High-power Led Devices, Thermal Studies

Since 1990s, the invention of the first blue LED, the power density of LED has continued to rise and LED has made its steps to general lighting. However, the increase of power density also leads to possibly higher operating temperature. As a semiconductor device, LED's lumen efficacy and life time greatly depends on its operating temperature, especially for high power LEDs. Thermal measurement of high power LED can be a useful source for the development of a LED lighting system with strong high performance as well as great reliability and should be studied.This thesis is based on the thermal problem of high power LEDs. All kinds of basic problems involved in Electrical Testing Method of LED's thermal characteristic are studied. The thesis mainly includes the following aspects:1. The relationship between K factor and measurement current is derived from theories of semiconductor device and the result is corroborated by experiment conducted by Keithley S2400 based measurement platform.2. A thermal measurement platform based on Keithley S2400 is established and junction temperature of an high power LED is measured under different sampling rates and measurement currents. The results are systematically analyzed; a theory of sampling rate and measurement current's effect on junction temperature measurement error is concluded and applied to explain why the junction temperature of LED could be underestimated with insufficient high sampling rate. Furthermore a method based on work current, low sampling rate and error compensation is proposed based on the theory.3. Another thermal measurement platform based on Nl 5922 digitizer is established according to ETM defined in JESD51-1. Several main non-ideal factors introduced by performance limitation of measurement equipments are studied and eliminated by data processing program in LABVIEW. The experiment on sampling rate and measurement current is repeated with the new platform and the results are quantitatively analyzed. The agreement with the former results, to some extent, verified the new platform.4. The RC network model of LED is introduced and used to explain the heating and cooling process of LED in junction temperature measurement. The forward voltage response of LED is not only solved from the theory but also measured and the relationship between VF response and thermal structure of LED system is qualitatively studied. Furthermore the thermal resistance of LED is acquired by fitting the measurement results and matches well with the one measured by ETM and the one calculated from the real physical model.