PATENT DOCUMENT

Publication Number: US-11061490-B2
Application Number: US-202016913948-A
Country: US
Kind Code: B2

Title: Capacitive wireless charging systems

Abstract:
A wireless power transmission system may include a wireless power transmitting device such as a tablet computer and a wireless power receiving device such as a computer stylus. A wireless power transmitting capacitor electrode may be formed in the tablet computer. A wireless power receiving capacitor electrode may be formed in the computer stylus. The transmitting capacitor electrode may be driven by a drive signal having a frequency of 900 MHz or greater to produce wireless power. The wireless power may be transmitted from the transmitting capacitor electrode to the receiving capacitor electrode on the stylus via near field capacitive coupling. The transmitting and receiving capacitor electrodes may each include conductive traces on dielectric substrates. The conductive traces may follow meandering paths to maximize the possible capacitive coupling efficiency between the capacitor electrodes and thus the end-to-end charging efficiency of the wireless power transmission system.

Claims:
What is claimed is: 
     
       1. An electronic stylus, wherein the electronic stylus is configured to receive wireless power from a wireless power transmitting device, the electronic stylus comprising:
 an elongated body having a tip and an opposing end coupled by a shaft that extends along a longitudinal axis and that has a circumference; 
 electrical components in the shaft; 
 a wireless power receiving capacitor electrode wrapped around at least some of the circumference of the shaft, wherein the wireless power receiving capacitor electrode is configured to receive the wireless power from the wireless power transmitting device via capacitive coupling; 
 rectifier circuitry that is configured to convert the wireless power received by the wireless power receiving capacitor electrode into a direct-current (DC) voltage; and 
 power management circuitry configured to power the electrical components using the DC voltage. 
 
     
     
       2. The electronic stylus of  claim 1 , wherein the wireless power receiving capacitor electrode comprises a conductive trace on a dielectric substrate. 
     
     
       3. The electronic stylus of  claim 2 , wherein the conductive trace comprises an electrode terminal coupled to the rectifier circuitry and wherein the conductive trace comprises a floating end. 
     
     
       4. The electronic stylus of  claim 3 , wherein the floating end is interposed between the electrode terminal and a first end of the longitudinal axis and wherein the electrode terminal is interposed between the floating end and a second end of the longitudinal axis. 
     
     
       5. The electronic stylus of  claim 4 , wherein the conductive trace follows a meandering path from the electrode terminal to the floating end. 
     
     
       6. The electronic stylus of  claim 5 , wherein the conductive trace comprises a plurality of repeating units. 
     
     
       7. The electronic stylus of  claim 1 , wherein the wireless power receiving capacitor electrode extends at least 30 degrees around the longitudinal axis. 
     
     
       8. The electronic stylus of  claim 1 , further comprising:
 an additional wireless power receiving capacitor electrode wrapped around at least some of the circumference of the shaft, wherein the additional wireless power receiving capacitor electrode is configured to receive the wireless power from the wireless power transmitting device via capacitive coupling. 
 
     
     
       9. The electronic stylus of  claim 1 , further comprising:
 a magnetic alignment structure on the shaft and positioned to magnetically couple with a magnetic structure on the wireless power transmitting device, wherein the wireless power receiving capacitor electrode is positioned to align with a wireless power transmitting capacitor electrode on the wireless power transmitting device when the magnetic alignment structure is magnetically coupled with the magnetic structure on the wireless power transmitting device. 
 
     
     
       10. The electronic stylus of  claim 9 , wherein the magnetic alignment structure comprises a magnetic ring that extends around the circumference of the shaft. 
     
     
       11. An electronic stylus, wherein the electronic stylus is configured to receive wireless power from a wireless power transmitting device, the electronic stylus comprising:
 a wireless power receiving capacitor electrode, wherein the wireless power receiving capacitor electrode is configured to receive the wireless power from the wireless power transmitting device via capacitive coupling; and 
 rectifier circuitry coupled to the wireless power receiving capacitor electrode, wherein the rectifier circuitry is configured to convert the wireless power received by the wireless power receiving capacitor electrode into a direct-current (DC) voltage and wherein the wireless power receiving capacitor electrode comprises a conductive trace with a plurality of repeating units. 
 
