Patent Publication Number: US-2017358951-A1

Title: System and method for adjusting a response in a wireless power receiver

Description:
FIELD 
     The present disclosure relates generally to wireless power. More specifically, the disclosure is directed to adjusting a response in a wireless power receiver. 
     BACKGROUND 
     An increasing number and variety of electronic devices are powered via rechargeable batteries. Such devices include mobile phones, portable music players, laptop computers, tablet computers, computer peripheral devices, communication devices (e.g., Bluetooth devices), digital cameras, hearing aids, and the like. While battery technology has improved, battery-powered electronic devices increasingly require and consume greater amounts of power, thereby often requiring recharging. Rechargeable devices are often charged via wired connections that require cables or other similar connectors that are physically connected to a power supply. Cables and similar connectors may sometimes be inconvenient or cumbersome and have other drawbacks. Wireless power charging systems, for example, may allow users to charge and/or power electronic devices without physical, electrical connections, thus reducing the number of components required for operation of the electronic devices and simplifying the use of the electronic device. As such, wireless charging systems and methods that efficiently and safely transfer power for charging rechargeable electronic devices are desirable. 
     SUMMARY 
     Various implementations of systems, methods and devices within the scope of the appended claims each have several aspects, no single one of which is solely responsible for the desirable attributes described herein. Without limiting the scope of the appended claims, some prominent features are described herein. 
     Details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings, and the claims. Note that the relative dimensions of the following figures may not be drawn to scale. 
     One aspect of the disclosure provides a wireless power receiver including a receive antenna configured to generate charging current in response to a primary magnetic field, and a field adjustment element located between the receive antenna and a metal structure of the wireless power receiver, the field adjustment element configured to adjust an amount of a secondary magnetic field that may reach the receive antenna. 
     Another aspect of the disclosure provides a method for adjusting a response of a receive antenna in a wireless power receiver including generating a charging current in response to a primary magnetic field, and limiting an amount of a secondary magnetic field that reaches the receive antenna in the wireless power receiver. 
     Another aspect of the disclosure provides a wireless power receiver including a receive antenna configured to generate charging current in response to a primary magnetic field, wherein a secondary magnetic field is generated by an eddy current in the wireless power receiver. The wireless power receiver further includes a field adjustment element located between the receive antenna and a metal structure of the wireless power receiver, the field adjustment element configured to adjust an amount of the secondary magnetic field, wherein the field adjustment element is chosen from the group consisting of a field blocking element configured to reduce a strength of the secondary magnetic field in a direction toward the receive antenna, a field blocking element configured to reduce a strength of the primary magnetic field in a direction toward a metal structure of the wireless power receiver such that the secondary magnetic field is substantially reduced, and a field altering element configured to generate a third magnetic field, the third magnetic field configured to reduce a strength of the secondary magnetic field in a direction toward the receive antenna. 
     Another aspect of the disclosure provides a device for wireless power transfer including means for generating a charging current in response to a primary magnetic field, and means for limiting an amount of a secondary magnetic field that reaches the means for generating the charging current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a functional block diagram of an exemplary wireless power transfer system, in accordance with exemplary embodiments. 
         FIG. 2  is a functional block diagram of an example of a wireless power transfer system, in accordance with various exemplary embodiments. 
         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 exemplary embodiments. 
         FIG. 4  is a functional block diagram of a transmitter that may be used in the wireless power transfer system of  FIG. 1 , in accordance with exemplary embodiments. 
         FIG. 5  is a functional block diagram of a receiver that may be used in the wireless power transfer system of  FIG. 1 , in accordance with exemplary embodiments. 
         FIG. 6  is a schematic diagram of a portion of transmit circuitry that may be used in the transmit circuitry of  FIG. 4 . 
         FIG. 7  is a schematic diagram showing an exemplary receiver located on a wireless charging surface. 
         FIG. 8  is a schematic diagram showing an exemplary receiver located on a wireless charging surface. 
         FIG. 9  is a schematic diagram showing an exemplary embodiment of a wireless power transfer system having a receiver with an adjustable response. 
         FIG. 10  is a schematic diagram showing an exemplary embodiment of a wireless power transfer system having a receiver with an adjustable response. 
         FIG. 11  is a schematic diagram showing an exemplary embodiment of a wireless power transfer system having a receiver with an adjustable response. 
         FIG. 12  is a plan view of an exemplary embodiment of a wireless power transfer system having a receiver with an adjustable response. 
         FIG. 13  is a plan view of an exemplary embodiment of a wireless power transfer system having a receiver with an adjustable response 
         FIG. 14  is a plan view of an exemplary embodiment of a wireless power transfer system having a receiver with an adjustable response. 
         FIG. 15  is a schematic diagram showing an exemplary embodiment of a wireless power transfer system having a receiver with an adjustable response. 
         FIG. 16  is a plan view of the exemplary embodiment of the wireless power transfer system of  FIG. 15 . 
         FIG. 17  is a flowchart illustrating an exemplary embodiment of a method for adjusting a response in a wireless power receiver. 
         FIG. 18  is a functional block diagram of an apparatus for adjusting a response in a wireless power receiver. 
     
    
    
     The various features illustrated in the drawings may not be drawn to scale. Accordingly, the dimensions of the various features may be arbitrarily expanded or reduced for clarity. In addition, some of the drawings may not depict all of the components of a given system, method or device. Finally, like reference numerals may be used to denote like features throughout the specification and figures. 
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of the invention and is not intended to represent the only embodiments in which the invention may be practiced. The term “exemplary” used throughout this description means “serving as an example, instance, or illustration,” and should not necessarily be construed as preferred or advantageous over other exemplary embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the exemplary embodiments of the invention. In some instances, some devices are shown in block diagram form. 
     In this description, the term “application” may also include files having executable content, such as: object code, scripts, byte code, markup language files, and patches. In addition, an “application” referred to herein, may also include files that are not executable in nature, such as documents that may need to be opened or other data files that need to be accessed. 
     As used in this description, the terms “component,” “database,” “module,” “system,” and the like are intended to refer to a computer-related entity, either hardware, firmware, a combination of hardware and software, software, or software in execution. For example, a component may be, but is not limited to being, a process running on a processor, a processor, an object, an executable, a thread of execution, a program, and/or a computer. By way of illustration, both an application running on a computing device and the computing device may be a component. One or more components may reside within a process and/or thread of execution, and a component may be localized on one computer and/or distributed between two or more computers. In addition, these components may execute from various computer readable media having various data structures stored thereon. The components may communicate by way of local and/or remote processes such as in accordance with a signal having one or more data packets (e.g., data from one component interacting with another component in a local system, distributed system, and/or across a network such as the Internet with other systems by way of the signal). 
     Drawing elements that are common among the following figures may be identified using the same reference numerals. 
     Wireless power transfer may refer to transferring any form of energy associated with electric fields, magnetic fields, electromagnetic fields, or otherwise from a transmitter to a receiver without physical electrical conductors connecting the transmitter to the receiver to deliver the power (e.g., power may be transferred through free space). The power output into a wireless field (e.g., a magnetic field or an electromagnetic field) may be received, captured by, or coupled to by a power receiving element to achieve power transfer. 
     It is desirable to have the ability to efficiently and safely transfer power for wirelessly charging rechargeable electronic devices of various sizes, shapes, and form factors. Some wireless receiver devices have attributes that may make wireless charging difficult. For example, a large receiver containing metal plate, or a receiver having a small receive resonator located near the center of a metal plate in the receiver may give rise to inconsistencies in the wireless charging field that is used to transfer power. Such an inconsistency may sometimes be referred to as a “hole” or a “peak” in the wireless charging field, and the resultant magnetic coupling will be either higher or lower than expected due to eddy current effects from the metal plate in the receiver. This leads to a wide variation in magnetic coupling and power transfer, which complicates receiver antenna design. 
