Power-transmitting circuit, power-transmitting device, power-receiving device, and contactless power supply system

Abstract

A power-transmitting circuit (120-120O) for contactlessly supplying power to a power-receiving device (200) comprises: a first resonant circuit (10) comprising a first coil (Ls1) connected to a power source device (110) for supplying alternating-current power of a predetermined operating frequency, and a first variable-impedance element (20) that is connected in series with the first coil between the first coil and the power source device and is for switching a state of the first resonant circuit between a resonant state and a non-resonant state; and a second resonant circuit (12) comprising a second coil (Ls2) magnetically coupled to the first coil, and a second variable-impedance element (22) that is connected in parallel with the second coil and is for switching a state of the second resonant circuit between a resonant state and a non-resonant state. One of two end sections of the second coil is connected to the power source device.

Description

Power transmission circuit, power transmission device, power receiving device, and contactless power supply system CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Japanese Application No. 2023-215344, filed on December 21, 2023, the contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a power transmission circuit, a power transmission device, a power receiving device, and a contactless power supply system.

[0003] A contactless power transfer system that contactlessly supplies power from a power transmission device to an electric vehicle equipped with a power receiving device is known. The contactless power transfer system described in Patent Document 1 improves power transfer efficiency by providing a tertiary coil that is magnetically coupled to a primary coil of the power transmission device and a secondary coil of the power receiving device.

[0004] Japanese Patent Application Laid-Open No. 2022-130305

[0005] However, if the primary coil and the tertiary coil are placed close to each other in order to magnetically couple them with each other, stray capacitance will occur between the coils, causing leakage current in the tertiary coil and potentially increasing the leakage electromagnetic field from the tertiary coil.

[0006] The present disclosure can be realized in the following forms.

[0007] According to one aspect of the present disclosure, there is provided a power transmitting circuit for contactlessly supplying power to a power receiving device, the power transmitting circuit including: a first resonant circuit having a first coil connected to a power supply device that supplies AC power at a predetermined operating frequency, and a first variable impedance element connected in series with the first coil between the first coil and the power supply device for switching a state of the first resonant circuit between a resonant state and a non-resonant state, and a second resonant circuit having a second coil magnetically coupled to the first coil and a second variable impedance element connected in parallel with the second coil for switching a state of the second resonant circuit between a resonant state and a non-resonant state, wherein one of both ends of the second coil is connected to the power supply device.

[0008] In this power transmission circuit, since one of the ends of the second coil is connected to the power supply, the leakage current flowing through the second coil is passed to the power supply line via the capacitance generated between the first coil and the second coil, thereby shortening the current path of the leakage current flowing through the second coil, or equalizing the potential of the first coil and the second coil, thereby suppressing the leakage current flowing through the second coil and suppressing an increase in the leakage electromagnetic field caused by the leakage current.

[0009] The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 1 is an explanatory diagram showing a schematic configuration of a contactless power supply system of the first embodiment, FIG. 2 is an explanatory diagram showing a schematic configuration of a power supply device of the first embodiment, FIG. 3 is an explanatory diagram showing a circuit configuration of the contactless power supply system of the first embodiment, FIG. 4 is a circuit diagram showing a coil configuration of a power transmission device of the first embodiment, FIG. 5 is a circuit diagram showing a schematic coil configuration of a power transmission device of the first embodiment, FIG. 6 is a circuit diagram showing a coil configuration of a power transmission device of the second embodiment, FIG. 7 is a circuit diagram showing a schematic coil configuration of a power transmission device of the second embodiment, FIG. 8 is a circuit diagram showing a coil configuration of a power transmission device of another embodiment, FIG. 9 is a circuit diagram showing a coil configuration of a power transmission device of another embodiment, FIG. 10 is a circuit diagram showing a coil configuration of a power transmission device of another embodiment, FIG. 11 is a circuit diagram showing a coil configuration of a power transmission device of another embodiment, FIG. 12 is a circuit diagram showing a schematic coil configuration of a power transmission device of another embodiment, and FIG. 21 is a circuit diagram showing the coil configuration of a power transmission device of another embodiment; FIG. 22 is a circuit diagram showing the coil configuration of a power transmission device of another embodiment; FIG. 23 is a circuit diagram showing the coil configuration of a power transmission device of another embodiment; FIG. 24 is a circuit diagram showing the coil configuration of a power transmission device of another embodiment; and FIG. 25 is a circuit diagram showing the coil configuration of a power transmission device of another embodiment.

[0010] A. First Embodiment: A-1. Device Configuration: A contactless power transfer system 1000 of this embodiment shown in FIG. 1 includes a power transmission device 100 and a power receiving device 200. In this embodiment, the power transmission device 100 is buried under a road 105. The power receiving device 200 is mounted on an electric vehicle 202 as a mobile body that travels on the road 105. In this embodiment, the electric vehicle 202 is configured as an AGV (Automatic Guided Vehicle) that travels within a factory or warehouse. Note that the electric vehicle 202 may also be configured as an electric vehicle or a hybrid vehicle.

[0011] In the contactless power supply system 1000, the power transmission device 100 supplies power to the power receiving device 200 when the electric vehicle 202 travels on a track 105. "Traveling on the track 105" includes cases where the electric vehicle 202 is moving, as well as cases where the electric vehicle 202 is stopped near fixed equipment such as a transport robot or conveyor for transferring transported goods, etc. In Fig. 1 , the x-axis direction indicates the traveling direction of the electric vehicle 202, the y-axis direction indicates the width direction of the electric vehicle 202, and the z-axis direction indicates the vertically upward direction.

[0012] The power transmission device 100 includes a power supply device 110 and a plurality of power transmission circuits 120. The power supply device 110 outputs AC power at a predetermined operating frequency. As shown in FIG. 2 , the power supply device 110 includes a system power supply PS, a noise filter 111, a power factor correction (PFC) circuit 112, an inverter 113, and a high-frequency filter 114. In this embodiment, one set of the noise filter 111, the PFC circuit 112, the inverter 113, and the high-frequency filter 114 is connected to the system power supply PS, but two or more sets of the noise filter 111, the PFC circuit 112, the inverter 113, and the high-frequency filter 114 may be connected in parallel to the system power supply PS.

[0013] The noise filter 111 removes noise from the AC power supplied from the power grid PS. The PFC circuit 112 is a well-known circuit that eliminates the phase difference between the input voltage and output current generated by the noise removal in the noise filter 111, suppresses harmonics, and improves the power factor to approach 1. The noise filter 111 and the PFC circuit 112 convert the AC power supplied from the power grid PS into DC power and smooth it. The inverter 113 converts the supplied DC power into high-frequency AC power. The high-frequency filter 114 extracts and passes AC power of a predetermined operating frequency from the supplied high-frequency AC power. The AC power that has passed through the high-frequency filter 114 is supplied to each power transmission circuit 120.

