A Discrete-Time Domain Modeling of LLC Resonant Converter Considering the Nonlinearity of Voltage-Controlled Oscillator

The LLC resonant converter, featured with soft switching realization, magnetic components simplicity, high conversion efficiency, high-power density and low EMI noise, has been widely used in diverse fields. Voltage-controlled oscillator (VCO), which shows strong nonlinear characteristics, plays a crucial role in output voltage regulating of the LLC resonant converter. However, in small-signal modeling for LLC resonant converter, VCO is usually treated as a constant gain, which will relegate the model accuracy especially in the high-frequency region. In this paper, an accurate small-signal model that can reflect the nonlinearity of VCO is proposed. The discrete-time small-signal model of the power stage is also deduced by exploiting the half-cycle symmetry for simplifying the derivation. Then the transfer function from the modulation signal to output voltage of the LLC resonant converter is developed. Finally, the simulation results verify the accuracy of the proposed model.