What Is Zero Voltage Switching?

Zero voltage switching (ZVS) is a soft-switching technique in power electronics in which a semiconductor switch is turned on only when the voltage across it has dropped to zero. By ensuring this condition through resonance, ZVS eliminates the capacitive energy that would otherwise be dissipated in the switch at every turn-on event, reducing switching losses and suppressing the electromagnetic interference associated with hard-switched transitions. The technique became practical in the 1980s alongside advances in high-speed MOSFETs and resonant converter design, and it remains the dominant soft-switching strategy in high-frequency power conversion.

ZVS is complementary to zero current switching (ZCS), which targets the turn-off transition rather than turn-on. Because MOSFETs suffer primarily from capacitive losses at turn-on, ZVS is generally better suited to MOSFET-based converters, while ZCS is preferred for insulated-gate bipolar transistors (IGBTs) whose reverse-recovery behavior makes zero-current turn-off more valuable. Together, the two techniques define the primary approaches to soft-switching in modern power supplies.

Resonant Operation and the Body Diode

ZVS exploits the MOSFET's own internal body diode and output capacitance (Coss) to create the zero-voltage condition. Before the switch turns on, a resonant inductor discharges the switch's output capacitance, pulling the drain-source voltage to zero. The body diode then clamps the voltage at approximately zero and conducts briefly, during which the gate drive fires to turn on the channel. This sequence ensures that no stored capacitive energy is dissipated as heat in the device. As IEEE analysis of resonant ZVS converters shows, formulating an accurate ZVS criterion must account for the energy stored in the resonant tank relative to the switch's total capacitance, a more nuanced requirement than the simple energy-based criterion used for non-resonant converters.

Phase-Shifted and Half-Bridge Topologies

Two topologies dominate ZVS implementation in isolated DC-DC converters: the phase-shifted full-bridge (PSFB) and the LLC resonant half-bridge. In the PSFB converter, the magnetizing energy of the transformer and the leakage inductance form the resonant tank that achieves ZVS for all four switches over a defined load range. The LLC resonant converter uses a series inductor-capacitor-parallel inductor (LLC) network that provides both ZVS at turn-on and near-ZCS at turn-off across a wide input voltage range, making it the preferred topology for server power supplies and consumer adapters. A ZVS boost converter study for power factor correction illustrates how ZVS can be extended to non-isolated topologies by adding a small auxiliary circuit that creates the resonant transition without significantly altering the main power path.

Switching Frequency and Efficiency

ZVS enables converters to operate at switching frequencies in the hundreds of kilohertz to low megahertz range with efficiency levels that would be unattainable in hard-switched designs. At 10 MHz, the experimental results for zero-voltage-switched quasi-resonant buck and flyback converters demonstrated the practicality of ZVS at frequencies that reduce passive component size dramatically. Higher switching frequency allows for smaller magnetic cores and capacitors, reducing overall converter volume and weight. The trade-off is that ZVS condition may be difficult to maintain at very light loads, where the resonant inductor current may not be sufficient to fully discharge the switch capacitance before turn-on.

Applications

Zero voltage switching has applications in a range of power electronics fields, including:

• Server and data center power supplies (LLC resonant converters)
• Electric vehicle onboard chargers and DC fast-charging stations
• Photovoltaic inverters and battery energy storage systems
• Wireless power transfer circuits operating in the 6.78 MHz and 13.56 MHz bands
• Telecom rectifiers and DC-DC converters requiring high efficiency at high frequency