Patent Publication Number: US-2022238271-A1

Title: Transformer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national stage of PCT/JP2020/018499 filed on May 7, 2020, which claims priority of Japanese Patent Application No. JP 2019-099186 filed on May 28, 2019, the contents of which are incorporated herein. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a transformer. 
     BACKGROUND 
     There is a known transformer in which a primary coil and a secondary coil included therein are both stacked. For example, JP 2015-192082A discloses a thin transformer in which a primary coil and a secondary coil are disposed in the same plane. In this thin transformer, a plurality of planar coils are arranged one on top of one another. 
     However, with the configuration described in JP 2015-192082A, the primary coil and the secondary coil are provided in the same plane, and therefore the same plane in which these coils are disposed requires a wide area. On the other hand, simply stacking the primary coil and the secondary coil is likely to result in a significant leakage flux in their respective end portions. When a leakage flux is increased, the leakage inductance is increased. 
     Therefore, it is an object of the present disclosure to provide a transformer including stacked primary and secondary coils with a reduced leakage inductance, in which the leakage inductance is reduced. 
     SUMMARY 
     A transformer according to the present disclosure includes a primary winding and a secondary winding that are insulated from each other, the secondary winding including a first coil and a second coil that are conductively connected to each other, the primary winding including a third coil, and all of the first coil, the second coil, and the third coil at least partially encircling one axis extending along a first direction. 
     The first coil, the second coil, and the third coil are stacked in the first direction. The first coil includes a first end and a second end. The second coil includes a third end and a fourth end. The secondary winding further includes a first conductor and a second conductor. 
     The first conductor is sandwiched between the first end and the third end in the first direction so as to be conductively connected to the first end and the third end. The second conductor is sandwiched between the second end and the fourth end in the first direction so as to be conductively connected to the second end and the fourth end. A second direction is orthogonal to the first direction. 
     The first end and the second end are arranged adjacent to, but not in contact with, each other in the second direction. The third end and the fourth end are arranged adjacent to, but not in contact with, each other in the second direction. The first conductor and the second conductor are arranged adjacent to, but not in contact with, each other in the second direction. 
     A first position is different from at least one of a second position and a third position. The first position is a position in the second direction of a boundary between the first conductor and the second conductor. The second position is a position in the second direction of a boundary between the first end and the second end. The third position is a position in the second direction of a boundary between the third end and the fourth end. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     The transformer according to the present disclosure includes stacked primary and secondary coils, in which the leakage inductance is reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view illustratively depicting the configuration of a transformer according to Embodiment 1. 
         FIG. 2  is a perspective view illustratively depicting the configuration of the transformer according to Embodiment 1. 
         FIG. 3  is a perspective view illustratively depicting the configuration of a core. 
         FIG. 4  is an exploded perspective view illustratively depicting a primary winding and a secondary winding. 
         FIG. 5  is a side view illustratively depicting the configuration of the transformer according to Embodiment 1. 
         FIG. 6  is a cross-sectional view illustratively depicting the configuration of the secondary winding. 
         FIG. 7  is a circuit diagram illustratively depicting a converter in which the transformer according to Embodiment 1 is used. 
         FIG. 8  is a plan view illustratively depicting the configuration of a transformer according to Embodiment 2. 
         FIG. 9  is an exploded perspective view illustratively depicting a primary winding and a secondary winding. 
         FIG. 10  is a cross-sectional view illustratively depicting the configuration of the secondary winding. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First, aspects of the present disclosure will be listed and described. 
     The present disclosure is as follows. 
     A transformer includes a primary winding and a secondary winding that are insulated from each other, the secondary winding including a first coil and a second coil that are conductively connected to each other, the primary winding including a third coil. All of the first coil, the second coil, and the third coil at least partially encircle one axis extending along a first direction. 
     The first coil, the second coil, and the third coil are stacked in the first direction. The first coil includes a first end and a second end. The second coil includes a third end and a fourth end. The secondary winding further includes a first conductor and a second conductor. 
     The first conductor is sandwiched between the first end and the third end in the first direction so as to be conductively connected to the first end and the third end. The second conductor is sandwiched between the second end and the fourth end in the first direction so as to be conductively connected to the second end and the fourth end. A second direction is orthogonal to the first direction. 
     The first end and the second end are arranged adjacent to, but not in contact with, each other in the second direction. The third end and the fourth end are arranged adjacent to, but not in contact with, each other in the second direction. The first conductor and the second conductor are arranged adjacent to, but not in contact with, each other in the second direction. 
     A first position is different from at least one of a second position and a third position. The first position is a position in the second direction of a boundary between the first conductor and the second conductor. The second position is a position in the second direction of a boundary between the first end and the second end. The third position is a position in the second direction of a boundary between the third end and the fourth end. 
     Due to the first position being different from at least one of the second position and the third position, the action of increasing the magnetic resistance to the leakage flux occurs, resulting in a reduction in the leakage flux, and hence the leakage inductance. 
