Abstract:
A high leakage inductance transformer core device, and method of forming same, that has a core made of tape wound material, at least one set of concentric primary and secondary windings, and at least one flux shunt between the primary and secondary windings which is also made of tape wound material. The transformer core and flux shunts are arranged so that the transformer has a low external magnetic field, and substantially no excess core losses due to principal core flux flowing from one part of the core structure to another through the broad surface of the core tape.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electromagnetic transformers used in power converters and, more specifically, to transformers with tape wound cores and a high leakage inductance between a primary and a secondary winding. 
     BACKGROUND OF THE INVENTION 
     Transformers are used for galvanic isolation between an input and an output, and/or to ‘transform’ the impedance; i.e., the ratio of voltage to current at a given power level. Such transformers typically consist of at least two coupled windings on a common ferromagnetic core, a nominal “primary” winding to which input power is conventionally applied, and a “secondary” winding which provides the output power. 
     Transformer Core Materials 
     Various transformer 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 high leakage inductance transformers with tape wound cores. 
     The distinction between core laminations and tape (also called “ribbon”) 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. 
     Tape wound cores may be used in the “as wound” state, but are often cut into two pieces (cut cores) for assembly with windings. “Bars” (or “bricks”) may also be cut from sections of a wound core, and core assemblies may be made from some combination of bars and/or cut cores. 
     Comparison of Ferrite and Nanocrystalline Tape Cores 
     Ferrite is a well-known transformer 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. Although amorphous cores have a somewhat higher saturation flux density, modern nanocrystalline materials have lower hysteressis losses, lower than ferrites up to about 200 kHz and can still 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). Other tape wound materials with superior properties may yet be developed. 
     A drawback to nanocrystalline (and other tape wound and laminated core) materials is that the losses are low only when flux flows along the direction of the tape surface; any significant flux which flows normal to the tape surface (e.g., between tape layers, or into the external broad surface of the tape) creates large eddy current losses in the core. Ferrite, on the other hand, has the advantage of being an isotropic ceramic material, allowing flux to flow in any direction in the core without excess losses. (Various “distributed gap” core materials, such as powdered iron, also have the isotropic advantages of ferrite, but their permeabilities are generally too low for most transformer applications.) 
     Transformer Leakage Inductance 
     All transformers have a finite leakage inductance between windings, which is due to the energy in the magnetic flux produced by a primary winding which is not coupled to a secondary winding. One manifestation of leakage inductance is that, if the secondary winding is “shorted out”, a finite inductance is still seen at the primary winding. In effect, the leakage inductance of a transformer is electrically equivalent to placing inductors in series with one or both of the transformer windings. 
     The relative magnitude of the leakage inductance of a transformer can be defined as the ratio of reactive power circulating in the leakage inductance divided by the output power, at the full rated output power of the transformer. This relative leakage impedance can also be expressed as X L /R, where X L  is the impedance of the leakage inductance, and R is the secondary load impedance, both viewed from the same winding. For most transformers this ratio is on the order of 2% to 10%, and is often considered a non-ideal and undesirable characteristic. 
     In other applications, however, the leakage inductance can be of considerable benefit. In power distribution transformers, it will limit the current under fault conditions, such as downed and shorted power lines. If the leakage impedance is 4%, for example, the fault current is limited to 25 times (1/0.04) the full rated load current, which limits the current that fuses or circuit breakers must interrupt. High leakage transformers are also used to limit or control output current in arc welders and gas tube illumination transformers. 
     In electronic power converters, a high leakage inductance may also be useful. In various “resonant” converters, the leakage inductance can form all or part of a resonant inductance in a circuit. Leakage inductance can also be used to aid in “soft switching” of converters, where energy stored in leakage inductance is used, for example, to bring the transistor voltage to zero before turn-on after another transistor turns off. 
