Wireless power feeding system

An object is to provide a wireless power feeding system using the resonance method, which can increase power transmission efficiency. The wireless power feeding system includes a power transmission coil electrically connected to a high-frequency power supply, a power transmission resonance coil for transmitting power by electromagnetic induction with the power transmission coil, a power reception resonance coil for exciting high-frequency power by magnetic resonance, a load coil for exciting high-frequency power by electromagnetic induction with the power reception resonance coil, a load, and a variable element. The load includes a microprocessor for controlling the impedance of the load, a battery charger, and a battery. The battery charger is configured to charge the battery with the high-frequency power excited by the load coil.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless power feeding system including a power-receiving device.

2. Description of the Related Art

A method called a magnetic resonance method attracts, attention as a method for feeding power to an object (hereinafter also called a power-receiving device) not in contact with a power supply (hereinafter also called a power-transmitting device) (such a method is also called a contactless power feeding method, a wireless power feeding method, or the like). The magnetic resonance method is a method in which resonance coils provided in a power-transmitting device and a power-receiving device are put in magnetic resonance with each other to form an energy propagation path. The magnetic resonance method yields a longer power transmittable distance than other methods (e.g., an electromagnetic induction method and a field induction method). For example, Non-Patent Document 1 discloses that in the magnetic resonance method, transmission efficiency is about 90% when the distance between the resonance coils is 1 m, and is about 45% when the distance between the resonance coils is 2 m.

REFERENCE

SUMMARY OF THE INVENTION

In the above-described magnetic resonance method, power feeding is accomplished by magnetic resonance which is produced when a pair of resonance coils having the same resonance frequency is in resonance with each other. Further, the conditions for high efficiency of power transmission vary depending on the distance between a power transmission resonance coil in a power-transmitting device and a power reception resonance, coil in a power-receiving device (hereinafter also called coil-to-coil distance), making it difficult to stably transmit power with high efficiency.

FIGS.8A1to8C2illustrate the relation between the coil-to-coil distance and power transmission efficiency. FIG.8A1illustrates the case where the coil-to-coil distance is too short. FIG.8B1illustrates the case where the coil-to-coil distance is appropriate. FIG.8C1illustrates the case where the coil-to-coil distance is too long. FIGS.8A2,8B2, and8C2each illustrate the relation between transmission efficiency and frequency.

In the case where the coil-to-coil distance is appropriate as shown in FIG.8B1, the maximum power transmission efficiency is obtained when the frequency is a resonance frequency f0as shown in FIG.8B2. However, in the case where the coil-to-coil distance is too short as shown in FIG.8A1, peak splitting in the power transmission efficiency occurs such that a peak appears with a frequency f0′ and the lowest point between peaks appears with the resonance frequency f0, which decreases the power transmission efficiency. In the case where the coil-to-coil distance is long as shown in FIG.8C1, peak splitting does not occur as shown in FIG.8C2but magnetic coupling between the resonance coils is lower and power transmission efficiency with the resonant frequency f0is therefore lower than that in FIG.8B2. Note that FIGS.8A1to8C2only illustrate, for easy understanding, a high-frequency power supply111, a power transmission resonance coil112, a power transmission coil117, a load121, a power reception resonance coil122, and a load coil129.

FIGS.8A1to8C2show that the maximum efficiency cannot be obtained with the resonance frequency f0if the coil-to-coil distance is not appropriate. Therefore, magnetic resonance wireless power feeding, which is seemingly less spatially limited as long as the coil-to-coil distance is short, is, in reality, problematic in that the power transmission efficiency dramatically decreases when strong magnetic coupling between the coils occurs.

When the load is a secondary battery, the secondary battery (e.g., lithium-ion battery) is generally charged during a constant-current charging period and a constant-voltage charging period. In the constant-current charging period, which comes immediately after charging is started, applied voltage is gradually increased, so that impedance gradually increases as the charging proceeds. During the subsequent constant-voltage charging period, the amount of charging current rapidly decreases, which rapidly increases impedance. A fluctuation in the impedance of the load further makes it difficult to feed power by magnetic resonance.

