Patent Publication Number: US-11043323-B2

Title: Variable inductor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of priority to Japanese Patent Application 2015-154009 filed Aug. 4, 2015, the entire content of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to variable inductors and, in particular, relates to a variable inductor that can vary an inductance value by varying the magnetic permeability in a portion through which a magnetic flux passes. 
     BACKGROUND 
     Variable inductors of interest to the present disclosure include, for example, a variable inductor described in Japanese Unexamined Patent Application Publication No. 2010-135699 or a variable inductor described in Japanese Unexamined Patent Application Publication No. 2009-152254. 
     Japanese Unexamined Patent Application Publication No. 2010-135699 describes a variable inductor that includes a first coil, a second coil that produces a magnetic flux in a direction that cancels out a magnetic flux produced by the first coil, a movable core that moves between the first coil and the second coil so as to block the magnetic fluxes produced by the first coil and the second coil, and a magnetic core of a closed magnetic circuit structure that encloses the first coil, the second coil, and the movable core. 
     Japanese Unexamined Patent Application Publication No. 2009-152254 describes an on-chip variable inductor provided as a wafer level package that is constituted by a semiconductor substrate, an integrated circuit layer on the semiconductor substrate, an insulation layer on the integrated circuit layer, and a redistribution layer on the insulation layer. In this variable inductor, a first inductor is formed in the integrated circuit layer, a second inductor is formed in the redistribution layer, and a current control circuit is connected to the first inductor. As the amplitude and/or the phase of a current input to the first inductor are/is controlled, a magnetic flux that passes through the second inductor is varied. 
     SUMMARY 
     However, the variable inductor described in Japanese Unexamined Patent Application Publication No. 2010-135699 needs to be configured such that the movable core is mechanically moved while being held stably and the magnetic fluxes produced by the first coil and the second coil are selectively blocked. Thus, the operation stability of the movable unit is likely to become a problem. In addition, the movable core has a relatively large mass, and thus a problem arises in that moving the movable core requires a relatively large amount of electric power and the reaction speed of an operation is low. 
     In the meantime, in the variable inductor described in Japanese Unexamined Patent Application Publication No. 2009-152254, since the amplitude and/or the phase of the current input to the first inductor are/is controlled, the current has to be passed continuously, but a DC current component ceases to contribute to the control after the inductance value is varied and thus can be considered to be a wasted current. Accordingly, there is a problem in that the power efficiency of the current control circuit deteriorates and the energy efficiency of the variable inductor deteriorates in turn. 
     Accordingly, it is an object of one embodiment of the present disclosure to provide a variable inductor that enables the above-described problems to be reduced, or in other words, a variable inductor that can vary an inductance value stably and quickly and that does not require much energy for achieving a desired operation. 
     According to one embodiment of the present disclosure, a variable inductor includes at least one coil that produces a magnetic flux. Then, the variable inductor further includes a receptacle portion that defines a space traversing at least a portion of the magnetic flux produced by the at least one coil, and a magnetic powder contained in the receptacle portion so as to occupy a portion of the space. The magnetic powder can move within the space, and this movement produces a change in the magnetic flux. Herein, a change in the magnetic flux corresponds to a change in how easily the magnetic flux passes, a change in the path of the magnetic flux, or the like. Such a change in the magnetic flux appears in the form of a change in an inductance value. 
     According to another embodiment of the present disclosure, it is preferable that the space defined by the receptacle portion include a first region in which a magnetic field provided by the at least one coil is relatively strong and a second region in which the magnetic field is relatively weak and that the magnetic powder can move between the first region and the second region. According to this configuration, a change in the inductance value can be obtained more efficiently. 
     According to another embodiment of the present disclosure, it is preferable that the at least one coil include first and second coils that are disposed coaxially with a gap provided therebetween. In this case, the first coil and the second coil are configured to mutually cancel out the magnetic fields produced thereby, and at least a portion of the space is located between the first coil and the second coil. In this manner, when the variable inductor includes two coils, the amount of change in the inductance value can be increased as compared to a case in which the variable inductor includes only one coil. 
     According to another embodiment of the present disclosure, preferably, the magnetic powder is coated with a resin having an electrostatic property, the variable inductor further includes an electric field generating electrode for applying a voltage so as to generate an electric field within the space, and the magnetic powder is moved within the space by applying a voltage to the electric field generating electrode. 
