Abstract:
A common mode filter includes a core, and first and second wires wound around the core. Each of the first and second wires includes at least i−1 th  turn, i th  turn, and i+1 th  turn. The i th  turn of the first wire intersects with the i th  turn of the second wire without intersecting each of the i−1 th  and i+1 th  turns of the second wire.

Description:
[0001]    The present application is a Continuation Application of U.S. patent application Ser. No. 14/045,393, filed on Oct. 3, 2013, which is based on and claims priority from Japanese Patent Application No. 2012-223249, filed on Oct. 5, 2012, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a common mode filter, and more particularly relates to a common mode filter configured by using a drum core. 
         [0004]    2. Description of Related Art 
         [0005]    There is a known common mode filter that is provided on each of two signal lines constituting a transmission path using a differential transmission method, and that is configured by two inductances magnetically coupled with each other. By inserting the common mode filter into the transmission path using a differential transmission method, it is possible to selectively remove only a common-mode noise current. 
         [0006]    It is known that a toroidal core or a drum core is used as a specific structure of the common mode filter. In the case of using the toroidal core, a leakage flux can be suppressed as compared to the case of using the drum core, and therefore high noise-removal performance can be obtained. On the other hand, because automatic coil winding is difficult for the toroidal core, it inevitably requires manual coil winding, thereby increasing variations in characteristics of the common mode filter. In contrast to this, in the case of using the drum core, it is difficult to obtain as high noise-removal performance as that of the toroidal core. On the other hand, an automatic coil winding method can be used, thereby lessening variations in characteristics of the common mode filter. Further, because the automatic coil winding method can be utilized, a drum-core type common mode filter is suitable for mass production. 
         [0007]    Japanese Patent Nos. 4789076 and 3973028 disclose an example of a common mode filter configured by using a drum core. In the example of Japanese Patent No. 4789076, two wires each of which constitutes an inductance are wound with a double-layer structure. In contrast, in the example of Japanese Patent No. 3973028, two wires each of which constitutes an inductance are wound together as a pair of wires. Generally, the former winding method is referred to as “layer winding”, and the latter winding method is referred to as “bifilar winding”. Furthermore, Japanese Patent No. 4737268 discloses an example of an automatic coil winder that is used to wind a wire around a drum core. 
         [0008]    In recent years, Ethernet has been widely adopted as an in-vehicle LAN. A common mode filter used in in-vehicle Ethernet is required to have more stable characteristics and higher noise-reduction performance than ever before. In this respect, a drum-core type common mode filter has a feature of being able to lessen variations in its characteristics, as described above. Therefore, when noise-reduction performance of the drum-core type common mode filter can be improved, it is possible to obtain the optimized common mode filter for in-vehicle Ethernet. 
         [0009]    What is specifically required as high noise-reduction performance is reduction in mode conversion characteristics (Scd) which indicate the rate of a differential signal component, input to a common mode filter, to be converted into a common mode noise and to be output. As a result of extensive studies by the present inventors in order to satisfy the requirement, it has been found that a capacitance caused between different turns (hereinafter, “capacitance between different turns”) is closely associated with the reduction in the mode conversion characteristics in a common mode filter configured by using a drum core. The mode conversion characteristics are reduced by reducing the capacitance between different turns. 
         [0010]    As disclosed in Japanese Patent No. 3973028 for example, the capacitance between different turns can be reduced by providing a given space (a “b” portion in FIG. 2 in Japanese Patent No. 3973028) between pairs of wires wound by bifilar winding. Such a winding method is employed in Japanese Patent No. 3973028 for the purpose of increasing a cutoff frequency, and there is no description of the mode conversion characteristics in Japanese Patent No. 3973028. On the other hand, in the winding method as described in Japanese Patent No. 3973028, each of spaces of equal width is provided between any two adjacent turns, and therefore the number of windings is decreased. The actual number of windings in the common mode filter in Japanese Patent No. 3973028 is only four turns, and an inductance (300 μH), required for a common mode filter utilized in in-vehicle Ethernet, cannot be obtained from the four turns. 
       SUMMARY 
       [0011]    Therefore, an object of the present invention is to provide a drum-core type common mode filter that can realize a high inductance, while achieving reduction in mode conversion characteristics by reducing a capacitance between different turns. 
         [0012]    In order to achieve the above object, a common mode filter of the present invention comprises a drum core that includes a winding core portion and a pair of flange portions provided at both ends of the winding core portion, and first and second wires that are wound around the winding core portion so as to form a pair-wire for each turn, wherein the first and second wires include one or a plurality of sparsely-wound portions in which the first and second wires are wound with adjacent pair-wires spaced from each other, and one or a plurality of closely-wound portions in which the first and second wires are wound with adjacent pair-wires in close contact with each other. 
         [0013]    The present inventors have found that in view of reducing mode conversion characteristics, sufficient reduction in a capacity between different turns can be achieved by providing a space between only a part of adjacent pair-wires, without having a configuration in which a space is provided between each of adjacent pair-wires as disclosed in Japanese Patent No. 3973028. The present invention is based on these new findings, in which while the one or plurality of sparsely-wound portions are provided to reduce the mode conversion characteristics, the one or plurality of closely-wound portions are provided to enable a higher inductance to be obtained as compared to the case where a space is provided between each of adjacent pair-wires. 
         [0014]    In the above common mode filter, the first and second wires can be wound in order that a relationship between the number of each of the pair-wires counted from one of the pair of flange portions and arrangement of the one or plurality of sparsely-wound portions, and a relationship between the number of each of the pair-wires counted from the other flange portion and arrangement of the one or plurality of sparsely-wound portions are substantially the same with each other. With this configuration, the mounting directionality can be reduced. 
         [0015]    In each of the above common mode filters, the one or the plurality of sparsely-wound portions can include a first sparsely-wound portion, the one or the plurality of closely-wound portions can include first and second closely-wound portions, and the first sparsely-wound portion can be arranged between the first and second closely-wound portions. With this configuration, the number of spaces can be reduced to a minimum (=1), and accordingly it is possible to increase the number of windings. 