In view of the foregoing, an object of one embodiment of the present invention is to provide a wireless power feeding system using the resonance method, which can increase power transmission efficiency.

One embodiment of the present invention is, a wireless power feeding system including a power transmission coil, a power transmission resonance coil for transmitting power by electromagnetic induction with the power transmission coil, a power reception resonance coil for exciting high-frequency power by magnetic resonance, a load coil for exciting high-frequency power by electromagnetic induction with the power reception resonance coil, a load electrically connected to the load coil, a variable element for controlling the amount of charging current supplied to the load, and a Microprocessor for controlling the variable element. The load includes a battery charger and a battery. The battery charger is configured to charge the battery with the high-frequency power excited by the load coil.

One embodiment of the present invention is a wireless power feeding system including: a high-frequency power supply, a power transmission coil, a power transmission resonance coil for transmitting power by electromagnetic induction with the power transmission coil, a first microprocessor for controlling output of the high-frequency power supply, a power reception resonance coil for exciting high-frequency power by magnetic resonance, a load coil for exciting high-frequency power by electromagnetic induction with the power reception resonance coil, a load electrically connected to the load coil, a variable element for controlling the amount of charging current supplied to the load, and a second microprocessor. The load includes a battery charger and a battery. The battery charger is configured to charge the battery with the high-frequency power excited by the load coil. The second microprocessor is configured to control the variable element and, transmit a signal to the first microprocessor. The signal is used for controlling supply current from the high-frequency power supply.

In the above wireless power feeding system, the variable element is preferably a switch provided in the battery charger.

In the above wireless power feeding system, the load preferably includes a rectifier circuit electrically connected to the load coil.

One embodiment of the present invention can provide a wireless power feeding system in which the impedance of a load is adjusted by external control, so that power feeding appropriate to the situation is achieved under conditions for the maximum power transmission efficiency regardless of positional relationship, between a power transmission resonance coil in a power-transmitting device and a power reception resonance coil in a power-receiving device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below with reference to the drawings. Note that the embodiments can be implemented in various different ways. It will be readily appreciated by those skilled in the art that modes and details of the embodiments can be modified in various ways without departing from the spirit and scope of the present invention. The present invention therefore should not be construed as being limited to the description of the embodiments. Note that in structures of the present invention described below, reference numerals denoting the same portions are used in common in different drawings.

Note that, the size, layer thickness, and signal waveform of each object shown in the drawings and the like in the embodiments, are exaggerated for simplicity in some cases. Each object therefore is not necessarily in such scales.

In this specification and the like, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components, and the terms do not limit the components numerically.

This embodiment describes a wireless power feeding system according to one embodiment of the present invention, which achieves wireless power feeding by the resonance method.

<Example of Configuration of Wireless Power Feeding System>

FIG. 1illustrates a configuration of a wireless power feeding system according to one embodiment of the present invention. The wireless power feeding system inFIG. 1uses the magnetic resonance type power transmission method. The wireless power feeding system inFIG. 1includes a power-transmitting device140and a power-receiving device150. InFIG. 1, power feeding can be achieved by electromagnetic waves when a power transmission resonance coil112in the power-transmitting device140and a power reception resonance coil122in the power-receiving device150are in resonance with each other.

The power-transmitting device140includes a high-frequency power supply111, a power transmission resonance coil112, a power transmission coil117, and a capacitor118. In the power-transmitting device140, the high-frequency power supply111is connected to the power transmission coil117. Electromagnetic induction occurs between the power transmission resonance coil112and the power transmission coil117. One terminal of the power transmission resonance coil112is connected to one terminal of the capacitor118. The other terminal of the power transmission resonance coil112is connected to the other terminal of the capacitor118.

