Readers of this journal are well aware that the junction temperature TJ of LEDs plays a key-role for their performance; not only for light output (efficacy) but also for wavelength stability. The cooler the LED, the better will be the performance. However, the temperature of the LED is not only a function of its own termal power loss, but also depends on the other heating components on the board and the cooling and heating paths due to ambient conditions, i.e. the mechanical and thermal environment in the final application. These ambient conditions might be quite harsh e.g. in car entertainment systems where high ambient air temperatures can occur, other heat sources are near-by and space for cooling is precarious.

Since high power LEDs are more and more used in general lighting there is an increasing demand for off-line power supplies to drive these. Due to the V-I characteristic of LED such a power supply must have a constant current output. The following article describes a PSU based on a Fairchild Power Switch (FPS™) that realizes constant secondary current with primary-side regulation. The absence of operational amplifiers and an optocoupler for the stabilization of the output current makes this PSU extremely cost effective in case safety isolation is needed.