High efficient isolated switching converter and control method thereof

An isolated switching converter, comprising:

• a transformer having a primary winding, a secondary winding and an auxiliary winding, wherein the primary winding is configured to receive an input voltage, the secondary winding is coupled to provide an output voltage to a load;
• a primary switch coupled to the primary winding;
• a secondary switch coupled to the secondary winding;
• a feedback circuit coupled to the auxiliary winding, wherein the feedback circuit is configured to generate a feedback signal indicative of the output voltage;
• a primary controller configured to generate a primary control signal to control the primary switch;
• and
• a secondary controller configured to generate a secondary control signal to control the secondary switch;
• wherein
• under normal operation, the primary controller controls the primary switch based on the feedback signal, the secondary controller controls the secondary switch based on the status of the primary switch wherein the secondary switch is turned on after the primary switch is off, and the secondary switch is turned off before the primary switch is on;
• and wherein
• under light load condition, the secondary controller controls the secondary switch based on the output voltage wherein the secondary switch is turned on to generate a negative secondary current flowing through the secondary winding and turned off when the negative secondary current reaches a secondary current threshold, the primary controller turns on the primary switch based on a negative primary current and turns off the primary switch when the primary current reaches a primary current threshold.