Patent Publication Number: US-11398752-B2

Title: Zero-crossing slotted foreign object detection

Description:
TECHNICAL FIELD 
     The present invention relates generally to wireless charging of batteries, including batteries in mobile computing devices, and more particularly to detection of foreign objects during a charging operation. 
     BACKGROUND 
     Wireless charging systems have been deployed to enable certain types of devices to charge internal batteries without the use of a physical charging connection. Devices that can take advantage of wireless charging include mobile processing and/or communication devices. Standards, such as the Qi standard defined by the Wireless Power Consortium enable devices manufactured by a first supplier to be wirelessly charged using a charger manufactured by a second supplier. Standards for wireless charging are optimized for relatively simple configurations of devices and tend to provide basic charging capabilities. 
     Improvements in wireless charging capabilities are required to support continually increasing complexity of mobile devices and changing form factors. For example, there is a need for improved wireless transmission power control, including detection of foreign objects that may affect wireless transmission of power when placed on or near a charging device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of a charging cell that may be employed to provide a charging surface in accordance with certain aspects disclosed herein. 
         FIG. 2  illustrates the arrangement of power transfer areas provided by a charging surface that employs multiple layers of charging cells configured in accordance with certain aspects disclosed herein. 
         FIG. 3  illustrates a wireless transmitter that may be provided in a charger base station in accordance with certain aspects disclosed herein. 
         FIG. 4  illustrates a phase-modulated wireless charger configured in accordance with certain aspects of this disclosure. 
         FIG. 5  illustrates an example of a pulse-width modulation charger configured in accordance with certain aspects of this disclosure. 
         FIG. 6  illustrate the operation of the pulse-width modulation charger of  FIG. 5 . 
         FIG. 7  illustrates an example of a wireless charging system that employs a class-D wireless transmitter configured in accordance with certain aspects disclosed herein. 
         FIG. 8  illustrate the operation of the class-D wireless transmitter of  FIG. 7 . 
         FIG. 9  illustrates zero-crossing, slotted foreign object detection in accordance with certain aspects of the disclosure. 
         FIG. 10  illustrates a wireless charging system that employs zero-crossing detection to obtain measurements at one or more points in each cycle of current or voltage in a resonant circuit in accordance with certain aspects of the disclosure. 
         FIGS. 11 and 12  illustrate phase-based ASK demodulation that supports using zero-crossing detection in a wireless charging system configured in accordance with certain aspects of the disclosure. 
         FIG. 13  illustrates one example of an apparatus employing a processing circuit that may be adapted according to certain aspects disclosed herein. 
         FIG. 14  illustrates a method for operating a charging device in accordance with certain aspects of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring such concepts. 
     Several aspects of wireless charging systems will now be presented with reference to various apparatus and methods. These apparatus and methods will be described in the following detailed description and illustrated in the accompanying drawing by various blocks, modules, components, circuits, steps, processes, algorithms, etc. (collectively referred to as “elements”). These elements may be implemented using electronic hardware, computer software, or any combination thereof. Whether such elements are implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. 
     By way of example, an element, or any portion of an element, or any combination of elements may be implemented with a “processing system” that includes one or more processors. Examples of processors include microprocessors, microcontrollers, digital signal processors (DSPs), field programmable gate arrays (FPGAs), programmable logic devices (PLDs), state machines, gated logic, discrete hardware circuits, and other suitable hardware configured to perform the various functionality described throughout this disclosure. One or more processors in the processing system may execute software. Software shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software modules, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. The software may reside on a processor-readable storage medium. A processor-readable storage medium, which may also be referred to herein as a computer-readable medium may include, by way of example, a magnetic storage device (e.g., hard disk, floppy disk, magnetic strip), an optical disk (e.g., compact disk (CD), digital versatile disk (DVD)), a smart card, a flash memory device (e.g., card, stick, key drive), Near Field Communications (NFC) token, random access memory (RAM), read only memory (ROM), programmable ROM (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM), a register, a removable disk, a carrier wave, a transmission line, and any other suitable medium for storing or transmitting software. The computer-readable medium may be resident in the processing system, external to the processing system, or distributed across multiple entities including the processing system. Computer-readable medium may be embodied in a computer-program product. By way of example, a computer-program product may include a computer-readable medium in packaging materials. Those skilled in the art will recognize how best to implement the described functionality presented throughout this disclosure depending on the particular application and the overall design constraints imposed on the overall system. 
     Overview 
     Certain aspects of the present disclosure relate to systems, apparatus and methods applicable to wireless charging devices and techniques. Charging cells may be configured with one or more inductive coils to provide a charging device that can charge one or more devices wirelessly. The location of a device to be charged may be detected through sensing techniques that associate location of a device to changes in a physical characteristic centered at a known location on a surface of the charging device. Sensing of location may be implemented using capacitive, resistive, inductive, touch, pressure, load, strain, and/or another appropriate type of sensing. 
     In one aspect of the disclosure, an apparatus has a battery charging power source, a plurality of charging cells configured in a matrix, a first plurality of switches in which each switch is configured to couple a row of coils in the matrix to a first terminal of the battery charging power source, and a second plurality of switches in which each switch is configured to couple a column of coils in the matrix to a second terminal of the battery charging power source. Each charging cell in the plurality of charging cells may include one or more coils surrounding a power transfer area. The plurality of charging cells may be arranged adjacent to a surface of the device without overlap of power transfer areas of the charging cells in the plurality of charging cells. Devices placed on the surface may receive power that is wirelessly transmitted through one or more of the charging cells. 
