Patent ID: 6738266
Filing Date: 2004-05-18
Classification: H02M,Y02B

Abstract:
A switching power supply unit comprising:a first switching circuit including a first switching element, a first diode, and a first capacitor, which are connected in parallel with each other; a second switching circuit including a second switching element, a second diode, and a second capacitor, which are connected in parallel with each other; a capacitor that is connected in series with the second switching element to define a series circuit, the series circuit being connected to one end of the first switching circuit; a transformer including a primary winding, a secondary winding that is provided with a rectifying/smoothing circuit, a first drive winding, and a second drive winding, wherein the first drive winding generates a voltage for bringing the first switching element into conduction and the second drive winding generates a voltage for driving the second switching element; a switching control circuit that controls an ON-time of the first switching element by alternately turning on and off the first and second switching elements with a period in which the first and second switching elements are both turned off therebetween, thereby controlling stabilization of an output voltage; and an input power supply; wherein the first switching circuit, the transformer, and the input power supply are connected in series with each other; the switching power supply unit performs self-excitation oscillation by storing energy in the primary winding of the transformer in a period in which the first switching element is turned on and by releasing energy from the secondary winding of the transformer in a period in which the first switching element is turned off; the switching control circuit includes a first switching control circuit which includes a first ON-time control circuit and which is connected between a control terminal of the first switching element and the first drive winding, and a second switching control circuit which includes a second ON-time control circuit and which is connected between a control terminal of the second switching element and the second drive winding; wherein, when the output voltage reaches a predetermined value at light load, the first ON-time control circuit prevents a voltage generated across the first drive winding from turning on the first switching element to thereby stop oscillation, and, at rated load or heavy load, the first ON-time control circuit allows a voltage generated across the first drive winding to turn on the first switching element, thereby controlling the On-time of the first switching element; the second ON-time control circuit controls an ON-time of the second switching element such that energy release from the secondary winding is completed after the end of the ON-time of the second switching element; and the switching power supply unit performs an intermittent oscillation operation, in which an oscillation period and non-oscillation period are alternately repeated, at light load.