       FIG. 1  is a functional block diagram of an example of a wireless power transfer system  100 . Input power  102  is provided to a transmitter  104  from a power source (not shown) to generate a wireless field  105  (e.g., magnetic or electromagnetic) for performing energy transfer. A receiver  108  couples to the wireless field  105  and generates output power  110  for storing or consumption by a device (not shown in this figure) that is coupled to receive the output power  110 . The transmitter  104  and the receiver  108  are separated by a distance  112 . The transmitter  104  includes a power transmitting element  114  configured to transmit/couple energy to the receiver  108 . The receiver  108  includes a power receiving element  118  configured to receive or capture/couple energy transmitted from the transmitter  104 . 
     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, transmission losses between the transmitter  104  and the receiver  108  are reduced compared to the resonant frequencies not being substantially the same. As such, wireless power transfer may be provided over larger distances when the resonant frequencies are substantially the same. Resonant inductive coupling techniques allow for improved efficiency and power transfer over various distances and with a variety of inductive power transmitting and receiving element configurations. 
     The wireless field  105  may correspond to the near field of the transmitter  104 . The near field corresponds to a region in which there are strong reactive fields resulting from currents and charges in the power transmitting element  114  that do not significantly radiate power away from the power transmitting element  114 . The near field may correspond to a region that is within about three wavelengths, or even within about one wavelength (or a fraction thereof), of the power transmitting element  114 . Efficient energy transfer may occur by coupling a large portion of the energy in the wireless field  105  to the power receiving element  118  rather than propagating most of the energy in an electromagnetic wave to the far field. 
     The transmitter  104  may output a time-varying magnetic (or electromagnetic) field with a frequency corresponding to the resonant frequency of the power transmitting element  114 . When the receiver  108  is within the wireless field  105 , the time-varying magnetic (or electromagnetic) field may induce a current in the power receiving element  118 . As described above, with the power receiving element  118  configured as a resonant circuit configured to resonate at the frequency of the power transmitting element  114 , energy may be efficiently transferred. An alternating current (AC) signal induced in the power receiving element  118  may be rectified to produce a direct current (DC) signal that may be provided to charge an energy storage device (e.g., a battery) or to power a load. 
       FIG. 2  is a functional block diagram of an example of a wireless power transfer system  200 . The system  200  includes a transmitter  204  and a receiver  208 . The transmitter  204  is configured to provide power to a power transmitting element  214  that is configured to transmit power wirelessly to a power receiving element  218  that is configured to receive power from the power transmitting element  214  and to provide power to the receiver  208 . 
     The transmitter  204  includes transmit circuitry  206  that includes an oscillator  222 , a driver circuit  224 , and a front-end circuit  226 . The oscillator  222  may be configured to generate an oscillator 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 power transmitting element  214  at, for example, a resonant frequency of the power transmitting element  214  based on an input voltage signal  225  (V D ). The driver circuit  224  may be a switching amplifier configured to receive a square wave from the oscillator  222  and output a sine wave. 
     The front-end circuit  226  may include a filter circuit configured to filter out harmonics or other unwanted frequencies. The front-end circuit  226  may include a matching circuit configured to match the impedance of the transmitter  204  to the impedance of the power transmitting element  214 . As will be explained in more detail below, the front-end circuit  226  may include a tuning circuit to create a resonant circuit with the power transmitting element  214 . As a result of driving the power transmitting element  214 , the power transmitting element  214  generates a wireless field  205  to wirelessly output power at a level sufficient for charging a battery  236 , or powering a load. 
     The transmitter  204  further includes a controller  240  operably coupled to the transmit circuitry  206  and configured to control one or more aspects of the transmit circuitry  206 , or accomplish other operations relevant to managing the transfer of power. The controller  240  may be a micro-controller or a processor. The controller  240  may be implemented as an application-specific integrated circuit (ASIC). The controller  240  may be operably connected, directly or indirectly, to each component of the transmit circuitry  206 . The controller  240  may be further configured to receive information from each of the components of the transmit circuitry  206  and perform calculations based on the received information. The controller  240  may be configured to generate control signals (e.g., signal  223 ) for each of the components that may adjust the operation of that component. As such, the controller  240  may be configured to adjust or manage the power transfer based on a result of the operations performed by the controller  240 . The transmitter  204  may further include a memory (not shown) configured to store data, for example, such as instructions for causing the controller  240  to perform particular functions, such as those related to management of wireless power transfer. 
     The receiver  208  (also referred to herein as power receiving unit, PRU) includes receive circuitry  210  that includes a front-end circuit  232  and a rectifier circuit  234 . The front-end circuit  232  may include matching circuitry configured to match the impedance of the receive circuitry  210  to the impedance of the power receiving element  218 . As will be explained below, the front-end circuit  232  may further include a tuning circuit to create a resonant circuit with the power receiving element  218 . The rectifier circuit  234  may generate a DC power output from an AC power input to charge the battery  236 , as shown in  FIG. 3 . 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 . 
     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 . The transmitter  204  may be configured to generate a predominantly non-radiative field with a direct field coupling coefficient (k) for providing energy transfer. The receiver  208  may directly couple to the wireless field  205  and may generate an output power for storing or consumption by a battery  236  (or load) coupled to the output or receive circuitry  210 . 
     The receiver  208  further includes a controller  250  that may be configured similarly to the transmit controller  240  as described above for managing one or more aspects of the wireless power receiver  208 . The receiver  208  may further include a memory (not shown) configured to store data, for example, such as instructions for causing the controller  250  to perform particular functions, such as those related to management of wireless power transfer. 
     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 try to reduce transmission losses between the transmitter  204  and the receiver  208 . 
       FIG. 3  is a schematic diagram of an example of a portion of the transmit circuitry  206  or the receive circuitry  210  of  FIG. 2 . As illustrated in  FIG. 3 , transmit or receive circuitry  350  includes a power transmitting or receiving element  352  and a tuning circuit  360 . The power transmitting or receiving element  352  may also be referred to or be configured as an antenna such as a “loop” antenna. The term “antenna” generally refers to a component that may wirelessly output energy for reception by another antenna and that may receive wireless energy from another antenna. The power transmitting or receiving element  352  may also be referred to herein or be configured as a “magnetic” antenna, such as an induction coil (as shown), a resonator, or a portion of a resonator. The power transmitting or receiving element  352  may also be referred to as a coil or resonator of a type that is configured to wirelessly output or receive power. As used herein, the power transmitting or receiving element  352  is an example of a “power transfer component” of a type that is configured to wirelessly output and/or receive power. The power transmitting or receiving element  352  may include an air core or a physical core such as a ferrite core (not shown). 
     When the power transmitting or receiving element  352  is configured as a resonant circuit or resonator with tuning circuit  360 , the resonant frequency of the power transmitting or receiving element  352  may be based on the inductance and capacitance. Inductance may be simply the inductance created by a coil and/or other inductor forming the power transmitting or receiving element  352 . Capacitance (e.g., a capacitor) may be provided by the tuning circuit  360  to create a resonant structure at a desired resonant frequency. As a non-limiting example, the tuning circuit  360  may comprise a capacitor  354  and a capacitor  356 , which may be added to the transmit or receive circuitry  350  to create a resonant circuit. 
