Abstract:
A tape wound inductor core device, inductors including same and methods of manufacture. Tape wound material may be cut and/or shaped into “pucks” that have an exterior surface made up of or defined substantially by the edge surfaces of the layers of the constituent conductive material, with all or most of the broad surfaces disposed inwardly, thereby reducing eddy currents and associated losses. Various puck configurations, inductor arrangements and fabrication techniques are disclosed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electric inductors and, more specifically, to the configuration of a tape wound core proximate an air gap in the core of these inductors. 
     BACKGROUND OF THE INVENTION 
     Inductors are used in power converters to store energy in a magnetic field during one part of an operating cycle, and to return all or part of that energy during another part of the cycle. Such inductors are typically comprised of a winding on an easily magnetized or “ferromagnetic” core. One or more so-called “air gaps” in the core are usually required to maximize the energy which can be stored in the inductor. These air gaps may be ‘distributed’ throughout the core, in such materials as “powdered iron” type cores, or may consist of one or more ‘discrete’ air gaps in the core. The faces of a discrete air gap in an inductor are conventionally flat, parallel to each other, and at right angles to the surface of the core outside the air gap. 
     Various inductor core materials and configurations are known in the art. These materials include silicon-steel (Si-steel) in laminated or tape wound form, ferrite, and amorphous and nanocrystalline alloys in tape wound form, with benefits and drawbacks to each of these materials in various applications. The present invention applies to tape wound type inductor cores with one or more intentional discrete air gaps in the magnetic path, and with an alternating current (AC) in a conventional winding (not always shown in figures) on the core, and the resultant AC flux in the core. 
     The distinction between core laminations and tape is largely based on thickness and the method of assembly. Core laminations are relatively thick, typically greater than 0.1 mm, and are stacked or assembled flat. Core tape materials are generally somewhat thinner than 0.1 mm, and are typically wound around a suitable form or mandrel to provide the desired shape. Sections of wound tape cores may be cut out and reassembled to form new shapes, as noted in [1]. 
     The energy storage capability of an inductor is influenced significantly by the length of the air gap(s) in its core, there being an optimum air gap length at which the maximum core flux and winding current occur simultaneously, and where energy storage is at a maximum. A “fringe” flux field develops adjacent (but external) to such an air gap, extending from the surfaces of the core on one side of the gap to that of the other side. This fringe field is strongest at the edge of the air gap, and drops off approximately inversely with distance from the center of the air gap. 
     Referring to  FIG. 1A , a perspective view of a conventional inductor core  110  that illustrates this flux fringe field  150  is shown for one surface of the core. A problem associated with an AC flux fringe field is that, as noted in references [2], [3] and [4], at high frequencies and/or flux densities the fringe field  150  induces large eddy currents  117 ,  118  to flow on the broad surfaces  119 ,  120  of the tape core sections  112 ,  114 . These eddy currents induce losses in the core near the air gap, as illustrated by the shaded regions  122  and  132  in  FIG. 1B . These losses reduce the ability of the inductor to store and return energy at high frequencies, as the losses are proportional to the square of the induced eddy currents, and thus of both the AC flux density and the frequency. The overall result is a significantly lower allowable maximum power density (rate of energy storage and recovery) for the inductor before overheating occurs. (A similar fringe field enters the core on the edges of the tape, but this field does not induce excess eddy currents in the core.) 
     In the related field of Si-steel laminated core transformers, prior art attempts to reduce similar broad core surface eddy currents from the leakage flux field between primary and secondary windings entering the core are known. In this attempt, slots were made in the broad surfaces of the core laminations near the ends of the windings where the leakage flux would enter the core on the broad surface of the laminations. Application of this prior art technique to inductor cores is illustrated in  FIG. 2 , as taught by the inventor in [5], where slots  287  are cut into the broad surfaces of the laminated core sections  212 ,  214  near the air gap  226 . These slots  287  ‘break up’ the eddy currents, as shown by the eddy currents paths in phantom  227 ,  228 , at the ends of the illustrative flux line  261 , reducing their magnitude and the associated losses. 