[0014] As shown in FIG. 1 , the multiple power transmission circuits 120 are installed underground along the x-axis direction of the track 105. Note that some of the multiple power transmission circuits 120 may be installed in locations other than underground on the track 105, for example, on the side of fixed equipment such as a transport articulated robot or a conveyor. The power transmission circuits 120 are connected in parallel to the power supply device 110 and are supplied with AC power from the power supply device 110. Each power transmission circuit 120 has a primary-side resonant circuit 10. The primary-side resonant circuit 10 is supplied with AC power from the power supply device 110 and transmits the AC power in a wireless manner to a secondary-side resonant circuit 240, which will be described later. The specific configuration of the power transmission circuit 120, including the primary-side resonant circuit 10, will be described later.

[0015] The power receiving device 200 includes a battery 210, an auxiliary battery 215, a power receiving-side control unit 220, a power receiving circuit 230, a secondary-side resonant circuit 240, a DC / DC converter circuit 260, an inverter circuit 270, a motor generator 280, and an auxiliary device 290. The power receiving device 200 does not necessarily include the auxiliary device 290. In this case, the power receiving device 200 does not necessarily include the auxiliary battery 215 or the DC / DC converter circuit 260. In this embodiment, the secondary-side resonant circuit 240 is provided in a position facing the road 105, for example, on the underside of the electric vehicle 202. In addition, if the power transmitting circuit 120 is disposed on the side of the fixed equipment, the secondary-side resonant circuit 240 may be provided on the side of the electric vehicle 202. A specific configuration of the power receiving device 200 including the secondary-side resonant circuit 240 will be described later.

[0016] The secondary-side resonant circuit 240 is connected to the power receiving circuit 230, and the AC power received by the secondary-side resonant circuit 240 is converted into DC power by the power receiving circuit 230. The output of the power receiving circuit 230 is connected to the battery 210, the high-voltage side of the DC / DC converter circuit 260, and the inverter circuit 270. The low-voltage side of the DC / DC converter circuit 260 is connected to the auxiliary battery 215 and the auxiliary device 290. The inverter circuit 270 is connected to the motor generator 280. The DC power output from the power receiving circuit 230 can be used to charge the battery 210 or drive the motor generator 280 via the inverter circuit 270. Furthermore, by stepping down the voltage of the DC power output from the power receiving circuit 230 using the DC / DC converter circuit 260, the DC power can also be used to charge the auxiliary battery 215 or drive the auxiliary device 290.

[0017] Battery 210 is a secondary battery that outputs relatively high DC power, for example, a voltage of several tens to several hundreds of volts, for driving motor generator 280. Motor generator 280 operates as a three-phase AC motor and generates driving force for running electric vehicle 202. Motor generator 280 operates as a generator and regenerates electric power when electric vehicle 202 decelerates. When motor generator 280 operates as a motor, inverter circuit 270 converts the power of battery 210 into three-phase AC and supplies it to motor generator 280. When motor generator 280 operates as a generator, inverter circuit 270 converts the three-phase AC regenerated by motor generator 280 into DC and supplies it to battery 210.

[0018] The DC / DC converter circuit 260 converts the output of the battery 210 to a voltage lower than the output voltage of the battery 210, for example, 12 V, and supplies the voltage to the auxiliary battery 215 and the auxiliary device 290. The auxiliary battery 215 is a secondary battery for driving the auxiliary device 290, and has a relatively low voltage. The auxiliary device 290 includes peripheral devices of the electric vehicle 202 and various accessories of the electric vehicle 202.

[0019] The power receiving side control unit 220 controls the inverter circuit 270 and other units in the electric vehicle 202 .

[0020] A-2. Circuit Configuration: As shown in Fig. 3, the power transmitting circuit 120 has, in addition to the above-mentioned primary side resonant circuit 10, a tertiary side resonant circuit 12, a power transmitting side detection circuit 30, and a power transmitting side control circuit 40. Note that Fig. 3 shows only one of the multiple power transmitting circuits 120 connected in parallel to the power supply device 110, and the other power transmitting circuits 120 are not shown.

[0021] The primary-side resonant circuit 10 includes a primary-side coil Ls1 and a first variable impedance element 20. The first variable impedance element 20 is connected in series with the primary-side coil Ls1 between the power supply device 110 and the primary-side coil Ls1, and together with the primary-side coil Ls1, constitutes the primary-side resonant circuit 10. The first variable impedance element 20 includes two capacitors C11 and C12 and a first switch SW1. The capacitor C11 and the primary-side coil Ls1 are connected in series. The capacitor C12 and the first switch SW1 are connected in series, and the series-connected capacitor C12 and first switch SW1 are connected in parallel with the capacitor C11. The first switch SW1 may be configured to switch a mechanical contact such as a relay in response to an external command, or may be configured using a semiconductor element such as a MOS-FET or an analog switch.

[0022] The capacitance of the first impedance variable element 20 changes when the first switch SW1 is switched on and off. When the first switch SW1 is on, the capacitor C12 is connected to the primary coil Ls1. At this time, the capacitance of the first impedance variable element 20 is equal to the sum of the capacitances of the capacitors C11 and C12. When the first switch SW1 is off, the capacitor C12 is disconnected from the primary coil Ls1. At this time, the capacitance of the first impedance variable element 20 is equal to the capacitance of the capacitor C11. As the capacitance of the first impedance variable element 20 changes in this way, the impedance of the primary resonant circuit 10 when the first switch SW1 is on decreases compared to when the first switch SW1 is off. As the impedance of the primary resonant circuit 10 changes, the resonant state of the primary resonant circuit 10 also changes. In this embodiment, when the first switch SW1 is on, the primary side resonant circuit 10 is in a resonant state at the operating frequency and in a power transmission state. When the first switch SW1 is off, the primary side resonant circuit 10 is in a non-resonant state at the operating frequency and in a standby state. In the standby state, the power transmitting circuit 120 generates magnetic flux by passing a standby current, which is smaller than the current flowing in the power transmission state, through the primary side coil Ls1. In this embodiment, the first switch SW1 is configured as a normally open switch, and the primary side resonant circuit 10 normally maintains the standby state, more specifically, when the magnitude of the magnetic flux is less than a threshold value.

[0023] The primary side resonant circuit 10 described above corresponds to the "first resonant circuit" in the present disclosure. The primary side coil Ls1 corresponds to the "first coil" in the present disclosure. The capacitor C11 corresponds to the "first capacitor" in the present disclosure. The capacitor C12 corresponds to the "second capacitor" in the present disclosure. The first switch SW1 corresponds to the "switch element" in the present disclosure.