     It is preferable that the secondary winding further includes a third conductor, the first coil further includes a fifth end, the second coil further includes a sixth end, the fifth end and the second end are arranged adjacent to, but not in contact with, each other in the second direction, the sixth end and the fourth end are arranged adjacent to, but not in contact with, each other in the second direction, the third conductor is sandwiched between the fifth end and the sixth end in the first direction so as to be conductively connected to the fifth end and the sixth end, the third conductor and the second conductor are arranged adjacent to, but not in contact with, each other in the second direction, a fourth position is different from at least one of the first position and the second position, and the fourth position is a position in the second direction of a boundary between the third conductor and the second conductor. The reason being that the fifth end and the sixth end are center taps of the secondary winding. 
     It is preferable that the third coil is sandwiched between the first coil and the second coil in the first direction, and the third coil, the first conductor, and the second conductor are located aligned in the first direction. The reason being that this reduces the leakage flux. 
     Specific examples of the transformer according to the present disclosure will be described below with reference to the drawings. It should be noted that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications which fall within the scope of the claims and the meaning and scope of equivalents thereof. 
     Embodiment 1 
     In the following, a transformer according to Embodiment 1 will be described.  FIG. 1  is a plan view illustratively depicting the configuration of a transformer  100  according to Embodiment 1. The transformer  100  includes a core  5 . 
       FIG. 2  is a perspective view illustratively depicting the configuration of the transformer  100 . However, to facilitate viewing,  FIG. 2  only depicts a portion located on the core  5  side relative to a position indicated by the line CC in  FIG. 1 . 
       FIG. 3  is a perspective view illustratively depicting the configuration of the core  5 . The core  5  is a so-called EI core. The core  5  includes an I-core  51  and an E-core  52 . The E-core  52  includes three legs  501 ,  502 , and  503 . The legs  501 ,  502 , and  503  are arranged along a second direction X. The leg  502  is located between the legs  501  and  503 . The second direction X is orthogonal to a first direction Z. 
     The transformer  100  includes a primary winding  1  and a secondary winding  2 . The primary winding  1  includes third coils  11 ,  12 ,  13 , and  14 . The secondary winding  2  includes a first coil  21  and a second coil  22 . The first coil  21 , the second coil  22 , and the third coils  11 ,  12 ,  13 , and  14  are stacked in the first direction Z. The number of layers in which the third coils of the primary winding  1  are stacked is not limited to four. 
     All of the first coil  21 , the second coil  22 , and the third coils  11 ,  12 ,  13 , and  14  at least partially encircle an axis J. The axis J is a virtual axis passing through the leg  502  along the first direction Z, and is indicated by the dot-dash line in all of the drawings. 
     The first coil  21  includes an encircling portion  210 , and ends  21   a  and  21   c.  The encircling portion  210  encircles the leg  502 , and hence the axis J. The ends  21   a  and  21   c  extend continuously from the encircling portion  210 , and are arranged adjacent to, but not in contact with, each other in the second direction X. Hereinafter, being arranged in this manner is expressed as being “adjacent to each other in a non-contact manner”. A boundary  21   ac  is a boundary between the ends  21   a  and  21   c  in the second direction X. 
     The second coil  22  includes an encircling portion  220 , and ends  22   b  and  22   c.  The encircling portion  220  encircles the leg  502 , and hence the axis J. The ends  22   b  and  21   c  extend continuously from the encircling portion  220 , and are adjacent to each other in a non-contact manner in the second direction X. A boundary  22   bc  is a boundary between the ends  22   b  and  22   c  in the second direction X. 
     In Embodiment 1 and Embodiment 2, which will be described below, a third direction Y is introduced that is orthogonal to both the first direction Z and the second direction X. The encircling portion  210  is located on the third direction Y side relative to the ends  21   a  and  21   c.  The encircling portion  220  is located on the third direction Y side relative to the ends  22   a  and  22   c.    
     The first coil  21  encircles the leg  502  once, excluding the boundary  21   ac.    
     The second coil  22  encircles the leg  502  once, excluding the boundary  22   bc.  The third coils  11 ,  12 ,  13 , and  14  encircle the leg  502  a plurality of times. 
       FIG. 4  is an exploded perspective view illustratively depicting the primary winding  1  and the secondary winding  2  in the first direction Z. In  FIG. 4 , the core  5  is omitted. Also in  FIG. 4 , the region located on the core  5  side relative to the position indicated by the line CC is shown, as in the case of  FIG. 2 . 
     In the first direction Z, the second coil  22 , the third coils  14 ,  13 ,  12 ,  11 , and the first coil  21  are stacked in this order. 
     The third coil  11  includes ends  11   d  and  11   e.  The third coil  12  includes ends  12   d  and  12   e.  The third coil  13  includes ends  13   d  and  13   e.  The third coil  14  includes ends  14   d  and  14   e.    
