Abstract:
A power transfer system that transfers electric power from a power transmission device to a power reception device through electrical coupling. The power transmission device and the power reception device structurally designed such that the power transfer system is able to stabilize reference potentials of the power transmission device and the power reception device when the power reception device is placed on the power transmission device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of PCT/JP2015/065185 filed May 27, 2015, which claims priority to Japanese Patent Application No. 2014-114868, filed Jun. 3, 2014, the entire contents of each of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present disclosure relates to a power transfer system in which electric power is transferred from a power transmission device to a power reception device through capacitive coupling (electrical coupling). 
       BACKGROUND OF THE INVENTION 
       [0003]    Power transfer systems employing electrical coupling methods are known. In these power transfer system, active electrodes of a power transmission device and a power reception device are located in proximity to each other via a gap, and passive electrodes of the power transmission device and the power reception device are located in proximity to each other via a gap, so that large capacitances are formed between the electrodes, and the electrodes are electrically coupled to each other. Accordingly, electric power transfer between the power transmission device and the power reception device is enabled with high transfer efficiency even if the electrodes of the power transmission device and the power reception device are not in contact with each other. 
         [0004]    Examples of the power reception device of the power transfer system include electronic devices such as a cellular phone. In recent years, an electrostatic capacity type input unit (touch panel) having good operability is used in these electronic devices and the like in many cases. In the case where a power reception device is equipped with a touch panel, a situation is possible in which the touch panel is operated while the power reception device is placed on the power transmission device to be charged. In this case, a reference potential at the power reception device side varies, so that the power reception device may malfunction. 
         [0005]    Patent Document 1 discloses a power transfer system that stabilizes a reference potential of a power reception device by connecting the reference potential of the power reception device to a power transmission device having relatively small variation of a reference potential. In the power transfer system disclosed in Patent Document 1, each of the power transmission device and the power reception device has a reference potential electrode connected to the reference potential, and the reference potential electrodes are opposed to each other. Thus, the reference potential of the power reception device is connected to the reference potential of the power transmission device, so that the potential of the power reception device is stabilized and operation of the power reception device is stabilized. 
         [0006]    Patent Document 1: International Publication No. 2013/054800. 
         [0007]    Meanwhile, when the power reception device is placed on the power transmission device, for example, a capacitance (hereinafter, referred to as “cross capacitance”) may be formed between different types of electrodes, for example, between the active electrode of the power transmission device and the passive electrode of the power reception device. The cross capacitance increases particularly if a displacement occurs when the power reception device is placed on the power transmission device. In Patent Document 1, the influence of such a cross capacitance is not taken into consideration, and the reference potential at the power reception device side varies due to the cross capacitance, so that the power reception device may malfunction. 
       SUMMARY OF THE INVENTION 
       [0008]    Therefore, an object of the present disclosure is to provide a power transfer system that allows reference potentials of a power transmission device and a power reception device to be stabilized when the power reception device is placed on the power transmission device. 
         [0009]    A power transfer system is provided that includes a power transmission device including a first power-transmission-side electrode, a second power-transmission-side electrode, a power-transmission-side reference potential electrode connected to a power-transmission-side reference potential, and a voltage applying circuit configured to apply a voltage to the first power-transmission-side electrode and the second power-transmission-side electrode; and a power reception device including a first power-reception-side electrode, a second power-reception-side electrode, a power-reception-side reference potential electrode connected to a power-reception-side reference potential, and a load circuit configured to supply a voltage generated in the first power-reception-side electrode and the second power-reception-side electrode, to a load. Moreover, the power reception device is placed on the power transmission device to oppose the first power-transmission-side electrode and the first power-reception-side electrode to each other, to oppose the second power-transmission-side electrode and the second power-reception-side electrode to each other, and to oppose the power-transmission-side reference potential electrode and the power-reception-side reference potential electrode to each other, thereby to transfer electric power from the power transmission device to the power reception device through electrical coupling. In the exemplary embodiment, the power transmission device includes a first power-transmission-side capacitor and a second power-transmission-side capacitor connected in series between the first power-transmission-side electrode and the second power-transmission-side electrode, a connection point between the first power-transmission-side capacitor and the second power-transmission-side capacitor is connected to the power-transmission-side reference potential electrode, the power reception device includes a first power-reception-side capacitor and a second power-reception-side capacitor connected in series between the first power-reception-side electrode and the second power-reception-side electrode. 
