Current sensing in wireless power transfer systems

Abstract

A current sensing method measures a fractional current through a coil having a plurality of coil windings by measuring a current through a subset of the plurality of coil windings using a current sense resistor and measuring a voltage drop across the current sense resistor using a voltage sensor. The measured current and voltage values are provided to a processor to determine the fractional current and phase of the coil. For example, the fractional 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 of the coil, V is the measured voltage, and Rx is the impedance of the current sense resistor. The coil can be a secondary winding used in a wireless power transfer system.

Description

Technical Field

[0001] This disclosure relates to current measurement, and more particularly to current measurement in wireless power transmission systems. Background Technology

[0002] Resonant inductive wireless charging utilizes an air-core transformer comprising two concentric coils displaced along a common coil axis. Power is transmitted from the transmitting device (i.e., the primary coil) to the receiving device (i.e., the secondary coil) via a magnetic flux linkage between the two transmitting coils. Alternating current flowing in the primary coil induces alternating current in the secondary coil.

[0003] One option for constructing the coil is to use stranded (also known as Litz) wire or other conductive wires. Stranded wire comprises individually insulated wires stranded or braided into a consistent pattern, and its main benefit is reduced AC losses in high-frequency windings. Alternatively, as described in published PCT patent application WO / 2018 / 222669 A1 (PCT / US2018 / 035060) “WIRELESSPOWER TRANSFER THIN PROFILE COIL ASSEMBLY”, the coil conductor may comprise multiple conductive traces stacked in an insulating dielectric substrate (e.g., a printed circuit board).

[0004] Current is defined as a moving electric charge (e.g., an electron). Current is dq / dt, or the rate of change of charge over time. The flow rate of current is measured in amperes. The unit ampere (A) is defined as the flow rate equal to one coulomb of charge per second. Current in a circuit can be measured directly (e.g., using a sensing resistor) or indirectly (e.g., using a Hall effect sensor or an inductive sensor).

[0005] It is desirable to provide a current measuring device for large currents that may exist in wireless power transmission systems without adversely affecting the operation of the wireless power transmission systems. Summary of the Invention

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

[0007] In an example embodiment, a current sensing device is provided for measuring the current through a coil having multiple coil windings (e.g., stranded wire, printed circuit board traces, or conductive wires). The coil windings are impedance-matched and tightly coupled to each other via mutual inductance. The current sensing device includes: a current-sensing resistor connected to a subset of the multiple coil windings to measure the current through the subset of the multiple coil windings; a voltage sensor that measures the voltage drop across the current-sensing resistor; and a processor that determines the fractional current and phase of the coil based on the measured current and voltage. In an example embodiment, the coil is a secondary winding used in a wireless power transmission system (inductive or capacitively coupled system) that includes a rectifier that converts alternating current on the secondary winding into direct current to be applied to a load. An analog-to-digital converter may also be provided to digitize the voltage measured by the voltage sensor and provide the measured voltage to the processor.

[0008] In the example application, the processor determines the total current I through the secondary winding as... 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 alternating current on the secondary winding. Alternatively, the processor can determine the total current I through the secondary winding as... 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 windings. The processor determines the fractional current and phase of the coil by calculating the total current of the coil as I = n(V / Rz), where n is the number of coil windings, V is the measured voltage, and Rz is the impedance of the current-sensing resistor.

[0009] In the example implementation, the sensing resistor has a value R >> X, where, Where L is the inductance of the secondary winding, and f is the frequency of the alternating current on the secondary winding.

[0010] A method for measuring current through a coil comprising multiple coil windings is also provided. The method includes: using a current-sensing resistor to measure current through a subset of the multiple coil windings; measuring a voltage drop across the current-sensing resistor; and determining a fractional current and phase of the coil based on the measured current and voltage. This method can be implemented in an embodiment where the coil is a secondary winding used in a wireless power transmission system including a rectifier that converts alternating current on the secondary winding into direct 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 fractional current and phase of the coil and controls the operation of the rectifier.

[0011] In an example implementation of the method, the method includes: calculating the total current I through the secondary winding as... 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 alternating current on the secondary winding. The method may further include: calculating the total current I through the secondary winding as... Where Vn is the measured voltage of each sampled secondary winding, Rn is the resistance of each current-sensing resistor in each sampled secondary winding, n is the number of sampled coil windings, and m is the total number of coil windings. The method may further include: calculating the total current of the coil as I = n(V / Rz), where n is the number of coil windings, V is the measured voltage, and Rz is the impedance of the current-sensing resistor.

