Abstract:
Disclosed is a current sensor that senses current flow in a conductor by coupling a first magnetic field generated by the conductor to a sense element. The current sensor includes a shield including a first material that sandwiches the sense element to define a stack and a second material that sandwiches the stack. The shield is configured to generate a second magnetic field, responsive to a third magnetic field external to the current sensor that opposes the third magnetic field. The shield is further configured to prevent production of a magnetic field that opposes the first magnetic field generated by the flow of current in the conductor.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure generally relates to current sensors. More specifically, the disclosure is directed to devices, systems, and methods related to current sensors using magnetic induction. 
       BACKGROUND 
       [0002]    Accurate current measurement can be important in electronic systems. For example, in a wireless power transfer system, accurate measurement of the current injected into the antenna coil of a power transmit unit (PTU) may be used to maintain proper levels of electromagnetic (EM) radiation into the environment. Some solutions may be based on measuring the voltage drop across two series capacitors, one capacitor attached to each of the coil leads of the antenna. Measuring the voltage directly can create technical challenges related to the design of the differential voltage buffer and amplifier circuits because both coil leads are at a high voltage. In addition, the measurement process can be complex, requiring the measurement of the voltage across the antenna coil behind the series capacitors and then measuring the voltage after the series capacitors, along with fast switching of voltages that feed into low pass filters. The approach has some disadvantages: the circuitry may require costly components to implement; and the process of taking measurements can create a good amount of electromagnetic interference (EMI) due to switching noise which can be injected into the antenna. 
       SUMMARY 
       [0003]    The present disclosure describes a current sensor operative to sense a flow of current in a conductor. In various embodiments, the current sensor includes a sense element configured to couple to a first magnetic field generated by the flow of current in the conductor and to produce a signal that is representative of the flow of current in the conductor. The current sensor further includes a shield comprising a first material that sandwiches the sense element to define a stack, and a second material that sandwiches the stack. The shield is configured to generate a second magnetic field, responsive to a third magnetic field external to the current sensor, that opposes the third magnetic field. The shield is further configured to prevent production of a magnetic field that opposes the first magnetic field generated by the flow of current in the conductor. 
         [0004]    In some embodiments, the shield is further configured to close a path for the first magnetic field. 
         [0005]    In some embodiments, the first material may be a ferrite material and the second material may be an electrically conductive material. 
         [0006]    In some embodiments, the current sensor may include a capacitive shield disposed adjacent the sense element to avoid capacitive coupling of an electric field between the conductor and the sense element. In some embodiments, the capacitive shield may be a conductive lead having a free first end and a second end configured for a connection to ground potential, thereby providing a path to ground for the electric field. 
         [0007]    In some embodiments, the sense element may include a coil disposed on a substrate. In some embodiments, the substrate may be a layer of a multi-layer printed circuit board (PCB). 
         [0008]    In some embodiments, the sense element may include a first coil disposed on a first plane and at least a second coil disposed on at least a second plane spaced apart from the first plane. In some embodiments, a first capacitive shield may be disposed adjacent the first coil and the conductor, and a second capacitive shield may be disposed adjacent the second coil and the conductor. In some embodiments, the first coil may be connected in series with the second coil. In some embodiments, the first coil may be a trace formed on a layer of a multi-layer PCB and the second coil may be a trace formed on another layer of the multi-layer PCB. 
         [0009]    In some embodiments, the sense element may be a first coil arranged to be adjacent the conductor; and a second electrically conductive coil disposed in opposed relation to the first electrically conductive coil and arranged to be adjacent the conductor. In some embodiments, the first coil and the second coil may be substantially coplanar. 
         [0010]    In some embodiments, the current sensor may include an amplifier circuit connected to the sense element to generate an output voltage based on the signal produced by the sense element. 
         [0011]    In some embodiments, the conductor constitutes a portion of or is configured to drive a transmit coil configured to generate an external magnetic field for wireless power transfer, wherein the external magnetic field constitutes the third magnetic field. 
         [0012]    The present disclosure describes a method for sensing current. In some embodiments, the method includes generating an output voltage representative of the current flowing in the conductor by magnetically coupling, at a sensing area, to a first magnetic field generated by the current flowing in the conductor. The method further includes shielding the sensing area from an external magnetic field including generating a second magnetic field that opposes the external magnetic field so that the output voltage generated by magnetically coupling to the first magnetic field is substantially free of influence from the external magnetic field. The method further includes preventing production of a magnetic field that opposes the first magnetic field generated by the flow of current in the conductor. 
         [0013]    In some embodiments, preventing production of the magnetic field that opposes the first magnetic field includes coupling the first magnetic field to a ferrite material that at least partially encloses the sensing area. 
         [0014]    In some embodiments, the method may further include shielding the sensing area from an electric field generated by the current flowing in the conductor so that the generated output voltage is substantially free of influence from the electric field. 
         [0015]    In some embodiments, magnetically coupling to the first magnetic field may include disposing a coil of electrically conductive material adjacent the conductor. In some embodiments, the method may include shielding the sensing area from an electric field generated by the current flowing in the conductor by disposing a conductive lead adjacent the conductor and the coil of electrically conductive material and connecting the conductive lead to ground potential. 
