A control method for a power convert is disclosed. The power convert uses an active-clamp flyback topology and has low-side and high-side switches. The low-side switch is switched to generate consecutive switching cycles including a modified flyback cycle and a normal flyback cycle. Each of the consecutive switching cycles is not less than a blanking time generated in response to a load of the power converter. The high-side switch is constantly turned OFF during the normal flyback cycle. The high-side switch is turned ON after the blanking time during the modified flyback cycle to perform zero-voltage switching for the low-side switch.