US3448421A

US3448421A - Shielded magnetic core

Info

Publication number: US3448421A
Application number: US657327A
Authority: US
Inventors: Robert S Berg; Bradford Howland
Original assignee: Massachusetts Institute of Technology
Current assignee: Massachusetts Institute of Technology
Priority date: 1967-07-31
Filing date: 1967-07-31
Publication date: 1969-06-03
Anticipated expiration: 1986-06-03

Description

June 3, 1969 R BERG ET AL 3,448,421

SHIELDED MAGNETIC CORE Original Filed June 7, 1962 s 9 g 7 K 2 6 7 vi N F x 513:

FIG. I g, 3

55 7 2 3 v 4 M W 0:.

FIG. 2

7 l7 rL I \Q\\V 2 3 1 I5 ML FIG. 5

INVENTCRS.

BRADFORD HOWLAND BY ROBERT S. BERG United States Patent US. Cl. 336-84 8 Claims ABSTRACT OF THE DISCLOSURE A low leakage flux magnetic core suitable for use as the core of an inductor or transformer is described. A ferrite core has deposited on it two electrically conductive layers which are separated by an electrically insulating layer of plastic. Each conductive layer has a continuous insulating gap which is parallel to the flux path in the core with the gaps in each layer being on opposite sides of the core from each other.

The invention herein described was made in the course of work performed under a contract with the Electronic Systems Division, Air Force Systems Command.

This application is a division of our co-pending application No. 200,748, filed June 7, 1962, now Patent No. 3,336,662, for Shielding a Magnetic Core.

This invention relates to a low leakage-inductance transformer and in particular to a high frequency transformer with a magnetic core shielded from the windings by a chemically and electrically deposited electrostatic shield.

A transformer capable of operating over a frequency band of approximately 1 to- 100 megacycles finds many applications in electrical circuits which cover this frequency range. Satisfactory frequency response has been obtained by transformers constructed in accordance with the invention of this application in hybrid junction or magic T circuits where a turns ratio approximating 2:1 is required. Distributed transformer techniques, while providing the necessary bandwidth were not used because of the difliculty in getting the required turns ratio. The use of the eddy-current shield of the present invention reduces the leakage inductance by an order of magnitude with consequent improvement in the high frequency response of a transformer Wound by conventional techniques.

Prior art transformers have used eddy current shielding to reduce leakage inductance. One type of eddy current shielding which is used on a toroidal core consists of a copper screen having a circumferential gap along the inner surface of the toroid. Such a shield is discussed in an article in the Wireless Engineer, June, 1947, pp. 175-176 where it is stated that the windings of the transformer may be placed outside the screen. The general outline of such a shield is similar to that shown in FIGURE 3 of this application. Another type of shield which is described in the General Radio Experimenter, vol. XXX, No. 11, April 1956, consists of two copper toroidal cups of different diameters which slip over a toroidal magnetic core to form a shield similar to that of FIGURE 5 of this application. Neither of these techniques are suitable for application to magnetic cores especially suited for high frequency transformers. These cores are physically of small dimensions since transformers in general improve in high frequency performance as the size decreases. Since space for the transformer windings is at a premium, further reduction of this winding space by eddy current shielding must be kept to a minimum. The copper screen shield is 3,448,421 Patented June 3, 1969 bulky and not suitable for small cores. The copper cups are diflicult to fabricate in small sizes and with the thin walls required to avoid occupying a large portion of the center hole winding space of the toroidal core. In addition, copper cups are restricted to use with toroidal cores since fabrication of the cups for other shapes by machining techniques would be diflicult.

Accordingly, it is an object of this invention to provide a means for eddy current shielding of small magnetic cores which is easy to fabricate and which reduces the available winding space by a negligible degree.

It is a further object of this invention to provide an eddy current shield which may be easily fabricated on cores of shape other than toroidal.

These objects are obtained in the present invention by chemical and electrical deposition of a conducting surface over the entire core and by removing said conductor along a circumferential path to avoid a short circuited turn. A second conducting surface applied on a nonconductive coating on said first conducting surface and similarly modified to avoid a short circuited turn will provide additional reduction in leakage inductance.

The novel features of the invention together with further objects and advantages thereof will become apparent from the following description taken in connection with the accompanying drawings wherein:

FIGURE 1 shows in cross section a single layer conductive material eddy current shield on a toroidal core.

FIGURE 2 shows in cross section a two layer shield.

FIGURE 3 shows in cross section another single layer shield with a gap on a circumference of the core.

