Performance degradation of high-brightness light emitting diodes under DC and pulsed bias

This paper presents the results of an experimental investigation of the performance of commercially available high-brightness light emitting diodes (HBLEDs). Three different families of white HBLEDs from three different manufacturers are considered. The main issues taken into account and reported in detail are the following: quality of the emitted light, impact of the driving strategy on the expected device lifetime, thermal management and related aging effects. The execution of a large number of accelerated stress tests reveals the weaknesses of the technology with respect to thermal degradation and the sensitivity of the device performance degradation to the adopted driving strategy. Furthermore, square-wave driving has been compared to conventional de driving in terms of device performance and reliability. Comparison has been carried out for the same average current value of the driving waveforms. It has been found that square-wave driving can be an effective alternative to dc driving in terms of device efficiency only for high duty cycles. For low duty cycles, worse performance was detected due to the saturation of efficiency at high peak current levels. Reliability tests did not univocally indicate whether the use of pulsed bias can be more convenient than dc driving in terms of lumen maintenance. The three families of devices submitted to dc and pulsed stresses showed different behaviors, indicating that stress kinetics strongly depends on the LED technological structure and package thermal design.