A system comprising: a DC-DC converter (unit 1, unit 2, C1, C2, Q5, Q6),and a pre-charging circuit, wherein the DC-DC converter comprises: a security device (Q5, Q6), and wherein the pre-charging circuit comprises: an auxiliary power supply generating means, which is connectable to a first power supply (P14V), and configured to generate an auxiliary power supply; a constant current source circuit configured to supply a charging current based on the auxiliary power supply; and a switch part connected between the auxiliary power supply generating means and the constant current source circuit, wherein an output terminal of the constant current source circuit is connected to the security device of the DC-DC converter, and wherein the security device comprises two common-source security MOSFETs, and the output terminal of the constant current source circuit is connected to a middle point of the two security MOSFETs. a DC-DC converter (unit 1, unit 2, C1, C2, Q5, Q6),and a pre-charging circuit, wherein the DC-DC converter comprises: a security device (Q5, Q6), and wherein the pre-charging circuit comprises: an auxiliary power supply generating means, which is connectable to a first power supply (P14V), and configured to generate an auxiliary power supply; a constant current source circuit configured to supply a charging current based on the auxiliary power supply; and a switch part connected between the auxiliary power supply generating means and the constant current source circuit, wherein an output terminal of the constant current source circuit is connected to the security device of the DC-DC converter, and wherein the security device comprises two common-source security MOSFETs, and the output terminal of the constant current source circuit is connected to a middle point of the two security MOSFETs. a security device (Q5, Q6), and wherein the pre-charging circuit comprises: an auxiliary power supply generating means, which is connectable to a first power supply (P14V), and configured to generate an auxiliary power supply; a constant current source circuit configured to supply a charging current based on the auxiliary power supply; and a switch part connected between the auxiliary power supply generating means and the constant current source circuit, wherein an output terminal of the constant current source circuit is connected to the security device of the DC-DC converter, and wherein the security device comprises two common-source security MOSFETs, and the output terminal of the constant current source circuit is connected to a middle point of the two security MOSFETs. an auxiliary power supply generating means, which is connectable to a first power supply (P14V), and configured to generate an auxiliary power supply; a constant current source circuit configured to supply a charging current based on the auxiliary power supply; and a switch part connected between the auxiliary power supply generating means and the constant current source circuit, wherein an output terminal of the constant current source circuit is connected to the security device of the DC-DC converter, and wherein the security device comprises two common-source security MOSFETs, and the output terminal of the constant current source circuit is connected to a middle point of the two security MOSFETs.