Patent Publication Number: US-9412511-B2

Title: Transformer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Japanese Patent Application No. 2010-284803 filed Dec. 21, 2010 in the Japan Patent Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     The present invention relates to a transformer, specifically a transformer having a gap between a primary coil and a secondary coil. 
     As shown in  FIG. 24 , as a transformer having a gap between a primary coil  10  and a secondary coil  20 , a following configuration is known: an E-type core  30   e  (an iron core or a magnetic core) having an E-shaped cross section is wound with, for example, the secondary coil  20 , and the E-type core  30   e  with the secondary coil  20  being wound therearound is arranged to face the primary coil  10 . In this connection, however, following problems exist: it is difficult to manufacture the integrally-formed E-type core  30   e ; and even if such manufacturing can be achieved, manufacturing costs thereof are costly. 
     In order to solve the above problems, it has been suggested to form an E-type core by stacking plate-like cores on one another, thereby facilitating the manufacturing of the E-type core and reducing the manufacturing costs (see, for example, Unexamined Japanese Patent Application Publication No. 2008-120239; hereinafter, referred to as “Patent Document 1”). 
     Patent Document 1 also suggests, as an embodiment of an E-type core capable of handling a small amount of an electric power compared with the integrally-formed E-type core, a transformer having a configuration in which parts of the core are removed in a striped manner. This configuration can provide an advantage of reducing materials to be used for the core, thereby reducing a weight of the transformer. 
     Moreover, since there is a problem in which the E-type core is heavy, it is suggested to use a flat-plate like core for the purpose of reducing a weight of a transformer (see, for example, Unexamined Japanese Patent Application Publication No. 2008-087733; hereinafter, referred to as “Patent Document 2”). As shown in  FIG. 25 , the flat-plate like core  30   p  is arranged adjacent to a secondary coil  20  such that the secondary coil  20  is located between the flat-plate like core  30   p  and a primary coil  10 . Alternatively, the flat-plate like core  30   p  is arranged adjacent to the primary coil  10  such that the primary coil  10  is located between the flat-plate like core  30   p  and the secondary coil  20  (this arrangement is not shown). 
     SUMMARY 
     In the E-type core such as described in Patent Document 1, a magnetic saturation phenomenon is less likely to occur due to a large volume of the core. In this case, there is an advantage in which a performance of the transformer having the E-type core can be improved, compared with a transformer having a core in which a magnetic saturation phenomenon may occur due to a small volume thereof. The E-type core, however, involves a following problem: since the E-type core has the large volume, the weight of the core is heavy; accordingly, a weight of the transformer is also heavy. 
     Moreover, as described in Patent Document 1, when the E-type core is formed by stacking the plate-like cores, an air gap is likely to be formed between contact faces of the respective plate-like cores. Since this air gap becomes a magnetic resistance, the performance of the transformer may be degraded. 
     Furthermore, as described in Patent Document 1, when the configuration in which the parts of the core are removed in the striped manner is adopted, a following problem arises: efficiency of the transformer decreases depending on a removal ratio of the core, compared with a core in which parts thereof are not removed. When an electric power to be used (transmitted) is limited depending on the removal ratio of the core, a problem of decrease in the efficiency of the transformer will not occur. However, when the electric power to be used is not limited, a problem occurs in which the efficiency of the transformer decreases because of a greater loss of the electric power in the transformer. 
     Meanwhile, use of the flat-plate like core such as described in Patent Document 2 can provide an advantage in which a volume of the core is small, thereby making it possible to reduce a weight of the core, compared with the E-type core. In this case, however, a following problem exists: an electromagnetic gap becomes greater, compared with when using the E-type core, and thus, efficiency of the transformer may be decreased. Here, the electromagnetic gap is a gap between the primary coil and the flat-plate like core arranged facing to each other. 
     In one aspect of the present invention, it is preferable to provide a transformer with improved performance, and a reduction of weight and manufacturing costs. 
     The transformer of the present invention includes a primary coil and a secondary coil, and a core. The core is to be used in combination with the primary coil or the secondary coil. Each of the primary coil and the secondary coil may be formed by winding a conductor formed into an elongated shape, for example, a linear shape. The core includes a conductor storage part and a central part. In the conductor storage part, a conductive wire portion of the primary coil or the secondary coil is to be arranged. The conductor storage part is a concave recess formed in an annular manner. In other words, the conductor storage part is capable of storing the conductive wire portion of the primary coil or the secondary coil therein. The central part is located in a region inward from the conductive wire portion of the primary coil or the secondary coil (a region inward from the conductor storage part). 
     The central part has a recessed shape having an opening that opens toward a direction opposite to a direction of an opening of the concave recess of the conductor storage part. The central part has an end, at a side opposite to a side of the opening of the central part, which is generally flat so as to be contiguous with an inner opening edge of opening edges of the conductor storage part. 
     By adopting the above configuration, the core of the transformer in the present invention can reduce the electromagnetic gap compared with the flat-plate like core such as described in Patent Document 2. The electromagnetic gap is a distance between the core (specifically, the central part) and one coil of the primary coil and the secondary coil, which is positioned opposing to the other coil to which the core is provided. Moreover, the core in the present invention is formed of the conductor storage part and the central part; therefore, parts which do not contribute to a flux path are reduced. By the above configuration, it is possible to reduce a volume of the core, compared with the E-type core such as described in Patent Document 1. Moreover, it is preferable that the core in the present invention has a thickness in which magnetic saturation of the core does not occur and distribution of a magnetic flux inside the core is uniform. 
     Further, the core may include a flange portion. The flange portion extends outward from an outer opening edge of the opening edges of the conductor storage part. By adopting this configuration, the core in the present invention can effectively collect a magnetic flux extending between the primary coil and the secondary coil, compared with a core without the flange portion. Consequently, an improved performance of the transformer in the present invention can be achieved. 
     The conductor storage part may have a thickness larger than a thickness of the central part. By constituted as above, the core in the present invention can inhibit magnetic saturation from occurring in the conductor storage part arranged at a position close to the conductor (coil) where an electric current flows. Meanwhile, in the central part, magnetic saturation is less likely to occur. Accordingly, by making the thickness of the central part be thinner than the thickness of the conductor storage part, weight reduction of the core in the present invention can be achieved. 
     The central part may include an aperture which penetrates through a member constituting the central part. The aperture may be a through-hole. The through-hole may be a slit-like hole. As a result of having the aperture in the central part, the weight of the core in the present invention can be reduced. Moreover, the central part has a low efficiency in collecting the magnetic flux existing between the primary coil and the secondary coil, compared with the conductor storage part. Therefore, even if the aperture is formed in the central part, influence due to decrease of an inductive voltage in the core of the present invention can be minimized. 
