From equation (1) we see that under light loading, IL1 is small, so that ZVS operation is not easily achieved, but as the load is increased ZVS operation is more easily realized.

As in the explanations of Modes (5) and (6) and Modes (12) and (13), in the lagging leg, if the amount of energy stored in LS is not greater than that stored in COSS for a MOSFET, the MOSFET charging and discharging are not completed, and so ZVS operation is not achieved.

Q2 turns on. At this time DQ2 is conducting, so the Q2 drain-source voltage VDS_Q2 is essentially zero. That is, ZVS operation is achieved, and so a turn-on loss does not occur