Note that the high-frequency power supply111may have any structure which enables generation of high-frequency, power having a frequency equal to the self-resonant frequency of the power transmission resonance coil112.

The power-receiving device150inFIG. 1includes a load121, the power reception resonance coil122, a microprocessor125, a rectifier circuit127, a load coil129, and a capacitor130. The load121includes a battery charger123, a battery124, and a variable element126. In the power-receiving device150, electromagnetic induction between the power reception resonance coil122and the load coil129occurs, and the load coil129is connected to the load121via the rectifier circuit127. The variable element126in the load121is included in the battery charger123. One terminal of the battery124is connected to the battery charger123, and the other terminal of the battery124is grounded.

Note that the capacitor118and the capacitor130may bother stray capacitances of the power transmission resonance coil112and the power reception resonance coil122, respectively. Alternatively, the capacitor118and the capacitor130may be provided independently of these coils. The variable element126may instead be outside of the battery charger123and connected to the battery charger123.

High-frequency power, which is AC power from the high-frequency power supply111, is applied to, and then rectified in the rectifier circuit127. DC voltage and direct current produced by rectification are applied to the load121.

Although not illustrated, in the power-receiving device150, an A/D converter may be provided between the rectifier circuit127and the microprocessor125or included in the microprocessor125.

A DCDC converter may be provided between the rectifier circuit127and the load121. DC voltage produced by rectification in the rectifier circuit is supplied to the DCDC converter where the magnitude of the DC voltage is adjusted, and then appears at the output of the DCDC converter.

The variable element126is capable of controlling, the impedances of the battery charger123and the battery124by controlling the amount of charging current supplied to the battery124in the power-receiving device150. The variable element126may be, for example, a switch which is included in the battery charger123and the switching of which is controlled by the microprocessor125. When the variable element126is a switch, high-frequency power may be applied thereto; therefore, the variable element126is preferably a mechanical switch (e.g., a mechanical relay or a MEMS switch) which controls presence or absence of a connecting point.

In a power transfer technique using coils, there is a parameter k×Q (k is a coupling coefficient and Q is a Q value of a resonance coil) as a parameter that represents an index of high transmission efficiency. The coupling coefficient k is a coupling coefficient that represents the degree of magnetic coupling between the resonance coil on the power-transmitting side and the resonance coil on the power-receiving side. Further, the Q value is a value showing sharpness in a resonance peak of a resonance circuit. In resonance type wireless power feeding technology, the power transmission resonance coil112and the power reception resonance coil122are preferably resonance coils having extremely high Q values (for example, the Q is larger than 100, or k×Q is larger than 1) in order to achieve high transmission efficiency.

<Power Feeding Method for Wireless Power Feeding System>

Next, a power feeding method for a wireless power feeding system according to one embodiment of the present invention will be described with reference toFIG. 2.FIG. 2is a flow Chart illustrating an example of a power feeding method for the wireless power feeding system.

First, when the power-receiving device150′ is placed at an appropriate position with respect to the power-transmitting device140, power feeding is started by turning on the high-frequency power supply111in the power-transmitting device140(see the step S1inFIG. 2). At this time, efficient power transmission is not necessarily achieved. In other words, the maximum transmission efficiency is not necessarily achieved. Note that this embodiment describes the case where impedance is adjusted by controlling the amount of charging current supplied to the battery124in the power-receiving device150.

When the power-transmitting device140starts to transmit power to, the power-receiving device150, the power is transmitted from the power transmission resonance coil112in the power-transmitting device140to the power reception resonance coil122in the power-receiving device150through magnetic coupling, then converted into DC voltage and direct current with the rectifier circuit127, and then applied to the load121(including at least a secondary battery, an LED, or an IC chip, for example). At this time, the values of DC voltage and direct current applied to the load121in the power-receiving device150are obtained. The product of the DC voltage value and the direct current value at this time is a power value P1. Data D1which is data of the obtained product of the DC voltage value and the direct current value (power value P1) is stored in the microprocessor125(seethe step S21inFIG. 2).

