Q-factor measurement

1. A method of determining a Q-factor in a transmit circuit with a resonant circuit, comprising:

setting a system voltage;

performing a coarse scan to determine a coarse resonant frequency;

performing a fine scan based on the coarse scan to determine a resonant frequency;

performing a final measurement at the resonant frequency to determine an average system voltage and an average peak voltage of the resonant circuit;

calculating a Q parameter from the average system voltage and the average peak voltage; and

calculating the Q-factor from the Q parameter.

2. The method of claim 1 , wherein setting the system voltage includes adjusting a buck voltage converter to produce the system voltage.

3. The method of claim 1 , wherein the system voltage is set at a low enough voltage that a receiver in the proximity of the transmit circuit will not be activated.

4. The method of claim 1 , wherein performing a coarse scan comprises:

setting a coarse frequency range and a coarse step size;

sweeping a frequency through the frequency range at the coarse step size while monitoring a peak voltage of the resonant circuit; and

determining the coarse resonant frequency wherein the peak voltage is at a maximum.

5. The method of claim 1 , wherein performing a fine scan comprises:

setting a fine frequency range based on the coarse resonant frequency;

setting a fine step size based on the fine frequency range;

sweeping a frequency through the fine frequency range at the fine step size while monitoring a peak voltage of the resonant circuit; and

determining the resonant frequency where the peak voltage is at a maximum.

6. The method of claim 5 , wherein the fine frequency range is set from a percentage of the coarse resonant frequency less than the coarse resonant frequency to the percentage of the coarse resonant frequency higher than the coarse resonant frequency.

7. The method of claim 6 , wherein the percentage is 25%.

8. The method of claim 6 , wherein the percentage is 10%.

9. The method of claim 1 , wherein performing a final measurement comprises:

setting a frequency at the resonant frequency;

setting a duty cycle;

measuring the average system voltage VBRG and the average peak voltage at the frequency and the duty cycle.

10. The method of claim 1 , wherein calculating the Q parameter as the ratio of the average peak voltage to the average system voltage.

11. The method of claim 10 , wherein the average peak voltage is adjusted according to an ADC parameter.

12. The method of claim 1 , wherein calculating the Q-factor from a 2 nd order polynomial or linear piece-wise equation fitting to the Q parameter.

13. A transmitter that determines a Q-factor, comprises:

a switching bridge circuit;

a resonant circuit that includes a transmit coil and a transmit capacitor coupled to the switching bridge circuit; and

a controller coupled to the switching bridge circuit to control operation of the resonant circuit and coupled to the resonant circuit to receive a peak voltage from the resonant circuit, wherein the controller executes instructions to set a system voltage,

perform a coarse scan to determine a coarse resonant frequency,

perform a fine scan based on the coarse scan to determine a resonant frequency,

perform a final measurement at the resonant frequency to determine an average system voltage and an average peak voltage of the resonant circuit,

calculate a Q parameter from the average system voltage and the average peak voltage, and

calculate the Q-factor from the Q parameter.

14. The transmitter of claim 13 , further including a buck voltage converter to provide the system voltage and wherein the controller sets the system voltage by adjusting the buck voltage converter.

15. The transmitter of claim 14 , wherein the system voltage is set at a low enough voltage that a receiver in the proximity of the transmit circuit will not be activated.

16. The transmitter of claim 13 , wherein the controller performs a coarse scan by

setting a coarse frequency range and a coarse step size;

sweeping a frequency through the frequency range at the coarse step size while monitoring a peak voltage of the resonant circuit; and

determining the coarse resonant frequency wherein the peak voltage is at a maximum.

17. The transmitter of claim 13 , wherein the controller performs a fine scan by

setting a fine frequency range based on the coarse resonant frequency;

setting a fine step size based on the fine frequency range;

sweeping a frequency through the fine frequency range at the fine step size while monitoring a peak voltage of the resonant circuit; and

determining the resonant frequency where the peak voltage is at a maximum.

18. The transmitter of claim 17 , wherein the fine frequency range is set from a percentage of the coarse resonant frequency less than the coarse resonant frequency to the percentage of the coarse resonant frequency higher than the coarse resonant frequency.

19. The transmitter of claim 18 , wherein the percentage is 25%.

20. The transmitter of claim 18 , wherein the percentage is 10%.

21. The transmitter of claim 13 , wherein the controller performs a final measurement by

setting a frequency at the resonant frequency;

setting a duty cycle;

measuring the average system voltage VBRG and the average peak voltage at the frequency and the duty cycle.

22. The transmitter of claim 13 , wherein the controller calculates the Q parameter as the ratio of the average peak voltage to the average system voltage.

23. The transmitter of claim 22 , further including

a select circuit that provides a divided voltage from the peak voltage, and

a scale circuit that receives the divided voltage and provides a coil voltage to an analog-to-digital converter of the controller,

wherein the average peak voltage is adjusted according to an ADC parameter.

24. The transmitter of claim 23 , wherein the controller calculates the Q-factor from a 2 nd order polynomial or linear piece-wise equation fitting to the Q parameter.