Switching power supply device

A switching power supply device comprising:

• a first switching element to which an input voltage is supplied;
• an output terminal detachable from and reattachable to an input terminal of a load device;
• an input/output conversion circuit that converts the input voltage switched by the first switching element into an output voltage to output the output voltage to the output terminal;
• a switching control circuit that controls switching operation of the first switching element, and has an OFF mode for stopping the switching operation in response to a signal input to an OFF terminal of the switching control circuit;
• a first rectifier circuit connected between the input/output conversion circuit and the output terminal;
• a second rectifier circuit connected to the output terminal;
• a flyback transformer having a secondary winding wire whose dot side terminal is connected to the second rectifier circuit;
• a second switching element connected to a dot side terminal of a primary winding wire of the flyback transformer;
• a pulse generator that supplies a pulse signal to a gate terminal of the second switching element;
• a waveform detection circuit that is connected to a first connecting point being a connecting point between the dot side terminal of the primary winding wire of the flyback transformer and the second switching element, and detects a voltage or a current at the first connecting point to output a detection signal to the OFF terminal;
• and
• a capacitive element that is connected to a connecting point between the output terminal and the second rectifier circuit, and stores energy output from the secondary winding wire of the flyback transformer by a single switching of the second switching element,
• wherein a capacitance value of the capacitive element is set such that the waveform detection circuit is capable of detecting a voltage value difference or a current value difference at the first connecting point by detachment and reattachment of the input terminal of the load device from and to the output terminal, the input terminal being connected to a discharge circuit.