     
     
       12. The electronic stylus of  claim 11 , wherein the conductive trace has an electrode terminal coupled to the rectifier circuitry and a floating end opposite the electrode terminal. 
     
     
       13. The electronic stylus of  claim 12 , wherein the wireless power receiving capacitor electrode follows a meandering path from the electrode terminal to the floating end. 
     
     
       14. The electronic stylus of  claim 11 , wherein each repeating unit in the plurality of repeating units comprises a thick portion having a first width, a thin portion coupled to a first edge of the thick portion and having a second width that is less than the first width, and a short path coupled between the thin portion and a second edge of the thick portion. 
     
     
       15. The electronic stylus of  claim 11 , further comprising:
 a battery; and 
 power management circuitry configured to charge the battery using the DC voltage. 
 
     
     
       16. The electronic stylus of  claim 11 , further comprising:
 an elongated body having a tip and an opposing end coupled by a shaft; 
 electrical components in the shaft; and 
 power management circuitry configured to power the electrical components using the DC voltage. 
 
     
     
       17. An electronic stylus, wherein the electronic stylus is configured to receive wireless power from a wireless power transmitting device, the electronic stylus comprising:
 a wireless power receiving capacitor electrode, wherein the wireless power receiving capacitor electrode is configured to receive the wireless power from the wireless power transmitting device via capacitive coupling; 
 rectifier circuitry coupled to the wireless power receiving capacitor electrode, wherein the rectifier circuitry is configured to convert the wireless power received by the wireless power receiving capacitor electrode into a direct-current (DC) voltage; and 
 a magnetic alignment structure positioned to magnetically couple with a magnetic structure on the wireless power transmitting device, wherein the wireless power receiving capacitor electrode is positioned to align with a wireless power transmitting capacitor electrode on the wireless power transmitting device when the magnetic alignment structure is magnetically coupled with the magnetic structure on the wireless power transmitting device. 
 
     
     
       18. The electronic stylus of  claim 17 , further comprising:
 an elongated body having a tip and an opposing end coupled by a shaft, wherein the magnetic alignment structure is on the shaft. 
 
     
     
       19. The electronic stylus of  claim 17 , wherein the wireless power receiving capacitor electrode comprises a conductive trace with a plurality of repeating units. 
     
     
       20. The electronic stylus of  claim 17 , further comprising:
 an antenna configured to transmit radio-frequency signals to the wireless power transmitting device.