     High Leakage Inductance Transformers 
     In many of these applications, however, the practical leakage inductance obtainable with conventional transformer designs is often less than that desired. Referring to  FIG. 1 , the prior art transformer  10  has ferromagnetic core  11 , with primary  12  and secondary  13  wound on the center leg of an E-E or E-I core (so called from the shape of the core pieces or segments). In this construction, the maximum practical leakage impedance may be on the order of 5% to 10%, whereas a leakage impedance of 50% to 100% or more may be required. 
     Another prior art transformer construction is shown in  FIG. 2 , where transformer  20  comprises primary and secondary windings  22  and  23  respectively, placed on the outer legs of a so called U-U or C-C core. This construction has several benefits, including more winding cooling area and lower high frequency losses, but the leakage impedance is about half of that of  FIG. 1 . 
     A prior art transformer construction with higher leakage inductance is shown in  FIG. 3 , where transformer  30  has primary  32  and secondary  33  wound side-by-side on core  31 . This construction may double or triple the leakage impedance over that of transformer  10  in  FIG. 1 , but this is still inadequate for many high leakage applications. 
     A prior art construction with relatively high leakage inductance is shown in  FIG. 4 , where transformer  40  has primary and secondary windings  42  and  43  placed on opposite legs of core  41 . The leakage inductance may be further increased with the construction of  FIG. 5 . This construction is similar to  FIG. 4 , with the primary and secondary windings  52  and  53  placed on the outer legs of core  51 . In this case, a “flux shunt”  54  with air gap  55  is added between windings  52  and  53 , which allows leakage impedances to be three to 10 times higher than even that of  FIG. 4 . 
     The transformers of  FIGS. 4 and 5  do have a major drawback in generating a large external magnetic “leakage” field, however, as illustrated in  FIG. 6 . Here transformer  60  again has primary and secondary windings  62  and  63  on outside legs of core  61 . This is easily seen if secondary  63  is shorted out. The voltage on a winding is proportional to the rate of change of internal magnetic flux, so a shorted winding, which has essentially zero voltage, must have essentially zero ac flux in the core beneath the winding. The shorted winding  63  thus “blocks” the core flux from the leg under winding  63 . This in a sense removes the winding and that part of the core from the magnetic structure, so they are shown in phantom lines in  FIG. 6 , and the core beneath shorted winding  63  becomes effectively a core air gap  65 . The magnetic flux produced by current in primary  62  must form a closed path, so the return flux forms a large external dipole magnetic field, illustrated by flux lines  69  (a similar field develops with the transformer of  FIG. 5 ). Secondary winding  63  need not be shorted out for this external field to develop; any load current flowing in winding  63  will cause an external field  69  proportional to the secondary current. Such external fields can cause severe electromagnetic interference (EMI) problems in higher frequency power converters, and is to be avoided. 
     A prior art high leakage transformer construction with reduced external field is shown in  FIG. 7 , where transformer  70  has primary and secondary windings  72  and  73  side-by-side, as in  FIG. 3 , but now flux shunts  74  are placed between the windings with air gaps  75  on each end of the flux shunts. This construction is very popular in many line frequency (50 Hz and 60 Hz) applications, including ferroresonant transformers. Drawbacks are a somewhat limited winding surface area for cooling, and higher eddy current (so called “skin and proximity effect”) losses in high frequency (HF) transformers. 
     An improved prior art construction is shown in  FIGS. 8A and 8B . In this figure, and in  FIGS. 9A and 9B , and  FIGS. 11A-11B  through  FIGS. 21A-21B , the “A” figure is a perspective view of the transformer core and flux shunts, without the windings for clarity. The “B” figures show the location of the primary and secondary windings in a cross section through the core. 
     The transformer  80  of  FIG. 8  is similar to that of  FIG. 1 , but now flux shunts  84 , with air gaps  85 , are placed between primary winding  82  and secondary  83 . This construction increases the winding cooling area and decreases HF eddy current losses, with less of an external field that the transformer of  FIG. 7 . 