     The core according to the present invention may be formed of a plurality of plate members stacked upon one another. Each of the plurality of plate members may include one or more slits. The plurality of plate members may be stacked such that the one or more slits of any given one of the plurality of plate member does not overlap with the one or more slits of each of the plurality of plate members that is adjacent to the given one of the plurality of plate members. By adopting this configuration, if the core is expanded or contracted due to temperature change around the core and therefore, temperature change in the core, such an expansion or contraction can be absorbed by spaces in the slits. For this reason, it is possible to alleviate compression stress and tensile stress acting on the plate members constituting the core. Thereby, occurrence of breakage or the like of the core can be suppressed. 
     Each of the primary coil and the secondary coil may have at least two sides substantially parallel to each other. The primary coil may be formed to have a length longer than a length of the secondary coil in an extending direction of the two sides. In this case, the core arranged at a side of the secondary coil may include a plurality of core segments each extending in a direction substantially horizontally perpendicular to the two sides. The core segments may be arranged to be spaced apart from one another in the extending direction of the two sides. 
     By constituted as above, the core can be formed of a plurality of relatively small core segments. Thus, compared with forming the integrally-formed core, it is possible to use a material (plate member) which is relatively small as a material to be used for manufacturing the core. Moreover, compared with obtaining a larger plate member, obtaining the aforementioned small plate member is easy and less expensive. Thus, manufacturing costs of the transformer in the present invention can be reduced. 
     Moreover, it is preferable that, compared with the plate member constituting the integrally-formed core, each of the plate members each constituting the core segments has a thick plate thickness, depending on a distance at which the core segments are arranged apart from one another (hereinafter, “arrangement distance”). That is to say, it is preferable that even if the core is formed of the core segments arranged at the distance apart from one another, the core is configured to have substantially the same volume as a volume of the integrally-formed core. For example, when a ratio of a width of the core segment to a length of the arrangement distance in the aforementioned extending direction of the two sides is 1:1, the core segment may preferably have the plate thickness as twice as the plate thickness of the integrally-formed core. By constituted as above, even if the core is formed of the core segments spaced apart from one another, it is possible to inhibit the performance of the core from decreasing. 
     The conductor storage part of the core, which is to be used in combination with the secondary coil, may have a size such that only a predetermined portion of the secondary coil is arranged in the conductor storage part. 
     More specifically, each of the primary coil and the secondary coil may have at least two sides substantially parallel to each other. The primary coil may be formed to have a length longer than a length of the secondary coil in the extending direction of the two sides. In this case, the conductor storage part may be formed to have a length shorter than the length of the secondary coil in the extending direction of the two sides. A part of the secondary coil may be exposed from the conductor storage part in areas at both ends of the conductor storage part in the extending direction of the two sides. In other words, the conductor storage part may be formed to have a shape such that an outer portion thereof is removed while at least a portion on a side of the central part is not removed. 
     By the above constitution, the weight reduction of the core can be achieved. Moreover, the above-mentioned outer portion of the conductor storage part, which is located outward from the secondary coil, has a low efficiency in collecting the magnetic flux existing between the primary coil and the secondary coil, compared with other portions. Therefore, even if the conductor storage part is formed by removing the above-mentioned outer portion thereof, influence due to decrease of an inductive voltage in the secondary coil can be minimized. 
     Here, the core may be arranged to both at a side of the primary coil and the side of the secondary coil. 
     According to the transformer of the present invention, it is possible to shorten the electromagnetic gap, compared with the flat plate-like core such as described in Patent Document 2. Furthermore, since the core according to the present invention is formed of the conductor storage part and the central part, the parts which do not contribute to the flux path are reduced. Because of this, it is possible to reduce the volume of the core, compared with the E-type core such as described in Patent Document 1. Consequently, following advantageous effects can be obtained: the performance of the transformer in the present invention can be improved; and the weight as well as the manufacturing costs of the transformer in the present invention can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described below, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view showing a configuration of a transformer according to a first embodiment of the present invention; 
         FIG. 2  is a plan view showing an overall configuration of the transformer in  FIG. 1 ; 
         FIG. 3  is a view showing a core in  FIG. 1  seen from a side of a primary coil; 
         FIG. 4  is a view showing a core according to another example when seen from the side of the primary coil; 
         FIG. 5  is a cross-sectional view showing a configuration of a transformer according to a second embodiment of the present invention; 
         FIG. 6  is a view showing the core in  FIG. 5  seen from a side of a primary coil; 
         FIG. 7  is a graph illustrating a relationship between an overhang length F of a flange portion and an inductive voltage; 
         FIG. 8  is a view showing a configuration of the core of  FIG. 6  according to another example seen from the side of the primary coil; 
         FIG. 9  is a cross-sectional view showing a configuration of a transformer according to a third embodiment of the present invention; 
         FIG. 10  is a cross-sectional view showing a configuration of a transformer according to a fourth embodiment of the present invention; 
         FIG. 11  is a plan view showing an overall configuration of the transformer in  FIG. 10 ; 
         FIG. 12  is a graph illustrating a relationship between a ratio of a width W 4  of an aperture to a width W 2  of a central part, and an inductive voltage in a secondary coil; 
         FIG. 13  is a plan view showing an overall configuration of another example of the core in  FIG. 11 ; 
         FIG. 14  is a cross-sectional view showing a configuration of a transformer according to a fifth embodiment of the present invention; 
         FIG. 15  is a plan view showing an overall configuration of a transformer according to a sixth embodiment of the present invention; 
         FIG. 16  is a graph illustrating a relationship between a distance between core segments in  FIG. 15 , and an inductive voltage ratio; 
         FIG. 17  is a view showing an overall configuration of a transformer in  FIG. 15 , according to another example; 
         FIG. 18  is a plan view showing an overall configuration of a transformer according to a seventh embodiment of the present invention; 
         FIG. 19  is a view showing a modification of the present invention; 
         FIG. 20  is a view showing a modification of the present invention; 
         FIG. 21  is a view showing a modification of the present invention; 
         FIG. 22  is a view showing a modification of the present invention; 
         FIG. 23  is a view showing a modification of the present invention; 
         FIG. 24  is a schematic view showing a configuration of a transformer having a conventional E-type core; and 
         FIG. 25  is a schematic view showing a configuration of a transformer having a conventional flat plate-like core. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Hereinafter, a transformer  1  according to a first embodiment of the present invention will be described with reference to  FIGS. 1 to 4 . 
     The transformer  1  according to the first embodiment shown in  FIGS. 1 to 4  is mainly composed of a primary coil  10 , a secondary coil  20 , and a core  30 . An alternating current is to be supplied from outside to the primary coil  10 . In the secondary coil  20 , an inductive voltage is induced by a magnetic flux generated as a result of an electric current flowing into the primary coil  10 . The core  30  is provided in a vicinity of the secondary coil  20 . 
     In the transformer  1 , a voltage which is different from a voltage applied to the primary coil  10  is induced in the secondary coil  20 . The first embodiment will be explained with regard to an example in which the core  30  is provided only to the secondary coil  20 ; however, the core may be also provided to the primary coil  10  and thus, the present invention should not be limited to this example. 
