Single-stage single-switch isolated PFC regulator with unity power factor, fast transient response and low voltage stress

In this paper a simple control method is presented for a single-stage single-switch isolated PFC regulator that can simultaneously achieve unity power factor and fast output voltage regulation, while keeping the voltage stress of the storage capacitor low. The power converter topology comprises essentially a cascade combination of a discontinuous-mode boost converter and a continuous-mode forward converter. The proposed control utilises variation of both duty cycle and frequency. The role of varying the duty cycle is mainly to regulate the output voltage. Changing the frequency, moreover, can achieve unity power factor as well as low voltage stress. Basically, the switching frequency is controlled such that it has a time periodic component superposed on top of a static value. While the time periodic component removes the harmonic contents of the input current, the static value is adjusted according to the load condition so as to maintain a sufficiently low voltage stress across the storage capacitor. The theory is first presented which shows the possibility of meeting all three requirements using a combined duty-cycle and frequency control. An experimental prototype circuit is presented to verify the controller´s functions