Modeling of secondary LLC resonant converter based on full frequency range extended describing function method

The LLC resonant converter is widely used in different industries such as photovoltaic and electric vehicles due to the high efficiency. In order to obtain a high gain boost in conventional LLC circuits, the resonant inductor design value ought to be very small, which is challenging to achieve in engineering. To address this issue, the secondary LLC resonant circuit is suggested. However, it is in fact a multi-resonant converter with a complex modelling process. Therefore, it is necessary to model its small signal for improved circuit control. The extended describing function method accurately establishes the small signal model of resonant converters, but is typically limited to applications near the resonance point and ignores small signal models in other states. Hence, the accurate control of the circuit is achieved by developing a small-signal model of the secondary LLC resonant converter over the full frequency range. The correctness of the model is verified through simulation and experiment.