Abstract:
Metallic coils sheets ( 34, 36, 38 ) are planar and include center windows ( 34   a,    36   a,    38   a ). Slits ( 34   b,    36   b,    38   b ) extend outward through the respective sheets from the windows. Connection terminals ( 34   c,    34   d;    36   c,    36   d;    38   c,    38   d ) are provided on the sheets at locations facing across the respective slits. The metallic coil sheets are stacked, and adjacent ones of the stacked metallic coil sheets are electrically connected by means of the connection terminals. A core ( 60, 62 ) is disposed in the windows of the stacked metallic coil sheets. The metallic coil sheets are individually covered with an insulating coating.

Description:
This invention relates to a coil that may be used, for example, as a component of a transformer or as a choke. 
     BACKGROUND OF THE INVENTION 
     The applicant of the present application filed U.S. patent application Ser. No. 10/006,478 on Dec. 6, 2001, entitled “High-Frequency Large Current Handling Transformer”, which was published on Jun. 13, 2002 under US-2002-0070836-A1. The transformer disclosed in the U.S. application includes coil sheets or planar coil members  1 ,  2 ,  3 ,  4 ,  5  and  6  of metal, e.g. copper, as shown in FIG.  1 . The metallic coil sheets  1 ,  2 ,  3 ,  4 ,  5  and  6  are formed in a rectangular shape with windows  1   a ,  2   a ,  3   a ,  4   a ,  5   a  and  6   a  in their center portions. One side of each coil sheet is cut to form a slit  1   b ,  2   b ,  3   b ,  4   b ,  5   b ,  6   b  therein. Tabs  1   c  and  1   d  extend outward from the portions facing across the slit  1   b . Similarly, tabs  2   c  and  2   d ,  3   c  and  3   d ,  4   c  and  4   d ,  5   c  and  5   d , and  6   c  and  6   d  extend outward from the portions of the respective sheet coils  2 ,  3 ,  4 ,  5  and  6  facing each other across the slits  2   b ,  3   b ,  4   b ,  5   b  and  6   b . The tabs  1   c ,  2   c ,  3   c ,  4   c ,  5   c  and  6   c  provide winding start terminals, while the tabs  1   d ,  2   d ,  3   d ,  4   d ,  5   d  and  6   d  provide winding end terminals. The coil sheets  1 ,  2  and  3  are stacked, with the tabs  1   d  and  2   c  interconnected and with the tabs  2   d  and  3   c  interconnected, to thereby provide a primary winding of the transformer. Similarly, the coil sheets  4 ,  5  and  6  are stacked, with the tabs  4   c ,  5   c  and  6   c  interconnected and with the tabs  4   d ,  5   d  and  6   d  interconnected, to thereby provide a secondary winding. Insulating sheets  9 ,  10 ,  11  and  14  are disposed in such a manner that each coil sheets  1 ,  2  and  3  are sandwiched between two of the insulating sheets. An insulating sheet  17  is disposed on the stack of the coil sheets  4 ,  5  and  6  so as to sandwich them between the insulating sheets  17  and  14 . The insulating sheets  9 ,  10 ,  11 ,  14  and  17  have center windows  9   a ,  10   a ,  11   a ,  14   a  and  17   a , respectively. Two core halves of, for example, ferrite,  18  and  19  are used. The core halves  18  and  19  have center legs  18   a  and  19   a , respectively, with grooves  18   b  and  18   c , and  19   b  and  19   c  located on opposite sides of the respective legs  18   a  and  19   a . Outward of the grooves  18   b  and  18   c  are outer legs  18   d  and  18   e , respectively, and outward of the grooves  19   b  and  19   c  are outer legs  19   d  and  19   e , respectively. The core halves  18  and  19  are combined in such a manner that the center legs  18   a  and  19   a  can be placed to extend through the center windows  1   a - 6   a  in the coil sheets  1 - 6  and the center windows  9   a - 14   a  and  17   a  in the insulating sheets  9 - 14  and  17 . 
