Patent Publication Number: US-9431166-B2

Title: Inductor and method of manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-044023, filed March 6 and No. 2013-229702, filed Nov. 5, 2013; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relates to an inductor and a method of manufacturing the same. 
     BACKGROUND 
     Many recent apparatuses adopt wireless power transmission systems that wirelessly transmit electric power in a noncontact manner by using mutual inductance between a power transmitting coil and a power receiving coil. A power transmitting coil used in such a wireless power transmission system includes a ferrite core, a coil wire wound around the ferrite core, and a resin covering the ferrite core and the coil wire. The coil wire is a stranded wire having low loss, such as a Litz wire. 
     When the ferrite core with the Litz wire wound therearound is covered with the resin, a space between turns of the Litz wire or a vicinity of the Litz wire may not be filled with the resin, and a void (cavity) may be formed. If a void is formed in the resin, the electrical field can be concentrated in the void to produce a discharge, thereby causing a dielectric breakdown. In addition, there is a possibility that heat is not uniformly diffused, the thermal conductivity decreases, and the resin deteriorates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a wireless power transmission system according to a first embodiment; 
         FIG. 2  is a top view of an inductor according to the first embodiment; 
         FIG. 3  is a cross-sectional view taken along the line A-A in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view taken along the line B-B in  FIG. 2 ; 
         FIG. 5  is a top view of an inductor according to a second embodiment; 
         FIG. 6  is a cross-sectional view taken along the line A-A in  FIG. 5 ; 
         FIG. 7  is a cross-sectional view taken along the line B-B in  FIG. 5 ; 
         FIG. 8  shows process cross-sectional views for illustrating a method of manufacturing the inductor according to the second embodiment; 
         FIG. 9  is a top view of an inductor according to a third embodiment; 
         FIG. 10  is a cross-sectional view taken along the line A-A in  FIG. 9 ; 
         FIG. 11  is cross-sectional view taken along the line c-c in  FIG. 9 ; 
         FIG. 12  is a top view of an inductor according to a fourth embodiment; 
         FIG. 13  is a cross-sectional view taken along the line D-D in  FIG. 12 ; 
         FIG. 14  is a cross-sectional view taken along the line E-E in  FIG. 12 ; 
         FIG. 15  is a cross-sectional view taken along the line F-F in  FIG. 12 ; 
         FIG. 16  is a top view of an inductor according to a modification; 
         FIG. 17  is a top view of an inductor according to a fifth embodiment; 
         FIG. 18  is an enlarged view of a region “R” surrounded by the dashed line in  FIG. 17 ; 
         FIG. 19  shows process cross-sectional views for illustrating a method of manufacturing the inductor according to the fifth embodiment; 
         FIG. 20  shows process cross-sectional views for illustrating a method of manufacturing an inductor according to a modification of the fifth embodiment; 
         FIG. 21  is a diagram showing a surface of a bobbin according to the modification of the fifth embodiment; 
         FIG. 22  is a cross-sectional view of the inductor according to the modification of the fifth embodiment; and 
         FIG. 23  is a cross-sectional view of the inductor according to the modification of the fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to an embodiment, there is an inductor, including: a magnetic core; a winding formed around the magnetic core; a first resin provided between turns of the winding; and a second resin covering the winding and the first resin, wherein the second resin has higher filler content than the first resin. 
     In the following, embodiments of the present invention will be described with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a block diagram showing a configuration of a wireless power transmission system according to a first embodiment of the present invention. The wireless power transmission system includes a power transmitter  1  and a power receiver  2  to which electric power is wirelessly transmitted from the power transmitter  1 . The power receiver  2  supplies the electric power transmitted thereto to a load  28  of an electrical apparatus. The power receiver  2  may be provided in the electric apparatus, integrated with the electric apparatus, or attached to the exterior of the main body of the electrical apparatus. For example, the electric apparatus may be a mobile terminal or an electric automobile, and the load  28  may be a rechargeable battery. 