         [0016]    In some embodiments, magnetically coupling to the first magnetic field may include disposing a first coil adjacent the conductor and a second coil adjacent the conductor. In some embodiments, the first coil may be coplanar with the second coil. In some embodiments, the first coil may be on a plane separate from the second coil. 
         [0017]    The present disclosure describes a current sensor having first means for magnetically coupling, at a sensing area proximate a conductor, to a first magnetic field generated by a current flow in the conductor. In some embodiments, the current sensor may include a second means for generating a second magnetic field that opposes an external magnetic field to shield the sensing area from the external magnetic field so that the output of the first means is substantially free of influence from the external magnetic field. In some embodiments, the current sensor may include third means for shielding the sensing area from the second means so that the output of the first means is substantially free of influence from effects of the second means. 
         [0018]    In some embodiments, the second means may include an electrically conductive material that at least partially encloses the sensing area. In some embodiments, the third means may include a ferrite material that at least partially encloses the sensing area and is disposed within the electrically conductive material. 
         [0019]    In some embodiments, the current sensor may include a fourth means for shielding an electric field generated by the current flow in the conductor so that the output of the first means is substantially free of influence from the electric field. In some embodiments, the fourth means may include a conductive lead configured to be disposed adjacent the first means and the conductor. 
         [0020]    In some embodiments, the first means may be a loop of electrically conductive material disposed on a substrate. In some embodiments, the loop may have a plurality of turns. 
         [0021]    In some embodiments, the present disclosure describes an apparatus for wirelessly transmitting charging power to a receiver device. The apparatus includes a transmit coil configured to generate a first magnetic field for wirelessly transmitting charging power to the receiver device in response to being driven by an alternating current. The apparatus further includes a driver circuit electrically coupled to the transmit coil via a conductor, the driver circuit configured to drive the transmit coil with the alternating current via the conductor. The apparatus further includes a current sensor configured to sense a flow of current in the conductor. The current sensor includes a sense coil configured to couple to a second magnetic field generated by the alternating current in the conductor to produce a signal that is indicative of the flow of current in the conductor. The current sensor further includes a shield comprising a ferromagnetic material that sandwiches the sense coil to define a stack and comprising an electrically conducting material that sandwiches the stack. 
         [0022]    The following detailed description and accompanying drawings provide a better understanding of the nature and advantages of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    With respect to the discussion to follow and in particular to the drawings, it is stressed that the particulars shown represent examples for purposes of illustrative discussion, and are presented in the cause of providing a description of principles and conceptual aspects of the present disclosure. In this regard, no attempt is made to show implementation details beyond what is needed for a fundamental understanding of the present disclosure. The discussion to follow, in conjunction with the drawings, makes apparent to those of skill in the art how embodiments in accordance with the present disclosure may be practiced. In the accompanying drawings: 
           [0024]      FIG. 1  is a functional block diagram of a wireless power transfer system, in accordance with an illustrative embodiment. 
           [0025]      FIG. 2  is a functional block diagram of a wireless power transfer system, in accordance with an illustrative embodiment. 
           [0026]      FIG. 3  is a schematic diagram of a portion of transmit circuitry or receive circuitry of  FIG. 2  including a transmit or receive antenna, in accordance with an illustrative embodiment. 
           [0027]      FIGS. 4A and 4B  represent illustrative configurations that embody a current sensor in accordance with the present disclosure. 
           [0028]      FIG. 5  shows an illustrative embodiment of a current sensor in accordance with aspects of the present disclosure. 
           [0029]      FIG. 6  shows an illustrative embodiment of a current sensor in accordance with aspects of the present disclosure. 
           [0030]      FIG. 6A  illustrates an example of an end-to-end connected capacitive shield. 
           [0031]      FIG. 7  shows an illustrative embodiment of a magnetic shield in accordance with the present disclosure. 
           [0032]      FIGS. 7A and 7B  illustrate side views of a magnetic shield in accordance with the present disclosure. 
           [0033]      FIG. 8  demonstrates an aspect of the magnetic shield of  FIG. 7 . 
           [0034]      FIG. 9  shows an illustrative embodiment of a current sensor in accordance with aspects of the present disclosure. 
           [0035]      FIG. 9A  shows an illustrative embodiment of a current sensor in accordance with aspects of the present disclosure. 
           [0036]      FIGS. 10A, 10B, 10C, and 10D  show illustrative configurations of current sensors in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be evident, however, to one skilled in the art that the present disclosure as expressed in the claims may include some or all of the features in these examples, alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein. 
         [0038]      FIG. 1  is a functional block diagram of a wireless power transfer system  100 , in accordance with an illustrative embodiment. An input power  102  may be provided to a transmitter  104  from a power source (not shown in this figure) to generate a wireless (e.g., magnetic or electromagnetic) field  105  for performing energy transfer. A receiver  108  may couple to the wireless field  105  and generate an output power  110  for storing or consumption by a device (not shown in this figure) coupled to the output power  110 . The transmitter  104  and the receiver  108  may be separated by a distance  112 . 
         [0039]    In one illustrative embodiment, the transmitter  104  and the receiver  108  may be configured according to a mutual resonant relationship. When the resonant frequency of the receiver  108  and the resonant frequency of the transmitter  104  are substantially the same or very close, transmission losses between the transmitter  104  and the receiver  108  are minimal. As such, wireless power transfer may be provided over a larger distances. Resonant inductive coupling techniques may thus allow for improved efficiency and power transfer over various distances and with a variety of inductive coil configurations. 