     According to this configuration, the magnetic powder can be moved only by applying a voltage to the electric field generating electrode from the outside, and the inductance value can be varied accordingly. At this point, electric power necessary for moving the magnetic powder is comparatively smaller than the electric power necessary for moving the movable core described in Japanese Unexamined Patent Application Publication No. 2010-135699. In addition, since the magnetic powder has an electrostatic property, the magnetic powder does not easily move even when the voltage ceases to be applied to the electric field generating electrode. Thus, no electric power is required to keep the position of the magnetic powder. Accordingly, the power consumption can be reduced. 
     In the embodiments described above, it is preferable that the electric field generating electrode include a substantially comb-shaped portion spreading along a wall surface of the receptacle portion that defines the space. With this, an occurrence of an eddy current that reduces a Q value of the inductor can be suppressed. 
     The variable inductor according to another embodiment of the present disclosure may include a configuration that allows the magnetic powder to move within the space by its own weight, aside from the configuration for moving the magnetic powder by an electric field, as described above. 
     According to one embodiment of the present disclosure, as the magnetic powder moves through the space within the receptacle portion, the magnetic flux produced by the coil varies, and the inductance value provided by the coil can be varied. In this manner, movement of the relatively lightweight magnetic powder is used to vary the inductance value, and merely the receptacle portion for housing the magnetic powder needs to be prepared in order to movably hold the magnetic powder. Thus, a problem that could be faced when operating a movable unit such as the movable core having a relatively large mass can be avoided advantageously. In other words, a mechanism for operably holding a movable unit such as the movable core is not necessary. In addition, since the magnetic powder is relatively lightweight, advantageously, the variable inductor can be expected to excel in the operation stability, to have a high operation reaction speed, and not to require much energy for achieving a desired operation. 
     Other features, elements, characteristics and advantages of some embodiments of the present disclosure will become more apparent from the following detailed description of some embodiments of the present disclosure with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view illustrating a variable inductor according to a first embodiment of the present disclosure, and illustrates the variable inductor in two typical states in which an inductance value is varied in accordance with a first operation principle. 
         FIG. 2  is a sectional view illustrating a variable inductor according to a second embodiment of the present disclosure, and illustrates a configuration in which an inductance value can be varied in accordance with a second operation principle. 
         FIG. 3  is a perspective view illustrating an appearance of a variable inductor according to a third embodiment of the present disclosure. 
         FIG. 4  is an equivalent circuit diagram of the variable inductor illustrated in  FIG. 3 . 
         FIG. 5  is a sectional view of the variable inductor illustrated in  FIG. 3  taken along the V-V line. 
         FIGS. 6A through 6F  are sectional views of the variable inductor illustrated in  FIG. 3 , in which  FIG. 6A  is a sectional view taken along the  6 - 6  line indicated in  FIG. 5 ,  FIG. 6B  is a sectional view taken along the  7 - 7  line indicated in  FIG. 5 ,  FIG. 6C  is a sectional view taken along the  8 - 8  line indicated in  FIG. 5 ,  FIG. 6D  is a sectional view taken along the  9 - 9  line indicated in  FIG. 5 ,  FIG. 6E  is a sectional view taken along the  10 - 10  line indicated in  FIG. 5 , and  FIG. 6F  is a sectional view taken along the  11 - 11  line indicated in  FIG. 5 . 
         FIG. 7  is a sectional view of a variable inductor according to a fourth embodiment of the present disclosure, which corresponds to  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a variable inductor  1  according to a first embodiment of the present disclosure. The variable inductor  1  can take two typical states illustrated in sections ( 1 ) and ( 2 ) of  FIG. 1  and thus varies the inductance value. 
     The variable inductor  1  includes a first coil  2  and a second coil  3 . The first coil  2  and the second coil  3  are disposed coaxially with a gap provided therebetween. The first coil  2  and the second coil  3  are configured to mutually cancel out magnetic fields provided thereby. 
     The variable inductor  1  further includes a receptacle portion  5  that defines a space  4  traversing at least a portion of magnetic fluxes produced by the first and second coils  2  and  3 , and a magnetic powder  6  contained in the receptacle portion  5  so as to occupy a portion of the space  4 . For example, a ferrite powder or a metal powder in general, such as a carbonyl iron powder or a nickel powder, that is used in a magnetic fluid can be used as the magnetic powder  6 . 