Description:
This application is a division of U.S. patent application Ser. No. 15/909,769, filed Mar. 1, 2018, which claims the benefit of provisional patent application No. 62/491,103, filed Apr. 27, 2017, each of which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD 
     This relates generally to wireless systems, and, more particularly, to systems in which devices are wirelessly charged. 
     BACKGROUND 
     In a wireless charging system, a wireless power transmitting device wirelessly transmits power to a wireless power receiving device. The receiving device receives the wirelessly transmitted power and uses this power to charge an internal battery and to power components in the receiving device. In practice, it can be challenging to ensure that the wireless power is transferred from the transmitting device to the receiving device with satisfactory efficiency. 
     SUMMARY 
     A wireless power transmitting device such as a tablet computer may include a wireless power transmitting capacitor electrode coupled to wireless power transmitting circuitry. The tablet computer may have a display cover layer that forms a front face of the tablet computer. The tablet computer may have a housing that includes conductive housing sidewalls extending from a rear face of the tablet computer to the display cover layer. The wireless power transmitting circuitry may supply drive signals to the wireless power transmitting capacitor electrode to produce wireless power (e.g., wireless power signals). The wireless power transmitting capacitor electrode may transmit the wireless power to a wireless power receiving device via near field capacitive coupling. The wireless power transmitting circuitry may supply the drive signals at a relatively high frequency such as 900 MHz or greater. 
     The wireless power transmitting capacitor electrode may be mounted behind a dielectric window in a given one of the conductive housing sidewalls or in a conductive rear wall that forms the rear face of the tablet computer. The wireless power transmitting capacitor electrode may transmit the wireless power through the dielectric window. If desired, the wireless power transmitting capacitor electrode may be mounted behind the display cover layer and may transmit the wireless power through the display cover layer. The wireless power transmitting electrode may include conductive traces on a dielectric substrate. The conductive traces may follow a meandering path to maximize the perimeter of the wireless power transmitting capacitor electrode and thus the possible capacitive coupling efficiency between the wireless power transmitting capacitor electrode and a wireless power receiving capacitor electrode on the wireless power receiving device. 
     The wireless power receiving device may be a computer stylus capable of providing a user input to the tablet computer. The computer stylus may include an elongated body having a tip and an opposing end coupled by a shaft that extends along a longitudinal axis and that has a circumference. Electrical components such as a battery may be mounted within the shaft. The computer stylus may include a wireless power receiving capacitor electrode wrapped around at least some of the circumference of the shaft. The wireless power receiving capacitor electrode may receive the wireless power transmitted by the wireless power transmitting capacitor electrode on the tablet computer via near field capacitive coupling. The computer stylus may include rectifier circuitry that converts the received wireless power received into a direct-current (DC) voltage. Power management circuitry may use the DC voltage to power the electrical components. The wireless power receiving capacitor electrode may include a conductive trace on a dielectric substrate. The conductive trace may follow a meandering path to maximize the perimeter of the wireless power receiving capacitor electrode and thus the possible capacitive coupling efficiency between the transmitting and receiving capacitor electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative capacitive wireless charging system in accordance with embodiments. 
         FIG. 2  is a perspective view of an illustrative capacitive wireless charging system having a computer and associated computer stylus in accordance with an embodiment. 
         FIG. 3  is a circuit diagram of an illustrative capacitive wireless charging system in accordance with an embodiment. 
         FIG. 4  is a side view of an illustrative computer stylus having one or more wireless power receiving capacitor electrodes in accordance with an embodiment. 
         FIGS. 5-7  are cross-sectional side views of an illustrative computer stylus having one or more wireless power receiving capacitor electrodes in accordance with an embodiment. 
         FIG. 8  is a top-down view of an illustrative tablet computer having a wireless power transmitting capacitor electrode in accordance with an embodiment. 
         FIGS. 9 and 10  are cross-sectional side views of an illustrative tablet computer having a wireless power transmitting capacitor electrode in accordance with an embodiment. 
         FIG. 11  is a side view of an illustrative tablet computer having a wireless power transmitting capacitor electrode aligned with a window in a conductive sidewall in accordance with an embodiment. 
         FIG. 12  is a cross-sectional side view of an illustrative tablet computer having a wireless power transmitting capacitor electrode aligned with a window in a conductive sidewall in accordance with an embodiment. 
