On the influence of an upstream heat source on the thermal structure function of an LED

The determination and evaluation of the thermal structure function is a well established method to characterize the thermal path of a LED from its junction to the heat sink [1]. For this, the junction is used both as heat source and as temperature sensor. For the usual interpretation of the measured temperature curve, the junction is the first element in the chain of heat capacities and thermal resistances as it is described by the structure function. Real LEDs often possess phosphors or primary optics in front of the junction. These components act as a thermal capacity and even as an additional heat source in front of the junction, i.e. upstream. The influence of such elements on the structure function and its evaluation will be shown. For this, well defined temperature curves will be simulated and evaluated. The findings will be used to evaluate and interpret the structure functions of really measured LEDs.