Patent Publication Number: US-10319513-B2

Title: Common mode choke coil

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of priority to Japanese Patent Application 2015-243692 filed Dec. 15, 2015, the entire content of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a common mode choke coil, in particular, a wire-wound common mode choke coil with two wires wound around a winding core part having two end portions. 
     BACKGROUND 
     Referring to  FIGS. 11 and 12 , a general configuration of a common mode choke coil  31  will be described. 
     As illustrated in  FIG. 11 , the common mode choke coil  31  includes a core  32 , and first and second wires  33  and  34  each forming an inductor. The core  32  is made of an electrical insulating material, more specifically, a material such as alumina as an example of a dielectric, a Ni—Zi-based ferrite as an example of a magnetic material, or resin. The core  32  has a substantially rectangular cross-sectional shape as a whole. The wires  33  and  34  are formed by, for example, a copper wire with an insulating coating. 
     The core  32  has a winding core part  35 , and first and second flange parts  36  and  37  provided at opposite end portions of the winding core part  35 . The first and second wires  33  and  34  are wound around the winding core part  35  in a substantially helical manner with a substantially equal number of turns while running parallel to each other from a first end portion  38  where the first flange part  36  is located toward a second end portion  39  where the second flange part  37  is located. 
     First and second terminal electrodes  41  and  42  are provided in the first flange part  36 , and third and fourth terminal electrodes  43  and  44  are provided in the second flange part  37 . The terminal electrodes  41  to  44  are formed by a method such as baking of an electrically conductive paste or plating of an electrically conductive metal. As can be appreciated from the locations of the terminal electrodes  41  to  44 ,  FIG. 11  depicts the common mode choke coil  31  in such a position that its mounting surface, which is orientated toward the mount board, faces upward. 
     Opposite end portions of the first wire  33  are connected to the first and third terminal electrodes  41  and  43 , and opposite end portions of the second wire  34  are connected to the second and fourth terminal electrodes  42  and  44 . These connections are made by, for example, thermo-compression bonding. 
     The common mode choke coil  31  further includes a top plate  45 . Like the core  32 , the top plate  45  is made of an electrical insulating material, more specifically, a material such as alumina as an example of a dielectric, a Ni—Zi-based ferrite as an example of a magnetic material, or resin. If the core  32  and the top plate  45  are each made of a magnetic material, when the top plate  45  is disposed so as to connect the first and second flange parts  36  and  37  with each other, the core  32  forms a closed magnetic circuit in cooperation with the top plate  45 . 
     The common mode choke coil  31  configured as described above gives an equivalent circuit as illustrated in  FIG. 12 . In  FIG. 12 , elements corresponding to those illustrated in  FIG. 11  are denoted by the same reference signs. 
     Referring to  FIG. 12 , the common mode choke coil  31  includes a first inductor  46 , and a second inductor  47 . The first inductor  46  is formed by the first wire  33  connected between the first and third terminal electrodes  41  and  43 . The second inductor  47  is formed by the second wire  34  connected between the second and fourth terminal electrodes  42  and  44 . The first and second inductors  46  and  47  are magnetically coupled to each other. 
     Although not clearly illustrated in  FIG. 11 , the first wire  33  is wound so as to form a first layer that contacts the peripheral surface of the winding core part  35 , and the second wire  34  is wound so as to form a second layer on the outer side of the first layer, with a part of the second wire  34  fitting in the recess defined between adjacent turns of the first wire  33 . 
     A problem often encountered by the common mode choke coil  31  mentioned above with increased frequency of signals input to the common mode choke coil  31  is the increased mode conversion characteristics, which represent the proportion of input differential signal components that are converted into and output as common mode noise. For example, Japanese Unexamined Patent Application Publication No. 2014-120730 cites imbalance in stray capacitance (distributed capacitance) generated between different turns of the first and second wires  33  and  34  as the cause of this problem. 
     Accordingly, the technique disclosed in Japanese Unexamined Patent Application Publication No. 2014-120730 employs, for example, the manner of winding the wires  33  and  34  as illustrated in  FIG. 13 . 
     In  FIG. 13 , the cross-sections representing the first wire  33  are shaded to clearly distinguish the first wire  33  from the second wire  34 . Further, the ordinal numbers of turns “1” to “12” as counted from the first end portion  38  of the winding core part  35  are written within the respective cross-sections of the first and second wires  33  and  34  illustrated in  FIG. 13 . 
