A simple digital DCM control scheme for boost PFC operating in both CCM and DCM

Boost power factor correctors (PFCs) and digital average current controllers are usually designed for continuous conduction mode (CCM). However, discontinuous conduction mode (DCM) appears at light loads in the inductor current near zero crossings of the input current, resulting in input current distortion. It is caused by inaccurate average current values obtained in DCM. The inductor current goes to zero before the end of the switching period. Thus, the average is not given by sampling in the middle of the rising edge of the inductor current. In addition, the linear CCM PFC controller is unable to ensure input current shaping in DCM whereby the converter exhibits non-linear characteristic. This paper proposes a simple digital DCM control scheme for boost PFC. DCM current control is achieved with minimal changes to the CCM control structure, and is easy to implement digitally. Good input current shaping is achieved in both CCM and DCM.