Non-contact power supply system, control method for same, power transmission device, and power reception device

Abstract

The present invention increases power efficiency by reducing loss of a power reception device in accordance with the charging mode of a secondary battery connected to the power reception device. This non-contact power supply system includes a first converter, a first resonant circuit, a second resonant circuit electromagnetically coupled with the first resonant circuit, and a second converter. The non-contact power supply system comprises: a switching transformer circuit that is inserted between the second resonance circuit and the second converter and performs impedance conversion so that an impedance conversion ratio changes by selecting any of a plurality of transformers or selectively switching whether or not to insert a transformer; and a control circuit which performs control to selectively switch any of the plurality of transformers so as to increase transmission efficiency when power is transmitted from the first converter to a rectification circuit on the basis of a charging state or a charging mode of the secondary battery which is a load device connected to at a stage subsequent to the second converter.

Description

Contactless power supply system, control method thereof, power transmitting device, and power receiving device

[0001] The present invention relates to a contactless power supply system including, for example, a power transmitting device and a power receiving device, a control method for the contactless power supply system, a power transmitting device for the contactless power supply system, and a power receiving device for the contactless power supply system.

[0002] Conventionally, mobile objects such as radio-controlled transport vehicles (AGVs (Automatic Guided Vehicles)) are equipped with rechargeable batteries such as lithium-ion batteries. When charging these rechargeable batteries, the AGV is moved to a charging station, and then a power receiving coil mounted on the AGV is electromagnetically coupled to a power transmitting coil of the charging station to perform contactless charging in a contactless charging system.

[0003] Contactless power transfer systems using the above-mentioned contactless power transfer technology have been widely studied for their improved convenience and safety. In certain applications, the relative position between the power transmitting and receiving coils may fluctuate, resulting in changes in magnetic properties that are one of the factors that hinder maintaining high power conversion (supply) efficiency. Specifically, there is a stopping error in the AGV, which causes misalignment in the coil arrangement, resulting in a decrease in power transfer efficiency (hereinafter referred to as "transmission efficiency"). Furthermore, fluctuations in the transmission distance and misalignment of the power transmitting and receiving coils cause impedance mismatch between the contactless power transfer coils, resulting in a decrease in transfer efficiency. To address these issues, the following two patent documents have been disclosed.

[0004] For example, Patent Document 1 discloses a wireless power transmission device that suppresses reflected power to a high-frequency power source by quickly switching the turns ratio of a variable transformer for impedance conversion. In this wireless power transmission device, a power transmitter that transmits power to a receiver using magnetic resonance includes a variable transformer 22 for impedance conversion between a high-frequency power source and a power transmission LC resonator. The turns ratio of the variable transformer is switched by five AC switch circuits using MOSFETs. The control unit calculates the impedance at the output end of the variable transformer from the high-frequency voltage, high-frequency current, and phase difference detected by the RF detection unit, determines which AC switch circuit to turn on based on the calculated value and the five turns ratios, and controls only that AC switch circuit to be turned on by a switch drive unit.

[0005] Furthermore, for example, Patent Document 2 discloses an electromagnetic inductive power transmission device. This electromagnetic inductive power transmission device includes a power source having an active state and an inactive state and configured to switch between the active state and the inactive state with a selectable duty cycle, and a power transmitting inductor coupled to the power source. Here, the electromagnetic inductive power transmission device includes an inductive power transmitter, a power receiving inductor disposed in inductive proximity to the power transmitting inductor, a programmable load, and an impedance controller coupled to the power receiving inductor. The impedance controller is configured to increase or decrease the electrical impedance of the inductive power receiver in response to changes in power demand of the programmable load.

[0006] Patent No. 6049605 Patent No. 6499185

[0007] Yuyu Geng, et al., "A High Efficiency Charging Strategy for a Supercapacitor Using a Wireless Power Transfer System Based on Inductor / Capacitor / Capacitor (LCC) Compensation Topology", [online], October 2017, Energies, Vol. 10, No. 135, (2017), [Retrieved February 20, 2024], Internet, <URL: http: / / www.mdpi.com / 1996-1072 / 10 / 1 / 135.

[0008] However, in Patent Document 1, the impedance matching circuit is located in the power transmitting device, which makes it impossible to reduce the loss in the power receiving device. Furthermore, in Patent Document 2, a DC / DC converter is added to the power receiving device to perform impedance conversion in order to reduce the loss in the power receiving device, but adding a DC / DC converter significantly increases the number of components, which results in an increase in the size and cost of the device. Furthermore, there is a problem that the loss in the power receiving device may decrease depending on the charging mode of a secondary battery, such as a battery, connected to the power receiving device.

[0009] The object of the present disclosure is to solve the above problems and to provide a contactless power supply system, a control method therefor, a power transmitting device, and a power receiving device, which, compared to conventional technology, can reduce losses in the power receiving device and increase power efficiency depending on the charging mode of a secondary battery, such as a battery, connected to the power receiving device, and which has a simple circuit configuration and can be made compact.

[0010] A contactless power supply system according to one aspect of the present disclosure is a contactless power supply system including a power transmitting device and a power receiving device, wherein the power transmitting device includes: a first converter that converts DC power into AC power; and a first resonant circuit including a first inductor and a first capacitor, the first resonant circuit receiving AC power converted by the first converter; the power receiving device includes: a second resonant circuit including a second inductor electromagnetically coupled to the first inductor and a second capacitor, the second resonant circuit outputting AC power; and a second converter that converts AC power from the second resonant circuit into DC power; the power receiving device further includes: a switching transformer circuit inserted between the second resonant circuit and the second converter, the switching transformer circuit selecting one of a plurality of transformers or selectively switching whether or not to insert a transformer to perform impedance conversion so that an impedance conversion ratio is changed; and the contactless power supply system further includes: A control circuit is provided that controls the switching transformer circuit to select one of a plurality of transformers or selectively switch whether or not to insert a transformer so as to increase the transmission efficiency when power is transmitted from the first converter to the second converter, based on the charging state or charging mode of a secondary battery that is a load device connected downstream of the second converter.

[0011] Therefore, according to a contactless power supply system according to one aspect of the present disclosure, compared to conventional techniques, it is possible to reduce losses in the power receiving device and increase power efficiency, and furthermore, the circuit configuration is simple and can be made smaller.

[0012] 1B is a block diagram showing an example of a basic configuration of a contactless power supply system for deriving a formula for calculating the power transfer efficiency between power transmitting and receiving coils. FIG. 1C is a block diagram showing an example of a configuration of a contactless power supply system when a DC / DC converter 23 and a load device 24 are equivalently converted into one equivalent load resistance Req1 in the contactless power supply system of FIG. 1A. FIG. 1D is a block diagram showing an example of a configuration of a contactless power supply system when a rectifier circuit 22 and an equivalent load resistance Req1 are equivalently converted into one equivalent load resistance Req2 in the contactless power supply system of FIG. 1C. FIG. 1E is a circuit diagram showing an example of a circuit configuration of an LCCS type resonant circuit disclosed in Non-Patent Document 1. 9A is a graph showing the transmission efficiency versus equivalent resistance, illustrating the optimal load resistance in the resonant circuit of FIG. 4A. It is a graph showing the transmission efficiency versus equivalent load resistance when position condition 1 (where the mutual inductance M is large) in which the distance between the power transmitting and receiving coils of the contactless power transfer system is close and position condition 2 (where the mutual inductance M is small) in which the distance is far. It is a circuit diagram showing an example of the configuration of an LCCS resonant circuit for calculating the voltage gain of the LCCS resonant circuit. It is a graph showing the transmission efficiency versus mutual inductance (voltage gain) when the transformer is switched in the LCCS resonant circuit of FIG. 6. It is a block diagram showing an example of the configuration of a contactless power transfer system according to the first embodiment. It is a circuit diagram showing an example of the configuration of the inverter 13 of FIG. 8. It is a timing chart showing a method of controlling the switch elements Q1 to Q4 of FIG. 9A and the inverter output voltage Voutinv. It is a circuit diagram showing an example of the configuration of a switching transformer circuit 30A for the contactless power transfer system of FIG. 8. It is a circuit diagram showing an example of the configuration of a switching transformer circuit 30B for the contactless power transfer system of FIG. 8. It is a circuit diagram showing an example of the configuration of a switching transformer circuit 30C for the contactless power transfer system of FIG. 8. 9 is a circuit diagram showing a configuration example of a switching transformer circuit 30D for the contactless power supply system of FIG. 8.12A . 12B is a flowchart showing a power receiving process executed by the control circuit 20 of the power receiving device 200 of FIG. 8 . 12C is a flowchart showing a power transmission process executed by the control circuit 10 of the power transmitting device 100 of FIG. 8 . 12D is a circuit diagram showing a configuration example of a switching transformer circuit 30AA according to Modification 1. 12E is a circuit diagram showing a configuration example of a switching transformer circuit 30BA according to Modification 2. 12F is a graph showing transmission efficiency versus mutual inductance in the switching transformer circuit 30AA of FIG. 12A . 12G is a flowchart showing a power receiving process executed by a power receiving device 200B of a contactless power supply system including the switching transformer circuit 30AA of FIG. 12A . 12H is a block diagram showing a configuration example of a contactless power supply system according to Embodiment 2. 12H is a graph showing time characteristics (charging profile) of voltage and current when performing CC charging and then CV charging on a load device in a contactless power supply system according to a conventional technique. 12I is a graph showing a charging profile of a load device in the charging method of FIG. 16 . 19 is a graph showing the transmission efficiency versus equivalent load resistance when the transformer used in the switching transformer circuit 30 is not switched (hereinafter referred to as CCCV switching) during transition from CC charging to CV charging in a contactless power supply system according to a conventional technique. FIG. 19 is a graph showing a charging profile of a load device when CCCV switching is performed in a contactless power supply system according to a third embodiment. FIG. 19 is a graph showing the transmission efficiency versus equivalent load resistance when the CCCV switching of FIG. 19 is performed. FIG. 19 is a flowchart showing a power receiving process executed by a control circuit 20 of a power receiving device 200C in a contactless power supply system according to a third embodiment. FIG. 19 is a flowchart showing a power transmission process executed by a control circuit 10 of a power transmitting device 100C in a contactless power supply system according to a third embodiment. FIG. 19 is a flowchart showing a power receiving process executed by a control circuit 20 of a power receiving device 200D in a contactless power supply system according to a fourth embodiment. FIG. 19 is a flowchart showing a power transmission process executed by a control circuit 10 of a power transmitting device 100D in a contactless power supply system according to a fourth embodiment. FIG. 19 is a circuit diagram showing a configuration example of a switching transformer circuit 30AC according to a third modification. 10 is a circuit diagram showing a configuration example of a switching transformer circuit 30BC according to Modification 4. FIG. 11 is a circuit diagram showing a configuration example of a switching transformer circuit 30CC according to Modification 5. FIG. 12 is a circuit diagram showing a configuration example of a switching transformer circuit 30DC according to Modification 6.Fig. 10 is a circuit diagram showing an example of a configuration of a switching transformer circuit 30AD according to Modification 7. Fig. 11 is a circuit diagram showing an example of a configuration of a switching transformer circuit 30BD according to Modification 8. Fig. 12 is a circuit diagram showing an example of a configuration of a switching transformer circuit 30DD according to Modification 9. Fig. 13 is a circuit diagram showing an example of a configuration of a switching transformer circuit 30ADC according to Modification 10. Fig. 14 is a circuit diagram showing an example of a configuration of a switching transformer circuit 30BDC according to Modification 11. Fig. 15 is a circuit diagram showing an example of a configuration of a switching transformer circuit 30DDC according to Modification 12.

