Current detection in wireless power transfer systems

Abstract

[Solution] A current sensing method measures the partial current through a coil having multiple coil windings by measuring the current through a subset of multiple coil windings using a current sensing resistor and measuring the voltage drop across the current sensing resistor using a voltage sensor. The measured current and voltage values ​​are provided to a processor to determine the partial current and phase of the coil. For example, the partial current and phase of the coil can be determined by calculating the total current of the coil as I=n(V / Rx), where n is the number of coil windings in the coil, V is the measured voltage, and Rx is the impedance of the current sensing resistor. The coil can be a secondary winding used in a wireless power transfer system.

Description

Technical Field

[0001] The present disclosure relates to current measurement, and more specifically, to current measurement in a wireless power transmission system.

Background Art

[0002] For resonant inductive wireless charging, an air-core transformer composed of two concentric coils arranged along a common coil axis is used. Power is sent from a transmitting device (i.e., a primary coil) to a receiving device (i.e., a secondary coil) by magnetic flux linkage between the two transmission coils. The alternating current flowing through the primary coil induces an alternating current in the secondary coil.

[0003] As one option for constructing the coil, Litz wire (also known as Litzendraht wire) or other conductive filaments are used. Litz wire is composed of individually insulated wires twisted or braided in a uniform pattern and has a major advantage in reducing the AC loss of high-frequency windings. Alternatively, as described in the international patent application PCT / US2018 / 035060 "WIRELESS POWER TRANSFER THINPROFILE COIL ASSEMBLY", the coil conductor can be composed of a plurality of conductive traces stacked on an insulating dielectric substrate (such as a printed circuit board, etc.).

[0004] Current is defined as moving charges (such as electrons). Current is dq / dt, that is, the time rate of change of charge. The magnitude of the electric current is expressed in amperes. The unit ampere (A) is defined as equal to the flow of 1 coulomb of charge per second. The measurement of current in an electrical circuit can be performed directly (e.g., by using a detection resistor, etc.) or indirectly (e.g., by using a Hall effect sensor or an inductive sensor, etc.).

[0005] It is desirable to provide a current measuring device for high currents that may appear in wireless power transmission systems without adversely affecting the operation of the wireless power transmission system. [Overview of the project] [Means for solving the problem]

[0006] Various details of embodiments of the subject matter of the present invention are described in the accompanying drawings and the following detailed description.

[0007] In an exemplary embodiment, a current sensing device is provided for measuring the current flowing through a coil having a plurality of coil windings (e.g., Litz wire, printed circuit board traces, or conductive filaments). The coil windings are impedance-matched and tightly coupled to one another via mutual inductance. The current sensing device includes a current sensing resistor connected to a subset of the plurality of coil windings for measuring the current flowing through the subset of coil windings; a voltage sensor for measuring the voltage drop across the current sensing resistor; and a processor for determining the partial current and phase of the coil from the measured current and voltage. In an exemplary embodiment, the coil is a secondary winding used in a wireless power transmission system (inductive or capacitively coupled system), and the wireless power transmission system includes a rectifier that converts the AC current of the secondary winding into a DC current for application to a load. An analog-to-digital converter may be provided to digitize the voltage measured by the voltage sensor and provide the measured voltage to the processor.

[0008] In an exemplary application, the processor determines the total current I flowing through the secondary winding as I = nV / sqrt(2R + 2π*L*f), where n is the number of coil windings in the secondary winding, V is the measured voltage, R is the resistance of the current sensing resistor, L is the inductance of the secondary winding, and f is the frequency of the AC current in the secondary winding. Alternatively, the processor can determine the total current I flowing through the secondary winding as I = {Σ[(Vn / Rn / n)]}*m, where Vn is the measured voltage of each secondary winding, Rn is the resistance of each current sensing resistor in each secondary winding, n is the number of sampled coil windings, and m is the total number of coil turns in the coil. The processor determines the partial current and phase of the coil by calculating the total current of the coil as I = n(V / Rz). Here, n is the number of coil windings in the coil, V is the measured voltage, and Rz is the impedance of the current sensing resistor.

[0009] In an exemplary embodiment, the sensing resistor has a value R >> X, where X = 2π * L * f, where L is the inductance of the secondary winding and f is the frequency of the AC current of the secondary winding. For example, R > nX, where n is the number of coil windings in the coil.