         [0040]    The receiver  108  may receive power when the receiver is located in the wireless field  105  produced by the transmitter  104 . The wireless field  105  corresponds to a region where energy output by the transmitter  104  may be captured by the receiver  108 . The wireless field  105  may correspond to the “near field” of the transmitter  104  as will be further described below. The transmitter  104  may include a transmit antenna or coil  114  for transmitting energy to the receiver  108 . The receiver  108  may include a receive antenna or coil  118  for receiving or capturing energy transmitted from the transmitter  104 . The near-field may correspond to a region in which there are strong reactive fields resulting from the currents and charges in the transmit coil  114  that minimally radiate power away from the transmit coil  114 . The near-field may correspond to a region that is within about one wavelength (or a fraction thereof) of the transmit coil  114 . 
         [0041]    As described above, efficient energy transfer may occur by coupling a large portion of the energy in the wireless field  105  to the receive coil  118  rather than propagating most of the energy in an electromagnetic wave to the far field. When positioned within the wireless field  105 , a “coupling mode” may develop between the transmit coil  114  and the receive coil  118 . 
         [0042]    In  FIG. 1 , the transmitter  104  may output a time varying magnetic (or electromagnetic) field with a frequency corresponding to the resonant frequency of the transmit coil  114 . When the receiver  108  is within the wireless field  105 , the time varying magnetic (or electromagnetic) field may induce a current in the receive coil  118 . As described above, if the receive coil  118  is configured to resonate at the frequency of the transmit coil  114 , energy may be efficiently transferred. The AC signal induced in the receive coil  118  may be rectified as described above to produce a DC signal that may be provided to charge or to power a load. 
         [0043]      FIG. 2  is a functional block diagram of a wireless power transfer system  200 , in accordance with another illustrative embodiment. The system  200  may include a transmitter  204  and a receiver  208 . The transmitter  204  (also referred to herein as power transfer unit, PTU) may include transmit circuitry  206  that may include an oscillator  222 , a driver circuit  224 , and a filter and matching circuit  226 . The oscillator  222  may be configured to generate a signal at a desired frequency that may adjust in response to a frequency control signal  223 . The oscillator  222  may provide the oscillator signal to the driver circuit  224 . The driver circuit  224  may be configured to drive the transmit antenna  214  at, for example, a resonant frequency of the transmit antenna  214  based on an input voltage signal (VD)  225 . The driver circuit  224  may be a switching amplifier configured to receive a square wave from the oscillator  222  and output a sine wave. 
         [0044]    The filter and matching circuit  226  may filter out harmonics or other unwanted frequencies and match the impedance of the transmitter  204  to the transmit antenna  214 . As a result of driving the transmit antenna  214 , the transmit antenna  214  may generate a wireless field  205  to wirelessly output power at a level sufficient for charging a battery  236 , or otherwise powering a load. 
         [0045]    The receiver  208  (also referred to herein as power receiving unit, PRU) may include receive circuitry  210  that may include a matching circuit  232  and a rectifier circuit  234 . The matching circuit  232  may match the impedance of the receive circuitry  210  to the receive antenna  218 . The rectifier circuit  234  may generate a direct current (DC) power output from an alternating current (AC) power input to charge the battery  236 , as shown in  FIG. 2 . The receiver  208  and the transmitter  204  may additionally communicate on a separate communication channel  219  (e.g., Bluetooth, Zigbee, cellular, etc.). The receiver  208  and the transmitter  204  may alternatively communicate via in-band signaling using characteristics of the wireless field  205 . 
         [0046]    The receiver  208  may be configured to determine whether an amount of power transmitted by the transmitter  204  and received by the receiver  208  is appropriate for charging the battery  236 . Transmitter  204  may be configured to generate a predominantly non-radiative field with a direct field coupling coefficient (k) for providing energy transfer. Receiver  208  may directly couple to the wireless field  205  and may generate an output power for storing or consumption by a battery (or load)  236  coupled to the output or receive circuitry  210 . 
         [0047]    As discussed above, transmitter  204  and receiver  208  may be separated by a distance and may be configured according to a mutual resonant relationship to minimize transmission losses between the transmitter and the receiver. 
         [0048]      FIG. 3  is a schematic diagram of a portion of the transmit circuitry  206  or the receive circuitry  210  of  FIG. 2 , in accordance with illustrative embodiments. As illustrated in  FIG. 3 , transmit or receive circuitry  350  may include an antenna  352 . The antenna  352  may also be referred to or be configured as a “loop” antenna  352 . The antenna  352  may also be referred to herein or be configured as a “magnetic” antenna, or an induction coil, or a resonator. The term “antenna” generally refers to a component that may wirelessly output or receive energy for coupling to another “antenna.” The antenna may also be referred to as a coil of a type that is configured to wirelessly output or receive power. As used herein, the antenna  352  is an example of a “power transfer component” of a type that is configured to wirelessly output and/or receive power. The antenna  352  may include an air core or a physical core such as a ferrite core (not shown in this figure). 
         [0049]    As stated, efficient transfer of energy between the transmitter  104  (transmitter  204  as referenced in  FIG. 2 ) and the receiver  108  (receiver  208  as referenced in  FIG. 2 ) may occur during matched or nearly matched resonance between the transmitter  104  and the receiver  108 . However, even when resonance between the transmitter  104  and receiver  108  are not matched, energy may be transferred, although the efficiency may be affected. For example, the efficiency may be less when resonance is not matched. 
