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 particularly to current measurement in wireless power transfer systems. [Background technology]

[0002] Resonant inductive wireless charging uses an air-core transformer consisting of two concentric coils aligned along a common coil axis. Power is transferred from the transmitting device (i.e., primary coil) to the receiving device (i.e., secondary coil) by magnetic flux linkage between the two transmitting coils. Alternating current flowing through the primary coil induces alternating current in the secondary coil.

[0003] One option for constructing the coil is to use Litz wire (also known as Litz) or other conductive filaments. Litz wire consists of individually insulated wires twisted or braided into a uniform pattern, which offers a key advantage in reducing AC losses in high-frequency windings. Alternatively, as described in International Patent Application PCT / US2018 / 035060, "WIRELESS POWER TRANSFER THINPROFILE COIL ASSEMBLY," the coil conductor can be constructed from multiple conductive traces superimposed on an insulating dielectric substrate (e.g., a printed circuit board).

[0004] Electric current is defined as an electrical charge (such as electrons) in motion. Current is dq / dt, or the time rate of change of charge. The magnitude of electrical flow is measured in amperes. The unit ampere (A) is defined as equal to the flow of one coulomb of charge per second. Current in an electrical circuit can be measured directly (for example, with a sense resistor) or indirectly (for example, with a Hall Effect or inductive sensor).

[0005] It is desirable to provide a current measurement device for the high currents that may appear in a wireless power transfer system that does not adversely affect the operation of the wireless power transfer system. The prior art documents relevant to the invention of this application are as follows (including documents cited in the international phase after the international filing date and documents cited when the application entered the national phase in other countries). (Prior art document) (Patent document) (Patent Document 1) U.S. Patent Publication No. 2014 / 0339916 (Patent Document 2) U.S. Patent No. 6,072,708 (Patent Document 3) U.S. Patent Publication No. 2015 / 0194836 (Patent Document 4) U.S. Patent Publication No. 2013 / 0282312 (Patent Document 5) U.S. Patent Publication No. 2007 / 0021937 (Patent Document 6) U.S. Patent Publication No. 2009 / 0026845 (Patent Document 7) International Publication No. 2016 / 108949 (Patent Document 8) Taiwan Patent No. I684847 (Patent Document 9) Chinese Patent Application Publication No. 103675401 (Patent Document 10) U.S. Patent No. 6,954,060 (Patent Document 11) U.S. Patent No. 4,182,982 (Patent Document 12) U.S. Patent No. 6,522,517 (Non-patent literature) (Non-Patent Document 1) EUROPEAN PATENT OFFICE. Extended European Search report issued for EP Application No. 21771897.2-1001, dated August 22, 2023. Summary of the Invention [Means for solving the problem]

[0006] Various details of embodiments of the present subject matter are set forth in the accompanying drawings and the detailed description below.

[0007] In an exemplary embodiment, a current sensing device is provided for measuring current through a coil having multiple 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 multiple coil windings and measuring the current through the subset of the multiple coil windings; a voltage sensor 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 transfer system (inductive or capacitively coupled system), and the wireless power transfer system includes a rectifier that converts the AC current in the secondary winding to 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 calculates the total current I through the secondary winding as I= m V / sqrt( R 2 + ( 2π*L*f) 2 ) where, m is the number of coil windings in the secondary winding Total number, V teeth The measured Current sensing resistor Voltage Descent where R is the resistance of the current sensing resistor and L is Parasitic resistance of the current detection resistor and f is the Measured voltage drop across the current-sensing resistor is the frequency.

[0009] In an exemplary embodiment, the sense resistor but With value R>>X If you want to, then X=2π*L*f, where L is the parasitic inductance of the current sense resistor and f is the frequency of the AC current in the secondary winding. in this case, the processor may determine the instantaneous current I through the secondary winding as I=m*V / R. Alternatively, the processor may determine the total current I through the secondary winding as I=[Σ(Vj / Rj / nj)]*m, where j is the current sense resistor number, nj is the number of coil turns connected to each current sense resistor, Rj is the resistance of each current sense resistor, and Vj is the voltage measured across each current sense resistor.