[0010] A method is also provided for measuring the current flowing through a coil having a plurality of coil windings. The method includes the steps of: measuring the current flowing through a subset of the plurality of coil windings using a current sensing resistor; measuring the voltage drop across the current sensing resistor; and determining the partial current and phase of the coil from the measured current and voltage. The method may be carried out in an embodiment in which the coil is a secondary winding used in a wireless power transmission system, and the wireless power transmission system has a rectifier that converts the alternating current of the secondary winding into a direct current for application to a load. The method further includes the steps of: digitizing the measured voltage; and providing the measured voltage to a rectifier control unit, the rectifier control unit determining the partial current and phase of the coil and controlling the operation of the rectifier.

[0011] In an exemplary embodiment of the above method, the method includes the step of calculating the total current I flowing through the secondary winding as I = nV / sqrt(2R + 2π * L * f), where n is the number of coil windings in the secondary winding, V is the measured voltage, R is the resistance of the current sensing resistor, L is the inductance of the secondary winding, and f is the frequency of the AC current in the secondary winding. The method may further include the step of calculating the total current I flowing through the secondary winding as I = {Σ[(Vn / Rn / n)]} * m, where Vn is the measured voltage of each sampled secondary winding, Rn is the resistance of the current sensing resistor of each sampled secondary winding, n is the number of sampled coil windings, and m is the total number of coil windings in the coil. Furthermore, the method may include a step of calculating the total current of the coil as I=n(V / Rz), where n is the number of coil windings in the coil, V is the measured voltage, and Rz is the impedance of the current sensing resistor.

[0012] The method may further include the step of selecting the current sensing resistor such that (a) the inductance is limited to prevent subtraction due to phase shift when the measured current returns to the plurality of coil windings, and / or (b) the inductance effect is reduced while maintaining minimal heating and power loss. In an exemplary embodiment, the current sensing resistor is selected such that the value R >> X, where X = 2π * L * f, where L is the inductance of the secondary winding and f is the frequency of the AC current in the secondary winding. For example, R is R > The number of coil windings in the coil may be selected to be nX, where n is the number of coil windings in the coil.

[0013] This summary is provided to give a simplified overview of aspects of the subject matter of the present invention, and a further description of the subject matter of the present invention follows in the detailed description. This summary is not intended to identify essential or necessary features of the subject matter in the claims, nor is any particular combination or order of elements listed in this summary intended to limit the elements of the subject matter in the claims. Rather, it will be understood that the following section provides a summary of some of the embodiments described below in the detailed description of the invention. [Brief explanation of the drawing]

[0014] The aforementioned and other beneficial features and advantages of the present invention will become apparent from the following detailed description, which is accompanied by the attached drawings. [Figure 1] Figure 1 schematically shows current measurement in a wireless power transmission system in an exemplary embodiment. [Figure 2] Figure 2 functionally illustrates a wireless power transmission system using current measurement in an exemplary embodiment. [Figure 3] Figure 3 shows the hardware for current measurement in an exemplary embodiment. [Modes for carrying out the invention]

[0015] The current sensing and related methods for wireless power transmission described herein will be more readily understood by referring to the following detailed description relating to the accompanying figures and examples that form part of this disclosure. This description is not limited to any specific product, method, condition, or parameter described and / or shown herein, and it should be understood that the terms used herein are intended solely to illustrate specific embodiments as examples and are not intended to limit the subject matter in the claims. Similarly, any description of possible mechanisms, modes of operation, or reasons for modification is for illustrative purposes only, and the subject matter described herein is not limited by whether such suggested mechanisms, modes of operation, or reasons for modification are accurate or inaccurate. Throughout this specification, this description refers to both methods and systems / software that implement such methods.

[0016] Hereinafter, exemplary embodiments will be described in detail with reference to Figures 1 to 3. The following description provides details of possible embodiments; however, it should be noted that such detailed descriptions are for illustrative purposes only and do not define the scope of the subject matter of the present invention.

[0017] Indirect current sensing is typically used in circuits with load currents in the range of 100A to 1000A. Indirect current sensing allows for galvanic isolation from the conductor under measurement. When using Hall effect-based sensors, the sensor is placed between the anode and the load. Hall effect sensors utilize the Lorentz force (the force exerted on electrons moving through a magnetic field), which creates a voltage difference (Hall voltage) across the ends of an electrical conductor transversely to the current in the conductor and the magnetic field applied perpendicular to that current, thereby determining the flow of alternating current (AC) and direct current (DC). Inductive current sensors or current sensing transformers use Faraday's law of induction, where the conductor acts as the primary and the voltage output acts as the secondary, measuring the alternating current (AC) in the conductor based on the generated magnetic flux.

