Power feeding system, power feeding device, and power feeding method

1. A power feeding system, comprising a power feeding device comprising a feeding coil and a power receiving device comprising a receiving coil, for supplying electric power from the power feeding device to the power receiving device via electromagnetic induction,

the power receiving device comprising a resonant circuit, the resonant circuit comprising:

the receiving coil to be supplied with electric power from the feeding coil;

a resonant capacitor configured to resonate with the receiving coil; and

a first switching element configured to change an electrical connection state of the resonant capacitor to control a resonant state of the resonant circuit,

the power feeding device comprising:

a second switching element connected in series to the feeding coil;

a drive signal generation section configured to generate a drive signal for driving the feeding coil by changing a state of the second switching element between a conductive state and a non-conductive state;

a variation detection section configured to detect the change in the electrical connection state of the resonant capacitor as a periodic waveform variation in a voltage excited in the feeding coil; and

a drive control section configured to determine whether or not electric power is suppliable to the power receiving device based on the periodic waveform variation detected by the variation detection section, and control whether or not to continuously supply the drive signal to the second switching element based on a result of the determination.

2. A power feeding system according to claim 1 , wherein the drive control section is configured to:

supply the drive signal to the second switching element in a predefined first period, to thereby cause the variation detection section to detect the periodic waveform variation;

continue, when it is determined in the first period that electric power is suppliable to the power receiving device, the supply of the drive signal to the second switching element in a predefined second period after the predefined first period, to thereby continuously drive the feeding coil; and

stop, when it is determined in the first period that electric power is not suppliable to the power receiving device, the supply of the drive signal to the second switching element in the second period, to thereby intermittently drive the feeding coil.

3. A power feeding system according to claim 2 , wherein the drive control section comprises a timing generation section configured to generate the first period and the second period alternately and regularly.

4. A power feeding system according to claim 2 , wherein the second period is defined to be longer than the first period so that an increased temperature of a foreign object caused by heat generation when the feeding coil and the foreign object are electromagnetically coupled to each other becomes a predetermined temperature or less.

5. A power feeding system according to claim 3 , wherein the second period is defined to be longer than the first period so that an increased temperature of a foreign object caused by heat generation when the feeding coil and the foreign object are electromagnetically coupled to each other becomes a predetermined temperature or less.

6. A power feeding system according to claim 1 , wherein:

the periodic waveform variation comprises a variation in peak voltage of the excited voltage;

the variation detection section detects the change in the electrical connection state of the resonant capacitor as the variation in the peak voltage; and

the drive control section determines whether or not electric power is suppliable to the power receiving device based on the variation in the peak voltage detected by the variation detection section.

7. A power feeding system according to claim 2 , wherein:

the periodic waveform variation comprises a variation in peak voltage of the excited voltage;

the variation detection section detects the change in the electrical connection state of the resonant capacitor as the variation in the peak voltage; and

the drive control section determines whether or not electric power is suppliable to the power receiving device based on the variation in the peak voltage detected by the variation detection section.

8. A power feeding system according to claim 3 , wherein:

the periodic waveform variation comprises a variation in peak voltage of the excited voltage;

the variation detection section detects the change in the electrical connection state of the resonant capacitor as the variation in the peak voltage; and

the drive control section determines whether or not electric power is suppliable to the power receiving device based on the variation in the peak voltage detected by the variation detection section.

9. A power feeding system according to claim 6 , wherein the variation detection section comprises:

a peak hold circuit configured to hold the peak voltage of the excited voltage; and

a detection circuit configured to detect the variation in the peak voltage based on a voltage held by the peak hold circuit and the peak voltage of the excited voltage.

10. A power feeding system according to claim 7 , wherein the variation detection section comprises:

a peak hold circuit configured to hold the peak voltage of the excited voltage; and

a detection circuit configured to detect the variation in the peak voltage based on a voltage held by the peak hold circuit and the peak voltage of the excited voltage.

11. A power feeding system according to claim 8 , wherein the variation detection section comprises:

a peak hold circuit configured to hold the peak voltage of the excited voltage; and

a detection circuit configured to detect the variation in the peak voltage based on a voltage held by the peak hold circuit and the peak voltage of the excited voltage.