[0010] Also provided is a method for measuring current through a coil having multiple coil windings. The method includes measuring the current through a subset of the multiple coil windings using a current sense resistor, measuring a voltage drop across the current sense resistor, and determining a partial current and phase of the coil from the measured current and voltage. The method may be implemented in an embodiment where the coil is a secondary winding used in a wireless power transfer system and the wireless power transfer system has a rectifier that converts AC current in the secondary winding to DC current for application to a load. The method further includes digitizing the measured voltage and providing the measured voltage to a rectifier controller, which determines the partial current and phase of the coil and controls operation of the rectifier.

[0011] In an exemplary embodiment of the method, the method further comprises: determining a total current I flowing through the secondary winding as I= m V / sqrt( R 2 + ( 2π*L*f) 2 ) The method includes a step of calculating the following: m is the number of coil windings in the secondary winding Total and V is the measured Current sensing resistor Voltage Descent where R is the resistance of the current sensing resistor and L is the Current Sense Resistor of parasitic resistance and f is the Measured voltage drop across each current-sensing resistor is the frequency.

[0012] The method 、 Additionally, (a) limiting the inductance to reduce the loss due to phase shift when the measured current returns to the multiple coil windings. Small and / or (b) selecting the current sense resistor to prevent overheating and power loss while reducing the effects of inductance. but is chosen to have values ​​R>>X If , where X=2π*L*f where the processor may determine the instantaneous current I through the secondary winding as I=m×V / R. Alternatively, the processor may determine the total current I through the secondary winding as I=[Σ(Vj / Rj / nj)]*m, where j is the current sense resistor number, nj is the number of coil turns connected to each current sense resistor, Rj is the resistance of each current sense resistor, and Vj is the voltage measured across each current sense resistor.

[0013] This Summary is provided to introduce aspects of the inventive subject matter in a simplified form, with further description of the inventive subject matter following the Detailed Description. This Summary is not intended to identify essential or essential features of the claimed subject matter, nor is the particular combination and order of elements listed in the Summary intended to limit the elements of the claimed subject matter. Rather, it will be understood that the following provides an illustrative summary of some of the embodiments described in the Detailed Description below. [Brief explanation of the drawings]

[0014] The foregoing and other beneficial features and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings. [Figure 1] FIG. 1 illustrates a schematic diagram of current measurement in a wireless power transfer system in an exemplary embodiment. [Figure 2] FIG. 2 functionally illustrates a wireless power transfer system with current measurement in an exemplary embodiment. [Figure 3] FIG. 3 shows the current measurement hardware in an exemplary embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] The current detection and related methods for wireless power transfer described herein will be more readily understood by reference to the following detailed description in conjunction with the accompanying figures and examples that form a part of this disclosure. It should be understood that this description is not limited to the specific products, methods, conditions, or parameters described and / or illustrated herein, and that the terminology used herein is intended to describe particular embodiments by way of example only and is not intended to limit the subject matter of the claims. Similarly, any description of possible mechanisms, modes of operation, or reasons for improvement is for illustrative purposes only, and the subject matter described herein is not constrained by the precision or inaccuracy of such suggested mechanisms, modes of operation, or reasons for improvement. Throughout this specification, the description refers to both methods and systems / software that implement the methods.

[0016] Exemplary embodiments will now be described in detail with reference to Figures 1 to 3. While the following description provides details of possible example embodiments, it should be noted that such detailed description is intended for illustrative purposes only and does not define the scope of the present subject matter.

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

[0018] High power and low Impedance Load in the case of , wireless power transfer (WPT) system teeth Because it is always a current source (i.e., a current source with controlled AC voltage), and high-power WPT systems generate stray magnetic flux, the use of indirect measurement techniques and technologies is problematic, especially as part of a closed control loop for an active rectifier.

[0019] Direct current measurement methods use a sense resistor. A sense resistor is a resistor with a nominal low ohm value, typically placed in series with a circuit network near circuit ground or a battery cathode. The total current through the sense 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 in amperes as a function of time, v(t) is the voltage across the sense resistor as a function of time in volts, and R is the resistance of the sense resistor in ohms. The sense resistor is chosen to have minimal resistance to avoid excessive heating and perturbations in the power delivery to the load (since power loss in the sense resistor is proportional to its resistance).

[0020] For the same reason, in AC systems, such as those used in magnetic resonance-based WPT systems, the sense resistor selected should minimize the reactive component of the total impedance. The requirement for accurate AC signal measurements (e.g., current, frequency, phase, etc.) at the frequencies of interest dictates that the reactance must be small relative to the resistance.