[0018] At high power levels with high impedance loads, wireless power transfer (WPT) systems constantly become current sources (i.e., AC voltage-controlled current sources), and high-power WPT systems generate stray flux, making the use of indirect measurement techniques and technologies problematic, especially as part of a closed-control loop of an active rectifier.

[0019] A direct method of measuring current uses a sensing resistor. A sensing resistor is a resistor with a low nominal ohm value, typically placed in series with the circuit network near the circuit ground or the battery cathode. The total current flowing through the sensing resistor is then measured by measuring the voltage drop across the resistor and calculating the current as I(t) = v(t) / R, where i(t) is the current (amperes) as a function of time, v(t) is the voltage (volts) across the sensing resistor as a function of time, and R is the resistance (ohms) of the sensing resistor. The sensing resistor is selected to have minimal resistance to avoid excessive heating and perturbation in the power supply to the load (since power loss in the sensing resistor is proportional to its resistance).

[0020] For the same reason, in AC systems, such as when used in magnetic resonance-based WPT systems, the selected sensing resistor should minimize the reactance component of the total impedance. The requirement for accurate AC signal measurement at the target frequency (e.g., current, frequency, phase) is that the reactance must be small relative to the resistance.

[0021] Measuring the total AC current using current sensing resistors in high-current systems (e.g., 125 amperes RMS or higher) is prohibited due to heating caused by power consumption. Another problem lies in the structural physics of resistors large enough to handle the current without generating unavoidable large inductance. When parasitic inductance is added, the phase angle applied to the AC signal can become too large, potentially preventing accurate measurements.

[0022] However, a method for detecting partial currents can be constructed for a WPT system, which separates a subset of conductors by utilizing a plurality of conductor structures of the secondary side coil winding, and the subset of the conductors is connected via a resistor with a low-ohm rating. The conductor current passes through a resistor (having inherent parasitic inductance) with a very small value, and the voltage drop across the resistor is measured to determine the partial current and the phase.

[0023] The current detection resistor is low-cost and maintains reliability even in the presence of stray magnetic flux generated by the WPT system. However, since the current detection resistor is a resistive element (with some self-inductance), the heat generated is proportional to the square of the current passing through it, and its usefulness is limited in high-current power supplies such as those used in WPT systems. However, by utilizing the multi-element structure of the magnetic coil, the detection resistor arranged on a single conductor provides measurement of the partial current without generating excessive heat and without affecting the power supply capacity of the entire system. The measurement of this partial current provides accurate phase measurement by multiplication and accurate current measurement for a control system (such as an active rectification system and power control feedback, etc.).

[0024] In reality, any resistor includes inevitable parasitic inductance. This inductance leads to a phase shift in the output. Since the measured conductors are returned to a tightly coupled parallel bundle of conductors and the total current load is shared through mutual inductance, the power reduction may be much greater than just resistive heating loss.

[0025] Due to the various structures of resistors, a low-inductance model can be selected. In a partial current measurement system, the relative influence of the self-inductance component can be reduced by selecting a (relatively) larger ohmic resistor because the influence of the resistor on the supplied power (and dissipated heat) decreases with the ratio of the detected conductors to the total number of conductors. The larger the resistance value, the larger the dynamic range of the voltage, and a more accurate voltage can be obtained, thus enabling more accurate detection of the partial current level and current phase.

[0026] In an exemplary embodiment, the sense resistor is selected to have a low or high inductance but still a relatively low value to limit the inductance while minimizing heating and power losses. The higher the resistance, the higher the dynamic range of the voltage, enabling more accurate current detection. The inductance is also limited to prevent subtraction by phase shift when the measured current returns to a tightly coupled bundle of conductors.

[0027] Figure 1 In FIG. 1, a plurality of conductors (e.g., Litz wire, printed circuit board traces, or conductive filaments, etc.) are used in the secondary coil of a WPT system (e.g., an inductive system, a magnetic resonance coupled inductive system, or a capacitive coupling system, etc.). The secondary side 101 is represented in the circuit as a bank of independent current sources each having its own conductor 105. Each conductor 105 is impedance-matched and tightly coupled to other conductors 105 by mutual inductance. FIG. 1 uses 10 conductors 105 for illustrative purposes only. Power is supplied to the load 106 from the secondary side 101 via a power bus 107 that couples the individual conductors 105 to a single conductor or power bus. This circuit is completed by a return electrical bus 108 from the load 106 to the secondary side 101.