         [0050]    Transfer of energy occurs by coupling energy from the wireless field  105  (wireless field  205  as referenced in  FIG. 2 ) of the transmit coil  114  (transmit coil  214  as referenced in  FIG. 2 ) to the receive coil  118  (receive coil  218  as referenced in  FIG. 2 ), residing in the vicinity of the wireless field  105 , rather than propagating the energy from the transmit coil  114  into free space. 
         [0051]    The resonant frequency of the loop or magnetic antennas is based on the inductance and capacitance. Inductance may be simply the inductance created by the antenna  352 , whereas, capacitance (e.g., a capacitor) may be added to create a resonant structure at a desired resonant frequency. As a non limiting example, a capacitor  354  and a capacitor  356  may be added to the transmit or receive circuitry  350  to create a resonant circuit. Accordingly, for larger diameter antennas, the size of capacitance needed to sustain resonance may decrease as the diameter or inductance of the loop increases. 
         [0052]    Furthermore, as the diameter of the antenna increases, the efficient energy transfer area of the near field may increase. Other resonant circuits formed using other components are also possible. As another non limiting example, a capacitor (not shown) may be placed in parallel between the two terminals of the circuitry  350 . For transmit antennas, the signal  358 , with a frequency that substantially corresponds to the resonant frequency of the antenna  352 , may be an input to the antenna  352 . For receive antennas, the signal  358 , with a frequency that substantially corresponds to the resonant frequency of the antenna  352 , may be an output from the antenna  352 . 
         [0053]    The discussion will now turn to a description of current sensors in accordance with the present disclosure, which may be used in the transmitter  104  of  FIG. 1  or transmitter  204  of  FIG. 2 .  FIG. 4A  shows circuitry comprising a power amplifier  40  connected to a load  44  via a current-carrying conductor  42 . A current sensor  402  in accordance with the present disclosure may be configured to sense the flow of current in the current-carrying conductor  42  and produce a signal that is representative of the flow of current in the conductor. Merely as an example to illustrate a usage case, the current sensor  402  may be incorporated in the wireless power transfer system  200  shown in  FIG. 2 . In this example, the power amplifier  40  may correspond to the driver circuit  224  in transmitter  204 , and the load  44  may correspond to the transmit coil  214 . The current sensor  402  may detect load changes in the transmit coil  214  during wireless power transfer as a consequence of variations in the amount of power that is being coupled to the receiver (e.g., PRU) via the magnetic field. For example, variations in power coupling may arise from the amount of power a PRU draws, the number of PRUs engaged in wireless power transfer with the PTU, and so on. The current-carrying conductor  42  may correspond to a connection (e.g., a wire) that provides current from the driver circuit  224  to the transmit coil  214 . It will be appreciated, of course, that current sensors in accordance with the present disclosure may be readily adapted for use in other circuit configurations. 
         [0054]    The current sensor  402  may include connections  404  and  406  to provide points of connection for the current-carrying conductor  42 . The current sensor  402  may include outputs  408  that output a signal in response to the flow of current in conductor  42 . 
         [0055]    The outputs  408  may be connected to a suitable amplifier  48 , for example, to produce a signal that represents the flow of current in the current-carrying conductor  42 . In some embodiments, the output of amplifier  48  may be a current signal that represents the flow of current in the current-carrying conductor  42 . In other embodiments, such as shown in  FIG. 4A , the output of amplifier  48  may be an output voltage V out  that represents the flow of current in the current-carrying conductor  42 . In some embodiments, the output of amplifier  48  may be used as a feedback signal to control the flow of current out of the power amplifier  40 . In other embodiments, the output of amplifier  48  may be used to monitor the operating conditions of the system. For example, in the context of the wireless power transfer system  200  shown in  FIG. 2 , in some embodiments, the current sensor  402  may be used to detect an overload condition. In other embodiments, the current sensor  402  may be used to detect placement of a PRU on the charging surface of the PTU, and so on. 
         [0056]    The power amplifier  40  in  FIG. 4A  represents an example of a single-ended output. Referring to  FIG. 4B , a power amplifier  40   a  may have a differential output, providing power on two current-carrying conductors  42   a  and  42   b . Accordingly, a current sensor  412  in accordance with some embodiments of the present disclosure may be configured to provide current sensing on multiple current-carrying conductors (e.g.,  42   a ,  42   b ). In some embodiments, for example, the current sensor  412  may include connections  404   a ,  404   b  and  406   a ,  406   b  to provide points of connection for the current-carrying conductors  42   a ,  42   b.    
         [0057]    The discussion will now turn to a description of an illustrative embodiment of current sensor  402  shown in  FIG. 4A .  FIG. 5  shows details of current sensor  402  in accordance with the present disclosure, along with some circuit elements shown in  FIG. 4A  included for context. In some embodiments, for example, current sensor  402  may comprise a sensing element  502  and a main (target) conductor  504  disposed on a plane, for example, as defined by a substrate  532 . The sensing element  502  may be disposed adjacent the main conductor  504 . In some embodiments, the sensing element  502  may comprise a coil  512  (or loop) of conductive material. The main conductor  504  may connect to the current-carrying conductor  42  at connection points  504   a ,  504   b ; e.g., by way of conductive pads formed at connection points  504   a ,  504   b.    
