Primary side feedforward control for TRIAC dimmable light emitting diode driver with constant power

This study proposes a primary side feedforward control scheme for low-power Triode AC semiconductor switch dimmable light emitting diode (LED) driver. The LED driver is a Flyback converter operated in constant frequency (CF) discontinuous current mode (DCM). With the proposed control scheme, the input power of the Flyback converter can be controlled by the TRIAC dimming angle, which is not affected by the AC input voltage. That is to say, the input power is constant with certain dimming angle. Besides, the output power is determined by the input power with a given conversion efficiency. For LED load, since the output voltage is almost constant, the output current can be regulated by TRIAC dimming angle without any secondary side feedback circuit. Almost linear dimming curve can be achieved. Also, the input current automatically follows the input voltage because of CF DCM operation, resistive input characteristic can be achieved and no dummy load is required for TRIAC dimming angle detection. Considering the LED forward voltage drop tolerance existed in the practical application, the related compensation method is also discussed in this study, which can further improve the performance of the proposed control scheme. A prototype with 27 V/0.5 A output and 180-265 V AC input has been built. Moreover, the experimental results from the prototype are presented to verify the theoretical analysis.