Ferrite transformer cores

Magnetite is the ancient lodestone used as an early compass. More recently, ferrites have found use as memory devices in computers, as magnetic particles on recording tapes and as transformer cores. 

Soft ferriles have a slender S-shaped hysteresis loop with low rcntanencc and low ciicreivc force permitting easy magnetization and demagnetization with little magnetic loss. These ferrites are uniquely suited to low-ioss inductor and transformer cores for radio, television, and carrier telephony. 

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 .)...

Ferrites are compounds of the type MeO. Fc203 exhibiting the technically important ferromagnetic properties. The main advantage of ferrites, as compared with ferromagnetic metals, is an electrical conductivity lower by orders of magnitude, so that the power losses due to the effect of eddy currents in coil and transformer cores are... 

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. 

A recent use for zinc oxide is its application to the manufacture of magnetic ferrites, which have been developed over the past 25-30 years. They usually are composed of ferric oxide in combination with zinc oxide (of high chemical purity) and any one or more of several other oxides of bivalent metals. The amount of zinc oxide used varies from 10-35%, depending npon the characteristics desired in the finished magnetic ferrite. Having as their prime properties high permeability and low hysteresis, they are used in the field of electronics for such devices as high frequency transformer cores for television receivers. 

Silicon Strengthens ferrite, raises the critical temperatures has a strong graphitizing tendency always present to some extent because it is used, with manganese, as a deoxidizer Silicon steel (0.07% C 4% SI) used for transformer cores used with chromium (3.5%Si 8% Cr) for its high-temperature oxidation resistance in internal combustion engine valves... 

A ferrite toroid or E core can be used for a drive transformer. No gap is needed since the input coupling capacitor guarantees that the core will operate in a bipolar fashion. A high permeability core is also suitable for this purpose. The wire that is going to be used will be in the range of 32 to 36 AWG. The core size will be approximately 0.4 to 0.6 inches (10 to 15mm). 

Parylene is used in the manufacture of high quality miniature stepping motors, such as those used in wristwatches, and as a coating for the ferrite cores of pulse transformers, magnetic tape-recording heads, and miniature inductors. 

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 combination of the above factors has rendered the nanocrystalline solution competitive, not only with amorphous Co-based alloys, but also with classical crystalline alloys and ferrites. The consequence is a steadily increasing level of applications in magnetic cores for ground fault interrupters, common mode chokes and high frequency transformers. Fig. 14 shows some typical examples. The worldwide production rate meanwhile approaches an estimated 1000 tons/year, and the trend is increasing. The only drawback of the nanocrystalline material appears to be the embrittlement that occurs upon crystallization, which requires final shape annealing and, thus, restricts application mainly to toroidally wound cores. 

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