Due to internal parasitic effects, a MOSFET requires a certain amount of time to fully turn on and turn off. During hard switching, the rising current and falling voltage during MOSFET turn-on overlap, resulting in instantaneous switching losses. In addition, the rapid voltage and current transitions associated with hard switching generate high-frequency noise, leading to significant EMI issues. To address these drawbacks, some power converters employ specific design techniques to enable MOSFETs to operate under soft-switching conditions, such as Zero-Voltage Switching (ZVS) turn-on or Zero-Current Switching (ZCS) turn-off.

Taking an LLC converter as an example. Over the operating frequency range of an LLC converter, the resonant tank presents an inductive load to the primary side, and the primary-side current lags the voltage. During the dead time when all primary-side switches are turned off, the circulating current charges and discharges the MOSFET junction capacitances (Coss), allowing the MOSFET voltage to fall to 0V before the next turn-on, thereby achieving ZVS.

Figure 1-1 provides a detailed illustration of the primary-side ZVS process of an LLC converter. Subfigure (a) shows the voltage and current waveforms of the switches during the dead time. Subfigure (b) shows the circulating path of the resonant inductor current ILR on the primary side during the dead time. For simplicity, only the Coss capacitors and body diodes of the four full-bridge MOSFETs are shown in the figure. When Qp1,3 turn off, the voltage on VHB1 gradually decreases from its initial value VS to 0, while the voltage on VHB2 gradually increases from 0V to VS. Note that IDS in the figure is defined as the MOS drain-source terminal current, which is the sum of the channel current, body diode current, and Coss current.

The total time TT required to complete the charging and discharging of Coss is:

After t = TT, the junction capacitor charge transfer is complete, and VHB1 and VHB2 are clamped by the body diodes of Qp2 and Qp4 at -VD and VS+VD, respectively (VD is the body diode forward voltage drop), until the switches turn on again. The necessary and sufficient conditions for primary-side ZVS in an LLC converter are:

1. The MOSFET dead time TD must be greater than TZ to ensure complete transfer of the junction capacitor charge.
2. The bridge-leg current must lag the voltage and must not reverse direction during the dead time.

For secondary-side ZCS in an LLC converter, based on the characteristics of its circuit, the secondary-side current operates in Boundary Conduction Mode (BCM) under heavy load and Discontinuous Conduction Mode (DCM) under medium and light loads. Therefore, the current is close to or equal to 0 when the rectifier MOSFETs turn on and turn off, achieving Zero-Current Switching (ZCS) for both turn-on and turn-off.