     In  FIG. 13 , among various portions of the first and second wires  33  and  34  wound around the winding core part  35 , the portions located forward of the winding core part  35  and the portions hidden behind the winding core part  35  are schematically indicated respectively by solid and broken lines. It is to be noted that  FIG. 13  does not depict all of the portions of the wires  33  and  34  located forward of the winding core part  35  and hidden behind the winding core part  35 . 
     Referring to  FIG. 13 , the winding core part  35  has a first winding region A, a switching region C, and a second winding region B in this order along the axis of the winding core part  35 . 
     (1) In the first winding region A, the respective same-numbered turns of the first and second wires  33  and  34  lie adjacent to each other with each turn of the first wire  33  being located closer to the first end portion  38  than the corresponding same-numbered turn of the second wire  34 . 
     (2) In the second winding region B, the respective same-numbered turns of the first and second wires  33  and  34  lie adjacent to each other with each turn of the first wire  33  being located closer to the second end portion  39  than the corresponding same-numbered turn of the second wire  34 . 
     (3) In the switching region C located between the first winding region A and the second winding region B, the first wire and the second wire  34  cross each other such that the relative positions of the turns of the first wire  33  and the turns of the second wire  34  are switched. 
     In addressing the problem of increased mode conversion, the technique described in Japanese Unexamined Patent Application Publication No. 2014-120730 makes the winding structure of the wires  33  and  34  in the first winding region A and the winding structure of the wires  33  and  34  in the second winding region B symmetric about a centerline C1 of the switching region C in order to balance out stray capacitances (distributed capacitances) generated between different turns of the first and second wires  33  and  34 . In other words, the number of turns of each of the wires  33  and  34  in the first winding region A, and the number of turns of each of the wires  33  and  34  in the second winding region B are made substantially equal to each other. 
     According to Japanese Unexamined Patent Application Publication No. 2014-120730, the winding structure of the wires  33  and  34  is made symmetric as mentioned above so that the distributed capacitance in the first winding region A and the distributed capacitance in the second winding region B are respectively generated in parallel to the first and second inductors  46  and  47  (see  FIG. 12 ). This causes the resonance point of the LC circuit formed by the first wire  33  and the resonance point of the LC circuit formed by the second wire  34  to both change, but the balance between the two resonance points remains unchanged, thus making it possible to reduce mode conversion. 
     SUMMARY 
     The technique described in Japanese Unexamined Patent Application Publication No. 2014-120730 employs the symmetric winding structure of the wires  33  and  34  mentioned above to reduce mode conversion. In actuality, however, it is nearly impossible to achieve a perfect symmetry of the winding structure for the common mode choke coil  31 . 
     For example, the wires  33  and  34  are wound in a substantially helical manner, which means that any attempt to physically position the wires  33  and  34  in a laterally symmetrical fashion does not result in prefect symmetry. 
     Further, since the two wires  33  and  34  are wound so as to substantially maintain a positional relationship such that the first wire  33  is always located at the inner side and the second wire  34  is always located at the outer side, a difference in inductance value is maintained between the first inductor  46  formed by the first wire  33  and the second inductor  47  formed by the second wire  34 . Thus, the resonant frequency does not match between the first inductor  46  and the second inductor  47 . 
     To mount the common mode choke coil  31  of a chip type onto a mount board, the common mode choke coil  31  is soldered by use of the terminal electrodes  41  to  44 . At this time, owing to various factors such as the shapes of the terminal electrodes  41  to  44  or the shapes of electrically conductive lands on the mount board, the soldering applied to each of the terminal electrodes  41  to  44  tends to become uneven, which also introduces asymmetry. 
     It has been found that the asymmetry introduced to the winding structure or portions other than the winding structure in this way also introduces asymmetry, that is, directionality to electrical characteristics such as inductance and capacitance. Such directionality makes it impossible to achieve mode-conversion reduction as suggested by theory. 
     Accordingly, it is an object of the present disclosure to provide a common mode choke coil that allows mode conversion to be reduced without pursuing symmetry. 
     A common mode choke coil according to one embodiment of the present disclosure includes a core having a winding core part, and a first flange part and a second flange part respectively provided in a first end portion and a second end portion of the winding core part, the first and second end portions being located opposite to each other, a first wire and a second wire that are wound around the winding core part in a substantially helical manner with substantially equal number of turns while running parallel to each other, a first terminal electrode and a second terminal electrode that are provided in the first flange part, the first terminal electrode and the second terminal electrode being respectively connected with a first end of the first wire and a first end of the second wire, and a third terminal electrode and a fourth terminal electrode that are provided in the second flange part, the third terminal electrode and the fourth terminal electrode being respectively connected with a second end of the first wire and a second end of the second wire. 