[0024] The tertiary resonant circuit 12 includes a tertiary coil Ls2 and a second impedance variable element 22 connected in parallel with the tertiary coil Ls2. The tertiary coil Ls2 is arranged so that, in a resonant state, it is magnetically coupled to a primary coil Ls1 of the primary resonant circuit 10 and a secondary coil Lr (described later) of the secondary resonant circuit 240. The specific positional relationship between the primary coil Ls1 and the tertiary coil Ls2 will be described later.

[0025] Furthermore, depending on the positional relationship between the primary coil Ls1 and the tertiary coil Ls2, either of the two ends of the tertiary coil Ls2 is connected to the power supply device 110. A specific manner in which the ends of the tertiary coil Ls2 are connected to the power supply device 110 will be described later.

[0026] The second variable impedance element 22 has two capacitors C21 and C22 and a second switch SW2. The capacitor C22 and the second switch SW2 are connected in series, and the series-connected capacitor C22 and the second switch SW2, the capacitor C21, and the tertiary coil Ls2 are connected in parallel. The second switch SW2 may be configured to switch a mechanical contact such as a relay in response to an external command, or may be configured to use a semiconductor element such as a MOS-FET or an analog switch.

[0027] The capacitance of the second impedance variable element 22 changes when the second switch SW2 is switched on and off. The principle by which the capacitance of the second impedance variable element 22 changes when the second switch SW2 is switched is the same as that of the first impedance variable element 20, and therefore a detailed description thereof will be omitted. In this embodiment, the second switch SW2 is configured as a normally open switch, and the tertiary-side resonant circuit 12 normally maintains a standby state, more specifically, when the magnitude of the magnetic flux is less than a threshold value.

[0028] The tertiary-side resonant circuit 12 described above corresponds to the "second resonant circuit" in the present disclosure. The tertiary-side coil Ls2 corresponds to the "second coil" in the present disclosure. The capacitor C21 corresponds to the "first capacitor" in the present disclosure. The capacitor C22 corresponds to the "second capacitor" in the present disclosure. The second switch SW2 corresponds to the "switch element" in the present disclosure.

[0029] The power transmitting side detection circuit 30 includes a magnetic flux sensor that detects the magnitude of the magnetic flux interlinked with the primary coil Ls1 and a current sensor that detects the magnitude of the current flowing through the primary coil. For convenience of illustration, the power transmitting side detection circuit 30 is shown in FIG. 3 as being located away from the primary coil Ls1, but in reality, it is located near the primary coil Ls1. In this embodiment, the magnetic flux sensor detects the magnitude of the magnetic flux by utilizing changes in voltage across a detection coil magnetically coupled to the primary coil Ls1. The current sensor detects the magnitude of the current by utilizing changes in voltage across the capacitor C11. The power transmitting side detection circuit 30 outputs a signal indicating the magnitude of the detected magnetic flux and a signal indicating the magnitude of the current to the power transmitting side control circuit 40.

[0030] The power transmitting side control circuit 40 drives the first switch SW1 and the second switch SW2 in response to the signal output from the power transmitting side detection circuit 30, thereby switching the first switch SW1 and the second switch SW2 on and off. More specifically, the power transmitting side control circuit 40 turns on the first switch SW1 and the second switch SW2 when the magnitude of the magnetic flux indicated by the signal output from the power transmitting side detection circuit 30 is equal to or greater than a preset threshold. Furthermore, the power transmitting side control circuit 40 turns off the first switch SW1 and the second switch SW2 when the magnitude of the current indicated by the signal output from the power transmitting side detection circuit 30 is equal to or less than the threshold.

[0031] The magnitude of the current and the magnitude of the magnetic flux change depending on the degree of magnetic coupling between the power transmitting circuit 120 and the power receiving device 200. In a non-resonant state, the magnitude of the magnetic flux increases as the power transmitting circuit 120 and the power receiving device 200 approach each other. In a resonant state, the magnitude of the current decreases as the power transmitting circuit 120 and the power receiving device 200 approach each other. The threshold values ​​of the magnitude of the magnetic flux and the magnitude of the current are specified and set in advance by performing simulations or the like as values ​​when the power receiving device 200 enters a power transmission area. The "power transmission area" refers to an area predetermined for each power transmitting circuit 120 as an area in which power transmission to the power receiving device 200 is performed. In other words, the primary-side control circuit 24 controls the switching of the first switch SW1 and the second switch SW2 so as to start power transmission when the power receiving device 200 enters the power transmission area. Therefore, it is possible to suppress the supply of power to the power transmission circuit 120 in the power transmission area of ​​which the power receiving device 200 is not located, and it is possible to suppress the power consumption in the contactless power supply system 1000 .

[0032] The circuit configuration of the power receiving device 200 will be described. Note that Fig. 3 illustrates the circuit configuration of the power receiving device 200, particularly the circuit configuration related to power supply to the battery 210, and omits other components. As shown in Fig. 3 , the secondary-side resonant circuit 240 includes a secondary-side coil Lr and a secondary-side capacitor Cr connected in series. The secondary-side resonant circuit 240 is configured to be in a resonant state at the operating frequency of the power transmitting device 100. In the resonant state, the secondary-side resonant circuit 240 receives AC power transmitted from the power transmitting device 100 and supplies the received AC power to the power receiving circuit 230.

[0033] The power receiving circuit 230 has a filter circuit 232 and a rectifier circuit 234. The filter circuit 232 suppresses the passage of noise components in the AC power input from the secondary-side resonant circuit 240 and passes AC power in a target frequency band, while supplying a constant amount of current to the rectifier circuit 234. The rectifier circuit 234 is configured as a diode bridge. The rectifier circuit 234 rectifies the AC power supplied from the secondary-side resonant circuit 240 and supplies the rectified DC power to the battery 210. The battery 210 is charged by the DC power supplied from the rectifier circuit 234.

[0034] A-3. Coil Configuration: With reference to Figure 4, the positional relationship between the primary coil Ls1 and the tertiary coil Ls2 in this embodiment and the manner in which the end of the tertiary coil Ls2 is connected to the power supply device 110 in accordance with this positional relationship will be described. Note that Figure 4 illustrates only the system power supply PS of the power supply device 110, and does not illustrate other components. Also, the power transmitting side detection circuit 30 and the power transmitting side control circuit 40 are not illustrated.

[0035] 4, the primary coil Ls1 is formed such that the planar coil Lp11 and the planar coil Lp12 overlap each other in the stacking direction. The planar coil Lp11 and the planar coil Lp12 are arranged at a predetermined distance d1 along the stacking direction. In this embodiment, the planar coil Lp11 is located above the planar coil Lp12 in the stacking direction.

