Abstract:
A high-frequency large current handling transformer includes a stack of plural metal planar coil members with a window formed in a center portion of each of the planar coil member. A slit extends outward from the window in each planar coil member. First and second terminals are provided for each planar coil member at locations on opposite sides of the slit. An insulating sheet having a window formed in its center portion is disposed between adjacent ones of the planar coil members. Some of the planar coil members are connected in series to provide a higher-voltage side coil, and the remaining planar coil members are connected in parallel to provide a lower-voltage side coil. An 8-shaped high-frequency core is operatively combined with the coils.

Description:
[0001]    This invention relates to a transformer which can handle a high-frequency large current, which may be used, for example, with an inverter.  
         BACKGROUND OF THE INVENTION  
         [0002]    An example of prior art transformer handling a high-frequency large current is shown in FIGS. 1A and 1B. In FIG. 1A, primary and secondary coils of ribbon-shaped conductors are wound on a bobbin  41 . The primary coil has winding start terminal  42  and a winding end terminal  43 . The secondary coil has a winding start terminal  44  and a winding end terminal  45 . These components form a coil unit  47 . E-shaped core halves  48  and  49  are inserted into a center hole of the bobbin  41  from opposite sides of the hole to such an extent that the front ends of the core halves  48  and  49  abut against each other. This complete a transformer shown in FIG. 1B.  
           [0003]    As is seen from FIG. 1B, the thickness H of the transformer is the sum of the thickness T of the core formed by the core halves  48  and  49 , the thickness U of the coils on one side and the thickness V of the coils on the opposite side of the bobbin  41 . Coils of transformers handling a large current, however, have an increased cross-sectional area, resulting in increased coil thicknesses U and V, which leads to increase of the overall thickness H of the transformer. In some cases, a heat sensing device, e.g. a thermistor, is disposed in intimate contact with the coils to avoid burnout of the coils. This causes a gap to be produced between coil layers, resulting in further increase of the coil thicknesses U and V.  
           [0004]    Another example is shown in FIG. 2. The example shown in FIG. 2 is a transformer disclosed in U.S. Pat. No. 5,010,314, which is issued to A. Estrov on Apr. 23, 1991, entitled “LOW-PROFILE PLANAR TRANSFORMER FOR USE IN OFF-LINE SWITCHING POWER SUPPLIES”.  
           [0005]    The transformer of Estrov uses planar conductors for coil windings to reduce the thickness of the coils. The transformer includes a printed circuit board  51  having a center window  52 . Coil conductors  53  and  54  formed in loop are disposed on opposite major surfaces of the board  51 . The conductors  53  and  54  are connected in series by soldering them through a through-hole  55 .  
           [0006]    The printed circuit board  51  has a tab  56  on which a winding start terminal  57  and a winding end terminal  58  are disposed. Disposed over the opposite major surfaces of the printed circuit board  51  are insulating sheets  61  and  62  having respective windows  59  and  60  and having the same shape and size as the printed circuit board  51  excluding the tab  56 . In this manner, a stack  63  is formed.  
           [0007]    A plurality of similar stacks  63  are prepared and stacked on the first stack to thereby form a coil unit  64 . The winding start terminal  57  of one board  51  and the winding end terminal  58  of adjacent board  51  in the coil unit  64  are soldered together, whereby primary and secondary coils having desired numbers of conductor turns are formed.  
           [0008]    Bobbins  67  and  68  each in the form of a short rectangular tube having flanges  65  and  66 , respectively, are inserted into the window of the coil unit  64  from opposite sides of the unit  64 . Then, E-shaped high-frequency core members  69  and  70  are inserted into the window to thereby complete the transformer.  
           [0009]    The dimensions of the windows  52 ,  59  and  60  in the printed circuit board  51  and the respective ones of the insulating sheets  61  and  62  are equal to the outer dimensions of the rectangular tubular bobbins  67  and  68 . The distance between the flanges  65  and  66  with the front end surfaces of the bobbins  67  and  68  abutting against each other is equal to the height of the coil unit  64 . The shapes and sizes of the center leg of the core members  69  and  70  are conformal to the windows in the bobbins  67  and  68 .  