     Prior art high leakage transformers have traditionally been constructed with either laminated cores (where the orientation of the laminations is shown as  56  in  FIGS. 5, and 76  in  FIG. 7 ) or isotropic materials such as ferrite which have no “orientation”, as illustrated in  FIGS. 4 and 8 . These constructions cannot be directly applied to tape wound cores, where the “as wound” orientation of the tape is at right angles to that of laminations. The problem this creates is illustrated in  FIG. 9 , where an attempt is made to realize the transformer of  FIG. 8  with a tape wound core. Here transformer  90  has a conventional tape wound core  91 , with additional flux shunts  94  (and air gaps  95 ) cut as “bars” or “bricks” from a tape wound core. Primary  92  and secondary  93  are arranged as in  FIG. 8 . Magnetic flux in the flux shunts  94  now flows into transformer core  91  where they join at  98 . This flux is normal to the surface of the tape in core  91 , and causes large eddy current losses in core  91  at those points. 
     Thus high leakage transformers with tape wound cores are desired which meet two objectives: a low magnetic field external to the transformer, and principal core flux which flows from one core segment to another along the direction of the tape; i.e., principal core flux does not flow normal to the tape surface. 
     One potential or seeming prior art approach to meeting the second objective is shown in  FIG. 10 , redrawn from [2]. This core is said to be made from “ . . . rectangular shapes of amorphous metal cores . . . ”, but windings are not shown, nor is a function for the core stated. It might be that the core is intended for high leakage transformers, as it resembles the core shown in  FIG. 5 , with transformer core  101  and possible flux shunt  104 , with the air gap  105  shown at one end of the flux shunt. However, this construction would still exhibit the same large external field as that of  FIG. 5 . 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide high leakage inductance transformers with tape wound cores, in which the principal flux in all parts of the core flows predominantly in directions parallel to the broad surface of the core tape. 
     It is another object of the present invention to develop high leakage transformers with minimal external magnetic field. 
     These objectives are accomplished by meeting three principle criteria:
     1) Primary and secondary windings are “concentric” (as defined below);   2) At least one flux shunt is placed between the primary and secondary windings, with the principal flux in the flux shunt returning though the transformer core;   3) Principal core flux flows from one core segment to another through tape edges, and not through the broad surface of the tape.   

     It is also desirable, but not essential to the invention, that air gaps in the flux shunt paths be relatively uniformly distributed to minimize fringe field losses in the windings. This can be realized as a single air gap near the center of the flux shunt(s), as in  FIGS. 11A and 13A , or with similar gaps at the ends of the flux shunt(s) as in  FIGS. 14A, 17A and 18A , or multiple air gaps distributed along the flux shunt(s) as in  FIG. 12A , where five air gaps are placed in each flux shunt. 
     These and related objects of the present invention are achieved by use of a high leakage transformer with tape wound core as described herein. 
     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. 1  is an illustration of a prior art transformer construction, with a single winding set on the center leg of a three leg core. 
         FIG. 2  is an illustration of a prior art transformer construction, with a dual winding set on the opposite legs of a two leg core. 
         FIG. 3  is an illustration of a prior art higher leakage transformer construction, with a primary and a secondary winding placed side-by-side on the center leg of a three leg core. 
         FIG. 4  is an illustration of a prior art high leakage transformer construction, with a primary and secondary windings placed separately on the opposite legs of a two leg core. 
         FIG. 5  is an illustration of a prior art high leakage transformer construction, with primary and secondary windings placed separately on the outer legs of a three leg core, where the third leg between the two windings forms a flux shunt to increase the leakage inductance. 
         FIG. 6  is an illustration of the external magnetic field which occurs when a current flows in the secondary of the high leakage transformer of  FIG. 4   
         FIG. 7  is an illustration of a prior art high leakage transformer construction, with a primary and a secondary winding placed side-by-side on the center leg of a three leg core, with flux shunts between the windings to increase the leakage inductance. 
         FIGS. 8A-8B  is an illustration of the core and winding arrangement of a prior art high leakage transformer without a significant external magnetic field. 
