| CPC H01F 27/29 (2013.01) [H01F 27/2885 (2013.01)] | 13 Claims |

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1. A planar winding structure, comprising:
an insulating planar board having a winding portion and a terminal portion, with a through hole at a central portion of the winding portion;
a plurality of conductive layers embedded in the winding portion of the insulating planar board and electrically connected with each other through one or more buried vias, the conductive layers being patterned to constitute a transformer winding around the through hole;
first and second terminals at the terminal portion of the insulating planar board, each being electrically connected to a respective one of the conductive layers;
a shielding layer coated on outer surfaces of the winding portion of the insulating planar board; and
a grading ring structure embedded in the terminal portion of the insulating planar board, wherein the grading ring structure and the shielding layer share a same potential.
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