Summer 2012 Intern Project- Benjamin Campo

TEMPERATURE EVOLUTION OF LIGHT EMITTING DIODE EFFICIENCY


Benjamin Campo
Electrical Engineering
UC Santa Barbara

Mentor: Nathan Pffaf
Faculty Advisor: Steve DenBaars
Departments: Electrical and Computer Engineering and Materials

Current widespread lighting technologies—incandescent and florescent—are inefficient light emitters. While energy demands remain high and consumption continues to rise, high efficiency light emitting diodes (LEDs) provide an attractive solution to the problem of inefficient lighting and promise energy and associated cost savings. However, LED performance is strongly temperature dependent; this study examined ways to optimize LED efficiency with respect to temperature. In order to accomplish this, LEDs of various emission wavelengths, chip mounts, and packages were first calibrated with a temperature stage to establish a forward voltage vs. temperature relationship. Then, when driven with a high duty cycle square current wave (to simulate regular operation), light output data was taken using an integrating sphere, and the temperature was calculated using the LED calibration data with forward voltage measurements. LED efficiency was calculated using the light output power and the electrical input power.  To verify these results, a pulsed current experiment was also used, with the LEDs artificially heated with a temperature controlled stage inside the integrating sphere. This study shows that LED efficiency is inversely proportional to temperature. In blue unpackaged LEDs mounted with silver paste, wallplug efficiency dropped from 31.7% at 33.7 °C to a mere 15.4% at a temperature of 109.1 °C. For optimal efficiency, LEDs should be operated at low temperatures. The data from this study can contribute to the overall understanding of how, and in what setting, LEDs should operate for maximum energy efficiency.

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