     In manufacturing this transformer, work for stacking the metallic coil sheets and the insulating sheets alternately is necessary, which increases the cost of the transformer. Furthermore, with this arrangement, the metallic coil sheets are exposed to air and, therefore, may be oxidized and rust after long use. In addition, in order to fulfill safety standards for transformers, it must be so arranged that a sufficient creepage distance can be kept even when the insulating sheets  9 ,  10 ,  11 ,  14  and  17  are displaced more or less with respect to is the metallic coil sheets. For that purpose, larger insulating sheets must be used, which makes transformers larger in size. 
     An object of the present invention is to provide a coil that requires fewer steps in manufacturing it, is hardly oxidized and is small in size. 
     SUMMARY OF THE INVENTION 
     A coil according to one embodiment of the present invention includes a coil section having a plurality of metallic coil sheets. The coil sheets are planar and each have a window in the center portion thereof. A slit is formed in each coil sheet, which extends from a location on the periphery of the window through the sheet to the outer periphery of the sheet. Connection terminals are formed on the sheet at locations facing each other across the slit. The coil sheets are stacked, and adjacent coil sheets are electrically connected with each other by the connection terminals. A core is disposed within the windows in the coil sheets. Each of the metallic coil sheets is individually coated completely with an insulating coating before the metallic coil sheets are stacked. 
     With the above-described arrangement, since each of the metallic coil sheet of the coil is individually pre-coated with an insulating coating, there is no need for placing an insulating sheet between adjacent coil sheets when the metallic coils sheets are stacked, which can reduce the manufacturing steps, which, in turn, can reduce the manufacturing cost. Furthermore, by covering the entire surface of each of the metallic coil sheets with an insulating coating, the metallic coil sheets are hardly oxidized and rusted. In addition, since each of the metallic sheets is individually pre-coated with an insulating coating, there is no need to take care to keep that insulating sheets are not displaced relative to the metallic coil sheets when the metallic coil sheets are stacked. Accordingly, it is not necessary to take such displacement into account when setting a creepage distance, and, therefore, the creepage distance can be set small. Then, the size of transformers can be reduced. 
     A plurality of coil sections may be used. The core is disposed to extend through the windows in the metallic coil sheets of the coil sections, so that the plural coil sections are inductively coupled with each other. This arrangement provides a transformer which can be manufactured at a low cost and hardly rust, and is small in size. 
     The insulating coatings may be formed by applying an insulative resin directly over the metallic coil sheet. Alternatively, an insulating film may be bonded to the metallic coil sheet to cover part of or the entirety of the surface of the metallic coil sheet before stacking the metallic coil sheets. The insulating resin may be used as an adhesive to bond the pre-formed insulating film to the metallic coil sheet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a prior art transformer. 
     FIG. 2 is an exploded perspective view of a transformer according to a first embodiment of the present invention. 
     FIGS. 3 a ,  3   b ,  3   c  and  3   d  illustrate steps for manufacturing a metallic coil sheet useable in the transformer shown in FIG.  2 . 
     FIG. 4 a  is a plan view of a metallic coil sheet useable in the transformer of FIG. 2, 
     FIG. 4 b  is a cross-sectional view of the metallic coil sheet shown in FIG. 4 a  along a line  4   b — 4   b , and 
     FIG. 4 c  is a cross-sectional view of the metallic coil sheet of FIG. 4 a  along a line  4   c — 4   c.    
     FIG. 5 a  is a cross-sectional view of a metallic coil sheet useable in the transformer of FIG. 2, and 
     FIG. 5 b  is a cross-sectional view of a metallic coil sheet used in a prior art transformer. 
     FIG. 6 is an exploded perspective view of a choke manufactured using a coil of the present invention. 
    
    
     DESCRIPTION OF EMBODIMENTS 
     The present invention may be embodied in a high-frequency large current handling transformer, as shown in FIG.  2 . The transformer includes a plurality, two, for example, of coil sections, or windings  30  and  32 . 