Next, the microprocessor125controls the variable element126to change the amount of charging current supplied to the battery124. This changes the impedances of the battery charger123and the battery124, so that the impedance of the entire load121is changed. At the same time, the microprocessor125obtains the values of DC voltage and direct current applied to the load121and calculates a power value P2, the product of the DC voltage value and the direct current value. Data D2which is data of the power value P2is stored in the microprocessor125.

After n types of impedance changes, data of power values obtained every change (D1, D2, . . . , Dn) are stored in the microprocessor125(see the step S2ninFIG. 2).

The microprocessor125changes the amount of charging current supplied to the battery124such that power feeding is performed with any one of stored power values (D1, D2, . . . , Dn) which enables the maximum power transmission efficiency, and the battery124is charged (see the steps S3and S4inFIG. 2).

After a certain period (e.g., 10 sec) of charging, whether the impedance at the instant enables the charging to be performed under the conditions for the maximum power transmission efficiency is confirmed.

The steps S21to S2nand the step S3inFIG. 2are taken again to change the amount of charging current supplied to the battery124. Then, settings are made to optimize the impedance of the entire load121; thus, the battery124is charged.

Subsequently, a loop consisting of the steps S21to S2n, the step S3, and the step S4is repeated. When the charging is completed, the high-frequency power supply111is turned off.

By repeating the loop in this manner, the battery124can be charged efficiently even when the distance between the power-transmitting device140and the power-receiving device150changes during the charging.

The use of the power feeding method inFIG. 2for the wireless power feeding system inFIG. 1increases power transmission efficiency in accordance with the positional relationship between the power-transmitting device140and the power-receiving device150, resulting in efficient power feeding. Therefore, the power feeding system can be more convenient for users.

This embodiment describes a configuration of a wireless power feeding system that is partly different from that inFIG. 1.

<Example of Configuration of Wireless Power Feeding System>

FIG. 3illustrates a configuration of a wireless power feeding system according to one embodiment of the present invention. A power-transmitting device160is the same as the power-transmitting device inFIG. 1in that it includes a high-frequency power supply111; a power transmission resonance coil112, a power transmission coil117, and a capacitor118. The power-transmitting device160is different from the power-transmitting device inFIG. 1in that it includes an antenna113, a microprocessor115connected to the high-frequency power supply111, and a first transmission/reception circuit119connected to the microprocessor115and the antenna113.

A power-receiving device170is the same as the power-receiving device inFIG. 1in that it includes a load121, a power reception resonance coil122, a microprocessor125, a rectifier circuit127, a load coil129, and a capacitor130and in that the load121includes a battery charger123, a battery124, and a variable element126. The power-receiving device170is different from the power-receiving device inFIG. 1in that it includes an antenna133and a second transmission/reception circuit128connected to the microprocessor125and the antenna133.

In the configuration of the wireless power feeding system inFIG. 3, data of the values of DC voltage and direct current generated in the power-receiving device170are sent back to the power-transmitting device160by using the antenna133. The power-transmitting device160receiving data of the DC voltage value and the direct current value obtained in the power-receiving device170can adjust the output of the high-frequency power supply111as needed.

When the battery124is a lithium-ion secondary battery, for example, the system is designed to prevent voltage exceeding a certain level, from being applied to the battery124in order to inhibit application of excess voltage. Therefore, when voltage applied to the battery124increases to a higher level than required, a charging control circuit included in the battery charger123eventually reduces the voltage and power corresponding to the reduced voltage is consumed in vain. For this reason, it is preferable to reduce the output power of the high-frequency power supply111in the power-transmitting device160and supply the minimum power for maintaining the charging state.

<Power Feeding Method for Wireless Power Feeding System>

Next, a power feeding method for a wireless power feeding system according to one embodiment of the present invention will be described with reference toFIG. 4.FIG. 4is a flow chart illustrating an example of a power feeding method for the wireless power feeding system.

