Transformer

1. A transformer comprising a pair of first coils and at least one second coil that are stacked so that the at least one second coil is interposed between the first coils in a common winding axial direction of the first and second coils, wherein

each of the first coils is covered by insulating films and integrated with the insulating films into an integrated body, so that the first coils are electrically insulated from the at least one second coil, and

wherein

the integrated bodies, each of which is comprised of one of the first coils and the insulating films covering the one of the first coils, are integrally formed of a single coil sheet,

in the coil sheet, the integrated bodies are connected with each other via a connecting portion,

the coil sheet is folded at the connecting portion so that the integrated bodies are superposed in the winding axial direction with the at least one second coil interposed therebetween in the winding axial direction,

the connecting portion of the coil sheet includes therein a connecting electric conductor that connects the first coils included in the respective integrated bodies,

the connecting electric conductor is integrally formed with the first coils into one piece,

each of the first coils is comprised of a plurality of coil segments that are stacked in the winding axial direction,

between each adjacent pair of the coil segments, there is interposed an insulating film so as to electrically insulate the coil segments from each other, and

each of the coil segments is interposed between an adjacent pair of the insulating films in the winding axial direction.

2. The transformer as set forth in claim 1 , wherein the integrated bodies, each of which is comprised of one of the first coils and the insulating films covering the one of the first coils, are substantially annular-shaped,

the at least one second coil is also substantially annular-shaped, and

a radially inner periphery of the at least one second coil is positioned radially outside of radially inner peripheries of the insulating films of the integrated bodies, and a radially outer periphery of the at least one second coil is positioned radially inside of radially outer peripheries of the insulating films.

3. The transformer as set forth in claim 1 , wherein the at least one second coil is bonded by an adhesive to a corresponding one of the integrated bodies each of which is comprised of one of the first coils and the insulating films covering the one of the first coils.

4. The transformer as set forth in claim 1 , wherein in the coil sheet, there are provided two coil terminals that protrude respectively from the integrated bodies in a direction perpendicular to both the winding axial direction and an extending direction of the connecting portion of the coil sheet.

5. The transformer as set forth in claim 1 , further comprising a core that is comprised of a pair of core pieces, wherein

the integrated bodies, each of which is comprised of one of the first coils and the insulating films covering the one of the first coils, and the at least one second coil are together interposed between and thereby covered by the pair of core pieces in the winding axial direction.

6. The transformer as set forth in claim 5 , wherein the integrated bodies and the at least one second coil are each substantially annular-shaped,

each of the core pieces of the core has a center portion that extends in the winding axial direction,

the center portions of the core pieces are inserted in a space formed radially inside of the integrated bodies and the at least one second coil, and

the insulating films of the integrated bodies have extensions that extend in the winding axial direction along outer surfaces of the center portions of the core pieces, so as to be radially interposed between the outer surfaces of the center portions of the core pieces and a radially inner surface of the at least one second coil.

7. The transformer as set forth in claim 1 , wherein each of the first coils is comprised of a large-linewidth coil segment and a small-linewidth coil segment that are stacked in the winding axial direction,

the at least one second coil is directly thermally connected to a heat sink, and

the first coils and the at least one second coil are stacked so that the large-linewidth coil segments of the first coils face the at least one second coil.

8. The transformer as set forth in claim 1 , wherein between each adjacent pair of the coil segments, there is interposed only one insulating film.