Abstract: A power supply device of a switching mode comprises a transformer, a direct current power supply circuit connected at an output end thereof with a first end of a primary winding of the transformer, a switching element connected at a first end thereof with a second end of the primary winding of the transformer and at a second end thereof with a ground, a control circuit connected at an output end thereof with the control end of the switching element for providing the switching element with a driving signal, a direct current output circuit connected at an input end thereof with the output end of a secondary winding of said transformer, a leakage current detection circuit connected at an input end thereof with a secondary side ground end of the transformer and at an output end thereof with a primary side ground end of the transformer for forming a leakage current loop and transmitting a leakage current signal at such time when a leakage current takes place.