A novel constant frequency quasi-CRM control scheme of three-phase single-switch boost PFC converter

Three-phase single-switch boost PFC converter operates in discontinuous current mode (DCM) to achieve PFC. If the duty cycle is constant in a line cycle, the average value of the inductor current in a line cycle is not sinusoidal. The PF is low and the input current contains rich 5^th and 7^th harmonics. Variable switching frequency control can reduce the input current harmonic distortion. However, this control scheme suffers from a serious defect that the switching frequency is load and input voltage dependent. The large variable frequency range complicates inductor design, device selection, and EMI filter design. By modulating the on-time of switch, a novel constant frequency quasi-critical conduction mode (CRM) three-phase single-switch boost PFC converter is proposed in this paper. Compared with traditional constant on-time (COT) control, the proposed control scheme achieves constant frequency at a line cycle which will simplify the design of EMI filter and improve the effect of filter. At the same time, the switching frequency is reduced also which will lower the switching losses, leading to a higher efficiency.