US3750069A

US3750069A - Low reluctance inductor

Info

Publication number: US3750069A
Application number: US00228086A
Authority: US
Inventors: J Renskers
Original assignee: Coilcraft Inc
Current assignee: Coilcraft Inc
Priority date: 1972-02-22
Filing date: 1972-02-22
Publication date: 1973-07-31
Anticipated expiration: 1990-07-31

Abstract

A closed magnetic circuit, high inductance device characterized by minimum reluctance and low cost of manufacture.

Description

[111 3,750,069 [1451 July 31, 1973 United States Patent [1 1 Renskers LOW RELUCTANCE mnucronx 3 on 6 3 3 Mrickclroy et Howell ct a1......r. 4/1962 Tay1or............. 6/1928 FOREIGN PATENTS OR APPLICATIONS [73] Assignee:

221 Filed:

- 662,909 12/1951 Great 897,517 3/1945 France............ 1,564,556 7/1969 Germany [21] Appl. No.2 228,086

Prima ry Exqminer-Thomas J. Kozma Attorney-William F. Gradolph and Howard H.

24 3 P-9 mp cl w 31 2 2 6 9 33 1 83 6 6 3 3 3 3 3 [52] U.S. [51] Int. [58] Field of Search.........

Rogers C .m V e d C c n m c H d .m m wh mu U mm c .w a n g a m d e S .m c HA1 S T N m M s E M e mm e D E n N U Q U characterizedby minimum reluctance and low cost of manufacture.

1,815,380 7/1931 Porter et 336/212 X 3,500,274 3/1970 Malsuura ct 336/83 X 1 Claim, 8 Drawlng Figure Y I l LOW RELUCTANCE INDUCTOR BACKGROUND OF THE INVENTION Field of the Invention As inductors are shrunk to meet the needs of microcircuitry. it is desirable to make the magnetic circuit of" an inductor as perfect as possible, or in other words, to hold the reluctance to the least possiblefigure in order to obtain maximum inductive values. This, of course, is achieved by a reduction of air gaps to the least possible extent. As is true with virtually all electronic components, almost any desired'degree of perfection can be attained if there is no limit on the cost of the component. In consumer electronics notably, however, cost factors are of prime importance; the field of circuit element manufacture is highly competitive, as is the field of the appliances themselves, and volume sales of the appliances are a direct consequence of low cost in the circuit elements thereof.

SUMMARY OF THE INVENTION This invention contemplates an inductor including a core and a magnetically permeable shell wherein the magnetic circuit achieves an extremely low reluctance with the least possible air gap at very little cost.

BRIEF DESCRIPTION-OF THE DRAWINGS,

FIG. I is a perspective view of an inductor incorporating the present invention;

FIG. 2 is an end elevation of the'inductor of FIG. 1;

FIG. 3 is a-section taken along the lirie-3-3 of FIG.

I 2 looking in the direction of the arrows;

FIG. 4 is a section taken along the line '4-4 of FIG. 2 looking in the direction of the arrows;

FIG. 5 is a bottom plan view of the inductor of FIG.

FIG. 6 is a section taken along the line 6--6 of FIG. 4 looking in the direction of the arrows;

FIG. 7 is a perspective of half of the inductor shell of FIG. 1 divided in an alternative fashion; and

FIG. 8 is a perspective similar to FIG. 7 showing a shell half formed for a different orientation of the center core. v

DESCRIPTION OF THE PREFERRED I EMBODIMENT The present invention is directed to a core andshell structure for an induction deviceeThe materi'al'of the structure is preferably a high resistivity, high permeability material such as a ferrite or powdered iron, although the shell may be ceramic where low inductive values are wanted. The induction device 10 includes a cylindrical core 12 and ashell '14 engaging the ends 16 of the cylindrical core and providing a cylindrical cavity 18 about the interior portion 20 of the cylindrical core to accommodate an induction or transformer winding 22 on the central portion'20 of the core 12. The shell is divided into two halves substantially on a plane 24 extending through the axis'ofthe core 12. In other words, the shell may be regarded as being longitudinally split with reference to the core 12. The division 24 separates the shell into a top half 26 and a bottom half 28.

The top half 26 is a plain block with a semicylindrical cavity 30 formed in the underside thereof leaving planar ends 32 having notches 34 in'the free, bottom edges dius to the cylindrical core 12.

The bottom half 28 likewise possesses a semicylindrical cavity 36 in the top surface-thereof leaving planar ends 38 with, again, slightly less than semicircular notches 40 formed'in the upper edge thereof to confront the notches 34 in the top half, and, with the "notches 34, to embrace the'ends 16 of the cylindrical core 12.

The bottom half 28 has downwardly protruding integral feet 42 at the comers of the bottom surface 44 thereof. The feet 42have transverse grooves46therein extending across the feet to the outside'e'dge of the bottom half28. Slots 48 are formed in the bottom of the semicylindrical cavity 36 at'each end-thereofto extend through the bottom shell and open between the end pairs of feet 42.

The bottoms of the feet 42rnay be coated with a conductive solder or conductivecementfor attachment to a hybrid 'microcircuit thick film" ceramic plate.

This invention will be best appreciated from some illustrative dimensions. A commercial model has been disigned'wherein the shell has alength 0.187'inches, a width of 0.125 inches and an overall height of about 0.125 inches. The core designed to be accommodated in the

notches

34 and 40 isequal inoverall length to the shell, that is 0. 187 inches, and has a diameter of 0.032 inches. The thickness of the end walls of the shell halvesis 0.035 inches. v

The inductive device illustrated willbe formed in the following fashion. The shell halves l0 and 28 will be molded and sintered or fired with the riotches'formed.