     The winding  30  includes a plurality, three, for example, of metallic coil sheets  34 ,  36  and  38 , which are formed in a rectangular shape and have the same size. The metallic coil sheets  34 ,  36  and  38  have windows  34   a ,  36   a  and  38   a , respectively, in their center areas. The windows  34   a ,  36   a  and  38   a  have the same size. The metallic coil sheets  34 ,  36  and  38  are formed of metal, e.g. copper. Each of the coil sheets  34 ,  36  and  38  includes a slit  34   b ,  36   b ,  38   b  in one of the four sides around the window. The sides in which the slits are formed are on the same side of the completed transformer, but the locations of the slits  34   b ,  36   b  and  38   b  are offset with respect to each other. On the portions of the coil sheet  34  facing each other across the slit  34   b , terminals  34   c  and  34   d  are provided. Similarly, terminals  36   c  and  36   d  and terminals  38   c  and  38   d  are provided on the portions of the coil sheets  36  and  38  facing each other across the respective slits  36   b  and  38   b . The terminals  34   c ,  36   c  and  38   c  provide winding start terminals, and the terminals  34   d ,  36   d  and  38   d  provide winding end terminals. The metallic coil sheets  34 ,  36  and  38  are stacked up with the windows  34   a ,  36   a  and  38   a  therein aligned with each other. The locations of the slits  34   b ,  36   b  and  38  are determined such that, when the coil sheets are stacked, the terminals  34   d  and  36   d  are vertically aligned, and the terminals  36   d  and  38   c  are vertically aligned. 
     The winding  32  includes metallic coil sheets  40 ,  42  and  44  configured similarly to the metallic coil sheets  34 ,  36  and  38  of the winding  30 . The metallic coil sheets  40 ,  42  and  44  have respective windows  40   a ,  42   a  and  44   a , respective slits  40   b ,  42   b  and  44   b , respective pairs of terminals  40   c  and  40   d ,  42   c  and  42   d , and  44   c  and  44   d . The metallic coil sheets  40 ,  42  and  44 , too, are stacked in such a manner that the windows  40   a ,  42   a  and  44   a  therein are vertically aligned. The locations of the slits  40   b ,  42   b  and  44   b  are determined such that the terminals  40   d  and  42   c  can be vertically aligned and the terminals  42   d  and  44   c  can be vertically aligned when the metallic coil sheets  40 ,  42  and  44  are stacked. 
     Each of the metallic coil sheets  34 ,  36 ,  38 ,  40 ,  42  and  44  has an insulating coating ( 46 ) thereon, as represented by the metallic coil sheet  38  shown in detail in FIGS. 4 a ,  4   b  and  4   c . The insulating coating  46  covers the entire surface of the metallic coil sheet  38 . FIG. 4 b  is a cross-sectional view of the metallic coil sheet  38  with the insulating coating shown in FIG. 4 a  along a line  4   b — 4   b , and FIG. 4 c  is a cross-sectional view along a line  4   c — 4   c.    
     The insulating coating  46  is formed of an insulating film and an epoxy resin layer, and is formed in the following manner. First, the metallic coil sheet  38  is formed by punching a copper sheet  50  along broken lines, as shown in FIG. 3 a . At this stage, holes  52  and  54  are also formed in the terminals  38   c  and  38   d , respectively. Next, as shown in FIG. 3 b , two insulating films, e.g. polyimide films  56  with an insulating adhesive layer, e.g. an epoxy resin layer  58 , are prepared by applying epoxy resin over one surface of each polyimide film  56 . The polyimide films  56  are rectangular and larger in size than the metallic coil sheet  38 . 
     When the epoxy resin layers  58  are partly dried, the polyimide films  56  are joined to opposing two major surfaces of the metallic coil sheet  38 , by placing, as shown in FIG. 3 c , the epoxy resin layers  58  to contact with the major surfaces of the metallic coil sheet  38 . Thus, the metallic coil sheet  38  is sandwiched. As is seen from FIG. 3 c , the terminals  38   c  and  38   d  are not covered with the polyimide films  56 . 
     Then, as shown in FIG. 3 d , downward and upward pressures are applied to the polyimide films  56  joined to the metallic coil sheet  38 , by means of a press (not shown), e.g. a press with silicone rubber pressing surfaces, and the metallic coil sheet  38  and the polyimide films  56  are heated at a temperature between about 150° C. and about 180° C. for a time period of from three (3) hours to five (5) hours, to thereby cure the epoxy resin  58 . After that, unnecessary peripheral and center portions of the polyimide films  56  and epoixy resin layers  58  are punched and removed, which results in the metallic coil sheet  38  with the polyimide films  56 , shown in FIG. 4 a . The holes  52  and  54  in the terminals  38   c  and  38   d  are used in positioning the metallic coil sheet  38  for this punching step. The other metallic coil sheets are also provided with an insulating coating in the same manner as described above. It should be noted that the thickness of the polyimide films  56  and epoxy resin layers  58  is exaggerated in FIGS. 3 a - 3   d  and  4   a - 4   c.    
