This topology is used at power levels up to 150W. Operation is at variable frequency and switching losses are reduced to produce good efficiency of 85% and above. The topology also works well down to light load levels.
Above 25W an external MOSFET is typically used, although some integrated solutions exist. Above 70W PFC is a requirement, but it may also be included at lower power levels such as in lighting circuits for example.
Synchronous rectification on the secondary side is used to improve efficiency where output currents are relatively high.
QUASI-RESONANT FLYBACK CONVERSION CONTROLLER
NCP1340/1: ON Semiconductor
BD7682/3/4/5FJ-LB: Rohm Semiconductor
SYNCHRONOUS RECTIFICATION CONTROLLER
TEA1999TK: NXP Semiconductors
NCP4306: ON Semiconductor