This paper discusses coil structures made from a folded flat sheet which is used for an inductor in high frequency operated battery chargers for future electrified vehicles. For AC loss reduction, a Litz wire is known to be a solution, however, since Litz wires are based on a cable type structure, planar type designs are desired for miniaturization and efficient fabrication of inductors. There exists a paper proposing planar type Litz windings using a printed circuit board for planar inductors. Our unit cell FEM analysis indicates that the stacking effect of multiple winding layers, i.e. the proximity effect between layers, was significant to its overall resistance and therefore the Litz wire pitch has to be much finer than the single board case. Thus, an effective Litz wire design, within our design requirements, requires a very fine strand width that goes beyond the fabrication capability of the vias, i.e. metalized through holes to connect different layers. In this paper, we propose novel planar Litz wire structure without vias by introducing a PCB folding technique by implementing a development chart for a coil structure with twisted strand bundle on one single sheet.

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