Intermediate-scale instability in two-stage power-factor correction converters

Intermediate-scale instability is investigated for two-stage power-factor correction (PFC) converters, which are practical and popular design solutions for medium-to-high power applications. The circuits employ a cascade configuration of a pre-regulator Boost PFC converter with an average current mode control to achieve a near unity power factor and a post-regulator DC-DC Buck converter with a voltage feedback control to regulate the output voltage. The intermediate-scale instability manifests itself as local oscillations within a line cycle, whose oscillation frequency is between the line frequency and the switching frequency. Based on the assumptions that the rectified line voltage can be represented by its root mean square value and the post-regulator DC-DC Buck converter can be considered as a constant power sink, the small-signal model is established, and the underlying mechanism of such intermediate-scale instability is studied further. Finally, experimental results are given for the verification of analytical results.