Abstract: A switching mode power supply including a transformer, a power transistor, rectifiers, filters, a feedback portion, a power transistor controlling portion, and a synchronous signal controlling portion is provided. The transformer includes an input winding to one end of which an input voltage is applied and a feedback winding, one end of which is grounded, and to which energy is fed back from a secondary winding, in a primary winding, and at least one secondary winding. The power transistor has a first electrode and a second electrode connected between the other end of the input winding and a ground. The rectifiers rectify the voltage generated by the secondary windings to direct current. The filters filter the direct current voltage output from the rectifiers and output it as an output voltage. The feedback portion feeds back some of the output voltage to the power transistor controlling portion.