The core pins 12 will be molded and sintered and trued to the formed n'otch diameter by centerless grinding,

7 probably on a batch basis to conform to the lots of shell halves. Since the core extends to the externalsurface of theshelland is cylindrical and of uniform diameter over its length, it will be appreciated that its length is not critical as long asthe necessaryminimu'm is provided. 4

The notches and shell half edges are interrelated such that, in the dimensionally specified embodiment described, the center of the notcharc*is 0.00l of an inch outside the line of the facing edge such that if the two halves conform exactly to specification and'are assembled on opposite sides of a core 12, there will be a 0.002 separation between'the shell halves. The. tolerance assigned to the outward displacement of the center of the arc in' the above embodiment actually is i 0.001 inches. Thus, combined notches describing a complete circle is a limiting case, andin'practice', there is always a positive gap existing between the shell halves when closed on a core.

In view of the smallness of the unit, the coil or transformer will probably be wound directly upon the core 12 although, of course, there is always the possibility that the coil may be wound on atubular form to encompass the core. In view of the likelihood of direct winding, the extension of the core to the exterior of the lodge within the grooves 46 in the feet 42 ofthebottom half and soldered therewithin. The leads thus become conductively connected to the conductive'material deposited on the bottom of the feet but are recessed into the surface of the feet allowing the feet to make broad contact with the appropriate pads on the thick film for connection thereto.

A polymerizing cement such as epoxy or polyester will be applied to the facing edges of the shell halves to span the separation imposed by the core. The two shell halves are then pressed together to obtain a pressure bearing of the arcuate notches against the core 12 and the adhesive cured. The adhesive is preferably not applied in the area of the notches to avoid an adhesive film which necessarily has some thickness and thus enlarges the air gap to that extent.

An optional shell structure or method of making the shell is illustrated in FIG. 7. Here, the shell is verticallyinstead of horizontally divided. Thus, two identical shell halves 50 are employed to make up the whole shell 14. In this situation, the line of division separates the slots 48 into open-ended notches 52.

FIG. 8 shows a shell structure designed to have the center core thereof extending perpendicularly to the mounting structure. In this situation, the shell halves 56 have the feet 58 formed on the bottom end walls 60 and the divided slots for the passage of coil leads to the feet again present as notches 62 in that same bottom end wall.

Although the shell halves of FIGS. 7 and 8 might appear to be adaptable to a simple compression molding, in actuality, the feet present difficulties. Accordingly, the feet are best formed by molding ribs to extend to the parting line of the shell halves, firing, and grinding off the inner ends of the ribs to leave the outer ends to constitute the feet. The grinding operation, while an additional step in manufacture, is simple and nonprecise and, therefore, inexpensive.

The separation of the slots into the facing notches 52 or 62 may readily facilitate assembly of the complete inductor. Laying the leads of the winding of the inductor in open notches may in many cases be a simpler and quicker procedure than threading them through the slots 48 of the principal described form, and thus offset the added expense of the shell halves incurred from the grinding operation, above.

Some of the advantages of this invention have been set forth above. Others going directly to the matter of improved permeability of the whole magnetic circuit or minimal reluctance are first that the division of the shell into the two halves is axial or longitudinal with respect to the core rather than transverse to the axis thereof. Thus, the fact of the division of the shell into halves imposes no air gap reluctance in itself to the development of flux therewithin. The division is parallel to the lines of force.

Second, theformation of the arcuate notches to less than a semicircle provides for direct and positive contact between the shell halves and the cores to reduce the air gap to an absolute minimum. It will be appreci-v ated that this device as described interposes not even an adhesive film therebetween. The curingof the adhesive while the shell halves are held under pressure results in an effective frictional binding of the ends of the core between the shell halves.

Third, the containment of the ends of the cylindrical core between the notches in the shell halves results in a maximum of confronting surfaces of area of gap, so minimizing reluctance.

These advantages of this invention will be best understood by consideration of alternative forms presently employed. One of these is the center post type pot core closed either by a plate or by a second opposing similar pot core. This configuration has the advantage of only two air gaps in the magnetic circuit, as does the present invention. The confronting area of the center post air gap in either case is r r whereas with the contained core end of the present invention the area is 1r dh. In order to obtain the same gap area from the center post pot core configuration as is present in the specified inv ductor above, the center post would have to be four times the diameter of the center core of the present invention, or be equal in diameter to the given shell itself.

Other problems pertain. The center post pot core requires jig grinding to obtain true aligned surfaces which is expensive. Adhesive or dirt enlarges the width of the gap. Avoidance of adhesive requires an outside clamp, adding substantially to the cost.

A second existant option is a pair of facing cup cores with holes in. theirbottoms anda cylindrical center core contained in the holes to give the 1r dh. gap area. This structure, however, imposes first, a third air gap. Second, there is the conflict of tolerances. The minimum diameter of the holes must be at least equal to the maximum diameter of the center core, so the normal assembly will include. a gap of positive width between the core and holes.

I claim:

1. An inductive device comprising a brittle, sintered cylindrical core, a winding thereon, and a brittle, sintered shell having side wall, end walls and a cavity therewithin containing said core and enclosing said winding, said shell being longitudinally divided on a plane containing the axis of said core and having arcuate notches in the free edges of the end walls thereof conforming to and encompassing the cylindrical ends of said core, said notches being less than semicircular so that the facing edges of said shell parts are spaced narrowly apart when said core is contained in said notches, adhesive means spanning said facing edges of said shell parts to secure said shell parts together and in direct contact with said cylindrical ends of said core, said device being characterized by the absence of adhesive between said core and said notches.

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