A high leakage inductance transformer core device, and method of forming same, that has a core made of tape wound material, at least one set of concentric primary and secondary windings, and at least one flux shunt between the primary and secondary windings which is also made of tape wound material. The transformer core and flux shunts are arranged so that the transformer has a low external magnetic field, and substantially no excess core losses due to principal core flux flowing from one part of the core structure to another through the broad surface of the core tape.