Research On Accelerated Aging Tests Under Electric And Thermal Stresses For LED Lamps

As a new generation of lighting source,Light-emitting diode(LED)has been widely applied in various lighting fields.Currently LED accelerated aging tests are mainly based on several standards proposed by Illuminating Engineering Society.The recommended test time is at least 6000 hours,which is too long to meet the market requirement.For LED product,it is effective method to shorten the time of aging tests by loading higher levels than normal environmental stresses.LED lamps are affected by different environmental stresses in different application environments,but the thermal and electric stresses are common in every application.Therefore,the study on accelerated aging tests under electric and thermal stresses is meaningful for realizing the lifetime evaluation and reliability analysis rapidly for LED lamps.The accelerated aging test of electric stress based on Bayes estimation for LED lamps is studied.First of all,the test platform of accelerated aging test under electric stress is built,and then the Bayes estimation and the medium rank method are respectively used to calculate the failure probability.Using the least squares method to estimate the parameters of Weibull distribution,Bayes estimation is selected due to its better fitting precision.The reliability of the lamps under the normal working condition is estimated by use of the inverse power law model.Results show that the lifetime is 5677,8612,10353 and 18407 hours,at the failure probabilities of 1%,5%,10% and 63.2%,respectively.The drive circuit aging can be neglected by analyzing the change of output current during aging test which causes a luminous flux variation less than 0.35%.The accelerated aging test under electric and thermal double-stress for LED lamps is studied.Firstly,a short-time double crossed step-stress aging test is done to obtain the limit of stress level for LED lamps,and suitable stress combinations for aging test are selected.Secondly,the junction temperature of each stress combination is measured by the forward voltage method of "small current".Thirdly,the parameters of Eyring model are estimated by multiple linear regression analysis.According to the estimated parameters,the reliability of the lamps is obtained.The results show that the lifetime is 5817,8690,10372 and 18032 hours,at the failure probabilities of 1%,5%,10% and 63.2%,respectively.Comparing to the results of accelerated aging test of electric stress,the relative error of the lifetimes is 2.59%,0.92%,0.19% and 2.04%,respectively.Finally,the analysis of color shift shows that the degradation of the luminous efficiency of phosphor is the major source of the failure.According to the exponential decay law,Arrhenius equations between degradation rate and junction temperature at 50% failure probability are obtained,under 260 V voltage and under 300 V voltage.The differences between online and offline test are experimentally studied.A method to simulate the online and offline test condition under electric stress is proposed.An adjustable AC voltage stabilized power source connected with the lamp holder in integrating sphere is adopted to get the transformation of test modes from one to another.The experimental results show that the exponential fitting of the luminous flux degradation in online tests possesses a higher fitting degree for most of lamps,and the degradation rate of the luminous flux by online tests is always lower than that by offline tests.The relative error of lifetime estimation is 5.8%,4.2% and 3.5%,at the failure probabilities of 1%,5% and 10%,respectively.The t test is used to do statistical analysis for the data of CCT and color shift by online and offline tests.The CCT of online tests is always larger than that of offline tests,but the change rate of CCT and the color shift of Du'v' are not significantly different between the two test modes.In this paper,accelerated aging tests under electric stress,and electric and thermal double-stress for LED lamps are conducted.We have made some progresses,and the work plays a guiding role for rapid lifetime evaluation and reliability analysis for other types of LED lamps.