A Low-Cost Constant Current Control Method for DCM and CCM in Digitally Controlled Primary-Side Regulation Flyback Converter

Primary-side regulation (PSR) flyback converter with a discontinuous conduction modulation (DCM) mode is widely used in small-power applications. To introduce higher output power, a continuous conduction modulation (CCM) mode is used in which an analog to digital converter of high cost is always required. In this paper, to reduce the cost, a novel digital peak current control method is proposed to realize constant output current control in both DCM mode and CCM mode. However, there exists a time length calculation error that cannot be avoided in the critical condition between DCM mode and CCM mode. The output current will be lower than the nominal output current. Thus, a hysteretic multimode control method with two pulse width modulation modes and two pulse frequency modulation modes is utilized to eliminate the time length calculation error. In the proposed control, only a low-speed digital to analog converter and three comparators are used, which greatly reduced the cost, and high accuracy of output current can be obtained. The proposed control scheme was then verified through a field-programmable gate array (FPGA)-controlled 3.76-15 V/3.76 A PSR flyback converter. In the prototype, the accuracy of the output current is within ±1.5% for the universal load range.