Power supply apparatus and method for controlling power supply apparatus

1. A power supply apparatus comprising:

a resonant circuit that comprises a power supply coil and determines a resonant frequency;

a drive unit that drives the resonant circuit; and

a controller that causes the resonant frequency to change between a first resonant frequency and a second resonant frequency based on a phase difference between a phase of a drive signal of the drive unit and a phase of at least one of a current and a voltage flowing in the power supply coil being eliminated,

wherein the first resonant frequency is higher than a frequency of the drive signal, and

wherein the second resonant frequency is lower than the frequency of the drive signal.

2. The power supply apparatus according to claim 1 , wherein the controller causes the resonant frequency to change between the first resonant frequency and the second resonant frequency after a predetermined time period is elapsed from when the phase difference is eliminated.

3. The power supply apparatus according to claim 2 , wherein the predetermined time period is longer than a period of the drive signal.

4. The power supply apparatus according to claim 3 , wherein the predetermined time period is 1.5 times the period of the drive signal.

5. The power supply apparatus according to claim 2 , wherein the predetermined time period is shorter than a period of the drive signal.

6. The power supply apparatus according to claim 1 , wherein the controller causes the resonant frequency to change between the first resonant frequency and the second resonant frequency when a value of the current flowing in the power supply coil becomes zero after the phase difference is eliminated.

7. The power supply apparatus according to claim 1 , further comprising: a phase comparator that outputs the phase difference as a voltage value proportional to the phase difference.

8. The power supply apparatus according to claim 1 , further comprising:

a synchronizer; and

a period adjuster that sends, to the synchronizer, a signal for changing between the first resonant frequency and the second resonant frequency after a predetermined time period from when the phase difference is eliminated.

9. The power supply apparatus according to claim 1 , wherein the resonant circuit comprises a lumped parameter element.

10. The power supply apparatus according to claim 9 , wherein

the lumped parameter element comprises a first capacitor and a second capacitor connected to the power supply coil,

the first resonant frequency is determined by the power supply coil and the first capacitor, and

the second resonant frequency is determined by the power supply coil, the first capacitor and the second capacitor.

11. The power supply apparatus according to claim 10 , wherein

the first capacitor is connected in series to the power supply coil,

the second capacitor is connected in parallel to the first capacitor,

the power supply apparatus further comprises a switch connected to the first capacitor and the second capacitor, and

the switch switches between connection and disconnection between the first capacitor and the second capacitor.

12. The power supply apparatus according to claim 10 , wherein

the drive unit includes a first drive unit connected to the first capacitor and a second drive unit connected to the second capacitor,

the first resonant frequency is a resonant frequency when the first drive unit drives the resonant circuit, and

the second resonant frequency is a resonant frequency when the first drive unit and the second drive unit drive the resonant circuit.

13. The power supply apparatus according to claim 10 , wherein

the drive unit includes a first drive unit connected to the first capacitor and a second drive unit connected to the second capacitor,

the first resonant frequency is a resonant frequency when the first drive unit drives the resonant circuit, and

the second resonant frequency is a resonant frequency when the second drive unit drives the resonant circuit.

14. The power supply apparatus according to claim 9 , wherein

the lumped parameter element comprises a capacitor connected to the power supply coil and an inductor connected to the capacitor,

the first resonant frequency is determined by the capacitor and the power supply coil, and

the second resonant frequency is determined by the capacitor, the inductor, and the power supply coil.

15. The power supply apparatus according to claim 1 , further comprising: a voltage phase detector that detects the phase of the voltage applied to the power supply coil, wherein

the phase difference is a phase difference between the phase of the drive signal and the phase of the voltage applied to the power supply coil.

16. A method for controlling a power supply apparatus, the method comprising:

changing a resonant frequency of a resonant circuit between a first resonant frequency and a second resonant frequency based on a phase difference between a phase of a drive signal of a drive unit and a phase of at least one of a current and a voltage flowing in a power supply coil of the resonant circuit being eliminated, wherein

the first resonant frequency is higher than a frequency of the drive signal, and

the second resonant frequency is lower than the frequency of the drive signal.

17. The method according to claim 16 , wherein the changing causes a switch to change between the first resonant frequency and the second resonant frequency.

18. The method according to claim 16 , further comprising:

outputting, with a phase comparator, the phase difference as a voltage value proportional to the phase difference.

19. The method according to claim 16 , further comprising:

detecting, with a voltage phase detector, the phase of the voltage applied to the power supply coil, wherein

the phase difference is a phase difference between the phase of the drive signal and the phase of the voltage applied to the power supply coil.