A switching power supply unit includes first and second switching elements which are alternately turned on/off so as to perform self-excited oscillation. An inductor and a capacitor resonate during an OFF-period of the first switching element. After the second switching element is turned on, a resonance current applied to a series circuit including the second switching element and the indictor is interrupted before energy is completely emitted from a secondary winding. Accordingly, a voltage is generated at the inductor and a voltage at a transformer is reversed. The switching power supply unit also includes an ON-time control circuit in which a time constant is set so as to turn off the second switching element accordingly.
A power semiconductor device includes a half bridge circuit, a control circuit and a noise removal circuit. The half bridge circuit includes a switching element on a low voltage side and a switching element on a high voltage side, those switching elements serially connected. The control circuit controls a switching operation of the switching element on a low voltage side in the half bridge circuit. The control circuit has a supply-voltage receiving terminal connected to a high voltage side of a power supply for driving the control circuit. The noise removal circuit including, for example, a zener diode, is interposed between the supply-voltage receiving terminal of the control circuit and the low voltage side terminal of the switching element on a low voltage side, to remove noise generated on a conductor connected to a low voltage side terminal of the switching element on a low voltage side.
A switching power supply apparatus includes a first inductor that is serially connected to a primary winding of a transformer, and a second inductor that is arranged so as to apply a voltage of a capacitor with sine waves obtained by rectifying an AC input voltage for an on-period of a first switching circuit. A diode for preventing the inverse current to the second inductor and a capacitor that is charged by excitation energy charged to the second inductor and applies a voltage to the primary winding for on-period of the first switching circuit. Further, a capacitor is arranged so that the inductor, the primary winding, and a second switching circuit define a closed loop. Switching control circuits control the on-period of a first switching element to control an output voltage Vo, and further control an input voltage Vi by controlling the on-period of a second switching element.