[0036] Similarly, the tertiary coil Ls2 is formed such that the planar coil Lp21 and the planar coil Lp22 overlap each other in the stacking direction. The planar coil Lp21 and the planar coil Lp22 are arranged at a predetermined distance d2 along the stacking direction. In this embodiment, the planar coil Lp21 is located above the planar coil Lp22 in the stacking direction.

[0037] In this embodiment, the distance d1 and the distance d2 are approximately the same. Note that the primary coil Ls1 and the tertiary coil Ls2 may be formed so that either the distance d1 or the distance d2 is larger than the other.

[0038] The primary coil Ls1 and the tertiary coil Ls2 are formed to overlap each other with a predetermined gap d3 in the stacking direction. The tertiary coil Ls2 is located above the primary coil Ls1 in the stacking direction. In FIG. 4, "distance d3" refers to the distance between the planar coil Lp22 and the planar coil Lp11 in the stacking direction. That is, the gap d3 refers to the distance between the planar coil Lp22, which is the closest coil to the primary coil Ls1 in the stacking direction among the two planar coils Lp21 and Lp22 that form the tertiary coil Ls2, and the primary coil Ls1. More specifically, the gap d3 refers to the distance between the planar coil Lp11, which is the closest coil to the tertiary coil Ls2 in the stacking direction among the primary coil Ls1, and the planar coil Lp22.

[0039] In this embodiment, the primary coil Ls1 and the tertiary coil Ls2 are arranged such that the distance d3 is greater than the distances d1 and d2. This allows for a greater distance in the stacking direction between the primary coil Ls1 and the tertiary coil Ls2 than in a configuration in which the distance d3 is approximately the same as the distances d1 and d2, or in a configuration in which the distance d3 is smaller than the distances d1 and d2. This reduces the stray capacitance between the primary coil Ls1 and the tertiary coil Ls2, as described below. When the distances d1 and d2 are different in size, the primary coil Ls1 and the tertiary coil Ls2 are preferably arranged such that the distance d3 is greater than the smaller of the distances d1 and d2. In this embodiment, the primary coil Ls1 and the tertiary coil Ls2 are formed by conductive patterns formed on a printed circuit board, and the distances d1, d2, and d3 are determined by the thickness of the prepreg constituting the printed circuit board. The prepreg separating the primary coil Ls1 and the tertiary coil Ls2 in this manner corresponds to the "separating layer" in this disclosure.

[0040] In the primary coil Ls1 and the tertiary coil Ls2 having the above-described positional relationship, when a current flows through the primary coil Ls1, stray capacitance is generated between the planar coils Lp11 and Lp12 that form the primary coil Ls1 and the planar coils Lp21 and Lp22 that form the tertiary coil Ls2, and a leakage current is generated in the tertiary coil Ls2 via this stray capacitance. In the following description, the stray capacitance generated between the planar coils Lp11 and Lp21 is referred to as the "stray capacitance Cs11," the stray capacitance generated between the planar coils Lp11 and Lp22 is referred to as the "stray capacitance Cs12," the stray capacitance generated between the planar coils Lp12 and Lp21 is referred to as the "stray capacitance Cs21," and the stray capacitance generated between the planar coils Lp12 and Lp22 is referred to as the "stray capacitance Cs22." In this embodiment, among the planar coils that generate the stray capacitance, the distance between the planar coil Lp11 and the planar coil Lp22, which is the closest coil, is the shortest. Therefore, among the stray capacitances, the stray capacitance Cs12 generated between the planar coil Lp11 and the planar coil Lp22 has the largest electrostatic capacitance. In other words, in this embodiment, the magnitude of the leakage current through the stray capacitance Cs12 is larger than the magnitude of the leakage current through the other stray capacitances.

[0041] As shown in FIG. 4 , in this embodiment, the end of the planar coil Lp22, which is the closest coil, is connected to the power grid PS between the first impedance variable element 20 and the power grid PS. The effects achieved by this connection configuration will be described with reference to FIG. 5 , which schematically illustrates the connection configuration shown in FIG. 4 . By connecting the end of the planar coil Lp22 to the power grid PS, as shown in FIG. 5 , the current path of the leakage current passing through the tertiary coil Ls2 is the power grid PS - the planar coil Lp11 - the stray capacitance Cs12 - the planar coil Lp22 - the power grid PS. As such, according to the connection configuration of this embodiment, the maximum leakage current passing through the stray capacitance Cs12, which has the largest capacitance, is prevented from passing through the planar coil Lp21. This shortens the current path of the leakage current flowing through the tertiary coil Ls2, thereby suppressing an increase in the leakage electromagnetic field caused by the leakage current.

[0042] According to the wireless power supply system 1000 of the first embodiment described above, the end of the planar coil Lp22, which is the closest coil, is connected to the power supply device 110 between the power supply device 110 and the first impedance variable element 20. As a result, the current path of the leakage current passing through the tertiary coil Ls2 is system power supply PS - planar coil Lp11 - stray capacitance Cs12 - planar coil Lp22 - system power supply PS. This prevents the maximum leakage current passing through the stray capacitance Cs12, which has the largest capacitance, from passing through the planar coil Lp21, thereby shortening the current path of the leakage current flowing through the tertiary coil Ls2 and suppressing an increase in the leakage electromagnetic field caused by the leakage current.

[0043] Furthermore, since the spacing d3 is larger than the spacing d1 and the spacing d2, the distance along the stacking direction between the primary coil Ls1 and the tertiary coil Ls2 can be increased compared to a configuration in which the spacing d3 is approximately the same as the spacing d1 and the spacing d2, and a configuration in which the spacing d3 is smaller than the spacing d1 and the spacing d2, and the increase in the magnitude of the stray capacitance generated between the primary coil Ls1 and the tertiary coil Ls2 can be suppressed.

[0044] The power transmitting circuit 120 also includes a first impedance variable element 20 and a second impedance variable element 22, and the first impedance variable element 20 and the second impedance variable element 22 switch the state of the primary side resonant circuit 10 or the tertiary side resonant circuit 12 between a resonant state and a non-resonant state depending on the positional relationship with the power receiving device 200. Therefore, when the positional relationship with the power receiving device 200 is such that power transmission is not performed, the states of the primary side resonant circuit 10 and the tertiary side resonant circuit 12 are switched to the non-resonant state and power supply to the power transmitting circuit 120 is suppressed, thereby suppressing an increase in power consumption in the contactless power transfer system 1000.

[0045] Furthermore, since the primary coil Ls1 and the tertiary coil Ls2 are separated by a separation layer, the primary coil Ls1 and the tertiary coil Ls2 can be spaced apart more than in a configuration without a separation layer, and the increase in the size of the stray capacitance Cs11 that occurs between the primary coil Ls1 and the tertiary coil Ls2 can be suppressed.

