Comparison of Small-Signal Dynamics of BIFRED and Single-Stage Cascaded Boost-and-Flyback PFC Converters

Single-stage power-factor-corrected (PFC) regulators employing a discontinuous-mode boost converter and a flyback converter have been proposed recently. This paper gives a detailed analysis of the small-signal dynamical response of this type of converters, and explains how the relative sizes of the storage and output capacitors affect the dynamics. Complete sets of small-signal transfer functions are derived for two typical configurations, namely BIFRED and SSIPP. In particular, we have shown that when the boost part operates in discontinuous mode and the flyback part in continuous mode, the duty-ratio-to-output transfer function of BIFRED can be reduced to a simple second-order minimum phase function whereas that of the SSIPP can be reduced to a second-order non-minimum phase function under certain conditions. Moreover, due to the occurrence of pole-zero cancellation, the SSIPP can achieve fast (single-pole) response regardless of the size of the storage capacitor, when both the boost and flyback parts operate in discontinuous mode. Also, both the converters have a high dc output impedance.