To eliminate unevenness in temperature distribution that occurs when a winding and a magnetic part are operated, for enhanced safety and reliability.

A winding 1 arranged around cores 2 and 2 is formed by winding a band-like copper foils C1-C4 from inside toward outside. The copper foils C1-C4 have different thickness respectively. The copper foil C1 constituting the innermost turn has the largest cross section area, getting smaller gradually from the inside toward the outside. The copper foil C4 constituting the outermost turn has the smallest one. Since the magnetic part degrades in cooling efficiency around the center, compared to the outside portion exposed to the outside air, copper loss is suppressed by making the cross section area of the conductor around the center larger, for suppressed rising of temperature. Meanwhile the number of turns that is required is assured by making the cross section area of the conductor near the outside smaller, eliminating unevenness in temperature.

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