Soft ferrite

Soft ferrites are used for the manufacture of inductor cores (pot cores) for telecommunications, low-power transformers and high-flux transformers such as television line output transformers, and as television tube scanning yokes (Fig. 9.17). The more important material characteristics for these and other applications are now discussed with emphasis on the influence of composition and microstructure. The review paper by A. Broese van Groenou et al. [9] and the monograph by E.C. Snelling [10] are recommended to supplement the discussion. 

The way in which magnetic loss in a material is expressed depends upon the particular application. For example, in the case of pot cores, when currents are small and hence the flux density is also small (typically less than 1 mT), the loss factor used is (tan 5)//iri or its reciprocal jinQ where Q is the quality factor. 

The loss factor is usually expressed in terms of three main contributions, namely 

It is shown (Section 9.1.4) that the loss factor is independent of gaps introduced in the magnetic circuit. In the case of pot cores (Fig. 9.48), where air gaps are introduced both intentionally and because of the imperfect join, the loss factor is a particularly useful parameter. 

The resistivity p of an inductor core material is important because it determines eddy current losses. In general room temperature resistivities of ferrites lie in the range 10 —106Qm, many orders of magnitude higher than that of the most resistive of the ferromagnetic alloys (about 8x10 7 Qm). Typical resistivity-temperature data for MnZn and NiZn ferrites are shown in Fig. 9.22. For both 

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The most commonly used ferrites, the so-called soft ferrites, are used ia soft magnet and low field telecommunication appHcations, low power... 

Energy losses in soft magnetic materials arise due to both hysteresis and eddy currents, as described in the previous section. Eddy current losses can be reduced by increasing the electrical resistivity of the magnetic material. This is one reason why solid-solution iron-silicon alloys ( 4% Si) are used at power frequencies of around 60 Hz and why iron-nickel alloys are used at audio frequencies. Some magnetically soft ferrites (see Section 6.2.2.1) are very nearly electrical insulators and are thus immune to eddy current losses. Some common soft magnetic materials and their properties are listed in Table 6.19. Soft magnetic alloys are described further in Section 6.2.1.6. 

Fig. 9.17 Range of soft ferrite components (i) TV scanning yoke (components kindly supplied by Philips Components Ltd.) (ii) UR core and TV line output transformer (iii) E core for switched mode power supply (iv) wide band transformer core (v) core giving good magnetic shielding (vi) high Q (adjustable) filter core (cf. Fig. 9.48) (vii) precision ferrite antenna for transponder (viii) multilayer EMI suppressors (ix) toroids for laser and radar pulse applications (x) typical EMI shields for cables, ((ii)—(x) Courtesy of Ferroxcube UK .)...

The choice of composition of a soft ferrite is made to achieve one or more of the following properties ... 

A soft ferrite with complex relative permeability fi T = 2000 — 7j is in the form of a toroid of cross-sectional area 0.5 cm2 and inner radius 3 cm. A primary winding... 

Small amounts of Y or La are used to dope BaTiC>3 which is the main component of all PTCs (positive temperature coefficient). Demand for PTC thermistors is high. Yttrium iron garnets are used in soft ferrites at very high frequencies (microwave region) for radar equipment. 

Magneisum ferrite spinel. MgFe204 is one a representative of soft ferrites extensively used for high-frequency applications. Fully dense bulk ferrites have been synthesized by low-temperature sintering of fine powders, by thermal decomposition and by co-precipitation. Preparation from ultra-fine powder is more advantageous since the composition can be more easily controlled, and the electrical and thermal properties are improved as a result of the reduced grain size. 

Soft ferrites with low porosity are obtained by hot pressure processes... 

Isotropic hexaferrites are manufactured in a similar way to soft ferrites, using natural or waste iron oxide as a raw material. 

Natural and synthetic iron oxides not possessing pigment properties are used as raw materials in the production of hard and soft ferrites (see Section 5.5.5.2, Electro- and Magneto-Ceramics) for radio, television and telephone technology, for adhesive magnets, for rotors in dynamos, for low-loss magnetic layers, for DC-motors, for transformer cores, for electronic calculators and high frequency furnaces. This amounts to an annual worldwide production of more than 300 10 t/a. 

Snelling, E. C. Soft Ferrites, Properties and Applications , Second Edition, ISBN 0408027606 Butterworths Co. 

Fair-Rite Soft Ferrites , Databook, 13 Edition Fair-Rite Products Corp. NY 12589... 

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