     The tuning circuit  360  may include other components to form a resonant circuit with the power transmitting or receiving element  352 . As another non-limiting example, the tuning circuit  360  may include a capacitor (not shown) placed in parallel between the two terminals of the circuitry  350 . Still other designs are possible. For example, the tuning circuit in the front-end circuit  226  may have the same design (e.g.,  360 ) as the tuning circuit in the front-end circuit  232 . Alternatively, the front-end circuit  226  may use a tuning circuit design different than in the front-end circuit  232 . 
     For power transmitting elements, the signal  358 , with a frequency that substantially corresponds to the resonant frequency of the power transmitting or receiving element  352 , may be an input to the power transmitting or receiving element  352 . For power receiving elements, the signal  358 , with a frequency that substantially corresponds to the resonant frequency of the power transmitting or receiving element  352 , may be an output from the power transmitting or receiving element  352 . Although aspects disclosed herein may be generally directed to resonant wireless power transfer, persons of ordinary skill will appreciate that aspects disclosed herein may be used in non-resonant implementations for wireless power transfer. 
       FIG. 4  is a functional block diagram of a transmitter  404  that may be used in the wireless power transfer system of  FIG. 1 , in accordance with exemplary embodiments. The transmitter  404  may include transmit circuitry  406  and a transmit antenna  414 . The transmit antenna  414  may be the antenna  352  as shown in  FIG. 3 . The transmit antenna  414  may be configured as the transmit antenna  214  as described above in reference to  FIG. 2 . In some implementations, the transmit antenna  414  may be a coil (e.g., an induction coil). In some implementations, the transmit antenna  414  may be associated with a larger structure, such as a pad, table, mat, lamp, or other stationary configuration. Transmit circuitry  406  may provide power to the transmit antenna  414  by providing an oscillating signal resulting in generation of energy (e.g., magnetic flux) about the transmit antenna  414 . Transmitter  404  may operate at any suitable frequency. By way of example, transmitter  404  may operate at the 6.78 MHz ISM band. 
     Transmit circuitry  406  may include a fixed impedance matching circuit  409  for matching the impedance of the transmit circuitry  406  (e.g., 50 ohms) to the impedance of the transmit antenna  414  and a low pass filter (LPF)  408  configured to reduce harmonic emissions to levels to prevent interference with devices and self jamming of devices coupled to receivers  108  ( FIG. 1 ). Other exemplary embodiments may include different filter topologies, including but not limited to, notch filters that attenuate specific frequencies while passing others and may include an adaptive impedance match, that may be varied based on measurable transmit metrics, such as output power to the transmit antenna  414  or DC current drawn by the transmitter driver circuit  424 . Transmit circuitry  406  further includes a driver circuit  424  configured to drive a signal as determined by an oscillator  423 . The transmit circuitry  406  may be comprised of discrete devices or circuits, or alternately, may be comprised of an integrated assembly. 
     Transmit circuitry  406  may further include a controller  415  for selectively enabling the oscillator  423  during transmit phases (or duty cycles) for specific receivers, for adjusting the frequency or phase of the oscillator  423 , and for adjusting the output power level for implementing a communication protocol for interacting with neighboring devices through their attached receivers. It is noted that the controller  415  may also be referred to herein as a processor. The controller may be coupled to a memory  470 . Adjustment of oscillator phase and related circuitry in the transmission path may allow for reduction of out of band emissions, especially when transitioning from one frequency to another. 
     The transmit circuitry  406  may further include a load sensing circuit  416  for detecting the presence or absence of active receivers in the vicinity of the near-field generated by transmit antenna  414 . By way of example, a load sensing circuit  416  monitors the current flowing to the transmitter driver circuit  424 , that may be affected by the presence or absence of active receivers in the vicinity of the field generated by transmit antenna  414  as will be further described below. Detection of changes to the loading on the transmitter driver circuit  424  are monitored by controller  415  for use in determining whether to enable the oscillator  423  for transmitting energy and to communicate with an active receiver. 
     The transmit antenna  414  may be implemented with a Litz wire or as an antenna strip with the thickness, width and metal type selected to keep resistive losses low. 
     The transmitter  404  may gather and track information about the whereabouts and status of receiver devices that may be associated with the transmitter  404 . Thus, the transmit circuitry  406  may include a presence detector  480 , an enclosed detector  460 , or a combination thereof, connected to the controller  415  (also referred to as a processor herein). The controller  415  may adjust an amount of power delivered by the transmitter driver circuit  424  in response to presence signals from the presence detector  480  and the enclosed detector  460 . The transmitter  404  may receive power through a number of power sources, such as, for example, an AC-DC converter (not shown) to convert AC power present in a building, a DC-DC converter (not shown) to convert a DC power source to a voltage suitable for the transmitter  404 , or directly from a DC power source (not shown). 
     As a non-limiting example, the presence detector  480  may be a motion detector utilized to sense the initial presence of a device to be charged that is inserted into the coverage area of the transmitter  404 . After detection, the transmitter  404  may be turned on and the power received by the device may be used to toggle a switch on the receiver device in a pre-determined manner, which in turn results in changes to the driving point impedance of the transmitter  404 . 
     As another non-limiting example, the presence detector  480  may be a detector capable of detecting a human, for example, by infrared detection, motion detection, or other suitable means. In some exemplary embodiments, there may be regulations limiting the amount of power that a transmit antenna  414  may transmit at a specific frequency. In some cases, these regulations are meant to protect humans from electromagnetic radiation. However, there may be environments where a transmit antenna  414  is placed in areas not occupied by humans, or occupied infrequently by humans, such as, for example, garages, factory floors, shops, and the like. If these environments are free from humans, it may be permissible to increase the power output of the transmit antenna  414  above the normal power restrictions regulations. In other words, the controller  415  may adjust the power output of the transmit antenna  414  to a regulatory level or lower in response to human presence and adjust the power output of the transmit antenna  414  to a level above the regulatory level when a human is outside a regulatory distance from the wireless charging field of the transmit antenna  414 . 
     As a non-limiting example, the enclosed detector  460  (may also be referred to herein as an enclosed compartment detector or an enclosed space detector) may be a device such as a sense switch for determining when an enclosure is in a closed or open state. When a transmitter is in an enclosure that is in an enclosed state, a power level of the transmitter may be increased. 
     In exemplary embodiments, a method by which the transmitter  404  does not remain on indefinitely may be used. In this case, the transmitter  404  may be programmed to shut off after a user-determined amount of time. This feature prevents the transmitter  404 , notably the transmitter driver circuit  424 , from running long after the wireless devices in its perimeter are fully charged. This event may be due to the failure of the circuit to detect the signal sent from either the repeater or the receive antenna  218  that a device is fully charged. To prevent the transmitter  404  from automatically shutting down if another device is placed in its perimeter, the transmitter  404  automatic shut off feature may be activated only after a set period of lack of motion detected in its perimeter. The user may be able to determine the inactivity time interval, and change it as desired. As a non-limiting example, the time interval may be longer than that needed to fully charge a specific type of wireless device under the assumption of the device being initially fully discharged. 
       FIG. 5  is a functional block diagram of a receiver  508  that may be used in the wireless power transfer system of  FIG. 1 , in accordance with exemplary embodiments. The receiver  508  includes receive circuitry  510  that may include a receive antenna  518 . Receiver  508  further couples to device  550  for providing received power thereto. It should be noted that receiver  508  is illustrated as being external to device  550  but may be integrated into device  550 . Energy may be propagated wirelessly to receive antenna  518  and then coupled through the rest of the receive circuitry  510  to device  550 . By way of example, the charging device may include devices such as mobile phones, portable music players, laptop computers, tablet computers, computer peripheral devices, communication devices (e.g., Bluetooth devices), digital cameras, hearing aids (and other medical devices), wearable devices, and the like. 
