A switched mode power converter includes a transformer having a primary winding, a secondary winding, and a bias winding. A primary side power switch is coupled to the primary winding and is adapted to periodically apply an input voltage to the primary winding. An output filter is operatively coupled to the secondary winding to provide an output voltage and output current. A forward synchronous rectification device is operatively coupled in series between the secondary winding and the output filter. A free-wheeling synchronous rectification device is operatively coupled in shunt with the secondary winding and the output filter. A control circuit is coupled to the bias winding and the forward and free-wheeling synchronous rectification devices. The control circuit includes a forward driving device adapted to couple an activation terminal of the forward synchronous rectification device to a first end of the secondary winding and a free-wheeling driving device adapted to couple an activation terminal of the free-wheeling synchronous rectification device to a second end of the secondary winding. The control circuit activates the forward and free-wheeling driving devices responsive to voltage on the bias winding.

A switching power supply which can prevent the output voltage Vout from fluctuating or undershooting when the instruction for stopping the operation is issued is disclosed. A switching power supply according to the present invention employs a transformer having a primary coil and a secondary coil, a switching circuit connected between an input terminal and the primary coil of the transformer, a rectifier connected to the secondary coil of the transformer, a smoothing circuit located at a subsequent stage of the rectifier and including an output capacitor, a control circuit controlling the switching circuit, and an operating voltage generating circuit, responsive to an instruction for stopping a switching operation, for supplying an operating voltage to the control circuit using at least energy stored in the output capacitor. Thus, the switching power supply of the present invention can substantially linearly lower the output voltage Vout.

A DC power source voltage is supplied to a center tap of a primary winding, and first and second semiconductor switches alternately turned on are disposed between each of both ends of the primary winding and a common potential point, and a current flowing through a load is fed back and PWM control of each of the semiconductor switches is performed. Also, snubber circuits are respectively connected between a ground and the center tap of the primary winding, and an abnormal high voltage at the time of switching is reduced. Also, a parallel running of plural inverters is simply performed by disposing PWM comparators corresponding to the first and second semiconductor switches.

First and second semiconductor switches which are activated alternately are provided between ends of a primary winding and a common potential point, wherein a DC power supply voltage is supplied to a center tap. An electric current flowing into a load is fed back to thereby subject the semiconductor switches to PWM control. Series circuits consisting of capacitors and semiconductor switches are connected between the center tap of the primary winding and the ends of the same. The semiconductor switches are activated in synchronism with the first and second semiconductor switches, thereby preventing occurrence of an anomalous high voltage, which would otherwise be caused at the time of switching operation.