The invention provides a low-input-voltage power supply which comprises a first transformer winding N1, a third transformer winding N3 connected with the first transformer winding N1 in series, a second transformer winding N2, a fourth transformer winding N4 connected with the second transformer winding N2 in series, a first switch S1, a second switch S2, a first capacitor C1, a second capacitor C2, a first diode VD1, a second diode VD2, and inductor L. According to the low-input-voltage power supply, a switch power supply circuit has bidirectional excitation and is smaller in conduction losses and easy to drive.