     Receive antenna  518  may be tuned to resonate at the same frequency, or within a specified range of frequencies, as transmit antenna  414  ( FIG. 4 ). Receive antenna  518  may be similarly dimensioned with transmit antenna  414  or may be differently sized based upon the dimensions of the associated device  550 . By way of example, device  550  may be a portable electronic device having diametric or length dimension smaller than the diameter or length of transmit antenna  414 . In such an example, receive antenna  518  may be implemented as a multi-turn coil in order to reduce the capacitance value of a tuning capacitor (not shown) and increase the receive coil&#39;s impedance. By way of example, receive antenna  518  may be placed around the substantial circumference of device  550  in order to maximize the antenna diameter and reduce the number of loop turns (i.e., windings) of the receive antenna  518  and the inter-winding capacitance. 
     Receive circuitry  510  may provide an impedance match to the receive antenna  518 . Receive circuitry  510  includes power conversion circuitry  506  for converting received energy into charging power for use by the device  550 . Power conversion circuitry  506  includes an AC-to-DC converter  520  and may also include a DC-to-DC converter  522 . AC-to-DC converter  520  rectifies the RF energy signal received at receive antenna  518  into a non-alternating power with an output voltage. The DC-to-DC converter  522  (or other power regulator) converts the rectified energy signal into an energy potential (e.g., voltage) that is compatible with device  550  with an output voltage and output current. Various AC-to-DC converters are contemplated, including partial and full rectifiers, regulators, bridges, doublers, as well as linear and switching converters. 
     Receive circuitry  510  may further include RX matching and switching circuitry  512  for connecting receive antenna  518  to the power conversion circuitry  506  or alternatively for disconnecting the power conversion circuitry  506 . Disconnecting receive antenna  518  from power conversion circuitry  506  not only suspends charging of device  550 , but also changes the “load” as “seen” by the transmitter  404  ( FIG. 2 ). 
     When multiple receivers  508  are present in a transmitter&#39;s near-field, it may be desirable to adjust the loading and unloading of one or more receivers to enable other receivers to more efficiently couple to the transmitter. A receiver  508  may also be cloaked in order to eliminate coupling to other nearby receivers or to reduce loading on nearby transmitters. This “unloading” of a receiver is also known herein as a “cloaking.” Furthermore, this switching between unloading and loading controlled by receiver  508  and detected by transmitter  404  may provide a communication mechanism from receiver  508  to transmitter  404 . Additionally, a protocol may be associated with the switching that enables the sending of a message from receiver  508  to transmitter  404 . By way of example, a switching speed may be on the order of 100 μsec. 
     In an exemplary embodiment, communication between the transmitter  404  and the receiver  508  may take place either via an “out-of-band” separate communication channel/antenna or via “in-band” communication that may occur via modulation of the field used for power transfer. 
     Receive circuitry  510  may further include signaling detector and beacon circuitry  514  used to identify received energy fluctuations that may correspond to informational signaling from the transmitter to the receiver. Furthermore, signaling and beacon circuitry  514  may also be used to detect the transmission of a reduced signal energy (i.e., a beacon signal) and to rectify the reduced signal energy into a nominal power for awakening either un-powered or power-depleted circuits within receive circuitry  510  in order to configure receive circuitry  510  for wireless charging. 
     Receive circuitry  510  further includes controller  516  for coordinating the processes of receiver  508  described herein including the control of switching circuitry  512  described herein. It is noted that the controller  516  may also be referred to herein as a processor. Cloaking of receiver  508  may also occur upon the occurrence of other events including detection of an external wired charging source (e.g., wall/USB power) providing charging power to device  550 . Controller  516 , in addition to controlling the cloaking of the receiver, may also monitor beacon circuitry  514  to determine a beacon state and extract messages sent from the transmitter  404 . Controller  516  may also adjust the DC-to-DC converter  522  for improved performance. 
       FIG. 6  is a schematic diagram of a portion of transmit circuitry  600  that may be used in the transmit circuitry  406  of  FIG. 4 . The transmit circuitry  600  may include a driver circuit  624  as described above in  FIG. 4 . As described above, the driver circuit  624  may be a switching amplifier that may be configured to receive a square wave and output a sine wave to be provided to the transmit circuit  650 . In some cases the driver circuit  624  may be referred to as an amplifier circuit. The driver circuit  624  is shown as a class E amplifier, however, any suitable driver circuit  624  may be used in accordance with the embodiments described herein. The driver circuit  624  may be driven by an input signal  602  from an oscillator  423  as shown in  FIG. 4 . The driver circuit  624  may also be provided with a drive voltage V D  that is configured to control the maximum power that may be delivered through a transmit circuit  650 . To eliminate or reduce harmonics, the transmit circuitry  600  may include a filter circuit  626 . The filter circuit  626  may be a three pole (capacitor  634 , inductor  632 , and capacitor  636 ) low pass filter circuit  626 . 
     The signal output by the filter circuit  626  may be provided to a transmit circuit  650  comprising an antenna  614 . The transmit circuit  650  may include a series resonant circuit having a capacitance  620  and inductance (e.g., that may be due to the inductance or capacitance of the antenna or to an additional capacitor component) that may resonate at a frequency of the filtered signal provided by the driver circuit  624 . The load of the transmit circuit  650  may be represented by the variable resistor  622 . The load may be a function of a wireless power receiver  508  that is positioned to receive power from the transmit circuit  650 . 
       FIG. 7  is a schematic diagram  700  showing an exemplary receiver  508  located on a wireless charging surface  702  associated with a transmitter. The wireless charging surface  702  may comprise a pad, a table, a mat, a lamp, or other structure, and may comprise some or all of the elements described in the transmitter  404  of  FIG. 4 . In the embodiment shown in  FIG. 7 , the receiver  508  is smaller in area than the wireless charging surface  702 . In the embodiment shown in  FIG. 7 , the receiver  508  comprises a primary receive antenna  518 , and is relatively large compared to the size of the primary receive antenna  518 . As used herein, the term “antenna” is used interchangeably with the term “coil,” and, when implemented with a capacitor, may comprise a resonant structure and be referred to as a “resonator.” As shown in  FIG. 7 , the receiver  508  comprises an enclosure or other metal structure  704  that may be large relative to the size of the primary receive antenna  518 . In such an instance, the large metal plate causes a large reactance shift, and also causes a reduction in coupling whereby the field generated by the transmitter causes an eddy current, I E , to be induced in the metal structure  704 . The eddy current, I E , generates a secondary magnetic field which in turn generates a current, I CE , in the primary receive antenna  518  that opposes the charging current, I RX , in the primary receive antenna  518 . The current, I CE , refers to a counter eddy current that is induced in the primary receive antenna  518  by the eddy current, I E . This can cause the magnetic coupling from the transmitter  404  to the receiver  508  to be reduced when the receiver  508  is centered on the wireless charging surface  702 , since the metal structure  704  covers the maximum area of transmit antenna (not shown in  FIG. 7 ) and thus generates the maximum eddy current, I E , in the metal structure  704  and hence the maximum current, I CE , opposing the charging current, I RX , generated in the primary receive antenna  518 . 
     A transmit antenna (not shown) having a uniform field will exhibit a wider-than-expected range of magnetic coupling when a large metallic receiver having a relatively small primary receive antenna is used. This makes receiver and receive antenna design difficult due to a wide voltage range, and/or a receiver that cannot accept charge, or that can accept a reduced charge, at many locations on a wireless charging surface  702 . As a result, the overall magnetic coupling between the transmit antenna (not shown) and the primary receive antenna  518  is reduced, resulting in a reduction in the voltage available at the receiver  508  (which may result in a voltage too low to be usable) and in an increase in the effective source impedance to a load after the rectifier in the receiver  508 , thus possibly reducing available power. 
