Patent ID: 8582320
Filing Date: 2013-11-12
Classification: H02M

Abstract:
1. A self-excited switching power supply circuit comprising: a transformer with a primary winding, a secondary output winding, a first feedback winding, and a second feedback winding; an oscillation field effect transistor connected to a DC power supply while being connected in series to the primary winding, the oscillation field effect transistor performing ON operation when a gate voltage thereof is equal to or higher than a threshold voltage V a start-up resistor connected between a high-voltage side terminal of the DC power supply and a gate of the oscillation field effect transistor; an ON control capacitor connected between one side of the first feedback winding and the gate of the oscillation field effect transistor, an opposite side of the first feedback winding being connected to a low-voltage side terminal of the DC power supply, the ON control capacitor applying a voltage for controlling ON of the oscillation field effect transistor to the gate of the oscillation field effect transistor; a driver element connected between the gate of the oscillation field effect transistor and the low-voltage side terminal of the DC power supply, the driver element forming continuity between the gate of the oscillation field effect transistor and the low-voltage side terminal if a control terminal voltage is equal to or higher than an operating voltage, thereby controlling OFF of the oscillation field effect transistor; a shunt resistor connected between the oscillation field effect transistor and the low-voltage side terminal; and an OFF control capacitor connected between a control terminal of the driver element and the low-voltage side terminal, the OFF control capacitor being charged with a voltage of the shunt resistor in which an exciting current from the primary winding flows during ON operation period of the oscillation field effect transistor, thereby increasing the control terminal voltage to the operating voltage or higher, wherein the oscillation field effect transistor is turned off with a charging voltage of the OFF control capacitor during the an ON operation period of the oscillation field effect transistor, and is turned on with the charging voltage of the ON control capacitor during OFF operation period of the oscillation field effect transistor, thereby making the ON operation period and the OFF operation period appear repeatedly with a predetermined oscillation cycle, and wherein the self-excited switching power supply circuit further comprises: a cycle control capacitor connected between the control terminal of the driver element and the low-voltage side terminal; a backflow preventing element connected between the second feedback winding and the cycle control capacitor, the backflow preventing element causing a charging current to flow during the OFF operation period of the oscillation field effect transistor to charge the cycle control capacitor with a flyback voltage generated in the second feedback winding; a time constant resistor connected between the second feedback winding and the low-voltage side terminal of the DC power supply while being connected in series to the cycle control capacitor, the time constant resistor controlling a speed at which the cycle control capacitor is charged; and a shorting circuit connected in parallel to the cycle control capacitor, the shorting circuit forming a short circuit across the cycle control capacitor with a periodic cycle sufficiently longer than the oscillation cycle each time the charging voltage of the cycle control capacitor reaches a predetermined voltage, and the OFF control capacitor is charged with the charging voltage of the cycle control capacitor as well as with the voltage of the shunt resistor to shorten the ON operation period until when the driver element turns the oscillation field effect transistor off.