[0012] The method may further include: selecting a current-sensing resistor to (a) limit inductance to prevent out-of-phase subtraction when the measured current returns to the multiple coil windings, and (b) reduce the effect of inductance while minimizing heat generation and power loss. In an example implementation, the current-sensing resistor is selected to have a value R >> X, where, Where L is the inductance of the secondary winding, and f is the frequency of the alternating current on the secondary winding.

[0013] The Summary section provides a simplified overview of aspects of the invention, followed by further explanation of the subject matter in the detailed description. This Summary section is not intended to identify essential or essential features of the claimed subject matter, and the specific combinations and order of elements listed in the Summary section are not intended to limit the elements of the claimed subject matter. Rather, it should be understood that the following sections provide general examples of some embodiments described in the following detailed description. Attached Figure Description

[0014] The foregoing and other beneficial features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0015] Figure 1 The current measurement in a wireless power transmission system in an example embodiment is illustrated schematically.

[0016] Figure 2 Functionally, an example embodiment of a wireless power transmission system using current measurement is shown.

[0017] Figure 3 The current measurement hardware in an example implementation is shown. Detailed Implementation

[0018] The current sensing and associated methods for wireless power transmission described herein can be more readily understood by referring to the following detailed description, which is incorporated in conjunction with the accompanying drawings and examples forming a part of this disclosure. It should be understood that this specification is not limited to the specific products, methods, conditions, or parameters described and / or shown herein, and the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to limit any claimed subject matter. Similarly, any description of possible mechanisms or modes of operation or reasons for improvement is merely illustrative, and the subject matter described herein is not bound by the correctness or incorrectness of any such suggested mechanisms or modes of operation or reasons for improvement. Throughout this document, it should be recognized that the description relates to methods and systems / software for implementing such methods.

[0019] Now refer to Figures 1 to 3 A detailed description of illustrative embodiments is provided. Although this specification provides detailed examples of possible implementations, it should be noted that these details are intended to be exemplary and are in no way intended to limit the scope of the subject matter of the invention.

[0020] Indirect current sensing is typically used in circuits where the load current ranges from 100 A to 1000 A. Indirect current sensing allows for isolation from the current being measured in the conductor. When using a Hall effect-based sensor, the sensor is mounted between the anode and the load. Hall effect sensors use the Lorentz force (the force exerted on electrons moving through a magnetic field) to determine alternating current (AC) and direct current (DC). This Lorentz force generates a voltage difference (Hall voltage) across the conductor, transverse to the current in the conductor and transverse to an applied magnetic field perpendicular to the current. Inductive current sensors, or current-sensing transformers, use Faraday's law of induction (where the conductor acts as the primary coil and the voltage output acts as the secondary coil) to measure alternating current (AC) in the conductor based on the resulting magnetic flux.

[0021] Because wireless power transmission (WPT) systems will always be current sources (i.e., AC voltage-controlled current sources) under high power conditions with low impedance loads, and because high-power WPT systems will generate stray magnetic flux, there are problems with using indirect measurement methods and techniques, especially as part of the closed control loop of an active rectifier.

[0022] A straightforward method for measuring current is by using a sensing resistor. The sensing resistor (a resistor with a nominally small ohmic value) is set up in series with the circuit network, typically near circuit ground or the battery cathode. The total current 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 in amperes as a function of time, v(t) is the voltage across the sensing resistor in volts as a function of time, and R is the resistance of the sensing resistor in ohms. A sensing resistor with minimal resistance is chosen to avoid overheating and disturbances in the power delivered to the load (because power loss in the sensing resistor is proportional to its resistance).

[0023] For the same reason, for alternating current systems, such as those used in magnetic resonance-based WPT systems, the selected sensing resistor should have the minimum reactance component of the total impedance. The requirement for accurate AC signal measurements (e.g., current, frequency, phase) at relevant frequencies drives the requirement for a reactance that is relatively small relative to the resistance.

[0024] Due to heat dissipation from power consumption, current-sensing resistors are prohibited from being used to measure full AC current in high-current systems (e.g., 125 amps RMS and higher). Another problem is the physical construction of resistors that are physically large enough to handle this current without introducing large, unavoidable inductance. Then, for accurate measurements, the added parasitic inductance adds too much phase angle to the AC signal.

[0025] However, a fractional current sensing method can be constructed for WPT systems. This method utilizes a multi-conductor configuration of the secondary coil winding to separate a subset of conductors, which are then connected through a resistor with a low ohmic rating. The conductor current flows through a very small resistor (with inherent parasitic inductance), and the voltage drop across the resistor is measured to determine the fractional current and phase.

