A Critical Comparison of Ferrites with Other Magnetic Materials ® a Division of Spang and Company

® Division of Spang & Company

A Critical Comparison of Ferrites with Other Magnetic Materials ® A Division of Spang and Company

Basic Differences- Material Magnetic Losses The magnetic losses in an A.C. Composition Considerations- application can be represented by and Structure Magnetic and the familiar Legg equation: The difference in properties and Mechanical Rm = µfL(ef + aBm +c) performance of ferrites as compared Properties with most other magnetic materials where: Rm = total core loss in is due to the fact that the ferrites are Saturation Magnetization ohms oxide materials rather than metals. As mentioned previously, the e = eddy current Ferromagnetism is derived from the highest saturation values are found coefficient unpaired electron spins in only a few in the metals and alloys. Thus, if high a = hysteresis metal atoms, these being iron, flux densities are required in high coefficient cobalt, nickel, manganese, and power applications, the bulk metals, c = residual loss some rare earth elements. It is not iron, silicon-iron and cobalt-iron are coefficient surprising that the highest magnetic unexcelled. Since the flux in max- µ = magnetic moments and therefore the highest wells Ø = BA, where B = flux density permeability saturation magnetizations are to be in gausses and A = cross-sectional f = frequency in found in the metals themselves or in area in cm², obtaining high total flux hertz alloys of these metals. The oxides, in materials such as ferrites or per- L = inductance in on the other hand, suffer from a dilu- malloy powder cores can be accom- henries tion effect of the large oxygen ions in plished only by increasing the Bm = maximum flux the crystal lattice. In addition, the net cross-sectional area. Powdered iron density in moment resulting from ferromag- has a fairly high saturation value, but gausses netic alignment of the atomic spins exhibits low permeabilities. is reduced because a different, less Eddy current losses will increase efficient type of exchange mecha- Curie Temperatures quite rapidly with frequency. In bulk nism is operative. The oxygen ions All magnetic materials lose their metals, these high frequency losses do serve a useful purpose, however, ferromagnetism at the Curie temper- can be reduced by reducing the since they insulate the metal ions ature. One overriding consideration thickness of the material perpendic- and, therefore, greatly increase the for a magnetic material is that the ular to the flux flow. This is accom- resistivity. This property makes the Curie point of the material be well plished by using thin gauge tapes or ferrite especially useful at higher fre- above the proposed operating tem- laminations or by powdering and quencies. The purpose of this paper peratures. Table 1 lists the Curie insulating the particles. In ferrites, is to list the various considerations Temperatures of the various the same result is obtained by which enter into the choice of a ma- materials. The Curie point depends increasing the resistivity by many terial for a specific application and to only on composition and not on orders of magnitude. Thus, at the contrast pertinent ferrite properties geometry. Even though some of the highest operating frequencies where with those of bulk metal or powdered magnetic materials shown can be further gauge or particle reduction is metal materials. used at higher operating tempera- impractical, ferrites are the only tures than others, very often the tem- available materials. perature limitations of the accessory items (wire insulation, potting or damping compound) can be more limiting; in this case, no practical advantage may be gained by the higher curie point materials.