[0046] B. Second Embodiment: A power transmission circuit 120A included in a power transmission device 100A of the second embodiment differs from the power transmission circuit 120 of the first embodiment in that an end of the nearest coil is connected to the power system PS between the power system PS and the primary coil Ls1. The circuit configuration of the power transmission circuit 120A of the second embodiment is the same as that of the power transmission circuit 120 of the first embodiment, and therefore the same components are denoted by the same reference numerals and detailed description thereof will be omitted.

[0047] In the power transmission circuit 120A shown in Fig. 6, an end of the planar coil Lp22, which is the closest coil, is connected to the power grid PS between the power grid PS and the primary coil Ls1. The effects achieved by this connection will be described with reference to Fig. 7, which schematically illustrates the connection shown in Fig. 6. By connecting the end of the planar coil Lp22 to the power grid PS as shown in Fig. 7, the potentials of the planar coil Lp11, which generates the stray capacitance Cs12 with the largest electrostatic capacitance, and the planar coil Lp22 can be made the same. This prevents the maximum leakage current from flowing through the stray capacitance Cs12 in the tertiary coil Ls2, and prevents an increase in the leakage electromagnetic field caused by the leakage current.

[0048] In the wireless power supply system 1000 including the power transmission device 100A of the second embodiment described above, the end of the closest coil Lp22 is connected to the power grid PS between the power grid PS and the primary coil Ls1. Therefore, the potentials of the planar coil Lp11, which generates the stray capacitance Cs12 having the largest electrostatic capacitance, and the planar coil Lp22 can be made the same, which prevents the maximum leakage current from flowing through the stray capacitance Cs12 in the tertiary coil Ls2 and prevents an increase in the leakage electromagnetic field due to the leakage current.

[0049] C. Other Embodiments: (C1) In the above embodiments, the primary coil Ls1 and the tertiary coil Ls2 are formed to overlap each other in the stacking direction as shown in Figures 4 and 6, but the present disclosure is not limited to this. The primary coil Ls1 and the tertiary coil Ls2 may be arranged in the same layer, with the tertiary coil Ls2 being arranged in an empty space radially inward of the primary coil Ls1, as shown in Figures 8 and 9.

[0050] 8 and 9 have the same configuration except for the manner in which the end of the tertiary coil Ls2 is connected to the power supply system PS. Therefore, the configurations of the primary coil Ls1 and the tertiary coil Ls2 will be described in more detail with reference to FIG. 8. In FIG. 8, the primary coil Ls1 is formed by winding a planar coil Lp11 and a planar coil Lp12 in the same layer. The planar coil Lp11 and the planar coil Lp12 are formed so that the conductors forming each planar coil overlap each other with a predetermined radial gap d4 between them. In this embodiment, the planar coil Lp11 is located radially inward of the planar coil Lp12.

[0051] Similarly, the tertiary coil Ls2 is formed by winding the planar coil Lp21 and the planar coil Lp22 on the same layer. The planar coil Lp21 and the planar coil Lp22 are formed so that the conductors forming each planar coil overlap each other with a predetermined radial gap d5 between them. In this embodiment, the planar coil Lp21 is located radially inward of the planar coil Lp22. In this embodiment, the gaps d4 and d5 are approximately the same size. Note that the primary coil Ls1 and the tertiary coil Ls2 may be formed so that either the gap d4 or the gap d5 is larger than the other.

[0052] The primary coil Ls1 and the tertiary coil Ls2 are formed to overlap each other with a predetermined radial distance d6 between them. In FIG. 8 , "distance d6" refers to the radial distance between the conductors forming the planar coil Lp22 and the conductors forming the planar coil Lp11. That is, the distance d6 refers to the radial distance between the conductors forming the planar coil Lp22, which is the closest coil to the primary coil Ls1 among the two planar coils Lp21 and Lp22 forming the tertiary coil Ls2 and has the smallest radial distance from the primary coil Ls1, and the conductors forming the primary coil Ls1. More specifically, the distance d6 refers to the radial distance between the conductors forming the planar coil Lp11, which is the closest coil to the conductors forming the tertiary coil Ls2 among the conductors forming the primary coil Ls1, and the conductors forming the planar coil Lp22. In this positional relationship, the stray capacitance Cs12 formed between the planar coil Lp22, which is the closest coil, and the planar coil Lp11 has the largest electrostatic capacitance.

[0053] In this embodiment, the primary coil Ls1 and the tertiary coil Ls2 are arranged so that the distance d6 is larger than the distances d4 and d5. This makes it possible to increase the radial distance between the primary coil Ls1 and the tertiary coil Ls2 and to suppress an increase in the magnitude of stray capacitance occurring between the primary coil Ls1 and the tertiary coil Ls2, compared to a configuration in which the distance d6 is approximately the same as the distances d4 and d5, and a configuration in which the distance d6 is smaller than the distances d4 and d5.

[0054] In the power transmission circuit 120B shown in Fig. 8, an end of the planar coil Lp22, which is the closest coil, is connected to the power system PS between the first impedance variable element 20 and the power system PS. A schematic diagram of such a connection configuration is similar to the circuit configuration shown in Fig. 5 above. In this way, the contactless power transfer system 1000 including the power transmission device 100B having the power transmission circuit 120B shown in Fig. 8 also achieves the same effects as those of the first embodiment.

[0055] On the other hand, in the power transmission circuit 120C shown in Fig. 9, an end of the planar coil Lp22, which is the closest coil, is connected to the power system PS between the power system PS and the primary coil Ls1. A schematic diagram of such a connection configuration is similar to the circuit configuration shown in Fig. 7 above. In this way, the wireless power transfer system 1000 including the power transmission device 100C having the power transmission circuit 120C shown in Fig. 9 also achieves the same effects as the second embodiment.

[0056] 4 and 6, the planar coil Lp11 is disposed above the planar coil Lp12 in the stacking direction in the primary coil Ls1, and the planar coil Lp21 is disposed above the planar coil Lp22 in the stacking direction in the tertiary coil Ls2. Also, in the configurations shown in FIGS. 8 and 9, the planar coil Lp11 is disposed radially inside the planar coil Lp12 in the primary coil Ls1, and the planar coil Lp21 is disposed radially inside the planar coil Lp22 in the tertiary coil Ls2. However, the positional relationship between the planar coil Lp11 and the planar coil Lp12 in the primary coil Ls1 and the positional relationship between the planar coil Lp21 and the planar coil Lp22 in the tertiary coil Ls2 are not limited to the above-described positional relationship.

[0057] 10 and 11, the planar coil Lp22 is disposed above the planar coil Lp21 in the lamination direction. In this positional relationship, the planar coil Lp21 corresponds to the closest coil, and the stray capacitance Cs11 has the largest electrostatic capacitance.