[0028] Current measurement is achieved by selecting a subset of independent conductors 105 (a single conductor 109 in this example) connected via a current sensing resistor 102. The current sensing resistor 102 has both a resistive impedance component (reactance) 103 and an inductive impedance component 104. The voltage sensor 107 reads the voltage drop across the current sensing resistor 102.

[0029] The reading from the voltage sensor 107 and the impedance of the resistor 102 are used to obtain the current. Since the impedance-matched and tightly coupled conductor 105 shares the current generated on the secondary side 101, the total current supplied to the load 106 can be calculated. For example, when a single conductor 109 is measured, the total current is n(V / Rz), where n is the number of conductors (e.g., n=1 for a single conductor), V is the measured voltage across the sensing resistor 102, and Rz is the impedance of the sensing resistor 102.

[0030] Figure 2 Figure 2 shows a high-level circuit diagram of a DC battery charging circuit using magnetic induction with current measurement in an exemplary embodiment. The receiver 201 or secondary side has a secondary coil with multiple windings 202. The receiver 201 can be inductive, resonant inductive, or capacitive. The receiver 201 converts a magnetic field from a transmitter (not shown) into an alternating current. The alternating current 208 generated by the resonant network 201 is used to power a load 203. The first stage of the load is a passive (diode-based) or active (switch-based) rectifier 204 that can be used to convert the alternating current into a DC current required to charge a battery 206 (the battery can be wet, dry, solid, capacitive, or hybrid (e.g., a battery with a capacitive component)). To charge the battery 206, the rectified DC signal may be smoothed and level-converted by a conditioner circuit 205.

[0031] The current sensor 209 is used to monitor the AC 208 generated by the receiver 201. The current level, frequency, and phase are reported to the control unit 207. The control unit 207 can report the electrical signal characteristics to auxiliary systems such as displays, closed-loop control systems, safety systems, and active rectifier control switching.

[0032] Figure 3 Figure 3 shows an exemplary embodiment of a current sensor using a current sensing resistor 301. The current sensing resistor 301 has both resistive (ohm) 302 and inductive impedance components 303. In this embodiment, an analog-to-digital converter (ADC) 304 is used to digitize the voltage generated across the current sensing resistor 301. The ADC 304 is connected to other systems, such as an active rectifier control unit 207, using a digital interface 305. The active rectifier control unit 207 may include a processor that calculates the current from the measured values, as described herein.

[0033] In an example of a WPT system equipped with 60 conductive element receivers to measure a single conductor, the current flowing through the measured conductor is given by I = V / sqrt(R2 + 2π*L*f), where I is the current through the single conductor (amperes), V is the measured voltage (volts), R2 is the resistance (ohms), L is the inductance of the secondary coil (henries), and f is the frequency of the AC signal (hertz).

[0034] Alternative Embodiment – ​​Multiple Parallel Current Detection Once the current (i) of a single conductor is calculated, the system's current level is determined by multiplying this current by the number of tightly coupled conductors. That is, the current is i = nV / sqrt(R² + 2π*L*f), where n is the number of conductors. When measuring multiple conductors, the sum of the individually measured currents is averaged. Then, this average current per conductor is multiplied by the number of tightly coupled conductors to determine the system's overall current level.

[0035] In the simplest case, a single conductor is separated from a closely wound inductive coil winding for partial current detection. In some cases, multiple conductors can be detected, each having its own current detection resistor. In the case of multiple conductors, the detected currents are averaged and then multiplied. i(total) = {Σ[(Vn / Rn) / n]} * m Here, i(total) is the current of the system, Vn is the voltage sampled for conductors 1 to n, Rn is the current resistance impedance of each resistor, n is the number of sampled conductors, and m is the total number of conductors.

[0036] Alternative Implementation Methods - Unknown Parasitism Induction In the case of a high-power (i.e., large current) system, it is not desirable to detect the entire alternating current using a single current detection resistor due to the power consumption (due to heating) in that resistor. This heat generation can be reduced by using an arbitrarily small-value resistor. However, if the resistance value becomes too small, the impedance of the detection resistor begins to be dominated by the reactance, i.e., the parasitic inductance of the detection resistor. This reactance dominance leads to an undesirable phase shift and also affects the measured value of the alternating current as follows. v(t) = I(t) * [sqrt(R + 2π * L * f)] Here, v(t) is the voltage, i(t) is the current, R is the resistance value of the detection resistor, L is the inductance inherent in the detection resistor, and f is the frequency of the AC signal.

