PSFB DC-DC converters make use of parasitic elements in the circuit to ensure zero voltage across the MOSFET switches before turning them ON, providing soft switching. This considerably reduces the amount of switching losses associated with hard switching.

For the system discussed here, switching transitions for switches in the Q2- Q3 leg end the power transfer interval. Therefore this leg is called the Active to Passive leg. When transitions occur for switches in this leg, current in the primary winding is close to its maximum magnitude for that half PWM switching cycle. The reflected load current aids the circulating energy in the primary circuit during this time, which makes it possible for voltage across switches in this leg to approach zero volts. It is possible to achieve ZVS for switches in this Q2-Q3 leg across the complete load range. It should be noted that as the load decreases the amount of dead-time needs to be increased to achieve/approach ZVS.

Switching transitions for switches in the Q1- Q4 leg start the power transfer interval. Therefore this leg is called the Passive to Active leg. During these switching transitions, primary current decreases. It crosses zero current value and changes direction. This results in lower available energy for ZVS. In fact for operations under low load conditions, voltage across these switches may not go to zero before turning them on. Switching losses can be kept to a minimum by turning these switches ON at a time when the voltage across them is at a minimum. This is called Low Voltage Switching or low voltage switching (LVS). As the load changes the time at which the switch should be turned on to achieve LVS changes, requiring dead-time adjustment similar to the Q2-Q3 leg switches.