     The power transmitter  1  includes a power supply  11  that converts a commercial electric power into an RF electric power suitable for electric power transmission, a controller  12  that controls the amount of required electric power and controls each component of the power transmitter  1 , a sensing unit  13 , a communication unit  14 , and a power transmitting inductor  15 . The sensing unit  13  includes at least one of a temperature sensor that monitors heat generation of the power transmitter  1 , a temperature sensor that monitors heat of a foreign matter between the power transmitting inductor  15  and a power receiving inductor  21  described later, a sensor that monitors a foreign matter with an electromagnetic wave radar or an ultrasonic wave radar, a sensor that detects the position of the power receiving inductor  21 , such as an RFID, and a sensor used in wireless power transmission between the power transmitter  1  and the power receiver  2 , such as an ammeter or a voltmeter used for detecting the transmitted electric power, for example. The communication unit  14  is capable of communicating with a communication unit  27  in the power receiver  2  described later and receives a power reception status of the power receiver  2  or transmits a power transmission status of the power transmitter  1 . 
     The power receiver  2  includes the power receiving inductor  21  that receives electric power from the power transmitting inductor  15  of the power transmitter  1  according to the mutual inductance between the two, a capacitor unit  22  connected to the power receiving inductor  21 , a rectifier  23  that converts an alternating-current electric power received via the capacitor unit  22  to a direct-current electric power, a DC-DC converter  24  that changes a voltage conversion ratio based on an operating voltage of the load  28 , a controller  25  that controls each component of the power receiver  2 , a sensing unit  26 , and the communication unit  27 . In a case where the received electric power is controlled on the side of the power transmitter  1 , the DC-DC converter  24  can be omitted. 
     The sensing unit  26  includes at least one of a temperature sensor that monitors heat generation of the power receiver  2 , a temperature sensor that monitors heat of a foreign matter between the power receiving inductor  21  and the power transmitting inductor  15 , a sensor that monitors a foreign matter with an electromagnetic wave radar or an ultrasonic wave radar, a sensor that detects the position of the power transmitting inductor  15 , such as an RFID, and a sensor used in wireless power transmission between the power transmitter  1  and the power receiver  2 , such as an ammeter or a voltmeter used for detecting the transmitted electric power, for example. 
     The communication unit  27  is capable of communicating with the communication unit  14  in the power transmitter  1  and transmits the power reception status of the power receiver  2  or receives the power transmission status of the power transmitter  1 . 
     The controller  25  controls the received electric power (electric power supplied to the load  28 ) based on information acquired by the communication unit  27  communicating with the power transmitter  1  or a result of detection by the sensing unit  26 . 
       FIG. 2  is a top view of an inductor  100  according to the first embodiment. For the convenience of explanation, other components that are actually hidden under a second resin  110  described later are also shown in the top view of  FIG. 2 .  FIG. 3  is a vertical cross-sectional view taken along the line A-A in  FIG. 2 , and  FIG. 4  is a vertical cross-sectional view taken along the line B-B in  FIG. 2 . The inductor  100  is used as the power transmitting inductor  15  and the power receiving inductor  21  shown in  FIG. 1 . 
     As shown in  FIGS. 2 to 4 , the inductor  100  includes a tubular bobbin  102 , a ferrite core  104  inserted in a hole of the bobbin  102 , a Litz wire (winding)  106  wound around an outer periphery of the bobbin  102 , a first resin  108  that fills the spaces between the turns of the Litz wire  106 , the second resin  110  that covers the bobbin  102 , the ferrite core  104 , the Litz wire  106  and the first resin  108 , and a conductive plate  112  attached to one surface of the second resin  110 . A conductive paint (conductive material)  114  having a lower rigidity than the bobbin  102  and the ferrite core  104  may be applied to an inner wall of the bobbin  102 . The conductive paint  114  can prevent occurrence of a partial discharge in a space between the bobbin  102  and the ferrite core  104 , because a potential difference occurs between the Litz wire  106  and the conductive paint  114  on the inside of the bobbin  102 . 
     The bobbin  102  is made of a plastic, for example, and the Litz wire  106  is a copper wire, for example. The conductive paint (conductive material)  114  contains carbon, for example. The conductive plate  112  is an aluminum plate or a copper plate, for example. 
     The second resin  110  is an epoxy resin, for example, and contains an inorganic filler, such as silica, boron nitride, or aluminum nitride. On the other hand, the first resin  108  contains no filler or has lower filler content than the second resin  110 . Therefore, the first resin  108  has higher flowability (lower viscosity) than the second resin  110  and can readily fill the spaces between the turns of the Litz wire  106 . 