     In some instances, the apparatus may also be referred to as a charging surface. Power can be wirelessly transferred to a receiving device located anywhere on a surface of the apparatus. The devices can have an arbitrarily defined size and/or shape and may be placed without regard to any discrete placement locations enabled for charging. Multiple devices can be simultaneously charged on a single surface. The apparatus can track motion of one or more devices across the surface. 
     Certain aspects disclosed herein relate to improved wireless charging techniques. In various aspects of the disclosure, a method for operating a charging device includes providing a charging current to a resonant circuit when a receiving device is present on a surface of the wireless charging device, providing a zero-crossing signal that includes edges corresponding to transitions of a voltage measured across the resonant circuit through a zero volt level or to transitions of a current in the resonant circuit through a zero ampere level, providing a measurement slot by decreasing or terminating the charging current for a period of time, and determining whether an object other than the receiving device is present on a surface of the charging device based on measurements of samples of voltage or current captured based on timing provided by the zero-crossing signal, wherein the samples are captured during the measurement slot. 
     Charging Cells 
     According to certain aspects disclosed herein, a charging device may be provided using charging cells that are deployed adjacent to a surface of the charging device. In one example the charging cells are deployed in accordance with a honeycomb packaging configuration. A charging cell may be implemented using one or more coils that can each induce a magnetic field along an axis that is substantially orthogonal to the surface of the charging device and adjacent to the coil. In this description, a charging cell may refer to an element having one or more coils where each coil is configured to produce an electromagnetic field that is additive with respect to the fields produced by other coils in the charging cell, and directed along or proximate to a common axis. 
     In some implementations, a charging cell includes coils that are stacked along a common axis and/or that overlap such that they contribute to an induced magnetic field substantially orthogonal to the surface of the charging device. In some implementations, a charging cell includes coils that are arranged within a defined portion of the surface of the charging device and that contribute to an induced magnetic field within the substantially orthogonal portion of the surface of the charging device associated with the charging cell. In some implementations, charging cells may be configurable by providing an activating current to coils that are included in a dynamically-defined charging cell. For example, a charging device may include multiple stacks of coils deployed across a surface of the charging device, and the charging device may detect the location of a device to be charged and may select some combination of stacks of coils to provide a charging cell adjacent to the device to be charged. In some instances, a charging cell may include, or be characterized as a single coil. However, it should be appreciated that a charging cell may include multiple stacked coils and/or multiple adjacent coils or stacks of coils. 
       FIG. 1  illustrates an example of a charging cell  100  that may be deployed and/or configured to provide a charging device. In this example, the charging cell  100  has a substantially hexagonal shape that encloses one or more coils  102  constructed using conductors, wires or circuit board traces that can receive a current sufficient to produce an electromagnetic field in a power transfer area  104 . In various implementations, some coils  102  may have a shape that is substantially polygonal, including the hexagonal charging cell  100  illustrated in  FIG. 1 . Other implementations may provide coils  102  that have other shapes. The shape of the coils  102  may be determined at least in part by the capabilities or limitations of fabrication technology, and/or to optimize layout of the charging cells on a substrate  106  such as a printed circuit board substrate. Each coil  102  may be implemented using wires, printed circuit board traces and/or other connectors in a spiral configuration. Each charging cell  100  may span two or more layers separated by an insulator or substrate  106  such that coils  102  in different layers are centered around a common axis  108 . 
       FIG. 2  illustrates the arrangement of power transfer areas provided across a surface  200  of the charging device that employs multiple layers of charging cells configured in accordance with certain aspects disclosed herein. The charging device may be constructed from four layers of charging cells  202 ,  204 ,  206 ,  208 . In  FIG. 2 , each power transfer area provided by a charging cell in the first layer of charging cells  202  is marked “L1”, each power transfer area provided by a charging cell in the second layer of charging cells  204  is marked “L2”, each power transfer area provided by a charging cell in the third layer of charging cells  206 ,  208  is marked “L3”, and each power transfer area provided by a charging cell in the first layer of charging cells  208  is marked “L4”. 
       FIG. 3  illustrates a wireless transmitter  300  that may be provided in a charger base station. A controller  302  may receive a feedback signal filtered or otherwise processed by a filter circuit  308 . The controller may control the operation of a driver circuit  304  that provides an alternating current (AC) signal to a resonant circuit  306  that includes a capacitor  312  and inductor  314 . The resonant circuit  306  may also be referred to herein as a tank circuit, an LC tank circuit and/or as an LC tank, and the voltage  316  measured at an LC node  310  of the resonant circuit  306  may be referred to as the tank voltage. 
     The wireless transmitter  300  may be used by a charging device to determine if a compatible device has been placed on a surface of the charging device. For example, the charging device may determine that a compatible device has been placed on the surface of the charging device by sending an intermittent test signal (active ping) through the wireless transmitter  300 , where the resonant circuit  306  may receive encoded signals when a compatible device responds to the test signal. The charging device may be configured to activate one or more coils in at least one charging cell after receiving a response signal defined by standard, convention, manufacturer or application. In some examples, the compatible device can respond to a ping by communicating received signal strength such that the charging device can find an optimal charging cell to be used for charging the compatible device. 