           [0010]    The current-carrying capacity in the transformer shown in FIG. 2 depends on the cross-sectional area of the conductors formed on the printed circuit board  51 . Usually, the maximum thickness of a conductor realizable by the printed circuit board technology is 0.1 mm, and the manufacturing cost is proportional to the conductor thickness. With the conductor thickness of 0.1 mm or so, the board tends to warp or deform during the formation of the conductors, and, therefore, the thickness of the board itself cannot be less than 1.0 mm. When conductors 0.1 mm in thickness are formed on the opposite major surfaces of the board having a thickness of 1.0 mm, the ratio of the cross-sectional areas of the conductors to the cross-sectional area of the coil is 20% or less.  
           [0011]    Even when deformation or warpage of an individual board produced during the formation of the conductors is small, the coil unit  64  formed of a stack of a plurality of such boards may swell due to warpage of the individual boards, and, therefore, the unit  64  cannot be properly placed between the flanges  65  and  66  of the bobbins  67  and  68 . Also, if there are gaps between adjacent boards, vibrations and noise tend to be generated when current is supplied to the transformer. Also, such warpage will decrease reliability of soldered connections between conductors when a large current is supplied. For these reasons, the transformer shown in FIG. 2 has a limit in practical use. It can be used only with the primary input of 200 V and 2 A or so.  
           [0012]    Therefore, an object of the present invention is to provide a thin, high-frequency transformer which can handle a large current.  
         SUMMARY OF THE INVENTION  
         [0013]    A transformer according to an embodiment includes a plurality of planar coil members, each of which coil members is formed of a metal sheet. The planar coil member has a window in its center portion. A slit extends outward from the center window. First and second terminals are disposed on the sheet at locations on opposite sides of the slit.  
           [0014]    A higher-voltage coil is formed by stacking a plurality of such coil members with an insulating sheet disposed between adjacent coil members. Instead, coil members each having an insulating sheet bonded to its one or both surfaces may be used. The first terminal of one coil member is connected to the second terminal of the adjacent coil member so that the coil members in the stack are connected in series.  
           [0015]    A lower-voltage coil is formed of one or more coil members. The number of the coil members to be used is determined in accordance with a desired number of turns and desired current-carrying capacity. Specifically, for one turn of the lower-voltage coil, one planar coil member is used if it can provide a sufficient current-carrying capacity. If, on the other hand, the current-carrying capacity provided by one coil member is insufficient, a plurality of coil members connected in parallel are used as a coil member assembly for one turn. Further, if a plurality of turns are desired, a plurality of coil members or coil assemblies are stacked with an insulating sheet disposed between adjacent coil member or coil member assemblies like the higher-voltage coil. As in the high-voltage coil, coil members or coil member assemblies each having an insulating sheet bonded to its one or both surfaces can be used, without disposing an insulating sheet between adjacent coil members or coil assemblies.  
           [0016]    The higher-voltage coil and the lower-voltage coils are stacked into a tubular coil unit with a window in its center portion. The coil unit is combined with a core having a portion extending through the window in the coil unit.  
           [0017]    The planar coil members can be joined together by screwing, riveting, welding or brazing. When riveting is employed, coupling between terminals is more or less unreliable, causing increase of electrical resistance, but the resistance exhibited at the riveted portions can be reduced by applying solder over the riveted portions.  
           [0018]    The core is suitably in the form of an 8-shaped frame including two outer legs spaced from a center leg with a window disposed between the center leg and each outer leg. The coil unit is placed around the center leg, with the coil members extending through the windows in the core. The width of each insulating sheet is substantially equal to the distance between the two outer legs, and the shape and size of the window in each insulating sheet are substantially same as those of the cross-section of the center leg. It is desirable that the width of the planar coil members is smaller than that of the insulating sheets, and that the width and length of the window in the planar coil members are larger than the width and length of the window in the insulating sheets, respectively, so that the planar coils can be prevented from contacting the core.  