         FIGS. 9A-9B  is an illustration of a hypothetical tape wound core based on the construction of  FIG. 8 , with high eddy current losses in the core as flux flows from the flux shunts into the transformer core. 
         FIG. 10  is prior art tape wound core construction which may be used in a high leakage inductance transformer. 
         FIGS. 11A-11B  is an embodiment of the present invention showing how conventional tape wound cores may be used with a single winding set in a high leakage inductance transformer. 
         FIGS. 12A-12B  is another embodiment of the present invention showing how bars of tape wound cores may be used with a single winding set in a high leakage inductance transformer. 
         FIGS. 13A-13B  is another embodiment of the present invention showing how conventional tape wound cores may be cut and used with a single winding set in a high leakage inductance transformer. 
         FIGS. 14A-14B  is another embodiment of the present invention showing how “outrigger” flux shunts may be used with conventional tape wound cores and a single winding set in a high leakage inductance transformer. 
         FIGS. 15A-15B  is another embodiment of the present invention showing how bars of tape wound cores may be used with a dual winding set in a high leakage inductance transformer. 
         FIGS. 16A-16B  is an alternative embodiment of the present invention showing how bars of tape wound cores may be used with a dual winding set in a high leakage inductance transformer, wherein the flux shunts have been moved to the outside of the core. 
         FIGS. 17A-17B  is another embodiment of the present invention showing how “outrigger” flux shunts may be used with a conventional tape wound core and a dual winding set in a high leakage inductance transformer. 
         FIGS. 18A-18B  is another embodiment of the present invention showing how “outrigger” flux shunts may be cut from, and used with, a conventional tape wound core and a dual winding set in a high leakage inductance transformer. 
         FIGS. 19A-19B  is another embodiment of the present invention showing how bars of tape wound core may be used in a high leakage inductance planar winding transformer. 
         FIGS. 20A-20B  is another embodiment of the present invention showing how bars of tape wound core may be used in alternative orientations in a high leakage inductance planar winding transformer. 
         FIGS. 21A-21B  is another embodiment of the present invention showing how bars of tape wound core may be used in a high leakage inductance planar winding transformer, wherein interleaved windings are used. 
     
    
    
     DETAILED DESCRIPTION 
     Definitions 
     
         
         
           
             1) A transformer core is a ferromagnetic material which carries the majority of the magnetic flux generated by currents in a primary winding. 
             2) A “flux shunt” is a ferromagnetic core placed between a primary and secondary winding to increase leakage inductance between the two windings. Magnetic flux in the flux shunt has a return path through part of the transformer core. 
             3) An “air gap” in a core is understood to be a non-magnetic portion of the core, which contains most of the core flux, and which may consist partially or wholly of material other than air. 
             4) A “winding set” consists of at least one concentric primary and secondary winding pair. The usage of the terms “primary” and “secondary” herein are conventional, in that the primary need not be the “first” or innermost winding. 
             5) “Concentric” windings have the central axis of one winding located inside another winding. The two windings may or may not have the same central axis. 
             6) The “broad surface” of a tape or lamination is the surface with the greater dimensions. 
             7) The “principal flux” in a core is that magnetic flux flowing from one part of the core to another, which is not contained in a fringe field near an air gap in the core, nor in stray fields outside the core. 
             8) A “core segment” is one of various ferromagnetic pieces which may be used to assemble a transformer core, which may include flux shunts. 
           
         
       
    
     In one embodiment tape wound cores are assembled as shown in  FIGS. 11A-11B , including transformer core  111 , flux shunts  114  with air gaps  115 , primary winding  112  and secondary winding  113 . The orientation  117  of the tape is shown, although the thickness of the tape is not shown to scale. 
