The present invention is an AC/DC (alternating current to direct current) converter. The converter contains two semi-stages. One is the boost-flyback semi-stage that has excellent self-PFC property when operating in DCM even though the boost output is close to the peak value of the line voltage. Moreover, it allows part input power transferred to load directly through the flyback path so the power conversion efficiency is improved. The other one is the D/DC semi-stage that provides another parallel power flow path and improves the regulation speed.