Abstract :
The authors have investigated the effect of thermal cycling on the output intensity, peak position and chromaticity coordinates of a high-power AlGaInP light-emitting diode (LED) driven under pulse-width modulation (PWM; pulse width of 200 s to 20 ms at a constant repetition frequency of 50 Hz) and pulse-frequency modulation (PFM; repetition frequency of 50 5000 Hz at a constant pulse width of 200 s) modes. Experimental data on the junction temperature oscillation were obtained by using the transient forward voltage method and accounted for by a thermal model of a repetitive-pulse-driven LED with multi-exponent relaxation of the junction temperature. Compared to the PWM driving mode, the PFM mode, which causes a reduced junction temperature oscillation, was shown to offer an efficiency gain (5 at a 50 duty cycle) and a reduced non-linearity of output characteristics (deviation of the output intensity dependence on duty cycle from a self-heating-free one, as well as electroluminescence (EL) band peak position and chromaticity coordinates dependences on duty cycle). Based on the results of the investigation, the PFM driving mode is suggested as an advantageous alternative for controlling polychromatic LED clusters by temperature feed-forward compensation loops and dimming LEDs in lighting applications.
