Improved ZVS Criterion for Series Resonant Converters

Zero voltage switching (ZVS) of power MOSFETs is essential for resonant converters and the ZVS criterion should be formulated appropriately to achieve the ZVS condition. Conventionally, the energy-based criterion has been considered the most accurate ZVS criterion for non-resonant converters and thus has been directly applied to the resonant converters without any doubt. Through a theoretical investigation along with a few examples, this study shows that the energy-based ZVS criterion is not as accurate for resonant converters as it is for non-resonant converters. A more accurate ZVS criterion, considering energy stored in every energy tank element, the power dissipation of the load, non-zero current assumption at the end of dead time period, and the energy absorbed or supplied by the input dc voltage source, is proposed. The energy-equivalent inductance is introduced to represent resonant tank energy and is used in formulating ZVS criterion for various modulation strategies. In addition, the minimum dead time for successful ZVS transition is also investigated in this study. Experiments have been conducted to validate our new finding and the proposed method is shown to be more accurate than the conventional method.