Direct junction temperature measurement in high-power LEDs

The light quality and long-term stability of phosphor converted light-emitting diodes (LEDs) for luminaires depend on the temperature distribution inside the LED chip and the color conversion element. Therefore, a reliable and accurate method to establish the LED's junction temperature is required to further improve and optimize high quality LED luminaires. In this paper we describe the development and application of an innovative junction temperature measurement method which is based on a precise and universally applicable calibration procedure, allowing to use the calibrated LED itself as a temperature sensor under the respective operation condition of interest. This method is based on an extremely fast pulse measurement procedure allowing to record pairs of forward current and voltage drop values periodically every microsecond starting from the first microsecond of a pulse. From these experiments we reap two kinds of result: (1) Independent of the pulse shape in our experiments we observe a constant relation of current-to-voltage drop which we interpret as a constant junction conductivity. Depending on the type of LED (but independent of the packaging technology) we obtain a constant junction conductivity throughout several ten microseconds which we understand as a proof that the junction temperature did not change during this very first pulse phase. (2) The junction conductivity obtained in this moment is a measure for the junction temperature so that a calibration can be made by comparison with an independent steady-state temperature measurement made at zero-current condition. The method has been successfully applied to thermally characterize high-power LED modules as a 3 × 3 LED arrays with color conversion glob tops built-up on an insulated metal substrate (IMS).