Can someone rectify this. Thanks in advance.
\textbf{Sub interval 1:}($t_{o}-t_{1}$)\\\
This sub-interval starts when D1 and D4 are Forward biased, so in this sub-interval D1-D4 and D6-D7 are conducting Bridge voltages $V_{h1}$, $V_{h2}$ and primary current $i_{r1}$ is as shown in waveform. This sub-interval ends when current is commuted from D6-D7 to S6-S7and from D1-D4 to S1-S4. Hence for ZVS operation of S1 and S4, primary current \fbox{ $i_{r}(t_{0})<0$}.\\
