Abstract
A detailed study about the direct measurement of junction temperature Ti of off-the-shelf power light emitting diodes (LED) is presented. The linear dependence on temperature of the voltage drop across the device terminals at a constant current is in particular exploited and fully characterized, in the temperature range from T = 35 °C to 135 °C, with tests repeated at one thousand different probe currents. The accurate experimental data, obtained on several LED samples bearing two different part numbers, are reported, showing that they exhibit a high degree of linearity in wide current ranges, a circumstance that allows for a fast and reliable calibration as sensors. The measurement error is also fully characterized in terms of repeatability and stability over time, with measurements repeated after 600, 900, 1200 and 1800 hours of applied electro-thermal stress, demonstrating that the relevant sensor parameters stabilize after a few hundred hours of operation. A full set of parameters is provided for the two device models, allowing the direct use of each LED for the self-monitoring of the junction temperature and ensure compliance with their safe operating area over time. Moreover, a procedure and a simple circuit for the real time measurement of Ti, while the LED is on, are presented. The procedure does not require a stable current source, and relies instead on the application of a sub-threshold current ramp for such a short time that the change in the output light is not perceived by human eyes.
Highlights
The Solid State Lighting (SSL) technology has definitively emerged as the most efficient and long lasting technology for converting electricity into light in almost every application context, from luminaries to flat panel displays, to wide area image projection
We focused on two commercial white light power light emitting diodes (LED), denoted hereafter as LED-A [36] and LED-B [37], having respectively a normal operation current of 700 mA at 3.05 V and 1050 mA at 3.02 V
Typical IF -VF characteristics for both LED types are reported in Fig. 1, measured in the temperature range from 25 ◦C to 135 ◦C, in steps of 10 ◦C
Summary
The Solid State Lighting (SSL) technology has definitively emerged as the most efficient and long lasting technology for converting electricity into light in almost every application context, from luminaries to flat panel displays, to wide area image projection It has brought significant advancements in terms of energy and cost savings. Intensity output may slowly fade or the color slowly degrade as the end of life is approaching [1]–[5] It is well known, that LED performances, at least in terms of intensity, lifetime and emission spectrum, degrade faster as their operation temperature increases above certain limits [6]–[12], which are set well below those of incandescent bulbs and even of fluorescent lamps. It turns out that high quality solid-state light sources must always adopt more or less complex temperature control strategies, including passive or active cooling methods [13]
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