[0013] Hereinafter, embodiments and modifications of the present invention will be described with reference to the drawings, in which the same or similar components are designated by the same reference numerals.

[0014] (Inventor's Findings 1) In order to understand the relationship between load resistance and transmission efficiency in a contactless power transfer system, the inventors will now explain a formula for calculating the power transmission efficiency between the power transmitting and receiving coils.

[0015] Fig. 1A is a block diagram showing an example of a basic configuration of a contactless power transfer system 300 for deriving a formula for calculating the power transfer efficiency between power transmitting and receiving coils. Fig. 1B is a block diagram showing an example of a configuration of a contactless power transfer system when the DC / DC converter 23 and the load device 24 in the contactless power transfer system 300 of Fig. 1A are equivalently converted into one equivalent load resistance Req1. Fig. 1C is a block diagram showing an example of a configuration of a contactless power transfer system when the rectifier circuit 22 and the equivalent load resistance Req1 in the contactless power transfer system 300 of Fig. 1B are equivalently converted into one equivalent load resistance Req2.

[0016] 1A, the power transmitting device 100 includes an AC power source 11, an AC / DC converter 12, an inverter 13 which is a DC / AC converter, and an LCC-type resonant circuit 14 which includes a power transmitting coil L1. The power receiving device 200 includes an LC resonant circuit 21 which includes a power receiving coil L2, a rectifier circuit 22 which is an AC / DC converter, a DC / DC converter 23, and a load device 24 which includes a secondary battery such as a battery.

[0017] 1A, the transmission efficiency between the power transmitting and receiving coils refers to the transmission efficiency of power transmission from the power transmitting device 100 to the power receiving device 200. To calculate this transmission efficiency, the impedance when the load device 24 is viewed from the capacitor C2 of the power receiving device 200 is calculated. First, as shown in FIG. 1B, the DC / DC converter 23 and the load device 24 are converted into one equivalent load resistance Req1. The equivalent load resistance Req1 at this time is expressed by the following equation.

[0018] (1)

[0019] Here, R battery is the load resistance value of the load device 24, and D is the step-down conversion ratio of the DC / DC converter 23.

[0020] 1C, the rectifier circuit 22 and the equivalent load resistance Req1 are converted into one equivalent load resistance Req2. The equivalent load resistance Req2 at this time is expressed by the following equation.

[0021] (2)

[0022] Here, (8 / π ２ ) is the conversion ratio of the rectifier circuit 22. Therefore, as shown in FIG. 1C, the equivalent load resistance Req2 when looking at the load device 24 from the capacitor C2 of the resonant circuit 21 of the power receiving device 200 is expressed by the above formula.

[0023] Next, in the power receiving device 200, it is considered that a transformer circuit 40 for performing impedance conversion is inserted between the resonant circuit 21 and the equivalent load resistance Req2 in order to match the impedance of the resonant circuit 21 with that of the load device 24.

[0024] Fig. 2 is a block diagram of the contactless power supply system 300 when a transformer circuit 40 that performs impedance conversion is inserted between the resonant circuit 21 and the equivalent load resistance Req2 in the contactless power supply system 300 of Fig. 1C. Fig. 3A is a circuit diagram when the transformer circuit 40 of Fig. 2 has a secondary side impedance Z, and Fig. 3B is a circuit diagram when the secondary side impedance Z is converted to the primary side in the transformer circuit 40 of Fig. 2.

[0025] By inserting the transformer circuit 50 in the preceding stage of the load device 24, as is clear from FIGS. 3A and 3B, the equivalent load resistance Req3 can be changed to a value obtained by multiplying the equivalent load resistance Re2 by the square of the turns ratio N (the voltage or current conversion ratio of the transformer TR).

[0026] (3)

[0027] As described above, when the transformer circuit 50 is inserted, the impedance becomes the equivalent load resistance Req3. Here, it is considered that the transformer circuit 50 can transmit power with high efficiency by changing the impedance according to the circuit state of the load device 24.

[0028] Fig. 4A is a circuit diagram showing an example of the circuit configuration of an LCCS type resonant circuit disclosed in Non-Patent Document 1. Fig. 4B is a graph showing the transmission efficiency versus equivalent resistance, indicating the optimum load resistance in the resonant circuit of Fig. 4A.

[0029] The impedance after the capacitor C2 of the resonant circuit 21 of the power receiving device 200 is set as an equivalent load resistance, and the formula for the transmission efficiency can be obtained from the loss calculation of the resonant circuit 21 as follows: Here, the transmission efficiency η in the LCCS type resonant circuit of FIG. 4A is expressed by the following formula (see Non-Patent Document 1):

[0030] (4)

[0031] Here, the transmission efficiency η is differentiated by the equivalent load resistance Req, and the function value is set to zero.

[0032] (5)

[0033] In this case, the optimum equivalent load resistance Ropt (at the time of maximum efficiency) is expressed by the following equation.

[0034] (6)

[0035] Here, the self-resistance value R Ｐ , R Ｓ , R Ｌｒ By further reducing the equation, the above equation can be simplified to:

[0036] (7)

[0037] From the above equation (6), it can be seen that the transmission efficiency η depends on the mutual inductance M and the load resistance (equivalent load resistance) of the load device 24. Furthermore, as is clear from FIG. 4B , it can be seen that there is an optimal load point where the transmission efficiency is maximized. Here, since the constants other than the mutual inductance M are basically fixed, the equivalent load resistance Req at this optimal load point varies depending on the mutual inductance M. In other words, this means that the optimal load resistance value changes depending on the positional conditions between the transmitting coil and the receiving coil.

[0038] 5 is a graph showing the transmission efficiency versus equivalent load resistance under positional condition 1 (mutual inductance M is greater than a predetermined value) in which the distance between the power transmitting and receiving coils of the contactless power transfer system 300 is close, and positional condition 2 (mutual inductance M is smaller than a predetermined value) in which the distance is far. As is clear from the graph of FIG. 5 , the optimal load point depends on the mutual inductance M and the equivalent load resistance Req, and therefore the optimal load point changes depending on the positional condition between the power transmitting and receiving coils. For example, assuming that the resistance value of the equivalent load resistance Req2 obtained by equivalent load conversion is 20 Ω, the transmission efficiency is high under positional condition 1, but is significantly reduced under positional condition 2.

[0039] To avoid this decrease in transmission efficiency, the inventors use a switching transformer circuit. By using the switching transformer circuit, the equivalent load resistance Req2 can be changed to equivalent load resistance Req3-1 and equivalent load resistance Req3-2. Here, the resistance value after impedance conversion by the switching transformer circuit is represented as Req3, and for multiple switching cases using the switching transformer circuit, a hyphen is added before the positional condition number. That is, if the only positional conditions between the transmitting and receiving coils are positional condition 1 and positional condition 2, operation with high transmission efficiency can be achieved by adjusting the load resistances Req3-1 and Req3-2 after impedance conversion using the switching transformer circuit to the optimal load point. Furthermore, in cases where there are multiple positional conditions, optimal load operation cannot be achieved at all points, but high efficiency can be achieved at load points in specific cases, or overall high efficiency can be achieved in the wireless power transfer system 300.

[0040] Next, the concept of transformer switching in the switching transformer circuit will be explained below.

[0041] FIG. 6 is a circuit diagram showing an example of the configuration of an LCCS type resonant circuit for calculating the voltage gain of the LCCS type resonant circuit, and FIG. 7 is a graph showing the transmission efficiency versus mutual inductance (voltage gain) when the transformer is switched in the LCCS type resonant circuit of FIG. 6.

[0042] 6, the LCCS resonant circuit includes an LCC resonant circuit 14A, a power transmitting coil L1, and a series resonant circuit 21A including a power receiving coil inductor L2. Here, the LCC resonant circuit 14A includes an inductor L0 and capacitors C0 and C1, and the series resonant circuit 21A includes a power receiving coil inductor L2, a capacitor C2, and a load resistor R0. In this case, the input / output voltage gain of the LCCS resonant circuit is expressed by the following equation, where Vin is the input voltage and Vout is the output voltage.

[0043] Vout=(M / L0)・Vin (8)

[0044] Here, this gain calculation is valid only under the condition that inductor L0 and capacitor C0, and inductor L2 and capacitor C2, respectively, are resonating at the operating frequency of power transmitting device 100. If the positional conditions between the transmitting and receiving coils change and the inductance L2 of the receiving coil fluctuates, the resonant state is disrupted and this voltage gain is no longer obtained.