[0021] Measuring the total AC current with a current sensing resistor in high current systems (e.g., above 125 amps RMS) is prohibited due to heating caused by power dissipation. Another problem is the unavoidable large inductance. give rise to handles the current without possible Physically large resistor of structure of It's in physics. Additional Parasitic inductance but AC signal excessive There is a possibility that a phase angle will be added, making accurate measurements impossible.

[0022] However, a method for detecting partial currents can be constructed for WPT systems that utilizes the multi-conductor structure of the secondary coil winding to isolate a subset of conductors that are connected through a resistor with a low ohm rating. The conductor current passes through a very small value resistor (which has an inherent parasitic inductance), and the voltage drop across the resistor is measured to determine the partial current and phase.

[0023] Current-sensing resistors are low-cost and remain reliable in the presence of stray magnetic flux generated by WPT systems. However, because current-sensing resistors are resistive elements (albeit with some self-inductance), the heat they generate is proportional to the square of the current passing through them, limiting their usefulness in high-current power supplies such as those used in WPT systems. However, by utilizing a multi-element structure of the magnetic coil, a sensing resistor placed on a single conductor provides partial current measurement without affecting the overall system's power delivery capability and without generating excessive heat. This partial current measurement provides accurate phase measurement through multiplication and accurate current measurement for control systems (e.g., active rectification systems and power control feedback).

[0024] In reality, every resistor contains unavoidable parasitic inductance. This inductance leads to a phase shift in the output. The power drop can be much greater than that of simple resistive heating losses, since the measured conductor is returned to a bundle of closely coupled parallel conductors, sharing the total current load through mutual inductance.

[0025] Resistor structures vary, allowing the selection of low-inductance models. Since the effect of a resistor on the power delivered (and heat dissipated) decreases with the ratio of the detected conductors to the total number of conductors, the relative influence of the self-inductance component can be reduced by selecting resistors with (relatively) larger ohms in a partial current measurement system. Higher resistor values ​​result in a larger dynamic range of voltage, resulting in more accurate voltages and therefore more accurate detection of partial current levels and current phases.

[0026] In an exemplary embodiment, the sense resistor is selected to limit inductance or to use a high, but still relatively low, value to reduce the effects of inductance while keeping heating and power loss to a minimum. A higher resistance allows for a higher dynamic range of voltage, allowing for more accurate current detection. Inductance also reduces the amount of current flowing back into the tightly coupled conductor bundle. of Attenuation due to phase shift Small are restricted to prevent

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

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

[0029] The voltage sensor reading 107 and the impedance of resistor 102 are used to derive the current. Because the impedance-matched, tightly coupled conductors 105 share the current generated on the secondary 101, the total current delivered to the load 106 can be calculated. For example, if 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 sense resistor 102, and Rz is the impedance of sense resistor 102.

[0030] Figure 2 FIG. 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, includes 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 AC current. The AC 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 AC current into the DC current required to charge a battery 206 (which can be wet, dry, solid-state, capacitive, or hybrid (e.g., a battery with a capacitive component)). The rectified DC signal may be smoothed and level-shifted by a conditioner circuit 205 to charge the battery 206.

[0031] A current sensor 209 is used to monitor the alternating current 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 commutation control switching.

[0032] Figure 3 3 shows an exemplary embodiment of a current sensor using a current sense resistor 301. The current sense resistor 301 has both resistive (ohmic) 302 and inductive 303 impedance components. In this example, an analog-to-digital converter (ADC) 304 is used to digitize the voltage developed across the current sense resistor 301. The ADC 304 uses a digital interface 305 to connect to other systems, such as the active rectification controller 207. The active rectification controller 207 can include a processor to calculate the current from the measurements, as described herein.

[0033] In the example of a WPT system with 60 conductive element receivers measuring a single conductor, the current through the measured conductor is I=V / sqrt(R 2 + ( 2π*L*f ) 2 ) where I is the current in a single conductor in amperes, V is the measured voltage in volts, R is Resistance value (ohms), L is Current Sense Resistor of Parasitic where f is the frequency of the AC signal in Hertz.

[0034] Alternative Embodiment—Multiple Parallel Current Sensing Once the current in a single conductor (i) is calculated, it is multiplied by the number of tightly coupled conductors to determine the current level in the system. That is, the current is i= m V / sqrt( R 2 + ( 2π*L*f) 2 ) Here,m is the number of conductors. When measuring multiple conductors, of The sum of the measured currents is averaged. This average current per conductor is then multiplied by the number of tightly coupled conductors to determine the current level for the entire system.