           [0019]    Instead of dimensioning the planar coil members and the insulating sheets in the manner as described above, the stack of the planar coils and insulating sheets may be surrounded by an insulating frame. The frame is provided with an projection on its inward facing surface, which protrusion is brought into engagement with a recess formed at a corresponding location in the outer periphery of the stack of planar coil members and insulating sheets. This arrangement enables the positioning of the planar coil members with respect to the insulating sheets and, at the same time, can prevent the planar coils from contacting the inner surface of the outer legs of the core.  
           [0020]    An outwardly extending tab may be formed on one or more of planar coil members, with a heat sensing element mounted thereon to measure the temperature of the planar coils. With this arrangement, increase of the thickness of the coils due to the mounting of a heat sensing element can be avoided. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIGS. 1A and 1B are an exploded perspective view and a side view of an example of prior art high-frequency large current handling transformer, respectively;  
         [0022]    [0022]FIG. 2 is an exploded perspective view of another example of prior art high-frequency large current handling transformer;  
         [0023]    [0023]FIG. 3 is an exploded perspective view of a high-frequency large current handling transformer according to one embodiment of the present invention;  
         [0024]    [0024]FIG. 4 is an enlarged perspective view of some major components of the transformer shown in FIG. 3; and  
         [0025]    [0025]FIGS. 5A, 5B and  5 C are plan views of an insulating sheet, a planar coil and an insulating frame of a transformer according to another embodiment of the present invention, and FIG. 5D is a plan view of the completed transformer. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0026]    A high-frequency large current handling transformer according to one embodiment of the present invention is shown in FIG. 3. The transformer includes planar coil members  1 ,  2 ,  3 ,  4 ,  5  and  6 , insulating sheets  7 ,  8 ,  9 ,  10 ,  11 ,  12 ,  13 ,  14 ,  15 ,  16  and  17 , and high-frequency core members  18  and  19 .  
         [0027]    The planar coil members  1 - 6  each are formed of, for example, a rectangular sheet of copper having a thickness of 0.5 mm and of the same shape and size. The planar coil members  1 - 6  have rectangular windows  1   a,    2   a,    3   a,    4   a,    5   a  and  6   a  of the same size, respectively. Slits l b ,  2   b ,  3   b ,  4   b ,  5   b  and  6   b  are provided to divide one side, for example, one of shorter sides, of the respective planar coil member into two.  
         [0028]    Tabs  1   c,    2   c,    3   c,    4   c,    5   c  and  6   c  and tabs  1   d,    2   d,    3   d,    4   d,    5   d  and  6   d  extend outward from facing portions of the respective planar coil members on opposite sides of the respective slits  1   b - 6   b.  The tabs  1   c - 6   c  provide first terminals, e.g. winding start terminals, of the respective planar coil members  1 - 6 , and the tabs  1   d - 6   d  provide second terminals, e.g. winding end terminals, of the respective planar coil members.  
         [0029]    The planar coil members  1 - 6  are disposed in parallel with each other and stacked. The winding start terminal  2   c  of the planar coil member  2  is formed such that it can be positioned over the winding end terminal  1   d  of the planar coil member  1  in the stack of the planar coil members. Similarly, the winding start terminal  3   c  of the planar coil member  3  is formed such that it can be positioned over the winding end terminal  2   d  of the planar coil member  2  in the stack. As for the planar coil members  4 ,  5  and  6 , their tabs are so formed that their winding start terminals  4   c,    5   c  and  6   c  can be vertically aligned, with the winding end terminals  4   d,    5   d  and  6   d  vertically aligned when the planar coil members are stacked.  
         [0030]    The insulating sheets  7 - 17  have a thickness of, for example, 0.2 mm, and are heat resistant. They have the same shape. Windows  7   a - 17   a  of the same shape are formed in the center portions of the respective insulating sheets  7 - 17 .  
         [0031]    The planar coil members  1 - 6  and the insulating sheets  7 - 17  are stacked in the following order: the insulating sheets  7 ,  8  and  9 , the planar coil member  1 , the insulating sheet  10 , the planar coil member  2 , the insulating sheet  11 , the planar coil member  3 , the insulating sheets  12 ,  13  and  14 , the planar coil members  4 ,  5  and  6 , and the insulating sheets  15 ,  16  and  17  with the insulating sheet  15  disposed on the planar coil member  6 , whereby a rectangular tubular coil block results.  