       FIG. 8  is a schematic diagram  800  showing an exemplary receiver  508  located on a wireless charging surface  802  associated with a transmitter. The wireless charging surface  802  may comprise a pad, a table, a mat, a lamp, or other structure, and may comprise some or all of the elements described in the transmitter  404  of  FIG. 4 . In the embodiment shown in  FIG. 8 , the receiver  508  is larger in area than the wireless charging surface  802  and overhangs the wireless charging surface  802 . In the embodiment shown in  FIG. 8 , the receiver  508  comprises a primary receive antenna  518 , and is relatively large compared to the size of the primary receive antenna  518 . As used herein, the term “antenna” is used interchangeably with the term “coil,” and, when implemented with a capacitor, may comprise a resonant structure and be referred to as a “resonator.” As shown in  FIG. 8 , the receiver  508  comprises an enclosure or other metal structure  804  that may be large relative to the size of the primary receive antenna  518 . 
     In such an instance, the large metal plate causes a large reactance shift, and also causes an increase in coupling whereby the flux, referred to as eddy current, I E , induced in the metal structure  804 , generates a current, I CE , in the receive antenna  518  that reinforces the charging current, I RX , in the receive antenna. This means that magnetic coupling from the transmitter  404  to the receiver  508  is increased when the receiver  508  is centered on the wireless charging surface  802 . 
     Stated in terms of phase, the transmit antenna (not shown) transmits a signal at 0° phase, and outside of the transmit antenna (not shown) the phase inverts to 180°. When the receiver  508  does not overhang the transmit antenna  518  (such as in the embodiment shown in  FIG. 7 ) the eddy current, I E , generated within the receiver&#39;s conductive plane (not shown) is at a phase of 180°, so it generates a field at a phase of 180° that effectively reduces the coupling between the transmit antenna (not shown) and the receive antenna  518 . When the receiver  508  overhangs the transmitter (such as in the embodiment shown in  FIG. 8 ) the eddy current, I E  induced by the field outside of the receive antenna  518  is at 0° phase, so the induced field reinforces the charge coupling. 
     When this happens, the eddy current, I E , generated in the metal structure  804  is in the opposite direction from that described in  FIG. 7 , since the edges of the metal structure  804  extend into a “reverse field” region. In an exemplary embodiment, the “reverse field” region may be the area outside the charge area of the wireless charging surface  802  where the magnetic field wraps around the transmit antenna (not shown) in the wireless charging surface  802  and demonstrates a reverse polarity. This induces a current, I CE , at zero degrees (0°) phase, in the receive antenna  518  that reinforces the current, I RX , generated by the received field. As a result, the overall magnetic coupling between the transmit antenna (not shown) and the primary receive antenna  518  is increased. In some instances, receive voltage may increase beyond the limits that the receiver  508  can handle. This can damage the receiver  508  and/or result in expensive, large and inefficient designs. 
       FIG. 9  is a schematic diagram showing an exemplary embodiment of a wireless power transfer system  900  having a receiver with an adjustable response. The wireless power transfer system  900  includes a transmitter (also referred to as a power transmit unit (PTU)  404  and a receiver (also referred to as a power receiving unit (PRU)  508 . The transmitter  404  comprises a transmit antenna  414  having one or more coils  914 . The other elements of the transmitter  404  are not shown in  FIG. 9  for simplicity. 
     The receiver  508  comprises a receive antenna  518 , a ground plane  902 , a field adjustment element  905  and a ferrite element  906 . In an exemplary embodiment, the field adjustment element  905  may comprise a metal or a metallic element configured to adjust, modify, contain or otherwise adjust the magnetic field experienced by the receiver  508 . In an exemplary embodiment, the field adjustment element  905  may comprise one or more ferrite elements, in addition to the ferrite element  906 . The ground plane  902  may be substantially formed from a metal or metallic electrically conductive material and may also be part of a printed circuit board (PCB) (not shown) that contains circuitry related to the receiver  508 . The ferrite element  906  provides a magnetically conductive path for the magnetic field generated by the receive antenna, which may otherwise be blocked by the ground plane  902 . 
     In an exemplary embodiment, in general, the field generated by the transmit antenna  414  comprises a 0° phase field generally inside of the coils  914  in the region  950  and comprises a 180° phase field generally outside of the coils  914  in the region  960 . The field generally inside of the coils  914  in the region  950  may be referred to as a primary magnetic field. If the 0° phase field in the region  950  reaches the ground plane  902  the field in the region  970  that is generated by the ground plane  902  is generally generated at a 180° phase that opposes the original 0° phase field in the region  950  and decreases the coupling between the transmit antenna  414  and the receive antenna  518 . The wireless power receiver  508  is typically designed to accommodate this condition. The 180° phase field generated by the transmit antenna  414  in the region  960  that is generated generally outside of the coils  914  may reach the ground plane  902  and generate an eddy current  982  in the ground plane  902 . The eddy current  982  may generate a field at a phase of 0° in the region  980  that reinforces the original 0° phase field in the region  950  and which increases the coupling between the transmit antenna  414  and the receive antenna  518 . The field in the region  980  may be referred to as a secondary magnetic field. This condition can be problematic, such as by causing excessive coupling that may lead to an overvoltage condition in the wireless power receiver  508 , such that eliminating, reducing, blocking, or otherwise mitigating the effect of the field in the region  980  is desired. To eliminate or mitigate the effect of the field in the region  980  and its effect on the wireless power receiver  508 , it is desirable to prevent the field in the region  980  from reaching the receive antenna  518 . The regions  950 ,  960 ,  970  and  980  are illustrated as rectangular regions for convenience of illustration only. Those skilled in the art understand that magnetic fields are generally not rectangular in shape, but may take other shapes. 
     In an exemplary embodiment, the field adjustment element  905  may be configured as a center field blocking element configured to reduce the strength of, to block, or otherwise mitigate the effect of the field in the region  980  generated by the eddy current  982  on the receive antenna  518 . In exemplary embodiments, the field adjustment element  905  may comprise coils, segments or blocks of metallic or other conductive material, ferrite material, or other structures configured to adjust a response of the receiver  508 . For example, the field adjustment element  905  may cancel or diminish the strength of the magnetic field generated by the eddy current  982  in the region  980  that may otherwise reach the receive antenna  518 , and may help to cancel or diminish the likelihood that the eddy current  982  causes the detrimental effects described herein. Preventing or limiting the strength of the magnetic field generated by the eddy current  982  that may reach the transmit antenna  518  prevents the eddy current  982  from possibly causing an overvoltage condition in the wireless power receiver  508 . 
     It is desirable to prevent the field in the region  980  generated by the eddy current  982  from coupling to the receive antenna  518 . In this exemplary embodiment, the field adjustment element  905  prevents the field in the region  980  generated by the eddy current  982  from coupling to the receive antenna  518 , thus preventing excessive coupling that may lead to an overvoltage condition in the receiver  508 . 
       FIG. 10  is a schematic diagram showing an exemplary embodiment of a wireless power transfer system  1000  having a receiver with an adjustable response. The wireless power transfer system  1000  includes a transmitter  404  and a receiver  508 . The transmitter  404  comprises a transmit antenna  414  having one or more coils  914 . The other elements of the transmitter  404  are not shown in  FIG. 10  for simplicity. 