     Disadvantageous aspects of this approach include that it is not readily ascertained how long, deep or frequent the slots should be, nor on how to make them. Another disadvantageous aspect is that it is difficult to cut or otherwise form slots in laminated or tape wound material without creating electrical shorts between the cut layers, which increase eddy current losses. 
     Another prior art approach to minimizing the fringe field losses in Si-steel laminated cores was developed for large “shunt reactors” used in the power transmission industry [6]. This is shown in  FIG. 3 . The accompanying description states that “The [lamination] sheets are stacked tightly together to form “wedge” sections, which are inserted into a circular base to create each core element. Radial lamination [sic] prevents fringing flux from entering the flat surfaces of core-steel, eliminating eddy current, overheating and hot spots.”  FIG. 3A  shows such an arrangement  300  of wedge sections  310  of laminations  315 , which become a cylindrical “core element”  320  ( FIG. 3B ), which are stacked with spacers to form a “gapped core”  340  ( FIG. 3C ). In  FIG. 3D , a complete inductor  350  is formed by adding winding  352  around the gapped core, and core field return yoke  355 . An enlargement of the gapped core near the edge ( FIG. 3E ) shows the fringe field flux, near the air gap, entering the core element  320  at the lamination  315  edges. 
     Disadvantageous aspects of this approach include that it is labor intensive, and thus expensive, and is not feasible for the thin tapes used in tape wound cores, which are on the order of 25 micron (or 0.001″) thick for amorphous and nanocrystalline tape materials. 
     A need thus exists to reduce fringe field induced losses in a tape wound inductor core and, furthermore, to do so in a manner that is practical, effective, and economical, and that provides consistent and predictable results. 
     Ferrite and Nanocrystalline 
     Ferrite is a well-known inductor core material and has been one of the principal core materials of choice for frequencies above about 5 to 10 kHz due to low hysteressis and eddy current losses. Modern nanocrystalline materials, however, have lower hysteressis losses than ferrites up to about 200 kHz and can operate with 1.6 times the ac flux at 40 kHz and twice the ac flux at 20 kHz for the same loss (based on published data). Furthermore, the nanocrystalline material&#39;s saturation flux density B SAT  is about 3 times that of ferrites at elevated temperatures of 80-100 degrees C. (1.2 Tesla v. 400 mT). Ferrite, on the other hand, has the advantage of being an isotropic ceramic material, and thus ferrite cores do not exhibit the excess eddy current losses near an air gap experienced by laminated and tape wound metallic core materials. 
     A need further exists to provide inductors of significantly smaller size, for example, by taking advantage of the properties of nanocrystalline material (or other similar materials yet to be developed) to improve the overall power densities of switching converters, particularly when inductor currents include DC or low frequency (e.g., 50 Hz or 60 Hz) AC currents significantly greater than the allowable high frequency AC ripple current. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to minimize or eliminate eddy current losses induced in tape wound cores by the flux fringe fields near a core air gap. 
     It is another object of the present invention to allow inductors of smaller size and/or lower mass to be produced due to the reduced eddy current losses induced by the fringe fields near core air gaps. 
     In one embodiment, the present invention may include an inductor core device of square, rectangular or similar cross section where the broad surface of core tape is not substantially exposed on the surface of the core. 
     In another embodiment, the present invention may include an inductor core device of round cross section where the broad surface of core tape is not substantially exposed on the surface of the core. 
     In other embodiments, the present invention may include an inductor core device of rectangular, hexagonal, octagonal or other desired cross section where the broad surface of core tape is not substantially exposed on the surface of the core. 
     These and related objects of the present invention are achieved by cutting sections from tape wound cores, which are reconfigured to leave the broad surface of the tape unexposed on the surface of the core. 
     The attainment of the foregoing and related advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the following more detailed description of the invention taken together with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is an illustration of the eddy currents induced in the broad surface of the laminations or tape in a core with an air gap. 
         FIG. 1B  is an illustration of the losses in a core due to induced eddy currents in the core. 
         FIG. 2  illustrates a prior art method of reducing the core eddy current losses adjacent to an air gap. 
         FIGS. 3A-3E  illustrates another prior art method for eliminating the core eddy current losses in laminations adjacent to an air gap. 
