Current sensing in a wireless power transfer system

1. A method of measuring current through a coil comprising a plurality of coil windings, comprising:

connecting a current sensing resistor to a subset of the plurality of coil windings to directly measure a current through the subset of the plurality of coil windings;

measuring a voltage drop across the current sensing resistor; and

determining a fractional current and phase of the coil from the measured current and voltage.

2. The method of claim 1 , wherein the coil is a secondary winding used in a wireless power transfer system comprising a rectifier that converts an alternating current on the secondary winding into a direct current for application to a load.

3. The method of claim 2 , further comprising digitizing the measured voltage and providing the measured voltage to a rectifier controller that determines the fractional current and phase of the coil and controls the operation of the rectifier.

4. The method of claim 3 , further comprising calculating a total current I through the secondary winding as I={Σ[(Vn/Rn/n)]}*m, where Vn is a measured voltage for each sampled secondary winding, Rn is a resistance for each current sensing resistor of each sampled secondary winding, n is a number of coil windings sampled, and m is a total number of coil windings of the coil.

5. The method of claim 3 , further comprising calculating a total current of the coil as I=n(V/Rz), where n is the number of coil windings of the coil, V is the measured voltage, and Rz is an impedance of the current sensing resistor.

6. The method of claim 1 , further comprising selecting the current sensing resistor to at least one of (a) limit inductance to prevent out-of-phase subtraction when measured current returns to the plurality of coil windings and (b) reduce impact of inductance while keeping heating and power loss to a minimum.

7. A current sensing device for measuring current through a coil comprising a plurality of coil windings, comprising:

a current sensing resistor connected to a subset of the plurality of coil windings to directly measure current through the subset of the plurality of coil windings;

a voltage sensor that measures a voltage drop across the current sensing resistor; and

a processor that determines a fractional current and phase of the coil from the measured current and voltage.

8. The device of claim 7 , wherein the coil is a secondary winding used in a wireless power transfer system comprising a rectifier that converts an alternating current on the secondary winding into a direct current for application to a load.

9. The device of claim 8 , further comprising an analog to digital converter that digitizes the voltage measured by the voltage sensor and provides the measured voltage to the processor.

10. The device of claim 9 , wherein the processor determines a total current I through the secondary winding as I={Σ[(Vn/Rn/n)]}*m*, where Vn is the measured voltage for each secondary winding, Rn is a resistance for each current sensing resistor of each secondary winding, n is a number of coil windings sampled, and n is a total number of coil windings of the coil.

11. The device of claim 9 , wherein the processor determines the fractional current and phase of the coil by calculating a total current of the coil as I=n(V/Rz), where n is a number of coil windings of the coil, V is the measured voltage, and Rz is an impedance of the current sensing resistor.

12. The device of claim 7 , wherein the coil windings comprise one of Litz wire, printed circuit board traces, and conductive filaments.

13. The device of claim 7 , wherein the coil windings are impedance matched and tightly coupled via mutual inductance with each other.

14. The device of claim 8 , wherein the wireless power transfer system comprises one of an inductive system and a capacitively coupled system.