1 The hysteresis losses are propor- Permeability this phase is emphasized by the fact tional to the flux density and can be Permeability is a function of com- that the market value for computer depicted as the area inside the hys- position and processing. The highest magnetics is now greater in dollars teresis loop. High hysteresis losses initial permeabilities (those meas- to that of the power materials market. are accompanied by the presence of ured at very low flux levels) are found It is interesting to note that disk unwanted harmonics. The nickel- in the nickel-iron alloys, particularly media and bubble materials are fer- iron (permalloy) alloys have low hys- in supermalloy where the value is rite type oxides. teresis losses and a great asset to about 100,000. Powdered iron cores the permalloy powder core is that have low permeabilities (10-100) Brittleness these low losses are maintained with while permalloy powder cores are One drawback to the ferrite core the accompanying reduction in eddy somewhat higher (15-550). Ferrites as compared with metal cores is its current losses. can be made over a wide range of brittleness. Being ceramic in nature, The residual losses are not too permeabilities. The linear filter type care must be exercised in the han- well understood and perhaps repre- permeabilities vary from 100-2000, dling of these cores. Powder cores sent an expression of our ignorance while those used in power applica- are also somewhat brittle and similar of the system. They apparently are tions range from 3000-1 5,000. As the precautions are required. Although tied in partially to absorption of operating frequency increases, fer- metal tape cores are not brittle, energy from the system by gyromag- rites with lower permeabilities are (except amorphous metal cores), netic resonance. used because these have distinctly they nevertheless are sensitive to A listing of the various losses in the lower losses in these regions. The strain and mechanical shock, espe- materials under consideration is permeabilities for a variety of cially in the high permeability given in Table 1. materials are listed in Table 1. materials. Consequently, tape wound cores are often embedded in Figure of Merit a damping compound which pre- A useful figure of merit for linear vents the transfer of strain or shock core materials is the µQ product. to the cores. Values of this factor are tabulated in Table 1 . At frequencies of 100 KHz Hardness and above, the value for ferrites is Ferrites are very hard materials as considerably above all other compared with the other materials materials. under consideration. This property is especially useful in applications in Squareness which wear is a factor. Conse- The squareness ratio is defined as quently, ferrite material is being used the ratio of Br to Bm and is especially extensively in magnetic recorder important in memory and switching head applications. core applications. Magnesium- manganese ferrites can be produced with extremely high squareness ratios. While some metal tape and bobbin cores possess similar high ratios, their higher cost and difficulty in miniaturization made the ferrites the material of choice in large scale memory applications in early computer models. Thin magnetic film memories, which may be considered bulk metals, have become increas- ingly important, and along with semi- conductor, disk, tape and bubble memories, have replaced the old core memories. The importance of

2 MAGNETICS • BUTLER, PA ® A Division of Spang and Company

Geometry Tunability Magnetic Shielding Considerations An exact inductance is required in If magnetic components are rela- certain L-C circuits. If the shape of tively close in a circuit, the fields Formability the inductor is toroidal, the induc- produced by one component may The three types of materials- bulk tance can be trimmed only by the affect the performance of other metal, powdered metal and ferrite - addition or removal of turns, a time cores. One solution is to increase the are produced by widely varying tech- consuming and costly procedure. If space between components. This niques and consequently the availa- a ferrite pot core is used, the tuning increases the overall size of the sys- ble geometries also vary. can be accomplished by means of a tem. Another is to use a magnetic Bulk metals - These are screw-type trimmer core which shield which increases weight and produced mostly by standard changes the effective air gap of the size. A ferrite pot core is inherently metallurgical process involving core. Threaded rods of powdered self-shielding by nature of the melting followed by hot and cold iron or ferrite materials are used enclosed magnetic circuit. rolling. The sheet material extensively as tuning elements in produced is either slit and wound slug tuned inductors. into tape or bobbin cores or punched into laminations. Pho- Winding Considerations toetching, a new method of form- Winding turns on a toroid involves ing small complex parts, avoids specialized equipment and the proc- costly tooling, and produces ess involves winding each core stress-free parts. separately in a relatively time consuming operation. The bobbins used Powdered Iron and permalloy - in ferrite pot cores can be wound These materials are always die- many at a time on a rather simple pressed into toroids or slugs, machine. This ease of winding con- molypermalloy usually in toroids stitutes an important advantage for and powdered iron into slugs. ferrite pot cores.

Ferrites - Because ferrites are produced by a ceramic technique, they can be made in a large num- ber of shapes. Unlike the bulk metals, they can be molded directly, and unlike the powdered permalloy, they can be machined and ground to close tolerances after firing. Various forming processes for ferrites include die pressing, extrusion, hydrostatic pressing and hot pressing. The available shapes include toroids, E-l cores, U-l cores, pot cores, rods, tubes, beads and blocks.

3 Inductance Figure 1 — Ferrite Toroids Stability Initial Permeability (µo) Vs. Temperature Considerations Temperature Stability In telecommunications circuitry (tuned L-C filters), the maintenance of a near-constant inductance as a function of temperature and time is most critical. One method of achiev- ing this stability is by the insertion of an air gap. The gap may be dis- tributed as in powder cores or localized as in gapped ferrite pot cores. Gapping also results in a reduction in the effective permeability but often this is not a serious limitation. In gapped ferrite cores, the temperature coefficient (T.C.) can be linear to match a capacitor with an equal but opposite T.C. (polystyrene) or relatively flat if a flat T.C. capacitor (silver-mica) is used. ∆ T.C. = L L∆T

where ∆L and ∆T are corresponding changes in inductance and temperature and L is inductance at a standard temperature. Figure 1 illustrates the temperature characteristics of several ferrite materials.