     In the transformer  1 , each of the primary coil  10  and the secondary coil  20  is formed of a conductive material such as copper. Specifically, each of the primary coil  10  and the secondary coil  20  is formed such that a linear conductor is wound in a circular manner. The conductor constituting the primary coil  10  may be formed by a material having the same composition as or a material having a different composition from the conductor constituting the secondary coil  20 ; therefore, materials forming the conductors are not limited to the aforementioned material. 
     Both ends of the conductor constituting the primary coil  10  are connected to an external power source (not shown) so that the alternating current can be supplied to the primary coil  10  from the external power source. Both ends of the conductor constituting the secondary coil  20  are connected to an external device (not shown) to which the alternating current is supplied, so that an electric power can be supplied to the external device. A ratio of a number of winding turns of the primary coil  10  to a number of winding turns of the secondary coil  20  is determined based on a ratio of a voltage of the alternating current supplied to the primary coil  10  to a voltage of an alternating current supplied (outputted) from the secondary coil  20 . 
     The primary coil  10  and the secondary coil  20  are arranged side by side and adjacent to each other in an axial direction of the primary coil  10  and the secondary coil  20  (up-and-down direction in  FIG. 1 ), and arranged to have a predetermined distance apart from each other. The predetermined distance is preferably a following distance: the distance in which when the primary coil  10  and the secondary coil  20  are relatively moved in a direction (left-and-right direction in  FIG. 1 ) perpendicular to the aforementioned axial direction, the primary coil  10  and the secondary coil  20  do not physically come into contact with each other; and the distance in which the secondary coil  20  and the core  30  are capable of receiving a magnetic flux generated by the primary coil  10 . 
     Referring to  FIG. 3 , the core  30  is a plate-like iron core or magnetic core formed into a generally rectangular shape and configured to collect the magnetic flux generated by the primary coil  10 . The core  30  is provided adjacent to the secondary coil  20  such that the secondary coil  20  is located between the core  30  and the primary coil  10 . A plate-like member constituting the core  30  is preferably formed to have a thickness in which magnetic saturation of the core  30  does not occur and distribution of a magnetic flux inside the core  30  is uniform. As shown in  FIGS. 1 to 3 , the core  30  includes a conductor storage part  31  and a central part  32 . The conductor storage part  31  is configured to store the secondary coil  20  therein. The central part  32  constitutes a central region of the core  30 . The first embodiment will be explained with regard to an example in which the conductor storage part  31  is formed integrally with the central part  32 . 
     The conductor storage part  31  is configured to store the secondary coil  20  therein. That is, the conductor storage part  31  is configured to store the conductor constituting the secondary coil  20 . The conductor storage part  31  is formed of a plate member. The plate member has a concave cross section which extends in an annular manner. The conductor storage part  31  is provided such that an opening of the concave cross section opens in a direction from a side of the secondary coil  20  toward a side of the primary coil  10 . 
     The central part  32  is a part located in an inner region (hereinafter, referred to as “central region”) inward from the conductor storage part  31 . The central part  32  has a recessed shape opening toward a direction opposite to the direction of the opening of the concave cross section of the conductor storage part  31 . In other words, the central part  32  is a recessed part formed in an area where the conductor storage part  31  is not formed. The recessed part has a flat bottom face. The first embodiment will be explained with regard to an example in which the central part  32  is a flat plate-like part extending over the overall central region. 
     Next, operations in the transformer  1  constituted as above will be explained. 
     When an alternating current is supplied to the primary coil  10  from the external power source, a magnetic flux whose intensity changes as time progresses is generated around the primary coil  10 , as shown by thin-line arrows in  FIG. 1 , due to the alternating current flowing through the primary coil  10 . In an area where the magnetic flux is generated, the secondary coil  20  and the core  30  are arranged. The core  30  is configured to collect the magnetic flux generated by the primary coil  10 . 
     In the core  30  of the transformer  1  according to the first embodiment, a gap “G” from the primary coil  10  to the central part  32  can be made shorter than a gap “g” from the primary coil  10  to a flat plate-like core as shown in  FIG. 25 . By making the gap “G” be shorter as above, it becomes possible for the core  30  to more effectively collect the magnetic flux generated by the primary coil  10 . 
     In the secondary coil  20 , an alternating current is induced. The alternating current has an electric current value that changes as time progresses, according to a density of the magnetic flux that changes as time progresses. In other words, in the secondary coil  20 , a voltage, which is transformed based on the ratio of the number of the winding turns of the primary coil  10  to the number of the winding turns of the secondary coil  20 , is induced. 
     The core  30  of the transformer  1  according to the first embodiment makes it possible to shorten an electromagnetic gap, compared with the flat plate-like core shown in  FIG. 25 . The electromagnetic gap is a gap between the primary coil  10  and the core  30 , specifically, the central part  32 . Therefore, the core  30  can effectively collect the magnetic flux generated by the primary coil  10 , resulting in an improved performance of the transformer  1 . For example, the transformer  1  in the first embodiment can increase an inductive voltage by about 11%, compared with a transformer provided with the flat plate-like core shown in  FIG. 25 . 
     Moreover, since the core  30  in the first embodiment includes the conductor storage part  31  and the central part  32  which are formed of the plate member, a part of the core, which does not contribute to a flux path, is reduced. Thus, a volume of the core  30  can be reduced compared with the E-type core as shown in  FIG. 24 . Consequently, it is possible to reduce a weight of the core  30  and therefore, a weight of the transformer  1 . 
     Although the first embodiment has been explained with regard to an example in which the secondary coil  20  is wound in a generally rectangular shape and the core  30  is formed into a generally rectangular shape, the shape of the secondary coil  20  and the shape of the core  30  are not limited to the rectangular shape and may be, for example, a circular shape, an oval shape, or other shapes. 
     The core  30  may be formed such that the conductor storage part  31  has the concave cross section which extends in the annular manner, as in the above-explained embodiment. Alternatively, it may be possible to form the core  30  that does not include walls  32 A and  32 B shown in  FIG. 3 . Specifically, the core  30  may be formed as shown in  FIG. 4 . In other words, the conductor storage part  31  may be formed such that outer walls (the walls  32 A and  32 B) in a longitudinal direction are not provided. In this case, a cross section taken from a line C 1 -C 1  and a cross section taken from a line C 2 -C 2  in  FIG. 4  have a substantially L-shape. 
     Specifically, in a case where each of the primary coil  10  and the secondary coil  20  is formed into the generally rectangular shape and a length of long sides of the rectangular shape in the primary coil  10  (length in the left-and-right direction in  FIG. 4 ) is longer than a length of sides, which correspond to the long sides of the primary coil  10 , of the secondary coil  20  (length in the left-and-right direction in  FIG. 4 ), it is preferable that the core  30  (or the conductor storage part  31 ) does not include the walls  32 A and  32 B, as explained above. 