     In this way, formation of a void (cavity) between the turns of the Litz wire  106  and in the vicinity of the Litz wire  106  can be prevented. Since void formation is prevented, occurrence of a partial discharge and a dielectric breakdown can be prevented. 
     Since void formation is prevented, heat of the Litz wire  106  can be uniformly diffused. The second resin  110  covering the Litz wire  106  and the first resin  108  contains a filler and has high thermal conductivity and therefore can efficiently diffuse heat. Therefore, deterioration of thermal conductivity and deterioration of the resins caused thereby can be prevented. 
     Next, a method of manufacturing such an inductor  100  will be described. First, the Litz wire  106  is wound around the bobbin  102 . In a space-filling process, the spaces between the turns of the Litz wire  106  are then filled with the first resin  108 . Since the first resin  108  contains no filler or has extremely low filler content, the first resin  108  has high flowability (low viscosity) and can readily fill the spaces between the turns of the Litz wire  106 . Therefore, the first resin  108  pervades the spaces between the turns of the Litz wire  106  and other minute regions, so that formation of a void can be prevented. Following the space-filling process, a heating process is performed to cure the first resin  108 . 
     The conductive paint  114  may then be applied to an inner wall part of the bobbin  102 . After that, the ferrite core  104  is inserted into the hole of the bobbin  102 . 
     The assembly of the bobbin  102 , the ferrite core  104  and the Litz wire  106  is then housed in a mold (container), and the second resin  110  is poured into the mold in a vacuum and cured. 
     The resulting assembly is then removed from the mold, and the conductive plate  112  is attached to one surface of the second resin  110 . For example, the conductive plate  112  is applied to one surface of the second resin with a conductive paint (conductive material)  124  having lower rigidity than the conductive plate  112  interposed therebetween and fixed to the surface with a screw or the like. In this way, the inductor  100  shown in  FIGS. 2 to 4  can be manufactured. The applied conductive paint  124  can prevent occurrence of a partial discharge between the second resin  110  and the conductive plate  112 , because a potential difference occurs between the Litz wire  106  and the conductive paint  124 . Since the conductive paint  124  having lower rigidity than the conductive plate  112  is inserted, a void can be prevented from being formed between the conductive plate  112  and the second resin  110  because of peel off of the resin caused by vibration. 
     By filling the spaces between the turns of the Litz wire  106  with the first resin  108  having high flowability, void formation can be prevented, dielectric breakdown due to a partial discharge can be prevented, and heat of the Litz wire  106  can be uniformly diffused. In addition, by covering the bobbin  102 , the ferrite core  104  and the Litz wire  106  with the second resin  110  containing a filler and having high thermal conductivity, heat can be efficiently diffused, and deterioration of the resin can be prevented. In this way, the inductor according to this embodiment can be prevented from deteriorating in electric insulating properties and thermal conductivity. 
     In the embodiment described above, the conductive plate  112  is attached after the second resin  110  is cured. With such a configuration, the conductive plate  112  can be easily removed. 
     As an alternative, the conductive plate  112  may be housed in the mold (container) along with the bobbin  102 , the ferrite core  104  and the Litz wire  106 , and the second resin  110  may be then poured into the mold and cured. In that case, the adhesion between the conductive plate  112  and the second resin  110  can be improved. 
     As an alternative, the mold (container) may be a plastic case, which can be used as a housing of the inductor  100 . In that case, the step of removing the cured second resin  110  from the mold (container) can be omitted. 
     If the filling rate of the filler, such as boron nitride or aluminum nitride, in the second resin  110  is increased, the thermal conductivity can be further improved. 
     Second Embodiment 
       FIGS. 5 to 7  show a schematic configuration of an inductor according to a second embodiment of the present invention.  FIG. 5  is a top view of the inductor according to this embodiment,  FIG. 6  is a vertical cross-sectional view taken along the line A-A in  FIG. 5 , and  FIG. 7  is a vertical cross-sectional view taken along the line B-B in  FIG. 5 . 
     This embodiment differs from the first embodiment shown in  FIGS. 2 to 4  in that the second resin  110  is provided around the Litz wire  106 , and the second resin  110  is disposed between third resins  120  having lower filler content than the second resin  110 . In  FIGS. 5 to 7 , the same components as those in the first embodiment shown in  FIGS. 2 to 4  are denoted by the same reference numerals, and descriptions thereof will be omitted. 