     The winding core part has a first winding region, a switching region, and a second winding region in this order along an axis of the winding core part. 
     In the first winding region, the respective same-numbered turns of the first and second wires lie adjacent to each other, with each turn of the first wire being located closer to the first end portion than the corresponding same-numbered turn of the second wire. 
     In the second winding region, the respective same-numbered turns of the first and second wires lie adjacent to each other with each turn of the first wire being located closer to the second end portion than the corresponding same-numbered turn of the second wire. 
     In the switching region, the first wire and the second wire cross each other such that the relative positions of the turns of the first wire and the turns of the second wire are switched. 
     In the common mode choke coil configured as described above, number of turns of the first and second wires in the first winding region differs from number of turns of the first and second wires in the second winding region. 
     As described above, to address the above-mentioned problem, the present disclosure relies on asymmetry, which runs counter to the symmetry pursued by the technique described in Japanese Unexamined Patent Application Publication No. 2014-120730. It has been found that this configuration results in improved mode conversion characteristics compared to physically symmetric configurations. 
     The reliance on asymmetry as mentioned above introduces directionality to the electrical characteristics of the common mode choke coil. Accordingly, the common mode choke coil according to another embodiment of the present disclosure preferably further includes a mark that discriminates between the first flange part and the second flange part. 
     In the common mode choke coil according to another embodiment of the present disclosure, preferably, the number of turns of the first and second wires in one region of the first winding region and the second winding region is more than or equal to 1.5 times that in the other region of the first winding region and the second winding region. 
     In the common mode choke coil according to another embodiment of the present disclosure, preferably, a difference between the number of turns of the first and second wires in the first winding region and the number of turns of the first and second wires in the second winding region is more than or equal to five. 
     The common mode choke coil according to another embodiment of the present disclosure may employ any one of the first and second winding arrangements described below for the first and second wires. 
     With the first winding arrangement, the first wire is wound such that the first wire forms a first layer in contact with the peripheral surface of the winding core part, and in the first and second winding regions, the second wire is wound such that the second wire forms a second layer on the outer side of the first layer, with a part of the second wire fitting in a recess defined between adjacent turns of the first wire. 
     With the second winding arrangement, in the first and second winding regions, the first wire and the second wire are both wound in contact with the peripheral surface of the winding core part. 
     The common mode choke coil according to one embodiment of the present disclosure allows for improved mode conversion characteristics observed from a given direction compared to arrangements in which the first and second wires have a physically symmetric winding structure. 
     In a common mode choke coil, owing to its inherent structure, top-bottom or left-right asymmetry always develops with regard to inductance or capacitance in portions of the coil other than the wire windings. Such asymmetry is determined by factors such as the positional relationship between the first and second wires to be wound or the positional relationship between the terminal electrodes and the electrically conductive lands on the mount board, and thus directionality is always present. 
     The present disclosure actively makes the number of turns of the first and second wires different between the first winding region and the second winding region, thus compensating for the above-mentioned asymmetry to achieve improved characteristics. 
     Other features, elements, characteristics and advantages of the present disclosure will become more apparent from the following detailed description with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are respectively a plan view and a bottom view illustrating the outward appearance of a common mode choke coil according to a first embodiment of the present disclosure. 
         FIG. 2  is a cross-sectional view schematically illustrating the state of winding of first and second wires in the common mode choke coil illustrated in  FIG. 1B . 
         FIG. 3  is a cross-sectional view for explaining the procedure for winding the first wire illustrated in  FIG. 2 . 
         FIG. 4  is a cross-sectional view for explaining the procedure for winding the second wire illustrated in  FIG. 2 . 
         FIG. 5  illustrates the S-parameter frequency characteristics of a common mode choke coil in which the number of turns of each of the first and second wires in a first winding region and the number of turns of each of the first and second wires in a second winding region are substantially equal, with the solid line indicating a case in which a signal is input from the side where first and second terminal electrodes are located, and the broken line indicating a case in which a signal is input from the side where third and fourth terminal electrodes are located. 
         FIG. 6  illustrates the S-parameter frequency characteristics of a common mode choke coil in which the number of turns of each of the first and second wires in the second winding region is one less than the number of turns of each of the first and second wires in the first winding region, with the solid line indicating a case in which a signal is input from the side where the first and second terminal electrodes are located, and the broken line indicating a case in which a signal is input from the side where the third and fourth terminal electrodes are located. 