     Passive ping techniques may use the voltage and/or current measured or observed at the LC node  310  to identify the presence of a receiving coil in proximity to the charging pad of a device adapted in accordance with certain aspects disclosed herein. In many conventional wireless charger transmitters, circuits are provided to measure voltage at the LC node  310  or the current in the network. These voltages and currents may be monitored for power regulation purposes and/or to support communication between devices. In the example illustrated in  FIG. 3 , voltage at the LC node  310  is monitored, although it is contemplated that current may additionally or alternatively be monitored to support passive ping. A response of the resonant circuit  306  to a passive ping (initial voltage V 0 ) may be represented by the voltage (V LC ) at the LC node  310 , such that: 
     
       
         
           
             
               
                 
                   
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     According to certain aspects disclosed herein, coils in one or more charging cells may be selectively activated to provide an optimal electromagnetic field for charging a compatible device. In some instances, coils may be assigned to charging cells, and some charging cells may overlap other charging cells. In the latter instances, the optimal charging configuration may be selected at the charging cell level. In other instances, charging cells may be defined based on placement of a device to be charged on a surface of the charging device. In these other instances, the combination of coils activated for each charging event can vary. In some implementations, a charging device may include a driver circuit that can select one or more cells and/or one or more predefined charging cells for activation during a charging event. 
     Phase Modulated Charging 
     One aspect of this disclosure relates to the use of a phase-modulated wireless charger  400 , an example of which is illustrated in  FIG. 4 . A driver circuit  402  provides a charging current  410  to a resonant circuit  404  that includes a capacitor (C p ) and an inductor (L p ). The charging current  410  may be substantially the same as the current in the inductor (i.e., the L p  current), although some portion of the charging current  410  may be lost due to parasitic capacitance, or the like. The charging current  410  alternates at a frequency that may be closely matched to the resonant frequency of the resonant circuit  404  to improve efficiency of power transfer. In accordance with certain aspects of this disclosure, the level of power transferred through the resonant circuit  404  to a receiving device may be controlled through phase modulation of the charging current  410 . 
     The timing diagram  420  illustrates certain aspects of phase modulation as applied to the charging current  410  in certain implementations. Phase modulation enables fine control over the level of power delivery by the driver circuit  402 . The timing diagram  420  depicts three charging periods  422 ,  424  and  426  in which power is delivered at different levels, as indicated by the varying amplitude of the charging current  410 . 
     Phase control is obtained using a zero-crossing detector  406  and a phase modulator  408  that responds to a phase control signal  418  provided by a controller or other processor. The zero-crossing detector  406  is used to provide timing information used by the phase modulator  408 . In one example, the zero-crossing detector  406  may compare polarity of a measurement signal  412  representing the current flowing to the resonant circuit  404  with polarity of a delayed version of the measurement signal  412 , whereby a difference in polarity is detected when a zero-crossing occurs in the measurement signal  412 . The zero-crossing detector  406  provides a zero-crossing signal  414  (ZC) that includes timing information identifying zero-crossings of the measurement signal  412 . In one example, the zero-crossing signal  414  includes an edge for each zero-crossing of the measurement signal  412 . Direction of transition of the edge may indicate positive-going or negative-going zero-crossings. In another example, the zero-crossing signal  414  includes a pulse for each zero-crossing of the measurement signal  412 . 
     The phase modulator  408  uses the zero-crossing signal  414  to generate a phase modulation signal  416 . The phase modulation signal  416  may change the phase of a modulated current that contributes to the charging current  410 . The phase of the modulated current with respect to the phase of the current in the resonant circuit can cause an increase or decrease in the charging current  410 . In the first charging period  422 , the phase modulation signal  416  is closely synchronized to the zero-crossing signal  414 , and the effect of the modulated current is additive over each cycle of the charging current  410 . In this example, the driver circuit  402  provides maximum power transfer through the resonant circuit  404 . In the second charging period  424 , the phase modulation signal  416  has a phase shift of 90° with respect to the zero-crossing signal  414 , and the effect of the modulated current is additive and subtractive on alternating quarter cycles. In this example, the driver circuit  402  provides 50% of the maximum available power through the resonant circuit  404 . In the third charging period  426 , the phase modulation signal  416  has a phase shift with respect to the zero-crossing signal  414  that increases from 90° to 180° in the last-depicted cycle  428 . The effect of the modulated current is negative over an increasing portion of each cycle of the charging current  410  and driver circuit  402  provides power through the resonant circuit  404  that decreases from 50% of the maximum available power to no power transfer or minimal power transfer. 
     In certain implementations, the zero-crossing signal  414  is provided as a digital signal that provides the timing needed by the phase modulator  408  to add a phase-lead or phase-lag to the incoming zero-cross signal when indicated by the phase control signal  418 . In one example, the driver circuit  402  includes a half-bridge circuit. In one example, the phase control signal  418  is a multi-bit digital signal that indicates the amount of phase shift to be added to the zero-crossing signal  414  in order to directly affect the amount of power that flows in the resonant circuit  404  (i.e., Lp and Cp). 
     Resonant Pulse-Width Modulation 
       FIG. 5  illustrates an example of a PWM charger  500  and the timing diagrams  600 ,  620  in  FIG. 6  illustrate certain aspects of the operation of the PWM charger  500 . One aspect of this disclosure relates to the use of a pulse-width modulation (PWM) charging system to modulate a charging current  510  provided to a resonant circuit  504 . A driver circuit  502  provides a charging current  510  to a resonant circuit  504  that includes a capacitor (C p ) and an inductor (L p ). The charging current  510  may be substantially the same as the current in the inductor (i.e., the L p  current), although some portion of the charging current  510  may be lost due to parasitic capacitance, or the like. The charging current  510  alternates at a frequency that may be closely matched to the resonant frequency of the resonant circuit  504  to improve efficiency of power transfer. In accordance with certain aspects of this disclosure, the level of power transferred through the resonant circuit  504  to a receiving device may be controlled using PWM modulation to alter the charging current  510 . 
