After that, triggering voltage is applied to S1 and S3
these switches are turn ON. Now the voltage regularly applying
on L1,ip1 (inductor current) increases continuously and
passed S1, Current ir (Resonant Current) is above zero level
and passed S1 and S3. Before this operation S1 and S3
switches are off. At the switching-off time, ip1 (inductor
current) moved to maximum value, and ir (Resonant Current)
is positive. Current ir (Resonant Current) will freewheel
through D2 and D4 to charge Cdc1 and Cdc2.