         FIG. 7  illustrates the S-parameter frequency characteristics of a common mode choke coil in which the number of turns of each of the first and second wires in the second winding region is three less than the number of turns of each of the first and second wires in the first winding region, with the solid line indicating a case in which a signal is input from the side where the first and second terminal electrodes are located, and the broken line indicating a case in which a signal is input from the side where the third and fourth terminal electrodes are located. 
         FIG. 8  illustrates the S-parameter frequency characteristics of a common mode choke coil in which the number of turns of each of the first and second wires in the second winding region is five less than the number of turns of each of the first and second wires in the first winding region, with the solid line indicating a case in which a signal is input from the side where the first and second terminal electrodes are located, and the broken line indicating a case in which a signal is input from the side where the third and fourth terminal electrodes are located. 
         FIG. 9  illustrates the S-parameter frequency characteristics of a common mode choke coil in which the number of turns of each of the first and second wires in the second winding region is seven less than the number of turns of each of the first and second wires in the first winding region, with the solid line indicating a case in which a signal is input from the side where the first and second terminal electrodes are located, and the broken line indicating a case in which a signal is input from the side where the third and fourth terminal electrodes are located. 
         FIG. 10  is an illustration corresponding to  FIG. 2  for explaining a second embodiment of the present disclosure. 
         FIG. 11  is a perspective view illustrating the outward appearance of a common mode choke coil according to related art. 
         FIG. 12  is an equivalent circuit diagram of the common mode choke coil illustrated in  FIG. 11 . 
         FIG. 13  is a cross-sectional view schematically illustrating, for the common mode choke coil illustrated in  FIG. 11 , the state of winding of the first and second wires described in Japanese Unexamined Patent Application Publication No. 2014-120730. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A and 1B  illustrate a common mode choke coil  51  according to a first embodiment of the present disclosure. The common mode choke coil  51  illustrated in  FIGS. 1A and 1B  differs from the common mode choke coil  31  illustrated in  FIG. 11  mentioned above only in how the first and second wires  33  and  34  are wound, and is otherwise of substantially the same configuration as the common mode choke coil  31 . Accordingly, in  FIGS. 1A and 1B , elements corresponding to the elements illustrated in  FIG. 11  are denoted by the same reference signs to avoid repetitive description. 
     In  FIG. 1B , the first wire  33  and the second wire  34  are schematically depicted darkened and hollow, respectively, to clearly distinguish the first wire  33  and the second wire  34  from each other. 
     The state of winding of the first and second wires  33  and  34  in the common mode choke coil  51  illustrated in  FIGS. 1A and 1B  is depicted as a schematic cross-sectional view in  FIG. 2 . As can be appreciated from a comparison between  FIG. 1B  and  FIG. 2 , the number of turns of the wires  33  and  34  illustrated in  FIG. 1B  is less than the number of turns illustrated in  FIG. 2 , indicating that the wires  33  and  34  are depicted with some details omitted in  FIG. 1B . In  FIGS. 2 to 4 , the cross-sections representing the first wire  33  are shaded to clearly distinguish the first wire  33  from the second wire  34 . 
     The first and second wires  33  and  34  are wound around the winding core part  35  in a substantially helical manner with a substantially equal number of turns while running parallel to each other from the first end portion  38  where the first flange part  36  is located toward the second end portion  39  where the second flange part  37  is located. Further, the ordinal numbers of turns “1” to “27” as counted from the first end portion  38  of the winding core part  35  are written within the respective cross-sections of the first and second wires  33  and  34  illustrated in  FIG. 2 . The ordinal number of turns written within the respective cross-sections of the first and second wires  33  and  34  are also similarly written in  FIGS. 3, 4, and 10 . 
     The first wire  33  is wound so as to form a first layer that contacts the peripheral surface of the winding core part  35 , and the second wire  34  is wound so as to form a second layer on the outer side of the first layer, with the part of the second wire  34  fitting in the recess defined between adjacent turns of the first wire  33 . 
     The state of winding of the first and second wires  33  and  34  will be described in detail with reference to  FIGS. 3 and 4  in conjunction with  FIG. 2 . In  FIGS. 3 and 4 , among various portions of the first and second wires  33  and  34  wound around the winding core part  35 , the portions located forward of the winding core part  35  and the portions hidden behind the winding core part  35  are schematically indicated respectively by solid and broken lines. As for the broken lines indicating the portions of the wires  33  and  34  hidden behind the winding core part  35 , not all of the broken lines are depicted but only those for characteristic locations are depicted. 