     The timing diagrams  600 ,  620  illustrate certain aspects of PWM as applied to the charging current  510  in certain implementations. PWM enables fine control over the level of power delivery by the driver circuit  502 , although the timing diagrams  600 ,  620  depict a limited number of charging periods  602 ,  604 ,  606 ,  622 ,  624  and  626  in which power is delivered at different levels, as indicated by the varying amplitude of the charging current  510 . 
     The power provided in the charging current  510  may be controlled using a zero-crossing detector  506  and a PWM circuit  508  that responds to a control signal  518  provided by a controller or other processor. The zero-crossing detector  506  is used to provide timing information used by the PWM circuit  508 . In one example, the zero-crossing detector  506  may compare the polarity of a measurement signal  512  representing the current flowing to the resonant circuit  504  with the polarity of a delayed version of the measurement signal  512 , whereby a difference in polarity is detected when a zero-crossing occurs in the measurement signal  512 . The zero-crossing detector  506  provides a zero-crossing signal  514  (ZC) that includes timing information identifying zero-crossings of the measurement signal  512 . In one example, the zero-crossing signal  514  includes an edge for each zero-crossing of the measurement signal  512 . Direction of transition of the edge may indicate positive-going or negative-going zero-crossings. In another example, the zero-crossing signal  514  includes a pulse for each zero-crossing of the measurement signal  512 . 
     The PWM circuit  508  uses the zero-crossing signal  514  to generate a PWM signal  516 . The PWM signal  516  may control the contribution of energy to the charging current  510 . In one example, pulses in the PWM signal  516  are used to gate a current that is provided to a power inverter circuit that produces an alternating output used to provide the charging current  510 . 
     In the first charging period  602 ,  622 , the PWM signal  516  includes pulses that match the duration of a half-cycle of the charging current  510 , and provides a charging current  510  with maximum (100%) power. In this example, the driver circuit  502  provides maximum power transfer through the resonant circuit  504 . In the second charging period  604 ,  624 , the PWM signal  516  includes pulses that have a duration of approximately half the duration of a half-cycle of the charging current  510 , and the resultant charging current  510  with provides 50% of the maximum available power when provided to the resonant circuit  504 . In the third charging period  606 ,  626 , the PWM signal  516  includes pulses that decrease, initially having a duration of approximately half the duration of a half-cycle of the charging current  510 , and decreasing to almost an absence of a pulse. The driver circuit  502  provides power through the resonant circuit  504  that decreases from 50% of the maximum available power to no or minimal power transfer. 
     The timing of the pulses in the PWM signal  516  may be selected based on the method of generating the charging current  510  used in the driver circuit  502 . In the example illustrated by the first timing diagram  600  of  FIG. 6 , each pulse is initiated at a zero crossing and has a duration that may be determined by the width control signal  518 . The width control signal  518  may be provided as a multi-bit digital signal that configures a programmable delay circuit or selects an out of a delay line to provide a delay that determines the duration of a pulse in the width control signal  518 . 
     In the example illustrated by the second timing diagram  620  of  FIG. 6 , each pulse in the PWM signal  516  is centered on the mid-point of a corresponding pulse in the zero-crossing signal  514 . In other words, the center of each pulse is midway between zero crossings of the measurement signal  512 . The duration of these pulses may be determined by the width control signal  518 . The width control signal  518  may be provided as a multi-bit digital signal that configures a programmable delay circuit or selects an out of a delay line to provide a delay that determines the duration of a pulse in the width control signal  518 . The location of the pulses may be configured using counters, delay lines, lookup tables and/or other circuits. Centering the pulses in the PWM signal  516  between zero crossings of the measurement signal  512  can lower distortion of the AC signal in the charging current  510 . 
     In some implementations, resonant pulse width modulation may use a detected zero-crossing as a temporal reference to initiate a PWM drive cycle. In one example, a timer may be started to control with width of the pulse. In another example, a delay circuit may be used to control with width of the pulse. The charging current  510  flowing in the resonant circuit  504  is controlled by the width of the pulse. 
     In some implementations, PWM may be used to control the charging current  510  flowing in the resonant circuit  504  without zero-crossing synchronization. Accordingly, a current measurement circuit and a zero-crossing detector  506  may not be necessary, provided other information is known, including the values of L p  and C p , for example. 
     Resonant Class-D Wireless Transmitter 
       FIG. 7  illustrates an example of a wireless charging system  700  that employs a class-D wireless transmitter  702  provided in accordance with certain aspects disclosed herein. The timing diagram  800  in  FIG. 8  illustrate certain aspects of the operation of the class-D wireless transmitter  702 . The class-D wireless transmitter  702  includes a class-D amplifier that operates as a switching amplifier. The class-D wireless transmitter  702  generates a first signal that switches between voltage rails at a first frequency. The first signal is modulated by a second lower-frequency signal. In the illustrated example, the first signal is pulse-width modulated to obtain a PWM signal  718 . 
