In this presentation on Internal Thermal Resistance of LEDs we will look at the definition and significance of internal thermal resistance in LED packages, the path of heat transfer in some OSRAM LED packages, and the calculation of the junction temperature in LEDs.

1. LED Fundamentals
Internal Thermal
Resistance of LEDs
09-07-2011

2. Thermal Resistance (Rth )
(T1  T2 )
The thermal resistance (Rth) is defined as Rth 

Q
where:
Difference between the inner temperature of the heat
T1  T2   source (semiconductor chip) and an external constant
reference temperature.
f t t

Q Dissipated power
For an LED
 The temperature of the inner heat source refers to the temperature of the active semiconductor
zone in which the photons are generated. This is designated as the junction temperature (TJ).
 The appropriate selection of the external temperature reference point depends on the type of
housing and the respective rate of heat dissipation. This reference point is the "solder point
temperature" (TS).
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3. Two Definitions of Thermal Resistance (Rth JS)
For LEDs t e t e a resistance from ju ct o to so de po t ( thJS) can be de ed in t o ways
o s the thermal es sta ce o junction solder point (R ca defined two ays
Electrical thermal resistance Real thermal resistance
Electrical Rth JS Real Rth JS
T T
RJX el  RJX  real 
Pel Pel  Popt

Q  Pel  Popt

Q  Pel  VF  I F
l Q  Pel  1   LED 

where :
where :
Pel  Supplied electrical power
VF  Forward voltage
Pel  Supplied electrical power
I F  Forward current Popt  Supplied optical power
 LED  Optical efficiency of the LED
The OSRAM datasheets lists value of the
real thermal resistance (Rth JS) in K/W
LED Fundamentals | Internal Thermal Resistance | Page 3

4. Internal Thermal Resistance Rth JS
Heat Conduction and Heat Path of LED Packages
g
TOPLED Family DRAGON Family
LED
 Conductive heat transfer through leads to  Heat spreading within the internal heat
PCB spreader
 Thermal active and thermal inactive leads  Conductive heat transfer through heat
and solder pads
p spreader to PCB
 The chip is mounted on the thermal active  The heat spreader has electrical potential
lead. The bond wire goes to the thermal
inactive lead
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5. Internal Thermal Resistance Rth JS (contd…)
Heat Conduction and Heat Path of LED Packages
g
OSLON SSL Family
 The chip is mounted on a thermal pad
 The thermal pad is electrically isolated by the
g
ceramic housing
 Conductive heat transfer takes place though
the ceramic housing and all the three pads.
LED Fundamentals | Internal Thermal Resistance | Page 5

6. Thermal Resistances (Rth JS ) of OSRAM LEDs
150
sistance Rth [KW-1]
125
hJS
120 K/W
100
75
Real Thermal Res
50 40 K/W 36 K/W
25
7 K/W 6.5 K/W 4.2 K/W
Advanced Power OSRAM OSTAR-
OSTAR
DURIS E 3 DURIS E 5 OSLON SSL DRAGON
TOPLED Plus Lighting Plus
LED Fundamentals | Internal Thermal Resistance | Page 6

7. Significance of Thermal Resistance Rth JS
Knowledge of the thermal resistance (Rth JS ) of an LED serves to:
 Determine the junction temperature that arises in the LED under operating conditions.
 Determine the maximum allowable external reference temperature for a given internal
temperature and power dissipation.
 Evaluate
E l t measures f dissipation of h t i th LED system (th
for di i ti f heat in the t (thermal management).
l t)
LED Fundamentals | Internal Thermal Resistance | Page 7

8. Thermal Measurement
Estimation of Junction Temperature (TJ)
p (
LCW CP7P
80.00
If = 350 mA
70.00
Te m pe ra ture (C )
60.00
60 00 Vf = 3 2 V
3.2
50.00 Solder Point Temp (C) Ts (measured) = 75°C
RthJS = 7K/W
40.00 Ambient temp (C)
30.00 ФV(25°C) = 100 lm Ta (measured) = 20°C
20.00
10.00
Approximation (White LED): 100 lm ~ 320 mW =
0.00
Popt
0.00 50.00 100.00 150.00 200.00
Pel = 3 2 x 0 3 0 = 1 12 W
3.2 0.350 1.12
Time (min) PD = Pel – Popt
= 1.12 W - 0.320 W = 0.8 W
Calculation : T  T  Rth  P
J S JS D
= 75°C + 7 x 0.8 W
= 81°C Junction Temperature Estimation at Tamb(20°C) = 81°C
LED Fundamentals | Internal Thermal Resistance | Page 8

9. Thermal Management System
Thermal Resistance of System RthJA
Internal External
Thermal Resistance Thermal Resistance
RthJS RthSB RthBA
 Materials  Substrate Technology  Housing
 Package Geometry  Solder Pads  Heat Sink
 Thermal Spreader  Thermal Vias  Air Velocit
Velocity
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10. 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 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
GmbH s partners Furthermore,
Opto Semiconductors GmbH cannot be made liable for any damage that occurs in
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 | Internal Thermal Resistance | Page 10

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12. Thank you for your attention.
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