A primary side control Scheme for Triac dimmable LED driver based on indirect output current sensing

This paper proposes a primary side control scheme for triac dimmer compatible LED driver based on Flyback topology. By sensing the average primary side switch current and transformer auxiliary winding voltage signal, the output current can be derived using the proposed calculation method, which can be adopted in any operation mode with variable frequency and constant frequency control. Based on this indirect sensed output current signal, the output current can be regulated precisely without conventional isolated feedback circuit. For TRIAC dimming application, it is better to force the input current follow the input voltage to eliminate the dummy load for fire angle detection and achieve high power factor at no dimming condition. With critical conduction mode or discontinuous current mode operation, the proposed method can achieve all the desired functions as well as a linear dimming characteristic. The detailed operation principles are presented and the experimental results from a 15W prototype verify the theoretical analysis.