     The receiver  508  comprises a receive antenna  518 , a ground plane  1002 , a field adjustment element  1010 , a field adjustment element  1020 , and a ferrite element  1006 . In an exemplary embodiment, the field adjustment element  1010  and the field adjustment element  1020  may comprise a metal or a metallic element configured to adjust, modify, contain or otherwise adjust the magnetic field developed by the receiver  508 . The ground plane  1002  may be substantially formed from a metal or metallic electrically conductive material and may also be part of a printed circuit board (PCB) (not shown) that contains circuitry related to the receiver  508 . The ferrite element  1006  provides a magnetically conductive path for the magnetic field generated by the receive antenna  518 , which may otherwise be blocked by the ground plane  1002 . 
     In an exemplary embodiment, in general, the field generated by the transmit antenna  414  comprises a 0° phase field generally inside of the coils  914  in the region  1050  and comprises a 180° phase field generally outside of the coils  914  in the region  1060 . The field generally inside of the coils  914  in the region  1050  may be referred to as a primary magnetic field. If the 0° phase field in the region  1050  reaches the ground plane  1002  the field in the region  1070  that is generated by the ground plane  1002  is generally generated at a 180° phase that opposes the original 0° phase field in the region  1050  and decreases the coupling between the transmit antenna  414  and the receive antenna  518 . The wireless power receiver  508  is typically designed to accommodate this condition. The 180° phase field generated by the transmit antenna  414  in the region  1060  that is generated generally outside of the coils  914  may reach the ground plane  1002  and generate an eddy current  1082  in the ground plane  1002 . The eddy current  1082  may generate a field at a phase of 0° in the region  1080  that reinforces the original 0° phase field in the region  1050  and which increases the coupling between the transmit antenna  414  and the receive antenna  518 . The field in the region  1080  may be referred to as a secondary magnetic field. This condition can be problematic, such as by causing excessive coupling that may lead to an overvoltage condition in the wireless power receiver  508 , such that eliminating the field in the region  1080  is desired. To eliminate the field in the region  1080 , it is desirable to prevent the field in the region  1060  from reaching the ground plane  1002  and generating an eddy current  1082 . The regions  1050 ,  1060 ,  1070  and  1080  are illustrated as rectangular regions for convenience of illustration only. Those skilled in the art understand that magnetic fields are generally not rectangular in shape, but may take other shapes. 
     In an exemplary embodiment, the field adjustment element  1010  and the field adjustment element  1020  may be configured as edge field blocking elements configured to cancel or diminish the magnetic field generated by the transmit antenna  414  toward the periphery or edges of the receiver  508 , generally in the region  1060  and to prevent the field in the region  1060  from reaching the ground plane  1002  and generating the eddy current  1082 . In exemplary embodiments, the field adjustment elements  1010  and  1020  may comprise coils, segments of metallic material, ferrite material, or other structures configured to adjust a response of the receiver  508 . For example, the field adjustment element  1010  and the field adjustment element  1020  may prevent, cancel or diminish the strength of the magnetic field generated by the transmit antenna  414  outside of periphery of the transmit antenna  414  in the region  1060 , and prevent it from reaching the ground plane  1002 . Preventing or limiting the strength of the field in the region  1060  that may reach the ground plane  1002  prevents, or at least minimizes any eddy current  1082  from being developed in the ground plane  1002 , and thereby prevents the generation of a 0° phase field in the region  1080  which may couple with the field in the region  1050  and thus create an overvoltage condition in the wireless power receiver  508 . In an exemplary embodiment, the field adjustment elements  1010  and  1020  cancel or diminish the strength of the magnetic field generated by the transmit antenna  414  in the region  1060  so that the magnetic field in the region  1060  does not reach the ground plane  1002 , particularly above the field adjustment elements  1010  and  1020 . 
     In an exemplary embodiment, when the magnetic field generated by the transmit antenna  414  reaches the ground plane  1002 , an eddy current  1082  may be generated in the ground plane  1002 , possibly giving rise to a 0° phase field in the region  1080 , which may couple with the 0° phase field in the region  1050 , and result in excessive coupling that may lead to an overvoltage condition in the wireless power receiver  508 . The eddy current  1082  could exist anywhere that the current could flow in the ground plane  1002 , and in this exemplary embodiment, is shown toward the edges of the ground plane  1002  generated as a result of the field in the region  1060  reaching the ground plane  1002 . Therefore, it is desirable to prevent such an inverted field from being developed in the ground plane  1002  (or in any other metal structure associated with the wireless power receiver  508 ) and coupling to the receive antenna  518 . In the embodiment shown in  FIG. 10 , the field adjustment elements  1010  and  1020  prevent the eddy current  1082  from being generated in the ground plane  1002  above the field adjustment elements  1010  and  1020  and preventing the formation of a magnetic field in the region  1080 , thus preventing excessive coupling that may lead to an overvoltage condition in the receiver  508 . 
       FIG. 11  is a schematic diagram showing an exemplary embodiment of a wireless power transfer system  1100  having a receiver with an adjustable response. The wireless charging system  1100  includes a transmitter  404  and a receiver  508 . The transmitter  404  comprises a transmit antenna  414  having one or more coils  914 . The other elements of the transmitter  404  are not shown in  FIG. 11  for simplicity. 
     The receiver  508  comprises a receive antenna  518 , a ground plane  1102 , a field adjustment element  1110 , a field adjustment element  1120 , and a ferrite element  1106 . In an exemplary embodiment, the field adjustment element  1110  and the field adjustment element  1120  may comprise a metal or metallic elements configured to adjust, modify, contain or otherwise adjust the magnetic field developed by the receiver  508 . In an exemplary embodiment, the field adjustment element  1110  includes a portion  1116  and a portion  1117 . In an exemplary embodiment, the portion  1116  can be formed in such a way as to be inverted with respect to the portion  1117 . As used herein, the term “inverted” refers to the portion  1116  being wound or coiled in a direction opposite of the winding or coiling of the portion  1117 . Similarly, the field adjustment element  1120  includes a portion  1126  and a portion  1127 . In an exemplary embodiment, the portion  1126  can be formed in such a way as to be inverted with respect to the portion  1127 , similar to the portions  1116  and  1117  of the field adjustment element  1110 . The ground plane  1102  may be substantially formed from a metal or metallic electrically conductive material and may also be part of a printed circuit board (PCB) (not shown) that contains circuitry related to the receiver  508 . The ferrite element  1106  provides a magnetically conductive path for the magnetic field generated by the receive antenna  518 , which may otherwise be blocked by the ground plane  1102 . 
     In an exemplary embodiment, in general, the field generated by the transmit antenna  414  comprises a 0° phase field generally inside of the coils  914  in the region  1150  and comprises a 180° phase field generally outside of the coils  914  in the region  1160 . The field generally inside of the coils  914  in the region  1150  may be referred to as a primary magnetic field. If the 0° phase field in the region  1150  reaches the ground plane  1102  the field in the region  1170  that is generated by the ground plane  1102  is generally generated at a 180° phase that opposes the original 0° phase field in the region  1150  and decreases the coupling between the transmit antenna  414  and the receive antenna  518 . The wireless power receiver  508  is typically designed to accommodate this condition. The 180° phase field generated by the transmit antenna  414  in the region  1160  that is generated generally outside of the coils  914  may reach the ground plane  1102  and generate an eddy current  1182  in the ground plane  1102 . The eddy current  1182  may generate a field at a phase of 0° in the region  1180  that reinforces the original 0° phase field in the region  1150  and which increases the coupling between the transmit antenna  414  and the receive antenna  518 . The field in the region  1180  may be referred to as a secondary magnetic field. This condition can be problematic, such as by causing excessive coupling that can lead to an overvoltage condition in the wireless power receiver  508 , such that eliminating, reducing, blocking, or otherwise mitigating the effect of the field in the region  1180  is desired. To eliminate or mitigate the effect of the field in the region  1180  and its effect on the wireless power receiver  508 , it is desirable to prevent the field in the region  1180  from reaching the receive antenna  518 . The regions  1150 ,  1160 ,  1170  and  1180  are illustrated as rectangular regions for convenience of illustration only. Those skilled in the art understand that magnetic fields are generally not rectangular in shape, but may take other shapes. 