[0058] In the power transmission circuit 120D shown in FIG. 10 , an end of the planar coil Lp21, which is the closest coil, is connected to the power grid PS between the first impedance variable element 20 and the power grid PS. The effects achieved by this connection configuration will be described with reference to FIG. 12 , which schematically illustrates the connection configuration shown in FIG. 10 . By connecting the end of the planar coil Lp21 to the power grid PS, as shown in FIG. 12 , the current path of the leakage current passing through the tertiary coil Ls2 is the power grid PS - the planar coil Lp11 - the stray capacitance Cs11 - the planar coil Lp21 - the power grid PS. In this way, the connection configuration of this embodiment prevents the leakage current from passing through the planar coil Lp22, thereby shortening the current path of the leakage current flowing through the tertiary coil Ls2 and suppressing an increase in the leakage electromagnetic field caused by the leakage current passing through the stray capacitance Cs11. That is, the contactless power supply system 1000 including the power transmission device 100D having the power transmission circuit 120D shown in FIG. 10 also provides the same effects as those of the first embodiment.

[0059] In the power transmission circuit 120E shown in FIG. 11 , the end of the planar coil Lp21, which is the closest coil, is connected to the power grid PS between the power grid PS and the primary coil Ls1. The effects achieved by this connection will be described with reference to FIG. 13 , which schematically illustrates the connection shown in FIG. 11 . By connecting the end of the planar coil Lp21 to the power grid PS as shown in FIG. 13 , the potential of the planar coil Lp11, which generates the stray capacitance Cs11 with the largest capacitance, can be made the same as that of the planar coil Lp21. This prevents leakage current from flowing through the stray capacitance Cs11, thereby suppressing an increase in the leakage electromagnetic field caused by the leakage current. Thus, the contactless power transfer system 1000 including the power transmission device 100E having the power transmission circuit 120E shown in FIG. 11 also achieves the same effects as the second embodiment.

[0060] 14 and 15, the planar coil Lp22 is disposed radially inward of the planar coil Lp21. In this positional relationship, the planar coil Lp21 corresponds to the closest coil, and the stray capacitance Cs11 has the largest electrostatic capacitance.

[0061] In the power transmitting circuit 120F shown in Fig. 14, an end of the planar coil Lp21, which is the closest coil, is connected to the power system PS between the first impedance variable element 20 and the power system PS. A schematic representation of such a connection configuration results in the same circuit configuration as that shown in Fig. 12 above. In this way, the contactless power transfer system 1000 including the power transmitting device 100F having the power transmitting circuit 120F shown in Fig. 14 also achieves the same effects as those of the first embodiment.

[0062] In the power transmission circuit 120G shown in Fig. 15, an end of the planar coil Lp21, which is the closest coil, is connected to the power system PS between the power system PS and the primary coil Ls1. A schematic diagram of such a connection configuration results in a circuit configuration similar to that shown in Fig. 13 above. In this way, the contactless power transfer system 1000 including the power transmission device 100G having the power transmission circuit 120G shown in Fig. 15 also achieves the same effects as those of the second embodiment.

[0063] 16 and 17, the planar coil Lp12 is arranged above the planar coil Lp11 in the stacking direction, and the planar coil Lp22 is arranged above the planar coil Lp21 in the stacking direction. In this positional relationship, the planar coil Lp21 corresponds to the closest coil, and the stray capacitance Cs21 has the largest electrostatic capacitance.

[0064] In the power transmitting circuit 120H shown in FIG. 16 , an end of the planar coil Lp21, which is the closest coil, is connected to the power grid PS between the first impedance variable element 20 and the power grid PS. The effects achieved by this connection configuration will be described with reference to FIG. 18 , which schematically illustrates the connection configuration shown in FIG. 16 . By connecting the end of the planar coil Lp21 to the power grid PS, as shown in FIG. 18 , the current path of the leakage current passing through the tertiary coil Ls2 is the power grid PS - the planar coil Lp12 - the stray capacitance Cs21 - the planar coil Lp21 - the power grid PS. In this way, the connection configuration of this embodiment prevents the leakage current from passing through the planar coil Lp22, thereby shortening the current path of the leakage current flowing through the tertiary coil Ls2 and suppressing an increase in the leakage electromagnetic field caused by the leakage current passing through the stray capacitance Cs21. That is, the contactless power supply system 1000 including the power transmission device 100H having the power transmission circuit 120H shown in FIG. 16 also provides the same effects as those of the first embodiment.

[0065] In the power transmission circuit 120I shown in Fig. 17, an end of the planar coil Lp21, which is the closest coil, is connected to the power system PS between the power system PS and the primary coil Ls1. A schematic diagram of such a connection configuration results in a circuit configuration similar to that shown in Fig. 13 above. In this way, the contactless power transfer system 1000 including the power transmission device 100I having the power transmission circuit 120I shown in Fig. 17 also achieves the same effects as those of the second embodiment.

[0066] 19 and 20, the planar coil Lp12 is arranged radially inward of the planar coil Lp11, and the planar coil Lp22 is arranged radially inward of the planar coil Lp21. In this positional relationship, the planar coil Lp21 corresponds to the closest coil, and the stray capacitance Cs21 has the largest electrostatic capacitance.

[0067] In the power transmission circuit 120J shown in Fig. 19, an end of the planar coil Lp21, which is the closest coil, is connected to the power system PS between the first impedance variable element 20 and the power system PS. A schematic diagram of such a connection configuration results in the same circuit configuration as that shown in Fig. 18 above. In this way, the contactless power transfer system 1000 including the power transmission device 100J having the power transmission circuit 120J shown in Fig. 19 also achieves the same effects as those of the first embodiment.

[0068] In the power transmission circuit 120K shown in Fig. 20, an end of the planar coil Lp21, which is the closest coil, is connected to the power system PS between the power system PS and the primary coil Ls1. A schematic diagram of such a connection configuration results in a circuit configuration similar to that shown in Fig. 13 above. In this way, the contactless power transfer system 1000 including the power transmission device 100K having the power transmission circuit 120K shown in Fig. 20 also achieves the same effects as those of the second embodiment.

[0069] 21 and 22, the planar coil Lp12 is disposed above the planar coil Lp11 in the stacking direction. In this positional relationship, the planar coil Lp22 corresponds to the closest coil, and the stray capacitance Cs22 has the largest electrostatic capacitance.

