There is provided a synchronous rectifier circuit that makes it possible to secure sufficient driving voltage for the rectifier switch regardless of the voltage of the secondary winding. In this synchronous rectifier circuit, the primary winding is insulated from the secondary winding and a rectifier switch is provided on the secondary side. An auxiliary switch, a diode, and an auxiliary winding are also provided on the secondary side. The emitter of the auxiliary switch and the cathode of the diode are connected to the gate of the rectifier switch. One end of the auxiliary winding is connected to the base of the auxiliary switch and the anode of the diode. An end of the secondary winding is collected to the collector of the auxiliary switch. The other end of the auxiliary winding is connected to this one end of the secondary winding.
A commutation switch turning-on control switching element is controlled with a voltage generated at a fourth coil (auxiliary coil) of a main transformer and controls application of a control voltage to a control terminal of a commutation switching element. A commutation switch turning-off control switching element is provided which is connected to the control terminal of the commutation switching element and, when the commutation switch turning-off control switching element is turned on, controls a voltage at the control terminal of the commutation switching element to turn off the commutation switching element. A control switching element drive circuit turns on the commutation switch turning-off control switching element at a time when a primary switching element is turned on.
A first drive control signal regenerating circuit outputs an ON timing drive signal at turn-ON of a main switch element, and a second drive control signal regenerating circuit generates an OFF timing drive signal at turn-OFF of the main switch element. A rectifying switch controlling switch element connected between the gate and source of a rectifying switch element is driven by an output of the second drive control signal regenerating circuit. An output of the first drive control signal regenerating circuit connects to the gate of a commutating switch controlling switch element, which connects to one end of an auxiliary winding, the other end thereof being connected to the gate of a commutating switch element. Accordingly, the rectifying switch element is directly controlled from the primary side.
In one embodiment, an apparatus includes a transformer having a primary winding and a secondary winding and a switching circuit to facilitate a periodic application of an input voltage to the primary winding. The switching circuit includes an active switch and a capacitor commonly coupled to the primary winding. In one embodiment, the apparatus further includes a synchronous rectification circuit coupled to the secondary winding to generate a rectified signal, an output filter coupled to the synchronous rectification circuit to provide an output voltage in response to the rectified signal, and a control circuit coupled to the output filter to receive the output voltage and coupled in a switching control relationship with the active switch and the synchronous rectification circuit.