     By constituting as above, it is possible to minimize an influence due to a low efficiency in collecting the magnetic flux by the core  30 , while reducing the weight of the core  30 . That is, in the above-mentioned configuration, the efficiency in collecting the magnetic flux is lower in parts (the walls  32 A and  32 B) of the short sides of the core  30  than in other parts. Accordingly, the parts (the walls  32 A and  32 B) do not greatly affect the efficiency in collecting the magnetic flux. For example, even if the walls  32 A and  32 B are not provided, a lowered amount of the efficiency in collecting the magnetic flux is small. Thus, the influence due to the low efficiency in collecting the magnetic flux in the overall core  30  can be made minor. Meanwhile, since the volume of the core  30  can be made small by not providing the walls  32 A and  32 B, the weight of the core  30  can be reduced. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described with reference to  FIGS. 5 to 8 . A transformer according to the second embodiment has a basic configuration the same as that of the transformer in the first embodiment, except for a shape of the core. Therefore, in the second embodiment, explanations will be given with regard to the shape of the core and so on with reference to  FIGS. 5 to 8  and will not be repeated with regard to the other constituent elements and the like. 
     The transformer  101  according to the second embodiment is, as shown in  FIG. 5 , mainly composed of the primary coil  10 , the secondary coil  20 , and a core  130  provided in a vicinity of the secondary coil  20 . 
     The core  130  is, in the same manner as the core  30  in the first embodiment, a plate-like iron core or magnetic core formed into a generally rectangular shape and configured to collect the magnetic flux generated by the primary coil  10 . The core  130  is provided adjacent to the secondary coil  20  such that the secondary coil  20  is located between the core  130  and the primary coil  10 . A plate-like member constituting the core  130  is preferably formed to have a thickness in which magnetic saturation does not occur in the core  130  and distribution of a magnetic flux inside the core  130  is uniform. 
       FIG. 6  is a view showing the core  130  seen from the side of the primary coil  10  and illustrating a configuration of the core  130  of  FIG. 5 . 
     As shown in  FIGS. 5 and 6 , the core  130  includes the conductor storage part  31 , the central part  32 , and a flange portion  133  provided on a circumference of the core  130 . The flange portion  133  extends outward from an outer opening edge (an end part  32 AA of the wall  32 A and an end part  32 BB of the wall  32 B) of opening edges of the conductor storage part  31 , and is a flat plate-like member (part) extending outward from the outer opening edge of the conductor storage part  31 . The second embodiment will be explained with regard to an example in which the conductor storage part  31 , the central part  32 , and the flange portion  133  are together formed integrally as one member. 
     Operations in the transformer  101  constituted as above are generally the same as those in the transformer  1  of the first embodiment, and therefore, will not be explained here. 
     Now, explanations will be given with regard to a relationship between an overhang length F of the flange portion  133  and improvement of an inductive voltage in the transformer  101 , based on analysis results. The overhand length F is a length of a portion, protruding outward from the conductor storage part  31 , of the flange portion  133 . 
     The above-mentioned relationship will be explained with reference to  FIG. 7 . 
     A horizontal axis of a graph in  FIG. 7  shows the overhang length F expressed in percentage in relation to a coil width W of the secondary coil  20 . A vertical axis the graph in  FIG. 7  shows a ratio of the inductive voltage in the transformer  101  to the inductive voltage in the transformer  1  including the core  30  in the first embodiment. In the transformer  1 , the overhang length F is 0%, i.e., the flange portion  133  is not provided. 
     In the graph of  FIG. 7 , a following tendency is shown: as the overhang length F increases, the ratio of the inductive voltage in the transformer  101  increases. While it may be considered to increase the overhang length F so as to increase the inductive voltage, the overhang length F is restricted by a width TW of the entire core  130  including the flange portion  133 . 
     For example, when the overhang length F is around 30% of the coil width W of the secondary coil  20 , the transformer  101  including the core  130  of the second embodiment can provide following improvements: the inductive voltage can be improved by around 17%, compared with the transformer  1  including the core  30  of the first embodiment, and further, can be improved by around 30%, compared with the transformer including the flat plate-like core shown in  FIG. 25 . 
     According to the aforementioned configuration, the core  130  in the second embodiment can more effectively collect the magnetic flux extending between the primary coil  10  and the secondary coil  20 , compared with the core  30  without the flange portion  133  in the first embodiment. Consequently, an improved performance of the transformer  101  in the second embodiment can be achieved. 
     In other words, by providing the flange portion  133 , it becomes possible to reduce a magnetic resistance in the magnetic flux generated by the primary coil  10 , thereby reducing a leakage flux. Consequently, the improved performance of the transformer  101  in the second embodiment can be achieved. 
     Another example (another configuration) of the core  130  in  FIG. 6  will be described with reference to  FIG. 8 . 
     In the core  130  of  FIG. 8 , the flange portions  133  are provided only on a pair of opposite sides of the core  130 . 
     In the core  130  of  FIG. 8 , the flange portion  133  may be provided on the circumference of the core  130  as in the aforementioned embodiment. 
     Specifically, in a case where each of the primary coil  10  and the secondary coil  20  is formed into a generally rectangular shape and a length of long sides of the rectangular shape in the primary coil  10  (length in a left-and-right direction in  FIG. 8 ) is longer than a length of sides, which correspond to the long sides of the primary coil  10 , of the secondary coil  20  (length in the left-and-right direction in  FIG. 8 ), it is preferable to provide the flange portion  133  only on long sides of the core  130 , that is, upper and lower sides of the core  130  in  FIG. 8 . 
     Third Embodiment 
     Next, a third embodiment of the present invention will be described with reference to  FIG. 9 . A transformer according to the third embodiment has a basic configuration the same as that of the transformer in the second embodiment, except for a shape of the core. Therefore, in the third embodiment, explanations will be given with regard to the shape of the core and so on with reference to  FIG. 9  and will not be repeated with regard to the other constituent elements and the like. 
       FIG. 9  is a cross-sectional view showing a configuration of the transformer  201  according to the third embodiment. The transformer  201  according to the third embodiment is mainly composed of the primary coil  10 , the secondary coil  20 , and a core  230  provided in a vicinity of the secondary coil  20 . 
     The core  230  is, in the same manner as the core  30  in the first embodiment, a plate-like iron core or magnetic core formed into a generally rectangular shape and configured to collect the magnetic flux generated by the primary coil  10 . The core  230  is provided adjacent to the secondary coil  20  such that the secondary coil  20  is located between the core  230  and the primary coil  10 . 
     The core  230  includes, as shown in  FIG. 9 , a conductor storage part  231 , a central part  232 , and a flange portion  233  provided on a circumference of the core  230 . The conductor storage part  231  is configured to store the secondary coil  20  therein. The central part  232  constitutes a central region of the core  230 . 