         [0016]    In this common mode filter, the first and second wires can be wound by layer winding in which the second wire is wound as a first layer, and the first layer is wound as a second wire, and the first and second wires can be wound in order that a relationship between the number of each of the pair-wires counted from one of the pair of flange portions and a position at which the first wire falls to the first layer at an end of the first closely-wound portion, and a relationship between the number of each of the pair-wires counted from the other flange portion and a position at which the first wire falls to the first layer at an end of the second closely-wound portion are substantially the same with each other. With this configuration, the mounting directionality can further be reduced. 
         [0017]    In this common mode filter, the first and second wires are wound by bifilar winding, and the first and second wires can cross each other within the first sparsely-wound portion. With this configuration, the polarities are opposite to each other on both sides of the first sparsely-wound portion, and therefore it is possible to balance the polarities. 
         [0018]    In each of the above common mode filters, the one or the plurality of sparsely-wound portions can include first and second sparsely-wound portions, the one or the plurality of closely-wound portions can include a first closely-wound portion, and the first closely-wound portion can be arranged between the first and second sparsely-wound portions. With this configuration, a capacitance between different turns can be reduced at a position close to both ends of the winding core portion, which have a large influence on reducing the mode conversion characteristics, and therefore it is possible to reduce the capacitance between different turns efficiently. 
         [0019]    Also in this common mode filter, the one or the plurality of closely-wound portions further can include second and third closely-wound portions, and the first and second sparsely-wound portions and the first to third closely-wound portions can be arranged from one of the pair of flange portions to the other flange portion in order of the second closely-wound portion, the first sparsely-wound portion, the first closely-wound portion, the second sparsely-wound portion, and the third closely-wound portion. 
         [0020]    Further in this common mode filter, each of the first and second sparsely-wound portions can include a predetermined number of the pair-wires, distances between the adjacent pair-wires in the first sparsely-wound portion can be increasingly shorter in order from a position closer to one of the pair of flange portions, and distances between the adjacent pair-wires in the second sparsely-wound portion can be increasingly shorter in order from a position closer to the other flange portion. With this configuration, the width of spaces is increasingly larger as the spaces are positioned closer to both ends of the winding core portion, which have a large influence on reducing the mode conversion characteristics, and therefore it is possible to reduce the capacitance between different turns efficiently. 
         [0021]    In each of the above common mode filters, the first and second wires can be wound by layer winding. With this configuration, it is possible to increase the number of windings as compared to the case of bifilar winding. Certainly, in each of the above common mode filters, the first and second wires can also be wound by bifilar winding. 
         [0022]    According to the present invention, while one or a plurality of sparsely-wound portions are provided to achieve reduction in mode conversion characteristics, one or a plurality of closely-wound portions are provided to enable a higher inductance to be obtained as compared to the case where a space is provided between each of adjacent pair-wires. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The above and other objects, features and advantages of this invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, wherein: 
           [0024]      FIG. 1  is a schematic perspective view of an exterior structure of a surface-mount common mode filter according to a first embodiment of the present invention; 
           [0025]      FIGS. 2A to 2D  are plan views of the common mode filter shown in  FIG. 1  with a plate core removed, when viewed respectively from four directions in an x-z plane; 
           [0026]      FIG. 3  is an electric circuit diagram realized by the common mode filter shown in  FIG. 1 ; 
           [0027]      FIG. 4  is a schematic diagram showing a winding state of the wires in the common mode filter shown in  FIG. 1 ; 
           [0028]      FIGS. 5A and 5B  are explanatory diagrams of a winding method of the wires shown in  FIG. 4 ; 
           [0029]      FIG. 6  is a schematic diagram showing a modification of the winding state of the wires shown in  FIG. 4 ; 
           [0030]      FIG. 7  is a schematic diagram showing a winding state of the wires in the common mode filter according to a second embodiment of the present invention; 
           [0031]      FIG. 8  is a schematic diagram showing a modification of the winding state of the wires shown in  FIG. 7 ; 
           [0032]      FIG. 9  is a schematic diagram showing a winding state of the wires in the common mode filter according to a third embodiment of the present invention; 
           [0033]      FIG. 10  is an explanatory diagram of a winding method of the wires shown in  FIG. 9 ; 
           [0034]      FIG. 11  is a schematic diagram showing a modification of the winding state of the wires shown in  FIG. 9 ; 
           [0035]      FIG. 12  is a schematic diagram showing a winding state of the wires in the common mode filter according to a fourth embodiment of the present invention; and 
           [0036]      FIG. 13  is a schematic diagram showing a winding state of the wires in the common mode filter according to a fifth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0037]    Preferred embodiments of the present invention will now be explained in detail with reference to the drawings. 
         [0038]      FIG. 1  is a schematic perspective view of an exterior structure of a surface-mount common mode filter  10  according to a first embodiment of the present invention.  FIG. 2A  to  2 D are plan views of the common mode filter  10  with a plate core  12  (described later) removed, when viewed respectively from four directions in an x-z plane (a plane perpendicular to the y direction). In the present embodiments, as shown in  FIG. 1 , a direction in which a pair of flange portions  11   b  and  11   c  (described later) are opposed to each other is referred to as “y direction”, a direction perpendicular to the y direction in a plane of upper surfaces  11   bs  and  11   cs  (described later) is referred to as “x direction”, and a direction perpendicular to both the x direction and the y direction is referred to as “z direction”. 
