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Also shown is the resistivity of conventional high frequency magnetic materials as function of frequency. Shown is a cross-sectional view. This arrangement is used in one of the methods of making new high frequency, low loss composite material in which the stack is dipped in a molten or liquid insulating material. Alternately, a molten or insulating material is poured on the stack so that a thin film of the insulating material is formed between the adjacent magnetic plates.

Magnetic plates 20 comprise low hysteresis loss magnetic material, such as MnZn ferrites with hysteresis loss densities of 0. The thickness of magnetic plates 10 would be chosen such that the eddy currents flowing within a given magnetic plate 10 are not significant. For operation in the frequency range of MHz, a typical thickness of magnetic plates 10 is on the order of 0. Insulating films 20 comprise a material such as Kapton polyamide film manufactured by E.

A typical thickness for insulating films 20 is on the order of 10 to 30 microns. Insulating films 20 have high resistivity, high dielectric strength and, preferably, low dielectric constant.

High-Frequency Magnetic Components, 2nd Edition -- Marian K. Kazimierczuk -ѕ©¶«ФД¶Б-ФЪПЯФД¶Б

Insulating films 20 perform the same function in controlling the bulk eddy currents as that of insulating grain boundary layer in a typical state-of-the-art, high frequency magnetic materials, such as a ferrite, but are able to maintain their functionality to much higher frequencies. Typical range of operation of the present invention is MHz, although, advantageously, these materials can also be used in lower frequency ranges e.

Advantageously, the relative thickness of insulating films [] 20 relative to magnetic plates 10 is very large, when compared to the thickness of magnetic grains of the grain boundary layer in a typical conventional high frequency magnetic material.


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A typical relative thickness of insulating films 20 of the present invention is of the order of 0. This very large relative thickness of insulating films 20 results in a very large reduction in the overall capacitance and therefore, the material is able to retain high resistivity up to very high frequencies. As shown in the graph of FIG. In contrast, the resistivity of the current state-of-the-art high frequency materials such as a high frequency ferrite begins to drop at kHz range and continues to decrease with increasing frequencies in kHz to 10 MHz range.

The high resistivity of the new materials eliminates bulk eddy current loss even at very high frequencies, thus allowing the new material to be used at high frequencies. Advantageously, the problem of dielectric break down does not occur in the material made according to the present invention. This relatively large thickness means that the electric fields induced by the rapidly changing magnetic fields would be orders of magnitude smaller than those seen at the grain boundary in a conventional high frequency magnetic material.

Thus, the problem of dielectric breakdown does not occur in the materials made according to the present invention and the materials are able to maintain high resistivity up to very high flux levels and high frequencies. This allows the new materials to be operated at high flux levels and at high frequencies. Additionally, advantageously, these modern insulating materials can have extremely high resistivities, which results in much higher resistivities of the composite high frequency material of the present invention. This is shown in FIG.

The choice of insulating materials used at the grain boundary of the conventional high frequency magnetic material is very limited.

Typically only SiO [] 2 or CaO can be used in conventional material. Materials made according to present invention enable the use of modern insulating materials for making insulating films Modern materials, such as the polyamide Kapton films made by E. The use of such materials further eliminates the problem of dielectric breakdown at high flux levels and, advantageously, allows the material to be driven to high flux levels at high frequencies.

Advantageously, the structure of the composite, high frequency material of the present invention enables independent and simultaneous control and reduction of the high frequency eddy current loss and the hysteresis loss. It is very difficult to obtain such control in other high frequency materials. In particular, the resistivity of the magnetic material of magnetic plates [] 10 can be chosen to be relatively low.

A typical high frequency resistivity of the magnetic material of the magnetic plates 10 is on the order of 0. This is because the eddy current loss in the present invention is controlled independently by the insulating films 20 and by the thickness of the magnetic plates 10 and a high resistivity of the magnetic material of plates 10 is not needed. The very lax requirements on the resistivity of the magnetic materials of plates 10 allows the grains of the magnetic material to be made relatively large, thereby reducing the surface area of the grain boundaries and making it possible to achieve very low hysteresis losses.

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Advantageously, the thickness of magnetic plates [] 10 is typically an order of magnitude smaller than the size of a typical high frequency core of a magnetic component and, therefore, the problem of dimensional resonance is avoided up to much higher frequencies. In particular, the magnetic components made with the composite high frequency, low loss material of the present invention is able to function properly in the MHz range. This also allows the material made according to present invention to be used in very high power cores. Such an embodiment is suitable, for organic insulating films 10 , such as Kapton polyamide manufactured by E.