[0026] Current-sensing resistors are low-cost and remain reliable even in the presence of stray magnetic flux generated by WPT systems. However, because a current-sensing resistor is a resistive element (albeit one with a certain degree of self-inductance), the heat generated by it is proportional to the square of the current passing through it, thus limiting its usefulness in high-current power supplies such as those used in WPT systems. However, by utilizing a multi-element construction with a magnetic coil, a sensing resistor placed on a single conductor can produce fractional current measurements without generating excessive heat and without affecting the overall power transfer capability of the system. This fractional current measurement provides double the accuracy of phase measurements and precise current measurements for control systems, such as active rectifier systems and power control feedback.

[0027] In fact, any resistor includes unavoidable parasitic inductance. This inductance causes a phase shift in the output. Since the measured conductors return to the tightly coupled bundle of parallel conductors and the total current load is shared through mutual inductance, the power reduction can be far greater than the power reduction from resistive heat loss alone.

[0028] Due to the different resistor construction, a low-inductance model can be selected. In fractional current measurement systems, since the effect of the resistor on the delivered power (and heat dissipation) is reduced as a fraction of the sensed conductor relative to the total number of conductors, a larger ohmic (relative to ground) resistance can be selected to reduce the relative influence of the self-inductance component. A larger resistor value also results in a larger dynamic voltage range for more accurate voltage, and thus more accurate fractional current level and current phase sensing.

[0029] In the example implementation, the sensing resistor is selected to limit the inductance or to use a high but still relatively low value to reduce the influence of the inductor, while keeping heat generation and power loss to a minimum. A higher resistance provides a wider dynamic range of voltage, resulting in more accurate current sensing. The inductance is also limited to prevent out-of-phase subtraction as the measured current returns to the tightly coupled conductor bundle.

[0030] Figure 1

[0031] exist Figure 1 In a WPT system (such as an inductive system, a magnetically coupled inductive system, or even a capacitively coupled system), multiple conductors (such as stranded wires, printed circuit board traces, or conductive filaments) are used in the secondary coil. The secondary winding 101 presents itself in the circuit as a set of independent current sources, each with its own conductor 105. Each conductor 105 is impedance matched and tightly coupled to the other conductors 105 via mutual inductance. Figure 1Ten conductors 105 are used for illustrative purposes only. Power is supplied from the secondary winding 101 to the load 106 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 winding 101.

[0032] Current measurement is achieved by selecting a subset of individual conductors 105 connected via the current-sensing resistor 102 (in this example, a single conductor 109). The current-sensing resistor 102 has both a resistive impedance component (reasonable) 103 and an inductive impedance component 104. A voltage sensor 107 reads the voltage drop across the current-sensing resistor 102.

[0033] The current is derived using the voltage sensor reading 107 and the impedance of resistor 102. Since the impedance-matched, tightly coupled conductor 105 shares the current generated by the secondary winding 101, the total current delivered to the load 106 can be calculated. For example, when measuring a single conductor 109, 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.

[0034] Figure 2

[0035] exist Figure 2 The diagram shows a high-level schematic of a DC battery charging circuit using magnetic induction and current measurement in an example embodiment. The receiver 201 or secondary winding 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 alternating 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, which can be used to convert the AC current into DC current required to charge the battery 206 (the battery can be a wet battery, dry battery, solid-state battery, capacitive battery, or hybrid battery (e.g., a battery with capacitive components)). To charge the battery 206, the rectified DC signal can be smoothed and level-shifted by a regulator circuit 205.

[0036] Current sensor 209 is used to monitor the alternating current 208 generated by receiver 201. It reports the current level, frequency, and phase to controller 207. Controller 207 can then report the electrical signal characteristics to auxiliary systems such as displays, closed-loop control systems, safety systems, and active rectifier control switches.

[0037] Figure 3

[0038] Figure 3 An exemplary embodiment of a current sensor using a current-sensing resistor 301 is shown. The current-sensing resistor 301 has both a resistive (ohmic) impedance component 302 and an inductive impedance component 303. In this example, 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 controller 207, via a digital interface 305. The active rectifier controller 207 may include a processor that calculates the current based on measured values, as described herein.

[0039] As an example, for a WPT system with a receiver comprising 60 conductive elements and in the case of measuring a single conductor, the current through the conductor being measured would be: Where i is the current of a single conductor in amperes, V is the measured voltage in volts, R2 is the resistance in ohms, L is the inductance of the secondary coil in henries, and f is the frequency of the AC signal in hertz.

