High Power Factor Primary Side Controlled Flyback LED Driver

There are many international standards to limit power factor and harmonic content of AC/DC LED driver, for example, the American energy star and the IEC6100032 Class C standard. Therefore, such LED driver needs the function of power factor correction.As a single stage structure, flyback converter working in the critical conduction mode can regulate the output as well as realize power factor correction. This structure is widely used for the small and medium power field. The PSR technology can directly collect the signal of voltage or current from the primary side of isolation converter to regulate the output. Compared with the traditional control mode, the PSR eliminate the secondary side sampling circuit to make the system more reliable and efficient, but has low precision of output current. The topic is based on PSR CRM flyback PFC LED driver, and improved the LED driver’s output current accuracy and the PFC performance.In this paper, the working characteristic of CRM PSR flyback and the principle of constant current for LED driver is analyzed firstly, and the feasibility of the structure which is used for the LED driver is explained. However, it discovers that there will be some errors in the current precision of CRM PSR flyback LED driver and the errors is caused by the sampling precision of primary side peak current. In addition, the increase of input voltage will result larger errors. Based on the relationship between the input voltage and output current error, this paper tries to make the input voltage feed-forward compensation of output current to increase accuracy of output current. And then it provides a simplified control method which will realize higher accuracy. When traditional CRM PSR flyback LED driver carries out the power factor correction, the sine degrees of input current will be effected by the LED voltage and the input voltage. But the LED load is sensitive to temperature, the change of temperature will lead to LED voltage changes and that the distortion of drive power input current. Accordingly, it can well solve this problem by introducing a variable on-time control to expand the scope of LED driver. Finally, this paper designs a LED driver for experiment. According to the experimental results, it proved the correctness of the theoretical analysis and simulation study and the feasibility of control method.