Isolated flyback converter with efficient light load operation

A flyback converter comprising:

• a transformer with a primary winding and a secondary winding, the primary winding being coupled to a power source;
• a first transistor coupled to the primary winding for conducting a current through the primary winding when the first transistor is on;
• a second transistor for conducting a current through the secondary winding when the second transistor is on;
• a regulator coupled to the first transistor for controlling a duty cycle of the first transistor to regulate the output voltage of the converter, the regulator being configured to control the first transistor to have a primary side regulated duty cycle for load currents above a threshold level and a primary side minimum duty cycle for load currents below the threshold level;
• an output voltage sensor circuit coupled to the transformer for sensing an output voltage of the converter using primary side sensing;
• an output capacitor coupled to an output terminal of the converter;
• a synchronous rectifier controller coupled to the second transistor for controlling the second transistor to be on or off;
• a comparator having one input coupled to receive a voltage corresponding to the output voltage of the converter and having another input connected to a reference voltage representing a threshold voltage exceeding a regulated voltage of the converter, wherein triggering of the comparator signifies an over-voltage condition;
• during operation of the regulator at the minimum duty cycle with the load currents below the threshold level, an output of the comparator being coupled so as to control the synchronous rectifier controller to turn the second transistor on for an interval to conduct a reverse current through the secondary winding, upon the over-voltage condition being detected, to reduce the output voltage of the converter to mitigate the over-voltage condition;
• and
• a diode coupled to the primary winding to conduct a current through the primary winding after the interval without turning on the first transistor, such that power is transferred from a secondary side of the transformer to the power source while mitigating the over-voltage condition.