         [0032]    The high-frequency core members  18  and  19  are formed of, for example, ferrite. The ferrite core member  18  includes outer legs  18   d  and  18   e  spaced on opposite sides of a center leg  18   a,  with grooves  18   b  and  18   c  formed between the center leg  18   a  and the outer leg  18   d  and between the center leg  18   a  and the outer leg  18   e,  respectively. Similarly, the high-frequency core member  19  has outer legs  19   d  and  19   e  spaced on opposite sides of a center leg  19   a,  with grooves  19   b  and  19   c  formed between the center leg  19   a  and the outer leg  19   d  and between the center leg  19   a  and the outer leg  19   e,  respectively. In other words, each of the high-frequency cores  18  and  19  is E-shaped. The cores  18  and  19  are combined with the coil block, with their center legs  18   a  and  19   a  inserted into the windows  1   a - 17   a  from opposite sides of the coil block. The front distal ends of the center legs  18   a  and  19   a  abut against each other in the windows  1   a - 17   a,  to thereby form a square 8-shaped core.  
         [0033]    [0033]FIG. 4 illustrated, in an exaggerated form, the planar coil members  1  and  2 , the insulating sheets  9 ,  10  and  11 , and the core members  18  and  19  shown in FIG. 3.  
         [0034]    The length A and width B of the planar coil member  1  are a little smaller than the length C and width D of the insulating sheet  9 . The length E and width F of the window  1   a  in the planar coil member  1  are a little larger than the length G and width H of the window  9   a  in the insulating sheet  9 . Accordingly, when the planar coil member  1  is placed in position on the insulating sheet  9 , the outer peripheral portions of the insulating sheet  9  extend outward beyond the peripheral edges of the planar coil member  1 , and the inner peripheral portions around the window  9   a  of the insulating sheet  9  extend inward of the window  1   a  of the planar coil member  1 .  
         [0035]    The length J and width K of the center leg  18   a  of the core member  18  are equal to the length G and width H of the window  9   a  in the insulating sheet  9 , respectively. The distance L between the outer legs  18   d  and  18   e  of the core  18  is equal to the width D of the insulating sheet  9 . The core member  19  is dimensioned same as the core member  18 .  
         [0036]    Thus, by placing the insulating sheets  7 ,  8  and  9  in the named order, the planar coil member  1  on the insulating sheet  9 , the insulating sheet  10 , the planar coil member  2 , the insulating sheet  11  and the planar coil member  3  in the named order on the planar coil member  1 , the insulating sheets  12 ,  13  and  14  in the named order on the planar coil member  3 , the planar coil members  4 ,  5  and  6  in the named order on the insulating sheet  14 , and the insulating sheets  15 ,  16  and  17  in the named order on the planar coil member  6 , as shown in FIG. 3, the rectangular tubular coil block mentioned above results. After that, the center legs  18   a  and  19   a  of the core members  18  and  19  are inserted into the window, formed by the windows  1   a - 17   a,  in the coil block from its opposite sides. In this case, only the insulating sheets  7 - 17  contact the core members  18  and  19 , but the planar coil members  1 - 6  are spaced from the surfaces of the core members  18  and  19 .  
         [0037]    Alternatively, the insulating sheets  9 ,  10 ,  11 ,  14  and  15  may be bonded with an adhesive to the planar coil members  1 ,  2 ,  3 ,  4  and  6 , respectively, before stacking them. Another alternative is to bond insulating sheets to both major surfaces of the planar coil members  1 ,  2  and  3  before stacking them. Such arrangements can prevent the planar coil members from deviating from the proper position relative to the insulating sheets and, hence, from contacting the core members.  
         [0038]    The depth M of the grooves  18   b,    18   c,    19   b  and  19   c  is determined to be equal to a half of the height of the rectangular tubular coil block. If the height of the coil block is too large or small, the number of the insulating sheets  7 - 17  is adjusted to attain the proper height.  
         [0039]    The legs of core members  18  and  19  have been described to have the same length, but the lengths of the legs of one core member may be different from the length of the legs of the other core member.  
