A primary side feedforward control scheme for low power LED driver compatible with Triac dimmer

This paper proposes a primary side feedforward control scheme for low power Triac dimmable LED driver. The LED driver is based on the Flyback topology operated in constant frequency (CF) DCM mode. The input power is controlled by the proposed control scheme based on the Triac dimming angle, which is not affected by the AC input voltage variation. Therefore the output power and current can be regulated without any isolated feedback circuit. Also, the input current automatically follows the input voltage due to CF DCM operation. With the proposed control method, high efficiency and high power factor can be achieved by eliminating the isolated feedback circuit and the dummy load required for traditional Triac dimmable LED driver. Furthermore, considering the parameters tolerance existed in the practical application, such as LED forward voltage drop and Flyback transformer magnetizing inductance, the output voltage compensation and primary side inductance compensation method for the proposed control schemes are also presented in the paper, which can further improve the control accuracy. The experimental results from a 27V/0.5A prototype verify the theoretical analysis.