Characteristics of Thermal Resistance for High Power LEDs

In this paper the accurate and fast measurement equipment was developed and applied to study the thermal characteristics of high power LEDs. The forward-voltage based method was conducted to measure the junction temperature of high power LEDs. Conduction type method is adopted to measure the temperature sensitivity parameter (TSP) with small magnitude of error compared with the traditional method. The experiment time was reduced from 3~4 hours to 10 minutes for one sample. It was demonstrated that the repeatability of the measurement system was well after the repeatability test. LEDs used here were 5 W single chip LED and 50 W multi-chip LED with 36 chips inside the LED. Thermal resistance of junction-to-case as function of input power and case temperature was discussed. It was shown that the 5 W LED revealed an increasing trend of thermal resistance with the input power at each case temperature but the contrary trend of 5OW LED. The results also exhibited the dependency of thermal resistance and case temperature. With the increasing case temperature, the value of thermal resistance became higher under each input power. Three factors affected the thermal performance including: the first, the relation between light output efficiency and junction temperature; the second, the effect of internal series electrical resistance R_in and external electrical resistance R_ex; and the third, the materials degeneration of each part inside the LEDs package as the junction temperature increased. To combine the three factors could explain the thermal characteristics of high power LEDs.