A method of characterising the thermal resistance of high power LEDs

A method for determining the thermal resistance of high power LEDs is described. Unlike more complicated systems, this method simply uses a fast pulse to determine the junction temperature under actual operating currents, combined with a conventional thermocouple to measure the mounting face temperature. The literature can be somewhat confusing in describing the thermal resistance of an LED. Often an “apparent” thermal resistance is determined from the input power alone, ignoring the optical power output. This can provide a correct junction temperature as a guideline under a given condition. However, the real thermal resistance, which may be about twice as high as the apparent thermal resistance, is of little use unless the exact output power of the LED is known, so that the real heat dissipated can be determined. This is dependent on the operating current, temperature, and where the LED is on its longevity curve, and can also be affected by the light fitting as well. Therefore, it is necessary to model these LED effects which correctly describes the light output under real conditions.