     The space  4  defined by the receptacle portion  5  includes a first region  7  in which the magnetic field provided by the first and second coils  2  and  3  is relatively strong and a second region  8  in which the magnetic field is relatively weak. To be more specific, the space  4  has a substantially T-shaped section, the first region  7  is located at a position between the first coil  2  and the second coil  3 , and the second region  8  is located at a position that is on a side of the second coil  3  opposite to the position where the first coil  2  is located and that is spaced apart from the second coil  3 . 
     In the present embodiment, the posture of the variable inductor  1  is changed in order to vary the inductance value. As the posture of the variable inductor  1  is changed, the magnetic powder  6  can move reversibly by its own weight between the first region  7  and the second region  8  within the space  4 . 
     To be more specific, in the section ( 1 ) of  FIG. 1 , the variable inductor  1  assumes a posture in which the second region  8  of the receptacle portion  5  is located downward, and the magnetic powder  6  is settled in the second region  8  by its own weight. In the meantime, in the section ( 2 ) of  FIG. 1 , the variable inductor  1  assumes a posture in which the first region  7  of the receptacle portion  5  is located downward, and the magnetic powder  6  is settled in the first region  7  by its own weight. The receptacle portion  5  may be provided with a substantially conical guide surface  9  so that the magnetic powder  6  can move to the first region  7  smoothly. 
     As the magnetic powder  6  is displaced as described above, a change in the magnetic fluxes produced by the first and second coils  2  and  3  is produced. To be more specific, the movement of the magnetic powder  6  changes how easily the magnetic flux passes, in a similar manner to when the distance between the first coil  2  and the second coil  3  is changed. The change in the magnetic flux appears in the form of a change in the inductance value in the variable inductor  1 . In other words, the inductance value of the variable inductor  1  in a state in which the magnetic powder  6  is in the second region  8  in which the magnetic field provided by the first and second coils  2  and  3  is relatively weak as illustrated in the section ( 1 ) of  FIG. 1  is smaller than the inductance value of the variable inductor  1  in a state in which the magnetic powder  6  is in the first region  7  in which the magnetic field provided by the first and second coils  2  and  3  is relatively strong as illustrated in the section ( 2 ) of  FIG. 1 . 
     Such a change in the inductance value can be achieved repeatedly with reproducibility. Here, when focusing on the strength of the magnetic field provided by the first and second coils  2  and  3 , as the difference between the strength of the magnetic field in the first region  7  and the strength of the magnetic field in the second region  8  is greater, the amount of change in the inductance value can be made greater. 
     Now, with reference to  FIG. 2 , a variable inductor  11  according to a second embodiment of the present disclosure will be described. The variable inductor  11  illustrated in  FIG. 2  includes many elements that are common to those in the variable inductor  1  illustrated in  FIG. 1 . Therefore, in  FIG. 2 , the elements that are common to those illustrated in  FIG. 1  are given identical reference numerals, and duplicate descriptions thereof will be omitted. 
     The variable inductor  11  includes, in addition to the elements provided in the variable inductor  1  described above, electric field generating electrodes  12  through  14  for applying a voltage so as to generate an electric field within the space  4  defined by the receptacle portion  5 . The electric field generating electrode  12  is provided along an end wall of the receptacle portion  5  that defines a terminal of the second region  8  of the space  4 . The electric field generating electrodes  13  and  14  are provided along a side wall of the receptacle portion  5  that defines the periphery of the first region  7  of the space  4 . The electric field generating electrode  13  and the electric field generating electrode  14  are electrically connected in parallel and located so as to face each other. 
     A direct current power supply  15  is prepared separately from a signal system power supply (not illustrated) for the first and second coils  2  and  3 . The voltage supplied from the direct current power supply  15  and the polarity of the voltage can be varied. The direct current power supply  15  applies a voltage across the electric field generating electrode  12  and the electric field generating electrodes  13  and  14 , and thus an electric field is generated within the space  4 . 
