A Continuous Rectangular-Wave Method for Junction Temperature Measurement of Light-Emitting Diodes

Junction temperature is an important parameter for evaluating thermal property of light-emitting diodes (LEDs). How to accurately measure the junction temperature still remains controversial. In this paper, we propose a continuous rectangular-wave method (CRWM) for measuring the junction temperature of LEDs. This method uses a continuous rectangular wave to drive LED under either voltage mode or current mode, and acquires the transient voltage response, i.e., the heating curve, by a high-definition oscilloscope at the rising edge of each cycle. Compared with the conventional forward voltage method (FVM), where a switch is necessary to transit the heating state to the sensing state and a transient cooling curve is usually acquired, the switch-free-periodic-acquisition CRWM avoids the acquisition time delay caused by switch and decreases charging/discharging effect of LEDs effectively with rising time of the rectangular wave less than 40 μs. Besides, periodic acquisition of the heating curve further enhances the signal-to-noise ratio. Comparative experiments by FVM, CRWM, and micro-thermocouple show coincidence and accuracy of CRWM with relative error less than 3% in current mode and 1.5% in voltage mode. COMSOL simulation is applied to verify the validity of CRWM and discuss the impact of rising time of the rectangular wave.