[0045] For simplicity, the following description will be given assuming that the resonance conditions are not lost. Since the inductor L0 has a fixed value regardless of the positional conditions of the transmitting and receiving coils or the circuit conditions, the voltage gain varies depending on the mutual inductance M. By referring to this voltage gain equation (8), a voltage gain threshold is set at the optimal load point or the point where overall efficiency is high, and a transformer switching point is determined. Based on whether or not this switching point threshold is exceeded, the transformer to be used is selected so as to be one of multiple transformers, and control is exercised to switch to the selected transformer.

[0046] 7, for example, by determining a threshold value near the intersection of the transmission efficiency curves of the equivalent load resistances Req3-1 and Req3-2 of the transformers TR1 and TR2 and selectively switching the transformers, the contactless power transfer system 300 can be operated on a highly efficient operating line (solid line). Note that instead of selectively switching between multiple transformers, it is also possible to switch whether or not to insert a transformer.

[0047] Hereinafter, an embodiment based on the above-mentioned findings of the inventors will be described.

[0048] (Embodiment 1) Fig. 8 is a block diagram showing an example of the configuration of a contactless power supply system 300 according to embodiment 1. The contactless power supply system 300 in Fig. 8 is inserted between an AC power supply 11 and a load device 24, and is configured to include a power transmitting device 100 and a power receiving device 200. Here, the contactless power supply system 300 according to embodiment 1 is characterized in that a switching transformer circuit 30 that can selectively switch to one of a plurality of transformers and performs impedance conversion is inserted between a resonant circuit 21 and a rectifier circuit 22. Note that instead of selectively switching between a plurality of transformers, it may be possible to switch whether or not a transformer is inserted.

[0049] In FIG. 8 , the power transmitting device 100 includes an AC / DC converter 12 , an inverter 13 , a resonant circuit 14 , a control circuit 10 , a communication circuit 15 , a voltage detector 51 , and a current detector 52 .

[0050] The voltage detector 51 detects the voltage Vin and outputs a detection signal to the control circuit 10, and the current detector 52 detects the current Iin and outputs a detection signal to the control circuit 10. The voltage detector 51 and the current detector 52 may be omitted so that only the detectors necessary for the following control processing are provided.

[0051] The resonant circuit 14 is an LC resonant circuit including inductors L0 and L1 and capacitors C0 and C1. The inductor L1 is an inductor of the power transmitting coil 14C, and has a core such as an iron core. When the power receiving device 200 moves to a predetermined position on the power transmitting device 100, the inductor L1 is electromagnetically coupled to the inductor L2 of the power receiving coil 21C of the resonant circuit 21.

[0052] The control circuit 10 includes a memory 10m for recording and storing programs and data required for executing control processing, as well as data during calculation, and controls the operation of the AC / DC converter 12 and the inverter 13. The control circuit 10 controls the operation of the inverter 13 by generating a plurality of gate voltages of a plurality of switch elements of the inverter 13 as PWM voltages having a predetermined duty ratio d0. The control circuit 10 is also connected to a communication circuit 15, and communicates with the control circuit 20 of the power receiving device 200 via the communication circuit 15, a communication line 35, and a communication circuit 25 using a predetermined plurality of communication signals. The communication line 35 may be a wireless communication line or a wired communication line.

[0053] The power receiving device 200 also includes a resonant circuit 21 , a switching transformer circuit 30 , a rectifier circuit 22 , a DC / DC converter 23 , a control circuit 20 , a communication circuit 25 , a voltage detector 61 , and a current detector 62 .

[0054] The voltage detector 61 detects the output voltage Vout and outputs a detection signal to the control circuit 20, and the current detector 62 detects the output current Iout and outputs a detection signal to the control circuit 20. Note that the voltage detector 61 and the current detector 62 may be omitted so that only the detectors necessary for the following control processing are provided.

[0055] The resonant circuit 21 includes an inductor L2 and a capacitor C2. The inductor L2 is an inductor of the power receiving coil 21C, and has a core such as an iron core. When the power receiving device 200 moves to a predetermined position in the power transmitting device 100, the inductor L2 is electromagnetically coupled to the inductor L1 of the resonant circuit 14 of the power transmitting coil 14C.

[0056] The switching transformer circuit 30 has terminals T1 and T2 on the side of the resonant circuit 21 and terminals T3 and T4 on the side of the rectifier circuit 22. As will be described in detail later with reference to Figures 10A to 10D, the switching transformer circuit 30 has a plurality of transformers, and can selectively switch to one of the plurality of transformers based on gate control signals VG11 to VG14 from the control circuit 20 to perform impedance conversion.

[0057] The control circuit 20 includes a memory 20m for recording and storing programs and data required for executing control processing and data during calculation, and controls the operations of the switching transformer circuit 30, the rectifier circuit 22, and the DC / DC converter 23. The control circuit 20 is connected to a communication circuit 25, and communicates with the control circuit 10 of the power transmitting device 100 via the communication circuit 25, a communication line 35, and the communication circuit 15 using a predetermined number of communication signals.

[0058] The control circuits 10 and 20 cooperate with each other using the above-mentioned communication line 35, and execute the power receiving process of Fig. 11A, the power transmitting process of Fig. 11B, and the power receiving process of Fig. 14 described later, thereby selectively selecting and switching one of the multiple transformers in the switching transformer circuit 30, thereby controlling to increase the transmission efficiency in the contactless power supply system 300. Here, the control circuits 10 and 20 constitute a drive control device for the composite resonant circuit that drives and controls the composite resonant circuit including two resonant circuits 14 and 21 electromagnetically coupled to each other.

[0059] In the wireless power supply system 300 of FIG. 8 configured as described above, the AC voltage (AC power) from the AC power source 11 is converted into a predetermined DC voltage (DC power) by the AC / DC converter 12 and then input to the inverter 13 via the voltage detector 51 and the current detector 52. The inverter 13 switches the input DC voltage (DC power) in accordance with gate voltages VG1 to VG4 from the control circuit 10 to generate an AC voltage (AC power) with a rectangular waveform having a predetermined duty ratio d0, and outputs the AC voltage (AC power) to the rectifier circuit 22 via the resonant circuits 14 and 21 and the switching transformer circuit 30. The rectifier circuit 22 rectifies and smoothes the input AC voltage (AC power) to convert it into a predetermined DC voltage (DC power), and then outputs the DC voltage (DC power) to the DC / DC converter 23. The DC / DC converter 23 converts the input DC voltage (DC power) into a predetermined DC voltage (DC power), and then outputs the DC voltage (DC power) to the load device 24 via the voltage detector 61 and the current detector 62.

[0060] Fig. 9A is a circuit diagram showing an example of the configuration of the inverter 13 in Fig. 8, and Fig. 9B is a timing chart showing a method for controlling the switch elements Q1 to Q4 in Fig. 9A and the inverter output voltage Voutinv. In Fig. 9B, M1 to M4 indicate the respective switching modes.

[0061] 9A, inverter 13 includes switch elements Q1 to Q4, each of which is made up of, for example, four MOS field-effect transistors connected in a bridge configuration, and an input capacitor Cin. Note that input capacitor Cin may also be provided in the preceding circuit. Here, switch elements Q1 to Q4 are on / off controlled by gate control voltages VG1 to VG4 from control circuit 10.

[0062] In this embodiment, the control of the output characteristics using the inverter 13 uses, for example, frequency modulation control and phase shift control, as shown in FIG. 9B . Here, frequency modulation control controls the output by changing the switching frequency and changing the characteristics of the resonant circuit. Furthermore, phase shift control controls the output by applying a phase difference θ between the leg of the switching elements Q1 and Q2 and the leg of the switching elements Q3 and Q4. Here, the output voltage Voutinv=Vac is expressed by the following equation:

[0063] (9)

[0064] Therefore, when phase shift control is performed, the output voltage Vout generally has an uneven waveform. As shown in the above equation (9), phase shift control controls the input voltage Vin with a duty ratio d.

[0065] In this embodiment, the output voltage and output current of the controlled object are controlled using either or both of an AC / DC converter 12 in the upstream stage of the inverter 13 and a DC / DC converter 23 in the downstream stage of the inverter 13. Here, the AC / DC converter 12 is used when an AC power supply 11 is used, but a DC / DC converter is used when a DC power supply is used instead of the AC power supply 11. These converters 12, 23, etc. may be various converters such as a step-up / step-down converter, a step-up converter, or a step-down converter, and for example, a converter whose output characteristics can be adjusted by PWM or PFM is used.

[0066] 10A to 10D, a configuration example and operation of the switching transformer circuit 30 will be described below. In FIG. 10A to 10D, solid lines indicate the current flow in operation mode OP1, and dotted lines indicate the current flow in operation mode OP2.

[0067] Fig. 10A is a circuit diagram showing an example of the configuration of a switching transformer circuit 30A for the contactless power supply system 300 of Fig. 8. In Fig. 10A, the switching transformer circuit 30A includes, for example, four MOS field effect transistors, switch elements Q11 to Q14, and a transformer TR1A. The switch elements Q11 and Q12 form a pair of bidirectional switch elements, as do the switch elements Q13 and Q14, and so on. Here, the transformer TR1A has an inductor L of a primary winding Ｐ１ , L Ｐ２ and the inductor L of the secondary winding Ｓ１ and an inductor L Ｐ１ , L Ｐ２ is the inductor L of the secondary winding Ｓ１ and the mutual inductance M Ｐ１Ｓ , M Ｐ２Ｓ Here, the inductor L Ｐ１Inductance and inductor L ｐ２ The terminal T1 is connected to the switching elements Q1 and Q2 and the inductor L Ｐ１ and is connected to the terminal T2 via the switch elements Q1, Q2 and the inductor L Ｐ２ Terminal T3 is connected to terminal T2 via inductor L Ｓ１ The voltage between terminals T1 and T2 is V １ and the voltage between terminals T3 and T4 is V ２ and so on.