[0035] In the simplest case, a single conductor is isolated from the dense inductive coil winding for partial current sensing. In some cases, multiple conductors can be sensed, each with its own current sensing resistor. In the multiple conductor case, the sensed currents are averaged and then multiplied. i(total)={Σ[(Vn / Rn) / n]}*m where i(total) is the current in 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 conductors sampled, and m is the total number of conductors.

[0036] Alternative embodiment - unknown parasitic induction For high-power (i.e., high-current) systems, using a single current-sensing resistor to sense the entire AC current is undesirable due to the power dissipation (due to heating) in that resistor. This heating can be mitigated by using an arbitrarily small value resistor. However, if the resistance value becomes too small, the impedance of the sense resistor begins to be dominated by its reactance, i.e., the parasitic inductance of the sense resistor. This reactance dominance leads to an undesirable phase shift, which reduces the AC reading as follows: Size also affects v(t)= i (t)*[sqrt(R 2 +(2π*L*f) 2 )] where v(t) is the voltage and , i(t) is the current and , R is the sensing resistor Ohm Value where L is the parasitic inductance inherent in the sense resistor and f is the frequency of the AC signal.

[0037] Since the voltage v(t) is a quantity measured by a measurement system (see, for example, FIG. 3), it can be seen that by examining, as the quantity under the square root increases, the detected voltage v(t) increases. Therefore, although it calculates the current from the detected voltage, in the case of a detection resistor where R << X (where 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 performing 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 (a single conductor out of the total number), the resistance of the detection resistor can be made 60 times larger than when detecting the total current using a single detection resistor. More generally, R > is nX. Here, n is the number of coil windings in the coil. Of course, this is on the premise 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 also the amplitude of the voltage signal is dominated by the real component of the impedance, that is, the ohmic resistance. Therefore, in practice, as long as the resistance component is much larger than the reactance generated by the inductive component, there is no need to know the exact parasitic inductance of the detection resistor to calculate the exact voltage measurement value (and thus the current level, phase, and frequency).

[0040] conclusion Although 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 related to the system and method described above can employ any of the desirable functions described above. Therefore, the scope of the preferred embodiments should not be limited by any of the exemplary embodiments described above.

[0041] As described herein, logic, commands, or instructions implementing aspects of the methods described herein may be provided in a computer system, including any number of form factors for computer systems, such as desktop or notebook personal computers, mobile devices, e.g., tablets, netbooks, and smartphones, client terminals, and server host machine instances. 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, including apparatuses 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 of the sequence of electronic operations described herein, or other aspects shown in the accompanying drawings and the following detailed description. Such systems and computer-readable media containing instructions for performing the methods described herein also constitute exemplary embodiments.

[0042] In one embodiment, the monitoring and control functions of the controller 207 described herein can be implemented in software. Such software can have computer-executable instructions stored on a computer-readable medium or storage device, such as one or more non-transitory memories or other types of hardware-based storage devices, locally or over a network. Additionally, such functions correspond to modules, which can be software, hardware, firmware, or any combination thereof. Where appropriate, multiple functions may be performed by one or more modules; the described embodiments are merely exemplary. The software can be executed by a digital signal processor, ASIC, microprocessor, or other type of processor operating on a computer system, such as a personal computer, server, or other computer system, which can also transform such a computer system into a specially programmed machine.

[0043] Examples described herein may include or operate on a processor, logic, or a number of components, modules, or mechanisms (herein "modules"). A module is a tangible entity (e.g., hardware) that can perform specified operations and may be configured or arranged in a particular manner. In one example, a circuit may be arranged in a specified manner as a module (e.g., internally or relative to external entities such as other circuits). In one example, all or part of one or more computer systems (e.g., standalone, client, or server computer systems), or one or more hardware processors, may be configured with firmware or software (e.g., instructions, application portions, or applications) as modules that operate to perform specified operations. In one example, the software may reside on a machine-readable medium. The software, when executed by the underlying hardware of the module, causes the hardware to perform the specified operations.

[0044] Thus, the term "module" is understood to encompass tangible hardware and / or software entities that are physically configured, specifically configured (e.g., hardwired), or temporarily (e.g., transitorily) configured (e.g., programmed) to operate in a specified manner or to perform some or all of the operations described herein. In instances where modules are temporarily configured, each module need not be instantiated at any one time. For example, if the modules include a general-purpose hardware processor configured using software, the general-purpose hardware processor may be configured as each different module at different times. Thus, the software may configure the hardware processor, for example, to configure a particular module at one time and a different module at a different time.

