Series resonant converters with an adaptive secondary-side digital control for MHz 48V VRs: Circuit analysis and modeling

An adaptive secondary-side digital control is proposed for the series resonant converter to solve the loss of ZVS under certain input and load conditions, making it a good candidate for MHz 48V voltage regulators (VRs). By means of varying AC equivalent resistance, the output voltage regulation is achieved by controlling the duty cycle of the synchronous MOSFET. An additional control variable has added complexity to circuit analysis, modeling and controller design, since the state variables such as the inductor current and capacitor voltage are AC domain. A sine-cosine transformation is applied to these AC resonant state variables, correspondingly, the resonant circuit can be decomposed into two subcircuits under sine and cosine axes at each harmonics frequency of interest. The transformed state space model can be used for the large signal and small signal analysis for the series resonant converter with the secondary-side control. A power circuit is designed based on the large signal analysis and a feedback controller is designed based on the derived small signal model. Simulation and experimental results verify the validation of the design.