     In the primary winding  1 , the third coils  11 ,  12 ,  13 , and  14  are connected in series in this order. The end  11   e  is connected to the end  12   e  by a conductor  112 . The conductor  112  is sandwiched between the ends  11   e  and  12   e  in the first direction Z so as to be conductively connected to the ends  11   e  and  12   e.  The end  12   d  is connected to the end  13   e  by a conductor  123 . The conductor  123  is sandwiched between the ends  12   d  and  13   e  in the first direction Z so as to be conductively connected to the ends  12   d  and  13   e.  The end  13   d  is connected to the end  14   e  by a conductor  134 . The conductor  134  is sandwiched between the ends  13   d  and  14   e  in the first direction Z so as to be conductively connected to the ends  13   d  and  14   e.  The ends  11   d  and  14   d  function as opposite ends of the primary winding  1 . The ends  11   d  and  14   d  are located on the third direction Y side relative to the third coils  12  and  13 . 
       FIG. 5  is a side view illustratively depicting the configuration of the transformer  100 .  FIG. 5  shows a side as viewed from a direction opposite to the third direction Y. The hatching of the conductors  112 ,  123 , and  134  is provided for the sake of convenience in order to improve viewability, rather than to indicate a cross section. 
     The first coil  21  and the second coil  22 , and the third coils  11 ,  12 ,  13 , and  14  can each be realized in the form of a conductive pattern in a printed circuit board  60  in which a plurality of insulating layers are stacked. The conductive pattern is provided on a boundary or a surface of a stacked insulating layer. Each of the conductors  112 ,  123 , and  134  can be realized by a via hole that provides conductive connection in the corresponding insulating layer in the thickness direction. In  FIGS. 1, 2, and 4 , the insulating layers are omitted. By only depicting the contour of the printed circuit board  60  with a dot-dash line in  FIG. 5 , the viewability of the conductors  112 ,  123 , and  134  is enhanced. 
     The secondary winding  2  includes conductor groups  211 ,  212 ,  213 , and  214 . The conductor group  211  includes conductors  211   a,    211   b,  and  211   c.  The conductor group  212  includes conductors  212   a,    212   b,  and  212   c.  The conductor group  213  includes conductors  213   a,    213   b,  and  213   c.  The conductor group  214  includes conductors  214   a,    214   b,  and  214   c.    
       FIG. 6  is a cross-sectional view illustratively depicting the configuration of the secondary winding  2 .  FIG. 6  shows a cross section seen at a position indicated by the line DD shown in  FIG. 1  as viewed along the third direction Y. 
     The end  21   a  and the end  21   c  are arranged in a non-contact manner across the boundary  21   ac  along the second direction X. The conductor  211   a  and the conductor  211   c  are arranged in a non-contact manner across a boundary  211   ac  along the second direction X. The conductor  212   a  and the conductor  212   c  are arranged in a non-contact manner across a boundary  212   ac  along the second direction X. The conductor  213   a  and the conductor  213   c  are arranged in a non-contact manner across a boundary  213   ac  along the second direction X. The conductor  214   a  and the conductor  214   c  are arranged in a non-contact manner across a boundary  214   ac  along the second direction X. The end  22   a  and the end  22   c  are arranged in a non-contact manner across a boundary  22   ac  along the second direction X. 
     The end  21   c  and the end  21   b  are arranged in a non-contact manner across a boundary  21   bc  along the second direction X. The conductor  211   c  and the conductor  211   b  are arranged in a non-contact manner across a boundary  211   bc  along the second direction X. The conductor  212   c  and the conductor  212   b  are arranged in a non-contact manner across a boundary  212   bc  along the second direction X. The conductor  213   c  and the conductor  213   b  are arranged in a non-contact manner across a boundary  213   bc  along the second direction X. The conductor  214   c  and the conductor  214   b  are arranged in a non-contact manner across a boundary  214   bc  along the second direction X. The end  22   c  and the end  22   b  are arranged in a non-contact manner across the boundary  22   bc  along the second direction X. 
     In  FIGS. 2 and 4 , some or all of the reference numerals of the above-described boundaries are omitted in order to avoid complication of the illustration. 
     A conductor  611   a  is sandwiched between the end  21   a  and the conductor  211   a  in the first direction Z so as to be conductively connected to the end  21   a  and the conductor  211   a.  The conductor  611   a  can be realized by a via hole formed in the insulating layer sandwiched between the end  21   a  and the conductor  211   a.    
     A conductor  612   a  is sandwiched between the conductor  211   a  and the conductor  212   a  in the first direction Z so as to be conductively connected to the conductor  211   a  and the conductor  212   a.  The conductor  612   a  can be realized by a via hole formed in the insulating layer sandwiched between the conductor  211   a  and the conductor  212   a.    
     A conductor  623   a  is sandwiched between the conductor  212   a  and the conductor  213   a  in the first direction Z so as to be conductively connected to the conductor  212   a  and the conductor  213   a.  The conductor  623   a  can be realized by a via hole formed in the insulating layer sandwiched between the conductor  212   a  and the conductor  213   a.    