First, when the power-receiving device170is placed at an appropriate position with respect to the power-transmitting device160, the power-transmitting device160starts to feed power to the power-receiving device170. Power feeding is started by turning on the high-frequency power supply111in the power-transmitting device160(see the step S11inFIG. 4). At this time, efficient power feeding is not necessarily achieved. In other words, the maximum transmission efficiency is not necessarily achieved. Note that this embodiment describes the case where impedance is adjusted by controlling the amount of charging current supplied to the battery124in the power-receiving device170.

When the power-transmitting device160starts to transmit power to the power-receiving device170, the power is transmitted from the power transmission resonance coil112in the power-transmitting device160to the power reception resonance coil122in the power-receiving device170through magnetic coupling, then converted into DC voltage and direct current with the rectifier circuit127, and then applied to the load121(including at least either a secondary battery, an LED, or an IC chip, for example). At this time, the values of DC voltage and direct current applied to the load121in the power-receiving device170are obtained. The product of the DC voltage value and the direct current value at this time is a power value P1. Data D1which is data of the obtained product of the DC voltage value and the direct current value (power value P1) is stored in the microprocessor125(see the step S121inFIG. 4).

Next, the microprocessor125controls the variable element126to change the amount of charging current supplied to the battery124. This changes the impedances of the battery charger123and the battery124, so that the impedance of the entire load121is changed. At the same time, the microprocessor125obtains the values of DC voltage and direct current applied to the load121and calculates a power value P2, the product of the DC voltage value and the direct current value. Data D2which is data of the power value P2is stored in the microprocessor125.

After n types of impedance changes, data of power values, obtained every change (D1, D2, . . . , Dn) are stored in the microprocessor125(see the step S12ninFIG. 4).

The microprocessor125changes the amount of charging current supplied to the battery124such that power feeding is performed with any one of stored power values (D1, D2, . . . , Dn) which enables the maximum power transmission efficiency (see the step S13ainFIG. 4j.

Next, in order that optimal power can be supplied from the high-frequency power supply111, a signal for adjusting power is supplied from the microprocessor125to the second transmission/reception circuit128, sent from the second transmission/reception circuit128to the first transmission/reception circuit119via the antenna133and the antenna113, and then supplied from the first transmission/reception circuit119to the microprocessor115.

The microprocessor115commands the high-frequency power supply111to adjust the output. The high-frequency power supply111adjusts the output to supply optimal power. In order to determine if the output of the high-frequency power supply111is appropriate; the microprocessor115commands the microprocessor125via the first transmission/reception circuit119, the antenna113, the antenna133, and the second transmission/reception circuit128to monitor the DC voltage value. Monitoring of the DC voltage value is repeated; thus, optimal power is supplied (see the step S13binFIG. 4). In an instant before completion of the charging, for example, the high-frequency power supply111supplies less power than the instant, and then the supplied power is gradually reduced during repetition of processes for monitoring the DC voltage value, resulting in optimal power.

Then, the microprocessor115determines whether the charging is to be continued, based on the output of the high-frequency power supply111(see the step S14inFIG. 4). When the output of the high-frequency power supply111becomes 0 and it is determined that the charging is not to be continued, the charging is completed by turning off the high-frequency power supply111(see the step S15inFIG. 4). When it is determined that the charging is to be continued, the next step is taken.

After a certain period (e.g., 10 sec) of charging (see the step S16inFIG. 4), whether the impedance at the instant enables the charging to be performed under the conditions for the maximum power transmission efficiency is confirmed.

The steps S121to S12n, the step S13a, the step S13b, the step S14illustrated inFIG. 4are taken again to change the amount of charging current supplied to the battery124, make settings to optimize the impedance of the entire load121, make settings to optimize the output of the high-frequency power supply111, and then determine whether the charging is to be continued.

Subsequently, a loop consisting of the steps. S121to S12n, the step S13a, the step S13b, the step S14, and the step S16is repeated.