         [0039]    As shown in  FIG. 1 , the common mode filter  10  is configured by including a drum core  11 , the plate core  12  attached to the drum core  11 , and wires W 1  and W 2  (first and second wires) wound around the drum core  11 . The drum core  11  includes a bar-shaped winding core portion  11   a  that is rectangular in cross section, and the flange portions  11   b  and  11   c  that are provided at both ends of the winding core portion  11   a . The drum core  11  has a structure in which the winding core portion  11   a  and the flange portions  11   b  and  11   c  are integrated with each other. The drum core  11  is installed on a substrate for use, and is affixed to the substrate in a state where an upper surface  11   as  of the winding core portion  11   a , and the upper surfaces  11   bs  and  11   cs  of the flange portions  11   b  and  11   c  are opposed to the substrate. The plate core  12  is fixedly attached to lower surfaces of the flange portions  11   b  and  11   c  (opposite surfaces to the upper surfaces  11   bs  and  11   cs ). 
         [0040]    The drum core  11  and the plate core  12  are formed by sintering a magnetic material with relatively high permeability, such as Ni—Zn-based ferrite or Mn—Zn-based ferrite. The high-permeability magnetic material such as Mn—Zn-based ferrite is normally conductive with low specific resistance. 
         [0041]    Two terminal electrodes E 1  and E 2  are formed on the upper surface  11   bs  of the flange portion  11   b . Two terminal electrodes E 3  and E 4  are formed on the upper surface  11   cs  of the flange portion  11   c . The terminal electrodes E 1  and E 2  are arranged in this order from one-end side in the x direction. Similarly, the terminal electrodes E 3  and E 4  are also arranged in this order from one-end side in the x direction. Respective ends of the wires W 1  and W 2  are joined to the terminal electrodes E 1  to E 4  by thermocompression bonding. 
         [0042]    The wires W 1  and W 2  are covered conductive wires, and are both wound around the winding core portion  11   a  in the same winding direction to constitute a coil conductor. The number of turns of the wire W 1  and the number of turns of the W 2  are the same with each other and a pair-wire is formed for each turn. In the first embodiment, the wires W 1  and W 2  are wound by layer winding to have a double-layer structure. A space is provided between adjacent pair-wires positioned in the middle of the winding core portion  11   a , thereby constituting a sparsely-wound portion  51 . This point is explained again in detail later. In an area except the sparsely-wound portion S 1 , the wires W 1  and W 2  are wound with adjacent pair-wires in close contact with each other. One end W 1   a  of the wire W 1  (an end on the side of the flange portion  11   b ) and the other end W 1   b  (an end on the side of the flange portion  11   c ) are respectively joined to the terminal electrodes E 1  and E 3 . One end W 2   a  of the wire W 2  (an end on the side of the flange portion  11   b ) and the other end W 2   b  (an end on the side of the flange portion  11   c ) are respectively joined to the terminal electrodes E 2  and E 4 . 
         [0043]      FIG. 3  is an electric circuit diagram realized by the common mode filter  10 . As shown in  FIG. 3 , the common mode filter  10  has a configuration in which an inductor I 1 , connected between the terminal electrodes E 1  and E 3 , and an inductor  12 , connected between the terminal electrodes E 2  and E 4 , are magnetically coupled with each other. The inductors I 1  and  12  are configured by the wires W 1  and W 2 , respectively. With this configuration, when the terminal electrodes E 1  and E 2  are used as an input terminal, and the terminal electrodes E 3  and E 4  are used as an output terminal, a differential signal input from the input terminal is hardly affected by the common mode filter  10 , and is output from the output terminal. In contrast, a common mode noise input from the input terminal is attenuated to a large extent by the common mode filter  10 , and is hardly output to the output terminal. 
         [0044]    A common mode filter generally has properties of converting a part of a differential signal, input to an input terminal of the common mode filter, into a common mode noise, and outputting the common mode noise from an output terminal. Because these properties are certainly not desirable, it is necessary to reduce the rate of the differential signal to be converted into the common mode noise (the mode conversion characteristics (Scd) described above) to a given level or lower. Apart from that, it is also necessary for the common mode filter to increase the number of windings of a wire to as many as possible, in order to obtain a required inductance even from a small size. In the common mode filter  10  according to the first embodiment, while the sparsely-wound portion S 1  is provided to reduce a capacitance between different turns, the wires W 1  and W 2  are closely wound in an area other than the sparsely-wound portion S 1 , thereby simultaneously solving the two problems described above. This solution is explained below in detail. 
         [0045]      FIG. 4  is a schematic diagram showing a winding state of the wires W 1  and W 2  in the common mode filter  10 . Among constituent elements shown in  FIG. 4 , an area covering the wires W 1  and W 2  and the winding core portion I 1   a  is shown as a cross-sectional view taken along the line A-A shown in  FIGS. 2B and 2D . An area covering the flange portions  11   b  and  11   c  is shown as a top view shown also in  FIG. 2A . 
         [0046]    The number shown within each of the wires W 1  and W 2  in  FIG. 4  is an illustration of the order of the turn (the turn number of each of the pair-wires) when the number of pair-wires is counted from the end on the side of the flange portion  11   b . In the example in  FIG. 4 , the maximum value of the turn number is 11. However, the actual number of turns is approximately 40. In  FIG. 4 , because priority is given to ease of viewing the drawing, a significantly-reduced number of windings of the wires W 1  and W 2  are shown in a closely-wound portion (described later). In  FIG. 4 , the connection relationship between the wires W 1  and W 2  and the terminal electrodes E 1  to E 4  is schematically shown by thick straight lines. These points are also applied to the drawings explained later. 
         [0047]    As shown in  FIG. 4 , the wires W 1  and W 2  according to the first embodiment include a first sparsely-wound portion S 1  in which the wires W 1  and W 2  are wound with adjacent pair-wires spaced from each other, and first and second closely-wound portions D 1  and D 2  in which the wires W 1  and W 2  are wound with adjacent pair-wires in close contact with each other. The first closely-wound portion D 1 , the first sparsely-wound portion S 1 , and the second closely-wound portion D 2  are arranged in this order from the flange portion  11   b  to the flange portion  11   c . The wires W 1  and W 2  with the turn numbers  1  to  5  are included in the first closely-wound portion D 1 . The wires W 1  and W 2  with the turn number  6  are included in the first sparsely-wound portion S 1 . The wires W 1  and W 2  with the turn numbers  7  to  11  are included in the second closely-wound portion D 2 . 
