Failure causes and mechanisms of retrofit LED lamps

a r t i c l e i n f o Keywords: Reliability Retrofit LED lamps Diffusive bulb Phosphors LED driver This paper describes one of the first studies of the degradation of retrofit light bulbs based on white GaN light emitting diodes. The results indicate that the lifetime of LED lamps depends mostly on the stability of the driver and optical elements, rather than on the degradation of the LED chips, that have a stable output over stress time. By comparing lamps from four different manufacturers stressed at room and high temperature, we found that (i) long-term stress causes a change of the chromatic properties of the lamps, which is ascribed to the degradation of the phosphors or to the inner LED reflector; (ii) during aging the LED driver may degrade gradually and/or catastrophically, causing a reduction of the output optical power, or a complete failure; (iii) proper thermal management and heat dissipation reduce the degradation rate; (iv) spectral transmissivity measurements and visual inspection reveal the degradation of the diffusive optical elements, which is induced by the short wavelength side of the LED emission spectrum. White LEDs based on blue GaN chips and yellow phosphor conversion are efficient devices whose reliability and stability was proved up to tens of thousands of hours [1,2]. In order to exploit this performance, several countries and associations are promoting the production of LED retrofit light bulbs, able to fit in a common Edison screw socket, whose most renowned example was the " L Prize " competition run in 2008 by the United States Department of Energy with a US$10 million prize fund [3]. Retrofit light bulbs are now commonly sold in stores. Given the higher initial cost, it is important to ensure that their intensity and chromatic properties remain stable over the usable lifetime to take advantage of the reduced power consumption and longer lifetimes compared to traditional incandescence or fluorescence lamps [4]. Although several papers have analyzed the degradation of chip LEDs over high temperature [5,6], high current [7,9] and reverse-bias [7–13] stress, to date no paper has described the long-term degradation of complete lamps. This paper presents one of the first studies of the degradation mechanisms of commercially-available lamps; the results allow us to identify the critical parts of the overall system, and to correlate failure modes with specific failure mechanisms. We found that (i) the operating temperature accelerates the degradation …