     The PWM signal  718  is provided to a driver circuit  704  that generates a charging current to drive a resonant circuit  706  that includes an LC tank circuit including a capacitor (C p ) and an inductor (L p ). The charging current may be substantially the same as the current in the inductor (i.e., the L p  current  802 ). The resonant circuit  706  operates as a low-pass filter that converts the high frequency PWM signal  718  to obtain an amplified version of the modulating signal, which may be a sine wave. The PWM controller  710  may be operated to control the peak amplitude of the L p  current  802  using cumulative scaling in order to control the power transmitted to a wireless receiver  730 . For example, wider pulses in the PWM signal  718  may correspond to peaks in the L p  current  802  amplitude. 
     The power provided by the driver circuit  704  may be controlled using a zero-crossing detector  708  and the PWM controller  710 , which may respond to a control signal  720  provided by a controller or other processor. The PWM controller  710  receives a sinusoidal signal from a reference source  712  that provides a carrier signal that can be PWM modulated. The zero-crossing detector  708  is used to provide timing information used by the PWM controller  710 . In one example, the zero-crossing detector  708  may compare the polarity of a measurement signal  714  representing the current flowing to the resonant circuit  706  with the polarity of a delayed version of the measurement signal  714 , whereby a difference in polarity is detected when a zero-crossing occurs in the measurement signal  714 . The zero-crossing detector  708  provides a zero-crossing signal  716  (ZCS) that includes timing information identifying zero-crossings of the measurement signal  714 . In one example, the zero-crossing signal  716  includes an edge for each zero-crossing of the measurement signal  714 . Direction of transition of the edge may indicate positive-going or negative-going zero-crossings. In another example, the zero-crossing signal  716  includes a pulse for each zero-crossing of the measurement signal  714 . The PWM controller  710  may use the zero-crossing signal  716  to generate a PWM signal  718 , in which the PWM signal  718  is in phase alignment with the L p  current  802 . 
     Zero-Crossing Slotted Foreign Object Detection 
     Slotted foreign object detection may be used to detect a foreign object (FO) on the surface of a wireless charging device. A driver circuit in the wireless charging device is periodically turned off for a short period of time, which may be referred to as a slot, during which the energy in a resonant circuit driven by the driver circuit is allowed to decay. The Q factor of the resonant circuit can be determined by measuring the rate of decay. A high sample rate is typically required to accurately measure the AC waveform in the tank circuit without aliasing or artifacts that may spoil the measurement accuracy of the Q factor. The sample rate can be a factor of ten to twenty times the frequency of the current in the resonant circuit, and generally requires the use of a fast and expensive analog-to-digital converter (ADC). 
     In certain aspects of the disclosure, a zero-crossing detector is used to provide timing information that permits a low-cost ADC to reliably obtain an accurate measurement of the voltage at the same point in each cycle of the AC waveform in the resonant circuit, during a slot provided for foreign object detection. Zero crossing slotted foreign object detection can be used to detect the zero crossing of either the voltage and/or the current in the resonant circuit. The detection of the zero crossing starts a hold-off timer that triggers a sample and hold circuit in the ADC. In one example, the hold-off timer triggers the sample and hold circuit after a quarter cycle of the AC waveform in the resonant circuit. In this example, the ADC reads a sample taken at the peak of the AC wave. A sample frequency that is less than the fundamental frequency of the AC waveform can be used. 
       FIG. 9  includes timing diagrams  900 ,  920  that illustrate certain aspects of a zero-crossing, slotted foreign object detection. A measurement slot  906 ,  926  is provided between periods  904 ,  908  or  924 ,  928  of normal charging operation. The first timing diagram  900  relates to an example of a signal  902  representing energy, voltage or current in the resonant circuit when no foreign object is present, and the slow decay  912  in the signal  902  corresponds to a resonant circuit with a high Q factor. The second timing diagram  920  relates to an example of a signal  922  representing energy, voltage or current in the resonant circuit when a foreign object  1030  (see  FIG. 10 ) is present, and the decay  932  corresponds to a resonant circuit with a low Q factor. A zero-crossing, slotted foreign object detection technique according to certain aspects of the disclosure uses sample points  914 ,  934  identified based on detected zero crossings identified by a zero-crossing signal  910 ,  930 . 
       FIG. 10  illustrates an example of a wireless charging system  1000  that employs zero-crossing detection to obtain measurements  1028  at one or more points in each cycle of current or voltage in a resonant circuit  1004 . In one example, the measurements may be used for slotted foreign object detection in accordance with certain aspects disclosed herein. The wireless charging system  1000  includes a driver circuit  1002  that generates a charging current to drive a resonant circuit  1004  that includes an LC tank circuit including a capacitor (C p ) and an inductor (L p ). The charging current may be substantially the same as the current in the inductor. In some implementations, a voltage measurement signal  1006  representative of the voltage across the resonant circuit  1004  is provided to a first zero-crossing detector  1012 . The first zero-crossing detector  1012  produces an output  1016  (ZVS) indicating the timing of zero-crossings of the voltage across the resonant circuit  1004 . In some implementations, a current measurement signal  1008  representative of the current in the resonant circuit  1004  is provided to a second zero-crossing detector  1014 . The second zero-crossing detector  1014  produces an output  1018  (ZCS) indicating the timing of zero-crossings of the current in the resonant circuit  1004 . 
     A capture timing circuit  1020  may be used to track zero crossings and determine or manage the sample and hold circuit  1024 . In one example, the capture timing circuit  1020  may include or use a hold-off timer  1022  that can locate the peak amplitude of the voltage or current across the resonant circuit  1004  that occurs after period of time corresponding to a half cycle of the resonant circuit  1004 . In other examples, the capture timing circuit  1020  may include or use a hold-off timer  1022  that can locate one or more points of the voltage or current across the resonant circuit  1004 . The sample and hold circuit  1024  provides an output digitized by the ADC  1026  to obtain a measurement  1028 . The measurement  1028  may be used to track the rate of decay of the energy in the resonant circuit  1004 . 