         [0048]    In the first embodiment, one turn (the turn number  6 ) of each of the wires W 1  and W 2  is included in the first sparsely-wound portion S 1 . However, a turn of each of the wires W 1  and W 2  is not necessarily included in the first sparsely-wound portion S 1 . This point is also applied to other embodiments described later. 
         [0049]    By providing the first sparsely-wound portion S 1  as described above, it is possible for the common mode filter  10  according to the first embodiment to reduce the capacitance between different turns in the wires W 1  and W 2  as compared to the case where no sparsely-wound portion is provided. Therefore, the mode conversion characteristics are also reduced. Meanwhile, because the first and second closely-wound portions D 1  and D 2  are provided, it is possible for the common mode filter  10  according to the first embodiment to obtain a higher inductance as compared to the case where a space is provided between each of adjacent pair-wires. 
         [0050]      FIGS. 5A and 5B  are explanatory diagrams of a winding method of the wires W 1  and W 2  shown in  FIG. 4 . Each of a straight line and a broken line shown in  FIGS. 5A and 5B  connects between cross sections of a wire. The straight line schematically shows the wire located on the front side of the winding core portion  11   a  in the drawings. The broken line schematically shows the wire located on the rear side of the winding core portion  11   a  in the drawings. The method of winding the wires W 1  and W 2  that include the first and second closely-wound portions D 1  and D 2 , and the first sparsely-wound portion S 1  is briefly explained below with reference to  FIGS. 5A and 5B . 
         [0051]    The wires W 1  and W 2  are wound around the winding core portion  11   a  using an automatic coil winder (not shown). Specifically, as this automatic coil winder, it is preferable to use an automatic coil winder disclosed in Japanese Patent No. 4737268, for example. Assuming that the automatic coil winder disclosed in Japanese Patent No. 4737268 is used, in winding the wires W 1  and W 2 , first, one end of the wire W 2  wound as a first layer is joined to the terminal electrode E 2 , and then while the drum core  11  is rotated about a rotation axis along the y direction at a constant speed, a nozzle that feeds the wire W 2  is moved in the y direction ( FIG. 5A ). At this time, in an area covering the first and second closely-wound portions D 1  and D 2 , the moving speed of the nozzle is adjusted so as not to create a gap between adjacent pair-wires. In contrast, in an area covering the first sparsely-wound portion S 1 , the moving speed of the nozzle is adjusted so as to create a space of appropriate size between adjacent pair-wires. The moving speed becomes slowest when the wire W 2  is wound so as not to create a gap, and becomes faster as a larger space is formed. It is necessary to keep the moving speed of the nozzle constant in the first and second closely-wound portions D 1  and D 2 . However, the moving speed of the nozzle is not necessarily kept constant in the first sparsely-wound portion S 1 . When winding of the wire W 2  is finished, the drum core  11  is stopped rotating, and the other end of the wire W 2  is joined to the terminal electrode E 4 . 
         [0052]    Next, one end of the wire W 1  wound as a second layer is joined to the terminal electrode E 1 , and then while the drum core  11  is rotated again at a constant speed, a nozzle that feeds the wire W 1  is moved in the y direction ( FIG. 5B ). At this time, in an area covering the first and second closely-wound portions D 1  and D 2 , the moving speed of the nozzle is adjusted so as to precisely fit the wire W 1  between the wires W 2 . However, because the number of windings of the wire W 1  and the number of windings of the wire W 2  are the same with each other in either of the first and second closely-wound portions D 1  and D 2 , at one of the ends of each of the first and second closely-wound portions D 1  and D 2 , the wire W 1  falls to the first layer as shown in  FIG. 5B . In  FIG. 5B , the wire W 1  with the turn number  1 , positioned at the end of the first closely-wound portion D 1  on the side of the flange portion  11   b , falls to the first layer, and the wire W 1  with the turn number  7 , positioned at the end of the second closely-wound portion D 2  on the side of the flange portion  11   b , falls to the first layer. Meanwhile, in an area covering the first sparsely-wound portion S 1 , the moving speed of the nozzle is adjusted so as to wind the wire W 1  along the wire W 2 . That is, within the first sparsely-wound portion S 1 , the positional relationship between the wires W 1  and W 2  is the same as in the case of bifilar winding. When winding of the wire W 1  is finished, the drum core  11  is stopped rotating, and the other end of the wire W 1  is joined to the terminal electrode E 3 . Following the procedure described above, winding of the wires W 1  and W 2  around the winding core portion  11   a  is completed. 
         [0053]    Returning to  FIG. 4 , the wires W 1  and W 2  are wound in order that the relationship between the number of turns counted from the flange portion  11   b  and the arrangement of a sparsely-wound portion S 1 , and the relationship between the number of turns counted from the flange portion  11   c  and the arrangement of a sparsely-wound portion S 1  are the same with each other. In other words, the wires W 1  and W 2  are wound in order that the wires W 1  and W 2  wound on the side of the flange portion  11   b  and the wires W 1  and W 2  wound on the side of the flange portion  11   c  are symmetric with respect to the center of the winding core portion  11   a  in the y-direction. Specifically, according to the number of turns counted from the flange portion  11   b , the first sparsely-wound portion S 1  is arranged between the fifth turn and the seventh turn. And, according to the number of turns counted from the flange portion  11   c , the first sparsely-wound portion S 1  is also arranged between the fifth turn and the seventh turn. 
         [0054]    By setting the relationship between the number of turns and the arrangement of the first sparsely-wound portion S 1  as described above, the common mode filter  10  can be expected to exhibit almost the same characteristics both in the case where the terminal electrodes E 1  and E 2  are utilized as an input terminal and in the case where the terminal electrodes E 3  and E 4  are utilized as an input terminal. Therefore, at the time of installing the common mode filter  10  on a substrate, it is not necessary to care which of the flange portions corresponds to the terminal electrodes E 1  and E 2  (the mounting directionality is reduced), and accordingly it is possible to reduce the burden of installation work, and to prevent mistakes with the installation. 
