A power supply circuit for energy transfer between a battery of a battery pack and a smoothing capacitor, the battery pack further comprising a first main contactor connected between a negative terminal of the battery and one end of the smoothing capacitor, a second main contactor connected between a positive terminal of the battery and other end of the smoothing capacitor, the power supply circuit comprising: a transformer including a first winding and a second winding magnetically coupled to the first winding; a first switching circuit including a first switch connected in series to the first winding, wherein the first switching circuit is connected in parallel to the battery together with the first winding; a second switching circuit including a second switch connected in series to the second winding, wherein the second switching circuit is connected in parallel to the smoothing capacitor together with the second winding; and a switch controller configured to independently control each switch (S1, S2) included in the first switching circuit and the second switching circuit wherein the switch controller is configured: to perform, in a first control mode for pre-charging the smoothing capacitor using energy of the battery, a first operation of turning on the first switch and turning off the second switch, and subsequently perform a second operation of turning off the first switch and turning on the second switch; and to perform, in a second control mode for charging the battery using energy of the smoothing capacitor, a third operation of turning off the first switch and turning on the second switch, and subsequently perform a fourth operation of turning on the first switch and turning off the second switch; and wherein the transformer further includes a third winding magnetically coupled to the second winding, wherein the third winding is connected in series to the first winding, and the first switching circuit further includes: a third switch connected in series to the third winding; and a fourth switch connected in parallel to the third winding and the third switch. a transformer including a first winding and a second winding magnetically coupled to the first winding; a first switching circuit including a first switch connected in series to the first winding, wherein the first switching circuit is connected in parallel to the battery together with the first winding; a second switching circuit including a second switch connected in series to the second winding, wherein the second switching circuit is connected in parallel to the smoothing capacitor together with the second winding; and a switch controller configured to independently control each switch (S1, S2) included in the first switching circuit and the second switching circuit wherein the switch controller is configured: to perform, in a first control mode for pre-charging the smoothing capacitor using energy of the battery, a first operation of turning on the first switch and turning off the second switch, and subsequently perform a second operation of turning off the first switch and turning on the second switch; and to perform, in a second control mode for charging the battery using energy of the smoothing capacitor, a third operation of turning off the first switch and turning on the second switch, and subsequently perform a fourth operation of turning on the first switch and turning off the second switch; and wherein the transformer further includes a third winding magnetically coupled to the second winding, wherein the third winding is connected in series to the first winding, and the first switching circuit further includes: a third switch connected in series to the third winding; and a fourth switch connected in parallel to the third winding and the third switch. to perform, in a first control mode for pre-charging the smoothing capacitor using energy of the battery, a first operation of turning on the first switch and turning off the second switch, and subsequently perform a second operation of turning off the first switch and turning on the second switch; and to perform, in a second control mode for charging the battery using energy of the smoothing capacitor, a third operation of turning off the first switch and turning on the second switch, and subsequently perform a fourth operation of turning on the first switch and turning off the second switch; and wherein the transformer further includes a third winding magnetically coupled to the second winding, wherein the third winding is connected in series to the first winding, and the first switching circuit further includes: a third switch connected in series to the third winding; and a fourth switch connected in parallel to the third winding and the third switch. a third switch connected in series to the third winding; and a fourth switch connected in parallel to the third winding and the third switch.