Forced zero voltage switching flyback converter

A flyback converter, comprising:

• a transformer having a primary winding receiving an input voltage and a secondary winding providing an output voltage, the primary winding having a first terminal coupled to receive the input voltage and a second terminal;
• a primary switch having a first terminal coupled to the second terminal of the primary winding, a second terminal coupled to a power supply voltage and a control terminal;
• a synchronous rectifier coupled to the secondary winding;
• an output capacitor coupled across the secondary winding;
• a controller comprising a primary side controller and a secondary side controller coupled to generate control signals to drive the primary switch and the synchronous rectifier respectively;
• a first capacitor coupled in parallel with across the primary switch, the first capacitor being coupled between the first terminal and the second terminal of the primary switch;
• and a second capacitor coupled in parallel with the primary winding of the transformer, the second capacitor being coupled between the first terminal and the second terminal of the primary winding, wherein the controller generates a control signal to turn the primary switch and the synchronous rectifier on and off alternately within a switching cycle, and wherein the primary switch is turned off at a zero voltage across the primary switch in response to the first capacitor being coupled across the primary switch and in response to the second capacitor being coupled across the primary winding, and wherein a positive current detect circuit is configured to sense a secondary current flowing in the secondary winding and to generate a detection signal in response to detecting the secondary current having a non-zero positive current value, wherein the controller generates the control signal to turn on the synchronous rectifier for a second on-duration within the switching cycle in response to detecting the secondary current reaching a zero current value, the positive current detect circuit senses the secondary current during the second on-duration and generate the detection signal in response to detecting the secondary current having the non-zero positive current value, and in response to the detection signal, the controller generates the control signal to turn off the synchronous rectifier within the switching cycle.