“Hot-plugging” of led modules: electrical characterization and device degradation

GaN-based light-emitting diodes have recently demonstrated to be almost ideal devices for the realization of next generation light sources, thanks to their high performance and long expected lifetime. For a massive market penetration of these devices, their reliability must be significantly improved: if on one hand the physical mechanisms responsible for gradual degradation have been extensively described, on the other hand the origin of catastrophic failures still has to be investigated in detail. One of the most critical situations occurs when an LED module is directly connected to an energized power supply: this event (usually referred to as "hot-plugging") can generate current spikes up to several tens of amperes, that can potentially destroy or damage the LEDs. The aim of this work is (i) to analyze, for the first time, the nature of the current spikes generated during hot-plugging, (ii) to understand the failure mechanisms of LEDs submitted to high current spikes, and (iii) to present a simplified model to explain the hot plugging phenomenon. The study is based on transient electrical measurements, carried out on several LED modules (fabricated by different manufacturers), connected to three different power supplies. Results reveal that the amplitude and the time constants of the current spikes are directly determined by the number of LEDs connected in series and by the output capacitance of the current driver, and provide information on the gradual or catastrophic failure of LEDs submitted to current spikes