[0068] In the switching transformer circuit 30A configured as described above, (1) the switching elements Q11 and Q12 are turned on by applying, for example, H-level gate control signals VG11 and VG12 to the gates of the switching elements Q11 and Q12, respectively, and the switching elements Q13 and Q14 are turned off by applying, for example, L-level gate control signals VG13 and VG14 to the gates of the switching elements Q13 and Q14, respectively, so that a current in the operation mode OP1 flows. Ｐ１ and inductor L Ｓ１ (2) For example, by applying L-level gate control signals VG11 and VG12 to the gates of the switch elements Q11 and Q12, respectively, the switch elements Q11 and Q12 are turned off, and by applying H-level gate control signals VG13 and VG14 to the gates of the switch elements Q13 and Q14, respectively, the switch elements Q13 and Q14 are turned on, and a current in the operation mode OP2 flows. At this time, the inductor L Ｐ２ and inductor L Ｓ１ The transformer TR1A-2 to which the two are coupled is selected.

[0069] That is, by controlling the switch elements Q11, Q12 and the switch elements Q13, Q14 so that their on and off states are different from each other, it is possible to selectively switch between the transformer TR1A-1 and the transformer TR1A-2. This changes the impedance conversion ratio between the terminals T1, T2 and the terminals T3, T4 of the switching transformer circuit 30A, thereby controlling the impedance matching state in the contactless power supply system 300.

[0070] In this switching transformer circuit 30A, the input voltage V １ is expressed by the following equation:

[0071] (10)

[0072] Here, k Ｐ１Ｓ is the inductor L Ｐ１ and inductor L Ｓ１ is the voltage conversion coefficient of the transformer TR1A-1.

[0073] In addition, the input voltage V １ is expressed by the following equation:

[0074] (11)

[0075] Here, k Ｐ２Ｓ is the inductor L Ｐ２ and inductor L Ｓ１ is the voltage conversion coefficient of the transformer TR1A-2.

[0076] Fig. 10B is a circuit diagram showing a configuration example of a switching transformer circuit 30B for the contactless power supply system 300 of Fig. 8. In Fig. 10B, the switching transformer circuit 30B includes, for example, four MOS field effect transistors, switch elements Q11 to Q14, and a transformer TR1B. Here, the transformer TR1B has a primary winding inductor L Ｐ１ , L ｐ２ and the inductor L of the secondary winding Ｓ１ and an inductor L Ｐ１ , L ｐ２ is the inductor L of the secondary winding Ｓ１ and the mutual inductance M Ｐ１Ｓ, M Ｐ２Ｓ Here, the inductor L Ｐ１ Inductance and inductor L ｐ２ The inductances of the terminal T1 and the inductor L may be the same or different. Ｐ１ and is connected to the terminal T2 via the switch elements Q11 and Q12, and is connected to the inductor L Ｐ１ , L Ｐ２ and is connected to terminal T2 via switch elements Q13 and Q14. Terminal T3 is connected to inductor L Ｓ１ is connected to the terminal T4 via

[0077] In the switching transformer circuit 30B configured as above, (1) by applying, for example, H-level gate control signals VG11 and VG12 to the gates of the switching elements Q11 and Q12, respectively, the switching elements Q11 and Q12 are turned on, and by applying, for example, L-level gate control signals VG13 and VG14 to the gates of the switching elements Q13 and Q14, respectively, the switching elements Q13 and Q14 are turned off, whereby a current in the operation mode OP1 flows. Ｐ１ and inductor L Ｓ１ (2) For example, by applying L-level gate control signals VG11 and VG12 to the gates of the switch elements Q11 and Q12, respectively, the switch elements Q11 and Q12 are turned off, and by applying H-level gate control signals VG13 and VG14 to the gates of the switch elements Q13 and Q14, respectively, the switch elements Q13 and Q14 are turned on, and a current in the operation mode OP2 flows. At this time, the inductor L Ｐ１ , L Ｐ２ and inductor L Ｓ１ The transformer TR1B-2 to which the and are connected is selected.

[0078] That is, by controlling the switch elements Q11, Q12 and the switch elements Q13, Q14 so that their on and off states are different from each other, it is possible to selectively switch between the transformer TR1B-1 and the transformer TR1B-2. This changes the impedance conversion ratio between the terminals T1, T2 and the terminals T3, T4 of the switching transformer circuit 30B, thereby controlling the impedance matching state in the contactless power supply system 300. In the switching transformer circuit 30B, the switch elements Q11 to Q14 are switched to change the voltage gain of the switching transformer circuit 30B. Inductor L Ｐ１ Since the winding is always in use, the efficiency of winding usage is high and it is possible to switch impedance while suppressing winding loss.

[0079] In this switching transformer circuit 30B, the input voltage V １ is expressed by the following equation:

[0080] (12)

[0081] Here, k Ｐ１Ｓ is the inductor L Ｐ１ and inductor L Ｓ１ is the voltage conversion coefficient of the transformer TR1B-1.

[0082] In addition, the input voltage V １ is expressed by the following equation:

[0083] (13)

[0084] Here, k Ｐ１２Ｓ is the inductor L Ｐ１ , L Ｐ２ and inductor L Ｓ１ is the voltage conversion coefficient of the transformer TR1B-2.

[0085] Fig. 10C is a circuit diagram showing a configuration example of a switching transformer circuit 30C for the contactless power supply system 300 of Fig. 8. In Fig. 10C, the switching transformer circuit 30C includes, for example, two MOS field effect transistors, switch elements Q11 and Q12, and a transformer TR1C. Here, the transformer TR1C has a primary winding inductor L Ｐ１ and the inductor L of the secondary winding Ｓ１ and an inductor L Ｐ１ is the inductor L of the secondary winding Ｓ１ and the mutual inductance M ＰＳ The terminal T1 is electromagnetically coupled to the inductor L Ｐ１ and is connected to terminal T3 via switch elements Q11 and Q12. Terminal T3 is connected to inductor L Ｓ１ is connected to the terminal T4 via

[0086] In the switching transformer circuit 30C configured as described above, (1) by applying, for example, H-level gate control signals VG11 and VG12 to the gates of the switching elements Q11 and Q12, respectively, to turn off the switching elements Q11 and Q12, a current flows in the operation mode OP1. At this time, the transformer TR1C is selected. (2) by applying, for example, L-level gate control signals VG11 and VG12 to the gates of the switching elements Q11 and Q12, respectively, to turn on the switching elements Q11 and Q12, a current flows in the operation mode OP2. At this time, the transformer TR1C is not selected.

[0087] That is, by turning on or off the switch elements Q11 and Q12, it is possible to selectively select whether or not to select the transformer TR1C. This allows the impedance matching state in the contactless power supply system 300 to be controlled by changing the impedance conversion ratio between the terminals T1 and T2 and the terminals T3 and T4 of the switching transformer circuit 30C. In the switching transformer circuit 30C, the impedance is adjusted by whether or not to use the transformer TR1C. The efficiency of use of the windings of the transformer TR1C is high, and winding loss can be suppressed. Furthermore, a simple switching structure allows switching to be achieved by using only two switch elements Q11 and Q12.

[0088] In this switching transformer circuit 30C, the input voltage V １ is expressed by the following equation:

[0089] (14)

[0090] Here, k ＰＳ is the inductor L Ｐ and inductor L Ｓ The transformer TR1C, which is composed of the above, is the voltage conversion coefficient to be selected.

[0091] In addition, the input voltage V １ is expressed by the following equation:

[0092] (15)

[0093] Here, the above-mentioned equation (15) represents the voltage conversion coefficient when the transformer TR1C is not selected.

[0094] Fig. 10D is a circuit diagram showing a configuration example of a switching transformer circuit 30D for the contactless power supply system 300 of Fig. 8. In Fig. 10D, the switching transformer circuit 30D includes, for example, two MOS field effect transistors, switch elements Q11 and Q12, and a transformer TR1D. Here, the transformer TR1D has a primary winding inductor L ｐ１ , L ｐ２ and the inductor L of the secondary winding ｓ and an inductor L ｐ１ is the inductor L of the secondary winding ｓ１ and the mutual inductance M ｐ１ｓ and the inductor L ｐ２ is the inductor L of the secondary winding ｓ and the mutual inductance M ｐ２ｓ The inductor L ｐ１ and inductor L ｐ２ is the mutual inductance M ｐ１ｐ２ Here, the inductor L ｐ１ Inductance and inductor L ｐ２The inductances of the terminal T1 and the inductor L may be the same or different. ｐ１ and is connected to the terminal T2 via the switch elements Q1, Q2 and the inductor L ｐ２ Terminal T3 is connected to terminal T2 via inductor L ｓ１ is connected to the terminal T4 via

[0095] In the switching transformer circuit 30D configured as above, (1) when the gate control signals VG11 and VG12 of, for example, L level are applied to the gates of the switching elements Q11 and Q12, respectively, to turn off the switching elements Q11 and Q12, a current of the operation mode OP1 flows. ｐ１ and inductor L ｓ (2) When the gate control signals VG11 and VG12 of, for example, H level are applied to the gates of the switch elements Q11 and Q12, respectively, to turn on the switch elements Q11 and Q12, a current of the operation mode OP2 flows. At this time, the inductor L ｐ１ , L ｐ２ and inductor L Ｓ The transformer TR1D-2 to which the and are connected is selected.

[0096] That is, by controlling the switch elements Q11 and Q12 to be on or off, it is possible to selectively switch between the transformer TR1D-1 and the transformer TR1D-2. This changes the impedance conversion ratio between the terminals T1 and T2 and the terminals T3 and T4 of the switching transformer circuit 30D, thereby controlling the impedance matching state in the contactless power supply system 300. In the switching transformer circuit 30D, the two switch elements Q11 and Q12 can selectively set and adjust two impedances. The transformer TR1D can be formed as an integrated transformer structure, and is also composed of a single ferrite core (iron core).

[0097] In this switching transformer circuit 30D, the input voltage V １ is expressed by the following equation:

[0098] (16)

[0099] Here, k ＭＴ＿１ is expressed by the following equation:

[0100] (17)

[0101] In addition, the input voltage V １ is expressed by the following equation:

[0102] (18)

[0103] Here, k ＭＴ＿２ is expressed by the following equation: (19)

[0104] Next, the reason why the winding resistance can be reduced in the switching transformer circuits 30A and 30B will be explained below.