     The core  111  may include leg  118  that is coupled to other core segments  119  (that may be termed “bars” in, for example,  FIGS. 12, 15 and 16 ). The embodiments of  FIGS. 11A-11B and 12A-12B  have a top core segment  119 A, 129 A, respectively, and a bottom core segment  119 B, 129 B, respectively, though top and bottom are arbitrary designations as the core device  110 , 120  may be otherwise positioned. As shown in  FIG. 11A , leg  118  is coupled to top core segments  119 A and bottom core segment  119 B (ie, to the remainder of the core) through continuous tape layers. 
     In  FIGS. 11A-11B  and subsequent figures, all cores are made from tape wound material. One viable orientation of the core tape is shown for illustration; in some cases the tape orientation in core bars may be at right angles to that shown, as long as criteria (3) above is met. 
     Also in  FIGS. 11A-11B  and subsequent figures, a final reference number digit “0” refers to a complete transformer, consisting of a core and one or more winding sets, while a final digit “1” refers to a complete core only, without windings. Use of other reference number final digits is intended to be consistent (ie, referencing the same or similar component, respectively) within these remaining figures. 
     Another preferred embodiment is shown in  FIGS. 12A-12B , where core  121  and flux shunts  124  of transformer  120  are made from bars cut from wound tape. The basic geometry is similar to that of  FIGS. 8A-8B , with a single winding set consisting of primary  122  and secondary  123 . Here multiple distributed air gaps  125  in flux shunts  124  are illustrated. Leg  128  is coupled between top and bottom core segments  129 A, 129 B, respectively. Core segments  129 A, 129 B are cut “bars” in contrast to the continuous tape layer embodiment of  FIG. 11A  (and other figures).  FIG. 12A  (and  FIGS. 15A and 16A ) illustrate that leg  128  may be coupled into the remainder of the core with a first edge surface of leg  128  abutting an edge surface of the top core segment  129 A and a second edge surface of leg  128  abutting the bottom core segment  129 B. Primary winding  122  encircles leg  128  while secondary winding  123  encircles the shunts  124  and primary winding  122 . 
     In the figures that follow, the “A” and “B” have been left off the designation of the top and bottom core segments, though it is to be understood (by analogy) that that this designation is implied. 
     Referring to  FIG. 13A , another preferred embodiment is shown with “outrigger” flux shunts  134  that are configured to define air gaps  135  and are cut from tape wound cores similar to the transformer core  131 . Primary  132  and secondary windings  133  are placed on the core structure as shown in  FIG. 13B . 
     Leg  138  is coupled to top and bottom core segments  139  through continuous tape layers, and primary winding  132  encircles leg  138 . An edge surface of the shunts  134  is preferably coupled to the edge surface of the core  131  tape wound layers. The secondary winding  133  encircles the shunts  134 . 
     In  FIG. 14A , another preferred embodiment is illustrated where outrigger flux shunts  144  are made from bars and placed as shown, with air gaps  145  at each end of each flux shunt. Primary  142  and secondary  143  are placed on the core structure as shown in  FIG. 14B . 
     Leg  148  is coupled to top and bottom core segments  149  via continuous tape layers and is encircled by primary winding  142 . While spaced by a gap, the edge surface of the shunts preferably face an edge surface of the core. 
     In  FIGS. 15A-15B , another preferred embodiment is shown. Transformer  150  has a dual set of windings  152 ,  153  that are placed on transformer core  151  with flux shunts  154  (with central air gaps  155 ), all made with tape core bars. Two legs  158  are coupled between the top and bottom core bar segments  159  through their respective edge surfaces. A primary winding  152  encircles each of the legs  158 , and a secondary winding  153  encircles a primary winding and shunt. 
     In  FIGS. 16A-16B , a similar preferred embodiment to that of  FIGS. 15A-15B  is shown, with flux shunts  164  of transformer  160  moved to the outside of the core, and with dual primaries  162  and secondaries  163  placed as shown on the core structure. The dual primaries  162  respectively encircle legs  168  which are connected between the top and bottom bar segments  169 . 
