Junction temperature measurement of a LED street light using forward voltage method

Operating junction temperature of LEDs is linked directly to lifetime, long term reliability and failure modes of LED luminaires. It is important to measure the operating junction temperature to assess the health of a LED luminaire / solid state lighting product. Measuring the junction temperature of LEDs is a difficult task as in modern LED luminaires the tiny LEDs are surrounded by optical system (lens, diffusers and reflectors) and thermal management systems (heat sink). It is difficult to measure operating junction temperature using thermocouples and IR imaging as these methods will result in erroneous estimates of junction temperature. This paper discusses the prospect of junction temperature measurement of LEDs in a LED street light by using dependency of forward voltage drop across the LED junction on the temperature of that junction. Results of operating junction temperature measurements of a LED street light indicate that inherent forward voltage dependency on junction temperature of LEDs can be applied to measure junction temperature of LEDs in a LED based luminaire. The proposed methodology can be implemented to measure operating junction temperature of LEDs in other LED lighting products.