High-voltage MOSFET behavior in soft-switching converters: analysis and reliability improvements

High-frequency converters powered from high source voltage show significant improvements when operated with soft switching: reduced switching losses, which allow high switching frequency and size reduction of reactive components; reduced EMI/RFI noise; no need for complex and expensive snubbers; and exploitation of the parasitic circuit elements to help the resonant transition. Thanks to these characteristics, zero-voltage-switching (ZVS) topologies are now widely used in power electronics, and especially in telecom power systems. The MOSFET is the most common choice of controlled switch in the ZVS full-bridge converter. The MOSFET is capable of very fast commutations and its intrinsic body diode saves an additional external component that would otherwise be necessary to clamp the switch voltage to the input supply voltage. Both the internal body diode and the output capacitance become essential components for the resonant transition. Although in the ZVS full-bridge converter the MOSFET operates well inside its safe operating area and its body diode is never subjected to hard turn-off, some "unexplainable" failures do happen, due to the unavoidable usage of the intrinsic body-diode. This paper analyses the MOSFET's behavior in high-power and high-input-voltage ZVS converters, and presents an original theory of the MOSFET breakdown. New technical solutions to improve the ruggedness of the device and consequently the reliability of the equipment are proposed. The effectiveness of these solutions is proved in a 100 A, 6000 W telecom power supply.