     In  FIGS. 2 to 4 , the “first winding region A”, the “switching region C”, and the “second winding region B” are depicted in this order from the first end portion  38  of the winding core part  35  toward the second end portion  39 . That is, the winding core part  35  has the first winding region A, the switching region C, and the second winding region B in this order along the axis of the winding core part  35 . The state of winding of the first and second wires  33  and  34  will be described below separately for each of the regions A to C. 
     First, the start edge of the first wire  33  is connected to the first terminal electrode  41  (see  FIG. 1B ). 
     Then, as clearly illustrated in  FIG. 3 , the 1st to 15th turns of the first wire  33  are wound in the first winding region A with no gap between adjacent turns. 
     Then, in the switching region C, which is located at the position of the transition from the 16th turn of the first wire  33  to the 17th turn, a gap is created between the 16th and 17th turns of the first wire  33 . 
     Then, in the second winding region B, the 18th to 27th turns of the first wire  33  are wound, again with no gap between adjacent turns. 
     Then, the end edge of the first wire  33  is connected to the third terminal electrode  43  (see  FIG. 1B ). 
     As for the second wire  34 , its start edge is connected to the second terminal electrode  42  (see  FIG. 1B ). 
     Then, as clearly illustrated in  FIG. 4 , the 1st to 15th turns of the second wire  34  are wound in the first winding region A such that the 1st turn of the second wire  34  fits in the recess defined between, for example, the 1st and 2nd turns of the first wire  33 , or stated in a more generalized way, the nth turn of the second wire  34  fits in the recess defined between the nth and (n+1)-th turns of the first wire  33 . 
     Next, in the switching region C, the 16th turn of the second wire  34  is wound with a gap between the 16th turn and the 15th turn, and further, the 17th turn of the second wire  34  is wound with a gap between the 17th turn and the 16th turn. The 16th and 17th turns of the second wire  34  are wound in contact with the peripheral surface of the winding core part  35 . As can be appreciated from a comparison between  FIG. 3  and  FIG. 4 , the second wire  34  crosses the first wire  33  at this time. 
     Then, in the second winding region B, the 18th to 27th turns of the second wire  34  are wound such that after a gap is created between the 18th turn of the second wire  34  and the 17th turn of the second wire  34 , the 18th turn of the second wire  34  fits in the recess defined between the 17th and 18th turns of the first wire  33 , or stated in a more generalized way, the (n+1)-th turn of the second wire  34  fits in the recess defined between the nth and (n+1)-th turns of the first wire  33 . 
     Then, the end edge of the first wire  34  is connected to the fourth terminal electrode  44  (see  FIG. 1B ). 
     Characteristic features that can be found from the winding state mentioned above will be listed below. 
     First, in the first winding region A, the respective same-numbered turns of the first and second wires  33  and  34  lie adjacent to each other, with each turn of the first wire  33  being located closer to the first end portion  38  than the corresponding same-numbered turn of the second wire  34 . 
     In the second winding region B, the respective same-numbered turns of the first and second wires  33  and  34  lie adjacent to each other, with each turn of the first wire  33  being located closer to the second end portion  39  than the corresponding same-numbered turn of the second wire  34 . 
     In the switching region C, the first wire  33  and the second wire  34  cross each other so that the relative positions of the turns of the first wire  33  and the turns of the second wire  34  are switched. 
     Further, the number of turns of each of the first and second wires  33  and  34  differs from the number of turns of the first and second wires in the second winding region B. That is, in the first winding region A, the number of turns of the first wire  33  is “15”, and the number of turns of the second wire  34  is “15”, whereas in the second winding region B, the number of turns of the first wire  33  is “10”, and the number of turns of the second wire  34  is “10”. 
     From a comparison between the number of turns of each of the first and second wires  33  and  34  in the first winding region A and the number of turns of each of the first and second wires  33  and  34  in the second winding region B, the following observations can be made. 
     For the first wire  33 , its number of turns “15” in the first winding region A is exactly 1.5 times its number of turns “10” in the second winding region B. For the second wire  34 , its number of turns “15” in the first winding region A is exactly 1.5 times its number of turns “10” in the second winding region B. That is, the number of turns of each of the first and second wires  33  and  34  in the first winding region A is more than or equal to 1.5 times that in the second winding region B. 