By repeating the loop in this manner, the battery124can be charged efficiently even when the distance between the power-transmitting device160and the power-receiving device170changes during the charging.

The use of the power feeding method inFIG. 4for the wireless power feeding system inFIG. 3increases power transmission efficiency in accordance with the positional relationship between the power-transmitting device160and the power-receiving device170, resulting in efficient power feeding. Therefore, the power feeding system can be more convenient for users.

This embodiment describes a configuration of a wireless power feeding system that is partly different from those inFIG. 1andFIG. 3.

<Example of Configuration of Wireless Power Feeding System>

FIG. 5illustrates a configuration of a wireless power feeding system according to one embodiment of the present invention. A power-transmitting device180is the same as the power-transmitting device inFIG. 3in that it includes a high-frequency power supply111, a power transmission resonance coil112, a microprocessor115, a power transmission coil117, a capacitor118, and a first transmission/reception circuit119. The power-transmitting device180is different from the power-transmitting device inFIG. 3in that it includes a directional coupler114and a mixer116, in that the high-frequency power supply111is connected to the mixer116and the microprocessor115, in that the first directional coupler114is connected to the mixer116, the first transmission/reception circuit119, and the power transmission coil117, and in that the first transmission/reception circuit119is connected to the mixer116and the microprocessor115.

A power-receiving device190is, the same as the power-receiving device inFIG. 3in that it includes a load121, a power reception resonance coil122, a microprocessor125, a rectifier circuit127, a second transmission/reception circuit128, a load coil129, and a capacitor130. The load121includes a battery charger123, a battery124, and a variable element126. The power-receiving device190is, different from the power-receiving device inFIG. 3in that it includes a second directional coupler134between the rectifier circuit127and one terminal of the load coil129, a load135between one terminal of the load coil129and the second directional coupler134, and a transistor136between the load135and the other terminal of the load coil129. The second directional coupler134is connected to the second transmission/reception circuit128. The second transmission/reception circuit128is connected to a gate of the transistor136. One of a source and a drain of the transistor136is connected to the load135. The other of the source and the drain of the transistor136is connected, to the load coil129.

The transistor136may be any type of transistor, but is preferably a transistor formed using an oxide semiconductor, in particular. A transistor formed using a highly purified oxide semiconductor has an off-state current (a current flowing into a device in the off state) of less than 10 zA/μm per micrometer of channel width, and an off-state current of as low as less than 100 zA/μm at 85° C. That is, the off-state current can be reduced to around the measurement limit or lower values. Therefore, the impedance of the power-receiving device190relative to the power-transmitting device180can be changed according to the on/off of the transistor as appropriate.

The mixer116is an analog multiplier. When the mixer116receives the output of the high-frequency power supply111and the output of the first transmission/reception circuit119, two oscillatory waveforms are multiplied by each other, so that a modulated signal is superimposed on the resonance frequency.

The first directional coupler114and the second directional coupler134enables obtainment of a signal corresponding to power transmitted in the forward direction (traveling wave) and a signal corresponding to power transmitted in the reverse direction (reflected wave). In this embodiment, a signal input to the first directional coupler114is transmitted to the power transmission coil117and the first transmission/reception circuit119, while a signal input to the second directional coupler134is transmitted to the rectifier circuit127and the second transmission/reception circuit128.

In the configuration of the wireless power feeding system inFIG. 5, data of the values of DC voltage and direct current generated in the power-receiving device190are sent back to the power-transmitting device180according to the pattern of change in the impedance of the power-receiving device190relative to the power-transmitting device180. The power-transmitting device180receiving data of the DC voltage value and the direct current value obtained in the power-receiving device190can adjust the output of the high-frequency power supply111as needed.

