Inductors, typical examples of which include choke coils, are used in energy conversion circuits, such as switching power sources and DC/DC converters. As a structural example of an inductor, there is known an inductor which includes a dust core obtained by firing a soft magnetic powder compact and a coil obtained by winding a winding wire around the dust core.
The dust core is, for example, fabricated as follows (e.g., refer to PTL 1 or the like). First, a soft magnetic powder, which is an aggregate of composite magnetic particles, each including a soft magnetic particle and an insulating coating film provided on the surface thereof, is prepared. Then, the soft magnetic powder is compacted into a predetermined shape, and the resulting compact is heat-treated to produce a dust core. It is described that, in a dust core obtained by such a method, insulation between the soft magnetic particles is secured by insulating coating films of silicon oxide, and even when a large direct current is superimposed, the inductance is not decreased excessively.
Properties required for a dust core include a reduction in an energy loss referred to as “iron loss”. Iron loss is roughly represented by a sum of eddy current loss and hysteresis loss, and in particular, becomes noticeable in high-frequency use. The eddy current loss in the iron loss can be reduced by ensuring that there is insulation between the soft magnetic particles. On the other hand, the hysteresis loss can be reduced by adjusting the composition of the soft magnetic particles. For example, Fe—Si—Al alloys, i.e., sendust alloys, can reduce the hysteresis loss of dust cores and also can improve the relative magnetic permeability of dust cores, and therefore, they are suitably used as soft magnetic particles.
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