Research On Junction Temperature Measurement Method Of Light-Emitting Diodes By Continuous Rectangular-Wave Driving Signal

In modern society,light-emitting diodes(LED)have been widely applied in numerous fields,such as general lighting,indicator lights,display screens and optical communications,bringing great convenience to people's lives.In practical applications,the thermal management of LED will have a great impact on its performance,mainly reflecting that high junction temperature will result in the LED luminous efficacy droop,poor color rendering performance and reduced lifetime.Therefore,good thermal management is needed to enhance the heat dissipation ability of LED,reduce junction temperature,improve energy conversion efficiency and prolong lifetime.Accurate junction temperature measurement can help improving the thermal structure design of LED,so that LED devices can work more efficiently and durably.In view of the shortcomings of some existing junction temperature measurement methods based on electrical and spectral characteristics of LED,such as narrow applicability and signal oscillation caused by circuit switching,this paper proposes a continuous rectangular-wave method(CRWM)for measuring junction temperature of LED.The universality and accuracy of CRWM are proved by comparative thermocouple and Raman spectrum experiments under the same experimental conditions.In addition,simulation experiments about the effect of the rising time of rectangular-wave driving signal on CRWM junction temperature measurement results are conducted by COMSOL Multiphysics.This paper is organized as follows:1.The influence of junction temperature of LED on photoelectric performance and reliability of devices is mainly introduced,different methods of junction temperature measurement of LED are analyzed and their error sources and limitations of application are commented.2.A new method of measuring junction temperature of LED by CRWM is proposed.This CRWM experiment can be divided into current mode and voltage mode.Firstly,the voltage-temperature sensitivity coefficient of the LED is measured by using narrow pulse current or voltage to drive the LED(fixed on the heat sink).Secondly,the continuous rectangular-wave current or voltage signal with a certain period and duty cycle is applied to obtain the transient voltage response at the rising edge of each cycle.Then,the voltage response curve is used to acquire the voltage difference between the peak voltage at the rising edge and the steady voltage after heating LED,which is caused by the junction temperature variation.Next,the temperature difference between junction and heat sink is calculated by combining the voltage difference and the voltage-temperature sensitivity coefficient obtained by the pulse method.Finally,the junction temperature of LED at the corresponding current or voltage level can be determined.Compared with the traditional forward voltage method,CRWM does not need to switch the LED from heating(working)state to cooling(testing)state and can avoid the low junction temperature test result caused by the additional data acquisition delay caused by circuit switching and the fact that the measured voltage response lags behind the actual temperature response.In addition,the change of LED voltage response at the rising edge is need being collected in CRWM,while that at the falling edge is collected in the traditional forward voltage method.Because the small voltage signal at the falling edge is easily disturbed by environmental noise,relatively,the signal-to-noise ratio of the voltage signal collected at the rising edge is relatively stronger.With continuous rectangular-wave,voltage data can be continuously collected for many times,further enhancing the signal-to-noise ratio.Thus,the data processing is more accurate and reliable.By comparing the measured results of junction temperature obtained by CRWM with those obtained by thermocouple and Raman spectroscopy under the same conditions,it is found that the relative error is less than 3%in current mode and less than 1.5%in voltage mode.3.The influence of rising edge on CRWM is discussed by changing the rising time of rectangular-wave.The simulating LED model is built by COMSOL Multiphysics and the rectangular-wave simulation experiments with different rising time(10ns-100?s)are conducted,which are compared with the actual CRWM experiments.Both CRWM and simulation results show that when the rising time of rectangular-wave is less than 40?s,the interference of the charge/discharge effect of LED on voltage signal sampling can be effectively reduced At this time,the collected voltage variation data can reflect the temperature change more accurately,improving the reliability of the test results.