         FIGS. 4A-4D  is an embodiment of the present invention where a tape wound core is cut into segments and reconfigured as a square core to eliminate the eddy current losses in the core from the fringe field. 
         FIG. 5  illustrates how the reconfigured core embodiment of the invention of  FIG. 4  may be combined with tape wound “cut cores” to form a complete inductor core. 
         FIG. 6  illustrates how the reconfigured core embodiment of the invention of  FIG. 4  may be combined with other segments cut from tape wound cores to form a complete inductor core. 
         FIGS. 7A-7E  is an embodiment of the present invention where a tape wound core is cut into segments and reconfigured as a round core to eliminate the eddy current losses in the core from the fringe field. 
         FIGS. 8A-8B  illustrates how a reconfigured square core may be cut into smaller sections. 
         FIGS. 9A-9D  illustrates how multiple reconfigured cores may be combined to form a larger core. 
         FIGS. 10A-10E  illustrates how a reconfigured core may be cut directly from a tape wound core without reassembly. 
         FIGS. 11A-11E  are a perspective view of a tape core bar and four lateral cross-sectional views of cut/machined core bars, respectively, in accordance with the present invention. 
     
    
    
     DEFINITIONS 
     
         
         
           
             1) An “air gap” in a core is understood to be a non-magnetic portion of the core, which may consist partially or wholly of material other than air. 
             2) The “broad surface” of a core tape is the surface with the greater dimensions. 
             3) A core “puck” is a segment of reconfigured tape wound core used as part of an inductor core. 
             4) “Saw kerf” is the width of material removed by a saw in cutting, or by other means used to cut a tape wound core into two or more pieces. 
           
         
       
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1B , a side view of the air gap near one external surface of a conventional core  110  in a tape wound or laminated inductor is shown. Inductor core  110  includes a first and second section  112 ,  114  separated by an air gap  116 . Each core section is preferably formed of alternating layers of conductive ferromagnetic and relatively thin insulative material. The conductive ferromagnetic layers  111  are shown. It is to be understood that insulating layers separate each of the conductive ferromagnetic layers to minimize eddy currents within the core itself. It is also to be understood that the conductive nature of the ferromagnetic material is an undesirable but currently unavoidable property of such materials, without which eddy current losses in the core would not be a concern. 
     In use, a magnetic field is produced across air gap  116  and a fringe field  150  develops near the ends of the gap. The arced lines  151  indicate the direction of this field and their increased spacing indicates a weakening of the field away from the gap. Referring back to  FIG. 1A , this field forms eddy currents  117 ,  118  in the broad surfaces  119 ,  120  of the outer tape or lamination on each of the core sections  112 ,  114  as noted above. The eddy current in turn produces localized heating in the core sections  112 ,  114  as indicated by shaded areas  122 ,  132  in  FIG. 1B . This heating is greatest at the corners  113 ,  115 , decreasing essentially as the inverse square of the distance from the center of the air gap  116 . Thus it is most important to minimize the induced eddy current losses in the core proximate to an air gap, typically for distances removed from the air gap of several times the length of the air gap. 
     As described above, this eddy current is disadvantageous in that it reduces the strength of the magnetic field obtainable across the gap for an allowable total power dissipation or temperature rise, and hence the ability of the inductor to store and return energy at a high rate. 
     Referring to  FIGS. 4A-4D , steps in accordance with a preferred embodiment of the invention for reconfiguring a tape wound core to minimize eddy currents near an air gap are shown. In  FIG. 4A , tape wound core  410  is made with straight segments  400 , with the edges of the tape  415  shown for orientation. In  FIG. 4B , bars  420  are cut from the straight segments  400 , each bar ideally twice as wide as it is thick (neglecting the width of any “saw kerf”).  FIG. 4C  shows how the bars are cut or sliced longitudinally at  45  degrees to the tape surface to create right triangle shaped segments  430  and  440 . In  FIG. 4D , the two smaller triangular segments  430  and the one larger triangular segment  440  are reconfigured to create a square cross section bar  450 , where only core tape edges are exposed at the surface of the bar. The triangular segments may be joined with an epoxy adhesive or other means. 
