A control device for a multi-phase DC-DC converter is disclosed. The soft magnetic composite according to the present invention may include the insulating layer formed along the inter-particle boundary of the soft magnetic core metal particles, such that damage to a coating film caused by a molding of the existing soft magnetic powder having the insulation coating film formed therein may be prevented, whereby an eddy current loss may be minimized. The motor assembly has a motor mounted on the platform and a worm mounted to the motor.