This disclosure is related to the technical field of magnets, and in particular to a method for applying a coupled inductor to a DC-DC converter providing a DC current output, and based on the number of phases of the DC-DC converter, the coupled inductor is designed to have a corresponding number of windings, the windings are reversely coupled to cancel out the magnetizing fields to avoid flux saturation of the magnet material under high current excitation, and the coupled inductor has air gaps, the leakage flux in the air gap induced by each winding is in the same direction, the leakage magnetic flux is used to achieve the filtering of the output current.