Effects of the Junction Temperature on the Dynamic Resistance of White LEDs

This paper deals with the thermal characteristics of the $I$ – $V$ curve of GaN-based white light-emitting diodes (LEDs), focused on the variations of the dynamic resistance. The final goal of this study is to improve the static and dynamic operations of the LED driver within a wide range of temperatures. Four LEDs from different manufacturers were chosen for this study. The first part of this paper shows the thermal characterization of the forward voltage at a given injected current. After that, the experimental data are fitted in order to calculate the junction temperature accurately. Then, a small-signal analysis where the LEDs are supplied with dc current and an ac perturbation superimposed at the operation point under variable junction temperature is covered. This analysis allows the dynamic resistance to be experimentally determined for a wide junction temperature range. Furthermore, the experimental data have been fitted in order to establish the relationship between junction temperature and dynamic-resistance variation, so the dynamic resistance can be determined for a given operation point. Finally, an illustrative example is presented as a case study in order to analyze the implications of the dynamic resistance on the output-current ripple and on the closed-loop operation of a LED driver. The experimental results confirm that the junction temperature shift induces a variation in the dynamic resistance, which might have a significant effect on the output-current ripple and closed-loop performance in certain LED fixtures.