     The conductor storage part  231  is, in the same manner as the conductor storage part  31  in the first embodiment, configured to store the secondary coil  20  therein. The central part  232  is, in the same manner as the central part  32  in the first embodiment, a part located in an inner region inward from the conductor storage part  231 . The central part  232  forms a recessed shape opening toward a direction opposite to a direction of an opening of the concave cross section of the conductor storage part  231 . In the same manner as the flange portion  133  in the second embodiment, the flange portion  233  extends outward from an outer opening edge (an end part  232 AA of a wall  232 A and an end part  232 BB of a wall  232 B) of opening edges of the conductor storage part  231 , and is a flat plate-like member extending outward from the outer opening edge of the conductor storage part  231 . 
     The core  230  in third embodiment is different from the respective cores in the first and second embodiments with regard to a following point: a plate thickness “t 1 ” of a plate member constituting the conductor storage part  231  is thicker than a plate thickness “t 2 ” of a plate member constituting the central part  232  and also than a plate thickness “t 3 ” of a plate member constituting the flange portion  233 . 
     Here, the above-mentioned plate thickness t 2  and plate thickness t 3  may be a thickness in which distribution of a magnetic flux is uniform in the core  230 . For example, the plate thickness t 2  may be equal to or different from the plate thickness t 3 . Moreover, the plate thickness t 2  may be equal to or thinner than a thickness of the central part  32  in the first and second embodiments. Similarly, the plate thickness t 3  may be equal to or thinner than a thickness of the flange portion  133  in the second embodiment. 
     According to the above configuration, the core  230  according to the third embodiment makes it possible to inhibit magnetic saturation from occurring in the conductor storage part  231  arranged close to the secondary coil  20  where an electric current flows. 
     For example, in the core  130  of the transformer  101  in the second embodiment, if an electric current of 350 A-turn rms/mm (effective value) is applied, magnetic saturation occurs in the core  130 . When the inductive voltage becomes lower due to a lower magnetic permeability of the core  130  or when a resonance circuit is formed as a circuit to be connected to the secondary coil  20  (secondary circuit), there may be a problem in which the secondary current flowing in the secondary coil  20  and the secondary circuit is not stable due to change in inductance. 
     On the other hand, in the core  230  of the third embodiment, a plate thickness only of the conductor storage part  231  is made to be thick; the conductor storage part  231  is positioned at around the secondary coil  20  where a magnetic flux by the secondary current flowing into the secondary coil  20  is concentrated. Therefore, it is possible to inhibit magnetic saturation from occurring in the core  230  and achieve a uniform distribution of the magnetic flux in the core  230 . Moreover, since the plate thickness of the core  230  is made to be thick only at the part where the magnetic flux is concentrated, a weight increase of the core  230  can be inhibited, compared with a case where a plate thickness of the entire core  230  is thick. 
     When there is a uniform distribution of the magnetic flux in the core  230 , heat generation mainly due to hysteresis loss can be inhibited in the core  230 . Moreover, when there is a uniform distribution of the magnetic flux, generation of spots where the magnetic flux is particularly concentrated in the core  230  can be inhibited and therefore, the heat generation due to the hysteresis loss can be inhibited in the above-mentioned spots where the magnetic flux is concentrated. Consequently, an (localized) increase of temperature can be inhibited from occurring in the core  230  and restriction of the flowing current in the coil due to the increase of temperature is less likely to be occurred. As a result, it becomes possible to inhibit an efficiency of the transformer  201  from decreasing. 
     Fourth Embodiment 
     Next, a fourth embodiment of the present invention will be described with reference to  FIGS. 10 to 13 . A transformer according to the fourth embodiment has a basic configuration the same as that of the transformer in the third embodiment, except for a shape of the core. Therefore, in the fourth embodiment, explanations will be given with regard to the shape of the core and so on with reference to  FIGS. 10 to 13  and will not be repeated with regard to the other constituent elements and the like. 
     The transformer  301  according to the fourth embodiment is, as shown in  FIGS. 10 and 11 , mainly composed of the primary coil  10 , the secondary coil  20 , and a core  330  provided in a vicinity of the secondary coil  20 . 
     The core  330  is, in the same manner as the core  230  in the third embodiment, a plate-like iron core or magnetic core formed into a generally rectangular shape and configured to collect the magnetic flux generated by the primary coil  10 . The core  330  is provided adjacent to the secondary coil  20  such that the secondary coil  20  is located between the core  330  and the primary coil  10 . 
     The core  330  includes the conductor storage part  231 , the central part  232 , the flange portion  233 , and an aperture  334 . The conductor storage part  231  is configured to store the secondary coil  20  therein. The central part  232  constitutes a central region of the core  330 . The flange portion  233  is provided on a circumference of the core  330 . The aperture  334  is formed in the central part  232 . 
     The aperture  334  is a through-hole formed in the central part  232  for the purpose of reducing a weight of the core  330 . The fourth embodiment will be explained with regard to an example in which the aperture  334  is formed as one through-hole in a rectangular shape which is the same as an overall shape of the core  330 ; however, the shape of the aperture  334  is not limited to the rectangular shape and may be a circular shape or an oval shape. Moreover, a number of the aperture  334  formed in the central part  232  is not limited to one and may be more than one. 
     Here, explanations will be given with regard to a relationship between a reduced amount of a volume of the core  330  as a result of providing the aperture  334  and the inductive voltage in the secondary coil  20 , based on analysis results. Specifically, explanations will be given with regard to a relationship between a ratio of a width W 4  of the aperture  334  to a width W 2  of the central part  232  in  FIG. 11 , and the inductive voltage in the secondary coil  20 . 
       FIG. 12  is a graph illustrating the aforementioned relationship. 
     A horizontal axis of the graph in  FIG. 12  shows the ratio of the width W 4  of the aperture  334  to the width W 2  of the central part  232 . A vertical axis of the graph in  FIG. 12  shows a ratio of the inductive voltage in the secondary coil  20  of the fourth embodiment to the inductive voltage in the secondary coil  20  of the third embodiment. In the secondary coil  20  of the third embodiment, a ratio of the aperture  334  is 0, i.e., the aperture  334  is not provided. 
     In the graph of  FIG. 12 , a following tendency is shown: as the ratio of the aperture  334  increases from 0 toward 1, the ratio of the inductive voltage in the secondary coil  20  gradually decreases. For example, when the ratio of the aperture  334  is 0.3, i.e., the volume of the core  330  is reduced by around 30% in the central part  232 , the inductive voltage in the secondary coil  20  decreases by around 1%. 
     According to the aforementioned configuration, the weight of the core  330  in the fourth embodiment can be reduced as a result of forming the aperture  334  in the central part  232 . Moreover, the central part  232  has a lower efficiency in collecting the magnetic flux existing between the primary coil  10  and the secondary coil  20 , compared with the conductor storage part  231 . Therefore, even if the aperture  334  is formed in the central part  232 , influence due to decrease of the inductive voltage in the secondary coil  20  can be made minor. 