Suitable materials for the adhesive films comprise various fluorocarbon resins, such as Teflon, which is also manufactured by duPont. This embodiment is suitable for the manufacture of rounded core pieces, such as the center post and the outer walls of a pot core, RM core or PQ core. Such an embodiment is suitable for making the center post of a pot core or a RM core or PQ core.

Trends In High Frequency Magnetics Part 4 Circuit Design

There are many methods of making the composite, high frequency material of the present invention. One method involves separately preparing the magnetic plates [] 10 by either pressing a low hysteresis loss material in plate shapes prior to firing, or by machining the magnetic plates 10 from a pre-fired block of low hysteresis loss magnetic material.

After making the magnetic plates, an adhesive 30 is applied to the both sides of the insulating films 20 or the magnetic plates 10 , or to both, and a stack is built of alternating magnetic plates 10 and insulating films Finally mechanical pressure or heat or both are applied to the stack to bond the magnetic plates 10 and the insulating plates Such a method is suitable for use when the insulating film 10 is made of a polymer or polyamide such as Kapton films made by E.

A further method of making the composite, high frequency magnetic material of the present invention comprises separately making magnetic plates [] 10 by either a pressing a low Hysteresis loss material in plate shapes prior to firing and then firing, or b by machining the magnetic plates 20 from a block of pre-fired low Hysteresis loss material, disposing a small amount of an insulating material on at least one side of the magnetic plates 10 , stacking the plates on top of each other, applying heat and pressure to melt or soften the insulating material, and cooling the stack so that thin insulating films 20 are formed, and provide bonding, between adjacent magnetic plates The insulating material can be disposed in powder form or through thick or thin film processes.

This method is suitable when the insulating material is a glass silicon dioxide or a low melting temperature glass. Yet another method of making the composite, high frequency magnetic material of the present invention comprise co-firing magnetic plates [] 10 and the insulating films This method is suitable when the insulating material comprises a ceramic such as aluminum oxide, aluminum nitride or beryllium oxide.

Such materials have firing conditions similar to those of high frequency magnetic materials and can be fired along with the high frequency magnetic plates They are used mostly for electromagnetic interference filters and low-frequency chokes, mainly in switched-mode power supplies. Very low core loss, low hysteresis and therefore low signal distortion. Very good temperature stability.

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High cost. Maximum saturation flux of about 0. Used in high-Q filters, resonant circuits, loading coils, transformers, chokes, etc.

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The material was first introduced in , used in loading coils to compensate capacitance in long telephone lines. Underground lines, where temperature is more stable, tend to use ferrite cores due to their lower cost. High energy storage, saturation flux density of about 1. Residual flux density near zero. Used in applications with high DC current bias line noise filters, or inductors in switching regulators or where low residual flux density is needed e.

Core losses higher than MPP. Very low magnetostriction , makes low audio noise. Loses inductance with increasing temperature, unlike the other materials; can be exploited by combining with other materials as a composite core, for temperature compensation. Saturation flux of about 1 tesla. Good temperature stability. Used in switching power supplies, pulse and flyback transformers, in-line noise filters, swing chokes, and in filters in phase-fired controllers e.

Absence of nickel results in easier processing of the material and its lower cost than both high-flux and MPP. The material was invented in Japan in A nanocrystalline alloy of a standard iron-boron-silicon alloy, with addition of smaller amounts of copper and niobium. The grain size of the powder reaches down to nanometers. The material has very good performance at lower frequencies.


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It is used in chokes for inverters and in high power applications. It is available under names like e. Nanoperm, Vitroperm, Hitperm and Finemet. Ferrite ceramics are used for high-frequency applications. The ferrite materials can be engineered with a wide range of parameters. As ceramics, they are essentially insulators, which prevents eddy currents, although losses such as hysteresis losses can still occur.

A coil not containing a magnetic core is called an air core. This includes coils wound on a plastic or ceramic form in addition to those made of stiff wire that are self-supporting and have air inside them. Air core coils generally have a much lower inductance than similarly sized ferromagnetic core coils, but are used in radio frequency circuits to prevent energy losses called core losses that occur in magnetic cores.

The absence of normal core losses permits a higher Q factor , so air core coils are used in high frequency resonant circuits , such as up to a few megahertz. However, losses such as proximity effect and dielectric losses are still present.

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High-Frequency Magnetic Components, 2nd Edition

Air cores are also used when field strengths above around 2 Tesla are required as they are not subject to saturation. Most commonly made of ferrite or powdered iron, and used in radios especially for tuning an inductor. The coil is wound around the rod, or a coil form with the rod inside. Moving the rod in or out of the coil changes the flux through the coil, and can be used to adjust the inductance. Often the rod is threaded to allow adjustment with a screwdriver. In radio circuits, a blob of wax or resin is used once the inductor has been tuned to prevent the core from moving.