A Constant Current Digital Control Method for Primary-Side Regulation Active-Clamp Flyback Converter in CCM Mode

Abstract

To improve the output power and maintain small volume, active-clamp flyback converter in continuous conduction modulation (CCM) mode is often used as soft-switching technique is realized with low cost and simple control. However, opto-coupler in the feedback loop will introduce nonlinearity and temperature drift. To eliminate the opto-coupler, a constant current digital control method is proposed for primary-side regulation active-clamp (PSRAC) flyback converter in CCM mode. As the nonlinearity of the output diode current exits, traditional primary-side control method cannot estimate the output current directly. In this paper, the average current of the output diode is calculated based on the primary current and auxiliary winding voltage considering the charge balance principle. Only three comparators, a low speed digital-to-analog converter (DAC) and a digital control unit are required. Constant output current is realized. The proposed control scheme was verified by a FPGA controlled 6.50-20. 00V/3.20A ac-dc PSRAC flyback converter.