         [0010]    Moreover, according to the exemplary embodiment, a connection point between the first power-reception-side capacitor and the second power-reception-side capacitor is connected to the power-reception-side reference potential electrode, and when: a connection point between the first power-transmission-side electrode and the first power-transmission-side capacitor is represented by P1; a connection point between the first power-reception-side electrode and the first power-reception-side capacitor is represented by P2; a connection point between the second power-transmission-side electrode and the second power-transmission-side capacitor is represented by P3; a connection point between the second power-reception-side electrode and the second power-reception-side capacitor is represented by P4; the connection point between the first power-transmission-side capacitor and the second power-transmission-side capacitor is represented by P5; the connection point between the first power-reception-side capacitor and the second power-reception-side capacitor is represented by P6; a parasitic capacitance occurring between P1 and P4 is represented by C 14 ; a parasitic capacitance occurring between P1 and P6 is represented by C 16 ; a parasitic capacitance occurring between P2 and P3 is represented by C 23 ; a parasitic capacitance occurring between P2 and P5 is represented by C 25 ; a parasitic capacitance occurring between P3 and P6 is represented by C 36 ; a parasitic capacitance occurring between P4 and P5 is represented by C 45 ; capacitances of the first power-transmission-side capacitor and the second power-transmission-side capacitor are represented by C 15  and C 35 , respectively; capacitances of the first power-reception-side capacitor and the second power-reception-side capacitor are represented by C 26  and C 46 , respectively; a capacitance occurring between the first power-transmission-side electrode and the first power-reception-side electrode is represented by C A ; and a capacitance occurring between second power-transmission-side electrode and the second power-reception-side electrode is represented by C P . The power transmission device and power reception device are arranged such that the following formulas are satisfied according to the exemplary embodiment: 
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         [0011]    Moreover, according to the exemplary embodiment ΣG=C A +C P +C 14 +C 23 +C 26 +C 46 +C 25 +C 45 , and ΣL=C A +C P +C 14 +C 23 +C 16 +C 36 +C 15 +C 35 . 
         [0012]    In this configuration, by satisfying formulas (1) and (2) (hereinafter, referred to as stable condition) which take a cross capacitance into consideration, it is possible to cause the reference potential of the power reception device to approach the reference potential of the power transmission device. Thus, the disclosed system stabilizes the reference potentials of the power transmission device and the power reception device. Accordingly, the disclosed system prevents problems caused due to the reference potentials being unstable. 
         [0013]    Preferably, the first power-transmission-side electrode and the second power-transmission-side electrode have a flat plate shape and are provided on the same plane, the first power-reception-side electrode and the second power-reception-side electrode have a flat plate shape and are provided on the same plane, and the power-transmission-side reference potential electrode and the power-reception-side reference potential electrode have a flat plate shape and are opposed to each other with the first power-transmission-side electrode and the second power-transmission-side electrode and the first power-reception-side electrode and the second power-reception-side electrode being interposed therebetween. 
         [0014]    In this configuration, the disclosed power transfer system satisfies the stable condition for the reference potentials only through minimal adjustments of the shapes of the electrodes, the sizes of the electrodes, or the distance between the electrodes, etc. 
         [0015]    Preferably, the power transmission device includes a first insulating layer and a second insulating layer laminated inward from a placement surface of a housing that is in contact with the placed power reception device, the first power-transmission-side electrode and the second power-transmission-side electrode are formed between the first insulating layer and the second insulating layer, the power-transmission-side reference potential electrode is formed at an outermost layer of the second insulating layer at a side opposite to the first insulating layer, the power reception device includes a third insulating layer and a fourth insulating layer laminated inward from a placement surface of a housing that is in contact with the placed power transmission device, the first power-reception-side electrode and the second power-reception-side electrode are formed between the third insulating layer and the fourth insulating layer, the power-reception-side reference potential electrode is formed at an outermost layer of the fourth insulating layer at a side opposite to the third insulating layer, a ratio between a dielectric constant and a thickness of each of the second insulating layer and the fourth insulating layer is uniform in a planar direction, and the first insulating layer and the third insulating layer have dielectric constants equal to each other, and a ratio between the dielectric constant and a total thickness of the first insulating layer and the third insulating layer is uniform in the planar direction. 
         [0016]    According to this configuration, it is possible to satisfy the stable condition for the reference potentials by fine adjustment of the thickness and the dielectric constant of the insulating layer or addition of a small correction capacitance. 
         [0017]    Preferably, in the first insulating layer, a dielectric constant of a region at least either between the first power-transmission-side electrode and the power-transmission-side reference potential electrode or between the second power-transmission-side electrode and the power-transmission-side reference potential electrode is different from that of another region. 
         [0018]    In this configuration, even when the first power-transmission-side electrode or the second power-transmission-side electrode has a thickness and the thickness of the first insulating layer is not uniform in the planar direction, it is possible to make the ratio between the dielectric constant and the thickness uniform in the planar direction by changing the dielectric constant between the electrodes. 
         [0019]    Preferably, in the fourth insulating layer, a dielectric constant of a region at least either between the first power-reception-side electrode and the power-reception-side reference potential electrode or between the second power-reception-side electrode and the power-reception-side reference potential electrode is different from that of another region. 
         [0020]    In this configuration, even when the first power-reception-side electrode or the second power-reception-side electrode has a thickness and the thickness of the fourth insulating layer is not uniform in the planar direction, it is possible to make the ratio between the dielectric constant and the thickness uniform in the planar direction by changing the dielectric constant between the electrodes. 
         [0021]    Preferably, any one of the power transmission device and the power reception device includes a capacitance adjustment element, and formulas (1) and (2) are satisfied including a capacitance of the capacitance adjustment element. 
         [0022]    In this configuration, even after the electrodes are formed, the disclosed power transfer system can satisfy the stable condition for the reference potentials by adding a minimal capacitance element. 
         [0023]    Preferably, the power-transmission-side reference potential electrode has a recess, a projection, or a cavity in a portion thereof opposed to at least one of the first power-transmission-side electrode and the second power-transmission-side electrode. 
