A lifetime prediction approach for LED packages in paralleled under thermal-electronic coupling effects

For LED packages in parallel, the driving current depends on the catastrophic failure history, so LEDs break down faster than that with the constant current hypothesis. This work proposes a hybrid reliability assessment approach that is based on the combination of the symbol sequence with Physics of Failure (PoF) modelling for parallel LEDs. A symbol sequence is proposed to abstract lumen maintenance curves into several failure modes. According to the monotonicity of the proposed symbol sequence, the boundary lifetime and probability is required to obtain the reliability of the light source in terms of mean time to failure (MTTF). The PoF simulations are utilized to calculate lifetime, temperature and thus probability of each failure mode with consideration of interaction between the lumen decay and open circuit. A temperature- and time-dependent model is applied to describe the LED lumen depreciation. LED's open circuit probability is considered as functions of temperature. The parameters of the proposed methodology are extracted by experiments which select a commercial type of LED. With the combined effect of the lumen depreciation and open circuit failuires, the time of open circuit failure determines the failure mode of the light source. By finding 3 boundary points among failure modes, the proposed approach obtains accurate survival rates as function of lifetime and MTTF of LED packages in parallel than conventional approaches.