     According to this embodiment, the second resin  110  having higher filler content is provided in a region surrounding the Litz wire  106 . End parts of the ferrite core  104  in a direction (horizontal direction in  FIGS. 5 and 6 ) perpendicular to the direction of winding of the Litz wire  106  are covered with the third resins  120  having lower filler content than the second resin  110 . The filler content of the third resin  120  is approximately equal to or higher than the filler content of the first resin  108 . 
     Since the Litz wire  106 , which is a heat generation source of the inductor  100 , is covered with the second resin  110  having higher filler content and higher thermal conductivity, heat of the Litz wire  106  can be efficiently diffused. In addition, since the third resins  120  having lower filler content and higher flowability are provided in parts spaced apart from the Litz wire  106 , formation of a void can be prevented. Since the filler content is lower, the weight of the inductor  100  can be reduced accordingly. 
     Next, a method of manufacturing the inductor according to this embodiment will be described. First, the Litz wire  106  is wound around the bobbin  102 . In a space-filling process, the spaces between the turns of the Litz wire  106  are then filled with the first resin  108 . Since the first resin  108  contains no filler or has extremely low filler content, the first resin  108  has high flowability (low viscosity) and can readily fill the spaces between the turns of the Litz wire  106 . Therefore, the first resin  108  pervades the spaces between the turns of the Litz wire  106  and other minute regions, so that formation of a void can be prevented. Following the space-filling process, a heating process is performed to cure the first resin  108 . 
     The conductive paint  114  is then applied to the inner wall part of the bobbin  102 , and the ferrite core  104  is inserted into the hole of the bobbin  102 . 
     The assembly of the bobbin  102 , the ferrite core  104  and the Litz wire  106  is then housed in a mold  200  shown in  FIG. 8( a ) . In this step, the assembly is placed in the mold  200  with one end of the ferrite core  104  in the direction perpendicular to the direction of winding of the Litz wire  106  located at the bottom and the other end located at the top. As shown in  FIG. 8( b ) , the third resin  120  is then poured to a level slightly below the bobbin  102  and cured. As shown in  FIG. 8( c ) , the second resin  110  is poured until the bobbin  102  is covered, and cured. As shown in  FIG. 8( d ) , the third resin  120  is then poured again and cured. 
     The resulting assembly is then removed from the mold  200 , and the conductive plate  112  is attached to one surface of the second resin  110  and the third resins  120 . In this way, the inductor  100  shown in  FIGS. 5 to 7  can be manufactured. 
     According to this embodiment, as in the first embodiment described above, by filling the spaces between the turns of the Litz wire  106  with the first resin  108  having high flowability, void formation can be prevented, dielectric breakdown due to a partial discharge can be prevented, and heat of the Litz wire  106  can be uniformly diffused. In addition, by covering the Litz wire  106  (bobbin  102 ) with the second resin  110  containing a filler and having high thermal conductivity, heat can be efficiently diffused, and deterioration of the resin can be prevented. 
     In addition, by covering the end parts of the ferrite core  104  spaced apart from the Litz wire  106  with the third resins  120  having higher flowability, void formation can be prevented, and dielectric breakdown due to a partial discharge can be prevented. In addition, the weight of the inductor can be reduced compared with the first embodiment described above. 
     Third Embodiment 
       FIGS. 9 to 11  show a schematic configuration of an inductor according to a third embodiment of the present invention.  FIG. 9  is a top view of the inductor according to this embodiment,  FIG. 10  is a vertical cross-sectional view taken along the line A-A in  FIG. 9 , and  FIG. 11  is a vertical cross-sectional view taken along the line C-C in  FIGS. 9 and 10 . 
     This embodiment differs from the first embodiment shown in  FIGS. 2 to 4  in that the ferrite core has a two-layer structure. In  FIGS. 9 to 11 , the same components as those in the first embodiment shown in  FIGS. 2 to 4  are denoted by the same reference numerals, and descriptions thereof will be omitted. 
     As shown in  FIGS. 9 to 11 , the ferrite core  104  includes a first core  104 A inserted in the hole of the bobbin  102  and second cores  104 B provided at end parts of the first core  104 A in the length direction. Note that the length direction is a direction perpendicular (horizontal direction in  FIGS. 9 and 10 ) to the direction of winding of the Litz wire  106 . The second cores  104 B are disposed on the opposite side of the first core  104 A to the conductive plate  112 . 