When the battery124is a lithium-ion secondary battery, for example, the system is designed to prevent voltage exceeding a certain level from being applied to the battery124in order to inhibit application of excess voltage. Therefore, when voltage applied to the battery124increases to a higher level than required, a charging control circuit included in the battery charger123eventually reduces the voltage and power corresponding to the reduced voltage is consumed in vain. For this reason, it is preferable to reduce the output power of the high-frequency power supply111in the power-transmitting device180and supply the minimum power for maintaining the charging state.

<Power Feeding Method for Wireless Power Feeding System>

Next, a power feeding method for a wireless power feeding system according to one embodiment of the present invention will be described with reference toFIG. 4that has been used for the description of the above embodiment.

Refer to the power feeding method according to the above embodiment for the steps from initiation of power transmission to change in the impedance of the battery124(the steps S11to S13ainFIG. 4).

Then, the microprocessor125performs processing for obtainment of received power in accordance with a demodulated signal obtained through the second directional coupler134and the second transmission/reception circuit128. Subsequently, obtained received power information is output as a modulated signal via the second transmission/reception circuit128to change the voltage applied to the gate of the transistor136. Thus, the impedance of the power-receiving device190relative to the power-transmitting device180is changed according to the on/off of the transistor136.

Reverse power (power transmitted in the direction opposite to a desired direction) is amplitude-modulated through the power transmission resonance coil112and the power transmission coil117according to the pattern of change in the impedance of the power-receiving device190relative to the power-transmitting device180, and then a resulting signal is input to the first directional coupler114.

The reverse power that has been amplitude-modulated and then input to the first directional coupler114as described above is transmitted front the directional coupler114to the first transmission/reception circuit119, demodulated in the first transmission/reception circuit119, and then supplied to the microprocessor115. The microprocessor115commands the high-frequency power supply111in accordance with a demodulated signal obtained with the first transmission/reception circuit119to adjust the output. Consequently, the high-frequency power supply111supplies optimal power dependent on the progress of the charging (see the step S13binFIG. 4). At the same time, the microprocessor115supplies the supplied reverse power to the mixer116via the first transmission/reception circuit119.

Then, the microprocessor115determines whether the charging is to be continued, based on the output of the high-frequency power supply111(see the step S14inFIG. 4). When the output of the high-frequency power supply111becomes 0 and it is determined that the charging is not to be continued, the charging is completed by turning off the high-frequency power supply111(see the step S15inFIG. 4). When it is determined that the charging is to be continued, the next step is taken.

After a certain period (e.g., 10 sec) of charging (see the step S16inFIG. 4), whether the impedance at the instant enables the charging to be performed under the conditions for the maximum power transmission efficiency is confirmed.

The steps S121to S12n, the step S13a; the step S13b, the step S14illustrated inFIG. 4are taken again to change the amount of charging current supplied to the battery124, make settings to optimize the impedance of the entire load121, make settings to optimize the output of the high-frequency power supply111, and then determine whether the charging is to be continued.

Subsequently, a loop consisting of the steps. S121to S12n, the step S13a, the step S13b, the step. S14, and, the step S16is repeated.

By repeating the loop in this manner, the battery124can be charged efficiently even when the distance between the power-transmitting device180and the power-receiving device190changes during the charging.

The use of the above-described power feeding method for the wireless power feeding system inFIG. 5increases power transmission efficiency in accordance with the positional relationship between the power-transmitting device180and the power-receiving device190, resulting in efficient power feeding. Therefore, the power feeding system can be more convenient for users. In this embodiment, communication based on wireless IC technology is achieved through an interface for resonance type power feeding. Therefore, by applying the wireless power feeding system according to this embodiment to mobile devices such as cellular phones, data transmission/reception for wireless IC fare cards or wireless applications such as electronic money can be accomplished without an additional private communication interface.

This embodiment describes applications of the wireless power feeding system described in the above embodiments. Examples of applications of the wireless power feeding system according to one embodiment of the present invention include portable electronic devices, such as digital video cameras, personal digital assistants (e.g., mobile computers, mobile phones, portable game machines, and e-book readers), image reproducing devices including a recording medium (specifically digital versatile disc (DVD) players), and electric mobile units powered by electricity, such as electric cars. Examples will be described below with reference to drawings.

