ZVS modelling of the LLC converter operating as unity power factor front end

The series resonant, parallel loaded LLC with voltage in- and output has widely been studied and used for DC-DC power conversion applications. In contrast, this paper considers the standard LLC in the AC-DC case, which means a far wider input voltage range from the peak mains down to approximately zero. Beside the question whether and how the standard LLC converter can handle the resulting large gain ratios e.g., in the context of a power factor correction (PFC) application, it needs to be explored whether zero-voltage switching (ZVS) can always be achieved, especially regarding the highly nonlinear output capacitances of superjunction MOSFETs. To comprehensively deal with the variety of operation conditions, the converter design choices and the inverter devices, an analytical description is preferred over approaches merely based on circuit simulations. Thereto, a system of equations firstly describes the ideally switching steady state. This model is linked to a linearized inverter transition model for ZVS assessment is used to refine the solution space for the converter design. Three ways to extract that inverter model from the MOSFETs' output CV-characteristic are compared that all can predict ZVS but differ in complexity.