Voltage and current stress reduction in single-stage power factor correction AC/DC converters with bulk capacitor voltage feedback

Single-stage power factor correction (PFC) ac/dc converters integrate a boost-derived input current shaper (ICS) with a flyback or forward dc/dc converter in one single stage. The ICS can be operated in either discontinuous current mode (DCM) or continuous current mode (CCM), while the flyback or forward dc/dc converter is operated in CCM. Almost all single-stage PFC ac/dc converters suffer from high bulk capacitor voltage stress and extra switch current stress. The bulk capacitor voltage feedback with a coupled winding structure is widely used to reduce both the voltage and current stresses in practical single-stage PFC ac/dc converters. This paper presents a detailed analysis of the bulk capacitor voltage feedback, including the relationship between bulk capacitor voltage, input current harmonics, voltage feedback ratio, and load condition. The maximum bulk capacitor voltage appears when the dc/dc converter operates at the boundary between CCM and DCM. This paper also reveals that only the voltage feedback ratio determines the input current harmonics under DCM ICS and CCM dc/dc operation. The theoretical prediction of the bulk capacitor voltage as well as the predicted input harmonic contents is verified experimentally on a 60 W ac/dc converter with universal-line input.