     A conductor  634   a  is sandwiched between the conductor  213   a  and the conductor  214   a  in the first direction Z so as to be conductively connected to the conductor  213   a  and the conductor  214   a.  The conductor  634   a  can be realized by a via hole formed in the insulating layer sandwiched between the conductor  213   a  and the conductor  214   a.    
     A conductor  642   a  is sandwiched between the conductor  214   a  and the end  22   a  in the first direction Z so as to be conductively connected to the conductor  214   a  and the end  22   a.  The conductor  642   a  can be realized by a via hole formed in the insulating layer sandwiched between the conductor  214   a  and the end  22   a.    
     A conductor  611   b  is sandwiched between the end  21   b  and the conductor  211   b  in the first direction Z so as to be conductively connected to the end  21   b  and the conductor  211   b.  The conductor  611   b  can be realized by a via hole formed in the insulating layer sandwiched between the end  21   b  and the conductor  211   b.    
     A conductor  612   b  is sandwiched between the conductor  211   b  and the conductor  212   b  in the first direction Z so as to be conductively connected to the conductor  211   b  and the conductor  212   b.  The conductor  612   b  can be realized by a via hole formed in the insulating layer sandwiched between the conductor  211   b  and the conductor  212   b.    
     A conductor  623   b  is sandwiched between the conductor  212   b  and the conductor  213   b  in the first direction Z so as to be conductively connected to the conductor  212   b  and the conductor  213   b.  The conductor  623   b  can be realized by a via hole formed in the insulating layer sandwiched between the conductor  212   b  and the conductor  213   b.    
     A conductor  634   b  is sandwiched between the conductor  213   b  and the conductor  214   b  in the first direction Z so as to be conductively connected to the conductor  213   b  and the conductor  214   b.  The conductor  634   b  can be realized by a via hole formed in the insulating layer sandwiched between the conductor  213   b  and the conductor  214   b.    
     A conductor  642   b  is sandwiched between the conductor  214   b  and the end  22   b  in the first direction Z so as to be conductively connected to the conductor  214   b  and the end  22   b.  The conductor  642   b  can be realized by a via hole formed in the insulating layer sandwiched between the conductor  214   b  and the end  22   b.    
     A conductor  611   c  is sandwiched between the end  21   c  and the conductor  211   c  in the first direction Z so as to be conductively connected to the end  21   c  and the conductor  211   c.  The conductor  611   c  can be realized by a via hole formed in the insulating layer sandwiched between the end  21   c  and the conductor  211   c.    
     A conductor  612   c  is sandwiched between the conductor  211   c  and the conductor  212   c  in the first direction Z so as to be conductively connected to the conductor  211   c  and the conductor  212   c.  The conductor  612   c  can be realized by a via hole formed in the insulating layer sandwiched between the conductor  211   c  and the conductor  212   c.    
     A conductor  623   c  is sandwiched between the conductor  212   c  and the conductor  213   c  in the first direction Z so as to be conductively connected to the conductor  212   c  and the conductor  213   c.  The conductor  623   c  can be realized by a via hole formed in the insulating layer sandwiched between the conductor  212   c  and the conductor  213   c.    
     A conductor  634   c  is sandwiched between the conductor  213   c  and the conductor  214   c  in the first direction Z so as to be conductively connected to the conductor  213   c  and the conductor  214   c.  The conductor  634   c  can be realized by a via hole formed in the insulating layer sandwiched between the conductor  213   c  and the conductor  214   c.    
     A conductor  642   c  is sandwiched between the conductor  214   c  and the end  22   c  in the first direction Z so as to be conductively connected to the conductor  214   c  and the end  22   c.  The conductor  642   c  can be realized by a via hole formed in the insulating layer sandwiched between the conductor  214   c  and the end  22   c.    
     With such conductive connection, both the end  21   a  and the end  22   a  function as one end of the secondary winding  2 , both the end  21   b  and the end  22   b  function as the other end of the secondary winding  2 , and both the end  21   c  and the end  22   c  function as a center tap of the secondary winding  2 . 
     As compared with a configuration in which the primary coil and the secondary coil are provided in the same plane, such as the configuration disclosed in JP 2015-192082A, the area extending in the second direction X and the third direction Y of the transformer  100  can be reduced because the primary winding  1  and the secondary winding  2  are stacked in the first direction Z. This is advantageous in reducing the size of the transformer. 
     Upon applying a voltage to the ends  11   d  and  14   d  of the primary winding  1 , a current flows through the primary winding  1 , and a magnetic flux is generated from the current. This magnetic flux has a component constituting a so-called leakage flux, which is not interlinked with the secondary winding  2 . 
     A first component of the leakage flux is a magnetic flux that passes between the first coil  21  and the third coil  11  in the first direction Z, and between the second coil  22  and the third coil  14  in the first direction Z, and that is not interlinked with either of the first coil  21  and the second coil  22 . 
