Planar transformer and DC-DC converter

A planar transformer comprising:

• a primary side planar air core coil;
• a secondary side planar air core coil arranged so as to be spaced from the primary side planar air core coil in an winding center axis direction of the primary side planar air core coil, the secondary side planar air core coil having a non-facing portion configured not to face the primary side planar air core coil in the winding center axis direction;
• and
• a primary side planar core and a secondary side planar core stacked on outer sides of the primary side planar air core coil and the secondary side planar air core coil, respectively, in the winding center axis direction, wherein
• the primary side planar air core coil and secondary side planar air core coil respectively have facing portions at which the primary side planar air core coil and secondary side planar air core coil face with each other in the winding center axis direction,
• a position of a winding center axis of one of the primary side planar air core coil and the secondary side planar air core coil is arranged at outer side of an air core of another of the primary side planar air core coil and the secondary side planar air core coil, and
• the primary side planar core and the secondary side planar core are arranged so as not to be in contact with each other by being spaced from each other.