         [0024]    In this configuration, even when the first power-transmission-side electrode or the second power-transmission-side electrode has a thickness and the thickness of the first insulating layer is not uniform in the planar direction, it is possible to change the thickness of the first insulating layer by forming a recess or the like in the power-transmission-side reference potential electrode. Thus, it is possible to make the ratio between the dielectric constant and the thickness of the first insulating layer uniform in the planar direction. 
         [0025]    Preferably, the power-reception-side reference potential electrode has a recess, a projection, or a cavity in a portion thereof opposed to at least one of the first power-reception-side electrode and the second power-reception-side electrode. 
         [0026]    In this configuration, even when the first power-reception-side electrode or the second power-reception-side electrode has a thickness and the thickness of the fourth insulating layer is not uniform in the planar direction, it is possible to change the thickness of the fourth insulating layer by forming a recess or the like in the power-reception-side reference potential electrode. Thus, it is possible to make the ratio between the dielectric constant and the thickness of the fourth insulating layer uniform in the planar direction. 
         [0027]    According to the present invention, by configuring the power transmission and reception devices to satisfy the formulas (1) and (2), the reference potential of the power reception device will approach the reference potential of the power transmission device. Thus, the reference potentials of the power transmission device and the power reception device can be stabilized. Accordingly, problems caused due to unstable reference potentials can be prevented. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a circuit diagram of a power transfer system according to an embodiment. 
           [0029]      FIG. 2  is a schematic diagram of a circuit in a state where a power reception device is placed on a power transmission device. 
           [0030]      FIG. 3(A)  is a perspective plan view of a placement surface of the power transmission device and  FIG. 3(B)  is a cross-sectional view taken along the line III-III in  FIG. 3(A) . 
           [0031]      FIG. 4(A)  is a perspective plan view of a placement surface of the power reception device and  FIG. 4(B)  is a cross-sectional view taken along the line IV-IV in  FIG. 4(A) . 
           [0032]      FIG. 5  is a cross-sectional view for illustrating a cross capacitance. 
           [0033]      FIG. 6  is a circuit diagram of a portion of the power transfer system including parasitic capacitances. 
           [0034]      FIG. 7  is an equivalent circuit diagram of  FIG. 6  in the case where an active electrode and a passive electrode of the power reception device are short-circuited. 
           [0035]      FIG. 8  is an equivalent circuit diagram of  FIG. 6  in the case where an active electrode and a passive electrode of the power transmission device are short-circuited. 
           [0036]      FIG. 9  is a cross-sectional view of an electrode portion in a state where the power reception device is placed on the power transmission device. 
           [0037]      FIG. 10  is a diagram for illustrating a structure example in the case where the thickness of each electrode is taken into consideration. 
           [0038]      FIGS. 11(A) and 11(B)  are diagrams for illustrating a structure example in the case where a ratio between a dielectric constant and a thickness is not distributed uniformly in a planar direction of an insulating layer. 
           [0039]      FIGS. 12(A) and 12(B)  are diagrams for illustrating a structure example in the case where a wire for each electrode is taken into consideration. 
           [0040]      FIG. 13  is a schematic diagram of a circuit of the power transfer system in the case where a capacitor is added. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0041]      FIG. 1  is a circuit diagram of a power transfer system  1  according to an embodiment. 
         [0042]    The power transfer system  1  includes a power transmission device  101  and a power reception device  201 . The power reception device  201  includes a load circuit RL. The load circuit RL includes a charging circuit and a secondary battery. The secondary battery may be attachable to/detachable from the power reception device  201 . The power reception device  201  is, for example, a portable electronic device including the secondary battery. Examples of the portable electronic device include a cellular phone, a portable music player, a notebook PC, and a digital camera. The power transmission device  101  is a charging stand for charging the secondary battery of the power reception device  201  placed thereon. 
         [0043]    The power transmission device  101  includes a power supply  10  that outputs a DC voltage. The power supply  10  is an AC adapter. The AC adapter is connected to a commercial power supply and converts AC 100-240 V to, for example, DC 5 V or 19 V. 
         [0044]    An inverter circuit  11  is connected to the power supply  10 . The inverter circuit  11  includes four switching elements composed of MOS-FETs. The switching elements are subjected to PWM control by a driver that is not shown. The inverter circuit  11  converts the DC voltage from the power supply  10  to an AC voltage by the switching elements being turned on or off. 
         [0045]    The primary winding of a step-up transformer T 1  is connected to the output side of the inverter circuit  11 . The AC voltage resulting from the conversion by the inverter circuit  11  is applied to the step-up transformer T 1 . An active electrode  12  and a passive electrode  13  are connected to the secondary winding of the step-up transformer T 1 . The step-up transformer T 1  steps up the AC voltage applied from the inverter circuit  11 , and applies the AC voltage to the active electrode  12  and the passive electrode  13 . 
         [0046]    The active electrode  12  corresponds to a “first power-transmission electrode” according to the present disclosure. The passive electrode  13  corresponds to a “second power-transmission electrode” according to the present disclosure. 
         [0047]    In addition, the power transmission device  101  includes a reference potential electrode  14 . The reference potential electrode  14  is connected to a reference potential of the power transmission device  101 . The reference potential of the power transmission device  101  is equal to an absolute earth potential, and is desirably connected to earth (or a desk or the like on which the power transmission device  101  is put). The reference potential electrode  14  corresponds to a “power-transmission reference potential electrode” according to the present disclosure. 
         [0048]    Capacitors Ca 1  and Cp 1  connected in series are connected to the active electrode  12  and the passive electrode  13 . Although described in detail later, the capacitors Ca 1  and Cp 1  are provided for the purpose of stabilizing a reference potential of the power reception device  201 . A connection point between the capacitors Ca 1  and Cp 1  is connected to the reference potential of the power transmission device  101 . The capacitor Ca 1  corresponds to a “first power-transmission capacitor” according to the present disclosure, and the capacitor Cp 1  corresponds to a “second power-transmission capacitor” according to the present disclosure. 
