Research On Junction Temperature Measurement And Lifetime Evaluation For White LED Array

LEDs have been widely used for substituting the traditional lighting products,owing to the advantages of environment friendliness,energy saving and longer lifetime.Although the advantages in environment friendliness and energy saving have been approved,the longer lifetime is still being questioned.One of the main issues in LED reliability is to solve the thermal problems in LED PN junction.The lifetime and conversion efficiency of LED reduce as its junction temperature rises,which makes an obstacle for its development and application in the lighting field.Due to the junction temperature playing an important role in LED reliability,the accurate measurement of junction temperature and the evaluation of lifetime are meaningful for designing and building a reliable LED product,especially for nowadays LED products with higher power and larger density.This thesis gives four main research aspects as follows.1.The methods of junction temperature measurement for white LED array are studied,namely the forward voltage method and the optical method based on W/B ratio.1)A testing platform is built for junction temperature measurement with the forward voltage method,and the correspondent uncertainty is analyzed.In the actual lighting system,a method is proposed for modifying the junction temperature error introduced by the changes in the current output of LED driver circuit.2)For single phosphor-converted white LED,the traditional method based on W/B ratio for junction temperature measurement is verified,and a correspondent testing platform is built.Result shows that the variation of W/B ratio as junction temperature is of a satisfactory linearity with R-square higher than 0.99.3)For two phosphors-converted white LED,a new approach based on spectral analysis is proposed,which uses(B+Y+R)/B ratio to establish the correlation with junction temperature.The uncertainty in spectral analysis and junction temperature estimation by the proposed approach is analyzed.The error of junction temperature estimation can be reduced from 5.7? to 2.2? compared with traditional method.2.In LED thermal accelerated aging test,we propose a method to compare online test mode with offline test mode in the lifetime prediction,by means of junction temperature measurement.1)Before the aging,a short-time step-stress aging test is done previously,yielding a variation curve of LED junction temperature with respect to stress.There is a catastrophe point in the curve,indicating the position of the change of failure mechanism of LED.Then suitable stress level of the accelerated aging test can be determined.2)In the thermal accelerated aging test,the online test mode and the offline test mode are respectively achieved by attaching the sample to different heat sinks in the testing condition.The lumen degradation,color shift and predicted lifetime in the two test modes are then obtained.Results show that the lifetime predicted in online test mode is about 8% to 13% smaller than that in offline test mode.3.Due to the junction temperature showing an upward trend over the aging time,a modified model of lumen degradation is proposed,in which the lumen flux is normalized to the value correspondent to initial junction temperature.Thereby,the influence of variation in junction temperature on the lifetime prediction is eliminated.In this research,the junction temperature increases about 6?-8?for 5 samples after 3000 h aging under ambient temperature of 80?,which causes about 9% to 21% under-estimation of lifetime.4.We propose a method for lumen degradation analysis of LED lamps based on subsystem isolation method,in which the entire LED lamp is divided into three subsystems,namely driver,lampshade and LED light source.The parameters of output power,transmittance and lumen flux are respectively adopted in the analysis of the degradation of drive,lampshade and LED light source.For the white LED lamps tested in this research,LED light source is the most aggressive part of the three subsystems,which accounts for 70% of lumen degradation of LED lamp in average.The lampshade is the second degradation source which causes about 21% of the total amount.The degradation of LED driver gives little affect on the total amount.In this thesis,the measurement of LED junction temperature and LED thermal accelerated aging test are conducted,and the progress of lifetime prediction is made.The proposed methods and the experimental results have certain guidance for the standardization of junction temperature measurement and reliability analysis of LED in the future.