Research Of Junction Temperature And Phosphor Self-Heating In LED Packaging

Light emitting diode (LED), with a lot of advantages such as high efficiency, lowconsumption and long lifetime, has been widely used. As a light source, LED also generatesheat when working, and temperature greatly affects its performance. Therefore, it isnecessary to study the method of obtaining and controlling LED temperature. In this paper,the LED temperature measurement, junction temperature prediction and phosphortemperature controlling were investigated to improve the thermal management of LED.It was found the surface temperature measurement of LED was not accurate bythermocouple. The error could exceed148.51%when the drive current was1000mA. Thereason of the inaccuracy was proved to be the light absorption by thermocouple, and it wasderived that the measurement error was related with the light intensity of the measurementsurface and the light absorption coefficient of the thermocouple.Two engineering methods to predict junction temperatures of LED array packagingwere presented. The first method needed to measure the radiator temperature, and derivedthe junction temperature from the bottom to the top surface based on analytical models.Comparison shown the deviation between presented method and experiment was less than4%. The other method started with measuring the top surface temperature of structure closeto LED chips, and then derived junction temperatures based on analytical models. The twomethods complement each other, and expand the application field.The phosphor self-heating phenomenon was proved by experiments. It was found thephosphor self-heating would lead to a much higher phosphor temperature than LEDjunction temperature with certain conditions, and might cause destructive failure of LEDpackaging. The reason of phosphor self-heating was explained and a self-heating modelbased on phosphor particles was built, and simulations showed the highest temperature ofphosphor particles could reach540.16℃。The effects of different phosphor coating methods on phosphor temperature were compared, and the phosphor settling method was chosen as the solution to reduce thephosphor temperature. The condition of phosphor settling in silicone gel was analyzed andit was proved to be feasible by experiment. LED modules with phosphor settling weremanufactured, and it was found that the surface temperature of phosphor gel decreased84.1%compared with modules without phosphor settling, which proved the settling method couldeffectively reduce the phosphor temperature.