FIGURE 4 shows in cross section the preferred embodiment of the two layer shield.

FIGURE 5 shows in cross section a multiaperture core with windings on the legs thereof.

FIGURES 1 through 4 show in cross section a toroidal core 1 on which a copper plated shield 2 has been deposited. Copper shield 2 serves as the eddy current shield which is of a shape inconvenient for direct mechanical fabrication and is most easily formed by electroplating directly on the core. The purpose of shield 2 is to provide an eddy current barrier to a changing magnetic flux with a component normal to shield 2. This barrier prevents flux from leaving core 1 and looping around either coil 6 or 7 before reentering the core 1, thereby producing a leakage inductance in whichever coil is looped. It is apparent that shield 2 cannot be perfectly effective since it must be interrupted along some circumference to avoid a shorted turn. One way of effecting this interruption is shown in FIGURE 1 where a counter-sink has been used to remove the plating at the inside corner 5 of the core 1. Care must be exercised in controlling the amount of material removed from corner 5 lest too little material removal causes an inadvertent short circuit, while too much removal will cause an excessive amount of flux to escape from the core 1 through gap 9, thereby increasing the leakage inductance of either or both coils 6 and 7. Another difficulty with the removal of material from a corner of the core is that the core must be sufliciently uniform in inside diameter and thickness that material is removed uniformly from the corner 5 of the core 1.

FIGURE 2 is a cross section of a toroidal magnetic core 1 having a two layer shield consisting of

shields

2 and 4 separated by a nonconductive material 3. The short circuited turn produced by shield 4 is interrupted by using a countersink to remove enough of the shield at corner 8 to form gap 10 in the same manner as the gap 9 at corner 5. In FIGURE 2,

corners

5 and 8 are chosen as the inside corners of core 1. To reduce leakage flux to a minimum, the gaps 9 and 10 should be at opposite corners although for convenience of fabrication, both inside or both outside corners may be used.

FIGURE 3 shows a core 1 in which the copper shield 2 is interrupted by a circumferential cut 11 made by a fine slitting saw. The difiiculty of controlling the depth of the saw cut 11 makes this embodiment of the invention somewhat less desirable than FIGURE 1 for a two layer shield configuration. The width of the saw cut 11 should narrow to minimize flux leakage but for small sized cores, available saws make a wider cut than is desirable.

FIGURES l and 3 are about equally effective, giving a four fold reduction of leakage inductance for the case of oppositely disposed primary 6 and secondary 7 windings on a high permeabiilty toroidal core 1 as compared to the case where no shield 2 is used.

FIGURE 4 shows in cross section the preferred embodiment of this invention. A toroidal core 1 with interlocking separated

shields

2 and 4 is shown. The magnetic reluctance of the flux leakage path in FIGURE 4 is considerably greater than in FIGURES 1 and 3. An eightfold reduction of leakage inductance has been obtained with the construction of FIGURE 4 as compared to a core with no shielding. The construction of FIGURE 4 has the further advantage that no precision machining operations are required, and there is little possibility of a short circuit causing a shorted turn.

Magnetic core 1 is a ferrite core suitable for operation at high frequencies. Typically, successful transformers have been constructed using ferrites manufactured by the General Ceramics Company in toroidal core size CF-l02 (approximately OD. x /6" I.D. x /s), tumbled to remove sharp edges. The ferrites Q-l and Q-2 have high bulk resistivity and may be copper plated directly; type H is of lower resistivity, and for best results is coated with a thin film (3-5 mils) of a nonconductive plastic of good dielectric properties before copper plating. An acrylic type plastic coating has been found to work satisfactorily. The plastic may be applied by any process suited to the particular plastic used which will produce a thin, relatively uniform coating free of pin holes and preferably free of high spots. Spraying of this plastic has been found to produce satisfactory results.

The copper plating shields 2 and 4 are applied by a chemical deposition technique followed by conventional electroplating to the desired thickness, 4-5 mils in the case of the shield 2 of FIGURES 1 and 3 and 2-3 mils for the

shields

2 and 4 of FIGURE 4. A suitable deposition technique is described in the book Metallizing of Plastics by Narcus, pp. 14-39, Reinhold Publishing Corp., 1960.

