Wireless power transfer system and method

1. A system comprising:

a primary switch network coupled to a power source, wherein the primary switch network comprises a plurality of power switches;

a primary resonant tank coupled to the plurality of power switches, wherein a resonant capacitor of the primary resonant tank is formed by a first variable capacitance network, and wherein the first variable capacitance network is modulated to improve soft switching of the plurality of power switches through reducing a voltage level and a current level of a switch at a turn-on instant; and

a primary coil coupled to the primary resonant tank.

2. The system of claim 1 , wherein:

the first variable capacitance network comprises a first capacitor and a diode connected in series, and a plurality of switch-capacitor networks connected in parallel with the diode, and wherein switches in the plurality of switch-capacitor networks are configured to change a capacitance of the first variable capacitance network.

3. The system of claim 1 , further comprising:

a secondary coil magnetically coupled to the primary coil; and

a secondary resonant tank having a second variable capacitance network coupled to an output, wherein a capacitance of the second variable capacitance network is modulated to regulate a voltage or power at the output during normal operation.

4. The system of claim 3 , wherein:

the capacitance of one of the variable capacitance networks is adjusted to protect the system during a fault.

5. The system of claim 3 , wherein:

the capacitance of one of the variable capacitance networks is adjusted gradually to implement a soft start process.

6. The system of claim 1 , further comprising:

a soft switching observer having a first input receiving a feedback signal representing a current flowing through the primary switch network; and

a control and protection unit configured to reduce a switching current at a switching instant of the switch and generate gate drive signals for the plurality of power switches at a first output of the control and protection unit and a control signal for adjusting a capacitance of the first variable capacitance network at a second output of the control and protection unit.

7. The system of claim 6 , wherein:

the feedback signal is generated from a voltage across the switch.

8. The system of claim 1 , further comprising:

an EMI filter comprising a plurality of resonant circuits, wherein each resonant circuit has a capacitor and an inductor, and wherein the capacitor is trimmed so that a resonant frequency of a resonant circuit comprising the trimmed capacitor is approximately equal to a predetermined value.

9. A method for a power apparatus comprising:

detecting a signal representing a current level at a turn-on transition of a power switch of a primary switch network, wherein the primary switch network is coupled to a primary resonant tank having a resonant capacitor formed by a first variable capacitance network comprising a plurality of switch-capacitor networks; and

modulating a capacitance of the first variable capacitance network to improve soft switching of the power switch through reducing a voltage level and a current level at the turn-on transition of the power switch.

10. The method of claim 9 , further comprising:

gradually adjusting a capacitance of the first variable capacitance network to implement a soft start of the power apparatus.

11. The method of claim 9 , wherein:

the primary switch network and the primary resonant tank are magnetically coupled to a secondary side comprising a secondary resonant tank, wherein the secondary resonant tank comprises a secondary resonant capacitor formed by a second variable capacitance network.

12. The method of claim 11 , further comprising:

adjusting a capacitance of the second variable capacitance network so that an output voltage or output current of the secondary side is regulated at a predetermined level.

13. The method of claim 11 , further comprising:

modulating the capacitance of one of the variable capacitance networks; and

establishing a communication channel through sensing a signal change during the step of modulating the capacitance of one of the variable capacitance networks.

14. The method of claim 11 , further comprising:

adjusting the capacitance of the second variable capacitance network gradually to achieve a soft start process or a soft stop process of at an output of the secondary side.

15. The method of claim 11 , further comprising:

adjusting the capacitance of one of the variable capacitance networks to protect the power apparatus during abnormal operating conditions.

16. A method comprising:

providing a wireless power transfer system comprising a transmitter magnetically coupled to a first receiver, wherein:

the transmitter comprises a power amplifier coupled to an input power source, a transmitter resonant tank comprising a first variable capacitance network and a transmitter coil coupled to the transmitter resonant tank; and

the first receiver comprises a first receiver resonant tank comprising a first receiver variable capacitance network and a first receiver coil coupled to the first receiver resonant tank; and

modulating a capacitance of the first variable capacitance network so that switches of the power amplifier achieve an improved soft switching condition.

17. The method of claim 16 , further comprising:

detecting a current flowing through the transmitter; and

modulating the capacitance of the first variable capacitance network based upon the detected current to maintain a current in the transmitter at a predetermined level at a switching instant.

18. The method of claim 17 , wherein:

a control speed of the step of modulating the capacitance of the first variable capacitance network is based upon the detected current.

19. The method of claim 16 , further comprising:

coupling a second receiver to the transmitter, wherein the second receiver comprises a second receiver resonant tank comprising a second receiver variable capacitance network and a second receiver coil coupled to the second receiver resonant tank, and wherein the step of coupling the second receiver to the transmitter is implemented in a smooth manner by adjusting a capacitance of the second receiver variable capacitance network.

20. The method of claim 19 , further comprising:

decoupling the second receiver from the transmitter in a smooth manner by adjusting the capacitance of the second receiver variable capacitance network.