The single-stage forward-flyback power factor correction (PFC) converter with quasi-resonant (QR) control is studied in this paper. This converter merges flyback converter and forward converter through a common transformer. Only the flyback sub-converter works when the input voltage is lower than the reflected output voltage while both the flyback sub-converter and the forward sub-converter operate to share the output power in the rest region. The dead zones which exist in the AC input current of traditional forward converter are eliminated and high power factor can be achieved. Two different quasi-resonant (QR) control schemes have been studied. Detailed analysis, optimal design considerations and comparison for these two QR control modes are provided. Finally, two 120W experimental prototypes for LED driver were built up to verify the theoretical analysis.

The single-stage flyback power factor correction (PFC) converter is well applied in medium and low power occasion such as adapters or LED drivers. Usually, the flyback PFC converter is preferred to operating in boundary conduction mode (BCM) for achieving high power factor (PF) and high efficiency at the same time. However, the conventional BCM control scheme such as input voltage tracking control or constant on-time control (COT) could not achieve unit PF for the flyback PFC converter. As input voltage increases, the PF and the total harmonic distortion (THD) of flyback converter deteriorate seriously. In this paper, a variable-frequency one-cycle control (VF-OCC) for BCM flyback single-stage PFC converter is presented to achieve unit power factor. Detailed theoretical analysis and design consideration for the VF-OCC are described. At last, two 54W/1.5A laboratory prototypes with two operation strategies have been built up. Experiment results show that the BCM flyback converter with VF-OCC can achieve high PF and low THD over universal input voltage range.

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The single-stage Forward-flyback PFC converter is studied in this paper. This converter combines a flyback circuit with forward circuit through a common transformer. Only the flyback circuit works when the input voltage is lower than the reflected output voltage and both the flyback circuit and the forward circuit operate to share the output power in the rest region. The dead zones which exist in the AC input current of conventional forward converter are eliminated and high power factor can be achieved. Two quasi-resonant (QR) control schemes are proposed and compared. Optimum design considerations for the Forward-flyback PFC converter based on these two QR controls were presented. Detailed analysis and two experimental prototypes with 60W output power were built up to verify the theoretical analysis.