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applications of ferrites

General Information

Ferrite magnets are sintered permanent magnets composed of Barium or Strontium Ferrite. This class of magnets, aside from good resistance to demagnetization, has the popular advantage of low cost.

Ferrite magnets are very hard and brittle, and require specialized machining techniques. Moreover, they should be machined in an unmagnetized state. We are equipped to machine these materials to specifications.

Anisotropic grades are oriented in the manufacturing direction, and must be magnetized in the direction of orientation. Isotropic grades are not oriented and can be magnetized in any direction, although some degree of greater magnetic strength will be found in the pressing dimension, usually the shortest dimension.

Due to their low cost, Ferrite magnets enjoy a very wide range of applications, from motors and loudspeakers to toys and crafts, and are the most widely used permanent magnets today.

 

Manufacturing Methods

Pressing and Sintering:

Pressing and sintering involves pressing very fine ferrite powder in a die, and then sintering this pressed magnet. All fully dense Ferrite magnets are produced this way. Ferrite magnets can be wet pressed or dry pressed. Wet pressing yields better magnetic properties, but poorer physical tolerances. Generally, the powder is dry for grade 1 or 5 materials, and wet for grade 8 and higher materials. Sintering involves subjecting the material to high temperatures to fuse the pressed powder together, thus creating a solid material. Magnets produced through this process usually need to have some finish machining, otherwise surface finishes and tolerances are not acceptable. Some manufacturers extrude instead of press wet powder slurry and then sinter the material. This is sometimes done for arc segment shapes, where the arc cross-section is extruded in long lengths, sintered, and then cut to length.

Injection Molding: Ferrite powder is mixed into a compound and then injection molded in the same way as plastic.

Tooling for this manufacturing process is usually very costly. However, parts produced through this process can have very intricate shapes and tight tolerances. Injection molded ferrite properties are either lower or about the same as grade 1 Ferrite.

 

Assemblies

We are able to manufacture metal and other components of finished sub assemblies using our CNC machining facilities.

Assemblies using metal or other components and magnets can be fabricated by adhering magnets with adhesives to suit a range of environments, by mechanically fastening magnets, or by a combination of these methods. Due to the relatively brittle nature of these magnet materials, press fits are not recommended.

 

Surface Treatments

The corrosion resistance of Ferrite is considered excellent , and no surface treatments are required. However, Ferrite magnets may have a thin film of fine magnet powder on the surface and for clean, non-contaminated applications, some form of coating may be required.

 

Machining

Ferrite is brittle, and prone to chipping and cracking. Special machining techniques must be used to machine this material. We are fully equipped to machine these materials to your blueprint specifications.

 

Magnetizing and Handling

Ferrite magnets require magnetizing fields of about 10 kOe. They can be magnetized with multiple poles on one or both pole surfaces. No special handling precautions are required, except that large blocks of Ferrite magnets are powerful, and care should be taken to ensure that they do not snap towards each other.

 

Temperature Effects

Up to about 840F, changes in magnetization are largely reversible, while changes between 840F and 1800 F are re-magnetizable. For all Ferrite magnets, the degradation of magnetic properties is essentially linear with temperature. At 350 F, about 75% of room temperature magnetization is retained, and at 550 F, about 50% is retained.

 

Common Applications for Ferrite Magnets

Ferrite magnets are widely used in motors, magnetic couplings, for sensing, loudspeakers, holding-magnet systems, crafts, magnetic therapy, novelties, and toys.

 

courtesy:magnetsales.com/Ferrite/FERRI.htm

Ferrite Beads :

Ferrite beads are often used in gigahertz filtering applications. Ferrite beads come in two flavors: high-Q, resonant beads and low-Q, nonresonant beads, also called lossy, or absorptive beads.

The high-Q type has no place in a digital circuit. These beads are used to construct RF oscillators and filters and other circuits that need highly resonant circuit elements. In a digital power-filtering application, the last thing you want is resonance. The low-Q type is commonly used for power-supply filtering, in series with the power connection. Most often, this style of filter also has a capacitor to ground on either side of the inductor (Figure 1).


(courtesy EDN Magazine)

The bead manufacturer should provide you with a curve of impedance versus frequency for your bead. From this curve, you may ascertain the efficacy of a particular ferrite bead at your frequency of interest. For your filter to work properly, the impedance of L1 should greatly exceed the impedance of C2 .

The performance of some ferrite materials (particularly the very high-permeability materials) begins to deteriorate at high frequencies. Past some critical point, the impedance no longer rises proportional to frequency as quickly as you might like. However, for a filtering application, as long as the impedance remains high enough to do the filtering job, you don't need to worry about the efficiency of the ferrite.

Beware that the parasitic capacitance from the input wire to the output wire of the bead can defeat your purpose, especially when you use a double-hole-type core wound in a U-turn or multi-turn configuration. For gigahertz-speed filtering applications, you should stay with a straight-through-type (one turn) core. A good core will look like a long, skinny cylinder with a single hole through the central axis for the signal wire. This straight-through-type topology keeps the input and output circuits as far apart as possible.

The circuit in Figure 1 models the effect of a bead's parasitic capacitance. As frequency rises, the impedance of L1 rises while the impedance of CP falls. Beyond some crucial frequency, capacitor CP begins to short out inductor L1 . You will see this effect if you probe inductor L1 , mounted as it will be used in your actual layout, with a network analyzer. The effect of parasitic capacitance may not be included in the plots of impedance versus frequency that come from the manufacturer of the bead, because they don't know how you are going to lay it out.

Capacitor C2 also suffers from parasitic layout effects. Figure 1 models the inductance LP of this capacitor's physical layout in series with the capacitance C2 . As you go up in frequency, the impedance of C2 falls while the impedance of LP rises. Beyond some crucial frequency, inductor LP prevents C2 from acting as a good short to ground.

The filter ultimately fails at a frequency high enough that the extra series impedance of LP becomes comparable to the parasitic shunt impedance of CP .

All low-pass filters fall prey to parasitic effects. At extremes of frequency, the inductors all turn into capacitors and the capacitors into inductors, reversing the action of the filter. Beyond some threshold frequency, the filter no longer prevents noise from passing through. The response of such a circuit resembles a band-stop filter more than a low-pass filter.

For example, suppose L1 is a straight-through 50-nH bead with CP =0.1 pF. Let C2 be a 0.01-uF capacitor in an 0402 package with LP =0.5 nH. This combination attenuates noise more than 30 dB from about 50 MHz to 3 GHz. Beyond 3 GHz, it works progressively less well.

To extend the response you must either use better components (with better layouts) or use a multistage filter. A multistage filter couples a number of stages in series, with each stage covering only a limited range of frequencies. Two well-designed stages can band-stop twice the range (on a logarithmic frequency scale) of a single-stage design.

As with all filter designs, you should use a network analyzer to measure the results of your final filter architecture. Low-Q beads are not supposed to resonate, but you still might get a noticeable resonance near the low-pass cutoff frequency. It's worth checking.courtesy:sigcon.com/Pubs/edn/ferritebeads.htm


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Tue, 06/02/2009 - 09:48

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