         [0049]    A capacitor C 1  is connected to the secondary winding of the step-up transformer T 1 . The capacitor C 1  forms a series resonant circuit together with a leakage inductance L 1  of the step-up transformer T 1 . 
         [0050]    The power reception device  201  includes an active electrode  22  and a passive electrode  23 . The active electrode  22  corresponds to a “first power-reception electrode” according to the present disclosure, and the passive electrode  23  corresponds to a “second power-reception electrode” according to the present disclosure. When the power reception device  201  is placed (mounted) on the power transmission device  101 , the active electrodes  12  and  22  are opposed to each other via a gap, and the passive electrodes  13  and  23  are opposed to each other via a gap. Because of this opposed arrangement, electrostatic capacities are formed between the active electrodes  12  and  22  and between the passive electrodes  13  and  23  to achieve electrical coupling. Electric power is transferred from the power transmission device  101  to the power reception device  201  through this coupling. 
         [0051]    Capacitors Ca 2  and Cp 2  connected in series are connected to the active electrode  22  and the passive electrode  23 . Similarly to the capacitors Ca 1  and Cp 1 , the capacitors Ca 2  and Cp 2  are provided for the purpose of stabilizing the reference potential of the power reception device  201 . A connection point between the capacitors Ca 2  and Cp 2  is connected to the reference potential of the power transmission device  101 . The capacitor Ca 2  corresponds to a “first power-reception capacitor” according to the present disclosure, and the capacitor Cp 2  corresponds to a “second power-reception capacitor” according to the present disclosure. 
         [0052]    In addition, the power reception device  201  includes a reference potential electrode  24 . The reference potential electrode  24  is connected to the reference potential of the power reception device  201 . When the power reception device  201  is placed (mounted) on the power transmission device  101 , a portion of the reference potential electrode  24  is opposed to the reference potential electrode  14 . Accordingly, the reference potential of the power reception device  201  is connected to the reference potential of the power transmission device  101  via a capacitance formed between the reference potential electrodes  14  and  24  opposed to each other. The reference potential electrode  24  corresponds to a “power-reception reference potential electrode” according to the present disclosure. 
         [0053]    The primary winding of a step-down transformer T 2  is connected to the active electrode  22  and the passive electrode  23 . A diode bridge DB is connected to the secondary winding of the step-down transformer T 2 , and a smoothing circuit composed of a capacitor C 3  and an inductor L 2  is further connected to the secondary winding of the step-down transformer T 2 . The step-down transformer T 2  steps down a voltage induced by the active electrode  22  and the passive electrode  23 . The diode bridge DB and the smoothing circuit rectify and smooth the voltage stepped-down by the step-down transformer T 2 , and supply the voltage to the load circuit RL. 
         [0054]    In addition, a capacitor C 2  is connected to the primary winding of the step-down transformer T 2 . The capacitor C 2  forms a parallel resonant circuit together with the secondary winding of the step-down transformer T 2 . The resonant frequency of the parallel resonant circuit is set so as to be equal to the resonant frequency of the series resonant circuit formed in the power transmission device  101 . Since the resonant frequencies of the resonant circuits of the power transmission device  101  and the power reception device  201  are set so as to be equal to each other, it is possible to efficiently transfer electric power from the power transmission device  101  to the power reception device  201 . 
         [0055]      FIG. 2  is a schematic diagram of a circuit in a state where the power reception device  201  is placed on the power transmission device  101 . In  FIG. 2 , a portion of the circuit shown in  FIG. 1  is not shown. In addition, the power supply  10  and the inverter circuit  11  described with reference to  FIG. 1  are shown as a power supply Ein in  FIG. 2 . 
         [0056]    The power transmission device  101  includes a housing  101 A. The power reception device  201  includes a housing  201 A. When the housing  201 A of the power reception device  201  is placed on the housing  101 A of the power transmission device  101 , electric power is transferred from the power transmission device  101  to the power reception device  201 . Hereinafter, surfaces that are in contact with each other when the housing  201 A is placed on the housing  101 A are referred to as placement surfaces of the housings  101 A and  201 A. 
         [0057]    The active electrode  12  and the passive electrode  13  of the power transmission device  101  are provided on the same plane along the placement surface of the housing  101 A. The reference potential electrode  14  is provided parallel to the active electrode  12  and the passive electrode  13  such that the active electrode  12  and the passive electrode  13  are located at the placement surface side of the housing  101 A. In addition, the power transmission device  101  includes a shield electrode  15  provided along a surface of the housing  101 A opposite to the placement surface of the housing  101 A. The shield electrode  15  has the same potential as that of the reference potential electrode  14 . 
         [0058]    The active electrode  22  and the passive electrode  23  of the power reception device  201  are provided on the same plane along the placement surface of the housing  201 A. The reference potential electrode  24  is provided parallel to the active electrode  22  and the passive electrode  23  such that the active electrode  22  and the passive electrode  23  are located at the placement surface side of the housing  201 A. In addition, the power reception device  201  includes a shield electrode  25  provided along a surface of the housing  201 A opposite to the placement surface of the housing  201 A. The shield electrode  25  has the same potential as that of the reference potential electrode  24 . 
         [0059]    As described above, when the power reception device  201  is placed on the power transmission device  101 , the active electrodes  12  and  22  are opposed to each other, and the passive electrodes  13  and  23  are opposed to each other. In addition, the reference potential electrodes  14  and  24  are opposed to each other with the active electrodes  12  and  22  and the passive electrodes  13  and  23  interposed therebetween. Furthermore, the active electrodes  12  and  22 , the passive electrodes  13  and  23 , and the reference potential electrodes  14  and  24  are interposed between the shield electrodes  15  and  25 . Radiation of noise generated within the power transmission device  101  and the power reception device  201  is suppressed by the shield electrodes  15  and  25 . 
         [0060]      FIG. 3(A)  is a perspective plan view of the placement surface of the power transmission device  101 .  FIG. 3(B)  is a cross-sectional view taken along the line III-III in  FIG. 3(A) . 
