A new method to calculate LED's thermal impedance and a new voltage-current formula of LED

Abstract

The thermal impedance and voltage-current relation of LEDs can be obtained using a photodetector response to pulsed light emitted by LEDs. In this study, LEDs have been driven by pulsed high currents. Photodetector voltage is proportional to the radiant power of LED. A triangle has been drawn on the top of the photodetector voltage figure, and some geometrical magnitudes have been measured. The junction temperature can be calculated using these magnitudes and the LED's relative radiant power-junction temperature coefficient. Thermal impedance can be calculated if junction temperature and ambient temperature difference are divided by thermal power dissipated from the LED. In this study, the thermal impedance of LEDs and their error have been calculated and a new voltage-current formula for LEDs has been given. The formulas for LED junction temperature-time and thermal impedance-time variations have been obtained. Besides these, the maximum pulse duration of the current pulse that the LED can work without breaking down has been calculated.