         [0040]    When the coil block and the core members have been assembled, the winding end terminal  1   d  of the planar coil member  1  is connected to the winding start terminal  2   c  of the planar coil member  2 , and the winding end terminal  2   d  of the planar coil member  2  is connected to the winding start terminal  3   c  of the planar coil member  3 . Terminal fittings are attached to the winding start terminal  1   c  of the planar coil member  1  and to the winding end terminal  3   d  of the planar coil member  3 , which completes a higher-voltage primary coil.  
         [0041]    The winding start terminals  4   c,    5   c  and  6   c  of the planar coil members  4 ,  5  and  6  are connected together, and also, the winding end terminals  4   d,    5   d  and  6   d  are connected together, to thereby complete a lower-voltage secondary coil.  
         [0042]    It is necessary to reliably join the planar coil members together by means of screwing, riveting, welding or brazing, since heat tends to be generated due to large current. When the planar coil members are joined together with rivets, it is desirable to employ soldering in addition to riveting in order to reduce electrical resistance.  
         [0043]    In the above-described example, when planar coil members having a width B of 20 mm and a thickness of 0.5 mm are used as the planar coil members  1 - 6 , the cross-sectional area of each planar coil member is 10 mm 2 , and, therefore, the primary coil can conduct a current of about 50 A therethrough. As for the secondary coil, it is formed of three planar coil members coupled in parallel, it can conduct a current of about 150 A therethrough. Since the thickness of the coil unit can be less than 10 mm, a thin transformer inclusive of the core, having a total height of not more than 25 mm can be realized.  
         [0044]    The planar coil member  5  shown in FIG. 3 is provided with a tab  5   e,  on which a heat sensing element  20  is mounted. In FIG. 4, however, for ease of illustration, the planar coil member  1  is shown to have a tab  1   e,  and the heat sensing element  20  is shown to be mounted on the tab  1   e.  The heat sensing element  20  mounted on the coil conductor makes it possible to know a correct temperature of the coil without delay. Furthermore, since such tab is formed to extend outward of the coil unit, it is possible to sense the temperature of the coil without increasing the thickness of the coil.  
         [0045]    [0045]FIGS. 5A through 5D illustrate a transformer according to another embodiment of the present invention.  
         [0046]    The width B of the planar coil member  1  and the width D of the insulating sheet  9  shown in FIGS. 5A and 5B are equal. The length E and width F of the window  1   a  in the planar coil member  1  are larger than the length G and width H of the window  9   a  in the insulating sheet  9 . Notches  31  and  32  are provided at predetermined locations in the longer sides of the planar coil member  1 , and also notches  33  and  34  are provided at predetermined locations in the longer sides of the insulating sheet  9 .  
         [0047]    An insulating frame  35  has a toppled U-shaped member, as shown in FIG. 5C. The height (i.e. the dimension in the direction perpendicular to the plane of the drawing sheet) is twice the depth M of the grooves  18   b,    18   c,    19   b  and  19   c.  The distance N between the leg-like portions  35   a  and  35   b  is equal to the width B of the planar coil member  1  and the width D of the insulating sheet  9 . The distance O between the outer surfaces of the leg-like portions  35   a  and  35   b  is equal to the distance L between the inner surfaces of the outer legs  18   d  and  18   e  of the core member  18 . Projections  36  and  37  are formed on the inner surfaces of the leg-like portions  35   a  and  35   b,  respectively.  
         [0048]    When the planar coil member and the insulating sheet are stacked in the manner as shown in FIG. 3, the notches  31  and  32  are in alignment with the notches  33  and  34 , respectively. When the insulating frame  35  is fitted around the stack, the projections  36  and  37  fit into the aligned notches  31  and  33  and the aligned notches  32  and  34 .  
         [0049]    The stack of planar coil members and insulating members with the insulating frame  35  fitted on it is combined with the core member  18  and the core member  19  (not shown), as shown in FIG. 5D. Since the positional relationship of the planar coil members with the insulating sheets is defined by the notches  31 ,  32 ,  33  and  34  and the projections  36  and  37 , the planar coil members can be prevented from contacting the core even if the difference in window size between the planar coil members and the insulating sheets is small.