     A second component of the leakage flux is a magnetic flux that passes through the boundaries  21   ac,    211   ac,    212   ac,    213   ac,    214   ac,  and  22   ac  in this order, or in the reverse order. 
     A third component of the leakage flux is a magnetic flux that passes through the boundaries  21   bc,    211   bc,    212   bc,    213   bc,    214   bc,  and  22   bc  in this order, or in the reverse order. 
     The second component of the leakage flux moves back and forth in a region surrounded by a closed circuit obtained as a result of the ends  21   a  and  22   c  of the secondary winding  2  being connected to each other outside the transformer  100  via a load or directly. The third component of the leakage flux moves back and forth in a region surrounded by a closed circuit obtained as a result of the ends  21   b  and  22   c  of the secondary winding  2  being connected to each other outside the transformer  100  via a load or directly. 
     Accordingly, in view of reducing the second component of the leakage flux in order to reduce the leakage flux, it is desirable to increase the magnetic resistance to the second component in a path extending from the boundary  21   ac  to the boundary  22   ac  via the boundaries  211   ac,    212   ac,    213   ac,  and  214   ac.  In view of reducing the third component of the leakage flux in order to reduce the leakage flux, it is also desirable to increase the magnetic resistance to the third component in a path extending from the boundary  21   bc  to the boundary  22   bc  via the boundaries  211   bc,    212   bc,    213   bc,  and  214   bc.    
     In the second direction X, the boundaries  21   ac,    211   ac,    212   ac,    213   ac,    214   ac,  and  22   ac  are at positions x 2 , x 1 , x 3 , x 1 , x 3 , and x 2 , respectively. Also, the positions x 1 , x 2 , and x 3  are all different from one another. Accordingly, the second component of the leakage flux generally flows in the first direction Z or a direction opposite thereto, but flows in a serpentine manner along the second direction X. Such a serpentine magnetic path increases the magnetic resistance to the second component of the leakage flux. 
     In the second direction X, the boundaries  21   bc,    211   bc,    212   bc,    213   bc,    214   bc,  and  22   bc  are at positions x 5 , x 4 , x 6 , x 4 , x 6 , and x 5 , respectively. Also, the positions x 4 , x 5 , and x 6  are all different from one another. Accordingly, the third component of the leakage flux generally flows in the first direction Z or a direction opposite thereto, but flows in a serpentine manner along the second direction X. Such a serpentine magnetic path increases the magnetic resistance to the third component of the leakage flux. 
     The example shown in  FIG. 6  illustrates a case where the positions in the second direction X of the boundaries adjacent to each other in the first direction Z are necessarily different from each other. In this case, as compared with a case where the positions in the second direction X of the boundaries adjacent to each other in the first direction Z are all the same, the leakage flux is reduced by about 20 to 30 percent (provided that the ends  21   a  and  21   c  are shorted, and the end  21   b  is open). 
     The positional relationship between the conductors illustrated in  FIG. 6  is desirable in view of increasing the magnetic resistance; however, such a positional relationship is not necessarily required. 
     When any two of the positions in the second direction X of the boundaries  21   ac,    211   ac,    212   ac,    213   ac,    214   ac,  and  22   ac  are different from each other, the magnetic path is longer than that in the case where any two of the positions are not different from each other (i.e., none of the positions are different from one another), resulting in an increase in the magnetic resistance to the second component of the leakage flux. For example, the boundaries  212   ac  and  214   ac  may be at the position x 1 . Although the boundaries  21   ac  and  22   ac  are both located at the position x 2  in  FIG. 6 , the boundaries  21   ac  and  22   ac  may be at positions different from each other in the second direction X. 
     When any two of the positions in the second direction X of the boundaries  21   bc,    211   bc,    212   bc,    213   bc,    214   bc,  and  22   bc  are different from each other, the magnetic path is longer than that in the case where any two of the positions are not different from each other (i.e., none of the positions are different from one another), resulting in an increase in the magnetic resistance to the third component of the leakage flux. For example, the boundaries  212   bc  and  214   bc  may be at the position x 4 . Although the boundaries  21   bc  and  22   bc  are both located at the position x 5  in  FIG. 6 , the boundaries  21   bc  and  22   bc  may be at positions different from each other in the second direction X. 
     The foregoing can be expressed as follows. First, in a configuration in view of reducing the second component of the leakage flux: 
     (ia) the first position is different from at least one of the second position and the third position; 
     (iia) the first position is any one of: 
     the position x 1  in the second direction X of the boundary  211   ac  between the conductor  211   a  and the conductor  211   c;    
     the position x 3  in the second direction X of the boundary  212   ac  between the conductor  212   a  and the conductor  212   c;    
     the position x 1  in the second direction X of the boundary  213   ac  between the conductor  213   a  and the conductor  213   c;  and 
     the position x 3  in the second direction X of the boundary  214   ac  between the conductor  214   a  and the conductor  214   c;    
     (iiia) the second position is 
     the position x 2  in the second direction X of the boundary  21   ac  between the end  21   a  and the end  21   c;  and 
     (iva) the third position is 
     the position x 2  in the second direction X of the boundary  22   ac  between the end  22   a  and the end  22   c.    