[0105] The switching transformer circuit 30A has an inductor L with different windings for the operation modes OP1 and OP2. Ｐ１ , L Ｐ２ On the other hand, in the operation mode OP2, the switching transformer circuit 30B uses the inductors L Ｐ１ , L Ｐ２ In the operation mode OP, the switching transformer circuit 30B has a higher winding utilization rate than the switching transformer circuit 30A, and the winding resistance can be reduced. This allows for a reduction in winding loss. In addition, the switching transformer circuit 30C uses the inductor L of the primary winding. Ｐ１ The switching transformer circuit 30C can reliably reduce winding loss compared to the switching transformer circuits 30A and 30B, but whether the switching transformer circuit 30B can always achieve a loss reduction effect depends on the circuit design.

[0106] The transformer switching in the switching transformer circuit 30 is performed based on, for example, an input / output voltage gain Gv=Vout / Vin (or a current gain Gi=Iout / Iin). Furthermore, since the voltage gain Gv or the current gain Iv is determined by the output voltage or output current operation command value issued by the control circuit 10 to the inverter 13 when the input voltage Vin or the input current Iin is constant, the transformer switching in the switching transformer circuit 30 may be performed based on the output voltage or output current operation command value issued by the control circuit 10 to the inverter 13. A test mode for checking the voltage gain (or current gain) may also be provided. The switching of the transformer in the switching transformer circuit 30 is not limited to the voltage gain or current gain, and may be based on a gain based on the input current and output voltage, or a gain based on the input voltage and output current.

[0107] Next, the transformer used in the test mode is the transformer TR1, and when the voltage gain Gaintest exceeds an arbitrarily set predetermined threshold value Gainth, for example, the transformer is switched to the transformer TR2. The power receiving process and power transmitting process will be described below.

[0108] FIG. 11A is a flowchart showing the power receiving process executed by the control circuit 20 of the power receiving device 200 of FIG.

[0109] 11A , it is determined whether the power receiving device 200 has stopped at a predetermined charging position, and if the determination is YES, the process proceeds to step S2. In step S2, a stop notification signal indicating that the power receiving device 200 has stopped at the charging position is transmitted to the power transmitting device 100 via the communication circuits 25 and 15, and set in the transformer TR1. Next, in step S3, an information signal including an input voltage, an input current, and an input power is received from the power transmitting device 100 via the communication circuits 15 and 25. In step S4, a voltage gain (or a current gain) is calculated based on the information signal and detection information including the detected output voltage (or the detected output current), and the calculated gain is set as Gaintest.

[0110] Next, in step S5, it is determined whether Gaintest>Gainth, and if YES, the process proceeds to step S6, whereas if NO, the process proceeds to step S9. In step S6, a power supply stop command signal is transmitted to power transmitting device 100 via communication circuits 25 and 15, and in step S7, it is determined based on the detected voltage (or detected current) whether power supply from power transmitting device 100 has stopped. If YES, in step S8, the transformer is switched to TR2, and in step S9, a main power supply start notification signal is transmitted to power transmitting device 100 via communication circuits 25 and 15.

[0111] Next, in step S10, it is determined whether charging of the load device 24 has been completed. If the result is YES, in step S11, a power supply stop notification signal is sent to the power transmission device 100 via the communication circuits 25 and 15, and the power receiving process is terminated.

[0112] FIG. 11B is a flowchart showing the power transmission process executed by the control circuit 10 of the power transmission device 100 of FIG.

[0113] 11B , it is determined whether a stop notification signal has been received from the power receiving device 200 via the communication circuits 25 and 15. In step S22, in the test mode, power is supplied to the power receiving device 200 at a predetermined voltage (or current), and an information signal including the input voltage, input current, and input power is transmitted to the power receiving device 200 via the communication circuits 15 and 26. Next, in step S23, it is determined whether a power supply stop command signal has been received from the power receiving device 200 via the communication circuits 25 and 15. If the determination is YES, the process proceeds to step S24, whereas if the determination is NO, the process proceeds to step S25. In step S24, power supply in the test mode is stopped, and in step S25, it is determined whether a main power supply start notification signal has been received from the power receiving device 200 via the communication circuits 25 and 15. If the determination is YES, the process starts main power supply to the power receiving device 200 in step S26. Furthermore, in step S27, it is determined whether or not a main power supply stop notification signal has been received from the power receiving device 200 via the communication circuits 25, 15. If the answer is YES, in step S28, the main power supply to the power receiving device 200 is stopped, and the power transmission process is terminated.

[0114] As described above, according to this embodiment, in the contactless power supply system 300, the switching transformer circuit 30 is inserted into the power receiving device 200, and one of the multiple transformers in the switching transformer circuit 30 is selected or whether or not to insert a transformer is switched so as to increase the transmission efficiency based on the voltage gain or current gain between input and output, or the operation command value of the output voltage or output current from the control circuit 10 to the inverter 13, thereby achieving impedance matching through impedance conversion and increasing the transmission efficiency in the contactless power supply system 300. Furthermore, because only the switching transformer circuit 30 is inserted, there are advantages in that the circuit configuration is simple and can be made smaller.

[0115] (Modifications 1 and 2) In the above-described first embodiment, one of two transformers is selected, but the switching transformer circuit 30 may be configured to select one of three or more transformers. This will be described below. Note that a contactless power supply system 300B according to Modifications 1 or 2 includes a power transmitting device 100B and a power receiving device 200B.

[0116] 12A is a circuit diagram showing a configuration example of a switching transformer circuit 30AA according to Modification 1. In FIG. 12A, the switching transformer circuit 30AA includes n pairs of switching elements (Q11, Q12; Q13, Q14; ...; Q(2n+9), Q(2n+10)) and a transformer TR1E. Here, the transformer TR1E has an inductor L Ｐ１ , L Ｐ２， ..., L Ｐ（２ｎ＋９） and the inductor L of the secondary winding Ｓ１ and a primary winding inductor L Ｐ１ , L Ｐ２， ..., L Ｐ（２ｎ＋９） is the inductor L of the secondary winding Ｓ１ and the mutual inductance M Ｐ１Ｓ , M Ｐ２Ｓ , ..., M Ｐ（２ｎ－１）Ｓ Here, the inductor L of the primary winding Ｐ１ , L Ｐ２， ..., L Ｐ（２ｎ＋９）The inductances of the switches Q11 and Q12 and the inductor L Ｐ１ and the terminal T1 is connected to the terminal T2 via the switch elements Q13 and Q14 and the inductor L Ｐ２ Similarly, the terminal T1 is connected to the switching elements Q(2n+9), Q(2n+10) and the inductor L Ｐｎ Terminal T3 is connected to terminal T2 via inductor L Ｓ１ is connected to the terminal T4 via

[0117] In the switching transformer circuit 30AA, by turning on any one pair of switch elements among a plurality of n pairs of switch elements (Q11, Q12; Q13, Q14; ...; Q(2n+9), Q(2n+10) and turning off the other pairs of switch elements, impedance conversion can be performed so as to change the impedance conversion ratio between the terminals T1, T2 and the terminals T3, T4.

[0118] 12B is a circuit diagram showing a configuration example of a switching transformer circuit 30BA according to Modification 2. In FIG. 12B, the switching transformer circuit 30BA includes n pairs of switching elements (Q11, Q12; Q13, Q14; ...; Q(2n+9), Q(2n+10)) and a transformer TR1E. Here, the transformer TR1E has an inductor L Ｐ１ , L Ｐ２， ..., L Ｐ（２ｎ＋９） and the inductor L of the secondary winding Ｓ１ and a primary winding inductor L Ｐ１ , L Ｐ２， ..., L Ｐ（２ｎ＋９） is the inductor L of the secondary winding Ｓ１ and the mutual inductance M Ｐ１Ｓ , M Ｐ２Ｓ , ..., M Ｐ（２ｎ－１）Ｓ Here, the inductor L of the primary winding Ｐ１ , L Ｐ２， ..., L Ｐ（２ｎ＋９） The inductances of the inductors L may be the same or different from each other. Ｐ１and is connected to terminal T2 via switch elements Q11 and Q12, and terminal T1 is connected to inductor L Ｐ１， L Ｐ２ and is connected to terminal T2 via switch elements Q13 and Q14, and similarly, terminal T1 is connected to terminal L Ｐ１ , L Ｐ２， ..., L Ｐ（２ｎ＋９） and is connected to terminal T2 via switch elements Q(2n+9) and Q(2n+10). Terminal T3 is connected to inductor L Ｓ１ is connected to the terminal T4 via

[0119] In the switching transformer circuit 30BA, by turning on any one pair of switch elements among a plurality of n pairs of switch elements (Q11, Q12; Q13, Q14; ...; Q(2n+9), Q(2n+10)) and turning off the other pairs of switch elements, impedance conversion can be performed so as to change the impedance conversion ratio between the terminals T1, T2 and the terminals T3, T4.

[0120] FIG. 13 is a graph showing the transmission efficiency versus mutual inductance in the switching transformer circuit 30AA of FIG. 12A. In the graph of FIG. 13, three characteristic curves are present, corresponding to the equivalent load resistances Req3-1 to Req3-3 of the three transformers TR1 to TR3. Two threshold voltage gains, Gain1 and Gain2 (where Gain2 > Gain1), are provided at the two switching points for switching the transformers, dividing the mutual inductance M (corresponding to the voltage gain). The voltage gain of the contactless power transfer system is compared with each threshold voltage gain, Gain1, Gain2, and control is performed to select one of the three transformers that provides greater transmission efficiency. Here, by switching the transformers so as to trace the maximum of the three transmission efficiency curves, the transmission efficiency of the contactless power transfer system can be significantly increased.

[0121] FIG. 14 is a flowchart showing a power receiving process executed by the power receiving device 200B of the contactless power feeding system 300B including the switching transformer circuit 30AA of FIG. 12A.