[0037] Since the voltage v(t) is the quantity measured by the measurement system (see, for example, Figure 3), it can be seen that as the quantity under the square root increases, the detected voltage v(t) increases upon examination. Therefore, although it calculates the current from the detected voltage, in the case of a detection resistor where R << X (here, X = 2π * L * f), this detected voltage is dominated by the inductance. When the exact inductance is unknown, it is almost impossible to calculate the exact current. However, in the said system and method for partial current detection of closely coupled parallel conductors, this problem is avoided by making R >> X.

[0038] For example, in the case of n=60 conductors, when detecting 1 / 60 of the total current (from a single conductor among the total number), the resistance of the detection resistor can be 60 times greater than when detecting the total current using a single detection resistor. More generally, R > nX, where n is the number of coil windings in the coil. Of course, this assumes that the power consumption of the detection resistor is kept constant.

[0039] The advantage of the partial detection concept is that, when R >> X, not only is the phase angle very close to zero, but the amplitude of the voltage signal is governed by the real component of the impedance, i.e., the ohmic resistance. Therefore, in practice, as long as the resistive component is much larger than the reactance generated by the inductive component, it is not necessary to know the exact parasitic inductance of the detection resistor to calculate accurate voltage measurements (and thus current levels, phases, and frequencies).

[0040] conclusion While various embodiments have been described above, it should be understood that these embodiments are presented for illustrative purposes only and are not limiting. For example, any of the elements relating to the systems and methods described above may adopt any of the desirable functions described above. Therefore, the scope of preferred embodiments should not be limited by any of the exemplary embodiments described above.

[0041] As described herein, logic, commands, or instructions for implementing aspects of the methods described herein may be provided in computer systems including any number of form factors for computer systems such as machine instances of client terminals and server hosts, including desktop or notebook personal computers, mobile devices such as tablets, netbooks, and smartphones. Other embodiments described herein include incorporating the techniques described herein into other forms including other forms of programmed logic, hardware configurations, or dedicated components or modules, and including apparatus with respective means for performing the functions of such techniques. Each algorithm used to implement the functions of such techniques may include some or all sequences of electronic operations described herein, or other forms shown in the accompanying drawings and the following detailed description. Such systems and computer-readable media including instructions for performing the methods described herein also constitute exemplary embodiments.

[0042] The monitoring and control functions of the control unit 207 described herein can be implemented in software in one embodiment. Such software may have instructions executable by a computer stored in a computer-readable medium such as one or more non-temporary memory or other types of hardware-based storage devices on a local or network. Such functions may also correspond to modules that can be software, hardware, firmware, or any combination thereof. Multiple functions may be performed in one or more modules as needed, and the embodiments described herein are merely examples. The software may run on a digital signal processor, ASIC, microprocessor, or other type of processor running on a computer system such as a personal computer, server, or other computer system, and may also transform such a computer system into a specially programmed machine.

[0043] The examples described herein may include, or can operate on, processors, logic, or several components, modules, or mechanisms (hereinafter referred to as "modules"). A module is a tangible entity (e.g., hardware) capable of performing a specified operation and can be configured or arranged in a particular manner. In one example, a circuit may be arranged in a manner designated as a module (e.g., internally or relative to an external entity such as other circuits). In one example, one or more computer systems, or all or part thereof (e.g., a standalone, client, or server computer system), or one or more hardware processors may be configured by firmware or software (e.g., instructions, application portions, or applications) as modules that operate to perform a specified operation. In one example, the software may reside on a machine-readable medium. When the software is executed by the underlying hardware of the module, it causes the hardware to perform the specified operation.

[0044] Accordingly, the term “module” is understood to encompass tangible hardware and / or software entities that are physically configured, specifically configured (e.g., wired), or temporarily configured (e.g., programmed) entities that operate in a specified manner or perform some or all of the operations described herein. In examples where a module is temporarily configured, each module does not need to be instantiated at any given time. For example, if a module has a general-purpose hardware processor configured with software, the general-purpose hardware processor may be configured as each of several different modules at different times. Thus, software can configure a hardware processor, for example, to configure one module at one time and a different module at a different time.

