Temperature Sensing Characteristics and Long Term Stability of Power LEDs Used for Voltage vs. Junction Temperature Measurements and Related Procedure

Abstract

A detailed study about the direct measurement of junction temperature T J 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 T J , 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. INDEX TERMS LED, light emitting diodes, junction temperature, temperature sensors, diode sensors.

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

“…The tests were carried out in the temperature range from 35 • C to 145 • C. The I-V characteristics of the ten samples are reported in Figure 1 at the lowest and highest temperatures, clearly showing the existence of a dispersion among them. By firstly using the same technique described in [22], the values of S and Q, averaged over the ten samples, were calculated at each current together with their standard deviations. T J extraction tests were then run to assess and compare the quality of the two extraction techniques.…”

“…Studies have already demonstrated a strong linear relationship between the voltage drop across a semiconductor junction and its temperature for several solid-state devices, provided that a constant and proper forward current is set [25][26][27][28][29][30]. In particular, the forward voltage of a P-N junction, biased at a constant current level where the diffusion component is largely predominant, is given (neglecting the impact of the series resistance) by [22,28]:…”

“…where T J is the junction temperature, I D is the device current, k and q are the Boltzman constant and the elementary charge, respectively, and E G is the band gap energy of the semiconductor at 0 K. B and b are two constants with temperature. Using, e.g., LEDs as absolute temperature sensors over moderate domains, Equation (1) yields high sensitivities (in excess of 1.5 mV/ • C, depending on the bias current levels) and reasonable linearity [19][20][21][22]31,32], such that the above relation can be written as:…”

“…Other sensorless ways of measuring an LED T J consist of using some temperature-dependent electrical parameters, such as the reverse saturation current [18] or the forward junction voltage V J under a constant current [19][20][21]. Recently, a detailed study based on the measurement of V J was presented in [22]. There, a set of measurements was carried out to identify the most suitable bias (or probe) current range where the LEDs can work as highly linear temperature sensors.…”

“…The result of the characterization procedure is a linear ∆V-T fitting that provides current-dependent coefficients, which effectively reduces the average error. This method has been characterized and the results are discussed in comparison with the previous T J measurement procedure [22,23].…”