[0070] In the power transmitting circuit 120L shown in FIG. 21 , an end of the planar coil Lp22, which is the closest coil, is connected to the power grid PS between the first impedance variable element 20 and the power grid PS. The effects achieved by this connection configuration will be described with reference to FIG. 23 , which schematically illustrates the connection configuration shown in FIG. 21 . By connecting the end of the planar coil Lp22 to the power grid PS, as shown in FIG. 23 , the current path of the leakage current passing through the tertiary coil Ls2 is the power grid PS - the planar coil Lp12 - the stray capacitance Cs22 - the planar coil Lp22 - the power grid PS. In this way, according to the connection configuration of this embodiment, the leakage current via the stray capacitance Cs22 is prevented from passing through the planar coil Lp21, thereby shortening the current path of the leakage current flowing through the tertiary coil Ls2 and suppressing an increase in the leakage electromagnetic field due to the leakage current. In this way, the contactless power supply system 1000 including the power transmission device 100L having the power transmission circuit 120L shown in FIG. 21 also provides the same effects as those of the first embodiment.

[0071] In the power transmission circuit 120M shown in Fig. 22, an end of the planar coil Lp22, which is the closest coil, is connected to the power system PS between the power system PS and the primary coil Ls1. A schematic diagram of such a connection configuration results in a circuit configuration similar to that shown in Fig. 7 above. In this way, the contactless power transfer system 1000 including the power transmission device 100M having the power transmission circuit 120M shown in Fig. 22 also achieves the same effects as those of the second embodiment.

[0072] 24 and 25, the planar coil Lp12 is disposed radially inward of the planar coil Lp11. In this positional relationship, the planar coil Lp22 corresponds to the closest coil, and the stray capacitance Cs22 has the largest electrostatic capacitance.

[0073] In the power transmission circuit 120N shown in Fig. 24, an end of the planar coil Lp22, which is the closest coil, is connected to the power system PS between the first impedance variable element 20 and the power system PS. A schematic representation of such a connection configuration results in a circuit configuration similar to that shown in Fig. 23 above. In this way, the contactless power transfer system 1000 including the power transmission device 100N having the power transmission circuit 120N shown in Fig. 24 also achieves the same effects as those of the first embodiment.

[0074] In the power transmission circuit 120O shown in Fig. 25, an end of the planar coil Lp22, which is the closest coil, is connected to the power system PS between the power system PS and the primary coil Ls1. A schematic diagram of such a connection configuration results in a circuit configuration similar to that shown in Fig. 7 above. In this way, the contactless power transfer system 1000 including the power transmission device 100O having the power transmission circuit 120O shown in Fig. 25 also achieves the same effects as those of the second embodiment.

[0075] (C3) In the above-described configuration in which the primary coil Ls1 and the tertiary coil Ls2 are arranged overlapping each other in the stacking direction, the tertiary coil Ls2 is arranged above the primary coil Ls1 in the stacking direction, but the present disclosure is not limited to this. The tertiary coil Ls2 may be arranged below the primary coil Ls1 in the stacking direction. Even in this configuration, connecting the end of the closest coil to the power system PS between the first impedance variable element 20 and the power system PS provides the same effect as the first embodiment. Furthermore, connecting the end of the closest coil to the power system PS between the primary coil Ls1 and the power system PS provides the same effect as the second embodiment.

[0076] (C4) In the above-described embodiment in which the primary coil Ls1 and the tertiary coil Ls2 are arranged on the same layer, the tertiary coil Ls2 is arranged in the space radially inward of the primary coil Ls1, but the present disclosure is not limited to this. The primary coil Ls1 may be arranged in the space radially inward of the tertiary coil Ls2. Even in this embodiment, connecting the end of the closest coil to the power system PS between the first impedance variable element 20 and the power system PS provides the same effect as in the first embodiment. Furthermore, connecting the end of the closest coil to the power system PS between the primary coil Ls1 and the power system PS provides the same effect as in the second embodiment.

[0077] (C5) In the above embodiment, the primary coil Ls1 and the tertiary coil Ls2 are formed by conductive patterns formed on a printed circuit board, but the present disclosure is not limited to this. The primary coil Ls1 and the tertiary coil Ls2 may be formed by Litz wire. In such a configuration, the air gap between each winding layer of the wound Litz wire corresponds to the "separating layer" in the present disclosure. Note that each winding layer may be separated by a resin or the like, and in such a configuration, the resin layer between each winding layer corresponds to the separating layer.

[0078] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in each embodiment corresponding to the technical features in the form described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. (Mode 1) A power transmission circuit (120-120O) for contactlessly supplying power to a power receiving device (200), comprising: a first resonant circuit (10) having a first coil (Ls1) connected to a power supply device (110) that supplies AC power at a predetermined operating frequency, and a first impedance variable element (20) connected in series with the first coil between the first coil and the power supply device, for switching a state of the first resonant circuit between a resonant state and a non-resonant state; and a second resonant circuit (12) having a second coil (Ls2) magnetically coupled to the first coil, and a second impedance variable element (22) connected in parallel with the second coil, for switching a state of the second resonant circuit between a resonant state and a non-resonant state, wherein either one of both ends of the second coil is connected to the power supply device. (Mode 2) A power transmission circuit according to Mode 1, wherein the first coil and the second coil are each formed by a plurality of planar coils (Lp11, Lp12, Lp21, Lp22) overlapping in a stacking direction, one of the first coil and the second coil is disposed at a distance from the other coil in the stacking direction, and an end of a closest coil, which is the planar coil of the plurality of planar coils forming the second coil and has the smallest distance between the planar coil and the first coil in the stacking direction, is connected to the power supply device between the power supply device and the first impedance variable element.(Mode 3) The power transmitting circuit according to Mode 1, wherein the first coil and the second coil are each formed so that a plurality of planar coils overlap in a stacking direction, one of the first coil and the second coil is disposed at a distance from the other coil in the stacking direction, and an end of a closest coil, which is the planar coil among the plurality of planar coils forming the second coil and has a smallest distance between the planar coil and the first coil along the stacking direction, is connected to the power supply device between the power supply device and the first coil. (Mode 4) The power transmitting circuit according to Mode 2 or Mode 3, wherein a distance (d3) between the closest coil and the first coil along the stacking direction is greater than a smaller distance (d1) between the plurality of planar coils forming the first coil along the stacking direction and a distance (d2) between the plurality of planar coils forming the second coil along the stacking direction. (Mode 5) A power transmission circuit according to Mode 1, wherein the first coil and the second coil are each formed by a plurality of planar coils wound in the same layer, one of the first coil and the second coil is disposed in an empty space radially inside the other coil in the same layer, and an end of a closest coil, which is the planar coil among the plurality of planar coils forming the second coil and has the smallest radial distance between a conductor forming the planar coil and a conductor forming the first coil, is connected to the power supply device between the power supply device and the first impedance variable element. (Mode 6) A power transmission circuit according to Mode 1, wherein the first coil and the second coil are each formed by a plurality of planar coils wound in the same layer, one of the first coil and the second coil is disposed in an empty space radially inside the other coil in the same layer, and an end of a closest coil, which is the planar coil among the plurality of planar coils forming the second coil and has the smallest radial distance between a conductor forming the planar coil and a conductor forming the first coil, is connected to the power supply device between the power supply device and the first coil.(Mode 7) A power transmission circuit according to Mode 5 or Mode 6, wherein the radial distance (d6) between the conductors forming the nearest coil and the conductors forming the first coil is greater than the smaller of the radial distance (d4) between the conductors forming the first coil and the radial distance (d5) between the conductors forming the second coil. (Mode 8) The power transmitting circuit according to any one of modes 1 to 7, wherein the first variable impedance element and the second variable impedance element each have a first capacitor (C11, C21), a second capacitor (C12, C22) connected in parallel to the first capacitor, and a switch element (SW1, SW2), the second capacitor and switch element being connected in series to each other, the first variable impedance element switches a state of a first resonant circuit between a resonant state and a non-resonant state depending on a positional relationship with the power receiving device, and the second variable impedance element switches a state of a second resonant circuit between a resonant state and a non-resonant state depending on a positional relationship with the power receiving device. (Mode 9) The power transmitting circuit according to any one of modes 1 to 7, wherein the first coil and the second coil are separated by a separation layer. (Mode 10) The power transmitting circuit according to any one of modes 1 to 7, wherein the first coil and the second coil are formed of litz wire. (Mode 11) A power transmission circuit according to any one of Modes 1 to 7, wherein the first coil and the second coil are formed by a conductive pattern formed on a printed circuit board. (Mode 12) A power transmission device (100 to 100O) that supplies power to a power receiving device in a wireless manner, comprising: the power transmission circuit according to Mode 1; and a power supply device that supplies AC power at a predetermined operating frequency to the power transmission circuit. (Mode 13) A power receiving device that is supplied with power in a wireless manner from the power transmission device according to Mode 12. (Mode 14) A contactless power supply system (1000) comprising: the power transmission device according to Mode 12; and the power receiving device according to Mode 13.

