Disclosed herein is a variable-frequency flyback. The flyback converter comprises a flyback transformer having secondary windings for drive and detection for providing first and second induced voltages induced from an input voltage in first and second turn ratios with a primary winding, respectively, a voltage detector for detecting the second induced voltage, a switching controller for determining, on the basis of a level of the voltage detected by the voltage detector, whether a current load state is the mini load state and outputting a first switching signal according to a result of the determination, a first switch for switching the supply of the second induced voltage in response to the second induced voltage and the first switching signal, and a synchronous rectifier for performing a switching operation depending on a state of the first switch, the synchronous rectifier having its source and drain connected to any one of a voltage output line or ground line connected to the drive secondary winding and its gate connected to the first switch.
Disclosed herein is a flyback converter with a synchronous rectifier which is applied to a power supply of a portable computer such as a notebook PC. The flyback converter is operated in a critical conduction mode to turn on/off a main switch at a zero crossing point of an output voltage. The flyback converter is also adapted to supply a driving voltage to a synchronous switch using the output voltage. Therefore, there is no need for a secondary auxiliary coil of a transformer and for a Schottky diode to be connected in parallel with the synchronous switch, resulting in simplification in circuit design.