     In the meantime, in the variable inductor  11 , a powder coated with a resin having an electrostatic property is used as the magnetic powder  6 . To be more specific, a core material, such as magnetite, Mn-based soft ferrite, Mn—Mg-based soft ferrite, or Cu—Zn-based soft ferrite used as an electrophotographic carrier, coated with a resin is advantageously used as the magnetic powder  6 . Thus, as a voltage of, for example, about several ten volts is applied across the electric field generating electrode  12  and the electric field generating electrodes  13  and  14  by the direct current power supply  15 , the magnetic powder  6  moves within the space  4 . By changing the polarity of the voltage supplied from the direct current power supply  15 , the magnetic powder  6  can be moved toward the first region  7  or can be moved toward the second region  8  as indicated by double-headed arrows  16 . 
     To be more specific, when the direct current power supply  15  has a polarity as illustrated in  FIG. 2 , a positive potential is given to the electric field generating electrode  12 , and negative potentials are given to the electric field generating electrodes  13  and  14 . At this point, if the magnetic powder  6  is positively charged, the magnetic powder  6  is attracted toward the electric field generating electrodes  13  and  14  having negative potentials and moves to the first region  7 . As a result, the variable inductor  11  provides a relatively high inductance value. Thereafter, even if the direct current power supply  15  is turned off, a state in which the magnetic powder  6  remains in the first region  7  is retained. 
     In the meantime, when the inductance value of the variable inductor  11  is to be made relatively small, the polarity of the direct current power supply  15  is switched. In other words, a negative potential is given to the electric field generating electrode  12 , and positive potentials are given to the electric field generating electrodes  13  and  14 . As described above, if the magnetic powder  6  is positively charged, the magnetic powder  6  is attracted toward the electric field generating electrode  12  having a negative potential and moves to the second region  8 . As a result, the variable inductor  11  provides a relatively low inductance value. Thereafter, even if the direct current power supply  15  is turned off, a state in which the magnetic powder  6  remains in the second region  8  is retained. 
     Although the magnetic powder  6  is depicted as being present in both the first region  7  and the second region  8  in  FIG. 2 , in reality, the magnetic powder  6  is typically present in either one of the first region  7  and the second region  8 . 
     However, when a driving system of electronic paper, which is attracting attention as a display medium on which the display content can be electrically overwritten, is applied, only a specific portion of the magnetic powder  6  can be moved, and thus the magnetic powder  6  can be distributed in both the first region  7  and the second region  8  at a specific rate. In that case, an intermediate inductance value can also be achieved. This modification can also be applied to other embodiments described later. 
     The space  4  may be filled not only with a gas but also with a liquid. For example, when the space  4  is filled with a liquid such as a silicone oil, the speed at which the magnetic powder  6  moves is lower than the speed at which the magnetic powder  6  moves when the space  4  is filled with a gas. However, an electric field is more easily applied, and thus the voltage to be applied across the electric field generating electrode  12  and the electric field generating electrodes  13  and  14  can be reduced. This modification can also be applied to other embodiments described later. 
     With reference to  FIGS. 3 through 6F , a variable inductor  21  according to a third embodiment of the present disclosure will now be described. 
     In the variable inductors  1  and  11  described above, it is intended that the first and second coils  2  and  3  are constituted by windings, although not particularly limited thereto. In contrast, the variable inductor  21  is a chip type inductor that includes a coil of a laminate structure and is fabricated by applying a lamination technique. 
     The variable inductor  21  includes a rectangular parallelepiped component body  22  having a laminate structure. As illustrated in  FIG. 3 , opposing end surfaces  23  and  24  of the component body  22  are provided with first and second external terminal electrodes  27  and  28 , respectively, and opposing side surfaces  25  and  26 , which are each adjacent to the end surfaces  23  and  24 , are provided with third and fourth external terminal electrodes  29  and  30 , respectively. These external terminal electrodes  27  through  30  are provided so as to fill the cutouts that are formed in the end surfaces  23  and  24  and the side surfaces  25  and  26 , respectively, of the component body  22  so as to penetrate the component body  22  in the thickness direction thereof. 
     The above-described mode of the external terminal electrodes  27  through  30  results from the method of fabricating the variable inductor  21 . When the component body  22  is fabricated, a component body in the mother state that, when cut along cut lines in the X direction and the Y direction, can yield a plurality of component bodies  22  is fabricated. This component body in the mother state has through-holes having a rectangular planar shape for locating the cut lines formed therein on the center line, and the through-holes are filled with a conductor. Then, the component body in the mother state is cut along the cut lines, and thus a plurality of component bodies  22  are produced. At this point, since the cut lines pass through the center lines of the above-described through-holes, and thus the conductor filling the through-holes is divided as being cut, which results in the external terminal electrodes  27  through  30  described above. 