         [0055]    The relationship between the number of turns counted from the flange portion  11   b  and the arrangement of a sparsely-wound portion S 1 , and the relationship between the number of turns counted from the flange portion  11   c  and the arrangement of a sparsely-wound portion S 1  are not necessarily completely the same with each other. It suffices that these relationships are substantially the same with each other. “Substantially the same with each other” means that a difference between those relationships is allowable, from the realistic viewpoint, as long as the mounting directionality is sufficiently reduced. For example, in the case where the total number of turns is 40, when one of the 40 turns is arranged within the first sparsely-wound portion S 1  as shown in  FIG. 4 , then it is inevitable to arrange 19 turns within one of the first and second closely-wound portions D 1  and D 2 , and to arrange 20 turns within the other. In this case, the wires W 1  and W 2  are not wound in order that the relationship between the number of turns counted from the flange portion  11   b  and the arrangement of a sparsely-wound portion S 1 , and the relationship between the number of turns counted from the flange portion  11   c  and the arrangement of a sparsely-wound portion S 1  are completely the same with each other. However, from the realistic viewpoint, the mounting directionality is sufficiently reduced. Therefore, in this case, the wires W 1  and W 2  are thought to be wound in order that the relationship between the number of turns counted from the flange portion  11   b  and the arrangement of a sparsely-wound portion S 1 , and the relationship between the number of turns counted from the flange portion  11   c  and the arrangement of a sparsely-wound portion S 1  are substantially the same with each other. These points described above are also applied to other embodiments described later and to “the relationship between the number of turns counted from each of the flange portions  11   b  and  11   c  and the falling position”. 
         [0056]    As explained above, in the common mode filter  10  according to the first embodiment, the first sparsely-wound portion S 1  is provided, and also the first and second closely-wound portions D 1  and D 2  are provided. Therefore, both reducing the mode conversion characteristics and ensuring a high impedance can be achieved. Further, because the mounting directionality of the common mode filter  10  can be reduced, it is possible to reduce the operation burden of installing the common mode filter  10  on a substrate, and to prevent mistakes with the installation. Furthermore, because layer winding is employed, it is possible to increase the number of windings as compared to the case where bifilar winding is employed. 
         [0057]      FIG. 6  is a schematic diagram showing a modification of the winding state of the wires W 1  and W 2  shown in  FIG. 4 . In the modification shown in  FIG. 6 , in the second closely-wound portion D 2 , the wires W 1  and W 2  are wound in order that the wire W 1  with the turn number  7 , positioned at the end on the side of the flange portion  11   b , does not fall to the first layer, but the wire W 1  with the turn number  11 , positioned at the end on the side of the flange portion  11   c , falls to the first layer. With this configuration, the relationship between the number of turns counted from the flange portion  11   b  and the falling position, and the relationship between the number of turns counted from the flange portion  11   c  and the falling position are the same with each other. Therefore, it is possible to further reduce the directionality of the common mode filter  10 . 
         [0058]      FIG. 7  is a schematic diagram showing a winding state of the wires W 1  and W 2  in the common mode filter  10  according to a second embodiment of the present invention. The common mode filter  10  according to the second embodiment is the same as the common mode filter  10  according to the first embodiment, except for a winding mode of the wires W 1  and W 2 . In  FIG. 7 , similarly to  FIG. 4 , an area covering the wires W 1  and W 2  and the winding core portion  11   a  is shown as a cross-sectional view taken along the line A-A shown in  FIGS. 2B and 2D , and an area covering the flange portions  11   b  and  11   c  is shown as a top view corresponding to  FIG. 2A .  FIG. 7  is explained below while focusing on the differences from  FIG. 4 . 
         [0059]    As shown in  FIG. 7 , the wires W 1  and W 2  according to the second embodiment include first and second sparsely-wound portions S 1  and S 2  in which the wires W 1  and W 2  are wound with adjacent pair-wires spaced from each other, and first to third closely-wound portions D 1  to D 3  in which the wires W 1  and W 2  are wound with adjacent pair-wires in close contact with each other. The second closely-wound portion D 2 , the first sparsely-wound portion S 1 , the first closely-wound portion D 1 , the second sparsely-wound portion S 2 , and the third closely-wound portion D 3  are arranged in this order from the flange portion  11   b  to the flange portion  11   c . The wires W 1  and W 2  with the turn numbers  1  to  3  are included in the second closely-wound portion D 2 . The wires W 1  and W 2  with the turn number  4  are included in the first sparsely-wound portion S 1 . The wires W 1  and W 2  with the turn numbers  5  to  7  are included in the first closely-wound portion D 1 . The wires W 1  and W 2  with the turn number  8  are included in the second sparsely-wound portion S 2 . The wires W 1  and W 2  with the turn numbers  9  to  11  are included in the third closely-wound portion D 3 . The second and third closely-wound portions D 2  and D 3  are not necessarily provided, and can be replaced with one turn of each of the wires W 1  and W 2 . 
         [0060]    By providing the first and second sparsely-wound portions S 1  and S 2  as described above, it is also possible for the common mode filter  10  according to the second embodiment to reduce the capacitance between different turns in the wires W 1  and W 2  as compared to the case where no sparsely-wound portion is provided. Therefore, the mode conversion characteristics are also reduced. Further, because the first to third closely-wound portions D 1  to D 3  are provided, it is possible to obtain a higher inductance as compared to the case where a space is provided between each of adjacent pair-wires. 
         [0061]    Also in the second embodiment, the wires W 1  and W 2  are wound in order that the relationship between the number of turns counted from the flange portion  11   b  and the arrangement of sparsely-wound portions, and the relationship between the number of turns counted from the flange portion  11   c  and the arrangement of sparsely-wound portions are the same with each other. Specifically, according to either the number of turns counted from the flange portion  11   b  or the number of turns counted from the flange portion  11   c , the sparsely-wound portions are arranged between the third turn and the fifth turn, and between the seventh turn and the ninth turn. Therefore, similarly to the first embodiment, the mounting directionality can be reduced, and it is possible to reduce the burden of installation work, and to prevent mistakes with the installation. 
