Power Electronics

Inductor Core Material: The Heart of an Inductor

Inductors are deceptively simple. However, a closer look reveals underlying complexity. How else can you explain the many types of core materials, types of windings, sizes, geometries and wide range of applications? This article will discuss three examples of the numerous types of material: silicon steel, soft iron powder and ferrites. We'll also focus on understanding the properties of inductor core material — the heart of an inductor.

What's a Magnetic Core?

An inductor's magnetic core is made of specially formed material with “soft” magnetic properties. Although physically hard, a magnetic core is said to be “soft” when it doesn't retain significant magnetism. A magnetic core is usually surrounded by carefully arranged windings of wire. The combination of magnetic core and windings results in a measurable property called inductance. There are various types of “soft” magnetic materials as well as different types and shapes of magnetic cores. Magnetic cores plus their windings can be thought of as miniature electromagnets.

There are many possible inductor core geometries. A core's geometry depends on various factors, including the application; the available mounting area and volume; the allowable radiation; the limitations on windings; the operating temperature; and how the inductor will be mounted. Consequently, a core's geometrical shape can take the form of a cylinder, bobbin, toroid or several other complex shapes.

In addition, an inductor's magnetic core doesn't have to be made in one piece. Multi-piece cores, each piece made of the same magnetic material, are sometimes used for extremely complex shapes or larger inductors.

Cores must be constructed and finished with an understanding of how windings will be installed. Sometimes, windings are wound directly around the core. Other times, the windings may be wound on a sleeve that is slipped over the core. Note that the wire used for the inductor windings is usually insulated, because when closely wound, adjacent turns would short out. However, thin insulation is fragile. When wound directly on the core, the magnetic material must not subject the thinly insulated wire to rough surfaces or sharp edges that could cut through insulation. To accommodate direct windings, a well-designed magnetic core will have a smooth winding surface. If appropriate, the core will provide a corner radius.

Examples of core materials for inductors include silicon steel, iron powder and ferrites. Each of these different materials has different properties at different frequencies, temperatures and power levels. Cores can be fashioned by casting, machining or pressing. Material cost may become a significant factor for larger, higher power cores. Table 1 illustrates the variety of core materials.

What Does a Magnetic Core Do?

Functionally, an inductor's magnetic core stores recoverable energy. Circuit designers specify inductors that are capable of receiving and returning energy in prescribed intervals. Mechanically, an inductor's core provides support for its windings. Magnetically, an inductor's core provides the medium to concentrate and contain magnetic flux. The combination of winding turns and volume of magnetic material sets an upper limit on the maximum allowable magnetic flux a core can sustain. Flux density is important because it's related to energy. Higher flux densities imply greater amounts of stored energy. Magnetic flux is analogous to electrical current in a purely resistive electrical circuit. Magnetic reluctance is analogous to resistance. A core with low reluctance can support a relatively high flux density. The same size core with high reluctance can support a lower flux density.

Another important core parameter is called permeability. Permeability is inversely related to reluctance. A core with high reluctance has low permeability and vice versa. Permeability is an important parameter because it can be thought of as a flux multiplier. For reference, consider the flux multiplier of free space to be unity (cgs system). Core permeability is always relative to the permeability of free space. Thus, the relative permeability of useful magnetic materials ranges from 10 to 10,000. More practical values of relative permeability are in the range of 100 to 1000.

An inductor transforms electrical energy into magnetic energy. That magnetic energy is stored in the inductor's magnetic field. Consequently, energy stored at one instant in time can be retained in the core until it's needed later. By controlling the rate at which energy is stored and removed from the magnetic field, designers can implement switched-mode power supplies. For example, switching power supplies may operate in the range of tens of kilohertz to a few megahertz. Slower switching supplies must store more energy per cycle than higher frequency switchers. The result is that core size is larger for lower switching frequencies and smaller for higher switching frequencies.

For a given winding configuration and core size, an inductor's value of inductance will be higher for a core with higher permeability. For the same electrical conditions, an inductor with a higher value of inductance can store more energy than an inductor with a lower value of inductance. Table 2 illustrates a few of the applications where magnetic cores are required.

