Abstract: A switched-mode power supply including a primary rectifying circuit connected to an AC power supply, a transformer connected to the primary rectifying circuit, a secondary rectifying circuit connected to the secondary side of the transformer, a voltage fluctuation detector for sensing fluctuation in the output voltage, a signal generator for generating switching signals in response to the fluctuation detector, a switching tansistor for shunting the transformer to ground in response to the switching signals, a protective circuit responsive to the output voltage for stopping any abnormal operation of the switching signal generator, and an auxiliary power circuit for supplying power to the switching signal generator and to the protective circuit.