     As illustrated in  FIG. 4 , an inductance L is formed between the first and second external terminal electrodes  27  and  28 , and the inductance L can be varied in accordance with a voltage applied across the third and fourth external terminal electrodes  29  and  30 . 
     The variable inductor  21  includes elements corresponding to the elements provided in the variable inductor  11  illustrated in  FIG. 2 . In other words, the variable inductor  21  forms first and second coils  31  and  32  and electric field generating electrodes  33  and  34  inside the component body  22  and constitutes a receptacle portion  36  that defines a space  35  by a portion of the component body  22 . 
     As illustrated in  FIG. 5 , the component body  22  has a laminate structure in which a resin layer  39  made of polyimide or the like is sandwiched between first and second insulating substrates  37  and  38  made of alumina or the like. The first insulating substrate  37  is also depicted in  FIGS. 6A and 6B , the resin layer  39  is also depicted in  FIG. 6C , and the second insulating substrate  38  is also depicted in  FIGS. 6D through 6F . 
     As illustrated in  FIG. 6B  as well, the first coil  31  is constituted, for example, by a spiral pattern conductor made of copper and is provided in the first insulating substrate  37 . Here, the first coil  31  is located in the first insulating substrate  37  on a side that makes contact with the resin layer  39 . The first coil  31  is coated for insulation as necessary. The first insulating substrate  37  has a laminate structure composed of a plurality of insulator layers, and an extended conductor  40  is provided in an insulator layer different from the insulator layer in which the first coil  31  is located, as illustrated in  FIG. 6A . One end of the extended conductor  40  is electrically connected to an inner peripheral end of the first coil  31  with a via conductor  41  that penetrates a specific insulator layer interposed therebetween, and the other end of the extended conductor  40  is electrically connected to the first external terminal electrode  27 . 
     As illustrated in  FIG. 6D  as well, the second coil  32  is provided in the second insulating substrate  38 . Similarly to the first coil  31 , the second coil  32  is constituted, for example, by a spiral pattern conductor made of copper. In addition, the second coil  32  is located in the second insulating substrate  38  on a side that makes contact with the resin layer  39 . The second coil  32  is coated for insulation as necessary. The second insulating substrate  38  also has a laminate structure composed of a plurality of insulator layers, and an extended conductor  42  is provided in an insulator layer different from the insulator layer in which the second coil  32  is located, as illustrated in  FIG. 6E . One end of the extended conductor  42  is electrically connected to an inner peripheral end of the second coil  32  with a via conductor  43  that penetrates a specific insulator layer interposed therebetween, and the other end of the extended conductor  42  is electrically connected to the second external terminal electrode  28 . 
     As described above, an outer peripheral end of the first coil  31  located in the first insulating substrate  37  on the side that makes contact with the resin layer  39  and an outer peripheral end of the second coil  32  located in the second insulating substrate  38  on the side that makes contact with the resin layer  39  are electrically connected to each other by a via conductor  44  illustrated in  FIGS. 6B and 6D . The via conductor  44  is provided so as to penetrate the resin layer  39 . 
     As described thus far, the first coil  31  and the second coil  32  are disposed coaxially with a gap provided therebetween, and the first coil  31  and the second coil  32  are configured to mutually cancel out the magnetic fields provided thereby. 
     A through-hole  45  is provided in the resin layer  39  so as to penetrate the resin layer  39  in the thickness direction thereof. As illustrated in  FIG. 6C , the through-hole  45  has a substantially elliptic planar shape. In addition, a concave portion  46  is provided in the second insulating substrate  38  such that the concave portion  46  opens on a side that makes contact with the resin layer  39  and communicates with the through-hole  45 . As illustrated in  FIGS. 6C through 6E , the concave portion  46  is smaller than the through-hole  45  and has a substantially elliptic planar shape. The base of the concave portion  46  is located at a position sufficiently spaced apart from the second coil  32 . 