[0040] Alternative Implementation Methods – Multiple Parallel Current Sensing

[0041] Once the current (i) of a single conductor has been calculated, this current is multiplied by the number of tightly coupled conductors to determine the system current level. In other words, the current will be: Where n is the number of conductors. When measuring multiple conductors, the sum of the measured currents is averaged. Then, this average value of the current per conductor is multiplied by the number of tightly coupled conductors to determine the overall system current level.

[0042] In the simplest case, a single conductor is separated from the tightly wound induction coil for fractional current sensing. In some cases, multiple conductors can be sensed, each with its own current-sensing resistor. In the case of multiple conductors, the sensed currents are averaged and then multiplied:

[0043]

[0044] Where i(total) is the system current, Vn is the sampled voltage in conductors 1 to n, and Rn is the current resistor impedance of each resistor, where n is the number of sampled conductors, and m is the total number of conductors.

[0045] Alternative Implementation Methods – Unknown Parasitic Inductance

[0046] For high-power (i.e., high-current) systems, using a single current-sensing resistor to sense the entire alternating current is undesirable because the power is dissipated in the resistor (via heat). This heat can be mitigated by using a resistor of arbitrarily small value. However, when the resistance value becomes too small, the impedance of the sensing resistor begins to be dominated by reactance (i.e., the parasitic inductance of the sensing resistor). This reactance dominance causes an undesirable phase shift and also affects the magnitude of the alternating current measurement as follows:

[0047]

[0048] Where v(t) is voltage, i(t) is current, R is the ohm value of the sensing resistor, L is the inherent parasitic inductance in the sensing resistor, and f is the frequency of the AC signal.

[0049] Since the voltage v(t) is measured by the system (see example...) Figure 3 The measured quantity, upon examination, shows that if the quantity under the square root increases, the sensed voltage v(t) will increase. Therefore, in the case of a sensed resistor where R << X (where X = 2π*L*f), the sensed voltage, from which the current is calculated, will be dominated by the inductance. If the exact inductance is unknown, it is nearly impossible to calculate the precise current. However, systems and methods for fractional current sensing of tightly coupled parallel conductors overcome this problem by allowing R >> X.

[0050] For example, if n = 60 conductors, when sensing 1 / 60 of the total current (a single conductor out of the total conductors), the resistance of the sensing resistor can be 60 times that of the case where a single sensing resistor is used to sense the total current. More generally, R ≥ nX, where n is the number of coil windings. Of course, this assumes that the power loss of the sensing resistor remains constant.

[0051] The advantage of the fractional sensing concept is that when R >> X, not only is the phase angle very close to zero, but the amplitude of the voltage signal is also dominated by the real component of the impedance (i.e., the ohmic resistance). Therefore, practically speaking, as long as the resistive component is much larger than the reactance produced by the inductive component, it is not necessary to know the exact parasitic inductance of the sensing resistor to calculate the accurate voltage measurement (and thus the current level, phase, and frequency).

[0052] in conclusion

[0053] Although various implementations have been described above, it should be understood that they are presented as examples rather than limitations. For instance, any component associated with the systems and methods described above can employ any desired functionality outlined above. Therefore, the breadth and scope of preferred implementations should not be limited by any of the example implementations described above.

[0054] As discussed herein, logic, commands, or instructions for implementing aspects of the methods described herein can be provided in a computing system, including any number of formal elements relating to the computing system, such as desktop or laptop personal computers, mobile devices such as tablets, netbooks, and smartphones, client terminals, and server-hosted machine instances, etc. Another implementation discussed herein includes incorporating the techniques discussed herein into other forms, including incorporating them into other forms of programming logic, hardware configurations, or dedicated components or modules, including devices with corresponding means for performing such techniques. Corresponding algorithms for implementing the functions of such techniques may include some or all of the sequences of electronic operations described herein, or other aspects depicted in the accompanying drawings and the detailed description below. Such systems and computer-readable media including instructions for implementing the methods described herein also constitute exemplary implementations.

[0055] In one embodiment, the monitoring and control functions of the controller 207 described herein can be implemented in software. This software may include computer-executable instructions stored on a local or networked computer-readable medium or computer-readable storage device—e.g., one or more non-transitory memories or other types of hardware-based storage devices. Furthermore, such functionality corresponds to modules that can be software, hardware, firmware, or any combination thereof. Multiple functions can be performed in one or more modules as needed, and the described embodiments are merely examples. The software can be executed on 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, thereby transforming such a computer system into a specifically programmed machine.

