A Simplified Equivalent Circuit Model of Series Resonant Converter

Equivalent circuit models are useful design tools for control and have already well served their purposes in pulse width modulation dc-dc converters. However, no simple equivalent circuit model is available yet for resonant-type dc-dc converters. Up to now, the most successful equivalent circuit model of series resonant converter (SRC) is based on extended describing function concept, which was proposed by Yang et al. However, the equivalent circuit is a complicated fifth order with the cross-coupling effect and no analytical solution is provided for transfer functions. This paper proposes a simple third-order equivalent circuit model of SRC. The equivalent circuit model is derived by simplification of the original fifth-order equivalent circuit, based on the fact that the resonant capacitor behaves like an equivalent resonant inductor with respect to the modulation frequency. The equivalent circuit model can predict the dynamic behavior very well when the switching frequency is below, close to, or above the resonant frequency. Furthermore, for the first time, analytical expressions of all transfer functions, i.e., control-to-output, input-to-output, output impedance, and input impedance are provided. These analytical transfer functions will serve as a useful tool for the feedback design. The equivalent circuit model is verified by Simplis simulation and experimental results.