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# Basics of Heat Transfer: LED Fundamentals

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In this presentation on Basics of Heat Transfer we will look at the different modes of heat transfer in an LED system, thermal conductivity of typical materials used in LED system design, and the various thermal resistances involved in an LED system.

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### Basics of Heat Transfer: LED Fundamentals

1. 1. LED FundamentalsBasics of HeatTransferT f08-16-2011
2. 2. Cooling of LEDs Heat Path through a system g y PCB Heat Sink Housing Ambient Component Heat tracks out to the ambientLED Fundamentals | Basics of Heat Transfer | Page 2
3. 3. Basics of heat transfer Heat t a s e occu s when t o su aces have d e e t te pe atu es, t us caus g heat e e gy eat transfer occurs e two surfaces a e different temperatures, thus causing eat energy to 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 Flow represents the movement of thermal energy from one place to another  Temperature represents the amount of thermal energy available p gy Heat Q [J] Total amount of energy transferred  dQ Q Heat Flow (Heat Transfer Rate) [W] = [Js-1] dt Heat Transfer per unit time d2 Q Heat Flux q  [Wm-2] dt  dA Heat Transfer Rate per unit areaLED Fundamentals | Basics of Heat Transfer | Page 3
4. 4. Modes of Heat Transfer Heat Transfer Conduction Convection Radiation m e- v      m e-  Energy transfer through direct Heat transfer by liquid motion. The Heat Transfer through electro- molecular collisions (phonon fluid motion may be caused by magnetic radiation transfer) and by conduction band density differences (free electrons convection) or external mechanical 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.LED Fundamentals | Basics of Heat Transfer | Page 4
5. 5. Heat Transfer in a thermal system Conduction Convection Radiation Heat Transfer from the die to the surrounding environment by conduction, convection and radiation.LED Fundamentals | Basics of Heat Transfer | Page 5
6. 6. Fundamental law of heat conduction Fourier 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.  T  T2  Q  A 1  Q dT   L A dx  The heat transfer rate (dQ/dt) is directly proportional to ( ) yp p the area A and to the temperature difference along the path of heat flow.  Proportionality Ratio: thermal conductivity    i a material property is t i l tLED Fundamentals | Basics of Heat Transfer | Page 6
7. 7. Thermal Conductivity Thermal Conductivity of typical materials within LED-Systems y yp y Air Aluminum 0.026 200 PC Aluminum Alloy 0.2 02 Housing H i 120 - 180 PSA MCPCB 0.2 2.2 FR4 SAC Copper 0.3 60 385 Printed Circuit Board PI Thermal Adhesive 0.2 1 Resin Germanium AlN 0.20 60 170 LED Package Premold Al2O3 Copper Alloy 0.25 18 - 25 280 - 350 0.01 0.1 1 10 100 1000 Thermal C d ti it  [W -1K-1] Th l Conductivity [Wm 1 1LED Fundamentals | Basics of Heat Transfer | Page 7
8. 8. Electrical Analogy for 1D Heat Conduction By co pa g t e steady state heat flow equat o with O y comparing the eat o equation t Ohm’s Law for cu e t flow t oug a resistor, s a o current o through es sto , we see that they have similar forms: Fourier Law Ohm‘s Law    A  (T  T ) Q 1 I   (V1  V2 ) 1 2 L R Definition of Thermal Resistance Rth  Valid for steady state one dimensional heat transfer L (T1  T2 )  The thermal resistance isn‘t a real physical value. It is Rth   only a tool in the field of thermal engineering. A  Q  In reality heat flow and temperature distribution are 3- dimensional problemsLED Fundamentals | Basics of Heat Transfer | Page 8
9. 9. Heat Generation of LEDs  QLED  Pheat  Pel  Popt Popt   LED  Pel Pel  VF  I F Pheat  1   LED   Pel  LED = optical efficiency of the LED Temperature Dependencies Rule of Thumb for white LEDs VF(T), LED(T), Popt(T)  Pheat(T) Approximation: 100 lm ~ 320 mWLED Fundamentals | Basics of Heat Transfer | Page 9
10. 10. General Resistor Networks 1-dimensional heat conduction Serial Configuration Parallel Configuration  Q Rth,1 Rth,1 Rth,2  Q Rth,2 Rth,3 Rth,3 1 1 R th,s er   R th,i  i R th,par i R th,iLED Fundamentals | Basics of Heat Transfer | Page 10
11. 11. Simplified Thermal Resistor Network for a LED System y TJunction Rth JS Component TSolder Point Rth SB Substrate Technology TBoard Aluminium Plate Rth BA Cooling System Tambient Thermal System Configuration Thermal Resistor Network Thermal Resistor NetworkLED Fundamentals | Basics of Heat Transfer | Page 11
12. 12. 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 originating f i i ti from one of OSRAM Opto Semiconductor GmbHs partners, or in view of f O t S i d t G bH t i i f products being a combination of an OSRAM Opto Semiconductor GmbHs product and a product of one of OSRAM Opto Semiconductor GmbHs partners. Furthermore, OSRAM Opto Semiconductors GmbH cannot be made liable for any damage that occurs in p y g connection with the use of a product of one of OSRAM Opto Semiconductor GmbHs partners, or with the use of a combination of an OSRAM Opto Semiconductor GmbHs product and a product of one of OSRAM Opto Semiconductor GmbHs partners.LED Fundamentals | Basics of Heat Transfer | Page 12
13. 13. Disclaimer This document constitutes neither an offer to sell nor a solicitation to buy or subscribe for securities. Any such offer will be made solely on the basis of the Securities Prospectus yet to be approved by the German Financial Supervisory Authority (BaFin) and published thereafter. The information legally required to be provided to investors will be contained only in the Securities Prospectus. The information contained herein is not for distribution, directly or indirectly, in or into the United States of America (including its territories and possessions of any State of the U i d S S f h United States of A f America or the Di i of C l bi ) and must not b i h District f Columbia) d be distributed to U.S. persons (as defined in Regulation S under the U.S. Securities Act of 1933, as amended ("Securities Act")) or publications with a general circulation in the United States of America. This document is not an offer of sec rities for sale in the America doc ment securities United States of America. The securities have not been and will not be registered under the Securities Act and may not be offered or sold in the United States of America absent registration or an exemption from registration under the Securities Act The Act. Issuer does not intend to register any portion of the offering in the United States of America 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 Kingdom, Canada, Japan or Australia.LED Fundamentals | Basics of Heat Transfer | Page 13
14. 14. Thank you for your attention.LED Fundamentals | Basics of Heat Transfer | Page 14