[0056] As described herein, examples may include processors, logic, or components, modules, or mechanisms (“modules” herein), or that can operate on processors, logic, or components, modules, or mechanisms (“modules” herein). A module is a tangible entity (e.g., hardware) capable of performing a specified operation and may be configured or arranged in a certain way. In the examples, circuitry may be arranged as a module in a specified manner (e.g., internally or relative to an external entity such as other circuitry). In the examples, 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 by firmware or software (e.g., instructions, application portions, or applications) to operate to perform a specified operation. In the examples, the software may reside on a machine-readable medium. The software causes the hardware to perform the specified operation when executed by the underlying hardware of the module.

[0057] Therefore, the term "module" is understood to encompass tangible hardware and / or software entities that are physically constructed, specifically configured (e.g., hardwired), or temporarily (e.g., provisionally) configured (e.g., programmed) to operate in a specified manner or perform any of the operations described herein, in part or in whole. Consider examples where modules are provisionally configured; it is not necessary to instantiate each module within a module at any given time. For example, in the case where a module includes a general-purpose hardware processor configured using software, the general-purpose hardware processor can be configured as a correspondingly different module at different times. Thus, software can configure the hardware processor to, for example, constitute a specific module at one time instance and different modules at different time instances.

[0058] Those skilled in the art will understand that the topologies and circuit implementations described herein can be efficiently implemented as a single application-specific integrated circuit (ASIC). Furthermore, while the disclosure contained herein relates to providing power to vehicles, it should be understood that this is only one of many possible applications, and other implementations, including those for non-vehicle applications, are possible. For example, those skilled in the art will understand that there are many applications that provide current source safety circuitry in non-vehicle inductive charging applications, such as chargers for portable consumer electronics devices, such as those used to charge toothbrushes, cell phones, and other devices (e.g., PowerMat). TM Therefore, these and other such applications are included within the scope of the appended claims.

Claims

1. A method for measuring current through a coil comprising a plurality of coil windings, comprising: A current-sensing resistor is connected to a subset of the plurality of coil windings to directly measure the current through the subset of the plurality of coil windings; Measure the voltage drop across the current sensing resistor; as well as The phase of the current through the subset of the plurality of coil windings and the phase of the measured current are determined based on the measured current and voltage.

2. The method according to claim 1, wherein, The coil is a secondary winding used in a wireless power transmission system, which includes a rectifier that converts alternating current on the secondary winding into direct current to be applied to a load.

3. The method according to claim 2, further comprising: The measured voltage is digitized and provided to a rectifier controller, which determines the phase of the current through the subset of the plurality of coil windings and the measured current, and controls the operation of the rectifier.

4. The method according to claim 3, further comprising: The total current I passing through the secondary winding is calculated as follows: Where Vn is the measured voltage of each sampled secondary winding, Rn is the resistance of each 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 of the coil.

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

6. The method according to claim 1, further comprising: The current sensing resistor is selected to perform at least one of the following operations: (a) limiting inductance to prevent out-of-phase subtraction when the measured current returns to the plurality of coil windings, and (b) reducing the effect of inductance while keeping heat generation and power loss to a minimum.

7. A current sensing device for measuring the current passing through a coil comprising a plurality of coil windings, comprising: A current-sensing resistor is connected to a subset of the plurality of coil windings to directly measure the current through the subset of the plurality of coil windings; A voltage sensor that measures the voltage drop across the current sensing resistor; as well as The processor determines the current through the subset of the plurality of coil windings and the phase of the measured current based on the measured current and voltage.

8. The apparatus according to claim 7, wherein, The coil is a secondary winding used in a wireless power transmission system, which includes a rectifier that converts alternating current on the secondary winding into direct current to be applied to a load.

9. The apparatus according to claim 8, further comprising: An analog-to-digital converter (ADC) digitizes the voltage measured by the voltage sensor and provides the measured voltage to the processor.

10. The apparatus according to claim 9, wherein, The processor determines the total current I through the secondary winding as follows: Where Vn is the measured voltage of each secondary winding, Rn is the resistance of each current sensing resistor of each secondary winding, n is the number of sampled coil windings, and m is the total number of coil windings of the coil.

11. The apparatus according to claim 9, wherein, The processor determines the total current of the coil as I = n(V / Rz), where n is the number of coil windings, V is the measured voltage, and Rz is the impedance of the current sensing resistor, and determines the phase of the measured current based on the measured voltage and the impedance of the current sensing resistor.

12. The apparatus according to claim 7, wherein, The coil winding includes one of stranded wire, printed circuit board traces, and conductive wire.

13. The apparatus according to claim 7, wherein, The coil windings are impedance matched and tightly coupled to each other via mutual inductance.

14. The apparatus according to claim 8, wherein, The wireless power transmission system includes either an inductive system or a capacitively coupled system.

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