Power converter and controlling method thereof

A controlling method of a power converter, comprising:

• performing a first switch controlling step to control a primary switch of the power converter by a first controlling signal;
• and
• performing a second switch controlling step to control an active clamp switch of the power converter by a second controlling signal;
• wherein a number of first times the primary switch is turned on by the first controlling signal is greater than a number of second times the active clamp switch is turned on by the second controlling signal in each of a plurality of durations, and each of the durations comprises a plurality of power converter switching cycles;
• wherein in the second switch controlling step, the active clamp switch is turned on after a part of the power converter switching cycles, and the part of the power converter switching cycles comprises at least one of the power converter switching cycles.

A power converter, comprising:

• a primary switch controlled by a first controlling signal;
• and
• an active clamp switch connected to the primary switch and controlled by a second controlling signal;
• wherein the primary switch is turned on by the first controlling signal at each of a plurality of power converter switching cycles, the active clamp switch is not turned on by the second controlling signal at a part of the power converter switching cycles, and the active clamp switch is turned on by the second controlling signal at another part of the power converter switching cycles;
• wherein the active clamp switch is turned on after the part of the power converter switching cycles, and the part of the power converter switching cycles comprises at least one of the power converter switching cycles.