As pointed out, the use of an air gap greatly increases the temperature stability. The powder core toroid and ferrite pot core are thus used to good advantage. In the powder core, the T.C. is built into the toroid, whereas in the pot core, the T.C. can be varied by changing the gap. How- ever, in the latter, the effective permeability and therefore the inductance of the core is changed. By choice of the proper size core with the proper gap, the optimum inductance and T.C. can be obtained.

4 MAGNETICS • BUTLER, PA ® A Division of Spang and Company

Permeability VS. AC Flux Density It is often desirable to have a air gap in either permalloy powder show the relative change in induc- minimum change in permeability cores or ferrites can be used to tance for a ferrite toroid, ferrite pot with AC excitation. Here again, the advantage. Figures 2, 3 and 4 core, and permalloy powder toroid.

Relative Change of Inductance With AC Flux Level for Toroids and Pot Cores Figure 2 — Ferrite Toroid Figure 3 — Ferrite Pot Core

FLUX DENSITY – GAUSSES FLUX DENSITY – GAUSSES

Figure 4 — Permalloy Powder Toroid

FLUX DENSITY – GAUSSES

Figure 5 Inductance VS. DC Bias, Permalloy Powder Cores

DC Magnetizing Force (Oersteds)

5 Permeability VS. DC Bias Figure 6 Often an AC circuit has a super- Inductance VS. DC Bias Curves, High Flux Powder Cores imposed DC bias conditon. Minimum variation of permeability with DC is desirable. Powder cores are especially resistant to these changes. Figures 5, 6, and 7 show typical variations of µ with DC bias for permalloy, high flux and Kool Mµ ® powder toroids. Gapped ferrite pot cores show a similar effect, shown in Figure 8.

Permeability VS. Time In most magnetic materials there is a slight decrease in permeability with time after the material is DC Magnetizing Force (Oersteds) demagnetized or after it is first produced. This effect is known as disaccommodation. In non-linear Figure 7 applications this effect is not too Inductance VS. DC Bias Curves, Kool Mµ Powder Cores important. However, in low flux level circuits where a constant inductance is required, the effect must be considered. The effect is pronounced in low permeability materials and is negligible for high permeability materials. However, the effect can be minimized greatly by reduction of the effective permeability by insertion of an air gap. Thus in powder cores, the change of permeability due to this effect is less than . 1%. In ferrite pot cores, the localized gap reduces the effect in proportion to the effective permeability compared with the toroidal permeability. Since DC Magnetizing Force (Oersteds) the effect is logarithmic, most of the decrease occurs in the first few days after firing. If some aging of the cores Figure 8 occurs before usage, the change of Inductance VS. DC Bias for A Ferrite Pot Core inductance due to time will be negligible.

FLUX DENSITY – GAUSSES

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Application Considerations- Ferrite Advantages and Disadvantages

Application Advantages Disadvantages

Low Frequency (<1KHz) • Ease of forming shapes allows • Flux density low High Flux Applications possible use in inexpensive, • Relative cost high Generators high loss applications such as • Limited size of parts Motors relays, small motors. Power Transformers

Medium Frequency (1-100 KHz) • Cost much lower than Nickel- • Flux density lower than Nickel-Iron Non-Linear High Flux Iron alloys, especially thin alloys Applications tapes • Permeabilities lower than Nickel- Flyback Transformers • Moderately high permeabilities Iron alloys Deflection Yokes available • Curie Temperature fairly low Inverters • Low losses, especially in upper • Good mating surface necessary Wide Band Transformers half of this range for high inductance Recording Heads • Inherent shielding in pot cores • Smaller flux change than bobbin Pulse Transformers • Good wear resistance cores Memory Cores • Easily adapted to mass production Medium Frequency (1-100 KHz) • Permeabilities higher than • Low Curie point Permalloy Low Flux, Linear powdered iron or • Need precision grinding of air Applications cores gap Loading Coils • Gapped pot cores provide: • Brittleness Filter Cores 1. Adjustability • Mounting hardware needed Tuned Inductors 2. Stability - temperature, Wide Band Transformers time, A. C. flux density, Antenna Rods D.C. bias 3. Self-shielding • µQ Products higher than other materials • Wide choice of Inductance and Temperature Coefficient

Higher Frequencies (>200 KHz) • Low losses (especially eddy • Permeabi lity decreases with Low Flux, Linear current) frequency Applications • Ferrites, powdered iron and • Medium frequency disadvantages Filters moly-permalloy can operate at apply Inductors higher frequencies • Poor heat transfer Tuning Slugs • Medium frequency advantages apply