         [0061]    Each of the active electrode  12  and the passive electrode  13  has a flat plate shape. The active electrode  12  is rectangular, and the passive electrode  13  is formed so as to surround the active electrode  12 . In addition, the reference potential electrode  14  has such a size as to cover the entirety of the active electrode  12  and the passive electrode  13  in a plan view. 
         [0062]    In the power transmission device  101 , insulating layers  16  and  17  are formed inward from the placement surface of the housing  101 A. The active electrode  12  and the passive electrode  13  are provided between the insulating layers  16  and  17 . The reference potential electrode  14  is provided at the outermost layer of the insulating layer  17 . The insulating layers  16  and  17  may be any members as long as the insulating layers  16  and  17  are insulating members such as resin. The insulating layer  16  corresponds to a “first insulating layer” and the insulating layer  17  corresponds to a “second insulating layer” according to the present disclosure. 
         [0063]      FIG. 4(A)  is a perspective plan view of the placement surface of the power reception device  201 .  FIG. 4(B)  is a cross-sectional view taken along the line IV-IV in  FIG. 4(A) . 
         [0064]    Each of the active electrode  22  and the passive electrode  23  has a flat plate shape. The active electrode  22  is rectangular, and the passive electrode  23  is formed so as to surround the active electrode  22 . In addition, the reference potential electrode  24  has such a size as to cover the entirety of the active electrode  22  and the passive electrode  23  in a plan view. 
         [0065]    In the power reception device  201 , insulating layers  26  and  27  are formed inward from the placement surface of the housing  201 A. The active electrode  22  and the passive electrode  23  are provided between the insulating layers  26  and  27 . The reference potential electrode  24  is provided at the outermost layer of the insulating layer  27 . The insulating layers  26  and  27  may be any members as long as the insulating layers  26  and  27  are insulating members such as resin. The insulating layer  26  corresponds to a “third insulating layer” and the insulating layer  27  corresponds to a “fourth insulating layer” according to the present disclosure. 
         [0066]    When the power reception device  201  having the electrodes shown in  FIGS. 4(A) and 4(B)  is placed on the power transmission device  101  having the electrodes shown in  FIGS. 3(A) and 3(B) , a cross capacitance occurs due to the differences in size and shape between the electrodes in a plan view. Hereinafter, the cross capacitance will be described. 
         [0067]      FIG. 5  is a cross-sectional view for illustrating the cross capacitance. The cross-sectional view shown in  FIG. 5  corresponds to a view obtained by overlaying the cross-sectional view in  FIG. 4(B)  on the cross-sectional view in  FIG. 3(B)  such that the placement surfaces of the power transmission device  101  and the power reception device  201  are in contact with each other. 
         [0068]    When the power reception device  201  is placed on the power transmission device  101 , the active electrodes  12  and  22  are opposed to each other, and the passive electrodes  13  and  23  are opposed to each other. At this time, the active electrodes  12  and  22 , which are rectangular, are opposed to each other such that the longitudinal directions thereof are orthogonal to each other. Therefore, as shown in  FIG. 5 , the active electrode  22  has a portion that is not opposed to the active electrode  12 . The portion of the active electrode  22  that is not opposed to the active electrode  12  is opposed to the reference potential electrode (dotted-line regions SP 1  and SP 2  in the drawing), and a capacitance occurs in the regions SP 1  and SP 2 . This capacitance becomes a cross capacitance. 
         [0069]    Similarly, the passive electrode  23  has a portion that is not opposed to the passive electrode  13 , this portion is opposed to the reference potential electrode  14  (dotted-line regions SP 3 , SP 4 , SP 5 , and SP 6  in the drawing), and a capacitance occurs in the regions SP 3 , SP 4 , SP 5 , and SP 6 . This capacitance becomes a cross capacitance. 
         [0070]    The cross capacitance illustrated in  FIG. 5  is an example, and an occurring cross capacitance is different depending on a position at which the power reception device  201  is placed relative to the power transmission device  101 . Hereinafter, the cross capacitance occurring in the power transfer system  1  will be described.  FIG. 6  is a circuit diagram of a portion of the power transfer system  1  including parasitic capacitances. 
         [0071]    In the circuit in  FIG. 1 , a connection point between the capacitor Ca 1  and the active electrode  12  is denoted by P1, a connection point between the capacitor Ca 2  and the active electrode  22  is denoted by P2, a connection point between the capacitor Cp 1  and the passive electrode  13  is denoted by P3, and a connection point between the capacitor Cp 2  and the passive electrode  23  is denoted by P4. In addition, the connection point between the capacitors Ca 1  and Cp 1  is denoted by P5, and the connection point between the capacitors Ca 2  and Cp 2  is denoted by P6. 
         [0072]    In this case, a parasitic capacitance occurs each between P1 and P4, between P1 and P6, between P2 and P3, P2 and P5, between P3 and P6, and between P4 and P5. A parasitic capacitance C 14  occurs between P1 and P4, and a parasitic capacitance C 16  occurs between P1 and P6. A parasitic capacitance C 23  occurs between P2 and P3, and a parasitic capacitance C 25  occurs between P2 and P5. A parasitic capacitance C 36  occurs between P3 and P6, and a parasitic capacitance C 45  occurs between P4 and P5. 
         [0073]    In  FIG. 6 , for convenience of explanation, the capacitors Ca 1 , Cp 1 , Ca 2 , and Cp 2  described with reference to  FIG. 1  are represented by C 15 , C 35 , C 26 , and C 46 . In addition, a capacitance formed between the active electrodes  12  and  22  is represented by C A , a capacitance formed between the passive electrodes  13  and  23  is represented by C P , and a capacitance formed between the reference potential electrodes  14  and  24  is represented by C G . The capacitance of each capacitance is represented by the same reference sign for each capacitance. 
         [0074]    In the circuit shown in  FIG. 6 , by satisfying formulas (1) and (2) below, it is possible to cause the reference potential of the power reception device  201  to approach the reference potential of the power transmission device  101 . Since the reference potential of the power transmission device  101  is connected to earth (or a desk, etc.) and is stable, the reference potential of the power reception device  201  can also be stabilized by causing the reference potential of the power reception device  201  to approach the reference potential of the power transmission device  101 . The reference potential of the power transmission device  101  may not be connected to earth, and the reference potential of the power reception device  201  may be stabilized by shielding electrolysis leaking to earth using a shield electrode. 
         [0000]    
       
