What capacitor and inductor are the best for a switching power supply? - Inductor-Features of Various Power Inductors

－Following up on the basics of inductors, please describe the types of different inductors known as power inductors.

Inductors come in a very wide variety of both types and structures. Among these, power inductors include wire-wound open magnetic circuit type, wire-wound closed magnetic circuit type, and multilayer type components. Their features are summarized in the table below. In this table, “x” should be taken to mean “cannot be called satisfactory”. Moreover, evaluations are relative to other component types. Among these, the open magnetic circuit type is not much used in power supplies due to its high DC losses Rdc, and so here we shall discuss only the closed magnetic circuit type and multilayer type inductors.

We explain two types of wire-wound closed magnetic circuit type inductors. One has a “drum sleeve construction”, and the other has a “sleeveless construction”. They are explained using the following diagram.

The drum sleeve construction has windings on a core and a sleeve added, and also has an air gap. The ferrite core material has a magnetic flux saturation point. The air gap causes magnetic flux to leak, raising the saturation point, and adjusts the DC bias characteristics.

The sleeveless construction has recently come into widespread use; again it has windings on a core, and the periphery is coated with a resin with ferrite powder or iron powder intermixed to create a closed magnetic circuit. However, for the same reasons as above, an air gap is necessary. In this case, the resin enclosing the magnetic body plays the role of the air gap.

－Why is it that the sleeveless design is popular?

Without addressing its popularity per se, the design has a number of advantages. As its name implies, no sleeve is used, and so the inductor can be made smaller for the same core size. Another important advantage is the gradual saturation characteristic compared with the drum sleeve type. As you probably know, ferrite has a steep saturation characteristic. When the saturation point is passed, the inductance decreases rapidly, and so in power supply design, this is another inductor-related matter that must be carefully considered. With a drum sleeve inductor, the saturation point due to the preset air gap is one point, and when this is exceeded, the saturation characteristic depends on the ferrite material. That is, the characteristic is basically steep. In the case of a sleeveless inductor, by contrast, there are various gap distances until the magnetic bodies (powder) intermixed in the resin, and so multiple saturation points coexist, and consequently saturation is more gradual. The graph on the right side of the diagram above shows this conceptually. This is a welcome characteristic in a power supply circuit.

－So the sleeveless design is superior by virtue of its compactness and more gradual saturation.

Those are certainly advantages, but the design also has drawbacks, or at least, other aspects requiring consideration. Put simply, there is no sleeve, and the windings are simply coated with a magnetic resin, so that there is the possibility of cracking due to external forces and stresses. To cite an example from TAIYO YUDEN’s own product line, we are changing over from the previous resin to a resin with higher hardness, to enhance the strength of these components.

Moreover, a resin and a core, two different materials, are combined, and so differences in linear expansion coefficients can cause cracking. This can be addressed by making the linear expansion coefficient of the resin as close to that of ferrite as possible.

Through such measures, the strength of these inductors has in fact been greatly increased, but the extent of the improvement will be different depending on the manufacturer.

－Well then, please explain the multilayer-type inductors.

As a matter of fact, in the past it was thought that power inductors with a multilayer structure were impossible. They would reach saturation as soon as a smallest current was passed, and there were also heat generation issues. It was also difficult to obtain a large inductance value.

In order to improve the DC bias characteristics, innovations in the inductor construction were introduced to control the magnetic flux path so as not to concentrate the flux, and the air gap structure was created. Where heat generation was concerned, the internal electrodes were made thicker to lower the resistance values, and various other improvements were added, culminating in a multilayer type inductor that could be used in power supply circuits.

Features of these inductors include enhanced miniaturization compared with winding-type constructions. Inductance values range up to several μH or so, making them suitable for switching power supplies with high oscillation frequencies in the megahertz band. One application example is the power supplies of mobile devices.