         [0062]    As explained above, the common mode filter  10  according to the second embodiment can also achieve both reducing the mode conversion characteristics and ensuring a high impedance. Further, because the mounting directionality of the common mode filter  10  can be reduced, it is possible to reduce the operation burden of installing the common mode filter  10  on a substrate, and to prevent mistakes with the installation. Furthermore, because layer winding is employed, it is possible to increase the number of windings as compared to the case where bifilar winding is employed. 
         [0063]    In the second embodiment, spaces are formed nearer to the flange portions  11   b  and  11   c  as compared to the first embodiment. As the positions of the spaces are closer to the flange portions  11   b  and  11   c , a larger effect of reducing the mode conversion characteristics can be obtained. Therefore, in the common mode filter  10  according to the second embodiment, it is possible to obtain the effect of reducing the mode conversion characteristics more efficiently (with a narrower space) as compared to the first embodiment. 
         [0064]      FIG. 8  is a schematic diagram showing a modification of the winding state of the wires W 1  and W 2  shown in  FIG. 7 . In the example in  FIG. 7 , the wire W 1  with the turn numbers  1 ,  5 , and  9 , positioned at each end of the first to third closely-wound portions D 1  to D 3  on the side of the flange portion  11   b , falls to the first layer. However, in the present modification shown in  FIG. 8 , the wire W 1  with the turn numbers  7  and  11 , positioned at each end of the first and third closely-wound portions D 1  and D 3  on the side of the flange portion  11   c , falls to the first layer. With this configuration, the relationship between the number of turns counted from the flange portion  11   b  and the falling position, and the relationship between the number of turns counted from the flange portion  11   c  and the falling position are not exactly the same with each other, but are similar to each other (substantially the same with each other). Therefore, it is possible to further reduce the directionality of the common mode filter  10  as compared to the example in  FIG. 7 . 
         [0065]      FIG. 9  is a schematic diagram showing a winding state of the wires W 1  and W 2  in the common mode filter  10  according to a third embodiment of the present invention. The common mode filter  10  according to the third embodiment is the same as the common mode filter  10  according to the first embodiment, except for a winding mode of the wires W 1  and W 2 . In  FIG. 9 , similarly to  FIG. 4 , an area covering the wires W 1  and W 2  and the winding core portion  11   a  is shown as a cross-sectional view taken along the line A-A shown in  FIGS. 2B and 2D , and an area covering the flange portions  11   b  and  11   c  is shown as a top view corresponding to  FIG. 2A .  FIG. 9  is explained below while focusing on the differences from  FIG. 4 . 
         [0066]    As shown in  FIG. 9 , the wires W 1  and W 2  according to the third embodiment are wound not by layer winding, but by bifilar winding. Meanwhile, similarly to the first embodiment, the wires W 1  and W 2  according to the third embodiment include the first sparsely-wound portion  51  in which the wires W 1  and W 2  are wound with adjacent pair-wires spaced from each other, and the first and second closely-wound portions D 1  and D 2  in which the wires W 1  and W 2  are wound with adjacent pair-wires in close contact with each other, and the first closely-wound portion D 1 , the first sparsely-wound portion S 1 , and the second closely-wound portion D 2  are arranged in this order from the flange portion  11   b  to the flange portion  11   c . Therefore, similarly to the first embodiment, in the common mode filter  10  according to the third embodiment, the capacitance between different turns in the wires W 1  and W 2  is reduced as compared to the case where no sparsely-wound portion is provided, and the mode conversion characteristics are also reduced. Further, it is possible to obtain a higher inductance as compared to the case where a space is provided between each of adjacent pair-wires. 
         [0067]      FIG. 10  is an explanatory diagram of a winding method of the wires W 1  and W 2  shown in  FIG. 9 . In  FIG. 10 , similarly to  FIGS. 5A and 5B , each of a straight line and a broken line connects between cross sections of a wire. The straight line schematically shows the wire located on the front side of the winding core portion  11   a  in the drawing. The broken line schematically shows the wire located on the rear side of the winding core portion  11   a  in the drawing. The method of winding the wires W 1  and W 2  that include the first and second closely-wound portions D 1  and D 2 , and the first sparsely-wound portion S 1  by bifilar winding is briefly explained below with reference to  FIG. 10 . 
         [0068]    Similarly to the first embodiment, it is preferable to use the automatic coil winder disclosed in Japanese Patent No. 4737268 as an automatic coil winder used for winding the wires W 1  and W 2 . In the third embodiment, first, one end of the wire W 1  is joined to the terminal electrode E 1 , and one end of the wire W 2  is joined to the terminal electrode E 2 . Next, while the drum core  11  is rotated at a constant speed, two nozzles that feed the wires W 1  and W 2  respectively are moved in the y direction with their relative positional relationship maintained. At this time, in an area covering the first and second closely-wound portions D 1  and D 2 , the moving speed of each of the nozzles is adjusted so as not to create a gap between adjacent pair-wires. In contrast, in an area covering the first sparsely-wound portion S 1 , the moving speed of each of the nozzles is adjusted so as to provide a space of appropriate size between adjacent pair-wires. Also in the third embodiment, the moving speed becomes slowest when the wires W 1  and W 2  are wound so as not to create a gap, and becomes faster as a larger space is formed. Further, it is necessary to keep the moving speed of each of the nozzles constant in the first and second closely-wound portions D 1  and D 2 . However, the moving speed of each of the nozzles is not necessarily kept constant in the first sparsely-wound portion S 1 . When winding of the wires W 1  and W 2  is finished, the other end of the wire W 1  is joined to the terminal electrode E 3 , and the other end of the wire W 2  is joined to the terminal electrode E 4 . 