         FIGS. 13 and 14  are diagrams of illustrative conductive traces that may be used in forming wireless power transmitting and/or receiving capacitor electrodes in accordance with an embodiment. 
         FIG. 15  is a perspective view of a wireless power transmitting structure having at least one wireless power transmitting capacitor electrode formed on a half-cylindrical surface that receives a computer stylus in accordance with an embodiment. 
         FIG. 16  is a perspective view of a wireless power transmitting structure having at least one wireless power transmitting capacitor electrode formed on a cylindrical surface that receives a computer stylus in accordance with an embodiment. 
         FIG. 17  is a diagram showing how a wireless power transmitting structure of the type shown in  FIGS. 15 and 16  may be mounted to a cover for a tablet computer in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A wireless power system has a wireless power transmitting device that transmits power wirelessly to a wireless power receiving device. The wireless power transmitting device may be a device such as a tablet computer, cellular telephone, watch, media player, laptop computer, desktop computer, computer display device, or other wireless power transmitting equipment. The wireless power transmitting device has one or more capacitor electrodes that are used in transmitting wireless power to one or more wireless power receiving capacitor electrodes in the wireless power receiving device. The wireless power receiving device may be a device such as a computer stylus, cellular telephone, watch, media player, tablet computer, pair of earbuds, headphones or other headset device, remote control, laptop computer, other portable electronic device such as a peripheral or accessory electronic device, or other wireless power receiving equipment. 
     During operation, the wireless power transmitting device supplies alternating-current drive signals to one or more wireless power transmitting capacitor electrodes. This causes the capacitor electrode to transmit alternating-current electromagnetic signals (sometimes referred to as wireless power signals) to one or more corresponding capacitor electrodes in the wireless power receiving device (e.g., via near-field capacitive coupling). Rectifier circuitry in the wireless power receiving device converts received wireless power signals into direct-current (DC) power for powering the wireless power receiving device. 
     An illustrative wireless power system (wireless charging system) is shown in  FIG. 1 . As shown in  FIG. 1 , wireless power system  8  includes wireless power transmitting device  12  and one or more wireless power receiving devices such as wireless power receiving device  10 . Device  12  may be an electronic device such as a tablet computer, computer monitor containing an embedded computer, cellular telephone, wristwatch, media player, laptop computer or other portable electronic device, mousepad, trackpad, keyboard, desktop computer, embedded system such as a system in which electronic equipment is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, or other wireless power transmitting equipment. Device  10  may be a computer accessory such as a computer stylus, trackpad, computer mouse, headphones, ear buds, or headset, a portable electronic device such as a wristwatch, cellular telephone, tablet computer, laptop computer, or other electronic equipment. Illustrative configurations in which device  12  is a tablet computer and in which device  10  is a computer stylus that is used to provide user input to device  12  are sometimes be described herein as examples. 
     In order to wirelessly power device  10 , a user places device  10  in proximity to power transmitting device  12 . Power transmitting device  12  is coupled to a source of alternating-current voltage such as alternating-current power source  14  (e.g., a wall outlet that supplies line power or other source of mains electricity), has a battery such as battery  16  for supplying power, and/or is coupled to another source of power. A power converter such as alternating-current-to-direct current (AC-DC) power converter  18  can convert power from a mains power source or other alternating-current (AC) power source into direct-current (DC) power that is used to power control circuitry  20  and other circuitry in device  12 . During operation, control circuitry  20  uses wireless power transmitting circuitry  22  and one or more capacitor electrodes  24  coupled to circuitry  22  to transmit alternating-current electromagnetic signals to device  10  (as shown by path  26 ) and thereby convey wireless power to wireless power receiving circuitry  40  of device  10 . 
     Power transmitting circuitry  22  may have clocking circuitry that supplies AC signals (clocking or drive signals) to one or more of capacitor electrodes  24  during wireless power transfer operations. One or more capacitor electrodes  24  may be used at a time for wireless power transfer. Capacitor electrodes  24  may sometimes be referred to herein as wireless power transmitting capacitor electrodes, wireless power transmit capacitor electrodes, transmitting capacitor electrodes, transmit capacitor electrodes, wireless power transmitting capacitor plates, transmitting capacitor plates, transmitting electrodes, or transmit electrodes. As examples, a single capacitor electrode  24  may supply power to a single receiving device that overlaps that electrode, two capacitor electrodes  24  may supply power to a single receiving device overlapping those two capacitor electrodes or to a pair of receiving devices overlapping those electrodes, three or more capacitor electrodes may be driven to supply power to a single overlapping receiving device or to multiple overlapping receiving devices, etc. 
