General ZVS half bridge model regarding nonlinear capacitances and application to LLC design

Zero voltage switching (ZVS) of the half bridge inverter is analyzed regarding nonlinear transistor output capacitances. A general method is described to derive an equivalent linear switch node capacitance from the transistors´ CV characteristics that accurately models the commutation energy. The nonlinearity depends on device technology and can have a strong impact on the equivalent capacitance and thus, on the design of ZVS converters, particularly at high switching frequencies. The nonlinearity can be expressed by means of a co-energy factor, which is unity for linear capacitances and can be as high as eight in recent superjunction MOSFETs, whereas first data on GaN HEMTs again shows values close to unity. The model is applied to the resonant LLC converter for which the calculation of ZVS boundaries is discussed in low and high gain operation modes.