     The outer end parts of the second cores  104 B in the length direction are positioned closer to the respective inductor end faces than the respective end parts of the first core  104 A in the length direction. In other words, the second cores  1043  are disposed to protrude from the first core  104 A. 
     Since the ferrite core  104  has a two-layer structure, the distance to the inductor of the counterpart device involved with the wireless power transmission can be reduced, and the coupling coefficient between the inductors can be increased. 
     In  FIGS. 9 to 11 , the first core  104 A and the second cores  104 B have the same width (width in the vertical direction in  FIG. 9  or width in the horizontal direction in  FIG. 11 ). As an alternative, however, the second cores  104 B may have a width larger than the width of the first core  104 A. Since the coupling coefficient between coils is proportional to the outer width of the coils, the coupling coefficient between the coils can be increased by increasing the width of the second cores  104 B. 
     Fourth Embodiment 
       FIGS. 12 to 15  show a schematic configuration of an inductor according to a fourth embodiment of the present invention.  FIG. 12  is a top view of the inductor according to this embodiment,  FIG. 13  is a vertical cross-sectional view taken along the line D-D in  FIG. 12 ,  FIG. 14  is a vertical cross-sectional view taken along the line E-E in  FIG. 14 , and  FIG. 15  is a vertical cross-sectional view taken along the line F-F in  FIG. 12 . 
     This embodiment differs from the third embodiment shown in  FIGS. 9 to 11  in that the second cores (upper layer cores)  104 B of the ferrite core  104  have a gap  140  at the center thereof in the width direction, and a capacitor  142  is disposed in the gap  140 . The capacitor  142  is the capacitor unit  22  shown in  FIG. 1 , for example. In  FIGS. 12 to 15 , the same components as those in the third embodiment shown in  FIGS. 9 to 11  are denoted by the same reference numerals, and descriptions thereof will be omitted. Note that the configuration according to this embodiment can be applied to the first and second embodiments described earlier. 
     As the distance from an end face of the ferrite core  104  in the length direction of the ferrite core  104  increases, the electromagnetic field becomes weaken Although the electromagnetic field also becomes weaker as the distance from the ferrite core  104  in the width direction of the ferrite core  104  increases, the degree to which the electromagnetic field becomes weaker is greater when the distance from the ferrite core  104  in the length direction increases. 
     Since the gaps  140  are formed at positions spaced apart from each other in the length direction of the ferrite core  104 , the weight of the ferrite core  104  can be reduced while reducing the influence on the electrical characteristics (characteristics of the coupling with the inductor of the opposite wireless power transmission device, for example) of the inductor  100 . In addition, the capacitors  142  can be disposed in the gaps  140 . That is, the capacitors  142  can be incorporated in the inductor  100 . As a result, the size of the entire inductor can be reduced. The magnetic field of the inductor  100  is concentrated in a part where the ferrite core  104  exists. By forming the gaps  140 , the magnetic field in the parts where the gaps  140  exist can be weakened. 
     In the fourth embodiment, in addition to the capacitors  142 , rectifiers (rectifiers  23  in  FIG. 1 , for example) can also be disposed in the gaps  140 . 
     In the first to fourth embodiments described above, the bobbin  102  has a flat outer periphery. As an alternative, however, recesses and projections may be formed on the outer periphery of the bobbin  102 , and the Litz wire  106  can be disposed in the recesses. Since the first resin  108  has high flowability, the first resin  108  can pervade minute regions between the recesses on the bobbin  102  and the Litz wire  106  and prevent void formation. 
     In the first to fourth embodiments described above, the Litz wire  106  is wound around the ferrite core  104  with the bobbin  102  interposed therebetween. As an alternative, however, as shown in  FIG. 16 , the bobbin  102  may be omitted, and the Litz wire  106  may be directly wound around the ferrite core  104 . 
     Fifth Embodiment 
       FIGS. 17 and 18  show a schematic configuration of an inductor according to a fifth embodiment of the present invention.  FIG. 17  is a vertical cross-sectional view of the inductor according to this embodiment, and  FIG. 18  is an enlarged view of a region “R” surrounded by the dashed line in  FIG. 17 . 
