Each switch applies the input voltage across the transformer and the voltage is then divided down by the turns ratio and applied to the secondary side. Since the magnitude of the voltage across the windings is the input voltage, when the switch is off the primary switching MOSFETs will see twice the input voltage as the voltage stress plus some amount of ringing. This means the MOSFETs chosen for a push-pull topology should have a voltage rating of about 2.5 to 3 times higher than the input voltage.