Patent ID: 8791786

Claim:
A coil device comprising; a first bobbin provided with a first hollow cylinder on which a primary coil is wound at the outer periphery, and a second bobbin mounted on said first bobbin and provided with a second hollow cylinder on which a secondary coil is wound at the outer periphery, wherein; a winding center of said primary coil and a winding center of said secondary coil displace with a predetermined displacement along a predetermined reference direction; said first bobbin and said second bobbin are assembled so that a first through hole of said first hollow cylinder is connected with a second through hole of said second hollow cylinder, a middle leg of a ferrite core is inserted into these first through hole and second through hole, and a proportion of said predetermined displacement with respect to a predetermined reference length of the middle leg of said ferrite core along said predetermined reference direction is 0.05 to 0.30.