[0045] Those skilled in the art will understand that the topology and circuit implementation methodology described herein enable effective implementation as a single, application-specific integrated circuit. Furthermore, while the disclosures contained herein relate to power supply to vehicles, it should be understood that this is only one of many possible applications, and other embodiments, including those for non-vehicle applications, are possible. For example, those skilled in the art will understand that there are numerous applications providing a current source safety circuit in non-vehicle inductive charging applications, such as portable consumer electronic device chargers, such as chargers used to charge toothbrushes, mobile phones, and other devices (e.g., PowerMat®). Accordingly, these and other such applications are included in the following claims.

Claims

1. 1. A method for measuring current through a coil having multiple coil windings, comprising: measuring current through a subset of the plurality of coil windings using a current sensing resistor; measuring the voltage drop across the current sense resistor; determining the partial currents and phases of the coil from the measured currents and voltages; A method having the following.

2. 10. The method of claim 1, wherein the coil is a secondary winding used in a wireless power transfer system, the wireless power transfer system including a rectifier that converts AC current in the secondary winding into DC current for application to a load.

3. 3. The method of claim 2, further comprising digitizing the measured voltage and providing the measured voltage to a rectifier controller, the rectifier controller determining the partial currents and phases of the coils and controlling operation of the rectifier.

4. 4. The method of claim 3, further comprising calculating a total current I through the secondary winding as I=nV / sqrt(2R+2π*L*f), where n is a number of coil turns in the secondary winding, V is the measured voltage, R is the resistance of the current sense resistor, L is the inductance of the secondary winding, and f is the frequency of the AC current in the secondary winding.

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

6. 4. The method of claim 3, further comprising calculating a total current in the coil as I=n(V / Rz), where n is the number of coil turns in the coil, V is the measured voltage, and Rz is the impedance of the current sense resistor.

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

8. 3. The method of claim 2, further comprising selecting the current sense resistor to have a value of R>>X, where X=2π*L*f, where L is the inductance of the secondary winding and f is the frequency of the AC current in the secondary winding.

9. 9. The method of claim 8, wherein R>nX, where n is the number of coil turns in the coil.

10. 1. A current sensing device for measuring current through a coil having a plurality of coil windings, comprising: a current sensing resistor connected to a subset of the plurality of coil windings to measure current through the subset of the plurality of coil windings; a voltage sensor that measures the voltage drop across the current sensing resistor; a processor for determining the partial currents and phases of the coil from the measured currents and voltages; A device having

11. 11. The device of claim 10, wherein the coil is a secondary winding used in a wireless power transmission system, the wireless power transmission system including a rectifier that converts AC current in the secondary winding into DC current for application to a load.

12. 12. The device of claim 11, further comprising an analog-to-digital converter that digitizes a voltage measured by the voltage sensor and provides the measured voltage to the processor.

13. 13. The device of claim 12, wherein the processor determines a total current I through the secondary winding as I=nV / sqrt(2R+2π*L*f), where n is a number of coil turns in the secondary winding, V is the measured voltage, R is the resistance of the current sense resistor, L is the inductance of the secondary winding, and f is the frequency of the AC current in the secondary winding.

14. 13. The device of claim 12, wherein the processor determines a total current I through the secondary windings as I={Σ[(Vn / Rn / n)]}*m, where Vn is the measured voltage for each secondary winding, Rn is the resistance of each current sense resistor in each secondary winding, n is the number of sampled coil turns, and m is the total number of coil turns in the coil.

15. 13. The device of claim 12, wherein the processor determines the partial current and phase of the coil by calculating a total current in the coil as I=n(V / Rz), where n is the number of coil turns in the coil, V is the measured voltage, and Rz is the impedance of the current sensing resistor.

16. 12. The device of claim 11, wherein the sense resistor has a value R>>X, where X=2π*L*f, where L is the inductance of the secondary winding and f is the frequency of the AC current in the secondary winding.

17. 17. The device of claim 16, wherein R>nX, where n is the number of coil turns in the coil.

18. 11. The device of claim 10, wherein the coil winding comprises one of a Litz wire, a printed circuit board trace, or a conductive filament.

19. 11. The device of claim 10, wherein the coil windings are impedance matched and tightly coupled to one another via mutual inductance.

20. 12. The device of claim 11, wherein the wireless power transfer system comprises one of an inductive system and a capacitive coupling system.

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