In the construction of the embodiment of FIGURE 4, the first copper plating shield 2 is applied over a plastic coating (not shown) if required by the core 1 resistivity, otherwise directly on the core. The shield 2 is removed from surface 12 of core 1 by rubbing surface 12 on fine emery paper, being careful to remove sufficient copper plating to avoid a short circuited turn. Next, a layer of -7 mils of plastic coating 3 is applied as described previously. This is followed by a second copper plating of 2-3 mils thickness over the entire plastic coating 3. The plating is removed from surface 13, which is opposite to surface 12, by rubbing surface 13 on fine emery paper. Care should be exercised in this last operation to remove sufiicientplating to avoid a short circuited turn while not removing so much material that plating shield 2 is removed also. Windings 6 and 7 may then be wound on the completed core assembly to produce a transformer.

Although the embodiment shown in FIGURE 4 shows a core 1 of a toroidal form, the same technique for shielding has been successfully applied to transformers where a three-legged core has been used, shown in cross section in FIGURE 5. The third leg 14 of the core 1 is in the same plane as the legs 15, 16 of core 1 and of the same thickness. Thus, the copper plating may be removed from the

faces

12 and 13 in the same manner as described for the toroidal core of FIGURE 4. The

4

windings

6, 7, and 17, insulated from plating 4, are wound on legs 14, 15 and 16 to produce a transformer having low leakage inductance and good high frequency performance. The core of FIGURE 5 is approximately twice the size of the toroidal cores of FIGURES 1 through 4, but with about the same thickness.

Transformer construction with the cores of FIGURES 1 through 5 consists of winding the core 1 with Teflon or plastic insulated wires 6, 7 (and 17 in FIGURE 5) of thickness chosen to give the correct winding impedance. For most applications of these transformers, the outer plating shield 4, if a two layer shield is used, will be grounded to minimize electrostatic coupling between the Windings 6, 7 (and 17 in FIGURE 5). The inner shield 2 of FIGURES 2, 4 and 5 being imperfectly ex posed to the windings and having large capacitance to the outer shield 4, need not be grounded.

In order to avoid severe loss of Q with certain ferrites, such as the types Q-l and Q-Z ferrites, following the plating and interrupting operations, it was found desirable to dry the cores for a day at room temperature in a vacuum dessicator after each plating and interrupting operation. It is believed that the Q of the ferrite cores was reduced because of moisture absorption which caused a large decrease in the high frequency bulk resistivity of these porous ferrite materials.

Transformers for wide bandwidth and high frequencies generally require very few turns and may be handwound to a variety of specifications without difficulty. Several single-layer windings spaced above a grounded, shielded core will exhibit effective electrosatic isolation together with close magnetic coupling. It is seen that the construction of this invention relegates most of the difficulties to the preparation of the cores, of which only a few types and sizes would normally be required.

Table I includes the performance specifications of a transformer intended for use in a balanced mixer circuit.

Freq. response (3 db down) 0.3 to 135 mc.

While there has been shown and described what is considered to be the preferred embodiment of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined in the appended claims.

What is claimed is:

1. A low leakage flux magetic circuit comprising a core having a continuous flux path, a first electrically conductive non-magnetic film in complete encompassing contact with said core except for a continuous gap parallel to the flux path in said core, a layer of electrically insulating plastic in contact with said film and said insulating gap, a second electrically conductive nonmagnetic film in complete encompassing contact with said plastic layer except for a continuous gap parallel to said flux path in. said core, the gap in said second film being on the opposite side of said core from the gap in said first film.

2. The magnetic circuit of claim 1 wherein each continuous gap lies in a substantially planar surface.

3. The magnetic circuit of claim 1 wherein said conducting films are a few thousandths of an inch thick.

4. The magnetic circuit of claim 1 wherein said core comprises a ferrite completely encompassed by an electrically non-conductive plastic.

5. The magnetic circuit of claim 1 wherein said core is a multi-apertured core having two or more continuous flux paths within said bore, said core having two planar diametric surfaces on opposite sides of said core, each of which contact all flux paths, the gap in said second film being in the planar surface on the opposite side of said core from the gap in said first film.

6. The apparatus of claim 1 comprising in addition a plurality of electrical conductors wound around said core on said second film to provide transformer windings having low leakage inductance.

7. The apparatus of claim 1 wherein said core is rectangular in cross-section transverse to the flux path and said insulating gaps have Widths which are as wide as the side of the rectangle at which the gap exists.

References Cited UNITED STATES PATENTS 2,724,108 11/1955 Hayes et a1. 33684 XR 3,032,729 5/1962 Fluegel 336-84 3,041,561 6/1962 Hannon et a1. 336-229 XR 3,149,296 9/1964 COX 336-229 XR LEWIS H. MYERS, Primary Examiner. T. J. KOZMA, Assistant Examiner.

U.S. Cl. X.R. 336-229