In a half-bridge self-exciting switching power supply, an over-voltage detector receives the power supply output and generates an over-voltage signal when the power supply output exceeds a preset limit. An over-voltage cut-off circuit is connected to a pulse-width-modulated controller and a driving unit of the switching power supply. The over-voltage cut-off circuit applies a pull-down voltage to the driving unit so as to inhibit self-exciting operation of first and second self-excited switching circuits of a half-bridge self-excited circuit, and disables the pulse-width-modulated controller upon reception of the over-voltage signal from the over-voltage detector. Therefore, when the pulse-width-modulated controller or the driving unit becomes defective, the switching power supply can be cut-off automatically in order to prevent damage to a load and to the other components of the switching power supply.