         [0058]    In some embodiments, the substrate  532  may be an area of a printed circuit board (PCB) for a larger circuit. In other embodiments, the substrate  532  may be stand-alone, self-contained PCB. The coil  512  may be a trace or a plurality of trace segments formed on the substrate  532 . The main conductor  504  may likewise be a trace formed on the substrate  532 . The conductive material used to form the traces may be copper or any suitable electrically conductive material. The traces may be formed on the substrate  532  using any of a number of known techniques. 
         [0059]      FIG. 5  depicts the coil  512  formed on a first face of the substrate  532 . In some embodiments, the coil  512  may be a spiral having one or more turns. The outer end  512   b  of the coil  512  may terminate at a conductive pad B on the substrate  532 . The inner end  512   a  of the coil  512  may terminate at a conductive pad A on the substrate  532  by way of a return path that comprises vias  514  and  518  formed through the substrate  532 , and a trace  516  formed on a second face of the substrate  532  that connects via  514  to via  518 . A trace may connect the via  518  to pad A. 
         [0060]    In accordance with the present disclosure, the current sensor  402  may further comprise a capacitive shield  522  disposed adjacent to both the sensor element  502  and the main conductor  504 . In some embodiments, the capacitive shield  522  may comprise a conductive trace (lead) formed on the substrate  532 . One end  522   a  of the capacitive shield  522  may be “free,” or not otherwise connected. Another end  522   b  of the capacitive shield  522  may connect to a conductive pad C via a trace  524 . In some embodiments, the pad C may be connected to ground potential. In other embodiments, the pad B and the pad C may be connected to a common voltage reference. 
         [0061]    In operation, when an electric current flows through the current-carrying conductor  42 , the current will flow through the main conductor  504 . As current flows through the main conductor  504 , a magnetic field may arise around the main conductor, for example, when the current is a time-varying current such as an alternating current (AC). The sensor element  502 , being in the vicinity of the main conductor  504 , may magnetically couple to the magnetic field generated by the main conductor. The area between the sensor element  502  and the main conductor  504  may be referred to as the sensing area. A voltage may be induced in the sensor element  502  that results from magnetically coupling to the magnetic field generated by the main conductor  504 . The induced voltage may be amplified by amplifier  48  to generate an output voltage V out  representative of the current flowing in the main conductor  504 . 
         [0062]    The electric field generated by current flowing in the main conductor  504  may capacitively couple to the sensor element  502 . The energy that can be coupled to the sensor element  502  can create an error in the generated output voltage V out . However, the capacitive shield  522  can capacitively couple the electric field to ground potential, thus preventing the output voltage V out  from influence by the electric field. 
         [0063]      FIG. 6  represents an example of a current sensor  600  in accordance with some embodiments of the present disclosure. In some embodiments, the current sensor  600  may comprise a sensing element  602  and a main (target) conductor  604 . The sensing element  602  may be disposed adjacent the main conductor  604 . In some embodiments, the sensing element  602  may comprise a first coil (or loop) of conductive material  612 - 1  disposed on a first plane (e.g., as defined by a substrate  632 - 1 ) and a second coil of conductive material  612 - 2  disposed on a second plane (e.g., as defined by a substrate  632 - 2 ). The main conductor  604  may be disposed on substrate  632 - 1 . The main conductor  604  may connect to a current-carrying conductor (e.g.,  42  in  FIG. 4 ) at connection points  604   a ,  604   b ; e.g., by way of conductive pads formed at the connection points. 
         [0064]    In some embodiments, the substrates  632 - 1 ,  632 - 2  may be layers in a multilayer PCB. The coils  612 - 1 ,  612 - 2  may be traces formed respective layers of the PCB. The main conductor  604  may likewise be a trace formed on one of the layers; e.g.,  FIG. 6  shows the main conductor formed on substrate  632 - 1 . The conductive material used to form the traces may be copper or any suitable material. The traces may be formed on the substrates  632 - 1 ,  632 - 2  using any of a number of known techniques. 
         [0065]    In some embodiments, the coils  612 - 1 ,  612 - 2  may be connected in series, as shown in  FIG. 6  for example. The outer end  612 - 1   b  of the coil  612 - 1  may terminate at a conductive pad B on the substrate  632 - 1 . A via  614   b  can provide a connection of the inner end  612 - 1   a  of coil  612 - 1  on substrate  632 - 1  to the inner end  612 - 2   a  of coil  612 - 2  on substrate  632 - 2 . A via  614   c  can provide a connection of the outer end  612 - 2   b  of coil  612 - 2  on substrate  632 - 2  to a conductive pad A on substrate  632 - 1 . 
         [0066]    In accordance with the present disclosure, the current sensor  600  may further comprise a first capacitive shield  622 - 1  disposed adjacent to both the coil  612 - 1  of sensor element  602  and the main conductor  604 , and a second capacitive shield  622 - 2  disposed adjacent to both the coil  612 - 2  of sensor element  602  and the main conductor  604 . The second capacitive shield  622 - 2  may still be considered to be adjacent the main conductor  604 , even though the second capacitive shield and main conductor are in different layers of the multilayer PCB. In some embodiments, the first capacitive shield  622 - 1  may comprise a conductive trace (lead) formed on substrate  632 - 1  and likewise the second capacitive shield  622 - 2  may comprise a conductive trace (lead) formed on substrate  632 - 2 . 
