Internal Thermal Resistance of LEDs: LED Fundamentals

LED Fundamentals

Internal Thermal
Resistance of LEDs

09-07-2011

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).
LED Fundamentals | Internal Thermal Resistance | Page 2

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


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


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.


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


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)


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


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


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.


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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 S
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 United States of America or the District of Columbia) and must not 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
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America
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g p g
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This document is not an offer of securities for sale in the United Kingdom, Canada,
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Thank you for your attention.


Internal Thermal Resistance of LEDs: LED Fundamentals

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