         [0069]    Returning to  FIG. 9 , the wires W 1  and W 2  are wound in order that the relationship between the number of turns counted from the flange portion  11   b  and the arrangement of a sparsely-wound portion S 1 , and the relationship between the number of turns counted from the flange portion  11   c  and the arrangement of a sparsely-wound portion S 1  are the same with each other. Specifically, according to either the number of turns counted from the flange portion  11   b  or the number of turns counted from the flange portion  11   c , the first sparsely-wound portion S 1  is arranged between the third turn and the fifth turn. Therefore, similarly to the first and second embodiments, the mounting directionality can be reduced, and it is possible to reduce the burden of installation work, and to prevent mistakes with the installation. 
         [0070]    As explained above, the common mode filter  10  according to the third embodiment can also achieve both reducing the mode conversion characteristics and ensuring a high impedance. Further, because the mounting directionality of the common mode filter  10  can be reduced, it is possible to reduce the operation burden of installing the common mode filter  10  on a substrate, and to prevent mistakes with the installation. 
         [0071]      FIG. 11  is a schematic diagram showing a modification of the winding state of the wires W 1  and W 2  shown in  FIG. 9 . In  FIG. 11 , similarly to  FIG. 10 , each of a straight line and a broken line connects between cross sections of a wire. The straight line schematically shows the wire located on the front side of the winding core portion  11   a  in the drawing. The broken line schematically shows the wire located on the rear side of the winding core portion  11   a  in the drawing. In the present modification, the wires W 1  and W 2  cross each other (their positions are interchanged) within the first sparsely-wound portion S 1  (at a position X 1  shown in  FIG. 11 ). Accordingly, the wires W 1  and W 2  cross each other also at the end of the second closely-wound portion D 2  on the side of the flange portion  11   c  (at a position X 2  shown in  FIG. 11 ). The crossing of the wires W 1  and W 2  is realized by interchanging the positions of two nozzles at a corresponding position. 
         [0072]    When the wires W 1  and W 2  cross each other as described above, the polarities are opposite to each other on both sides of the first sparsely-wound portion S 1 . Therefore, it is possible to balance the polarities. Further, the wires W 1  and W 2  cross each other within the first sparsely-wound portion S 1  (the wires W 1  and W 2  with the turn number  4 , which is not in close contact with its adjacent pair-wires on both sides, cross each other). Therefore, it is possible to minimize winding disarray caused by the crossing of the wires W 1  and W 2 . 
         [0073]      FIG. 12  is a schematic diagram showing a winding state of the wires W 1  and W 2  in the common mode filter  10  according to a fourth embodiment of the present invention. The common mode filter  10  according to the fourth embodiment is the same as the common mode filter  10  according to the third embodiment, except for a winding mode of the wires W 1  and W 2 . In  FIG. 12 , similarly to  FIG. 4 , an area covering the wires W 1  and W 2  and the winding core portion  11   a  is shown as a cross-sectional view taken along the line A-A shown in  FIGS. 2B and 2D , and an area covering the flange portions  11   b  and  11   c  is shown as a top view corresponding to  FIG. 2A .  FIG. 12  is explained below while focusing on the differences from  FIG. 4 . 
         [0074]    Similarly to the third embodiment, the wires W 1  and W 2  according to the fourth embodiment are wound not by layer winding, but by bifilar winding. Meanwhile, similarly to the second embodiment, the wires W 1  and W 2  according to the fourth embodiment include the first and second sparsely-wound portions S 1  and S 2  in which the wires W 1  and W 2  are wound with adjacent pair-wires spaced from each other, and the first to third closely-wound portions D 1  to D 3  in which the wires W 1  and W 2  are wound with adjacent pair-wires in close contact with each other, and the second closely-wound portion D 2 , the first sparsely-wound portion S 1 , the first closely-wound portion D 1 , the second sparsely-wound portion S 2 , and the third closely-wound portion D 3  are arranged in this order from the flange portion  11   b  to the flange portion  11   c . Therefore, similarly to the second embodiment, in the common mode filter according to the fourth embodiment, the capacitance between different turns in the wires W 1  and W 2  is reduced as compared to the case where no sparsely-wound portion is provided, and the mode conversion characteristics are also reduced. Further, it is possible to obtain a higher inductance as compared to the case where a space is provided between each of adjacent pair-wires. 
         [0075]    Also in the fourth embodiment, the wires W 1  and W 2  are wound in order that the relationship between the number of turns counted from the flange portion  11   b  and the arrangement of sparsely-wound portions, and the relationship between the number of turns counted from the flange portion  11   c  and the arrangement of sparsely-wound portions are the same with each other. Specifically, according to either the number of turns counted from the flange portion  11   b  or the number of turns counted from the flange portion  11   c , the sparsely-wound portions are arranged between the second turn and the fourth turn, and between the fifth turn and the seventh turn. Therefore, similarly to the first to third embodiments, the mounting directionality can be reduced, and it is possible to reduce the burden of installation work, and to prevent mistakes with the installation. 
         [0076]    As explained above, the common mode filter  10  according to the fourth embodiment can also achieve both reducing the mode conversion characteristics and ensuring a high impedance. Further, because the mounting directionality of the common mode filter  10  can be reduced, it is possible to reduce the operation burden of installing the common mode filter  10  on a substrate, and to prevent mistakes with the installation. 
         [0077]    In the fourth embodiment, spaces are formed nearer to the flange portions  11   b  and  11   c  as compared to the third embodiment. Therefore, it is possible to obtain the effect of reducing the mode conversion characteristics more efficiently (with a narrower space) as compared to the third embodiment. 