     Zero-Crossing Amplitude Shift Key Demodulation 
     The measurements obtained using the zero-crossing detection techniques illustrated in  FIG. 10  may be used for Amplitude Shift Keying (ASK) demodulation. ASK modulation is commonly used to carry messages defined by the Qi protocol, which is used for wirelessly interconnecting a power transmitter to a power receiver. The Qi protocol permits the power receiver to control the power transmitter wirelessly. The measurements  1028  obtained at one or more points in each cycle of current or voltage in a resonant circuit  1004  may be used for ASK demodulation. One or more zero-crossing detectors  1012 ,  1014  provide reference timing for sampling voltage or current associated with the resonant circuit  1004 . Sampled data can be used to extract the ASK data that is modulated on the carrier power signal by the receiving device. 
     Data can be extracted from signals that have much higher frequencies than the sampling frequency when zero cross detection is used to provide timing for sampling. In some instances, sampling can be performed at the fundamental frequency of the current or voltage associated with the resonant circuit  1004 , or at double the frequency of the current or voltage associated with the resonant circuit  1004 . Conventional sampling circuits operate at ten times the fundamental frequency of the current or voltage associated with the resonant circuit  1004  or more to avoid aliasing and other distortion artifacts. 
     In one example, ASK demodulation is performed using measurements of voltage captured using timing provided by the output  1016  (ZVS) of the first zero-crossing detector  1012  to time the trigger of a sample and hold circuit  1024 . In another example, ASK demodulation is performed using measurements of current captured using timing provided by the output  1018  (ZCS) of the second zero-crossing detector  1014  to time the trigger of a sample and hold circuit  1024 . ASK demodulation can be performed using a single sample taken at the peak of a cycle of voltage or current. Zero-crossing ASK demodulation can reject any communications channels that may be in the same domain, provided the phase and/or frequency of the interfering carrier is different from the target carrier. 
       FIGS. 11 and 12  illustrate an example of a wireless charging system  1200  that employs zero-crossing detection to support phase-based ASK demodulation. Referring to the timing diagram  1100  of  FIG. 11 , zero-crossing phase demodulation includes detecting the phase difference between zero-volt crossings of the voltage  1108  and the current  1106  in the resonant circuit  1204 . Phase shifts between the voltage  1108  and the current  1106  may correspond to different modulation levels  1102  when the power receiving device  1206  uses ASK modulation to encode data through load or resonance shift. A digital phase detector  1212  can determine the phase difference between a current zero-crossing signal  1220  (ZCS) and a voltage zero-crossing signal  1222  (ZVS) provided by corresponding zero-crossing detector circuits  1208 ,  1210  respectively. Phase differences can be measured at one or more points in each cycle of current or voltage in a resonant circuit  1204 . The wireless charging system  1200  includes a driver circuit  1202  that generates a charging current  1104  to drive the resonant circuit  1204 , which includes a capacitor (C p ) and an inductor (L p ). The charging current  1104  may be substantially the same as the current in the inductor. In some implementations, a voltage measurement signal  1218  representative of the voltage across the resonant circuit  1204  is provided to a first zero-crossing detector  1210 . The first zero-crossing detector  1210  produces an output (ZVS) indicating the timing of zero-crossings of the voltage across the resonant circuit  1204 . A current measurement signal  1216  representative of the current in the resonant circuit  1204  is provided to a second zero-crossing detector  1208 . The second zero-crossing detector  1208  produces an output (ZCS) indicating the timing of zero-crossings of the current in the resonant circuit  1204 . 
     The phase detector circuit  1212  provides a signal representative of the phase difference between the current zero-crossing signal  1220  (ZCS) and the voltage zero-crossing signal  1222  (ZVS) to an ASK demodulator  1214 . 
     Example of a Processing Circuit 
       FIG. 13  illustrates an example of a hardware implementation for an apparatus  1300  that may be incorporated in a charging device or in a receiving device that enables a battery to be wirelessly charged. In some examples, the apparatus  1300  may perform one or more functions disclosed herein. In accordance with various aspects of the disclosure, an element, or any portion of an element, or any combination of elements as disclosed herein may be implemented using a processing circuit  1302 . The processing circuit  1302  may include one or more processors  1304  that are controlled by some combination of hardware and software modules. Examples of processors  1304  include microprocessors, microcontrollers, digital signal processors (DSPs), SoCs, ASICs, field programmable gate arrays (FPGAs), programmable logic devices (PLDs), state machines, sequencers, gated logic, discrete hardware circuits, and other suitable hardware configured to perform the various functionality described throughout this disclosure. The one or more processors  1304  may include specialized processors that perform specific functions, and that may be configured, augmented or controlled by one of the software modules  1316 . The one or more processors  1304  may be configured through a combination of software modules  1316  loaded during initialization, and further configured by loading or unloading one or more software modules  1316  during operation. 
     In the illustrated example, the processing circuit  1302  may be implemented with a bus architecture, represented generally by the bus  1310 . The bus  1310  may include any number of interconnecting buses and bridges depending on the specific application of the processing circuit  1302  and the overall design constraints. The bus  1310  links together various circuits including the one or more processors  1304 , and storage  1306 . Storage  1306  may include memory devices and mass storage devices, and may be referred to herein as computer-readable media and/or processor-readable media. The storage  1306  may include transitory storage media and/or non-transitory storage media. 
