The embodiment of the present invention discloses a reverse excitation circuit used for reducing the switching loss of a MOS transistor in a DC auxiliary source circuit. The reverse excitation circuit in the embodiment of the present invention includes a DC auxiliary source circuit and an AC auxiliary source circuit. The DC auxiliary source circuit includes a first power source inlet, a first primary-side winding, a first capacitor, a first MOS transistor, a first diode, and a first targeted resistor. The first capacitor and the first MOS transistor are connected in parallel and are then connected with the first power source inlet. The first MOS transistor, the first primary-side winding and the first diode are connected in series and are then connected with the first power source inlet. The first targeted resistor is connected in parallel with the first diode, then connected with the MOS transistor in series and then connected to the first power source inlet; the AC auxiliary source circuit comprises a second power source inlet, a second primary-side winding, a second capacitor, a second MOS transistor and a second diode, wherein the second capacitor is connected in parallel with the second MOS transistor and are then connected to the second power source inlet; and the second MOS transistor, the second primary-side winding, and the second diode are sequentially connected in series and are then connected with the second power source inlet.