A method for projecting useful life of LED lighting systems

Abstract

This study investigated a non-invasive method to determine the junction temperature of AlGaInP light-emitting diodes (LEDs) in a system. Because the primary cause for the AlGaInP LED degradation is junction temperature, this method can be used to predict LED life. Currently, life estimates of LED lighting systems quoted by manufacturers (commonly 100,000 hours) are based on the average life of a single LED measured under specific laboratory conditions. In reality, rates of degradation are much different for LEDs in a system than for those in a laboratory environment because the packaging and the environmental conditions in which the system operates can affect LED performance. Current practices for estimation require time-consuming life tests to accurately predict the life of LEDs. Therefore, a rapid estimation method for LED life is needed. Based on previous studies, the authors chose to focus on the measurement of junction temperature and its relationship to LED degradation. The primary objective of this study was to verify that wavelength shift could be used to estimate accurately the junction temperature of 5mm epoxy encapsulated AlGaInP LEDs. In this study, the junction temperature was increased by changing the drive current while holding the ambient temperature surrounding the LED constant, and by changing the surrounding temperature while holding the drive current steady. Experimental results from this study showed that for commercial LEDs, peak wavelengths shift proportionally to junction temperature regardless of how the temperature is created at the junction, and that this linear relationship could be used as a direct measure of the junction temperature. Because the primary cause for the degradation of AlGaInP LEDs is junction heat, the light output degradation rate of these types of LEDs can be predicted by measuring the spectral shift. Therefore, LED systems can be evaluated without disassembly in their intended application.