An Accelerated Test Method for Predicting the Useful Life of an LED Driver

TLDR

Estimated failure rates indicate the electrolytic capacitor at the LED driver output stage is the weakest link. This paper proposes an accelerated life‑test method for LED drivers that use electrolytic capacitors at the output stage. The accelerated test monitors output current ripple, a parametric indicator of capacitor degradation, with temperature as the acceleration factor. The study shows that increasing ripple reduces light output and efficacy, defines useful life as the time to maximum ripple rate of change, and finds that useful life decreases exponentially with capacitor temperature.

Abstract

This paper proposes an accelerated life-test method for LED drivers that use electrolytic capacitors at the output stage. The type of failure considered here is parametric. Estimated failure rates of power components suggest the electrolytic capacitor used at the driver output stage is the weakest link. As an electrolytic capacitor degrades, its capacitance decreases and its equivalent series resistance increases, contributing to an increase of output current ripple. Thus, the amplitude of the output current ripple is a good indicator of the degradation level of the electrolytic capacitor. Temperature was selected as the acceleration factor because LED drivers are usually exposed to elevated temperatures in typical applications. The current ripple was experimentally found to have a negative impact on light output and efficacy; therefore, the useful life of an LED driver is proposed as the time it takes for the output current ripple to reach its maximum rate of change. The relationship between the capacitor's positive pin temperature and the useful life of the LED driver was then established using the proposed definition. The accelerated life test shows that the LED driver's useful life is shortened exponentially, as the output electrolytic capacitor's operating temperature increases.