A Technique for the Direct Measurement of the Junction Temperature in Power Light Emitting Diodes

Measuring the actual junction temperature TJ of power light emitting diodes used in high-end luminaires is important for implementing measures to reduce the stress and therefore extend the lifetime of these fundamental light sources. The direct measurement of this key parameter can be performed by imposing a known and precise forward current Iprobe through the LED, and tracking the bias voltage appearing at its terminals. The technique proposed in this paper is based on the same approach, but does not require a stable current source, relying instead on the exponential fitting of random I-V measurements and on the estimation of the bias voltage at the proper Iprobe through a mathematical interpolation. The technique is implemented on a custom-designed circuit, and experimental data are obtained on commercial power LEDs, allowing to assess the impact of Iprobe and sampling current range on the measurement error.