     The above-mentioned embodiment has been explained with regard to an example in which the aperture  334  is formed only in the central part  232 ; however, in a case where each of the primary coil  10  and the secondary coil  20  is formed into a generally rectangular shape and a length of long sides of the rectangular shape in the primary coil  10  (length in a left-and-right direction in  FIG. 13 ) is longer than a length of sides, which correspond to the long sides of the primary coil  10 , of the secondary coil  20  (length in the left-and-right direction in  FIG. 13 ), a clearance part  334 A may be formed so as to divide the core  330  into two parts, thereby dividing the core  330  into two parts, i.e., an upper part and a lower part as shown in  FIG. 13 . 
     Fifth Embodiment 
     Next, a fifth embodiment of the present invention will be described with reference to  FIG. 14 . A transformer according to the fifth embodiment has a basic configuration the same as that of the transformer in the second embodiment, except for a shape of the core. Therefore, in the fifth embodiment, explanations will be given with regard to the shape of the core and so on with reference to  FIG. 14  and will not be repeated with regard to the other constituent elements and the like. 
     The transformer  401  according to the fifth embodiment is, as shown in  FIG. 14 , mainly composed of the primary coil  10 , the secondary coil  20 , and a core  430  provided in a vicinity of the secondary coil  20 . 
     The core  430  is, in the same manner as the core  30  in the first embodiment, a plate-like iron core or magnetic core formed into a generally rectangular shape and configured to collect the magnetic flux generated by the primary coil  10 . The core  430  is provided adjacent to the secondary coil  20  such that the secondary coil  20  is located between the core  430  and the primary coil  10 . 
     The core  430  mainly includes a conductor storage part  431 , the central part  32 , and the flange portion  133 . The conductor storage part  431  is configured to store the secondary coil  20  therein. 
     In the same manner as the conductor storage part  31  in the first embodiment, the conductor storage part  431  is configured to store the secondary coil  20  therein. The conductor storage part  431  is a part of a plate member and the part has a concave cross section extending in an annular manner. 
     The conductor storage part  431  is provided such that an opening of the concave cross section opens in a direction from a side of the secondary coil  20  toward a side of the primary coil  10 . 
     In the conductor storage part  431  of the fifth embodiment, a bottom plate part  432  (part located in an upper side in  FIG. 14 ) located between side walls of the concave cross section is formed of two plate members which are stacked together. In these two plate members constituting the bottom plate part  432 , a plurality of groove-like slits  433  are formed. The groove-like slits  433  are configured to divide the two plate members into a plurality of sections. The aforementioned two plate members are stacked together in such a manner that the slits  433  formed in one of the two plate members do not overlap with any of the slits  433  formed in the other of the two plate members, thereby constituting the bottom plate part  432 . 
     In the fifth embodiment, the plate member located at the side of the secondary coil  20  (a lower side in  FIG. 14 ) (hereinafter, “secondary coil  20 -side plate member”) has two slits  433  formed at an equal interval thereon, while the plate member located at an outer side (an upper side in  FIG. 14 ) (hereinafter, “outer-side plate member”) has three slits  433  at an equal interval thereon. Explanations will be given with regard to an example in which these two plate members are stacked together such that the slits  433  formed in the secondary coil  20 -side plate member and the slits  433  formed in the outer-side plate member are located in an alternating manner. 
     Next, operations in the conductor storage part  431  of the core  430 , which are features of the transformer  401  in the fifth embodiment, will be explained. The other operations in the transformer  401  are the same as those in the transformer  1  of the first embodiment and the transformer  101  of the second embodiment, and therefore, will not be explained. 
     For example, in a case where a large amount of the electric current flows in the secondary coil  20 , heat generation due to iron loss in the core  430  is large. Thus, heat expansion occurs in the core  430 . Thereafter, when the flow of the electric current in the secondary coil  20  stops, the heat generation in the core  430  also stops; then, the core  430  is contracted. Such a heat expansion and contraction in the core  430  are absorbed by expansion or contraction of widths of the respective slits  433  in the conductor storage part  431 . 
     Meanwhile, since the slits  433  formed in the secondary coil  20 -side plate member and the slits  433  formed in the outer-side plate member are positioned in an alternating manner, a flux path of a magnetic field in the conductor storage part  431  can be formed without passing through spaces in the slits  433 , i.e., formed by bypassing the slits  433 . 
     Specifically, the flux path of the magnetic field can be made to bypass the slits  433  formed in the secondary coil  20 -side plate member by passing through the outer-side plate member. Also, the flux path of the magnetic field can be made to bypass the slits  433  formed in the outer-side plate member by passing through the secondary coil  20 -side plate member. 
     According to the above configuration, when the core  430  is expanded or contracted due to a temperature change of the core  430 , such an expansion or contraction of the core  430  can be absorbed by the spaces in the respective slits  433 . It is, therefore, possible to alleviate compression stress and tensile stress acting on the plate members constituting the core  430 . Particularly, when the core  430 , the secondary coil  20  and others are fixed to a case (not shown) which is a chassis constituting the transformer  401 , compression stress and tensile stress due to the temperature change act strongly on the core  430 ; however, the core  430  according to the fifth embodiment can alleviate the above-mentioned stresses. Therefore, the core  430  according to the fifth embodiment can inhibit defects such as cracks in the core  430  due to the temperature change, etc. from occurring. 
     Furthermore, since the flux path of the magnetic field in the conductor storage part  431  is formed by bypassing the slits  433 , it is possible to minimize for the spaces in the slits  433  to become a large magnetic resistance. Accordingly, decrease in efficiency of the transformer  401  of the fifth embodiment can be minimized. 
     Sixth Embodiment 
     Next, the sixth embodiment of the present invention will be described with reference to  FIGS. 15 and 16 . A transformer according to the sixth embodiment has a basic configuration the same as that of the transformer in the first embodiment, except for a shape of the core. Therefore, in the sixth embodiment, explanations will be given with regard to the shape of the core and so on with reference to  FIGS. 15 and 16  and will not be repeated with regard to the other constituent elements and the like. 
     The transformer  501  according to the sixth embodiment is, as shown in  FIG. 15 , mainly composed of the primary coil  10 , the secondary coil  20 , and a core  530  provided in a vicinity of the secondary coil  20 . 
     The core  530  is composed of a plurality of core segments  530 A arranged to be spaced apart from one another. Each of the core segments  530 A is a plate-like iron core or magnetic core formed into a generally strip-like shape. The core  530  is configured to collect the magnetic flux generated by the primary coil  10 . The core  530  is provided adjacent to the secondary coil  20  such that the secondary coil  20  is located between the core  530  and the primary coil  10 . 
