The gaps in power inductors can be either discrete or distributed. Ferrites are at the low end of the available range for B sat, and they shift down in B sat significantly as temperature rises. Ferrite’s main advantage for inductor cores is low loss at high frequencies because it has a high resistivity compared with metal alloys. Commercially useful magnetic materials have a B sat that ranges from about 0.3 to 1.8 T. Thus the lower the value of μ, the greater the value of H (or current) that the core supports when B is below the maximum value of flux density ( B sat) inherent to the magnetic material. Recall from basic electromagnetism that μ can be expressed as the flux density, B, divided by the magnetic field, H. Permeability, μ, is a measure of how much magnetization a material receives in an applied magnetic field. Specifically, the air gap reduces and controls the effective permeability of the magnetic structure. The gap is used to boost the flux level at which the core saturates under load.
The magnetic cores used in power inductors frequently have an air gap within their structure. Mind the gap A typical magnetization curve for a soft magnet with key parameters labeled: M s, or the saturation magnetization M r, the magnetization remaining after an external field is removed H c, the value of the magnetic field necessary to remove magnetization after the magnetic material has saturated and Χ i, the initial magnetic susceptibility. Tapes of silicon-steel alloy don’t have this problem. Also, nickel-iron alloys can be brittle, so tape-wound core toroids wound with this material can be sensitive to shock and vibration. On the other hand, ferrites have lower core losses and cost less per unit weight. The thinner the tape material, the higher the usable frequency.Ī benefit of tape-wound cores is that they saturate at higher levels than ferrite cores so they can be physically smaller at high power levels. The maximum usable frequency is usually lower than for ferrites because their resistivity is lower, resulting in high eddy currents and higher core losses. Tape-wound devices can be useful up to 10 to 20 kHz depending their material. The strips can be as thin as 0.000125 in and may be comprised of silicon steel, nickel-iron, cobalt-iron, and amorphous metal alloys. Both powder cores and ferrites are commonly available shaped as toroids, but tape-wound (also called strip-wound or cut wound) cores can be used as toroidal transformers as well. The geometry often used for power inductors is the toroid because its shape maximally constrains the magnetic field while providing a large area for windings. Thus iron-powders are usually the first choice for inductor cores.
Iron-powder cores typically maintain their magnetic qualities in the presence of high dc currents, though the ripple current must be relatively small to avoid overheating. In contrast, most inductors handle a small amount or ripple current but a large average current. Because transformers typically have a high ripple current but zero average current, ferrite cores work well. Ferrites have a power loss comparable to that of iron powder but can handle higher ripple currents. One reason is the behavior of these materials in the presence of ripple currents. Of these, the most common go-to materials are ferrites for transformers, iron-powder for inductors.
There are three general types of materials used for inductor magnetic cores: powder cores comprised of various iron alloys, ferrites, and wound cores comprised of thin magnetic steel strips. Thus a few basic concepts may come in handy when working with these components. There can be a mystique surrounding the specs of magnetic cores used in power inductors, due partly to the fact that magnetic materials may not be well characterized for handling high levels of magnetic flux. It is helpful to know how the material properties and geometries of magnetic cores affect the ability of inductors to store energy or filter current.