White light-emitting diodes (LEDs) are rapidly evolving for use in general illumination applications. TECTONIC are now designing more efficient and reliable white LEDs than ever before. However, there is no easy way to estimate the life of white LEDs. Therefore, the goal of this study was to investigate how different types of commercial white LEDs degrade over time and to compare the effects of drive current and ambient temperature on the degradation rate. In the long run, this information will be useful in developing a life predictor that can reduce the need for time-consuming life tests.
Typically, LED performance is affected by the drive current and by the ambient temperature surrounding the LED. Both of these parameters contribute to the junction temperature of the LED, which is known to be a good predictor of LED life. Therefore, our objective was to determine whether drive current and ambient heat have similar effects on the degradation of white LEDs. To understand how the different LED packages are affected by these parameters, LED arrays were tested at different operating conditions using either the same current but different ambient temperatures, or the same ambient temperature but different currents. The details of the experiment and the results are presented.
The degradation rate changes as a function of junction temperature depending on the source of increasing heat (drive current or ambient heat). However, for 5 mm LEDs, current-induced junction temperature rise has a greater effect on the degradation rate than ambient heat-induced junction temperature rise. From a preliminary study, it was observed that for high-flux LEDs the ambient temperature has a greater effect than the current. Currently, other types of high-flux white LEDs are being life-tested at 35C and 50C common ambient temperatures. Even though it is too early to make conclusions about their life, different packages will likely produce different performance characteristics. The results will be discussed in a follow-up paper. By testing different commercial white LEDs, we can understand the degradation mechanisms for different types of LEDs and finally develop a metric for predicting the life of white LEDs.