     In each of the core segments  530 A, a part of the conductor storage part  31  and a part of the central part  32  are arranged, so that the conductor storage part  31  and the central part  32  are provided to the core  530  as a whole. 
     In the sixth embodiment, a length of the primary coil  10  in a long-side direction thereof is longer than a length in of the secondary coil  20  in a long-side direction; the core segments  530 A extend along an intersecting direction, more preferably an orthogonal direction, to a pair of two sides extending in the aforementioned long-side direction of the primary coil  10 . Furthermore, the plurality of core segments  530 A are arranged at a predetermined distance apart from one another along the aforementioned long-side direction (an extending direction of the pair of the two sides). 
     In order to inhibit increase of magnetic resistance, it is desirable that the above-mentioned predetermined distance is generally proportional to a plate thickness of plate members each constituting each of the core segments  530 A. In other words, in a cross section taken from a plane (line A-A in  FIG. 15 ) which is parallel to the aforementioned long-side direction and which passes through the secondary coil  20 , the predetermined distance and the plate thickness of each of the plate members are desirably set such that a cross-sectional area of the core  530  is substantially the same as a cross-sectional area of the core  30  in the first embodiment and so on. Moreover, in a cross section taken from a line B-B in  FIG. 15 , the predetermined distance and the plate thickness of each of the plate members may be set such that the cross-sectional area of the core  530  is substantially the same as the cross-sectional area of the core  30  in the first embodiment and so on. 
     The sixth embodiment will be explained with regard to an example in which the predetermined distance is substantially the same as a width of each of the core segments  530 A in the aforementioned long-side direction and in which the plate thickness of each of the plate members each constituting the core segments  530 A is generally twice as thick as a plate thickness of the core  30  in the first embodiment. 
     By constituting the core  530  as above, it is possible to inhibit performance of the transformer  501  with regard to an inductive voltage, etc. from decreasing, compared with a following case: the core is divided without increasing the plate thickness of the plate member constituting the core, into the core segments  530 A and these core segments  530 A are arranged at a distance, substantially the same as the width of each of the core segments  530 A, apart from one another. However, as discussed later, as the distance between the arranged core segments  530 A (hereinafter, “arrangement distance”) is increased, a leakage flux in the core  530  increases even if the plate thickness of each of the plate members constituting the respective core segments  530 A is made to be thick. Consequently, the inductive voltage decreases. 
     A relationship between the distance (“arrangement distance”) between the core segments  530 A of  FIG. 15  and an inductive voltage ratio will be explained with reference to  FIG. 16 . 
     A horizontal axis of a graph in  FIG. 16  shows the arrangement distance of the core segments  530 A, while a vertical axis shows a ratio of the inductive voltage of the sixth embodiment to an inductive voltage in a case where the arrangement distance is 0 mm. In  FIG. 16 , when the arrangement distance is 0 mm, the plate thickness is 3 mm; and when the arrangement distance is not 0 mm, the plate thickness is 6 mm. The width of the plate member is equal to the arrangement distance. Accordingly, for example, when the arrangement distance is 10 mm, the width of the plate member is 10 mm. When the arrangement distance is 30 mm, the width of the plate member is 30 mm. In other words, a ratio of the arrangement distance to the width of the plate member is 1:1, and therefore, a removal ratio is 50%. 
     In  FIG. 16 , a following tendency is shown: as the arrangement distance of the core segments  530 A increases, the leakage flux increases, causing a gradual decrease of the inductive voltage. For example, when the arrangement distance is 30 mm, the inductive voltage is decreased by about 1.5%. This decrease does not cause a problem in which the performance of the transformer  501  is decreased. 
     It may be configured that, in proportion to the increase of the arrangement distance, the plate thickness of the plate member is made to increase, so that a volume of a material constituting the core  530  stays a certain amount. However, the core  530  is not limited to this configuration. 
     According to the above-explained configuration, the core  530  can be composed of the plurality of core segments  530 A each of which is relatively small in size, compared with an integrally formed core as in the core  30  in the first embodiment. Therefore, relatively small plate members can be used to manufacture the core  530 . Compared with obtaining large plate members, obtaining the small plate members is easy and less expensive. Thus, manufacturing costs of the transformer  501  in the sixth embodiment can be reduced. 
     Moreover, compared with the core  30  of the first embodiment, the plate members each having a thicker plate thickness can be used to constitute the core. In this regard, there is a case where a required plate thickness of a plate member to constitute an integrally-formed core is thinner than a minimum plate thickness of a commercially-available plate member. In this case, it is necessary to obtain and grind such a commercially-available plate member to the extent that a plate thickness of this commercially-available plate member becomes the required plate thickness. Consequently, a problem arises in which manufacturing costs for the core becomes expensive. 
     However, in the configuration in which the core segments  530 A are arranged at the predetermined distance apart from one another as in the core  530  according to the sixth embodiment, a required plate thickness of the plate member constituting the core segments  530 A can be made thicker, compared with the required thickness in the integrally-formed core. In other words, the plate thickness of the plate member constituting the core segments  530 A can be made to be generally equal to the plate thickness of the commercially-available plate member. Accordingly, a manufacturing step for grinding the plate member as explained above can be omitted or simplified. As a result, cost reduction in manufacturing the transformer  501  of the sixth embodiment can be achieved. 
     For example, in a case where an electric current of about 800 A-turn rms (effective value) is applied to a winding of the secondary coil  20 , if the plate member constituting the integrally-formed core has a plate thickness of 2 mm, magnetic saturation would occur. In this case, it is necessary to have at least 3 mm of the plate thickness of the conductor storage part  31 . Meanwhile, since it is necessary to make the plate thickness of the plate member be thinner so as to achieve a weight reduction of the integrally-formed core, the plate member having a minimum required plate thickness of 3 mm may be selected. However, if ferrite as a member constituting the core has a standard manufactured thickness of more than 3 mm, e.g., more than 5 mm, it is necessary to grind the material having the thickness of more than 3 mm, e.g., more than 5 mm, to the extent that the thickness becomes 3 mm in order to manufacture the core using the plate member with the thickness of 3 mm. 
     However, in the case where the core segments  530 A are arranged at the predetermined distance apart from one another as in the core  530  of the sixth embodiment, the plate thickness of each of the plate members constituting the respective core segment  530 A can be set as 6 mm. Accordingly, as mentioned above, the step for grinding the obtained plate member so as to have the required plate thickness can be omitted or simplified. 
     As a method for adjusting the thickness of the core without the step for grinding, as described in Patent Document 1, a following method is known: rectangular plate members are stacked upon one another in an overlapping manner to form a core. However, when the plate members are overlapped and bonded together, there is a problem in which a performance of the transformer may be degraded due to a very small space between bonded faces. On the other hand, the core  530  of the sixth embodiment is composed of the core segments  530 A formed without bonding the plate members together, and therefore, can inhibit performance of the transformer  501  from degrading due to the space between the bonded face. 