         [0067]    In accordance with the present disclosure, the capacitive shields  622 - 1 ,  622 - 2  may be connected together so that each capacitive shield has a free end and a grounded end, so that the capacitive shields do not form a closed loop.  FIG. 6  shows a connection configuration in accordance with some embodiments, for example. One end  622 - 1   a  of the capacitive shield  622 - 1  may be “free,” or not otherwise connected. Another end  622 - 1   b  of the capacitive shield  622 - 1  may connect to a conductive pad C, for example, via a trace  624 . Likewise, one end  622 - 2   a  of the capacitive shield  622 - 2  may be “free,” or not otherwise connected. Another end  622 - 2   b  of the capacitive shield  622 - 2  may connect to a conductive pad C; for example, a via  614   a  may connect end  622 - 2   b  to end  622 - 1   a . In some embodiments, the pad C may be connected to ground potential. In other embodiments, the pad B and the pad C may be connected to a common voltage reference. 
         [0068]      FIG. 6A  shows a connection configuration in accordance with other embodiments. In the configuration shown in  FIG. 6A , the capacitive shields  622 - 1 ,  622 - 2  may be connected in end-to-end fashion to form a continuous trace. For example, one end  622 - 2   b  of capacitive shield  622 - 2  may be the free end. The other end  622 - 2   a  of capacitive shield  622 - 2  may connect to one end  622 - 1   a  of capacitive shield  622 - 1 , for example, using via  614   a . The other end  622 - 1   b  of capacitive shield  622 - 1  may connect to pad C, for example, using trace  624 . One of ordinary skill will appreciate that still other connection configurations in accordance with the present disclosure may be possible. 
         [0069]    One of ordinary skill will appreciate that in some embodiments, the sensor element  602  may comprise additional coils provided on respective additional layers of the multi-layer PCB. In some embodiments, each layer of the multi-layer PCB may be provided with a coil. For example,  FIG. 7B  described below depicts a two-layer PCB  732 ′ supporting a sensor element  743 ′ comprising a coil in each layer. In other embodiments, the substrate may be an N-layer PCB supporting a sensor element comprising N coils, one coil in each layer. Accompanying each additional coil may be a capacitive shield (trace lead) disposed adjacent to the coil on the same layer (e.g., co-planar with the coil) and also adjacent the main conductor  604 . 
         [0070]    In accordance with the present disclosure, current sensors in accordance with the present disclosure may further include magnetic shielding to shield the current sensor from the effects of external magnetic fields, as further discussed below. Referring to  FIG. 7 , for example, the current sensor  600  ( FIG. 6 ) may further include a magnetic shield  700 . The magnetic shield  700  may comprise layers of a first material  702   a ,  702   b  that sandwich the sense element  602 , thus defining a stack  712  comprising the layers of first material  702   a ,  702   b  and the first and second coils  612 - 1 ,  612 - 2  of the sense element  602 . In some embodiments, the layers of first material  702   a ,  702   b  may be a ferrite material or other ferromagnetic material. 
         [0071]    Further in accordance with the present disclosure, the magnetic shield  700  may comprise layers of a second material  704   a ,  704   b  that sandwich the stack  712 . In some embodiments, the layers of second material  704   a ,  704   b  may be an electrically conductive material. In particular embodiments, the electrically conductive material may be copper tape. 
         [0072]      FIGS. 7A and 7B  show schematic side views of magnetic shield  700  in accordance with some embodiments.  FIG. 7A  shows a portion of a substrate  732  having formed thereon the various traces  734  for components (e.g., coil  612 - 1 , capacitive shield  622 - 1 , etc. in  FIG. 6 ) that comprise a current sensor (e.g.,  600 ,  FIG. 6 ) according to the present disclosure. The magnetic shield  700  comprises first material  702  that sandwiches the substrate  732  and traces  734  to form stack  712 . In some embodiments, the first material  702  may be a ferrite material. The magnetic shield  700  further comprises second material  704  that sandwiches the stack  712 . In some embodiments, the second material  704  may be an electrically conductive material, such as copper tape for example. 
         [0073]      FIG. 7B  illustrates magnetic shield  700  in accordance with other embodiments. Instead of a single-layer substrate (e.g.,  732 ), the substrate  732 ′ represents an example of a multilayer PCB, in this case a two-layer PCB. Traces  734 ′ represent traces formed in the layers of the substrate  732 ′ for components comprising a current sensor (e.g., coil and capacitive shield) according to some embodiments of the present disclosure. 
         [0074]    The effect of magnetic shield  700  will now be discussed. Consider first, a configuration of a current sensor without a magnetic shield, such as current sensor  402  illustrated in  FIG. 5  for example. Suppose the current sensor  402  is configured to sense current in a conductor configured to drive a transmit coil in a wireless power transfer system. During operation, the transmit coil may be drive to generate an external magnetic field for coupling power to a receiver. This external magnetic field can couple to the sense element  502 . The voltage, which can be induced in the sense element  502  as a result of coupling to the external magnetic field, can introduce an error in the output signal V out . The error can be pronounced if the external magnetic field varies (e.g., due to varying load conditions at the receiver side) when the current flowing in main conductor  504  is constant; in other words, variations in the external magnetic field can produce variations in the output signal V out  even though the current flow in main conductor  504  is constant. Since the current sensor  402  is used to provide feedback to adjust the generated field or to detect foreign objects in the generated field, it may be beneficial to ensure that the generated field does not interfere with the sensed current. 
