Asymmetric half-bridge power supplies and control methods thereof

A control method of an asymmetric half-bridge power supply comprising:

• asymmetrically controlling a first ON time of a first switch and a second ON time of a second switch to convert an input voltage into an output voltage via a transformer, the transformer comprising a primary winding, a first auxiliary winding and a secondary winding, wherein the first switch and the second switch control a resonance circuit, the resonance circuit comprises the transformer and a resonance capacitor, and the secondary winding is coupled to a synchronous rectifier switch;
• providing a switching cycle time limit according to the output voltage;
• turning on the synchronous rectifier switch for an assisting duration after the switching cycle time limit is reached;
• turning on the first switch to start a switching cycle after turning on the synchronous rectifier switch for the assisting duration;
• detecting a winding voltage of the first auxiliary winding, wherein during the second ON time, the winding voltage of the first auxiliary winding is a substantially constant value;
• and
• turning on the first switch when the winding voltage deviates from the substantially constant value and a declining slope of the winding voltage exceeds a preset threshold.

An asymmetrical half-bridge power supply for converting an input voltage on a primary side to an output voltage on a secondary side, the asymmetrical half-bridge power supply comprising:

• a resonance circuit comprising:
• a transformer comprising a primary winding, a first auxiliary winding and an assisting winding, and
• a resonance capacitor coupled to the primary winding;
• a first switch and a second switch arranged on the primary side coupled in series between an input power line and an input ground line to control a magnetization current flowing through the resonance circuit;
• an assisting switch coupled to the assisting winding;
• and
• a power controller configured to first turn on the first switch for a first ON time and then turn on the second switch for a second ON time in a switching cycle, wherein the power controller turns on the first switch for the first ON time after a declining slope of a winding voltage of the first auxiliary winding exceeds a preset threshold;
• wherein, after a discharge duration of the transformer is completed, the assisting switch is turned on for an assisting duration to enable one of the first switch and the second switch to achieve zero-voltage switching in a next switching cycle.