     Further, for the first wire  33 , its number of turns “10” in the second winding region B is five less than its number of turns “15” in the first winding region A. For the second wire  34 , its number of turns “10” in the second winding region B is five less than its number of turns “15” in the first winding region A. That is, a difference between the number of turns of each of the first and second wires  33  and  34  in the first winding region A and the number of turns of each of the first and second wires  33  and  34  in the second winding region B is more than or equal to five. 
     The above-mentioned asymmetry introduced by making the number of turns of each of the first and second wires  33  and  34  differ between the first winding region A and the second winding region B introduces directionality to the electrical characteristics of the common mode choke coil  51 . The resulting configuration has been found to provide improved mode conversion characteristics compared to physically symmetric configurations. This will be described below with reference to  FIGS. 5 to 9 . 
       FIGS. 5 to 9  each illustrate the S-parameter frequency characteristics of a common mode choke coil. In each of  FIGS. 5 to 9 , the solid line indicates a case in which the signal is input in the forward direction, that is, from the side of the common mode choke coil where the first and second terminal electrodes are located, and the broken line indicates a case in which the signal is input in the reverse direction, that is, from the opposite side of the common mode choke coil where the third and fourth terminal electrodes are located. With regard to the number of turns of each of the first and second wires in the first winding region, T1, and the number of turns of each of the first and second wires in the second winding region, T2,  FIGS. 5 to 9  illustrate the following cases: 
       FIG. 5  illustrates a case in which T1=15 and T2=15 (equal-number-of-turns); 
       FIG. 6  illustrates a case in which T1=15 and T2=14 (one-turn-less); 
       FIG. 7  illustrates a case in which T1=15 and T2=12 (three-turns-less); 
       FIG. 8  illustrates a case in which T1=15 and T2=10 (five-turns-less); and 
       FIG. 9  illustrates a case in which T1=15 and T2=8 (seven-turns-less). 
     Lower S-parameter values illustrated in  FIGS. 5 to 9  indicate improved mode conversion characteristics, that is, reduced mode conversion. 
     With the “equal-number-of-turns” arrangement illustrated in  FIG. 5 , owing to the above-mentioned inability to achieve a perfectly symmetrical winding structure, S-parameter values do not match but very similar characteristics are obtained between the case of the forward direction indicated by the solid line and the case of the reverse direction indicated by the broken line. 
     With the “one-turn-less” arrangement illustrated in  FIG. 6 , at lower frequencies, there is substantially no difference in S-parameter values between the case of the forward direction indicated by the solid line and the case of the reverse direction indicated by the broken line, but at higher frequencies, slightly lower S-parameter values are obtained in the case of the reverse direction indicated by the broken line than in the case of the forward direction indicated by the solid line. 
     In the case of the three-turns-less arrangement illustrated in  FIG. 7 , at lower frequencies, there is substantially no difference in S-parameter values between the case of the forward direction indicated by the solid line and the case of the reverse direction indicated by the broken line, but at higher frequencies, slightly lower S-parameter values are obtained in the case of the forward direction indicated by the solid line than in the case of the reverse direction indicated by the broken line. 
     With the “five-turns-less” arrangement illustrated in  FIG. 8 , at lower frequencies, clearly lower S-parameter values are obtained in the case of the reverse direction indicated by the broken line than in the case of the forward direction indicated by the solid line, and at higher frequencies, clearly lower S-parameter values are obtained in the case of the forward direction indicated by the solid line than in the case of the reverse direction indicated by the broken line. The “five-turns-less” arrangement illustrated in  FIG. 8  corresponds to the embodiment illustrated in  FIG. 2 . 
     With the “seven-turns-less” arrangement illustrated in  FIG. 9 , at lower frequencies, clearly lower S-parameter values are obtained in the case of the reverse direction indicated by the broken line than in the case of the forward direction indicated by the solid line, and at higher frequencies, clearly lower S-parameter values are obtained in the case of the forward direction indicated by the solid line than in the case of the reverse direction indicated by the broken line. 
     As can be appreciated from the trend of S-parameter values mentioned above, when a difference between T1 and T2 is more than or equal to five, or in terms of the ratio between T1 and T2, when T1 is more than or equal to 1.5 times of T2 as in the case of the “five-turns-less” arrangement illustrated in  FIG. 8  and the “seven-turns-less” arrangement illustrated in  FIG. 9 , a noticeable difference appears between the case of the forward direction indicated by the solid line and the case of the reverse direction indicated by the broken line, and thus a clear directionality is observed. Therefore, according to the present disclosure, it is preferable that a difference between T1 and T2 is more than or equal to five, or T1 is more than or equal to 1.5 times of T2. It is to be noted, however, that some directionality is obtained as long as T1 and T2 are different from each other, even if their difference or ratio is outside the preferred range mentioned above. 