     This reconfigured tape wound core  450  can be incorporated into complete inductor cores in numerous ways, two of which are shown in  FIGS. 5 and 6 . In  FIG. 5 , the bar is cut into four smaller square “pucks”  510  (or four pucks are cut from the bar) which are stacked with three air gaps  516  between them, and then installed into tape wound cut cores  530  to form the complete inductor core  500 . In practice, two or more pucks  510  may be utilized in the stack, with an air gap  516  between each pair of pucks. A winding (not shown for clarity of the core construction) is then placed around stack of core pucks. 
     In  FIG. 6 , the bar  450  of  FIG. 4  is cut into at least eight pucks  610 , from which two stacks are fabricated with air gaps  616 , and assembled with two tape core bars  630  as shown to form the complete inductor core  600 . In practice, two or more pucks  610  may be utilized in each stack, with an air gap  616  between each pair of pucks. A winding (not shown for clarity of the core construction) is then placed around the stacks of core pucks. 
     In both  FIGS. 5 and 6 , the cut cores  530  and the core bars  630 , respectively, may have a “coupling face” or “edge surface” to which the pucks may be directly coupled. These coupling faces or edge surfaces (obscured from view by the top most or bottom most pucks) are preferably defined or made up of the edge surfaces of the layers that form the cut cores  530  or cure bars  630 . While the top most or bottom most pucks are preferably coupled directly to the coupling faces, they may be spaced by a gap without departing from the present invention. The cut cores  530  or core bars  630  or the like may be regarded as supplemental core members. 
     Referring to  FIGS. 7A-7E , steps in accordance with another preferred embodiment of the invention for reconfiguring a tape wound core are shown. In  FIG. 7A , tape core bars  720  similar to bars  420  in  FIG. 4  are shown. In  FIG. 7B , each bar is cut or sliced longitudinally into four right triangle shaped segments  730  of the same size. (Alternatively, a square bar similar to  720  can be cut into two right triangle shaped segments, or a wider bar may be cut into six or more triangular segments.) Eight of these segments  730  are then reconfigured into a square section  740  as shown in  FIG. 7C ; the triangular segments may be joined with an epoxy adhesive or other means. The square core section  740  may be cut into core pucks for assembly into complete inductor cores, as illustrated in  FIGS. 5 and 6 , or it may be further processed into other shapes. 
     In  FIG. 7D , the square section  740  is machined into a round cross section bar  750 , where only core tape edges are exposed at the surface of the bar. In  FIG. 7E , the square bar  740  is cut into an octagonal bar  760 . It should be obvious that the bar  740  may also be machined into nearly any other shape desired, such as hexagonal or oval (not shown), while retaining the benefit of only core tape edges exposed at the surface of the reconfigured bar. 
     Referring to  FIGS. 8A-8B , further steps in accordance with another preferred embodiment of the invention for reconfiguring a tape wound core are shown. In  FIG. 8A , a reconfigured tape core bar  850  similar to  420  in  FIG. 4  is shown. In  FIG. 8B , the bar  850  is further cut or sliced to form a rectangular bar  860 , with the removed material  865  shown in phantom. The bar  850  may be cut into other than rectangular shapes. 
     Referring to  FIGS. 9A-9D , further steps in accordance with another preferred embodiment of the invention for reconfiguring a tape wound core are shown. In  FIG. 9A , a reconfigured tape core bar  950  similar to  420  in  FIG. 4  is shown. In  FIG. 9B , two square bars  950  are assembled side-by-side to form a rectangular bar  960  of 2:1 aspect ratio and, in  FIG. 9C , three such bars  950  are assembled into a rectangular bar  970  with a 3:1 aspect ratio. In  FIG. 9D , for bars  950  are assembled into a larger square bar  980 . Of course, larger bars may be assembled with any number of rows and columns of smaller bars  950 . 
     Cutting a tape core bar into the said right triangle shapes generally involves the least amount of waste material, but other triangle shapes, such as equilateral triangles, may also be cut from a tape core bar and reconfigured into desired shapes without departing from the present invention. 
