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LED Fundamentals

Basics of Heat
Transfer
T    f

08-16-2011
Cooling of LEDs
  Heat Path through a system
                 g     y




                                      PCB             Heat Sink         Housing   Ambient
      Component

                                       Heat tracks out to the ambient

LED Fundamentals | Basics of Heat Transfer | Page 2
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 area



LED Fundamentals | Basics of Heat Transfer | Page 3
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
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
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        t

LED Fundamentals | Basics of Heat Transfer | Page 6
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 1

LED Fundamentals | Basics of Heat Transfer | Page 7
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 problems



LED Fundamentals | Basics of Heat Transfer | Page 8
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 mW


LED Fundamentals | Basics of Heat Transfer | Page 9
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,i



LED Fundamentals | Basics of Heat Transfer | Page 10
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 Network

LED Fundamentals | Basics of Heat Transfer | Page 11
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 GmbH's 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 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       g
 connection 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.




LED Fundamentals | Basics of Heat Transfer | Page 12
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
Thank you for your attention.




LED Fundamentals | Basics of Heat Transfer | Page 14

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Basics of Heat Transfer: LED Fundamentals

  • 1. LED Fundamentals Basics of Heat Transfer T f 08-16-2011
  • 2. Cooling of LEDs Heat Path through a system g y PCB Heat Sink Housing Ambient Component Heat tracks out to the ambient LED Fundamentals | Basics of Heat Transfer | Page 2
  • 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 area LED Fundamentals | Basics of Heat Transfer | Page 3
  • 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. 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. 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 t LED Fundamentals | Basics of Heat Transfer | Page 6
  • 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 1 LED Fundamentals | Basics of Heat Transfer | Page 7
  • 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 problems LED Fundamentals | Basics of Heat Transfer | Page 8
  • 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 mW LED Fundamentals | Basics of Heat Transfer | Page 9
  • 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,i LED Fundamentals | Basics of Heat Transfer | Page 10
  • 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 Network LED Fundamentals | Basics of Heat Transfer | Page 11
  • 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 GmbH's 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 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 g connection 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. LED Fundamentals | Basics of Heat Transfer | Page 12
  • 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. Thank you for your attention. LED Fundamentals | Basics of Heat Transfer | Page 14