Abstract: The invention relates to a mid-frequency high power transformer which is specially used as a welding transformer in resistance welding, comprising a primary winding penetrated by the limb of a transformer core and a secondary winding which are configured as concentrically overlapping cylindrical windings. The windings of the primary windings are formed by a plurality of spirally wound layers of a relatively thin electrically conductive metal strip material, and the windings of the secondary winding are formed by an appropriate number of spirally wound layers of a thicker electrically conductive metal strip material chosen according to the desired reduction ratio between the primary and the secondary windings.