     Also in a configuration in view of reducing the third component of the leakage flux, as in the case of (ia) to (iva) above: 
     (ib) the first position is different from at least one of the second position and the third position; 
     (iib) the first position is any one of: 
     the position x 4  in the second direction X of the boundary  211   bc  between the conductor  211   b  and the conductor  211   c;    
     the position x 6  in the second direction X of the boundary  212   bc  between the conductor  212   b  and the conductor  212   c;    
     the position x 4  in the second direction X of the boundary  213   bc  between the conductor  213   b  and the conductor  213   c;  and 
     the position x 6  in the second direction X of the boundary  214   bc  between the conductor  214   b  and the conductor  214   c;    
     (iiib) the second position is the position x 5  in the second direction X of the boundary  21   bc  between the end  21   b  and the end  21   c;  and 
     (ivb) the third position is the position x 5  in the second direction X of the boundary  22   bc  between the end  22   b  and the end  22   c.    
     Example of Application to Full-Bridge DC/DC Converter 
       FIG. 7  is a circuit diagram illustratively depicting a converter  200  in which the transformer  100  is used. The converter  200  is a full-bridge DC/DC converter. The transformer  100  is used as a transformer T in the converter  200 . 
     The ends  11   d  and  14   d  of the transformer  100  function as primary-side terminals of the transformer T. The ends  21   a,    21   b,  and  21   c  of the transformer  100  function as secondary-side terminals of the transformer T. The end  21   c  functions as a center tap of the transformer T. An inductor La having one end connected to the end  11   d  inside the transformer T equivalently indicates the leakage inductance of the transformer T on the primary side. 
     On the primary side of the transformer T, switching elements Q 1 , Q 2 , Q 3 , and Q 4  and diodes D 1  and D 2  are provided between power lines H 1  and L 1 . The power line H 1  has a higher potential than the power line L 1 . 
     The switching elements Q 1  and Q 2  are connected in series between the power lines H 1  and L 1 . The switching elements Q 3  and Q 4  are connected in series between the power lines H 1  and L 1 . 
     The diodes D 1  and D 2  are connected in series between the power lines H 1  and L 1 . The anode of the diode D 1  and the cathode of the diode D 2  are connected to the end  11   d.  The cathode of the diode D 1  is connected to the power line H 1 . The anode of the diode D 2  is connected to the power line L 1 . 
     The end  11   d  is connected, via an inductor Lb, to a connection point P 1  at which the switching elements Q 1  and Q 2  are connected to each other. The end  14   d  is connected to a connection point P 2  at which the switching elements Q 3  and Q 4  are connected to each other. 
     Switching elements Q 101  and Q 102 , an inductor Lc, and a capacitor Cd are provided on the secondary side of the transformer T. The capacitor Cd is provided between power lines H 2  and L 2 . The power line H 2  has a higher potential than the power line L 2 . The power line L 2  is grounded, for example. 
     One end of the switching element Q 101  is connected to the end  21   b,  and the other end thereof is connected to the power line L 2 . One end of the switching element Q 102  is connected to the end  21   a,  and the other end thereof is connected to the power line L 2 . One end of the inductor Lc is connected to the end  21   c,  and the other end thereof is connected to the power line H 2 . 
     All of the switching elements Q 1 , Q 2 , Q 3 , Q 4 , Q 101 , and Q 102  are realized by a field-effect transistor, for example. 
     Since the operations of the converter  200  having the above-described configuration, including, for example, the timing at which the switching elements Q 1 , Q 2 , Q 3 , Q 4 , Q 101 , and Q 102  are switched, are known, descriptions of the operations are omitted in the present embodiment. A description will be given of an advantage of using the transformer  100  in the transformer T to reduce the leakage inductance of the primary side. 
     The inductor Lb has the function of reducing the surge voltage on the secondary side of the converter  200 . Energy is regenerated to the power lines H 1  and L 1  via the diodes D 1  and D 2 . 
     The larger the ratio of the inductance of the inductor Lb to the inductance (the leakage inductance on the primary side of the transformer T) of the inductor La, the greater the effect of reducing the surge voltage on the secondary side is. Since the sum of the inductance of the inductor Lb and the inductance of the inductor La affects the resonant period of so-called soft switching, it is not desirable to increase the inductance of the inductor Lb without limitation. Therefore, it is desirable that the inductance of the inductor La is small. 
     The transformer  100  has the above-described features (i) to (iv), and the leakage inductance on the primary side thereof is reduced. Accordingly, the transformer  100  is suitably applied to a full-bridge DC/DC converter, such as the converter  200  in which the inductor Lb is used. 
