Patent ID: 6639811
Filing Date: 2003-10-28
Classification: H02M

Abstract:
A switching power supply unit operating in a current-continuation mode, comprising:a transformer including a primary winding, a secondary winding, a first drive winding, and a second drive winding; an input power supply supplying an input voltage to the primary winding; an inductor connected to the primary winding; a capacitor connected to the input power supply; a first switching circuit connected to the primary winding, the first switching circuit including a parallel circuit having a first switching element, a first diode, and a first capacitor; a second switching circuit connected to the first switching circuit, the second switching circuit including a parallel circuit having a second switching element, a second diode, and a second capacitor; a rectifying and smoothing circuit which includes a rectifying element and which is connected to the secondary winding; and first and second switching control circuits for alternately turning on/off the first and second switching elements, with a period when both the first and second switching elements are turned off therebetween, the first switching control circuit being connected between the first drive winding and a control terminal of the first switching element and the second switching control circuit being connected between the second drive winding and a control terminal of the second switching element; wherein energy is stored in the primary winding and the inductor during an ON-period of the first switching element and the energy is emitted from the secondary winding during an OFF-period of the first switching element such that the first and second switching elements perform self-excited oscillation; the inductor and the capacitor define a resonant circuit which resonates during the OFF-period of the first switching element; the first switching control circuit includes a first ON-time control circuit in which a time constant is set such that the first switching element is turned off at a desired time after the first switching element is turned on; and the second switching control circuit includes a second ON-time control circuit in which a time constant is set such that the second switching element is turned off in order to interrupt a resonant current applied to a series circuit including the second switching element and the inductor after the second switching element is turned on and before the energy is completely emitted from the secondary winding.