     The above-described space  35  is provided by the through-hole  45  and the concave portion  46 . Accordingly, the receptacle portion  36  that defines the space  35  is provided by a portion of the component body  22 . The space  35  is located so as to traverse at least a portion of magnetic fluxes produced by the first and second coils  31  and  32 . The magnetic powder is contained in the receptacle portion  36  so as to occupy a portion of the space  35 , but the magnetic powder is omitted from the drawings in  FIGS. 5 through 6F . In addition, in the variable inductor  21  as well, a powder coated with a resin having an electrostatic property is used as the magnetic powder, as in the case of the variable inductor  11  illustrated in  FIG. 2 . 
     The above-described electric field generating electrode  33  is provided in the second insulating substrate  38 , as clearly illustrated in  FIG. 6F . The electric field generating electrode  33  is provided in, among a plurality of insulator layers constituting the second insulating substrate  38 , an insulator layer that provides the base of the concave portion  46  and is partially exposed through the base of the concave portion  46 . The electric field generating electrode  33  includes a substantially comb-shaped portion spreading along the bottom wall of the receptacle portion  36  that defines the space  35 . Accordingly, an occurrence of an eddy current that reduces the Q value of the inductor can be suppressed. As illustrated in  FIG. 6F , the electric field generating electrode  33  is electrically connected to the third external terminal electrode  29  with an extended conductor  47  interposed therebetween. 
     The electric field generating electrode  34 , which is paired with the electric field generating electrode  33 , is located in the resin layer  39  and is provided so as to be exposed through the peripheral surface of the through-hole  45 . As can be seen from  FIG. 5 , the electric field generating electrode  34  includes a substantially comb-shaped portion spreading along the side wall of the receptacle portion  36  that defines the space  35 . Although a detailed illustration is omitted, the comb teeth of the substantially comb-shaped portion of the electric field generating electrode  34  are electrically connected to one another by a conductor that extends in the thickness direction of the resin layer  39 . In addition, as illustrated in  FIG. 6C , the electric field generating electrode  34  is electrically connected to the fourth external terminal electrode  30  with an extended conductor  48  interposed therebetween. 
     As described above, as the electric field generating electrodes  33  and  34  each include a substantially comb-shaped portion, an occurrence of an eddy current that reduces the Q value of the inductor can be suppressed. 
     With reference to  FIG. 5 , the above-described space  35  includes a first region  49  in which a magnetic field provided by the first and second coils  31  and  32  is relatively strong and a second region  50  in which the magnetic field is relatively weak. In the present embodiment, the first region  49  is located at a position between the first coil  31  and the second coil  32 , or in other words, at a position defined by the through-hole  45 ; and the second region  50  is located at a position that is on a side of the second coil  32  opposite to the side where the first coil is located and that is sufficiently spaced apart from the second coil  32 , or in other words, at a position in the vicinity of the base of the concave portion  46 . 
     When the variable inductor  21  is fabricated, the second insulating substrate  38  is obtained through the following processes. Specifically, the electric field generating electrode  33  and the extended conductor  47  are formed in a specific insulator layer that is to partially constitute the second insulating substrate  38 . Another insulator layer having a through-hole that is to partially constitute the concave portion  46  is laminated on the aforementioned specific insulator layer, and the extended conductor  42  is formed in the other insulator layer. Then, yet another insulator layer having a through-hole that is to constitute the remaining portion of the concave portion  46  and provided with the via conductor  43  is laminated on the aforementioned insulator layer, and the second coil  32  is formed in the yet another insulator layer. 
     In addition, when the variable inductor  21  is fabricated, the resin layer  39  performs a function of bonding the first and second insulating substrates  37  and  38  to each other, and before the first and second insulating substrates  37  and  38  are bonded, the resin layer  39  includes the through-hole  45  and has the electric field generating electrode  34 , the extended conductor  48 , and the via conductor  44  provided therein. The resin layer  39  has a laminate structure. When the electric field generating electrode  34  having a substantially comb-shaped portion is to be formed, the comb teeth are provided in different layers of the resin layer  39  and are connected to one another by a conductor that extends in the thickness direction of the resin layer  39 . In addition, preferably, the resin layer  39  is disposed between the first and second insulating substrates  37  and  38  in a semi-solidified state, and as this resin layer  39  is solidified, the first and second insulating substrates  37  and  38  become bonded to each other. 