     Embodiment 2 
     A transformer according to Embodiment 2 will be described. Note that the same constituent elements as those described in Embodiment 1 are denoted by the same reference numerals in the description of Embodiment 2, and the description thereof has been omitted. 
       FIG. 8  is a plan view illustratively depicting the configuration of a transformer  101  according to Embodiment 2. The transformer  101  includes a core  5 . The same configuration as that of the core  5  of the transformer  100  can be used for the core  5 . 
       FIG. 9  is an exploded perspective view illustratively depicting a primary winding  1  and a secondary winding  2  in a first direction Z. The core  5  is omitted in  FIG. 9 .  FIG. 9  shows a region located on the core  5  side relative to a position indicated by the line EE in  FIG. 8 . 
     The transformer  101  includes a primary winding  1  and a secondary winding  2 . The primary winding  1  includes third coils  11 ,  12 ,  13 , and  14 . The secondary winding  2  includes a first coil  21  and a second coil  22 . In the first direction Z, the second coil  22 , the third coils  14 ,  13 ,  12 , and  11 , and the first coil  21  are stacked in this order. 
     All of the first coil  21 , the second coil  22 , and the third coils  11 ,  12 ,  13 , and  14  at least partially encircle an axis J. As an example of the configuration of the third coils  11 ,  12 ,  13 , and  14  of the transformer  101 , the configuration of the third coils  11 ,  12 ,  13 , and  14  illustrated in the transformer  100  is used. The number of layers in which the coils of the primary winding  1  are stacked is not limited to four. 
     The first coil  21  includes an encircling portion  210 , and ends  21   a  and  21   b.  The encircling portion  210  encircles the axis J. As the encircling portion  210  of the transformer  101 , the encircling portion  210  of the transformer  100  is used, for example. The ends  21   a  and  21   b  extend continuously from the encircling portion  210 , and are adjacent to each other in a non-contact manner in the second direction X. A boundary  21   ab  is a boundary between the ends  21   a  and  21   b  in the second direction X. 
     The second coil  22  includes an encircling portion  220 , and ends  22   a  and  22   b.  The encircling portion  220  encircles the axis J. As the encircling portion  220  of the transformer  101 , the encircling portion  220  of the transformer  100  is used, for example. The ends  22   a  and  22   b  extend continuously from the encircling portion  210 , and are adjacent to each other in a non-contact manner in the second direction X. A boundary  22   ab  is a boundary between the ends  22   a  and  22   b  in the second direction X. 
     The encircling portion  210  is located on the third direction Y side relative to the ends  21   a  and  21   b.  The encircling portion  220  is located on the third direction Y side relative to the ends  22   a  and  22   b.    
     The first coil  21  encircles the axis J once, excluding the boundary  21   ab.  The second coil  22  encircles the axis J once, excluding the boundary  22   ab.    
     The first coil  21 , the second coil  22 , and the third coils  11 ,  12 ,  13 , and  14  can each be realized in the form of a conductive pattern in a printed circuit board  60  in which a plurality of insulating layers are stacked. The conductive pattern is provided on a boundary or surface of a stacked insulating layer. Each of the conductors  112 ,  123 , and  134  can be realized by a via hole that provides conductive connection in the corresponding insulating layer in the thickness direction. The insulating layers are omitted in  FIGS. 8 and 9 . 
     The secondary winding  2  includes conductor groups  211 ,  212 ,  213 , and  214 . The conductor group  211  includes conductors  211   a  and  211   b.  The conductor group  212  includes conductors  212   a  and  212   b.  The conductor group  213  includes conductors  213   a  and  213   b.  The conductor group  214  includes conductors  214   a  and  214   b.    
       FIG. 10  is a cross-sectional view illustratively depicting the configuration of the secondary winding  2 .  FIG. 10  shows a cross section seen at a position indicated by the line FF shown in  FIG. 8  as viewed along the third direction Y. 
     The end  21   a  and the end  21   b  are arranged in a non-contact manner across the boundary  21   ab  along the second direction X. The conductor  211   a  and the conductor  211   b  are arranged in a non-contact manner across a boundary  211   ab  along the second direction X. The conductor  212   a  and the conductor  212   b  are arranged in a non-contact manner across a boundary  212   ab  along the second direction X. The conductor  213   a  and the conductor  213   b  are arranged in a non-contact manner across a boundary  213   ab  along the second direction X. The conductor  214   a  and the conductor  214   b  are arranged in a non-contact manner across a boundary  214   ab  along the second direction X. The end  22   a  and the end  22   b  are arranged in a non-contact manner across the boundary  22   ab  along the second direction X. 
     In  FIGS. 8 and 9 , some or all of the reference numerals of the above-described boundaries are omitted in order to avoid complication of the illustration. 
     The conductors  611   a,    612   a,    623   a,    634   a,    642   a,    611   b,    612   b,    623   b,    634   b,  and  642   b  are configured and arranged in the same manner as the transformer  100 . The transformer  101  does not include the ends  21   c  and  22   c  included in the transformer  100 . Accordingly, the conductors  611   c,    612   c,    623   c,    634   c,  and  642   c  are not provided in the transformer  101 . 