[0045] Those skilled in the art will appreciate that the topology and circuit implementation methodology described herein allow for effective realization as a single application-specific integrated circuit. Furthermore, while the disclosure contained herein relates to powering a vehicle, it should be understood that this is only one of many possible applications, and other embodiments are possible, including non-vehicle applications. For example, those skilled in the art will appreciate that there are numerous applications for providing a current source safety circuit in non-vehicle inductive charging applications, such as portable consumer electronic device chargers, e.g., chargers used to charge toothbrushes, cell phones, and other devices (e.g., PowerMat™). Accordingly, these and other such applications are intended to be within the scope of the following claims.

Claims

1. A method for measuring a current through a coil, the coil having a plurality of coil windings, the method comprising: connecting a current sensing resistor to a portion of the plurality of coil windings, the current sensing resistor being connected to directly measure current flowing through the portion of the plurality of coil windings; measuring the voltage drop across the current sensing resistor; determining a current flowing through a portion of the coil from the current measured in the step of connecting the current sense resistor and the voltage measured in the step of measuring the voltage drop; 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 a current through a portion of the coil and controlling operation of the rectifier.

4. 4. The method of claim 3, further comprising: defining a total current I flowing through the secondary winding as I=mV / sqrt(R ２ +(2π*L*f) ２ ) where m is the total number of coil turns in the secondary winding, V is the measured voltage drop across the current sense resistor, R is the resistance of the current sense resistor, L is the parasitic resistance of the current sense resistor, and f is the frequency of the measured voltage drop across the current sense resistor.

5. 4. The method of claim 3, further comprising calculating a total current I through the secondary winding as I={Σ[(Vj / Rj / nj)]}*m, where m is the total number of coil turns in the secondary winding, j is the number of current sense resistors, nj is the number of coil turns connected to each current sense resistor, Rj is the resistance of each current sense resistor, and Vj is the voltage measured at each current sense resistor.

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

7. 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 parasitic inductance of the current sense resistor and f is the frequency of the AC current in the secondary winding.

8. A current sensing device for measuring current through a coil, the coil having a plurality of coil windings, the device comprising: a current sensing resistor connectable to a portion of the plurality of coil windings to directly measure current through the portion of the plurality of coil windings; a voltage sensor that measures the voltage drop across the current sensing resistor; a processor that determines a current through a portion of the coil from the current measured by the current sense resistor and the voltage measured by the voltage sensor; A device having

9. A wireless power transmission system comprising the current detection device of claim 8, a rectifier, and a coil having the plurality of coil winding portions, the coil being a secondary winding used in the wireless power transmission system, and the rectifier converting the alternating current of the secondary winding into a direct current for application to a load.

10. 10. The system of claim 9, further comprising an analog-to-digital converter that digitizes a voltage measured by the voltage sensor and provides the measured voltage to the processor.

11. 11. The system of claim 10, wherein the processor calculates the total current I through the secondary winding as I=mV / sqrt(R ２ +(2π*L*f) ２ ), where m is the total number of coil turns in the secondary winding, V is the measured voltage drop across the current sense resistor, R is the resistance of the current sense resistor, L is the parasitic resistance of the current sense resistor, and f is the frequency of the measured voltage drop across the current sense resistor.

12. 11. The system of claim 10, wherein the processor determines a total current I through the secondary winding as I={Σ[(Vj / Rj / nj)]}*m, where m is a total number of coil turns in the secondary winding, j is a current sense resistor number, nj is the number of coil turns connected to each current sense resistor, Rj is the resistance of each current sense resistor, and Vj is the voltage measured at each current sense resistor.

13. 10. The system of claim 9, wherein the current sense resistor has a value R>>X, where X=2π*L*f, where L is the parasitic inductance of the current sense resistor and f is the frequency of the AC current in the secondary winding.

14. 10. The system of claim 9, wherein the coil winding comprises one of a Litz wire, a printed circuit board trace, or a conductive filament.

15. 10. The system of claim 9, wherein the coil windings are impedance matched and tightly coupled to one another via mutual inductance.

16. 10. The system of claim 9, wherein the wireless power transfer system comprises one of an inductive system and a capacitively coupled system.

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