         
           
             
               
                 
                   
                     [ 
                     
                       Math 
                       . 
                       
                           
                       
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                       1 
                     
                     ] 
                   
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         [0075]    Moreover, ΣG=C A +C P +C 14 +C 23 +C 26 +C 46 +C 25 +C 45  and ΣL=C A +C P +C 14 +C 23 +C 16 +C 36 +C 15 +C 35 . 
         [0076]    Hereinafter, a method for deriving the conditions of formulas (1) and (2) and the reason why the reference potential of the power reception device  201  approaches the reference potential of the power transmission device  101  when the conditions are satisfied, will be described. 
         [0077]    First, the case where the active electrode  22  and the passive electrode  23  are short-circuited in the circuit in  FIG. 1  is considered.  FIG. 7  is an equivalent circuit diagram of  FIG. 6  when the active electrode  22  and the passive electrode  23  of the power reception device  201  are short-circuited. 
         [0078]    In this case, the potentials at P2 and P4 become equal to each other, so that a circuit shown in the upper part of  FIG. 7  is established. When the circuit shown in the upper part of  FIG. 7  is subjected to star-mesh conversion, a bridge circuit shown in the lower part of  FIG. 7  is established. Here, capacitors C 15 ′, C 16 ′, C 35 ′, and C 36 ′ of the bridge circuit are represented by formulas below. 
         [0000]    
       
         
           
             
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         [0079]    In the bridge circuit in  FIG. 7 , from a balance condition of the bridge circuit, by satisfying: 
         [0000]        C   35   ′/C   15   ′=C   36   ′/C   16 ′  (3),
 
         [0000]    the potential difference Vc between P5 and P6 becomes 0. 
         [0080]    Next, the case where the active electrode  12  and the passive electrode  13  are short-circuited in the circuit in  FIG. 1  is considered.  FIG. 8  is an equivalent circuit diagram of  FIG. 6  when the active electrode  12  and the passive electrode  13  of the power transmission device  101  are short-circuited. 
         [0081]    In this case, the potentials at P1 and P3 become equal to each other, so that a circuit shown in the upper part of  FIG. 8  is established. When the circuit shown in the upper part of  FIG. 8  is subjected to star-mesh conversion, a bridge circuit shown in the lower part of  FIG. 8  is established. Here, capacitors C 25 ′, C 26 ′, C 45 ′, and C 46 ′ of the bridge circuit are represented by formulas below. 
         [0000]    
       
         
           
             
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         [0082]    In the bridge circuit in  FIG. 8 , from a balance condition of the bridge circuit, by satisfying: 
         [0000]        C   45   ′/C   25   ′=C   46   ′/C   26 ′  (4),
 
