A self-oscillation type switching power supply apparatus comprises a transformer T having a primary winding N1, a secondary winding N2, and a feedback winding N.sub.B, a switching transistor Q1 to interrupt current in the primary winding supplied with an input power supply voltage, a feedback signal being provided from the feedback winding N.sub.B and a rectifying smoothing circuit connected to the secondary winding. The self-oscillation type switching power supply apparatus further comprises a controlling transistor Q3 to control a positive feedback signal for the switching transistor Q1 from the feedback winding N.sub.B, and a detector detecting a flyback voltage produced in the feedback winding N.sub.B when the switching transistor Q1 is turned off, and changing-over the state of the controlling transistor Q3 to keep the switching transistor Q1 in the off-state when the flyback voltage goes lower than a predetermined value.
This switching circuit includes a transformer having primary and secondary windings insulated from each other; a voltage-driven switching element which is connected in series to one of the primary and secondary windings and has a control terminal for controlling a switching operation thereof; a drive circuit which has an output connected to the control terminal and drives the main switching element; and an auxiliary winding which is provided in parallel to the primary winding of the transformer and has an output connected to the control terminal via a capacitor with positive feedback.
A self-excited switching power supply circuit is provided which reduces a discharge current generated when an oscillating field effect transistor (3) is turned on, whereby it is possible to reduce energy loss and generation of noise when switching is executed. A time constant of an ON-control circuit (12, 23) is set such that when a polarity of a voltage of a feedback winding (2b) has reversed, a gate voltage of the oscillating field effect transistor (3) exceeds a threshold voltage V.sub.TH. Accordingly, after a voltage of a primary winding (2a) has become equal to or less than a power supply voltage, the oscillating field effect transistor (3) is turned on, and electrical charge stored in stray capacitance between the windings of the primary winding (2a) and in parasitic capacitance of the oscillating field effect transistor (3) is discharged gradually.
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.
An example synchronous rectifier circuit includes a transformer having a primary winding and a secondary winding which are insulated from each other; a main switch provided at a primary side of the circuit, which includes the primary winding; a rectifying FET provided at a secondary side of the circuit, which includes the secondary winding; a commutating FET provided at the secondary side of the circuit; and an auxiliary FET provided between a gate and a source of the rectifying FET. The auxiliary FET has a threshold lower than a threshold of the commutating FET and a gate of the auxiliary FET is connected to a gate of the commutating FET, and a short circuit is produced between the gate and the source of the rectifying FET while the commutating FET is in an on-state.
