1. Introduction
Small signal modeling of power stage is important in derivation of controller for power conversion circuit design. The state space averaging method for conventional switched model converters can be dated back to 1976 [1]. A resonant type converter differs from the hard-switched converters in that the switching frequency is comparable to the oscillatory frequency of the passive network. Therefore the small ripple assumption, a pre-assumption for the state-space averaging method, does not stand in these converters. The energy transfers from the input to the output mainly at the fundamental frequency while the conventional converters transfer the majority of energy at DC frequency. Method based on discrete time domain modeling [2]–[5] has been proposed for low order resonant conveners. However it becomes too complicated for high order resonant converters modeling. Works in [6], [7], [8] tried to extend the small signal modeling to 3-order DC/DC converter either using state-plane method or describing function method. A modified averaging method transforming the ac signal into DC signal at multiple frequencies has been described in [9], [10] for DC/DC series/parallel resonant converters. The frequencies included in the model can be as many as expected, so theoretically the accuracy of the model can be arbitrarily high. The dominant frequency for modeling and analysis can be chosen from those frequency components that participate in energy transfer. Therefore its application is not limited to only resonant DC/DC converters.