Is the thermal resistance coefficient of high-power LEDs constant?

Abstract

In this paper, we discuss the variations of thermal resistance coefficient from junction to board (Rθ_jb) 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 Rθ_jb at different input power levels and ambient temperatures. Experimental results show that Rθ_jb 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, Rθ_jb increased but not as much as what was observed with drive current increase.