rate per unit area

2d Qqdt dA

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Modes of Heat Transfer

Heat Transfer

Conduction Convection Radiation

v

e-m

m

e-

Energy transfer through direct molecular collisions (phonon transfer) and by conduction band electrons

Heat transfer by liquid motion. The fluid motion may be caused by density differences (free convection) or external mechanical

Heat Transfer through electro-magnetic radiation

forces (forced convection).

Within solids the heat transfer is done by heat conduction. In gases (air) the heat transfer is dominated by convection and radiation.

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heat transfer is dominated by convection and radiation.

Heat Transfer in a thermal system

Conduction

Convection

Radiation

Heat Transfer from the die to the surrounding environment by conduction, convection and radiation.

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Fundamental law of heat conductionFourier Law

When there exists a temperature gradient within a body, heat energy will flow from the region of high temperature to the region of low temperature.

TT L

TTAQ 21 Q dTA dx

The heat transfer rate (dQ/dt) is directly proportional to ( ) y p pthe area A and to the temperature difference along the path of heat flow.

Proportionality Ratio: thermal conductivity i t i l t

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is a material property

Thermal ConductivityThermal Conductivity of typical materials within LED-Systemsy yp y

Air0.026

Aluminum Alloy120 180

Aluminum200

PC0 2H i 120 - 1800.2Housing

SAC CopperFR4

MCPCB2.2

PSA0.2

SAC60

Copper385

FR40.3

PI0.2

Thermal Adhesive1

Printed Circuit Board

Premold

Resin0.20

Al O

Germanium60

Copper Alloy

AlN170

LED Package

0.01 0.1 1 10 100 1000Th l C d ti it [W 1K 1]

Premold0.25

Al2O318 - 25

Copper Alloy280 - 350

LED Package

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Thermal Conductivity [Wm-1K-1]

Electrical Analogy for 1D Heat ConductionBy comparing the steady state heat flow equation with Ohm’s Law for current flow through a resistor,

Fourier Law Ohm‘s Law

y co pa g t e steady state eat o equat o t O s a o cu e t o t oug a es sto ,we see that they have similar forms:

)( 21 TTL

AQ

)(1

21 VVR

I

Definition of Thermal Resistance Rth

Valid for steady state one dimensional heat transfer The thermal resistance isn‘t a real physical value. It is

only a tool in the field of thermal engineering.TTLRth

)( 21 only a tool in the field of thermal engineering. In reality heat flow and temperature distribution are 3-

dimensional problems

QAth

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Heat Generation of LEDs

optPelPheatPLEDQ

elLEDopt PP FFel IVP

elLEDopt

elLEDheat PP 1

LED = optical efficiency of the LED

VF(T), LED(T), Popt(T) Pheat(T)

Rule of Thumb for white LEDsTemperature Dependencies

Approximation: 100 lm ~ 320 mW

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General Resistor Networks1-dimensional heat conduction

Serial Configuration Parallel Configuration

Rth,1

QRth,1

Rth,2

QRth,2

Rth,3

Rth,3

i

i,thser,th RR i i,thpar,th R

1R

1

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Simplified Thermal Resistor Networkfor a LED System

TJunction

Rth JS Component

y

TSolder Point

Rth SB SubstrateTechnology

Aluminium PlateTBoard

Rth BA CoolingSystem

Technology

Tambient

System

Thermal Resistor NetworkThermal System Configuration Thermal Resistor Network

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Disclaimer

All information contained in this document has been checked with the greatest care. OSRAM Opto Semiconductors GmbH can however, not be made liable for any damage that occurs in connection with the use of these contents.

OSRAM Opto Semiconductor GmbH makes no representations and warranties as to a possible interference with third parties' intellectual property rights in view of products

i i ti f f OSRAM O t S i d t G bH' t i i foriginating from one of OSRAM Opto Semiconductor GmbH's partners, or in view of products being a combination of an OSRAM Opto Semiconductor GmbH's product and a product of one of OSRAM Opto Semiconductor GmbH's partners. Furthermore, OSRAM Opto Semiconductors GmbH cannot be made liable for any damage that occurs in p y gconnection with the use of a product of one of OSRAM Opto Semiconductor GmbH's partners, or with the use of a combination of an OSRAM Opto Semiconductor GmbH's product and a product of one of OSRAM Opto Semiconductor GmbH's partners.

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DisclaimerThis document constitutes neither an offer to sell nor a solicitation to buy or subscribefor securities. Any such offer will be made solely on the basis of the SecuritiesProspectus yet to be approved by the German Financial Supervisory Authority(BaFin) and published thereafter. The information legally required to be provided toinvestors will be contained only in the Securities Prospectus.The information contained herein is not for distribution, directly or indirectly, in orinto the United States of America (including its territories and possessions of anyS f h U i d S f A i h Di i f C l bi ) d bState of the United States of America or the District of Columbia) and must not bedistributed to U.S. persons (as defined in Regulation S under the U.S. Securities Actof 1933, as amended ("Securities Act")) or publications with a general circulation inthe United States of America This doc ment is not an offer of sec rities for sale in thethe United States of America. This document is not an offer of securities for sale in theUnited States of America. The securities have not been and will not be registered underthe Securities Act and may not be offered or sold in the United States of Americaabsent registration or an exemption from registration under the Securities Act Theabsent registration or an exemption from registration under the Securities Act. TheIssuer does not intend to register any portion of the offering in the United States ofAmerica or to conduct a public offering of the securities in the United States of America.This document is not an offer of securities for sale in the United Kingdom Canada

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This document is not an offer of securities for sale in the United Kingdom, Canada,Japan or Australia.

Thank you for your attention.Thank you for your attention.

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