     Referring to  FIGS. 10A-10E , steps in accordance with another preferred embodiment of the invention for reconfiguring a tape wound core are shown. In  FIG. 10A , tape core bar  1020  similar to bars  420  in  FIG. 4  are shown. In  FIG. 10B , the bar is cut or sliced longitudinally to remove four triangle shaped segments  1030 , leaving in this example a square core section  1040 , shown in  FIG. 10C . (Alternatively, the core material in segments  1030  may be ground away or removed by other means.) The square core section  1040  may then be cut into core pucks for assembly into complete inductor cores, as illustrated in  FIGS. 5 and 6 . 
     The tape core bar  1020  may also be cut, sliced or ground to produce cores of other lateral cross-sectional shapes. In  FIG. 10D , the core bar  1020  is beveled longitudinally to expose the core tape edges in core  1050 . In  FIG. 10E , the bevel is increased to produce a hexagonal cross section core  1060 . A core with a round cross section may also be produced. These bar segments  1040 ,  1050 ,  1060  may then be cut into core pucks like the pucks  510  and  610  of  FIGS. 5 and 6 . 
     Referring to  FIGS. 11A-11E , a perspective view of a tape core bar  1120  and then four lateral cross-sectional views of cut/machined core bars in accordance with the present invention are respectively shown.  FIG. 11A  illustrates an initial tape core bar  1120  similar to bar  1020  of  FIG. 10A . This bar may be longitudinally cut, sliced or otherwise machined to remove the corners and produce the cross-sectional shapes illustrated in  FIGS. 11B-11E  or related shapes. While  FIGS. 10B-10E  illustrate bars (and resultant pucks) with exterior surfaces defined wholly or nearly wholly by the edges of the conductive layers, the bars/pucks of  FIGS. 11B-11D  illustrate a portion of the broad surface retained on the exterior surface.  FIG. 11E  illustrates a substantially round cross section. 
     For example, referring to  FIG. 11B  (bar  1140 ), if D 1  is the overall width of the bar, then D 2 , the length of the largest remaining broad surface, is shown as being approximately 25% or less of D 1 . Referring to  FIG. 11C , D 2  is larger, tending towards 30-40% or less of D 1  (bar  1150 ) and, in  FIG. 11D , D 2  is larger still, tending towards 50-60% or less of D 1  (bar  1160 ). While removing nearly all the broad surface (e.g.,  FIGS. 10A-10E ) provides significantly enhanced performance, removing less than all of the broad surface as taught with reference to FIGS.  11 B- 11 D improves performance over a conventional fuller broad surface. 
     Any of the many conventional metal-working methods might be used in cutting and shaping the cores in the current invention, including but not limited to milling, grinding, sanding, sawing, laser cutting and water jet cutting. Some of these methods may require secondary operations such as lapping and polishing to obtain a requisite smooth surface, and a final etching process may be required if primary or secondary shaping operations produce significant electrical short circuits between lamination or tape layers. 
     It will also be understood that the invention can be applied to inductor cores in more complex magnetic structures, including ‘hybrid’ or ‘integrated’ structures of one or more transformers and inductors. These structures include the so-called “flyback” transformer, where the transformer core contains one or more air gaps to increase energy stored in the magnetic field, effectively placing an inductance in parallel with the transformer windings. Also included are “high leakage inductance” transformers where a ferromagnetic core, with one or more air gaps, is placed between a primary and secondary winding. 
     While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as may be applied to the essential features herein before set forth, and as fall within the scope of the invention and the limits of the appended claims. 
     REFERENCES 
     
         
         [1] Extract from Hill Technical Sales Corp. brochure, available at: 
       
    
     www.hilltech.com/products/emc_components/Amorphous_Shielding.html
     [2] Extract from ‘Filter Inductor Design’ by Ruben Lee.   [3] Extract from “Design of Powder Core Inductors” by Hakan Skarrie   [4] “Effect of Eddy Current in the Laminations on the Magnet Field”, Y. Chung and J. Galayda, Argonne National Laboratory, Argonne, Ill. 60439, LS Note No. 200, April, 1992   [5] Extract from “High Frequency Conductor Losses in Switchmode Magnetics”, B. Carsten, Seminar presented for EJ Bloom Associates, Inc., and other venues.   [6] Extract from presentation on Shunt Reactors&#39; “Shunt.1ZSE954001EN-11.pdf” by ABB Power Transmission