Ferrite Core Type Transformer

The transformer working is mainly dependent on its core design. The magnetic core is the heart of the transformer. Generally speaking, there are some properties to be considered in core materials, which are

i) Permeability : Ability to conduct flux.

ii) Saturation : No increase in flux, though magnetic field strength increases.

iii) Electrical Resistivity : The electrical resistance of the core responsible for `eddy current losses.

iv) Hysteresis : The area of hysteresis loop is the measure of hysteresis loss.

The silicon steel alloys are commonly used in the core manufacturing for the transformers at power frequencies i.e. 60 to 400 Hz. But use of such alloys can be extended upto audio frequency range upto 100 KHz using thin gauge tap wound laminations. At the same time these cores become more expensive and difficult to handle. In case of higher frequency range upto 200 KHz and more, the problem of eddy currents becomes very serious. In such cases a material which is not really a metal or not even a bonded powered metal product, is used for the cores. It is a ceramic which is magnetic in its nature called ferrites. Such transformer using ferrites for the core construction are called ferrite core transformers. The ferrite core produces a lower price to performance ratio.

The ferrites are very hard, brittle and dark grey or black in appearence. The ferrites have high permeability. The ferrites appear in numerous shapes like bar, rods, hollow pots, toroids, cross format, E_s etc. Hence the transformers using such cores can be easily adapted to circuit board mounting. The ferrites have low leakage inductance, The property of high resistivity of the ferrites keeps the eddy current losses very small, at high frequencies. This eliminates the need of the ferrites keep the eddy current losses, at high frequencies. This eliminates the need for laminated construction for the cores, in case of ferrites core transformers.

Following table gives the performance comparison between the ferrites and alloys.

The table shows the values of various at 25 kHz. This clearly indicates the advantages of the ferrites over the alloys, in high frequency range.

Ferrites consist of an iron oxide, Fe₂O₃. There are different methods of producing ferrites viz the method of coprecipitation of hydroxides or carbonates and the composite method. Audio frequency ferrites are made up of nickel-zinc oxide and manganese-zinc oxide along with iron oxide. Manganese-zinc ferrites have low hysteresis loss and smaller temperature coefficient of permeability, which is advantages compared to nickel-zinc ferrites. Burt due to lower threshold frequency, manganese-zinc ferrites are used for low frequency applications. Nickel-zinc ferrites maintain their permeability over a wide range of frequency upto hundreds of megahertz, hence are used for high frequency transformer cores. These ferrites are temperature sensitive. The limiting feature of the ferrites is their low curie temperature (under 500^oC). The magnetic properties of the ferrites deteriorate gradually as the temperature approaches to the curie temperature. This fact must be taken into account when considering the ferrites.

The ferrites cores substantially reduce the size and weight of the transformers used for high frequency applications. The ferrites are used for flyback transformers in television sets, carrier telephony, television receivers, radio receivers and audio range pulse transformers.

The wide variety of ferrites shapes is shown in the Fig. 1.