Patent ID: 7746672

Claim:
A switching power supply apparatus which comprises: a DC power supply; an isolation transformer having primary, secondary and tertiary windings; and a switching element in which, by turning on and off the switching element, a high-frequency voltage appearing in the secondary windings of the isolation transformer is rectified to obtain a DC output; a voltage-stabilizing mechanism connected to the DC output; and control mechanism which executes on/off control of the switching element using as a control voltage a voltage obtained by rectifying and smoothing a voltage appearing in the tertiary windings of the isolation transformer; wherein the control mechanism includes a first control mechanism, which compares magnitudes of a feedback signal which holds an output voltage constant and a carrier signal, and controls an on-time of the switching element, a second control mechanism which control a control power supply voltage, and an operation mode switching mechanism which selects either the first control mechanism or the second control mechanism according to a load state of the DC output; and wherein the second control mechanism is provided with at least one comparison mechanism which compares the control voltage with a first threshold value and a second threshold value, and when the control voltage falls below the first threshold value, a switching-enabled signal is output to enable on/off control of the switching element, but when the control voltage rises above the second threshold value, a switching-prohibited signal is output to turn off the switching element.