Behavior of Different Core Materials

In this section, we'll review the behavior of silicon steel, iron powder, and ferrite materials. These soft magnetic materials have properties of permeability and resistivity. It's the disparity in these properties that make the different materials appropriate for different design applications. This is another way of saying there's no “best” material for all applications.

Silicon steel is relatively inexpensive and easy to form. In addition, silicon steel is a metal with low resistivity. Low-core resistivity means silicon steel readily conducts electrical current. The result is that undesirable eddy currents can flow in the core material. Eddy currents contribute to heating and core loss. In addition, a silicon steel core tends to reach the point of saturation rather easily. When saturated, a core is unable to store additional magnetic energy. Rapid saturation results in reduced operating range.

The solution to rapid saturation is to introduce an air gap in the magnetic flux path. An air gap increases the reluctance of the flux path, which has the effect of reducing the permeability and the inductance. Consequently, the amount of current the core can handle is extended.

Soft iron powder has higher resistivity than silicon steel. By special processing, iron particles are insulated from each other. The particles are mixed with a binder (such as phenolic or epoxy). The cores are then pressed into their final shape. Next, a baking process is used to cure the cores. After curing, many tiny air gaps combine to provide a distributed air gap effect. In other words, the air gap has been distributed throughout the core. Iron powder cores have found wide use when core loss is a consideration.

When compared to other magnetic materials, such as ferrites, the distributed air gap allows powder cores to store higher levels of magnetic flux. The distributed air gap also allows higher dc current levels before saturation occurs.

Ferrite is a crystalline magnetic material made of iron oxide and other elements. The mixture is processed at a high temperature and formed into a crystalline molecular structure. Unlike others, ferrites are ceramic materials with magnetic properties. Ferrites have high magnetic permeability and high electrical resistivity. Consequently, undesirable eddy currents are greatly reduced by ferrite cores. With their high resistivity, ferrites are ideal for use as inductors. For example, ferrite beads are frequently used to reduce parasitic oscillations and for general filtering at the component lead level. This type of broadband component requires a broadband low-Q in order to provide high impedance over a wide frequency range. Table 3 summarizes some of the important properties of these magnetic cores.

Magnetic Domains or Why Materials are Different

Different materials have different magnetic characteristics. Intuitively, there must be some underlying mechanism that's different for different materials. The answer is found in what we call “magnetic domains.” Magnetic domains are much more than the model of simple bar magnets that are either aligned or out of alignment. Although useful at the macro level, the bar magnet model is inadequate at the micro level. A magnetic domain is a volumetric space within a material. This volume has certain elemental properties. However, there are many magnetic domains of different sizes and shapes, within a single magnetic core. Furthermore, impurities and material imperfections contribute to the differences.

Work is required to alter the energy state of each (different) domain. Because the domains are of varying shapes and sizes, different amounts of work are required for different domains. Certainly, at the macro level, we can ignore the micro properties. However, it's these very properties that give rise to the particular characteristics of each material. Thus, we can understand why it's virtually impossible to provide two cores with identical properties.

Consider magnetic flux acting on and within magnetic domains. The magnetic domains expand and shrink, much like bubbles. Magnetic domains coalesce and meander around like rivers. Sometimes, the domains flow within set channels and sometimes they spread out, as in a flood. The sizes of the various domains, the proximity of the other domains and the various topological considerations ensure lack of uniformity. It's small wonder that different magnetic materials have different characteristics.

Core Materials

Table 1. Core materials.
Air
Alloys
Amorphous
Iron Powder
Manganese-Zinc Ferrite
Molybdenum Permalloy Powder
Nickel-Zinc Ferrite
Sendust
Silicon Steel


Applications Using Magnetic Cores

Table 2. Reliability test data.
Boost Switching Regulator
Buck Switching Regulator
Buck-Boost Switching Regulator
Common Mode Filter
EMI / RFI Filter
Forward Converter
Flyback Converter
Resonant Converter


Important Magnetic Parameters

Table 3. Important properties of magnetic cores.
Coercive Force
Curie Temperature
Flux Density
Permeability
Reluctance
Resistivity
Saturation


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