Junction Temperature Measurement And Application In Thermal Packaging For High-Power Light-Emitting Diodes

High-power light-emitting diodes(LEDs) have been widely used in many applications as a new solid-state light source due to its advantages in energy-saving and long life. For LEDs, even with the increasing luminous efficiency, there is still more than 70% of electrical power converted into heat, junction temperature and thermal resistance have become the main bottleneck for its further application. The high heat flux density makes junction temperature of chips significantly increase, which accelerates the deterioration of electro-optical property of LEDs. Meanwhile, packaging materials will be also degraded to cause device failure because of high junction temperature. Therefore, to achieve high reliability of LEDs, junction temperature should be effectively monitored. Accurate measurement for LEDs junction temperature is a challenging issue in the development of high-power LEDs. This dissertation mainly studies the junction tmeperature measurement and application in thermal packaging for high-power light-emitting diodes, and achieved some innovative works as follows:A dynamic junction temperature measurement system was developed, and the various testing methods were analyzed. The function relationship between the forward voltage and the junction temperature was selected as the temperature sensitive parameter(TSP). The nonlinear characteristics in the factor K was verified by experiments, and the standard using piecewise fitting method for factor K calibration was developed according to the magnitude of junction temperature. Two constant-current sources were controlled by pulse modulation circuit, and they could be switched promptly between the testing cycle and the heating cycle. The dynamic junction temperature curves were obtained in real time, and eventually, the instrument reliability was designed and evaluated and the anti-jamming performance of system was researched and handled. the instrument was compact and portable with low power consumptions, which can be used to accomplish in-situ measurement in the field. Experiments show that the measurement accuracy of the instrument can be up to 0.1℃, and the standard error of temperature measurement can be controlled within 1%.To evaluate the performance of instrument, some parameters, including TSP, testing current, testing time, pulse width of sampling, fast switching time and sampling delay were analyzed and limited. The testing errors are mainly come from three aspects, and that is instrument calibration, factor K calibration and the influence of switching time. The finite element method was used to evaluate the declined magnitude of temperature when the heating current is fast switched. Experiments of comparison were conducted, the junction temperature is obtained indirectly by thermocouple, thermal resistance tester and the infrared thermal imager, respectively, and the results show a good agreement between the experimental data and reference values.A prediction model of transient junction temperature was proposed, which can predict junction temperature at arbitrary operation current based on a simple test. A five-layer thermal resistance and capacitance network was established, and the thermal parameters of each layer were obtained through transient junction temperature. The error caused by the simplification in the prediction method was analyzed. The dynamic junction temperature curve with small current can be used to deduce the thermal parameters of each layer in the LED packaging.To evaluate the LEDs chips, the thermal resistance in various chip structure is calculated and the results show that little influence lies in steady junction temperature. But the velocity of tested transient temperature rise could be used to judge the quality of LEDs chips. Four kinds of LEDs packages were tested with junction temperature instrument, and the results indicated that the small size of CoB LED packaging form has the best cooling effects because its thinner volume and its higher thermal conductivity in the cold plate. The measured steady junction temperature has a good agreement with the theoretical value. The degradation characteristics and aging mechanisms are analyzed in detail by monitoring the fluctuation of junction temperature in evaluating reliability of LEDs. At last, to obtained the real-timely data in reliability test, the testing equipment integrated with optical, thermal and electrical parameters was designed to evaluate the reliability of LEDs, which can be used to accomplish in-situ measurement.