Low THD multipliers for BCM buck and cascaded buck-boost PFC converters

Boundary conduction mode control is a convenient and low cost method for realizing power factor correction in boost and flyback converters. A signal proportional to the input sinusoid is used as a reference for shaping an inductor current. For a boost converter, this renders the input current to be sinusoidal, but it causes distortion in Flyback and buck derived converters. Mathematically, it can be shown that a different multiplier is needed for buck derived and Flyback converters to achieve a sinusoidal input current. In this paper, we develop peak current reference multipliers for Boundary Conduction Mode (BCM) control of buck derived converters. These multipliers are to be implemented in typical low cost BCM controllers. We derive a new multiplier for a buck PFC, and extend this to a cascaded buck boost (also known as a four switch buck boost) PFC as a composite multiplier. For the cascaded buck boost, we also implement a control scheme to handle the transition between buck and boost modes. We provide experimental results in both topologies.