12. A power feeding system according to claim 1 , wherein:

the periodic waveform variation comprises a variation in pulse width of the excited voltage;

the variation detection section detects the change in the electrical connection state of the resonant capacitor as the variation in the pulse width; and

the drive control section determines whether or not electric power is suppliable to the power receiving device based on the variation in the pulse width detected by the variation detection section.

13. A power feeding system according to claim 2 , wherein:

the periodic waveform variation comprises a variation in pulse width of the excited voltage;

the variation detection section detects the change in the electrical connection state of the resonant capacitor as the variation in the pulse width; and

the drive control section determines whether or not electric power is suppliable to the power receiving device based on the variation in the pulse width detected by the variation detection section.

14. A power feeding system according to claim 3 , wherein:

the periodic waveform variation comprises a variation in pulse width of the excited voltage;

the variation detection section detects the change in the electrical connection state of the resonant capacitor as the variation in the pulse width; and

the drive control section determines whether or not electric power is suppliable to the power receiving device based on the variation in the pulse width detected by the variation detection section.

15. A power feeding system according to claim 1 , wherein the power receiving device further comprises a resonance control section configured to control the first switching element to control the resonant state of the resonant circuit, the resonance control section being configured to control the first switching element in accordance with a current flowing into a battery that is charged with DC electric power obtained by rectifying electric power received by the receiving coil.

16. A power feeding system according to claim 1 , wherein the drive signal generation section comprises:

a first signal generation section configured to generate the drive signal for setting, when the excited voltage falls within a predetermined threshold range, the second switching element to the conductive state in a predefined third period and thereafter setting the second switching element to the non-conductive state; and

a second signal generation section configured to generate the drive signal for setting, when the excited voltage becomes out of the predetermined threshold range, the second switching element to the conductive state after a lapse of a predefined fourth period.

17. A power feeding device for a power feeding system, for supplying electric power to a power receiving device via electromagnetic induction,

the power receiving device comprising a resonant circuit, the resonant circuit comprising: a receiving coil to be supplied with electric power from a feeding coil comprised in the power feeding device; a resonant capacitor configured to resonate with the receiving coil; and a first switching element configured to change a connection state of the resonant capacitor to control a resonant state of the resonant circuit,

the power feeding device comprising:

a second switching element connected in series to the feeding coil;

a drive signal generation section configured to generate a drive signal for driving the feeding coil by changing a state of the second switching element between a conductive state and a non-conductive state;

a variation detection section configured to detect a change in the resonant state of the resonant circuit, which occurs in accordance with the connection state of the resonant capacitor, as a periodic waveform variation in a voltage excited in the feeding coil; and

a drive control section configured to determine whether or not electric power is suppliable to the power receiving device based on the periodic waveform variation detected by the variation detection section, and control whether or not to continuously supply the drive signal to the second switching element based on a result of the determination.

18. A power feeding method for a power feeding system configured to supply electric power from a power feeding device to a power receiving device via electromagnetic induction,

the power feeding device comprising: a feeding coil; and a drive signal generation section configured to generate a drive signal for driving the feeding coil by changing a state of a second switching element connected in series to the feeding coil between a conductive state and a non-conductive state,

the power receiving device comprising a receiving coil, the power feeding method comprising:

varying, by the power receiving device, a resonant state of a resonant circuit,

the resonant circuit comprising the receiving coil to be supplied with electric power from the feeding coil, a resonant capacitor configured to resonate with the receiving coil, and a first switching element configured to change a connection state of the resonant capacitor to control the resonant state of the resonant circuit;

detecting, by the power feeding device, a change in the resonant state of the resonant circuit, which occurs in accordance with the connection state of the resonant capacitor, as a periodic waveform variation in a voltage excited in the feeding coil; and

determining, by the power feeding device, whether or not electric power is suppliable to the power receiving device based on the periodic waveform variation detected in the detecting, and controlling, by the power feeding device, whether or not to continuously supply the drive signal to the second switching element based on a result of the determination.