[0122] 14 , it is determined whether the power receiving device 200B has stopped at a predetermined charging position, and if the determination is YES, the process proceeds to step S2. In step S2, a stop notification signal indicating that the power receiving device 200B has stopped at the charging position is transmitted to the power transmitting device 100B via the communication circuits 25 and 15, and set in the transformer TR1. Next, in step S3, an information signal including an input voltage, an input current, and an input power is received from the power transmitting device 100B via the communication circuits 15 and 25. In step S4, a voltage gain (or a current gain) is calculated based on the information signal and detection information including the detected output voltage (or the detected output current), and the calculated gain is set as Gaintest.

[0123] Next, in step S5A, it is determined whether Gaintest>Gain1, and if YES, the process proceeds to step S9, whereas if NO, the process proceeds to step S5B. In step S5B, it is determined whether Gain1<Gaintes≦Gain2, and if YES, the process proceeds to step S6A, whereas if NO, the process proceeds to step S6B.

[0124] In step S6A, a power supply stop command signal is sent to the power transmission device 100B via the communication circuits 25 and 15, and in step S7A, it is determined based on the detected voltage (or detected current) whether or not the power supply from the power transmission device 100B has stopped. If the result is YES, in step S8A, switching is made to the switching transformer TR2, and then the process proceeds to step S9.

[0125] In step S6B, a power supply stop command signal is sent to the power transmission device 100B via the communication circuits 25 and 15, and in step S7B, it is determined based on the detected voltage (or detected current) whether or not the power supply from the power transmission device 100B has stopped. If the determination is YES, in step S8B, switching is made to the switching transformer TR3, and then the process proceeds to step S9.

[0126] Next, in step S9, a power supply start notification signal is sent to the power transmission device 100B via the communication circuits 25 and 15, and in step S10, it is determined whether charging of the load device 24 has been completed.If the result is YES, in step S11, a power supply stop notification signal is sent to the power transmission device 100B via the communication circuits 25 and 15, and the power receiving process is terminated.

[0127] The power transmission process by the power transmission device 100B executed in response to the power reception process is executed in the same manner as in FIG. 11B.

[0128] As described above, according to Modifications 1 and 2, in a contactless power transfer system, by inserting the switching transformer circuit 30 into the power receiving device 200B, and selecting one of the three (or more) transformers in the switching transformer circuit 30 or switching whether to insert a transformer so as to increase transmission efficiency based on the voltage gain or current gain between input and output, or the operation command value of the output voltage or output current from the control circuit 10 to the inverter 13, impedance matching through impedance conversion can be achieved, thereby increasing the transmission efficiency in the contactless power transfer system. Furthermore, because only the switching transformer circuit 30 is inserted, there is the advantage that the circuit configuration is simple and can be made smaller.

[0129] (Embodiment 2) Fig. 15 is a block diagram showing an example of the configuration of a contactless power supply system 300A according to embodiment 2. In Fig. 15, the contactless power supply system 300A according to embodiment 2 differs from the contactless power supply system 300 of Fig. 8 in the following respects: (1) The contactless power supply system 300A includes a power transmitting device 100A and a power receiving device 200A. (2) Instead of the AC power supply 11 and the AC / DC converter 12, a DC power supply 12A, which is a secondary battery such as a battery, is provided. (3) The DC / DC converter 23 between the rectifier circuit 22 and the load device 24 is eliminated. The differences will be described below.

[0130] 15 , a DC voltage (DC power) from a DC power supply 12A is input to an inverter 13 via a voltage detector 51 and a current detector 52. In addition, a DC voltage (DC power) from a rectifier circuit 22 is input to a load device 24 via a voltage detector 61 and a current detector 62.

[0131] The second embodiment configured as described above has the same effects as the first embodiment, except for the absence of the functions of the AC / DC converter 12 and the DC / DC converter 23. That is, in the contactless power supply system 300A, the switching transformer circuit 30 is inserted into the power receiving device 200A, and one of the multiple transformers in the switching transformer circuit 30 is selected or whether or not to insert a transformer is switched so as to increase the transmission efficiency based on the voltage gain or current gain between input and output, or the operation command value of the output voltage or output current from the control circuit 10 to the inverter 13. This realizes impedance matching through impedance conversion, and increases the transmission efficiency in the contactless power supply system 300A. Furthermore, because only the switching transformer circuit 30 is inserted, there are advantages in that the circuit configuration is simple and can be made smaller.

[0132] (Inventor's Findings 2) For example, in the contactless power supply system 300 of Fig. 8, when the load device 24 is a secondary battery such as a battery, the voltage and current of the battery change depending on the charging rate, and the load impedance of the battery also changes. Therefore, the application of the configuration according to the first embodiment to the control of the charging mode of the battery (third embodiment) will be described below.

[0133] Battery charging modes include, for example, the following: (A) CC (Constant Current) charging mode: A mode in which charging is performed with a constant current flowing through the battery. (B) CV (Constant Voltage) charging mode: A mode in which charging is performed with a constant voltage flowing through the battery. (C) CCCV charging mode: A mode in which charging starts in CC charging mode and then transitions to CV charging mode.

[0134] The relationship between the battery voltage Vbattery, the current Ibattery, and the load resistance Rbattery is expressed by the following equation according to Ohm's law.

[0135] Vbattery=Ibattery×Rbattery (20)

[0136] As is clear from the above formula (20), the load resistance Rbattery can be calculated from the battery voltage Vbattery and current Ibattery. Since the voltage or current of the battery changes depending on the charging rate, the load resistance Rbattery changes. Therefore, as described in Inventor's Findings 1, the transmission efficiency in the contactless power transfer system 300A changes depending on the change in load resistance Rbattery.

[0137] For example, FIG. 16 is a graph showing an example of the time characteristics (charging profile) of the voltage and current of the battery in the CCCV charging mode.

[0138] Fig. 17 is a graph showing the charging profile of the load device 24 in the charging method of Fig. 16. As is clear from Fig. 17, after switching from CC charging mode to CV charging mode, the load resistance Rbattery of the battery increases.

[0139] FIG. 18 is a graph showing the transfer efficiency versus equivalent load resistance when the transformer used in the switching transformer circuit 30 is not switched (hereinafter referred to as CCCV switching) during a transition from CC charging to CV charging in a conventional contactless power transfer system. As is clear from FIG. 18 , in CV charging mode, the transfer efficiency η decreases as the equivalent load resistance Req increases. That is, due to changes in the battery load resistance Rbattery, the transfer efficiency at operating points of the load resistance Rbattery that are far from the optimal load point P10 (points where impedance matching is not achieved) decreases significantly. Therefore, we have learned that the equivalent load resistance needs to be adjusted to increase the efficiency of the entire charging profile.

[0140] (Embodiment 3) In embodiment 3, a contactless power supply system 300C according to embodiment 3 uses the configuration of the contactless power supply system according to embodiment 1 or 2, or a modification thereof, and is characterized in that the equivalent load resistance Req is controlled by using the impedance adjustment function during impedance conversion of the switching transformer circuit 30. Note that the contactless power supply system 300C includes a power transmitting device 100C and a power receiving device 200C.

[0141] Fig. 19 is a graph showing a charging profile of a load device when CCCV switching is performed in the contactless power supply system 300C according to embodiment 3. As is clear from Fig. 19, by adjusting the impedance by changing the turns ratio of the transformer in the switching transformer circuit 30, it is possible to reduce the range of change in the load resistance compared to the load resistance without adjustment.

[0142] Figure 20 is a graph showing the transfer efficiency versus equivalent load resistance when switching to CCCV mode as shown in Figure 19. By changing the impedance conversion ratio when switching to CCCV charging mode, the transfer width of the load resistance can be reduced, and the decrease in transfer efficiency η can be suppressed. Comparing Figures 18 and 20, it can be seen that the transfer efficiency in CV charging mode can be increased from 72% to 80%, an 8% improvement in efficiency.

[0143] For example, in CCCV charging mode, when switching from CC charging mode to CV charging mode, the transformer of the switching transformer circuit 30 is switched by stopping the power supply. After the transformer is switched, power is supplied again until the battery is fully charged. When switching to CCCV charging, power supply is stopped once and the transformer to be used is switched. Then, power supply is resumed.

[0144] FIG. 21A is a flowchart showing a power receiving process executed by the control circuit 20 of the power receiving device 200C of the contactless power feeding system 300C according to the third embodiment.

[0145] In step S1 of FIG. 21A, it is determined whether the device has stopped at the charging position. If the determination is YES, in step S2A, a stop notification signal indicating that the device has stopped at the charging position is sent to the power transmission device 100C via the communication circuits 25, 15. In step S2B, the charging method, either CC charging mode or CV charging mode, is determined based on the charging state of the load device 24 (determined based on the output voltage and output current to the load device 24) (or a charging command signal from the control circuit of the load device 24), and it is determined here that the charging mode is CC charging mode.

[0146] Next, in step S2C, the switching transformer circuit 30 is set to a CC charging transformer, and in step S9, a main power supply start notification signal is sent to the power transmission device 100C via the communication circuits 25 and 15. In step S10, it is determined based on the voltage of the load device 24 whether the voltage has reached a level at which CC charging will be completed (or whether a command signal to change to CCCV charging mode has been received from the control circuit of the load device 24). If the determination is YES, in step S11, a main power supply stop notification signal is sent to the power transmission device 100C via the communication circuits 25 and 15.

[0147] Next, in step S12, it is determined whether or not a stop of main power feeding from power transmission device 100C has been detected, and if YES, in step S13, switching transformer circuit 30 switches to the CV charging transformer, and in step S14, a main power feeding start notification signal is sent to power transmission device 100C via communication circuits 25, 15. In step S15, it is determined whether or not charging of load device 24 has been completed, and if YES, in step S16, a main power feeding stop notification signal is sent to power transmission device 100C via communication circuits 25, 15, and the power receiving process ends.

[0148] FIG. 21B is a flowchart showing a power transmission process executed by the control circuit 10 of the power transmission device 100C of the contactless power supply system 300C according to the third embodiment.

[0149] 21B, it is determined whether a stop notification signal has been received from the power receiving device 200C via the communication circuits 25 and 15. If the determination is YES, it is determined in step S25 whether a main power feeding start notification signal has been received from the power receiving device 200C via the communication circuits 25 and 15. In step S26, main power feeding is started to the power receiving device 200C. Next, in step S27, it is determined whether a main power feeding stop notification signal has been received from the power receiving device 200C via the communication circuits 25 and 15. If the determination is YES, main power feeding to the power receiving device 200C is stopped in step S28.