     In an exemplary embodiment, the field adjustment element  1110  and the field adjustment element  1120  may be referred to as field altering elements that may be configured as field inverting elements configured to reduce, block, or otherwise mitigate the effect of the magnetic field in the region  1180  generated by the eddy current  1182  on the receive antenna  518 . Preventing or limiting the strength of a magnetic field in the region  1180  that may reach the receive antenna  518  prevents excessive coupling and may prevent an overvoltage condition from developing in the wireless power receiver  508 . 
     In an exemplary embodiment, when the magnetic field generated by the transmit antenna  414  reaches the ground plane  1102 , an eddy current  1182  may be generated in the ground plane  1102  possibly giving rise to a 0° phase field in the region  1180 , which may couple with the 0° phase field in the region  1150 , and result in excessive coupling that may lead to an overvoltage condition in the receiver  508 . The eddy current  1182  could exist anywhere that the current could flow in the ground plane  1102  and in this exemplary embodiment, is shown toward the edges of the ground plane  1102 . Therefore, it is desirable to prevent the field in the region  1180  from reaching the receive antenna  518 . 
     In an exemplary embodiment, in response to the magnetic field in the region  1180  generated by the eddy current  1182 , the field adjustment element  1110  and the field adjustment element  1120  may generate a second eddy current  1184  at a phase opposite the phase of the eddy current  1182 . The second eddy current  1184  may generate a magnetic field in the region  1190  that counteracts, or reduces the strength of, the magnetic field in the region  1180 , thus preventing the magnetic field in the region  1180  from excessively coupling to the receive antenna  518 . The field in the region  1190  may be referred to as a third magnetic field. 
       FIG. 12  is a plan view of an exemplary embodiment of a wireless power transfer system  1200  having a receiver with an adjustable response. The wireless power transfer system  1200  shows a transmitter  404  over which a receiver  508  having a receive antenna  518  may be located. The plan view also shows in phantom a ground plane  1202  and a ferrite element  1206 , which may typically located between the ground plane  1202  and the receive antenna  518 . The ground plane  1202  and the ferrite element  1206  are similar to the ground plane and ferrite element discussed in  FIGS. 9, 10 and 11 . 
     The wireless power transfer system  1200  also comprises field adjustment elements shown as exemplary shorted coils  1212 ,  1214 ,  1216  and  1218 . In an exemplary embodiment, the shorted coils  1212 ,  1214 ,  1216  and  1218  are shown as being located around a periphery of the receive antenna  518 . In an exemplary embodiment, the shorted coils  1212 ,  1214 ,  1216  and  1218  may be configured to prevent a magnetic field in the region  1260  from reaching the ground plane  1202 . In an exemplary embodiment, the shorted coils  1212 ,  1214 ,  1216  and  1218  are relatively large relative to the receive antenna  518  and are arranged as edge-blocking elements configured to prevent a magnetic field in the region  1260  from reaching the ground plane  1202  and to prevent the subsequent development of an eddy current in the ground plane  1202 , as described herein. In an exemplary embodiment, the shorted coils  1212 ,  1214 ,  1216  and  1218  can diminish or prevent the magnetic field generated in the region  1260  by the transmit antenna (not shown) from reaching the ground plane  1202 . 
       FIG. 13  is a plan view of an exemplary embodiment of a wireless power transfer system  1300  having a receiver with an adjustable response. The wireless power transfer system  1300  shows a transmitter  404  over which a receiver  508  having a receive antenna  518  may be located. The plan view also shows in phantom a ground plane  1302  and a ferrite element  1306 , which may typically located between the ground plane  1302  and the receive antenna  518 . The ground plane  1302  and the ferrite element  1306  are similar to the ground plane and ferrite element discussed in  FIGS. 9, 10, 11 and 12 . 
     The wireless power transfer system  1300  also comprises field adjustment elements shown as exemplary shorted coils  1312 ,  1313 ,  1314 ,  1315 ,  1316 ,  1317 ,  1318 ,  1319 ,  1320 ,  1321 ,  1322  and  1323 . In an exemplary embodiment, the shorted coils  1312  through  1323  are shown as being located around a periphery of the receive antenna  518 . In an exemplary embodiment, the shorted coils  1312  through  1323  may be configured to prevent a magnetic field in the region  1360  from reaching the ground plane  1302 . In an exemplary embodiment, the shorted coils  1312  through  1323  are somewhat smaller than the shorted coils described in  FIG. 12  and are arranged as edge-blocking elements configured to prevent a magnetic field in the region  1360  from reaching the ground plane  1302  and to prevent the subsequent development of an eddy current in the ground plane  1302 , as described herein. In an exemplary embodiment, the shorted coils  1312  through  1323  can diminish or prevent the magnetic field generated in the region  1360  by the transmit antenna (not shown) from reaching the ground plane  1302 . 
       FIG. 14  is a plan view of an exemplary embodiment of a wireless power transfer system  1400  having a receiver  1408  with an adjustable response. The wireless power transfer system  1400  shows a transmitter  404  over which a receiver  508  having a receive antenna  518  may be located. The plan view also shows in phantom a ground plane  1402  and a ferrite element  1406 , which may be typically located between the ground plane  1402  and the receive antenna  518 . The ground plane  1402  and the ferrite element  1406  are similar to the ground plane and ferrite element discussed in  FIGS. 9, 10, 11, 12 and 13 . 
     In an exemplary embodiment, conductive plane blocks, such as, for example, the ground plane blocks  1412 ,  1414  and  1416 , are shown as three segments in a field of twelve ground plane blocks that comprise the field adjustment element  1410 , and the ground plane blocks  1422 ,  1424  and  1426  are shown as three segments in a field of twelve ground plane blocks that comprise the field adjustment element  1420 . In an exemplary embodiment, the field adjustment elements  1410  and  1420  may be configured to prevent a magnetic field in the region  1460  and the region  1465  from reaching the ground plane  1402 . In an exemplary embodiment, the field adjustment elements  1410  and  1420  comprise small individual blocks (exemplary blocks being shown using reference numerals  1412 ,  1414 ,  1416 ,  1422 ,  1424  and  1426 ) of metal, or metallic material, arranged in a rectangular grid to prevent a magnetic field in the region  1460  and the region  1465  from reaching the ground plane  1402  and to prevent the subsequent development of an eddy current in the ground plane  1402 , as described herein. In an exemplary embodiment, the field adjustment elements  1410  and  1420  can diminish or prevent the magnetic field generated in the regions  1460  and  1465  by the transmit antenna (not shown) from reaching the ground plane  1402 . 
     Although shown as relatively large coils in  FIG. 12 , as smaller coils in  FIG. 13  and as discrete blocks in  FIG. 14 , the field adjustment elements may be implemented using other structures, other shapes, and in other configurations, depending on implementation. For example, the field adjustment elements may be made from, or may include ferrite structures covering the gaps between the different elements in a wireless power receiver. Specific sizes and numbers of field adjustment elements are dependent on the size of the wireless power receiver and on implementation. 