     The metallic coil sheets  34 ,  36  and  38  with the respective insulating coatings formed in the manner described above are stacked in such a manner that the terminal  36   c  is placed on the terminal  34   d  and the terminal  38   c  is placed on the terminal  36   d , whereby the winding  30  is formed. Similarly, the metallic coil sheets  40 ,  42  and  44  with the respective insulating coatings formed in the manner described above are stacked such that the terminal  42   c  is placed on the terminal  40   d  and the terminal  44   c  is placed on the terminal  42   d , whereby the winding  32  is formed. The terminals  34   d  and  36   c  of the winding  30  are electrically connected together, and also, the terminals  36   d  and  38   c  are electrically connected. Similarly, the terminals  40   d  and  42   c  of the winding  32  are electrically connected together, and the terminals  42   d  and  44   c  are electrically connected together. 
     The two windings  30  and  32  are stacked in such a manner that the windows  34   a ,  36   a ,  38   a ,  40   a ,  42   a  and  44   a  are vertically aligned, and cores  60  and  62  of, for example, ferrite, are placed to sandwich the vertically stacked windings  30  and  32 . More specifically, the upper core  60  has a center leg  60   a , two outer legs  60   d  and  60   e , and grooves  60   b  and  60   c  between the center leg  60   a  and the outer leg  60   d  and between the center leg  60   a  and the outer leg  60   e , respectively. Similarly, the lower core  62  has a center leg  62   a , two outer legs  62   d  and  62   e , and grooves  62   b  and  62   c  between the center leg  62   a  and the outer leg  62   d  and between the center leg  62   a  and the outer leg  62   e , respectively. The center legs  60   a  and  62   a  are adapted to be placed into the windows  34   a ,  36   a ,  38   a ,  40   a ,  42   a  and  44   a , and two opposing sides of each metallic coil sheet  34 ,  36 ,  38 ,  40 ,  42  and  44  are placed in the respective spaces defined by the grooves  60   b ,  60   c ,  62   b  and  62   c , when the cores  60  and  62  are placed over the stacked windings  30  and  32  from above and below the stack. 
     FIG. 5 a  is a cross-sectional view of the metallic coil sheet  38  provided with the insulating coating  46 . FIG. 5 b  is a cross-sectional view of the prior art metallic coil sheet  2  (FIG. 1) which does not have an insulating coating like the coating  46 , but is insulated by means of the insulating sheets  10  and  11 , for example. The metallic coil sheets  38  and  2  have the same size. As is understood from FIG. 5 b , the prior art metallic coil sheet  2  requires larger insulating sheets so as to provide a larger creepage distance “a” in order to secure its necessary creepage distance when the position of the coil sheet  2  relative to the insulating sheets  10  and  11  is deviates from the nominal position. In contrast, according to the present invention, as shown in FIG. 5 a , since the metallic coil sheet  38  is joined with the insulating coating  46 , the creepage distance “b” can be only what is required and need not be longer than required. Shorter creepage distance can make it possible to downsize the transformer. Furthermore, since the metallic coil sheets are individually covered with the insulating coatings  56 , working to place an insulating sheet between adjacent metallic coil sheets can be eliminated, which reduces the manufacturing cost. In addition, the insulating coatings  56  entirely covering the individual metallic coil sheets  38  can prevent the sheets  38  from rusting. 
     FIG. 6 shows a coil according to the present invention as used for forming a high-frequency choke. The structure of the high-frequency choke show is same as that of the transformer shown in FIG. 2 from which the coil  30  is removed. Therefore, the same reference numerals as used in FIG. 2 are used for equivalent portions, and detailed description of the choke is not given. 
     In place of the two windings  30  and  32  used for the transformer shown in FIG. 2, more windings may be used so that a transformer with one primary winding and a plurality of secondary windings may be formed. In place of polyimide and epoxy, other materials may be used for the insulating films and insulating adhesive.