• Microwave frequencies (>500 Low dielectric losses MHz) • Good gyromagnetic properties • Only bulk materials available

7 Table 1 — Properties of Soft Magnetic Materials

Initial Bmax Loss Coefficients Curie Material Perm. Kilogausses e x 10 6 a x 10 ³ c x 10 ³ Temp. Resistivity µ Q at Operating ° µ °C (ohm-cm) 100 kHz Frequencies ° Fe 250 22 --- 770 10x10 -6 - 60-1000 Hz

Si-Fe (unoriented) 400 20 870 120 75 740 50 x 10-6 - 60-1000 Hz

Si-Fe (oriented) 1500 20 --- 740 50x10-6 - 60-1000Hz

50-50 Ni Fe (grain-oriented) 2000 16 --- 360 40 x 10-6 - 60-1000Hz

79 Permalloy 12,000 8 173 --450 55 x 10 -6 8000 1 kHz-75kHz to to to 100,000 11 12,000

AMORPHOUS Alloy B 3000 15-16 --- 370 135x10-6 - to 250 kHz

AMORPHOUS Alloy E 20,000 5-6.5 - --205 140 x 10-6 - to 250 kHz

Permalloypowder 14 3 .01 .002 .05 450 1. 10,000 10 kHz-1 MHz to to to 550 .04 .1

High Flux powder 14 15 - - - 360 --10 kHz to 1 MHz to 160

Kool Mu ®powder 26 10 --- 740 --to 10 MHz to 125

Iron powder 5 10 .002 ,002 .2 770 10 4 2000 100 kHz-100 MHz to to to to to 80 .04 .4 1.4 30,000

Ferrite-MnZn 750 3 .OO1 .002 .01 100 10 100,000 10 kHz-2 MHz to to to to to 15,000 5 300 100 500,000

Ferrite-NiZn 10 3 - - - 150 10 6 30,000 200 kHz-100MHz to to to 1500 5 450

Co-Fe 50% 800 24 - - - 980 70 x 10-6 - -

8 MAGNETICS • BUTLER, PA Listed below are catalogs and brochures where additional details and design information on the products manufactured by Magnetics are available. Some can be obtained on our website. Otherwise, write, phone or e-mail us to request a copy. (See back cover for contact information).

Tape Wound Core Literature Ferrite Literature TWC-500 Design Manual FC-601 Design Manual TWC-S1 Fundamentals of Tape Wound Core FC-S1 Ferrite Material Selection Guide Design FC-S2 EMI/RFI Common Mode Filters TWC-S2 How to Select the Proper Core for FC-S3 Q Curves Saturating Transformers FC-S4 Step Gap E Cores Swing Chokes TWC-S3 lnverter Transformer Core Design and FC-S5 Common Mode Inductors for EMI Material Selection FC-S6 Planar Core Design and Use - TWC-S5 Composite Core Technical Papers SR-4 Mag Amp Cores and Materials FC-S7 Curve Fit Equations for Ferrite Materials Powder Core Literature FC-S8 Designing with Planar Ferrite Cores MPP-400 Design Manual, MPP Cores CG-01 A Critical Comparison of Ferrites HFC-1.0 High Flux Cores with other Magnetic Materials HFD-1.0 High Flux Cores Diskette CMF-2.1 Common Mode Filter Inductor Design Software MPP-Q1 Q Curves for Molypermalloy Powder Cores CAD-1.3 CLIP ART Diskette KMC-2.0 Kool Mu Cores KMD-2.0 Kool Mu Cores Diskette KMC-S1 Kool Mu Application Notes General Information KMC-E1 Kool Mu E Cores CG-04 Testing Magnetic Cores CG-03 Cores For Flybacks CG-05 Frequently-Asked Questions About PCD-2.3 Magnetics Inductor Design Software Magnetic Materials TID-100 Power Transformer and Inductor Nickel-Iron, Supermendur, & Amorphous Design SR-1A Inductor Design in Switching Cut Core Literature Regulators MCC-100T Design Manual PS-01 Cores for SMPS PS-02 Magnetic Cores for Switching Power Bobbin Core Literature Supplies BCC-1.1 Design Manual APB-2 All Products Bulletin BCD-1.1 Design Manual Diskette HED-01 Cores for Hall Effect Devices Division of Spang & Company

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