Primarily inductors consist of a coil. If we insert a core of magnetic material the inductive properties of the coil will increase. Such coils are then called chokes. This article explains the basic core materials, their losses and material permeability.
When we draw current through a choke electric currents are induced in the magnetic material that try to create a counteracting magnetic field. These currents are undesired both for that reason and because they create heat losses.
Homogeneous magnetic bodies are excluded; the induced current would be too high. Instead mutually isolated ribbons are used or a powder technology where the binder material between the magnetic granules limits the induced current by their resistivity.
Soft Iron Cores
The magnetic flux can be increased by means of a core of laminated soft iron or iron powder mixed with a binder. As stated in the introduction the distribution in electrically isolated disks or surface oxidized powder granules reduces the eddy current losses in the choke. The saturation induction according to Figure 1., is relatively high in chokes with iron materials in the core, up to 2.3 T. They can operate at high frequencies; metal alloys to 100 kHz and iron powders to more than 100 MHz. The temperature stability is good. The temperature range for iron powder cores is -55/+105 °C.
Amorphous Magnetic Materials
These non-crystalline magnetic materials consist of ferro or cobalt based powdered amorphous alloys that are baked into some shape impervious material. The core material with the magnetic powder granules constitutes a core with “distributed air gaps” that limits eddy currents. Another manufacturer heats up the magnetic foils/bands and precipitates the cooling process so rapidly that the metal becomes amorphous. The eddy current limitation is achieved by using these thin ribbons mutually isolated, in the cores. The ribbons are approximately 20 mm (0.8 mils) thick.
Amorphous core materials produce a narrow and high, nearly rectangular BH characteristic that has its highest inductance when the current passes through zero and a nearly negligible inductance as soon as the current reaches a certain strength. Thus the hysteresis losses will be small. The saturation flux is high (Figure 2.). For some laminated cores it is more than 1.5 T. The Curie temperature may, depending on material, vary between 200 and 400 °C. The initial permeability is in some materials very high, up to 100 000.
Ferrite Core Materials
The most common inductor material is still the ferrites. They are ferro magnetic and are ceramic materials. They consist of a mixed mass of pulverized oxides of iron and other alloys that is pressed to the desired shape and then sintered. The material resistivity is much higher than that of iron and the eddy current losses correspondingly low, which makes them suitable for high frequency applications.
They have a wide range of permeabilities from 10 to 20 000 but a fairly low saturation induction, < 0.5T. The temperature range is –55/+105 °C. The following figure shows a comparison of characteristics of metals, amorphous materials and ferrites.
There are a number of different manufactured styles and shapes. Some of the most common are shown in following figure 3. The different manufactured shapes are not shown to the same scale. Rods, E cores, toroids and pot cores are provided with a winding, either direct on the core or via a bobbin. Attenuation ferrites surround the current path.
· a) cylindrical rod core · b) toroid or “attenuation bead” · c) E core with yoke · d) attenuation ferrite for flat conductor cable · e) pot core · f) SMD ferrite.