Evaluation of Primary Side Control Schemes for Flyback Converter with Constant Current Output

Flyback converter has been widely used in low power offline applications due to its simplicity and cost-effectiveness. Conventionally, the output voltage or output current are sensed and regulated by an isolated feedback circuit with opto-coupler. In order to further simplify the circuit and reduce the cost, primary side control for flyback converter has been adopted to eliminate the opto-coupler feedback circuit. The primary side output voltage control is quite straightforward. However, the primary side output current control is more complex than the output voltage control, which is also required for many applications like LED drivers. In this paper, a thorough review of the existing output current control methods are presented, including pulse frequency modulation (PFM), current estimation, adaptive peak current value and duty cycle control. Finally, an improved primary side output current control method based on the current estimation is studied, which is more flexible and accurate. Experimental results from a 15W LED driver prototype are presented to verify the performance of the control scheme.