Small-signal analysis of single-stage cascaded boost-and-buck PFC converters

Single-stage power-factor-correction (PFC) regulators based on the cascade combination of a discontinuous-mode boost converter and a buck converter have been proposed recently. Much of the work on this type of single-stage regulators has been performed around a number of topologically equivalent configurations, e.g., BIBRED, SSIPP and decoupled Cuk converter. Circuit operation and steady-state design have been the main focus of study. This paper gives a detailed analysis of the dynamical response of this type of converter, 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 all common operating modes. In particular, the authors have shown that when the boost part operates in discontinuous mode and the buck part in continuous mode, the duty-ratio-to-output transfer function can be reduced to a simple second-order function when the storage capacitor is sufficiently large. Such order reduction (pole-zero cancellation) is important for achieving fast response. Also, the power converter has a high output impedance, in contrast to the normal continuous-mode buck stage. Moreover, when both the boost and buck parts operate in discontinuous mode, the system can achieve fast response for most practical situations regardless of the occurrence of pole-zero cancellation