         [0075]    Consider next, the magnetic shield  700  shown in  FIG. 7  with reference to  FIG. 8 . The effect of magnetic shield  700  can be explained in connection with the schematic representation depicted in  FIG. 8 . The illustration is a view looking down on the electrically conductive layer of second material  704   a  of the magnetic shield  700 . An external magnetic field can couple to the electrically conductive layer of second material  704   a . Eddy currents can be induced in the electrically conductive layer of second material  704   a  under the influence of the external magnetic field. The eddy currents induced in the electrically conductive layer of second material  704   a , in turn, can generate a magnetic field that opposes the external magnetic field and thus can have a cancelling effect on the external magnetic field. A similar effect occurs with the electrically conductive layer of second material  704   b . The electrically conductive layers of second material  704   a ,  704   b  can therefore shield the sensing element (e.g.,  602 ) so that the output voltage V out  can be substantially free of influence from the external magnetic field. 
         [0076]    The electrically conductive layers of second material  704   a ,  704   b  may also act on the magnetic field generated by current flowing in the main conductor (e.g.,  604 ,  FIG. 6 ). The electrically conductive layers of second material  704   a ,  704   b  can generate a magnetic field that opposes the magnetic field generated by current flowing in the main conductor, which can be an undesirable effect. Therefore, in accordance with the present disclosure, the layers of first material  702   a ,  702   b  may be a ferrite material. The ferrite layers  702   a ,  702   b  can serve to close the path for the magnetic field generated by current flowing in the main conductor (e.g.,  604 ,  FIG. 6 ) so that the magnetic shield  700 , in particular the layers of second material  704   a ,  704   b , does not respond with an opposing magnetic field, while at the same time shielding the external magnetic field as described above. Accordingly, the output voltage V out  can be substantially free of influence from the act of shielding the sensing area from an external magnetic field. 
         [0077]      FIG. 9  shows a current sensor  900  in accordance with some embodiments of the present disclosure. In some embodiments, the current sensor  900  may comprise a sensing element  902  and a main conductor  904  disposed on a plane, for example, as defined by substrate  932 . The sensing element  902  may comprise a first coil of conductive material  912 - 1  and a second coil of conductive material  912 - 2 . In some embodiments, the first and second coils  912 - 1 ,  912 - 2  may be substantially co-planar on the substrate  932  and in opposed relation to each other. The first and second coils  912 - 1 ,  912 - 2  may be connected in series. For example, vias may be used to route traces on an opposite face of the substrate  932  in order to connect the first and second coils  912 - 1 ,  912 - 2  in series. 
         [0078]    In accordance with the present disclosure, the current sensor  900  may further comprise a first capacitive shield  922 - 1  disposed adjacent to both the first coil  912 - 1  and the main conductor  904 , and a second capacitive shield  922 - 2  disposed adjacent to both the second coil  912 - 2  and the main conductor  904 . In some embodiments, the first and second capacitive shields  922 - 1 ,  922 - 2  may comprise conductive traces (leads) formed on the substrate  932 . One end of respective first and second capacitive shields  922 - 1 ,  922 - 2  may be “free,” or not otherwise connected. Another end of respective first and second capacitive shields  922 - 1 ,  922 - 2  may be connected to a common point (e.g., GND). Though not shown in  FIG. 9 , the current sensor  900  may further include a magnetic shield such as illustrated in  FIG. 7A , for example. 
         [0079]      FIG. 9A  shows a current sensor  900 ′ in accordance with some embodiments of the present disclosure. The current sensor  900 ′ can be used to sense current flowing in two main conductors  904   a ,  904   b . For example, the current sensor  900 ′ may be used to sense current flow in the conductive leads of a differential amplifier; see, for example, the configuration illustrated in  FIG. 4B . The sense element  902  may comprise first, second, and third coils  912 - 1 ,  912 - 2 ,  912 - 3  configured to be adjacent the main conductors  904   a ,  904   b . The current sensor  900 ′ may include capacitive shields  922 - 1 ,  922 - 2  configured to shield the coils  912 - 1 ,  912 - 2  from an electric field that can emanate from main conductor  904   a . The current sensor  900 ′ may further include capacitive shields  922 - 3 ,  922 - 4  configured to shield the coils  912 - 2 ,  912 - 3  from an electric field that can emanated from main conductor  904   b . Though not shown in  FIG. 9A , the current sensor  900 ′ may further include a magnetic shield such as illustrated in  FIG. 7A , for example. 
         [0080]    In accordance with the present disclosure, the single-conductor current sensors (e.g.,  402  in  FIG. 5 ) may be used with a differential power amplifier. Differential power amplifiers, for example, may be integrated in wireless power transmit circuitry to drive a transmit coil.  FIGS. 10A and 10B  schematically depict illustrative embodiments of differential power amplifier configurations.  FIG. 10A  for example, shows a differential power amplifier  1002  connected to loads  1004 ,  1006 . Current sensors  1000   a ,  1000   b  may be disposed along conductors  1042   a ,  1042   b  to sense a flow of current in the respective conductors. The current sensors  1000   a ,  1000   b  may be connected together in series to produce a single output (e.g.,  408 ,  FIG. 4B ) that can be connected to an amplifier (e.g.,  48 ,  FIG. 4B ). Referring to  FIG. 5 , for example, pad B of current sensor  1000   a  may be connected to pad A of current sensor  1000   b . Pad A of current sensor  1000   a  and pad B of current sensor  1000   b  may be the inputs to an amplifier (e.g.,  48 ). 