[0150] Next, in step S35, it is determined whether or not a main power feeding start notification signal has been received from the power receiving device 200C via the communication circuits 25, 15, and in step S36, main power feeding is started to the power receiving device 200C. Next, in step S37, it is determined whether or not a main power feeding stop notification signal has been received from the power receiving device 200C via the communication circuits 25, 15, and if the determination is YES, main power feeding to the power receiving device 200C is stopped in step S38, and the power transmission process is terminated.

[0151] 22A is a flowchart showing a power receiving process executed by the control circuit 20 of the power receiving device 200D of a contactless power supply system 300D according to a modification of embodiment 3. The contactless power supply system 300D according to the modification of embodiment 3 includes a power transmitting device 100D and a power receiving device 200D.

[0152] 22A, it is determined whether power receiving device 200D has stopped at the charging position, and if the determination is YES, in step S2A, a stop notification signal indicating that power receiving device 200D has stopped at the charging position is transmitted to power transmitting device 100D via communication circuits 25, 15. Next, in step S2B, based on the charging state of load device 24 (or a charge command signal from the control circuit of load device 24), it is determined whether the charging method is CC charging mode or CV charging mode, and it is determined here that the charging mode is CV charging mode.

[0153] Next, in step S13, the switching transformer circuit 30 switches to the CV charging transformer, and in step S14, a main power feeding start notification signal is transmitted to the power transmitting device 100D via the communication circuits 25 and 15. In step S15, it is determined whether or not charging of the load device 24 has been completed, and if the result is YES, in step S16, a main power feeding stop notification signal is transmitted to the power transmitting device 100D via the communication circuits 25 and 15, and the power receiving process is terminated.

[0154] FIG. 22B is a flowchart showing a power transmission process executed by the control circuit 10 of the power transmission device 100D of the contactless power supply system 300D according to a modification of the third embodiment.

[0155] 22B , it is determined whether a stop notification signal has been received from the power receiving device 200D via the communication circuits 25 and 15. If the determination is YES, it is determined in step S25 whether a main power feeding start notification signal has been received from the power receiving device 200D via the communication circuits 25 and 15. If the determination is YES in step S25, main power feeding to the power receiving device 200D is started in step S26. Next, in step S27, it is determined whether a main power feeding stop notification signal has been received from the power receiving device 200D via the communication circuits 25 and 15. If the determination is YES, main power feeding to the power receiving device 200D is stopped in step S28, and the power transmission process is terminated.

[0156] As described above, according to the third embodiment and its modifications, in the contactless power supply system 300D, the switching transformer circuit 30 is inserted into the power receiving device 200D, and one of the multiple transformers in the switching transformer circuit 30 is selected based on the battery charging mode so as to increase the transmission efficiency, or whether or not a transformer is inserted is switched, thereby achieving impedance matching through impedance conversion and increasing the transmission efficiency in the contactless power supply system 300D. Furthermore, because only the switching transformer circuit 30 is inserted, there are advantages in that the circuit configuration is simple and can be made smaller.

[0157] (Modification 3) Fig. 23A is a circuit diagram showing an example of the configuration of a switching transformer circuit 30AC according to Modification 3. In Fig. 23A, the switching transformer circuit 30AC according to Modification 3 differs from the switching transformer circuit 30A of Fig. 10A in the following respects. Note that in Figs. 23A to 23D, the upper side of the inductor of each transformer is referred to as one end, and the lower side is referred to as the other end. (1) The inductor L of the switching element Q12 and the transformer TR1A Ｐ１ (2) A coupling capacitor Cc1 for power factor adjustment is inserted between the switch element Q14 and one end of the inductor L of the transformer TR1A. Ｐ２ A coupling capacitor Cc2 for power factor adjustment was inserted between the first end of the power supply and the second end of the power supply.

[0158] According to the switching transformer circuit 30A configured as above, by inserting the coupling capacitors Cc1 and Cc2, it is possible to perform power factor adjustment in addition to impedance conversion, and further to increase the transmission efficiency.

[0159] 23B is a circuit diagram showing a configuration example of a switching transformer circuit 30BC according to Modification 4. In FIG. 23B, the switching transformer circuit 30BC according to Modification 4 is different from the switching transformer circuit 30B of FIG. 10B in the following respects: (1) The switching element Q11 and the inductor L of the transformer TR1B ｐ１ A coupling capacitor Cc11 for power factor adjustment was inserted between the other end of the transformer TR1B and the inductor L ｐ２ A coupling capacitor Cc12 for power factor adjustment is inserted between one end of the inductor Lp1 and the other end of the inductor Lp1. ｐ２ and the switch element Q14.

[0160] According to the switching transformer circuit 30B configured as above, by inserting the coupling capacitors Cc11 and Cc12, it is possible to perform power factor adjustment in addition to impedance conversion, and further to increase the transmission efficiency.

[0161] (Modification 5) Fig. 23C is a circuit diagram showing a configuration example of a switching transformer circuit 30CC according to Modification 5. In Fig. 23C, the switching transformer circuit 30CC according to Modification 5 differs from the switching transformer circuit 30C of Fig. 10C in the following respects: (1) A coupling capacitor Cc21 for power factor adjustment is inserted between the switch element Q12 and the terminal T3.

[0162] According to the switching transformer circuit 30Cc configured as above, by inserting the coupling capacitor Cc21, it is possible to perform power factor adjustment in addition to impedance conversion, and further to increase the transmission efficiency.

[0163] 23D is a circuit diagram showing a configuration example of a switching transformer circuit 30DC according to Modification 6. In FIG. 23D, the switching transformer circuit 30DC according to Modification 6 is different from the switching transformer circuit 30D of FIG. 10D in the following respects: (1) The terminal T1 and the inductor L of the transformer TR1D ｐ１ (2) A coupling capacitor Cc31 for power factor adjustment is inserted between one end of the switch element Q12 and the inductor L of the transformer TR1D. ｐ２ A coupling capacitor Cc32 for power factor adjustment was inserted between one end of the power supply and the other end of the power supply.

[0164] According to the switching transformer circuit 30D configured as above, by inserting the coupling capacitors Cc31 and Cc32, it is possible to perform power factor adjustment in addition to impedance conversion, and further to increase the transmission efficiency.

[0165] (Variation 7) Fig. 24A is a circuit diagram showing a configuration example of a switching transformer circuit 30AD according to Variation 7. In Fig. 24A, the switching transformer circuit 30AD has the switch elements Q11 to Q14 moved from the primary side to the secondary side of the transformer TR1A, as compared to the switching transformer circuit 30A of Fig. 10A. Here, when the ratio between the primary winding and the secondary winding of the transformer TR1A exceeds 1, it is possible to use switch elements with a lower withstand voltage for the switch elements Q11 to Q14, as compared to the switching transformer circuit 30A of Fig. 10A, thereby reducing losses in the switching transformer circuit 30AD and increasing transmission efficiency in the contactless power supply system 300, etc.

[0166] (Modification 8) Fig. 24B is a circuit diagram showing a configuration example of a switching transformer circuit 30BD according to Modification 8. In Fig. 24B, the switching transformer circuit 30BD has the switch elements Q11 to Q14 moved from the primary side to the secondary side of the transformer TR1B, as compared to the switching transformer circuit 30B of Fig. 10B. Here, when the ratio between the primary winding and the secondary winding of the transformer TR1B exceeds 1, it is possible to use switch elements with a lower withstand voltage for the switch elements Q11 to Q14, as compared to the switching transformer circuit 30B of Fig. 10B, which reduces losses in the switching transformer circuit 30BD and increases transmission efficiency in the contactless power supply system 300, etc.

[0167] (Modification 9) Fig. 24C is a circuit diagram showing a configuration example of a switching transformer circuit 30DD according to Modification 9. In Fig. 24C, the switching transformer circuit 30DD has the switch elements Q11 and Q12 moved from the primary side to the secondary side of the transformer TR1D, as compared to the switching transformer circuit 30D of Fig. 10D. Here, when the ratio between the primary winding and the secondary winding of the transformer TR1D exceeds 1, it is possible to use lower-voltage switch elements for the switch elements Q11 and Q12, as compared to the switching transformer circuit 30D of Fig. 10D, which reduces losses in the switching transformer circuit 30DD and increases transmission efficiency in the contactless power supply system 300, etc.

[0168] 25A is a circuit diagram showing a configuration example of a switching transformer circuit 30ADC according to Modification 10. In FIG. 25A, the switching transformer circuit 30ADC according to Modification 10 differs from the switching transformer circuit 30AD of FIG. 24A in the following respects: (1) The switching element Q12 and the inductor L of the transformer TR1A Ｐ１ (2) A coupling capacitor Cc1 for power factor adjustment is inserted between the switch element Q14 and one end of the inductor L of the transformer TR1A. Ｐ２ A coupling capacitor Cc2 for power factor adjustment was inserted between the first end of the power supply and the second end of the power supply.

[0169] According to the switching transformer circuit 30ADC configured as above, by inserting the coupling capacitors Cc1 and Cc2, it is possible to perform power factor adjustment in addition to impedance conversion, and further to increase the transmission efficiency.

[0170] 25B is a circuit diagram showing a configuration example of a switching transformer circuit 30BDC according to Modification 11. In FIG. 25B, the switching transformer circuit 30BDC according to Modification 11 is different from the switching transformer circuit 30BD of FIG. 24B in the following respects: (1) The switching element Q11 and the inductor L of the transformer TR1B ｐ１ A coupling capacitor Cc11 for power factor adjustment was inserted between the other end of the transformer TR1B and the inductor L ｐ２ A coupling capacitor Cc12 for power factor adjustment is inserted between one end of the inductor Lp1 and the other end of the inductor Lp1. ｐ２ and the switch element Q14.

[0171] According to the switching transformer circuit 30BDC configured as above, by inserting the coupling capacitors Cc11 and Cc12, it is possible to perform power factor adjustment in addition to impedance conversion, and further to increase the transmission efficiency.

