Ferrites are one of the main core materials used in inductors and transformers.
Inductor ferrite is used to provide an increase in the permeability of the medium around the coil to increase the inductance of the inductor.
Ferrites are widely used within inductor technology to improve the performance of the inductor.
What is ferrite?
Ferrites are basically iron based magnetic material in the form of a ceramic.
Ferrites are made from a powder and can therefore the ferrite cores used in inductors and other applications can be manufactured in a variety of shapes according to the requirements.
Ferrites, or as they are also known ferromagnetic materials can be classified into two categories based on their magnetic coercivity, or persistence of internal magnetism:
- Soft ferrites: Soft ferrites are ferrite materials that are able to easily reverse their polarity of their magnetisation without a significant amount of energy being needed to reverse the magnetic polarity. This means that there is only a relatively small loss of energy.
Soft ferrites also have a high electrical resistance and therefore, when used in inductors and transformers eddy current losses are also low.
Soft ferrites are often made from a blend of iron, nickel, zinc or manganese oxides. Manganese-zinc and nickel-zinc magnets are the most common of the soft ferrite magnets. As a result of their high resistance, soft ferrites are widely used in the cores of inductors or transformers because they result in minimal energy loss.
Generally soft ferrites are accepted as those having a coercivity of less than 1 kA.m.
- Hard ferrites: Hard ferrites may also be called permanent magnets. They retain the polarity of their magnetisation once the magnetising field has been removed, i.e. they have a high remanence level.
Hard ferrite magnets are typically made of barium, iron or strontium oxides. They are cheap to produce and are the magnets that are used in a wide number of applications, but may be most commonly seen in such applications as standard household magnets (e.g., kitchen magnets).
Generally hard ferrites are considered to be those with coercivity levels of greater than 10 kA/m.
Ferrites are generally chemically inert ceramic iron-based materials. They generally have the chemical structure of the format XFe2O4, where X is a transition material.
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To manufacture the ferrites used in inductors and other applications, the powers of the metals are mixed in proportions and then milled to give the required grain size and then pressed into shape.
Sintering entails heating the material to between about 1150°C and 1300°C.
Sintering is a process where a powdered ceramic material is held in a mold to give it the required shape and then heated to a temperature which is below the material melting point. It is found that the atoms in the powder particles diffuse across the particle boundaries, so that the particles become fused together. In this way a single solid item is created.
The sintered core of the inductor ferrite may still require further finishing – it may be ground to provide a very flat surface for situations where mating halves of a core are required. Here flat surfaces are essential to ensure that air gaps in inductors or transformers, etc., are as small as possible.
The finished ferrite material contains thousands of small crystals or grains. Typically these are around 10µm across. Within each grain or crystal there are many more smaller magnetic domains that can will have a random orientation after heating. With the application of an external field, these domains will tend to orientate in the same direction.
There are many parameters that are of importance when a ferrite is used within an inductor. However the chief parameter for an inductor ferrite is the permeability. The level of permeability of the inductor ferrite enables the inductor to have a much greater inductance than it would if only an air core were used.
The permeability of ferrites used within inductors varies considerably between different types of ferrite. They can have permeability levels that may range between 20 to more than 15,000, although some very specialised ones may be higher.
Inductor ferrite core losses
One major parameter of interest to the electronic engineer using ferrites in inductors is the core losses they exhibit and their frequency dependence.
The core losses of a ferrite core can be expressed in the following manner:
Pc = total core loss
Ph = hysteresis loss
Pe = eddy current loss
Pr = residual loss
It is found that he hysteresis loss increases linearly with increasing frequency and flux. The eddy current loss increases exponentially with increasing frequency and flux. However it is found that the hysteresis loss is the dominant core loss up to a frequency determined by the performance of the core. Above this the eddy current loss predominates.
To improve high frequency performance the grain size used in the preparation of the ferrite used for the inductor must be small, and also the mixture used must be free from impurities.