Uncertainty analysis in measurement for LED junction temperature and thermal resistance

LEDs are temperature sensitive devices. Many other characteristics of LEDs strongly depend on their thermal characteristics. So accurate measurement for the junction temperature and thermal resistance of LEDs is important. The uncertainty analysis in these measurements, which is important for accurate measurements, is given in this paper. The standard evaluation method for measurement uncertainty in GUM is used. The junction temperature of an LED is measured by indirect measurement for its forward voltage under constant current. The uncertainty components are from the measurement for the initial junction temperature, the steady forward voltage, the initial forward voltage and the calibration of the voltage-temperature coefficient. The thermal resistance is calculated with the measured junction temperature and the thermal power. The uncertainty components are from the measurement for the junction temperature, input current and forward voltage. Analysis results show that the uncertainty of the junction temperature is the most dominant uncertainty contribution in the measurement for the thermal resistance. In the junction temperature measurement, the uncertainty in the calibration of the voltage-temperature coefficient contributes significantly. Thermal equilibrium and good thermal conducting condition should be obtained. These results give a better understanding and practice guide to an accurate measurement for the junction temperature and thermal resistance of LEDs.