[0172] (Modification 12) Fig. 25C is a circuit diagram showing a configuration example of a switching transformer circuit 30DDC according to Modification 12. In Fig. 23D, the switching transformer circuit 30DDC according to Modification 12 differs from the switching transformer circuit 30DD of Fig. 24D in the following respects: (1) The terminal T1 and the inductor L of the transformer TR1D Ｐ１ (2) A coupling capacitor Cc31 for power factor adjustment is inserted between one end of the switch element Q12 and the inductor L of the transformer TR1D. Ｐ２ A coupling capacitor Cc32 for power factor adjustment was inserted between one end of the power supply and the other end of the power supply.

[0173] According to the switching transformer circuit 30DDC configured as above, by inserting the coupling capacitors Cc31 and Cc32, it is possible to perform power factor adjustment in addition to impedance conversion, and further to increase the transmission efficiency.

[0174] (Other Modifications) In the above embodiment and modifications, the AC / DC converter 12 and the DC / DC converter 23 are provided, but the present invention is not limited to this, and these may be omitted as necessary.

[0175] Furthermore, a DC power supply may be used instead of the AC power supply 11 and the AC / DC converter 12, and the DC power supply may be a secondary battery such as a battery. Furthermore, although examples of the resonant circuits 14 and 21 are shown in Figures 8 and 15, various LC resonant circuits may also be used.

[0176] (Differences from Patent Document 1) In FIG. 1 of Patent Document 1, an impedance conversion unit is inserted between the power supply unit and the power transmission unit, whereas in FIG. 8 of the present application, a switching transformer circuit 30 that performs impedance conversion is inserted between the resonant circuit 21 and the rectifier circuit 22 of the power receiving device 200. While Patent Document 1 can reduce only the loss in the power transmitting device, the present application can reduce the loss in the power transmitting device 100 and the power receiving device 200. Furthermore, in the present application, by setting the turn ratio between the primary winding and the secondary winding of each of the transformers TR1A to TR1D of the switching transformer circuit 30 to a value greater than 1, the loss in the winding of the inductor L2 of the resonant circuit 21 can be reduced, and the loss of the entire contactless power transfer system 300, etc. can be reduced.

[0177] 1C , the equivalent impedance when the power receiving device 200 is viewed from the output end of the inverter 13 of the power transmitting device 100 includes power transmitting and receiving components. As a result, matching the impedance as viewed from the power transmitting device 100 has the effect of reducing the reactive current flowing in the power transmitting device 100, and minimizing this reactive current minimizes losses and achieves high efficiency. In other words, because this impedance matching includes power transmitting and receiving components, it is possible to adjust not only the power transmitting device 100 but also the components of the power receiving device 200.

[0178] (Differences from Patent Document 2) In Fig. 3 of Patent Document 2, a step-up DC / DC converter is coupled to a power receiving inductor, and an impedance controller is configured to increase or decrease the electrical impedance of the inductive power receiver using the leakage inductance of the power receiving inductor as an inductor for the step-up converter in response to changes in the power demand of the programmable load. On the other hand, in Fig. 8 of the present application, a switching transformer circuit 30 that performs impedance conversion is inserted between the resonant circuit 21 and the rectifier circuit 22 of the power receiving device 200. As such, the configuration is different, and the present application has the advantages of reduced circuit size and cost.

[0179] As described above in detail, in the contactless power supply system, compared to the prior art, it is possible to reduce the loss in the power receiving device and increase the power efficiency, and furthermore, the circuit configuration is simple and can be made compact.

[0180] 10 Control circuit 10m Memory 11 AC power supply 12 AC / DC converter 12A DC power supply 13 Inverter 14 Resonant circuit 14C Power transmitting coil 15 Communication circuit 20 Control circuit 20m Memory 21 Resonant circuit 21C Power receiving coil 22 Rectifier circuit 23 DC / DC converter 24 Load device 25 Communication circuit 30, 30A, 30B, 30C, 30D, 30AA, 30BA, 30AC, 30BC, 30CC, 30DC, 30AD, 30BD, 30CD Switching transformer circuit 31, 32 Load device 40 Transformer circuit 51 Voltage detector 52 Current detector 61 Voltage detector 62 Current detector 100, 100A to 100D Power transmitting device 200, 200A to 200D Power receiving device 300, 300A to 300D Wireless power supply system Cin, C0, C1, C2, Cc1 to Cc32 Capacitors L0, L1, L2, L Ｐ１ , L Ｐ２ , L Ｓ１， L ｐ１ , L ｐ２ , L ｓ Inductor M, M Ｐ１Ｓ , M Ｐ２Ｓ , M ＰＳ , M ｐ１ｓ , M ｐ１ｐ２ , M ｐ２ｓMutual inductance Q1 to Q6, Q11 to Q14 Switch element R0 Load resistance Req1, Req2 Equivalent load resistance TR Ideal transformer TR1, TR2, TR1A to TR1E Transformer

Claims

1. A contactless power supply system comprising a power transmitting device and a power receiving device, wherein the power transmitting device comprises: a first converter that converts DC power into AC power; a first resonant circuit including a first inductor and a first capacitor, the first resonant circuit receiving AC power converted by the first converter; the power receiving device comprises: a second resonant circuit including a second inductor electromagnetically coupled to the first inductor and a second capacitor, the second resonant circuit outputting AC power; and a second converter that converts AC power from the second resonant circuit into DC power; the power receiving device further comprises: a switching transformer circuit inserted between the second resonant circuit and the second converter, the switching transformer circuit selecting one of a plurality of transformers or selectively switching whether or not to insert a transformer to perform impedance conversion so as to change an impedance conversion ratio; and the contactless power supply system further comprises: a control circuit that controls the switching transformer circuit to select one of a plurality of transformers or to selectively switch whether to insert a transformer, so as to increase transmission efficiency when power is transmitted from the first converter to the second converter, based on a charging state or a charging mode of a secondary battery that is a load device connected in a subsequent stage of the second converter.

2. The contactless power supply system according to claim 1, wherein the control circuit controls the switching transformer circuit to switch the transformer when switching from a constant current charging mode to a constant voltage charging mode.

3. The contactless power supply system according to claim 1, wherein the switching transformer circuit comprises: a primary winding having a plurality of windings; a secondary winding having one winding; and a switch element that selects one of the plurality of windings.

4. The contactless power supply system according to claim 1, wherein the switching transformer circuit comprises: a primary winding having a plurality of windings; a secondary winding having one winding; and a switch element that selects whether to select all of the plurality of windings or any one of the plurality of windings.

5. The contactless power supply system according to claim 1, wherein the switching transformer circuit comprises: a primary winding having a plurality of windings; a secondary winding having one winding; and a switch element that selects one or a predetermined number of the plurality of windings.

6. The contactless power supply system according to any one of claims 3 to 5, wherein the switching transformer circuit further comprises a power factor adjustment capacitor connected between the switch element and each of the primary windings.

7. The contactless power supply system according to claim 1, wherein the switching transformer circuit comprises: a primary winding having one winding; a secondary winding having a plurality of windings; and a switch element that selects one of the plurality of windings.

8. The contactless power supply system according to claim 1, wherein the switching transformer circuit comprises: a primary winding having one winding; a secondary winding having a plurality of windings; and a switch element that selects whether to select all of the plurality of windings or any of the plurality of windings.

9. The contactless power supply system according to claim 1, wherein the switching transformer circuit comprises: a primary winding having one winding; a secondary winding having a plurality of windings; and a switch element for selecting one or a predetermined number of the plurality of windings.

10. A contactless power supply system according to any one of claims 7 to 9, wherein the switching transformer circuit further comprises a power factor adjustment capacitor connected between the switch element and each of the secondary windings.

11. The contactless power supply system according to any one of claims 1 to 5 and 7 to 9, wherein the power transmission device further comprises an AC / DC converter provided in a stage preceding the first converter, for converting AC power into DC power.

12. The contactless power supply system according to any one of claims 1 to 5 and 7 to 9, wherein the power receiving device further comprises a DC / DC converter provided between the second converter and a load device, for converting DC power from the second converter into predetermined DC power and outputting it to the load device.

13. The contactless power supply system according to claim 10, wherein the power receiving device further comprises a DC / DC converter provided between the second converter and a load device, for converting DC power from the second converter into predetermined DC power and outputting it to the load device.

14. A power transmission device for a contactless power supply system according to any one of claims 1 to 5 and 7 to 9, comprising the first resonant circuit and the first converter.

15. The power transmission device according to claim 14, further comprising: an AC / DC converter provided in a stage preceding the first converter, for converting AC power into DC power.

16. A power receiving device for a contactless power supply system according to any one of claims 1 to 5 and 7 to 9, comprising the second resonant circuit and the second converter.

17. The power receiving device according to claim 16, further comprising: a DC / DC converter provided between the second converter and a load device, for converting DC power from the second converter into predetermined DC power and outputting it to the load device.

18. A control method for a contactless power transfer system including a power transmitting device and a power receiving device, wherein the power transmitting device comprises: a first converter that converts DC power into AC power; a first resonant circuit including a first inductor and a first capacitor, the first resonant circuit receiving AC power converted by the first converter; the power receiving device comprises: a second resonant circuit including a second inductor electromagnetically coupled to the first inductor and a second capacitor, the second resonant circuit outputting AC power; and a second converter that converts AC power from the second resonant circuit into DC power; the power receiving device further comprises: a switching transformer circuit inserted between the second resonant circuit and the second converter, the switching transformer circuit selecting one of a plurality of transformers or selectively switching whether or not to insert a transformer to perform impedance conversion so as to change an impedance conversion ratio; and the contactless power transfer system further comprises: a control circuit that controls the switching transformer circuit; The control method includes a step of controlling the switching transformer circuit so that the control circuit selects one of a plurality of transformers or selectively switches whether to insert a transformer, so as to increase transmission efficiency when power is transmitted from the first converter to the second converter, based on a charging state or a charging mode of a secondary battery that is a load device connected in a subsequent stage of the second converter.

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