     As shown in  FIGS. 17 and 18 , an inductor  200  includes a tubular bobbin  202 , a ferrite core  204  inserted in a hole of the bobbin  202 , a Litz wire (winding)  206  formed by a stranded wire of conductive strands wound around an outer periphery of the bobbin  202 , a first resin  208  that fills the spaces between the turns of the Litz wire  206  and covers the periphery of the Litz wire  206 , a second resin  210  that covers the bobbin  202  and the first resin  208 , and a conductive plate  212  attached to one surface of the second resin  210 . The inductor  200  is housed in a housing  250  made of a thermoplastic resin, such as polyphenylene sulfide (PPS). 
     The bobbin  202  is made of a plastic, for example, and the Litz wire  206  is formed by a stranded wire of copper strands, for example. The conductive plate  212  is an aluminum plate or a copper plate, for example. 
     The second resin  210  is an epoxy resin, for example, and contains an inorganic filler, such as silica, boron nitride, or aluminum nitride. On the other hand, the first resin  208  contains no filler or has lower filler content than the second resin  210 . Therefore, the first resin  208  has higher flowability (lower viscosity) than the second resin  210  and can readily fill the spaces between the turns of the Litz wire  206 . 
     In this way, formation of a void (cavity) between the turns of the Litz wire  206  and in the surroundings of the Litz wire  206  can be prevented. Since void formation is prevented, occurrence of a partial discharge and a dielectric breakdown can be prevented. 
     Since void formation is prevented, heat of the Litz wire  206  can be uniformly diffused. The second resin  210  covering the Litz wire  206  and the first resin  208  contains a filler and has high thermal conductivity and therefore can efficiently diffuse heat. Therefore, deterioration of thermal conductivity and deterioration of the resins caused thereby can be prevented. 
     The second resin  210  has only to cover at least the Litz wire  206  (in other words, the first resin  208  covering the Litz wire  206 ). Therefore, as shown in  FIG. 17 , the second resin  210  does not have to cover parts  204 _ 1  of the ferrite core  204  that protrude from the hole of the bobbin  202 . In other words, the second resin  210  does not have to cover the end parts  204 _ 1 , whose surfaces are exposed, in the length direction of the ferrite core  204  (direction perpendicular to the direction of winding of the Litz wire  206 ). By selectively providing the second resin  210  only in the surroundings of the Litz wire  206 , which tends to generate heat, weight increase of the inductor  200  can be reduced while maintaining the heat dissipation capability. 
     Next, a method of manufacturing such an inductor  200  will be described with reference to  FIG. 19( a ) to ( e ) . 
     First, as shown in  FIG. 19( a ) , the ferrite core  204  is inserted into the hole of the bobbin  202 . The Litz wire  206  is then wound around the bobbin  202 . 
     As shown in  FIG. 19( b ) , in a space-filling process, the spaces between the turns of the Litz wire  206  are then filled with the first resin  208 . The first resin  208  is also applied to the surroundings of the Litz wire  206  and the surface of the bobbin  202 . Since the first resin  208  contains no filler or has extremely low filler content, the first resin  208  has high flowability (low viscosity) and can readily fill the spaces between the turns of the Litz wire  206 . Therefore, the first resin  208  pervades the spaces between the turns of the Litz wire  206  and other minute regions, so that formation of a void can be prevented. Following the space-filling process, a heating process is performed to cure the first resin  208 . 
     As shown in  FIG. 19( c ) , a mold (container)  260  is then provided to cover the Litz wire  206  and the first resin  208  but not to cover the end parts  204 _ 1  of the ferrite core  204 . As shown in  FIG. 19( d ) , the second resin  210  is then poured into the mold  260  and cured. After the second resin  210  is cured, the mold  260  is removed. In this way, the second resin  210  can be selectively provided only around the Litz wire  206  as shown in  FIG. 19( e ) . 
     As shown in  FIG. 19( f ) , the conductive plate  212  is then attached to one surface of the second resin  210 , and the resulting assembly is housed in the housing  250 . In this way, the inductor  200  shown in  FIG. 17  can be manufactured. 
