Mode4 (t3–t4): Prior to the turn-off of S1, S3 of the
auxiliary switch Q3 is turned on at t3. Subsequently,
quasi resonance due to Lr1 and Cr1 occurs and IQ1
commutates to the anti parallel diode D1. While D1
conducting, S1 is turned-off at t4, so Zero Voltage and
Zero Current Switching (ZVZCS) turn-off commutation
of S1 is completed.