     The bus  1310  may also link various other circuits such as timing sources, timers, peripherals, voltage regulators, and power management circuits. A bus interface  1308  may provide an interface between the bus  1310  and one or more transceivers  1312 . In one example, a transceiver  1312  may be provided to enable the apparatus  1300  to communicate with a charging or receiving device in accordance with a standards-defined protocol. Depending upon the nature of the apparatus  1300 , a user interface  1318  (e.g., keypad, display, speaker, microphone, joystick) may also be provided, and may be communicatively coupled to the bus  1310  directly or through the bus interface  1308 . 
     A processor  1304  may be responsible for managing the bus  1310  and for general processing that may include the execution of software stored in a computer-readable medium that may include the storage  1306 . In this respect, the processing circuit  1302 , including the processor  1304 , may be used to implement any of the methods, functions and techniques disclosed herein. The storage  1306  may be used for storing data that is manipulated by the processor  1304  when executing software, and the software may be configured to implement any one of the methods disclosed herein. 
     One or more processors  1304  in the processing circuit  1302  may execute software. Software shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software modules, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, algorithms, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. The software may reside in computer-readable form in the storage  1306  or in an external computer-readable medium. The external computer-readable medium and/or storage  1306  may include a non-transitory computer-readable medium. A non-transitory computer-readable medium includes, by way of example, a magnetic storage device (e.g., hard disk, floppy disk, magnetic strip), an optical disk (e.g., a compact disc (CD) or a digital versatile disc (DVD)), a smart card, a flash memory device (e.g., a “flash drive,” a card, a stick, or a key drive), RAM, ROM, a programmable read-only memory (PROM), an erasable PROM (EPROM) including EEPROM, a register, a removable disk, and any other suitable medium for storing software and/or instructions that may be accessed and read by a computer. The computer-readable medium and/or storage  1306  may also include, by way of example, a carrier wave, a transmission line, and any other suitable medium for transmitting software and/or instructions that may be accessed and read by a computer. Computer-readable medium and/or the storage  1306  may reside in the processing circuit  1302 , in the processor  1304 , external to the processing circuit  1302 , or be distributed across multiple entities including the processing circuit  1302 . The computer-readable medium and/or storage  1306  may be embodied in a computer program product. By way of example, a computer program product may include a computer-readable medium in packaging materials. Those skilled in the art will recognize how best to implement the described functionality presented throughout this disclosure depending on the particular application and the overall design constraints imposed on the overall system. 
     The storage  1306  may maintain and/or organize software in loadable code segments, modules, applications, programs, etc., which may be referred to herein as software modules  1316 . Each of the software modules  1316  may include instructions and data that, when installed or loaded on the processing circuit  1302  and executed by the one or more processors  1304 , contribute to a run-time image  1314  that controls the operation of the one or more processors  1304 . When executed, certain instructions may cause the processing circuit  1302  to perform functions in accordance with certain methods, algorithms and processes described herein. 
     Some of the software modules  1316  may be loaded during initialization of the processing circuit  1302 , and these software modules  1316  may configure the processing circuit  1302  to enable performance of the various functions disclosed herein. For example, some software modules  1316  may configure internal devices and/or logic circuits  1322  of the processor  1304 , and may manage access to external devices such as a transceiver  1312 , the bus interface  1308 , the user interface  1318 , timers, mathematical coprocessors, and so on. The software modules  1316  may include a control program and/or an operating system that interacts with interrupt handlers and device drivers, and that controls access to various resources provided by the processing circuit  1302 . The resources may include memory, processing time, access to a transceiver  1312 , the user interface  1318 , and so on. 
     One or more processors  1304  of the processing circuit  1302  may be multifunctional, whereby some of the software modules  1316  are loaded and configured to perform different functions or different instances of the same function. The one or more processors  1304  may additionally be adapted to manage background tasks initiated in response to inputs from the user interface  1318 , the transceiver  1312 , and device drivers, for example. To support the performance of multiple functions, the one or more processors  1304  may be configured to provide a multitasking environment, whereby each of a plurality of functions is implemented as a set of tasks serviced by the one or more processors  1304  as needed or desired. In one example, the multitasking environment may be implemented using a timesharing program  1320  that passes control of a processor  1304  between different tasks, whereby each task returns control of the one or more processors  1304  to the timesharing program  1320  upon completion of any outstanding operations and/or in response to an input such as an interrupt. When a task has control of the one or more processors  1304 , the processing circuit is effectively specialized for the purposes addressed by the function associated with the controlling task. The timesharing program  1320  may include an operating system, a main loop that transfers control on a round-robin basis, a function that allocates control of the one or more processors  1304  in accordance with a prioritization of the functions, and/or an interrupt driven main loop that responds to external events by providing control of the one or more processors  1304  to a handling function. 
