Internal Thermal Resistance of LEDs: LED Fundamentals

LED Fundamentals

Internal Thermal Resistance of LEDsResistance of LEDs

09-07-2011

Thermal Resistance (Rth )

QTTRth

)( 21 The thermal resistance (Rth) is defined as

where:

21 TTDifference between the inner temperature of the heat source (semiconductor chip) and an external constant

f t t

where:

reference temperature.

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

LED Fundamentals | Internal Thermal Resistance | Page 2

temperature" (TS).

Two Definitions of Thermal Resistance (Rth JS)For LEDs the thermal resistance from junction to solder point (RthJS) can be defined in two ways o s t e t e a es sta ce o ju ct o to so de po t ( thJS) ca be de ed t o ays

Electrical thermal resistance Real thermal resistance

Real Rth JS

realJXTR

Electrical Rth JS

elJX PTR

optelrealJX PP

FFl IVPQ optel PPQ

elP

voltageForwardpower electrical Supplied

F

el

FFel

VPwhere

IVPQ:

power electrical Supplied:

1

el

LEDel

Pwhere

PQ

The OSRAM datasheets lists value of the

current Forwardvoltage Forward

F

FIV

LED theof efficiency Optical

power optical Supplied

LED

optP


real thermal resistance (Rth JS) in K/W

Heat Conduction and Heat Path of LED Packages

Internal Thermal Resistance Rth JS

LEDTOPLED Family DRAGON Family

g

Conductive heat transfer through leads to Heat spreading within the internal heat Conductive heat transfer through leads to PCB

Thermal active and thermal inactive leads and solder pads

Heat spreading within the internal heat spreader

Conductive heat transfer through heat spreader to PCBp

The chip is mounted on the thermal active lead. The bond wire goes to the thermal inactive lead

The heat spreader has electrical potential


Heat Conduction and Heat Path of LED Packages

Internal Thermal Resistance Rth JS (contd…)

OSLON SSL Family

g

The chip is mounted on a thermal pad The thermal pad is electrically isolated by the

ceramic housingg Conductive heat transfer takes place though

the ceramic housing and all the three pads.


Thermal Resistances (Rth JS ) of OSRAM LEDshJ

S[K

W-1

] 150

125 120 K/W

sist

ance

Rth

75

100

erm

al R

es 75

50 40 K/W 36 K/W

Rea

l Th

25

OSRAM OSTAR

6.5 K/W 4.2 K/W7 K/W

DURIS E 3 Advanced Power TOPLED Plus

DRAGONOSRAM OSTAR-

Lighting PlusOSLON SSLDURIS E 5


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.

E l t f di i ti f h t i th LED t (th l t) Evaluate measures for dissipation of heat in the LED system (thermal management).


Thermal MeasurementEstimation of Junction Temperature (TJ)p ( J)

If = 350 mA

Vf = 3 2 V

LCW CP7P

60 0070.0080.00

(C)

RthJS = 7K/W

Vf 3.2 V

30.0040.0050.0060.00

pera

ture

Solder Point Temp (C)

Ambient temp (C)Ts (measured) = 75°C

Ta (measured) = 20°CФV(25°C) = 100 lm

0.0010.0020.00

0.00 50.00 100.00 150.00 200.00

Tem

Approximation (White LED): 100 lm ~ 320 mW = Popt

P 3 2 0 3 0 1 12 WTime (min)

PRthTT

Pel = 3.2 x 0.350 = 1.12 WPD = Pel – Popt

= 1.12 W - 0.320 W = 0.8 WCalculation :

Junction Temperature Estimation at T (20°C) = 81°C

= 75°C + 7 x 0.8 W

81°C

DJSSJ PRthTT Calculation :


Junction Temperature Estimation at Tamb(20°C) = 81°C= 81°C

Thermal Management System

Thermal Resistance of System RthJA

Internal ExternalInternalThermal Resistance

RthJS

ExternalThermal Resistance

RthBARthSBthJS

Materials Package Geometry

Thermal Spreader

thBAthSB

Substrate Technology Solder Pads

Thermal Vias

Housing Heat Sink

Air Velocit Thermal Spreader Thermal Vias Air 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 OSRAMproduct of one of OSRAM Opto Semiconductor GmbH s partners. Furthermore, OSRAM 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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