     In order to facilitate winding of the Litz wire  206  around the bobbin  202  and filling of the spaces between the turns of the Litz wire  206  with the first resin  208 , the Litz wire  206  may be covered with an insulating material having a surface with a hole or a mesh of insulating material. For example, the Litz wire  206  may be covered with a heat-shrinkable tube having a surface with a hole. 
     In the method of manufacturing the inductor  200  shown in  FIG. 19( a ) to ( f ) , the ferrite core  204  is inserted into the hole of the bobbin  202  before the Litz wire  206  is wound around the bobbin  202 . However, insertion of the ferrite core  204  can be performed at any time before the assembly is housed in the housing  250 . 
     As an alternative, the ferrite core  204  may be provided by separately preparing the part to be housed in the hole of the bobbin  202  and the parts to protrude from the hole of the bobbin  202  (the end parts  204 _ 1  in  FIG. 17 ) and retrofitting the end parts  204 _ 1  to the part in the hole. A method of manufacturing the inductor  200  in the case where the end parts  204 _ 1  of the ferrite core  204  are retrofitted will be described with reference to  FIG. 20( a ) to ( f ) . 
     First, as shown in  FIG. 20( a ) , a ferrite core  204 _ 2  having approximately the same length as the bobbin  202  is inserted into the hole of the bobbin  202 . The Litz wire  206  is then wound around the bobbin  202 . 
     As shown in  FIG. 20( b ) , in a space-filling process, the spaces between the turns of the Litz wire  206  are then filled with the first resin  208 , and a heating process is performed to cure the first resin  208 . This step is the same as the step shown in  FIG. 19( b ) . 
     As shown in  FIG. 20( c ) , the mold (container)  260  is then provided to cover the Litz wire  206  and the first resin  208 . The mold  260  preferably has such a size that the end parts of the bobbin  202  are exposed. 
     As shown in  FIG. 20( d ) , the second resin  210  is then poured into the mold  260  and cured. After the second resin  210  is cured, the mold  260  is removed. 
     As shown in  FIG. 20( e ) , the end parts  204 _ 1  of the ferrite core  204  are then bonded to both the end faces of the ferrite core  204 _ 2 . 
     As shown in  FIG. 20( f ) , the conductive plate  212  is then attached to one surface of the second resin  210 , and the resulting assembly is housed in the housing  250 . In this way, the inductor  200  shown in  FIG. 17  can also be manufactured in the manner in which the end parts  204 _ 1  of the ferrite core  204  are retrofitted. 
     In the fifth embodiment described above, as shown in  FIG. 21 , recesses and projections may be formed on the surface of the bobbin  202 , and the Litz wire  206  can be disposed in the recesses. 
     In the fifth embodiment described above, as shown in  FIG. 22 , the conductive plate  212  may be attached to one surface of the second resin  210  with a conductive paint (conductive material)  224  having lower rigidity than the conductive plate  212  interposed therebetween. The applied conductive paint  224  can prevent occurrence of a partial discharge between the second resin  210  and the conductive plate  212 , because a potential difference occurs between the Litz wire  206  and the conductive paint  224 . In addition, since the conductive paint  224  having lower rigidity than the conductive plate  212  is inserted, a void can be prevented from being formed between the conductive plate  212  and the second resin  210  because of peel off of the resin caused by vibration. 
     As shown in  FIG. 23 , the ferrite core may have a two-layer structure. As shown in  FIG. 23 , the ferrite core  204  includes a first core  204 A inserted in the hole of the bobbin  202  and second cores  204 B provided at opposite end parts (end parts  204 _ 1 ) of the first core  204 A in the length direction. Note that the length direction is a direction perpendicular (horizontal direction in the drawing) to the direction of winding of the Litz wire  206 . The second cores  204 B are disposed on the opposite side of the first core  204 A to the conductive plate  212 . 
     The outer end parts of the second cores  204 B in the length direction are positioned closer to the respective inner walls of the housing  250  than the respective end parts of the first core  204 A in the length direction. In other words, the second cores  204 B are disposed to protrude from the first core  204 A. 
     Since the ferrite core  204  has a two-layer structure, the distance between the ferrite surface and the inductor of the counterpart device involved with the wireless power transmission can be reduced, and the coupling coefficient between the inductors can be increased. 
     The Litz wire  106  and the first resin  108  in the first to fourth embodiments described earlier may be configured in the same way as the Litz wire  206  and the first resin  208  in this fifth embodiment. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.