         [0000]    the potential difference Vc between P5 and P6 becomes 0. 
         [0083]    It is possible to derive the conditions of formulas (1) and (2) from formulas (3) and (4). Then, by satisfying the conditions of formulas (1) and (2), the potential difference Vc between P5 and P6 becomes 0. That is, the reference potentials of the power transmission device  101  and the power reception device  201  become equal to each other. As a result, the reference potential of the power reception device  201  becomes stable. 
         [0084]    Next, a structure example of the power transmission device  101  and the power reception device  201  for satisfying the conditions of formulas (1) and (2) will be described. 
         [0085]      FIG. 9  is a cross-sectional view of an electrode portion in a state where the power reception device  201  is placed on the power transmission device  101 .  FIG. 9  corresponds to the cross-sectional view shown in  FIG. 5 . 
         [0086]    To satisfy formulas (1) and (2), the ratio between the thickness and the dielectric constant of each of the insulating layers  16 ,  17 ,  26 , and  27  is made uniform in a planar direction. Between the active electrode  12  and the passive electrode  13  and the active electrode  22  and the passive electrode  23 , the two insulating layers  16  and  26  are regarded as a single insulating layer. 
         [0087]    Specifically, in the insulating layer  17  between the active electrode  12  and the passive electrode  13  and the reference potential electrode  14 , when the dielectric constant is represented by ∈1 and the thickness is represented by d1, ∈1/d1 is made uniform in the planar direction. In addition, in the insulating layer  27  between the active electrode  22  and the passive electrode  23  and the reference potential electrode  24 , when the dielectric constant is represented by ∈2 and the thickness is represented by d2, ∈2/d2 is made uniform in the planar direction. Furthermore, in the insulating layers  16  and  26  between the active electrode  12  and the passive electrode  13  and the active electrode  22  and the passive electrode  23 , when the dielectric constant is represented by ∈3 and the total thickness is represented by d3, ∈3/d3 is made uniform in the planar direction. 
         [0088]    In the case where the insulating layers  16  and  26  are opposed to each other at regular positions, the insulating layers  16  and  26  suffice to be uniform. If the dielectric constants of the respective insulating layers are equal to each other, the boundary surface between the insulating layers  16  and  26  does not necessarily need be a flat surface, and may have, for example, an uneven shape such that the insulating layers  16  and  26  are fitted to each other. In addition, if the thicknesses d1, d2, and d3 of the respective insulating layers  16 ,  17 ,  26 , and  27  are equal to each other, the dielectric constants of the respective insulating layers may be different from each other. 
         [0089]    As described above, when the thicknesses and the dielectric constants of the respective insulating layers are adjusted, formulas (1) and (2) are satisfied. In the structure shown in  FIG. 9 , the influence of the thickness of each electrode is neglected. Thus, hereinafter, a structure example in the case where the thickness of each electrode is taken into consideration will be described. 
         [0090]      FIG. 10  is a diagram for illustrating the structure example in the case where the thickness of each electrode is taken into consideration.  FIG. 10  shows only the electrodes and the insulating layers in the power reception device  201 . 
         [0091]    Each of the active electrode  22  and the passive electrode  23  has a thickness. Thus, in the insulating layer  27 , the thickness T1 of a portion where the reference potential electrode  24  is opposed to the active electrode  22  and the passive electrode  23  and the thickness T2 of a portion where the reference potential electrode  24  is not opposed to the active electrode  22  and the passive electrode  23  have a relationship of T1&lt;T2. Therefore, as shown in  FIG. 10 , a projection  24 A is provided in each of regions where the reference potential electrode  24  is opposed to the active electrode  22  and the passive electrode  23 , and at the upper side in  FIG. 10 . Accordingly, the thickness T1 increases, so that it is possible to make T1 and T2 substantially equal to each other. As a result, as described with reference to  FIG. 9 , in the planar direction of each insulating layer, it is possible to make the ratio between the dielectric constant and the thickness uniform. 
         [0092]    Regarding the ratio between the dielectric constant and the thickness being uniform, the value of the ratio does not necessarily need to be completely uniform in the planar direction of the insulating layer, and the ratio suffices to fall within a range where formulas (1) and (2) are satisfied and the reference potential of the power reception device  201  approaches the reference potential of the power transmission device  101 . In addition, the ratio between the dielectric constant and the thickness may not be uniformly distributed in the planar direction of the insulating layer. For example, a case is also included in which the ratio between the dielectric constant and the thickness is high in a portion and is low in another portion in the planar direction of the insulating layer. 
         [0093]      FIGS. 11(A) and 11(B)  are diagrams for illustrating structure examples in the case where the ratio of the dielectric constant and the thickness is not distributed uniformly in the planar direction of the insulating layer. These figures shows only the electrodes and the insulating layers in the power reception device  201 . 
         [0094]    In  FIG. 11(A) , in the insulating layer  27  and between the active electrode  22  and the reference potential electrode  24 , a low-dielectric-constant portion  27 A having a lower dielectric constant than the other portion of the insulating layer  27  is provided. Similarly, in the insulating layer  27  and between the passive electrode  23  and the reference potential electrode  24 , a low-dielectric-constant portion  27 B having a lower dielectric constant than the other portion of the insulating layer  27  is provided. According to the exemplary embodiment, each of the low-dielectric-constant portions  27 A and  27 B has a dielectric constant that makes the ratio between the dielectric constant and the thickness uniform in the planar direction of the insulating layer  27 . When the low-dielectric-constant portions  27 A and  27 B are provided, the power reception device  201  has a structure in which the dielectric constant of a portion is lower than the dielectric constant of another portion in the planar direction of the insulating layer  27 . In this case, even if the thickness of the insulating layer  27  is not uniform in the planar direction due to the thickness of the electrode, it is possible to make the ratio between the thickness and the dielectric constant of the insulating layer  27  uniform by changing the dielectric constant. 