     In one implementation, the apparatus  1300  includes or operates as a wireless charging apparatus that has a battery charging power source coupled to a charging circuit, a plurality of charging cells and a controller, which may be included in one or more processors  1304 . The plurality of charging cells may be configured to provide a charging surface. At least one transmitting coil may be configured to direct an electromagnetic field through a charge transfer area of each charging cell. The apparatus  1300  may include a resonant circuit comprising a transmitting coil, a driver circuit configured to provide a charging current to the resonant circuit, and a zero-crossing detector configured to provide a zero-crossing signal that includes edges corresponding to transitions of a voltage measured across the resonant circuit through a zero volt level or corresponding to transitions of a current in the resonant circuit through a zero ampere level. The controller may be configured to cause the driver circuit to provide the charging current to the resonant circuit when a receiving device is present on a surface of the charging device, provide a measurement slot by causing the driver circuit to decrease or terminate the charging current for a period of time, and determine whether an object other than the receiving device is present on a surface of the charging device based on measurements of samples of voltage or current captured based on timing provided by the zero-crossing signal. The samples may be captured during the measurement slot. 
     In one example, the zero volt level corresponds to a current amplitude midway between maximum and minimum amplitudes of an AC measured in the resonant circuit. The zero volt level may correspond to a voltage level midway between maximum and minimum amplitudes of an AC voltage measured across the resonant circuit. 
     In one example, the controller is further configured to determine that the object other than the receiving device is present on the surface of the charging device based on a rate of decrease in voltage or current measured using the samples of voltage or current. In one example, the controller is further configured to determine that the object other than the receiving device is present on the surface of the charging device based on a rate of decrease of energy stored in the resonant circuit. The energy stored in the resonant circuit may be indicated by the samples of voltage or current. 
     In one example, the controller is further configured to determine that the object other than the receiving device is present on the surface of the charging device based on a Q factor of the resonant circuit. The Q factor of the resonant circuit may be indicated by a plurality of the samples of voltage or current. 
     In certain examples, apparatus  1300  may include a sample and hold circuit configured to sample the voltage or current in the resonant circuit after a delay following each of a plurality of edges in the zero-crossing signal to obtain the samples of voltage or current. The delay may be calculated to cause sampling of the voltage or current when the voltage or current has a maximum amplitude. 
     In some implementations, the storage  1306  maintains instructions and information where the instructions are configured to cause the one or more processors  1304  to provide a charging current to a resonant circuit when a receiving device is present on a surface of the wireless charging device, provide a zero-crossing signal that includes edges corresponding to transitions of a voltage measured across the resonant circuit through a zero volt level or to transitions of a current in the resonant circuit through a zero ampere level, provide a measurement slot by decreasing or terminating the charging current for a period of time, and determine whether an object other than the receiving device is present on a surface of the charging device based on measurements of samples of voltage or current captured based on timing provided by the zero-crossing signal. The samples may be captured during the measurement slot. 
     In one example, the zero volt level corresponds to a current amplitude midway between maximum and minimum amplitudes of an AC current measured in the resonant circuit. In another example, the zero volt level corresponds to a voltage level midway between maximum and minimum amplitudes of an AC voltage measured across the resonant circuit. In some instances, it may be determined that the object other than the receiving device is present on the surface of the charging device based on a rate of decrease in voltage or current measured using the samples of voltage or current. 
     In some instances, it may be determined that the object other than the receiving device is present on the surface of the charging device based on a rate of decrease of energy stored in the resonant circuit. The energy stored in the resonant circuit may be indicated by the samples of voltage and/or current. 
     In some instances, it may be determined that the object other than the receiving device is present on the surface of the charging device based on a Q factor of the resonant circuit. The Q factor of the resonant circuit may be indicated by a plurality of the samples of voltage or current. 
     In certain examples, the voltage or current in the resonant circuit may be sampled after a delay following each of a plurality of edges in the zero-crossing signal to obtain the samples of voltage or current. The delay may be calculated to cause sampling of the voltage or current when the voltage or current has a maximum amplitude. 
       FIG. 14  is a flowchart  1400  illustrating a method for operating a charging device in accordance with certain aspects of this disclosure. The method may be performed by a controller in the charging device. At block  1402 , the controller may provide a charging current to a resonant circuit when a receiving device is present on a surface of the wireless charging device. At block  1404 , the controller may provide a zero-crossing signal that includes edges corresponding to transitions of a voltage measured across the resonant circuit through a zero volt level or to transitions of a current in the resonant circuit through a zero ampere level. At block  1406 , the controller may provide a measurement slot by decreasing or terminating the charging current for a period of time. At block  1408 , the controller may determine whether an object other than the receiving device is present on a surface of the charging device based on measurements of samples of voltage or current captured based on timing provided by the zero-crossing signal. The samples may be captured during the measurement slot. 
     In one example, the zero volt level corresponds to a current amplitude midway between maximum and minimum amplitudes of an AC current measured in the resonant circuit. In another example, the zero volt level corresponds to a voltage level midway between maximum and minimum amplitudes of an AC voltage measured across the resonant circuit. 
     In some instances, it may be determined that the object other than the receiving device is present on the surface of the charging device based on a rate of decrease in voltage or current measured using the samples of voltage or current. 
     In some instances, it may be determined that the object other than the receiving device is present on the surface of the charging device based on a rate of decrease of energy stored in the resonant circuit. The energy stored in the resonant circuit may be indicated by the samples of voltage and/or current. 
     In some instances, it may be determined that the object other than the receiving device is present on the surface of the charging device based on a Q factor of the resonant circuit. The Q factor of the resonant circuit may be indicated by a plurality of the samples of voltage or current. 
     In certain examples, the voltage or current in the resonant circuit may be sampled after a delay following each of a plurality of edges in the zero-crossing signal to obtain the samples of voltage or current. The delay may be calculated to cause sampling of the voltage or current when the voltage or current has a maximum amplitude. 
     The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” Unless specifically stated otherwise, the term “some” refers to one or more. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. § 112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.”