Lifetime test of pulse width modulated LEDs supplemented with thermal investigations

The ability to dim LEDs is a big advantage compared to previous light sources, but it can also lead to new aging mechanisms. Due to the different coefficients of thermal expansion of the individual structural layers, the temperature cycle can negatively affect the main thermal path. In this article, we investigate how different dimming frequencies influence the aging of LEDs and to what extent they impair the thermal conductivity of structural layers belonging to different thermal time constants. In this paper the methodology of the joint examination of the integral structure function and the time constant spectrum is described, i.e. the determination of the time constants and the frequency values that expose the individual structural layers to the most mechanical stress. The theoretical results are demonstrated using an LM-80 based LED lifetime test supplemented with thermal transient testing which is also presented in this paper.