         [0095]    In  FIG. 11(B) , the reference potential electrode  24  has cavities (portions where the reference potential electrode  24  is not formed)  24 B and  24 C in portions thereof opposed to the active electrode  22  and the passive electrode  23 . In this case, each of the dielectric constant between the active electrode  22  and the reference potential electrode  24  and the dielectric constant between the passive electrode  23  and the reference potential electrode  24  is equal to the dielectric constant of the insulating layer  27 , but the thickness of the insulating layer  27  in each of the regions where the active electrode  22  and the passive electrode  23  are provided is smaller than that in the other region in the planar direction of the insulating layer  27 . At this time, when the cavities  24 B and  24 C are formed so as to have appropriate sizes, it is possible to make the ratio between the thickness and the dielectric constant of the insulating layer  27  by adjusting the electrostatic capacity between the active electrode  22  and the reference potential electrode  24  and the electrostatic capacity of the passive electrode  23  and the reference potential electrode  24  so as to decrease the electrostatic capacities to equivalently decrease the dielectric constant. 
         [0096]    As described above, by providing the low-dielectric-constant portions  27 A and  27 B or forming the cavities  24 B and  24 C, it is possible to perform adjustment such that the stable condition for the reference potential is satisfied. In  FIGS. 11(A) and 11(B) , the positions at which the low-dielectric-constant portions  27 A and  27 B are provided, and the positions at which the cavities  24 B and  24 C are formed, are preferably positions opposed to the active electrode  22  and the passive electrode  23 . The provision of the low-dielectric-constant portions  27 A and  27 B or the like at the positions does not influence the parasitic capacitances formed with the partner side, that is, the power transmission device  101  side. 
         [0097]    In  FIGS. 10 and 11 , the power reception device  201  side has been described, but the same applies to the power transmission device  101  side. 
         [0098]      FIGS. 12(A) and 12(B)  are diagrams for illustrating structure examples in the case where a wire for each electrode is taken into consideration. As described with reference to  FIG. 2 , the wire is connected to each electrode. Thus, in the case where the influence of a parasitic capacitance other than the electrode portion, for example, the influence of the parasitic capacitance by the wire, cannot be neglected, formulas (1) and (2) are caused to be satisfied by deleting or adding a capacitance between the electrodes.  FIGS. 12(A) and 12(B)  show only the electrodes and the insulating layer in the power reception device  201 . 
         [0099]      FIG. 12(A)  is a configuration example in the case of deleting a capacitance. In this case, a projection  24 D is provided in the portion of the reference potential electrode  24  to which the active electrode  22  is opposed, so as to project upward in  FIG. 12(A) , and an cavity  24 E is provided in the portion of the reference potential electrode  24  to which the passive electrode  23  is opposed. In addition, a low-dielectric-constant layer  27 C is provided between the passive electrode  23  and the reference potential electrode  24 . 
         [0100]      FIG. 12(B)  is a configuration example in the case of adding a capacitance. In this case, a recess  24 F is provided in the portion of the reference potential electrode  24  to which the active electrode  22  is opposed, so as to be recessed downward in  FIG. 12(B) . In addition, a high-dielectric-constant layer  27 D is provided between the passive electrode  23  and the reference potential electrode  24 . 
         [0101]    As described above, there is a possibility that due to the influence of the wire or the like, formulas (1) and (2) are not satisfied, but it is possible to avoid this by deleting or adding a capacitance as appropriate. 
         [0102]    In the case of adding a capacitance, a capacitor may be incorporated into the insulating layer or may be connected via a wire on an insulating substrate surface, without changing the shapes of the electrodes. 
         [0103]      FIG. 13  is a schematic diagram of a circuit of the power transfer system  1  when a capacitor is added. In this case, in the power transmission device  101 , capacitors C 41  and C 42  are connected between the active electrode  12  and the passive electrode  13  and the reference potential electrode  14 . In addition, in the power reception device  201 , capacitors C 51  and C 52  are connected between the active electrode  22  and the passive electrode  23  and the reference potential electrode  24 . Each of the capacitors C 41 , C 42 , C 51 , and C 52  is composed of, for example, a multilayer ceramic capacitor and corresponds to a “capacitance adjustment element” according to an exemplary embodiment. In this case, even after the electrodes and the like are formed, by adding capacitors as appropriate, it is possible to satisfy formulas (1) and (2). Moreover, it should be understood that the positions where the capacitors C 41 , C 42 , C 51 , and C 52  are connected can be adjusted as appropriate. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1  power transfer system 
               10  power source (voltage applying circuit) 
               11  inverter circuit (voltage applying circuit) 
               12 ,  22  active electrode 
               13 ,  23  passive electrode 
               14 ,  24  reference potential electrode 
               15 ,  25  shield electrode 
               16 ,  17 ,  26 ,  27  insulating layer 
               24 A projection 
               24 B,  24 C cavity 
               24 D projection 
               24 E cavity 
               24 F recess 
               27 A,  27 B low-dielectric-constant portion 
               27 C low-dielectric-constant layer 
               27 D high-dielectric-constant layer 
               101  power transmission device 
               101 A,  201 A housing 
               201  power reception device 
             Ca 1 , Cp 1 , Ca 2 , Cp 2  capacitor 
             DB diode bridge 
             RL load circuit 
             T1 step-up transformer 
             T2 step-down transformer