     In actual use of the common mode choke coil, in the case of, for example, the “five-turns-less” arrangement illustrated in  FIG. 8  and the “seven-turns-less” arrangement illustrated in  FIG. 9 , it is recommended to mount the common mode choke coil in such a way that the signal flows in the reverse direction during use at lower frequencies, and mount the common mode choke coil in such a way that the signal flows in the forward direction during use at higher frequencies. 
     Since directionality develops in the electrical characteristics of the common mode choke coil as described above, as illustrated in  FIG. 1A , the common mode choke coil  51  preferably has a mark  52  provided on, for example, the top plate  45  to discriminate between the first flange part  36  and the second flange part  37 . This allows the common mode choke coil  51  to be mounted in an orientation (forward or reverse) that makes it possible to obtain desired frequency characteristics. From this point of view, the mark  52  may be any mark that can be identified by a mounting apparatus. If the mark  52  is made visually identifiable, this facilitates handling of the common mode choke coil  51  during the manufacturing process. Specifically, for example, even if an error such as an error in the transport of the common mode choke coil  51  occurs during the packaging process in which the common mode choke coil  51  is taped onto a reel, the orientation of the common mode choke coil  51  can be identified/corrected on the spot, thus allowing the process to be readily resumed. 
     The mark  52  is formed by, for example, a laser. The mark  52  may be made on, for example, the core  32  instead of the top plate  45  as illustrated in  FIG. 1A . The mark  52  may not necessarily be in the form of the geometrical figure as illustrated in  FIG. 1A . As long as the mark  52  allows the first flange part  36  and the second flange part  37  to be discriminated from each other, the mark  52  may be in the form of other geometrical figures, or may be in the form of, for example, numerals, letters, or symbols. Further, the mark  52  may be substituted for by numbers such as the product numbers or lot numbers to be assigned to individual products. 
     If, like the mark  52  of a substantially circular shape illustrated in  FIG. 1A , the mark used is one with no spatial distinctions such as top/bottom or left/right in itself, then the mark  52  is made at a position offset toward one side of the top plate  45 , for example. In this case, the orientation of the common mode choke coil  51  can be identified based on where the mark  52  is located. 
     If the number of turns of the wires  33  and  34  differs relatively greatly between the first winding region A and the second winding region B, the orientation of the common mode choke coil  51  can be easily recognized simply by visually checking the state of winding of the wires  33  and  34 . This allows the orientation of the common mode choke coil  51  to be identified at some midway point during the manufacturing process, for example, prior to attaching the top plate  45  to the core  32 . Although it can be said that there is no particular need to provide the mark  52  in this case, the following advantage can be also anticipated with such an arrangement: in providing the mark  52 , it is possible to prevent the mark  52  from being made in a wrong way. During mounting of the common mode choke coil  51 , the location of the top plate  45  coincides with the location where a nozzle picks up the common mode choke coil  51 . Therefore, if the mark  52  is made on the top plate  45 , pickup of the common mode choke coil  51  by the mounting apparatus and identification of the orientation of the common mode choke coil  51  can be performed simultaneously, allowing for a streamlined mounting process. 
     Next, referring to  FIG. 10 , a common mode choke coil  51   a  according to a second embodiment of the present disclosure will be described. Like  FIG. 2 ,  FIG. 10  illustrates the state of winding of the first and second wires  33  and  34  in the common mode choke coil  51   a . Accordingly, in  FIG. 10 , elements corresponding to the elements illustrated in  FIG. 2  are denoted by the same reference signs to avoid repetitive description. 
     In the common mode choke coil  51   a  illustrated in  FIG. 10 , the first wire  33  and the second wire  34  are both wound in contact with the peripheral surface of the winding core part  35  while running parallel to each other. 
     The common mode choke coil  51   a  illustrated in  FIG. 10  is the same as the common mode choke coil  51  illustrated in  FIG. 2  with regard to which one of the followings is to be located closer to the first or second end portion  38  or  39  in each of the first winding region A and the second winding region B: a given turn of the first wire  33  and the corresponding same-numbered turn of the second wire  34 . 
     That is, in the first winding region A, each turn of the first wire  33  is located closer to the first end portion  38  than the corresponding same-numbered turn of the second wire  34 . 