         [0078]      FIG. 13  is a schematic diagram showing a winding state of the wires W 1  and W 2  in the common mode filter  10  according to a fifth embodiment of the present invention. The common mode filter  10  according to the fifth embodiment is the same as the common mode filter  10  according to the third and fourth embodiments, except for a winding mode of the wires W 1  and W 2 . In  FIG. 13 , similarly to  FIG. 4 , an area covering the wires W 1  and W 2  and the winding core portion  11   a  is shown as a cross-sectional view taken along the line A-A shown in  FIGS. 2B and 2D , and an area covering the flange portions  11   b  and  11   c  is shown as a top view corresponding to  FIG. 2A .  FIG. 13  is explained below while focusing on the differences from  FIG. 4 . 
         [0079]    As shown in  FIG. 13 , the wires W 1  and W 2  according to the fifth embodiment are wound by bifilar winding, and include the first and second sparsely-wound portions S 1  and S 2  in which the wires W 1  and W 2  are wound with adjacent pair-wires spaced from each other, and the first closely-wound portion D 1  in which the wires W 1  and W 2  are wound with adjacent pair-wires in close contact with each other. The first sparsely-wound portion S 1 , the first closely-wound portion D 1 , and the second sparsely-wound portion S 2  are arranged in this order from the flange portion  11   b  to the flange portion  11   c . The wires W 1  and W 2  with the turn numbers  2  and  3  are included in the first sparsely-wound portion S 1 . The wires W 1  and W 2  with the turn numbers  10  and  11  are included in the second sparsely-wound portion S 2 . 
         [0080]    Also in the fifth embodiment, the wires W 1  and W 2  are wound in order that the relationship between the number of turns counted from the flange portion  11   b  and the arrangement of sparsely-wound portions, and the relationship between the number of turns counted from the flange portion  11   c  and the arrangement of sparsely-wound portions are the same with each other. Specifically, according to either the number of turns counted from the flange portion  11   b  or the number of turns counted from the flange portion  11   c , the sparsely-wound portions are arranged between the first turn and the fourth turn, and between the ninth turn and the twelfth turn. 
         [0081]    In the common mode filter  10  according to the fifth embodiment, the first and second sparsely-wound portions S 1  and S 2 , and the first closely-wound portion D 1  are provided, and therefore the capacitance between different turns in the wires W 1  and W 2  is reduced as compared to the case where no sparsely-wound portion is provided, and the mode conversion characteristics are also reduced, similarly to the first to fourth embodiments. Further, it is possible to obtain a higher inductance as compared to the case where a space is provided between each of adjacent pair-wires. Furthermore, the wires W 1  and W 2  are wound in order that the relationship between the number of turns counted from the flange portion  11   b  and the arrangement of sparsely-wound portions, and the relationship between the number of turns counted from the flange portion  11   c  and the arrangement of sparsely-wound portions are the same with each other. Therefore, the mounting directionality can also be reduced. 
         [0082]    In the fifth embodiment, the arrangement of the wires W 1  and W 2  (with the turn numbers  2  and  3 ) within the first sparsely-wound portion S 1  is set in order that there are distances L 1 , L 2 , and L 3  between adjacent pair-wires in order from the position closest to the flange portion  11   b , as shown in  FIG. 13 . Similarly, the arrangement of the wires W 1  and W 2  (with the turn numbers  10  and  11 ) within the second sparsely-wound portion S 2  is set in order that there are distances L 1 , L 2 , and L 3  between adjacent pair-wires in order from the position closest to the flange portion  11   c , as shown in  FIG. 13 . 
         [0083]    With this configuration, values of the distances L 1  to L 3  can be appropriately adjusted. Therefore, in the common mode filter  10  according to the fifth embodiment, it is easier to adjust the mode conversion characteristics to a desired value as compared to the first to fourth embodiments. In view of obtaining the effect of reducing the mode conversion characteristics more efficiently (with a narrower space), it is preferable to select specific values of the distances L 1  to L 3  so as to satisfy L 1 &gt;L 2 &gt;L 3  as exemplified in  FIG. 13 . With this configuration, as spaces are closer to the flange portions  11   b  and  11   c , the width of the spaces can be larger, and therefore it is possible to reduce the mode conversion characteristics efficiently. 
         [0084]    It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention. 
         [0085]    It is possible to apply the configuration as shown in the fifth embodiment for example ( FIG. 13 ), in which the width of plural spaces within a sparsely-wound portion is appropriately adjusted, not only to the bifilar winding shown in  FIG. 13 , but also to layer winding. For example, also in  FIG. 8 , two spaces are provided within each of the first and second sparsely-wound portions S 1  and S 2  (on both sides of the wires W 1  and W 2  with the turn number  4  and on both sides of the wires W 1  and W 2  with the turn number  8 ). The width of these spaces can be appropriately adjusted, thereby adjusting the mode conversion characteristics to a desired value. 
         [0086]    In the cross-sectional view of the wires W 1  and W 2  shown in  FIG. 4  and the like, the length of the common mode filter  10  in the y direction is different between the embodiments only for convenience of illustration. The size of the common mode filter  10  is decided according to JIS. At the time of commercialization, the size of the common mode filter  10  is decided from among several standardized sizes (such as 0403, 0605, and 0806). Therefore, it is necessary to select and employ the common mode filter  10  that has characteristics appropriate to its size from the above embodiments. For example, a larger number of windings are obtained by layer winding than by bifilar winding from the same size. Therefore, it suffices that when the size of the common mode filter  10  is small, the layer winding is employed. For another example, when there is room for increasing the size of the common mode filter  10 , it suffices that the examples in  FIGS. 7 and 12  are employed, in which sparsely-wound portions are provided at a position close to the flange portions  11   b  and  11   c . In contrast, when there is no room for increasing the size of the common mode filter  10 , it suffices that the examples in  FIGS. 4 and 9  are employed, in which one sparsely-wound portion is provided at the center. 
         [0087]    Other than that, it is preferable to appropriately select a most appropriate embodiment according to required conditions, such as to employ the example in  FIG. 6  in a case where reduction in the directionality is strictly required when the example in  FIG. 4  is to be employed, or to employ the example in  FIG. 12  in a case where fine adjustments of the mode conversion characteristics are required.