       FIG. 15  is a schematic diagram showing an exemplary embodiment of a wireless power transfer system  1500  having a receiver with an adjustable response. The embodiment shown in  FIG. 15  is an alternative embodiment of the wireless power transfer system  1100  shown in  FIG. 11 . The wireless power transfer system  1500  includes a transmitter (also referred to as a power transmit unit (PTU)  404  and a receiver  1508 . The transmitter  404  comprises a transmit antenna  414  having one or more coils  914 . The other elements of the transmitter  404  are not shown in  FIG. 15  for simplicity. 
     The receiver  1508  comprises a receive antenna  518 , a ground plane  1502 , and a ferrite element  1506 . The ground plane  1502  may be substantially formed from a metal or metallic electrically conductive material and may also be part of a printed circuit board (PCB) (not shown) that contains circuitry related to the receiver  1508 . The ferrite element  1506  provides a magnetically conductive path for the magnetic field generated by the receive antenna, which may otherwise be blocked by the ground plane  1502 . 
     The receiver  1508  also comprises exemplary field adjustment elements  1510  and  1520 . In an exemplary embodiment, the effect of the field adjustment elements  1510  and  1520  may be similar to the effect provided by the field adjustment elements  1110  and  1120  described above. In an exemplary embodiment, the field adjustment element  1510  may comprise one or more coils, an exemplary one of which is shown using reference numeral  1511 , wound in a first direction outside of the receive antenna  518 , and the field adjustment element  1520  may comprise one or more coils, an exemplary one of which is shown using reference numeral  1521 , wound in the first direction outside of the receive antenna  518 . The receiver  1508  also comprises a field adjustment element  1530 . In an exemplary embodiment, the field adjustment element  1530  may comprise one or more coils, an exemplary one of which is shown using reference numeral  1532  wound in a second direction inside of the receive antenna  518 . In an exemplary embodiment, the first direction is opposite the second direction such that the one or more coils  1511  and  1521  are wound in a direction opposite the direction in which the coil  1532  is wound. 
     In an exemplary embodiment, the one or more coils  1511  and  1521  and the coil  1532  can be configured to adjust a response of the receiver  1508 . For example, the one or more coils  1511  and  1521  and the coil  1532  can be configured to reduce, block, or otherwise mitigate the effect of the magnetic field in the region  1580  generated by the eddy current  1582  on the receive antenna  518 . Preventing or limiting a magnetic field in the region  1580  from reaching the receive antenna  518  prevents excessive coupling and may prevent an overvoltage condition from developing in the wireless power receiver  508 . 
     In an exemplary embodiment, when the magnetic field generated by the transmit antenna  414  reaches the ground plane  1502 , an eddy current  1582  may be generated in the ground plane  1502  possibly giving rise to a 0° phase field in the region  1580 , which may couple with the 0° phase field in the region  1550  (the primary magnetic field), and result in excessive coupling that may lead to an overvoltage condition in the receiver  1508 . The field in the region  1580  may be referred to as a secondary magnetic field. The eddy current  1582  could exist anywhere that the current could flow in the ground plane  1502  and in this exemplary embodiment, is shown toward the edges of the ground plane  1502 . Therefore, it is desirable to prevent the field in the region  1580  from reaching the receive antenna  518 . 
     In an exemplary embodiment, in response to the magnetic field in the region  1580  generated by the eddy current  1582 , the field adjustment elements  1510 ,  1520  and  1530  may generate a second eddy current  1584  at a phase opposite the phase of the eddy current  1582 . The second eddy current  1584  may generate a magnetic field in the region  1590  that counteracts, or reduces the strength of, the magnetic field in the region  1580 , thus preventing the magnetic field in the region  1580  from excessively coupling to the receive antenna  518 , and thus preventing excessive coupling that may lead to an overvoltage condition in the receiver  1508 . The field in the region  1590  may be referred to as a third magnetic field. 
       FIG. 16  is a plan view of the exemplary embodiment of the wireless power transfer system of  FIG. 15 . The plan view  1600  shows the receiver  1508  and shows in phantom the ground plane  1502  and the ferrite element  1506 . In an exemplary embodiment, the coils  1511  and  1521  are wound in a first direction and are located outside of the receive antenna  518 , and the coil  1532  is wound in a second direction and is located inside of the receive antenna  518 . In an exemplary embodiment, the first direction is opposite the second direction such that the one or more coils  1511  and  1521  are wound in a direction opposite the direction in which the coil  1532  is wound. In an exemplary embodiment, the one or more coils  1511  and  1521  and the coil  1532  may be configured to develop a magnetic field responsive to the magnetic field generated by the eddy current in the receiver  1508 , such that the magnetic field developed in the one or more coils  1511  and  1521  and the coil  1532  can counteract the effect of the magnetic field generated by the eddy current in the receiver  1508 , as described above. 
       FIG. 17  is a flowchart  1700  illustrating an exemplary embodiment of a method for adjusting a response in a wireless power receiver. The blocks in the method  1700  can be performed in or out of the order shown. 
     In block  1702 , in an exemplary embodiment, a charging current is generated in response to a primary magnetic field in a wireless power receiver antenna. 
     In block  1704 , an additional magnetic field is prevented from coupling to the receive antenna. 
       FIG. 18  is a functional block diagram of an apparatus  1800  for adjusting a response in a wireless power receiver. The apparatus  1800  comprises means  1802  for generating a charging current in response to a primary magnetic field in a wireless power receiver. In certain embodiments, the means  1802  for generating a charging current in response to a primary magnetic field in a wireless power receiver can be configured to perform one or more of the function described in operation block  1702  of method  1700  ( FIG. 17 ). In an exemplary embodiment, the means  1802  for generating a charging current in response to a primary magnetic field in a wireless power receiver may comprise the structure shown in any of  FIG. 9  through  FIG. 16 . 
     The apparatus  1800  further comprises means  1804  for limiting an additional magnetic field from coupling to the means for generating the charging current. In certain embodiments, the means  1804  for limiting an additional magnetic field from coupling to the means for generating the charging current can be configured to perform one or more of the function described in operation block  1704  of method  1700  ( FIG. 17 ). In an exemplary embodiment, the means  1804  for limiting an additional magnetic field from coupling to the means for generating the charging current may comprise the structure shown in any of  FIG. 9  through  FIG. 16 . 
     The various operations of methods described above may be performed by any suitable means capable of performing the operations, such as various hardware and/or software component(s), circuits, and/or module(s). Generally, any operations illustrated in the Figures may be performed by corresponding functional means capable of performing the operations. 
     In view of the disclosure above, one of ordinary skill in programming is able to write computer code or identify appropriate hardware and/or circuits to implement the disclosed invention without difficulty based on the flow charts and associated description in this specification, for example. Therefore, disclosure of a particular set of program code instructions or detailed hardware devices is not considered necessary for an adequate understanding of how to make and use the invention. The inventive functionality of the claimed computer implemented processes is explained in more detail in the above description and in conjunction with the FIGS. which may illustrate various process flows. 
     In one or more exemplary aspects, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted as one or more instructions or code on a computer-readable medium. Computer-readable media include both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A storage media may be any available media that may be accessed by a computer. By way of example, and not limitation, such computer-readable media may comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that may be used to carry or store desired program code in the form of instructions or data structures and that may be accessed by a computer. 
     Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (“DSL”), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. 
     Disk and disc, as used herein, includes compact disc (“CD”), laser disc, optical disc, digital versatile disc (“DVD”), floppy disk and Blu-Ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     Although selected aspects have been illustrated and described in detail, it will be understood that various substitutions and alterations may be made therein without departing from the spirit and scope of the present invention, as defined by the following claims.