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.
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
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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
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LED Fundamentals | Basics of Heat Transfer | Page 14