     Moreover, by forming the core segments  530 A in a strip-like shape (rectangular shape), it is easy to deal with the core  530  of a larger size, compared with forming the core segments  530 A in a square shape. A manufacturable size of the core is determined, for example, depending on a shape or an area of a forming board where a shape of the core is formed. If the forming board has a round face, the manufacturable size for the core to be formed into a rectangular shape is determined by a length of a diagonal line in the core. In this case, if the core segment  530 A is formed into the strip-like shape, it is possible to form a longer core segment  530 A compared with the core segment  530 A formed into the square shape. 
     As above, compared with the core segment  530 A formed into the square shape, the core segment  530 A formed into the strip-like shape can more easily deal with expansion of the distance between a pair of two sides (up side and bottom side in  FIG. 15 ) of the primary coil  10 . Moreover, since each of the core segments  530 A has the same shape, the core segment  530 A can be manufactured by using a single mold for molding. Therefore, mass manufacturing of the core segments  530 A can be easily achieved and cost reduction in manufacturing the core  530  can be achieved. 
     As in the above-explained embodiment, the core  530  in which the plurality of the core segments  530 A are arranged to be spaced apart from one another may be used. Also, as shown in  FIG. 17 , the core  530  may be divided into two parts by providing a clearance part  334 A such that each of the core segments  530 A is divided into two parts. 
     Seventh Embodiment 
     Next, the seventh embodiment of the present invention will be described with reference to  FIG. 18 . A transformer according to the seventh embodiment has a basic configuration the same as that of the transformer in the first embodiment, except for a shape of the core. Therefore, in the seventh embodiment, explanations will be given with regard to the shape of the core and so on with reference to  FIG. 18  and will not be repeated with regard to the other constituent elements and the like. 
     The transformer  601  according to the seventh embodiment is, as shown in  FIG. 18 , mainly composed of the primary coil  10 , the secondary coil  20 , and a core  630  provided in a vicinity of the secondary coil  20 . In the seventh embodiment, a length of the primary coil  10  in a long-side direction (left-and-right direction in  FIG. 18 ) is longer than a length of the secondary coil  20  in a long-side direction. 
     The core  630  is, in the same manner as the core  30  in the first embodiment, a plate-like iron core or magnetic core formed into a generally rectangular shape and configured to collect the magnetic flux generated by the primary coil  10 . The core  630  is provided adjacent to the secondary coil  20  such that the secondary coil  20  is located between the core  630  and the primary coil  10 . 
     The core  630  mainly includes conductor storage parts  631 A and  631 B, and the central part  32 . Each of the conductor storage parts  631 A and  631 B is configured to store the secondary coil  20  therein. 
     In the same manner as the conductor storage part  31  in the first embodiment, the conductor storage part  631 A is configured to store a coil winding forming the secondary coil  20  therein. The conductor storage part  631 A has a concave cross section (a cross section taken from a line D 1 -D 1 , and a cross section taken from a line D 2 -D 2  in  FIG. 18 ) and is arranged along a pair of two sides (upper side and lower side in  FIG. 18 ) of the primary coil  10  in the aforementioned long-side direction. Furthermore, the conductor storage part  631 A is provided such that an opening of the concave cross section opens in a direction from a side of the secondary coil  20  toward a side of the primary coil  10 . 
     The conductor storage part  631 B has an L-shaped cross section (a cross section taken from a line E 1 -E 1 , and a cross section taken from a line E 2 -E 2  in  FIG. 18 ) and is arranged along an intersecting direction, more preferably an orthogonal direction, to the aforementioned pair of two sides. The seventh embodiment will be explained with regard to an example in which, when the core  630  is seen in a plan view, the conductor storage part  631 B covers generally a half of a coil width of the secondary coil  20 . As mentioned above, the conductor storage part  631 B may generally cover the half of the coil width of the secondary coil  20 . Also, the conductor storage part  631 B may cover at least a part of the coil winding constituting the secondary coil  20 . Thus, a portion of the secondary coil  20  to be covered by the conductor storage part  631 B is not specifically limited to the above constitution. 
     Next, operations in the core  630 , which are features of the transformer  601  in the seventh embodiment, will be explained. The other operations in the transformer  601  are the same as those in the transformer  1  of the first embodiment, and therefore will not be explained. 
     An area of the conductor storage part  631 B does not contribute to increase of an interlinkage magnetic flux M 1  passing across the secondary coil  20 . Therefore, even if the conductor storage part having the concave cross section is provided in the aforementioned area as in the first embodiment, this area does not contribute to improvement of the performance of the transformer. Moreover, an outer part of the core, which is located outward from short sides of the secondary coil  20  (sides extending in an up-and-down direction in  FIG. 18 ), constitutes a flux path that does not interlink with the secondary coil  20 ; therefore, the outer part of the core does not contribute to improvement of the performance of the transformer. 
     On the other hand, if the aforementioned area of the conductor storage part  631 B is completely removed, a magnetic flux which does not interlink with the secondary coil  20  increases. Therefore, a voltage induced by the secondary coil  20  decreases, thereby lowering a performance of the transformer  601 . 
     Therefore, as in the core  630  of the seventh embodiment, the generally half of the coil winding of the secondary coil  20  is covered by the conductor storage part  631 B, so that a magnetic flux adjacent to the conductor storage part  631 B among a magnetic flux generated in the primary coil  10  is drawn toward the core  630 . Thereby, the drawn magnetic flux can be made as a magnetic flux interlinking with the secondary coil  20 . Consequently, the transformer  601  of the seventh embodiment can slightly increase the inductive voltage, compared with the transformer  1  of the first embodiment. 
     According to the above configuration, the conductor storage part  631 B has the shape in which the outer portion located outward from the secondary coil  20  is removed; thus, a weight of the core  630  can be reduced. Moreover, the aforementioned outer portion has low efficiency in collecting the magnetic flux existing between the primary coil  10  and the secondary coil  20 , compared with other portions of the core  630 . Therefore, even when the conductor storage part  631 B is formed such that the aforementioned outer portion is removed, influence due to decrease of the inductive voltage in the secondary coil  20  can be made minor. 
     The transformer of the present invention may be configured as shown in  FIGS. 19 to 23 . 
       FIG. 19  shows a configuration of the transformer  1  in  FIG. 1  in which another core  30  is also provided at the side of the primary coil  10 . 
       FIG. 20  shows a configuration of the transformer  101  in  FIG. 5  in which another core  130  is also provided at the side of the primary coil  10 . 
       FIG. 21  shows a configuration of the transformer  201  in  FIG. 9  in which another core  230  is also provided at the side of the primary coil  10 . 
       FIG. 22  shows a configuration of the transformer  301  in  FIG. 10  in which another core  330  is also provided at the side of the primary coil  10 . 
       FIG. 23  shows a configuration of the transformer  401  in  FIG. 14  in which another core  430  is also provided at the side of the primary coil  10 .