     The variable inductor  21  is made to function as an inductor by connecting the first and second external terminal electrodes  27  and  28  to a signal path and has its inductance value varied by applying a voltage having predetermined voltage value and polarity across the third and fourth external terminal electrodes  29  and  30 . 
     The mechanism for varying the inductance value is substantially the same as that of the case of the variable inductor  11  illustrated in  FIG. 2 . In simple terms, when a voltage having a specific polarity is applied across the electric field generating electrodes  33  and  34  with the third and fourth external terminal electrodes  29  and  30  interposed therebetween, the magnetic powder is attracted toward either one of the electric field generating electrodes  33  and  34  and moves toward either one of the first region  49  and the second region  50 . This state is retained even after the voltage ceases to be applied across the electric field generating electrodes  33  and  34 . 
     In the meantime, when the polarity of the voltage applied across the electric field generating electrodes  33  and  34  is switched, the magnetic powder is attracted toward the other one of the electric field generating electrodes  33  and  34  and moves toward the other one of the first region  49  and the second region  50 . This state is retained even after the voltage ceases to be applied across the electric field generating electrodes  33  and  34 . 
     Now, with reference to  FIG. 7 , a variable inductor  51  according to a fourth embodiment of the present disclosure will be described. As can be seen by comparing  FIG. 7  with  FIG. 5 , the variable inductor  51  illustrated in  FIG. 7  includes many elements that are common to those in the variable inductor  21  illustrated in  FIG. 5 . Therefore, in  FIG. 7 , the elements that correspond to those illustrated in  FIG. 5  are given identical reference numerals, and duplicate descriptions thereof will be omitted. 
     The above-described variable inductor  1 ,  11 , and  21  include two coils  2  and  3  (or  31  and  32 ) that are disposed coaxially with a gap provided therebetween and that are configured to mutually cancel out the magnetic fields provided thereby. Meanwhile, the variable inductor  51  illustrated in  FIG. 7  includes only a single coil  52 . 
     An outer peripheral end of the coil  52  is electrically connected to the first external terminal electrode  27  with an extended conductor  53  interposed therebetween, and an inner peripheral end of the coil  52  is electrically connected to the second external terminal electrode  28  with a via conductor  54  and an extended conductor  55  interposed therebetween. 
     In a space  56  that traverses at least a portion of a magnetic flux produced by the coil  52 , a first region  57  in which a magnetic field provided by the coil  52  is relatively strong is a portion enclosed by the coil  52 , or in other words, a portion corresponding to the inside of the concave portion  46 , and a second region  58  in which the magnetic field provided by the coil  52  is relatively weak is located at a position sufficiently spaced apart from the coil  52 , or in other words, a portion corresponding to a relatively upper portion inside the through-hole  45 . In the present embodiment, the positional relation between the first region  57  and the second region  58  in the space  56  is reversed from the positional relation between the first region  49  and the second region  50  in the space  35  of the variable inductor  21  described above. In addition, the concave portion  46  that serves as the first region  57  is shallower than the concave portion  46  that serves as the second region  50  in the variable inductor  21  described above. 
     When a voltage having a specific polarity is applied across the electric field generating electrodes  33  and  34 , the magnetic powder (not illustrated) is attracted toward either one of the electric field generating electrodes  33  and  34  and moves toward either one of the first region  57  and the second region  58 . In the meantime, when the polarity of the voltage applied across the electric field generating electrodes  33  and  34  is switched, the magnetic powder is attracted toward the other one of the electric field generating electrodes  33  and  34  and moves toward the other one of the first region  57  and the second region  58 . As a result of such movement of the magnetic powder, the inductance value changes. 
     According to the variable inductor  51  illustrated in  FIG. 7 , since only the single coil  52  is provided, the amount of change in the inductance value is smaller than that in the above-described variable inductor  21  that includes the two coils  31  and  32 . 
     Thus far, the present disclosure has been described in association with several illustrated embodiments, but various other modifications can also be made within the scope of the present disclosure. For example, the shape of a space defined by a receptacle portion can be modified as desired as long as a given shape traverses at least a portion of a magnetic flux produced by a coil. 
     In addition, the embodiments described in the present specification are illustrative in nature, and the configurations can be partially replaced or combined among different embodiments. 
     While the embodiments of the disclosure have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the disclosure. The scope of the disclosure, therefore, is to be determined solely by the following claims.