FIG. 6Aillustrates the case where the wireless power feeding system is used for a mobile phone or a personal digital assistant. The wireless power feeding system includes a power-transmitting device701, a mobile phone702A including a power-receiving device703A, and a mobile phone702B including a power-receiving device703B. The wireless power feeding system according to the above embodiments can be used between the power-transmitting device701and the power-receiving device703A and between the power-transmitting device701and the power-receiving device703B.

For example, the power-transmitting device701can have the configuration of the power-transmitting device140,160, or180according to the above embodiments, while the power-receiving device703A and the power-receiving device703B can have the configuration of the power-receiving device150,170, or190according to the above embodiments.

The use of the wireless power feeding system in one embodiment of the present invention can increase power transmission efficiency in accordance with the positional relationship between the power-transmitting device701and the power-receiving device703A and the positional relationship between the power-transmitting device701and the power-receiving device703B; thus, the power-transmitting device701can supply over efficiently to the power-receiving device703A and the power-receiving device703B.

FIG. 6Billustrates the case where the wireless power feeding system is used for an electric car, a type of electric mobile unit. The wireless power feeding system includes a power-transmitting device711and an electric car712including a power-receiving device713. The wireless power feeding system according to the above embodiments can be used between the power-transmitting device711and the power-receiving device713.

For example, the power-transmitting device711can have the configuration of the power-transmitting device140,160, or180according to the above embodiments, while the power-receiving device713can have the configuration of the power-receiving device150,170, or190according to the above embodiments.

The use of the wireless power feeding system in one embodiment of the present invention can increase power transmission efficiency in accordance with the positional relationship between the power-transmitting device711and the power-receiving device713; thus, the power-transmitting device711can supply power efficiently to the power-receiving device713.

As described above, the wireless power feeding system according to the above embodiments can be used in any object which is electrically driven.

This embodiment can be implemented in appropriate combination with any structure described in the other embodiments.

Example 1 describes how the impedance of a battery, with which the maximum power transmission efficiency between a power transmission resonance coil and a power reception resonance coil is obtained, varies depending on the distance between the power transmission resonance coil and the power reception resonance coil (also called coil-to-coil distance) with reference toFIGS. 7A and 7B.FIG. 7Ashows the case where the coil-to-coil distance is longer than the distance with which the maximum transmission efficiency between the coils is obtained and magnetic coupling between the coils is weak.FIG. 7Bshows the case where the coil-to-coil distance is shorter the distance with which the maximum transmission efficiency between the coils is obtained and magnetic, coupling between the coils is strong. InFIGS. 7A and 713, the horizontal axis represents frequency f [MHz], while, the vertical axis represents power transmission efficiency [%]. The measurement was made with load resistors with four types of resistances: 20Ω, 35Ω, 50Ω, and 100Ω which are used to change the impedance of the battery.

It was found that, as shown inFIG. 7A, the maximum transmission efficiency is obtained (f=13.56 MHz) when the impedance of the battery is about 35Ω to 50Ω in the case where the coil-to-coil distance is longer than the distance with which the maximum transmission efficiency between the coils is obtained.

It was also found that, as shown inFIG. 7B, the maximum power transmission efficiency is obtained (f=13.56 MHz) when the impedance of the battery is, about 20Ω in the case where the coil-to-coil distance, is, shorter than the distance with which the maximum transmission efficiency between the coils is obtained.

As shown inFIGS. 7A and 7B, charging can be achieved without consuming power in vain by adjusting the impedance of the battery with the load resistors such that the maximum power transmission efficiency is always obtained in accordance with positional relationship between the coils, that is, the degree of magnetic coupling between the coils.

This application is based on Japanese Patent Application serial no. 2011-135059 filed with Japan Patent Office on Jun. 17, 2011, the entire contents of which are hereby incorporated by reference.