         [0081]      FIG. 10B  illustrates a configuration in which the conductors  1042   a ,  1042   b  that are sensed by current sensors  1000   a ,  1000   b  may be disposed along the ground paths from respective loads  1004 ,  1006 . The current sensors  1000   a ,  1000   b  may be connected in series. The configuration shown in  FIG. 10B  may be advantageous in some applications, since the line voltage in conductors  1042   a ,  1042   b  is close to ground potential. 
         [0082]      FIG. 10C  illustrates a configuration of a dual-conductor single current sensor  1000   c , such as illustrated in  FIG. 9A  for example, for sensing the current flow in conductors  1042   a ,  1042   b  of the differential amplifier  1002 . The configuration shown in  FIG. 10C  shows the conductors  1042   a ,  1042   b  to be along the ground path. In other embodiments, however, the conductors  1042   a ,  1042   b  that are sensed by the current sensor  1000   c  may be at the outputs of the differential power amplifier  1002 . 
         [0083]    In still other embodiments, three or more current sensors may be used. For example, the configuration two single-conductor current sensors  1000   a ,  1000   b  shown in  FIG. 10B  may be combined in series fashion with the dual-conductor current sensor  1000   c  shown in  FIG. 10C .  FIG. 10D  illustrates an example of such a configuration. 
         [0084]    In accordance with the present disclosure, a current sensor may include first means for magnetically coupling, at a sensing area proximate a conductor, to a first magnetic field generated by a current flow in the conductor, the first means having an output representative of the current flow. The sensor element  502  shown in  FIG. 5  represents an illustrative example of the first means in accordance with some embodiments. The sensor element  602  in shown in  FIG. 6  represents an illustrative example of the first means in accordance with some embodiments. 
         [0085]    In accordance with the present disclosure, a current sensor may further include second means for generating a second magnetic field that opposes the external magnetic field to shield the sensing area from the external magnetic field so that the output of the first means is substantially free of influence from the external magnetic field. The magnetic shield  700  shown in  FIGS. 7, 7A, and 7B  represent illustrative examples of the second means in accordance with some embodiments. Moreover, the layers of electrically conductive second material  704   a ,  704   b  represent an illustrative example of the second means in accordance with some embodiments. 
         [0086]    In accordance with the present disclosure, a current sensor may further include third means for shielding the sensing area from the second means so that the output of the first means is substantially free of influence from effects of the second means. The magnetic shield  700  shown in  FIGS. 7, 7A, and 7B  represent illustrative examples of the third means in accordance with some embodiments. Moreover, the layers of first material  702   a ,  702   b  represent an illustrative example of the third means in accordance with some embodiments. 
         [0087]    In accordance with the present disclosure, a current sensor may further include fourth means for shielding an electric field generated by the current flow in the conductor so that the output of the first means is substantially free of influence from the electric field. The capacitive shield  522  shown in  FIG. 5  represents an illustrative example of the fourth means in accordance with some embodiments. The capacitive shield  622  shown in  FIG. 6  represents an illustrative example of the fourth means in accordance with some embodiments. 
         [0088]    Current sensors may be used in wireless power circuitry; e.g., to provide feedback for power control. Current sensors may be particularly useful for lost power determination. For example, current sensors may used detect an amount of power transmitted in order to determine the amount of power lost based on what the receiver is receiving, or to detect the presence of objects consuming power on the pad. 
         [0089]    Current sensors in accordance with the present disclosure are easy to implement. The sensor element (e.g.,  502 ,  FIG. 5 ) may be designed along with the other traces on the PCB. In some embodiments, they may only require a small about of PCB area and a correspondingly small amount of ferrite and copper tape. For example, in some embodiments, a current sensor in accordance with the present disclosure may only consume less than 1 cm 2  of PCB area, although the size is not relevant and may be larger or smaller in other embodiments. Current sensors in accordance with the present disclosure adapt nicely to mass production processes. 
         [0090]    Current sensors in accordance with the present disclosure do not interact directly with the current flow that is being sensed. Therefore, the current sensor creates no imbalance in the power amplifier that supplies the current. In addition, current sensors in accordance with the present disclosure can provide an output voltage that is isolated from the output of the power amplifier. 
         [0091]    Current sensors in accordance with the present disclosure do not emit EMI because there is no switching circuitry. 
         [0092]    Current sensors in accordance with the present disclosure create a voltage waveform that is 90 degrees out of phase with current and thus can provide a usable phase angle measurement of the current flow. In addition, the zero crossing of this waveform can be compared to that of the power amplifier output voltage to provide an accurate measure of phase angle. This phase angle can be used for both load power and impedance measurements. 
         [0093]    The above description illustrates various embodiments of the present disclosure along with examples of how aspects of the particular embodiments may be implemented. The above examples should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the particular embodiments as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents may be employed without departing from the scope of the present disclosure as defined by the claims.