     With such conductive connection, both the end  21   a  and the end  22   a  function as one end of the secondary winding  2 , and both the end  21   b  and the end  22   b  function as the other end of the secondary winding  2 . 
     Also in the transformer  101 , the magnetic flux generated upon application of a voltage to the ends  11   d  and  14   d  of the primary winding  1  includes a component constituting a leakage flux. 
     A first component of the leakage flux is a magnetic flux that passes between the first coil  21  and the third coil  11  in the first direction Z, and between the second coil  22  and the third coil  14  in the first direction Z, and that is not interlinked with either of the first coil  21  and the second coil  22 . 
     A second component of the leakage flux is a magnetic flux that passes through the boundaries  21   ab,    211   ab,    212   ab,    213   ab,    214   ab,  and  22   ab  in this order, or in the reverse order. 
     Accordingly, in view of reducing the second component of the leakage flux in order to reduce the leakage flux, it is desirable to increase the magnetic resistance in a path extending from the boundary  21   ab  to the boundary  22   ab  via the boundaries  211   ab,    212   ab,    213   ab,  and  214   ab.    
     In the second direction X, the boundaries  21   ab,    211   ab,    212   ab,    213   ab,    214   ab,  and  22   ab  are at positions x 8 , x 7 , x 9 , x 7 , x 9 , and x 8 , respectively. Also, the positions x 7 , x 8 , and x 9  are all different from one another. Accordingly, the second component of the leakage flux generally flows in the first direction Z or a direction opposite thereto, but flows in a serpentine manner along the second direction X. Such a serpentine magnetic path increases the magnetic resistance to the second component of the leakage flux. 
     The example shown in  FIG. 10  illustrates a case where the positions in the second direction X of the boundaries adjacent to each other in the first direction Z are necessarily different from each other. The positional relationship between the conductors illustrated in  FIG. 10  is desirable in view of increasing the magnetic resistance; however, such a positional relationship is not necessarily required. 
     When any two of the positions in the second direction X of the boundaries  21   ab,    211   ab,    212   ab,    213   ab,    214   ab,  and  22   ab  are different from each other, the magnetic path is longer than that in the case where any two positions are not different from each other (i.e., none of the positions are different from one another), resulting in an increase in the magnetic resistance to the second component of the leakage flux. For example, the boundaries  212   ab  and  214   ab  may be at the position x 7 . Although the boundaries  21   ab  and  22   ab  are both located at the position x 8  in 
       FIG. 10 , the boundaries  21   ab  and  22   ab  may be at positions different from each other in the second direction X. 
     The foregoing can be generally expressed as follows. 
     (i) the first position is different from at least one of the second position and the third position; 
     (ii) the first position is any one of: 
     the position x 7  in the second direction X of the boundary  211   ab  between the conductor  211   a  and the conductor  211   b;    
     the position x 9  in the second direction X of the boundary  212   ab  between the conductor  212   a  and the conductor  212   b;    
     the position x 7  in the second direction X of the boundary  213   ab  between the conductor  213   a  and the conductor  213   b;  and 
     the position x 9  in the second direction X of the boundary  214   ab  between the conductor  214   a  and the conductor  214   b;    
     (iii) the second position is the position x 8  in the second direction X of the boundary  21   ab  between the end  21   a  and the end  21   b;  and 
     (iv) the third position is the position x 8  in the second direction X of the boundary  22   ab  between the end  22   a  and the end  22   b.    
     APPENDIX 
     Not all of the third coils  11 ,  12 ,  13 , and  14  necessarily need to be sandwiched between the first coil  21  and the second coil  22  in the first direction Z. Any of the third coils  11 ,  12 ,  13 , and  14  may be sandwiched between the first coil  21  and the second coil  22 , or none of them need to be sandwiched between the first coil  21  and the second coil  22 . 
     In view of reducing the first component of the leakage flux, in a preferred example of the arrangement, all of the third coils  11 ,  12 ,  13 , and  14  are sandwiched between the first coil  21  and the second coil  22  in the first direction Z. 
     When the primary winding  1  and the secondary winding  2  are realized in the printed circuit board  60 , it is desirable that the first coil  21  is not sandwiched between any of the second coil  22  and the third coils  11 ,  12 ,  13 , and  14 . The reason being that the ends  21   a,    21   b,  and  21   c  are likely to be connected to an external device of the transformer  100 . Alternatively, it is desirable that the second coil  22  is not sandwiched between any of the first coil  21  and thew third coils  11 ,  12 ,  13 , and  14 . The reason being that the ends  22   a,    22   b,  and  22   c  are likely to be connected to an external device of the transformer  100 . 
     It should be appreciated that the configurations described in the embodiments and modifications above may be combined as appropriate as long as there are no mutual inconsistencies.