     In the second winding region B, each turn of the first wire  33  is located closer to the second end portion  39  than the corresponding same-numbered turn of the second wire  34 . 
     In the switching region C, the first wire  33  and the second wire  34  cross each other so that the relative positions of the turns of the first wire  33  and the turns of the second wire  34  are switched. 
     More specifically, in the first winding region A, the 1st to 15th turns of each of the first and second wires  33  and  34  are wound such that the respective same-numbered turns of the first and second wires  33  and  34  lie adjacent to each other, with the first wire  33  preceding the second wire  34 . 
     Then, in the switching region C, which is located at the position of the transition from the 16th turn of the first wire  33  to the 17th turn, a gap is created between the 16th and 17th turns of the first wire  33 . The switching region C is also located at the position of the transition from the 16th turn of the second wire  34  to the 17th turn, with a gap being created between the 16th and 17th turns of the second wire  34 . Further, in the switching region C, the second wire  34  crosses the first wire  33 . 
     Then, in the second winding region B, the 18th to 27th turns of each of the first and second wires  33  and  34  are wound such that the respective same-numbered turns of the first and second wires  33  and  34  lie adjacent to each other, with the second wire  34  preceding the first wire  33 . 
     In the case of the embodiment illustrated in  FIG. 10  as well, the number of turns of each of the first and second wires  33  and  34  is made to differ between the first winding region A and the second winding region B. That is, in the first winding region A, the number of turns of the first wire  33  is “15”, and the number of turns of the second wire  34  is “15”, whereas in the second winding region B, the number of turns of the first wire  33  is “10”, and the number of turns of the second wire  34  is “10”. 
     Therefore, for the first wire  33 , its number of turns “15” in the first winding region A is exactly 1.5 times its number of turns “10” in the second winding region B. For the second wire  34 , its number of turns “15” in the first winding region A is exactly 1.5 times its number of turns “10” in the second winding region B. That is, the number of turns of each of the first and second wires  33  and  34  in the first winding region A and the number of turns of each of the first and second wires  33  and  34  in the second winding region B have a ratio such that one of the numbers of turns in the first and second winding regions is about 1.5 times or more greater than the other. 
     Further, for the first wire  33 , its number of turns “10” in the second winding region B is five less than its number of turns “15” in the first winding region A. For the second wire  34 , its number of turns “10” in the second winding region B is five less than its number of turns “15” in the first winding region A. That is, the number of turns of each of the first and second wires  33  and  34  in the first winding region A and the number of turns of each of the first and second wires  33  and  34  in the second winding region B have a difference such that one of the numbers of turns in the first and second winding regions is about five or more less than the other. 
     As described above, asymmetry is also achieved for the common mode choke coil  51   a  according to the second embodiment such that the number of turns of each of the first and second wires  33  and  34  is made to differ between the first winding region A and the second winding region B. As in the first embodiment, this configuration introduces directionality to the electrical characteristics of the common mode choke coil  51   a . The resulting configuration provides improved mode conversion characteristics compared to physically symmetric configurations. 
     Although the present disclosure has been described above with reference to the illustrated embodiments of a common mode choke coil, various other modifications are possible within the scope of the present disclosure. 
     For example, the number of turns of each of the first and second wires included in the common mode choke coil may be increased or decreased to any value that satisfies the conditions set forth in the present disclosure. Accordingly, depending on how many turns each of the first and second wires is wound in the first winding region and how many turns each of the first and second wires is wound in the second winding region, the numbers of turns that is 1.5 times the numbers of turns in one region which is the first winding region or the second winding region may not be the same as the numbers of turns that is five less than the numbers of turns in the one region. 
     The direction in which the number of turns is counted may be reversed from the direction described above with reference to the embodiments. 
     Although the first and second wires  33  and  34  are located in immediate proximity to each other in the first winding region A and the second winding region B in the above embodiments, this is not to be construed restrictively. A slight gap may be present between the first and second wires  33  and  34 . 
     Although an explicit space is illustrated to exist between the first and second wires  33  and  34  in the switching region C in the above embodiments, this space is not always necessary. The only requirement in this regard is that in the switching region, the two wires cross each other to have their relative positions switched. Specifically, for example, the relative positions of the two wires may be switched by winding one of the wires in closely spaced turns while winding the other wire in widely spaced turns. 
     It is to be noted that the embodiments illustrated are intended to be illustrative, and among different embodiments, some of their features may be substituted for or combined with each other. 
     While some embodiments of the disclosure have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the disclosure. The scope of the disclosure, therefore, is to be determined solely by the following claims.