Claims

1. A power transmission circuit (120-120O) for contactlessly supplying power to a power receiving device (200), comprising: a first resonant circuit (10) having a first coil (Ls1) connected to a power supply device (110) that supplies AC power at a predetermined operating frequency; and a first impedance variable element (20) connected in series with the first coil between the first coil and the power supply device, for switching a state of the first resonant circuit between a resonant state and a non-resonant state; and a second resonant circuit (12) having a second coil (Ls2) magnetically coupled to the first coil; and a second impedance variable element (22) connected in parallel with the second coil, for switching a state of the second resonant circuit between a resonant state and a non-resonant state, wherein either one of both ends of the second coil is connected to the power supply device.

2. A power transmission circuit as described in claim 1, wherein the first coil and the second coil are each formed so that a plurality of planar coils (Lp11, Lp12, Lp21, Lp22) are overlapped in a stacking direction, one of the first coil and the second coil is arranged at a distance from the other coil in the stacking direction, and an end of a closest coil, which is the planar coil among the plurality of planar coils forming the second coil and has the smallest distance between the planar coil and the first coil in the stacking direction, is connected to the power supply device between the power supply device and the first impedance variable element.

3. A power transmission circuit as described in claim 1, wherein the first coil and the second coil are each formed so that a plurality of planar coils are overlapped in a stacking direction, one of the first coil and the second coil is disposed at a distance from the other coil in the stacking direction, and an end of a closest coil, which is the planar coil among the plurality of planar coils forming the second coil and has the smallest distance between the planar coil and the first coil in the stacking direction, is connected to the power supply device between the power supply device and the first coil.

4. A power transmission circuit as described in claim 2 or claim 3, wherein a distance (d3) between the nearest coil and the first coil along the stacking direction is greater than the smaller of a distance (d1) between the multiple planar coils forming the first coil along the stacking direction and a distance (d2) between the multiple planar coils forming the second coil along the stacking direction.

5. A power transmission circuit as described in claim 1, wherein the first coil and the second coil are each formed of a plurality of planar coils wound in the same layer, one of the first coil and the second coil is arranged in an empty space radially inside the other coil in the same layer, and an end of a closest coil, which is the planar coil among the plurality of planar coils forming the second coil and has the smallest radial distance between a conductor forming the planar coil and a conductor forming the first coil, is connected to the power supply device between the power supply device and the first impedance variable element.

6. A power transmission circuit as described in claim 1, wherein the first coil and the second coil are each formed of a plurality of planar coils wound in the same layer, one of the first coil and the second coil is disposed in an empty space radially inside the other coil in the same layer, and an end of a closest coil, which is the planar coil among the plurality of planar coils forming the second coil and has the smallest radial distance between a conductor forming the planar coil and a conductor forming the first coil, is connected to the power supply device between the power supply device and the first coil.

7. A power transmission circuit as described in claim 5 or claim 6, wherein the radial distance (d6) between the conductors forming the nearest coil and the conductors forming the first coil is greater than the smaller of the radial distance (d4) between the conductors forming the first coil and the radial distance (d5) between the conductors forming the second coil.

8. A power transmission circuit as claimed in any one of claims 1 to 3, 5 and 6, wherein the first impedance variable element and the second impedance variable element each have a first capacitor (C11, C21), a second capacitor (C12, C22) connected in parallel to the first capacitor, and a switch element (SW1, SW2), the second capacitor and switch element being connected in series with each other, the first impedance variable element switches the state of a first resonant circuit between a resonant state and a non-resonant state depending on its positional relationship with the power receiving device, and the second impedance variable element switches the state of a second resonant circuit between a resonant state and a non-resonant state depending on its positional relationship with the power receiving device.

9. A power transmission circuit according to any one of claims 1 to 3, 5 and 6, wherein the first coil and the second coil are separated by a separation layer.

10. A power transmission circuit according to any one of claims 1 to 3, 5 and 6, wherein the first coil and the second coil are formed from a Litz wire.

11. A power transmission circuit as claimed in any one of claims 1 to 3, 5 and 6, wherein the first coil and the second coil are formed by a conductive pattern formed on a printed circuit board.

12. A power transmission device (100-100O) that supplies power to a power receiving device in a non-contact manner, comprising: the power transmission circuit according to claim 1; and a power supply device that supplies AC power at a predetermined operating frequency to the power transmission circuit.

13. A power receiving device, comprising: a power transmitting device according to claim 12, the power receiving device being supplied with power in a wireless manner.

14. A contactless power supply system (1000), comprising: a power transmitting device according to claim 12; and a power receiving device according to claim 13.

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