     In the preferred embodiment of  FIGS. 17A-17B , a transformer  170  is shown with transformer core  171  and outrigger flux shunts  174  made from tape core bars, with air gaps  175  at each end of the flux shunts. Dual primaries  172  and secondaries  173  are placed on the core structure as shown. Legs  178  may be coupled between top and bottom core segments  179  via continuous layers of tape material. While spaced by a gap, an edge surface of the flux shunts  174  preferably faces an edge surface of the core  171 .  FIG. 17A  clearly shows the orientation  177  of the tape layers. 
     In the preferred embodiment of  FIGS. 18A-18B , a transformer  180  is shown with transformer core  181  and outrigger flux shunts  184  made from wound tape cores, with air gaps  185  at each end of the flux shunts. Dual primaries  182  and secondaries  183  are preferably placed on the core structure as shown. Legs  188  are preferably coupled between top and bottom core segments  179  via continuous layers of tape material. While spaced by a gap  185 , an edge surface of the flux shunts  184  preferably faces an edge surface of the core  181 . Reference numeral  187  designates the orientation of the tape wound layers in the core, and indicates an edge surface of core  181  in  FIG. 18A . 
     The term “planar transformer” applies to transformers with planar windings; i.e., winding layers are in a plane instead of forming a cylinder or solenoid. They basically have the geometry of  FIG. 3 , but usually with a height somewhat less than the width or depth. 
     One preferred embodiment of a planar transformer according to this invention is designated by reference numeral  190  in  FIGS. 19A-19B . The transformer core  191  and flux shunts  194  are preferably made from tape core bars. The flux shunts are placed between the planar transformer winding  192  and planar secondary winding  193 , with the orientation of the winding layers illustrated in  FIG. 19B . Core  191  may include a leg  198  that is coupled between core bar segments  199  in a manner similar to that discussed above for  FIGS. 12A-12B . Primary winding  192  encircles leg  198 , while secondary winding  193  is concentric as defined herein with the primary winding. 
     Another preferred embodiment of a planar transformer  200  is shown in  FIG. 20 . The construction is similar to that of  FIG. 19 , but with an alternative tape orientation. 
     In all cases it is possible to have an “interleaved” winding consisting of more than one primary and/or secondary, with suitable flux shunts between windings. Common arrangements are to split a primary winding into two halves “sandwiching” the secondary, or visa versa, and more complex arrangements are possible. An example is shown in  FIG. 21  for planar transformer  210 , with two sets of flux shunts  214  (with air gaps  215 ) between the split secondary  213  and the sandwiched primary  212 . 
     Core  211  may include a leg  218  that is coupled between top and bottom core bar segments  219  in a manner similar to that discussed above. Primary winding  212  encircles leg  218 , while secondary winding  213  is concentric as defined herein with the primary 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 “Design Considerations for High Frequency Linear Magnetics”, B. Carsten, Seminar presented at the PCIM Conference in Nurnberg, Germany, May 21, 2007, May 12, 2009, and other venues. 
         [2] Hill Technical Sales Corp. brochure, available at: www.hilltech.com/products/emc components/Amorphous Shie lding.html 
         [3] J. Biela, J. W. Kolar, “Electromagnetic Integration of High Power Resonant circuits Comprising High Leakage Inductance Transformers”, Power Electronic Systems Laboratory, ETH Zurich, Zurich, Switzerland 
         [4] A. E. Feinberg, U.S. Pat. No. 3,392,310: “High Leakage Transformer and Gaseous Discharge Lamp Circuit Regulated by such Transformer”, Jul. 9, 1968. 
         [5] Sayed-Amr El-Hamamsy, U.S. Pat. No. 4,902,942: “Controlled Leakage Transformer for Fluorescent Lamp Ballast Including Integral Ballasting Inductor”, Feb. 20, 1990 
         [6] Raets et al., U.S. Pat. No. 6,100,781: “High Leakage inductance Transformer”, Aug. 8, 2000 
         [7] Chi-Chip WU, US patent application US2010/0134230 A1, “Transformer with High Leakage Inductance” Jun. 3, 2010