Equivalent Circuit Modeling of LLC Resonant Converter

LLC resonant converter is widely used in industry. However, up to now, no simple and accurate small-signal equivalent circuit model is available. This article proposes an equivalent circuit model of LLC resonant converter. The simple equivalent circuit model is derived based on modification and simplification of extended describing function method, which has already been successful deriving equivalent circuit model in series resonant converter [15]. The model can well predict the small-signal behaviors observed in pulse-frequency-modulated LLC resonant converter, whenever switching frequency is below, close to, or above the resonant frequency. For the first time, analytical expressions for control to output voltage, input to output voltage, input impedance, and output impedance are provided to aid closed-loop feedback design. SIMPLIS simulation and experimental results are presented to prove the accuracy of the model.