In this paper, we discuss the variations of thermal resistance coefficient from junction to board (Rθjb) for high-power
light-emitting diodes (LEDs) as a result of changes in power dissipation and ambient temperature. Three-watt white and
blue LED packages from the same manufacturer were tested for Rθjb at different input power levels and ambient
temperatures. Experimental results show that Rθjb increases with increased input power for both LED packages, which
can be attributed to current crowding mostly and some to the conductivity changes of GaN and TIM materials caused by
heat rise. With increasing ambient temperature, Rθjb increased but not as much as what was observed with drive current
increase.
Heat at the junction of light-emitting diodes (LED) affects the overall performance of the LED in terms of light output,
spectrum, and life. Usually it is difficult to measure junction temperature of a LED directly. There are several techniques
for estimating LED junction temperature. One-dimensional heat transfer analysis is one of the most popular methods for
estimating the junction temperature. However, this method requires accurate knowledge of the thermal resistance
coefficient from the junction to the board or pin. An experimental study was conducted to investigate what factors affected
the thermal resistance coefficient from the junction to the board of high-power LED. Results showed that the thermal
resistance coefficient changed as a function of ambient temperature, power dissipation at the junction, the amount of heat
sink attached to the LED, and the orientation of the LED with the heat sink. This creates a challenge for using onedimensional
heat transfer analysis to estimate junction temperature of LEDs once incorporated into a lighting system.
However, it was observed that junction temperature and board temperature maintains a linear relationship if the power
dissipation at the junction is held constant.
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