Although soft-switching technology can largely avoid the overlapping switching losses associated with hard switching, the reverse conduction voltage of MOSFETs introduces a certain amount of reverse conduction loss during the dead time. Taking an LLC converter as an example, the loss mechanism during the primary-side ZVS dead time is shown in Figure 1-2:

At time t1, the switches of bridge leg A (Qp1, 3) turn off, and the remaining current in the loop flows through the switch Coss, causing the Vds voltage to enter the commutation state. At time t2, the Vds of all switches completes commutation and the Coss voltage is clamped. However, because bridge leg B (Qp2, 4) has not yet turned on, the remaining current in the loop continues to flow through the body diode until bridge leg B turns on at t3. Therefore, during the t2-t3 interval, losses caused by body diode conduction occur. This loss can be estimated using Equation 2:

where V_D is the MOSFET body diode forward voltage drop, I_Lr is the series resonant inductor current, T_D is the dead time, T_T is the transition time calculated using Equation (1), and f_SW is the switching frequency. In an LLC topology, I_Lr during the dead time can be calculated from the switching frequency and magnetizing inductance.

The loss mechanism during the LLC secondary-side ZCS dead time is shown in Figure 1-3:

The example in Figure 1-3 uses a current-controlled secondary-side synchronous rectification strategy. At time t1, the current of synchronous rectifier bridge leg A (Qs1, 3) falls below the synchronous rectification threshold It, causing bridge leg A to turn off. t2 is the actual zero-crossing point of the B bridge-leg current. During the t1-t3 interval, the secondary-side switch Coss provides a low-impedance current path for bridge leg A. Therefore, the Vds of all switches commutates during this interval until the switch voltage is clamped at t3, and the current of bridge leg B (Qs2, 4) returns through the body diode of bridge leg B. During the t3 to t4 interval, third-quadrant dead-time losses occur on the secondary side, which can be estimated using Equation 3:

where I_TR is the secondary-side current, whose magnitude is related to the load, and T_T is the transition time required for commutation of the secondary-side Coss. For simplicity, the effect of the t1-t2 interval is ignored in Equation 3. It should also be noted that in DCM, the zero-crossing point t2 of the B bridge-leg current occurs very close to t3, and the loss can be approximately estimated using Equation 4:

Under the same dead-time and switching-frequency conditions, the LLC primary-side dead-time loss is independent of the load, while the secondary-side dead-time loss generally increases with increasing load. Therefore, under light-load conditions, primary-side dead-time loss becomes a non-negligible factor.