     During power transfer operations, control circuitry  20  may provide control signals to wireless power transmitting circuitry  22  that control circuitry  22  to generate AC signals. Control circuitry  20  may control wireless power transmitting circuitry  22  to provide the AC signals to one or more capacitor electrodes  24  that have been selected for supplying wireless power. As the AC signals pass through one or more capacitor electrodes  24  that have been selected for supplying wireless power, alternating-current electromagnetic fields (wireless power signals) are produced that are received by corresponding capacitor electrodes(s)  42  coupled to wireless power receiving circuitry  40  in receiving device  10  (e.g., capacitor electrodes  24  and  42  may be wirelessly linked over path  26  through near field capacitive coupling). When the alternating-current electromagnetic fields are received by capacitor electrode  42 , corresponding alternating-current currents and voltages are produced on capacitor electrode  42 . Rectifier circuitry in circuitry  40  converts received AC signals (received alternating-current currents and voltages associated with wireless power signals) from capacitor electrode(s)  42  into DC voltage signals for powering device  10 . The DC voltages are used in powering components in device  10  such as sensors and other components  44  (e.g., buttons, accelerometers, force sensors, touch sensors, magnetic sensors, capacitive sensors, resistance sensors, temperature sensors, light sensors, pressure sensors, gas sensors, image sensors, moisture sensors, etc.), wireless communications circuits  46  for communicating wirelessly with corresponding wireless communications circuitry  28  in control circuitry  20  of wireless power transmitting device  12  and/or other equipment, audio components, and other components (e.g., input-output devices  48  and/or control circuitry  50 ), and are used in charging an internal battery in device  10  such as battery  52 . 
     Devices  12  and  10  include control circuitry  20  and  50 , respectively. Control circuitry  20  and  50  each include storage and processing circuitry such as microprocessors, power management units, baseband processors, digital signal processors, microcontrollers, and/or application-specific integrated circuits with processing circuits. Control circuitry  20  and  50  are configured to execute instructions for implementing desired control and communications features in system  8 . For example, control circuitry  20  and/or  50  may be used in determining power transmission levels, determining received power levels, processing sensor data, processing user input, processing other information such as information on wireless coupling efficiency from transmitting circuitry  22 , processing information from receiving circuitry  40 , using sensing circuitry to measure electrode capacitances and other parameters, processing measured capacitance values, using information from circuitry  22  and/or  40  such as signal measurements on output circuitry in circuitry  22  and other information from circuitry  22  and/or  40  to determine when to start and stop wireless charging operations, adjusting charging parameters such as charging frequencies, capacitor electrode settings (e.g., which capacitor electrodes are active), wireless power transmission levels, and performing other control functions. 
     Control circuitry  20  and/or  50  may be configured to perform these operations using hardware (e.g., dedicated hardware or circuitry) and/or software (e.g., code that runs on the hardware of system  8 ). Software code for performing these operations is stored on non-transitory computer readable storage media (e.g., tangible computer readable storage media). The software code may sometimes be referred to as software, data, program instructions, instructions, or code. The non-transitory computer readable storage media may include non-volatile memory such as non-volatile random-access memory (NVRAM), one or more hard drives (e.g., magnetic drives or solid state drives), one or more removable flash drives or other removable media, other computer readable media, or combinations of these computer readable media or other storage. Software stored on the non-transitory computer readable storage media may be executed on the processing circuitry of control circuitry  20  and/or  50 . The processing circuitry may include application-specific integrated circuits with processing circuitry, one or more microprocessors, or other processing circuitry. 
     Control circuitry  20  and  50  may be configured to support wireless communications between devices  12  and  10  (e.g., control circuitry  50  may include wireless communications circuitry such as circuitry  46  and control circuitry  20  may include wireless communications circuitry such as circuitry  28 ). Wireless communications circuitry  28  may include one or more antennas (e.g., antennas that are separate from capacitor electrodes  24 ). Wireless communications circuitry  46  may include one or more antennas (e.g., antennas that are separate from capacitor electrodes  42 ). Antennas in communications circuitry  28  and  46  may include one or more monopole antennas, dipole antennas, patch antennas, slot antennas, loop antennas, helical antennas, inverted-F antennas, planar inverted-F antennas, combinations of these, or any other desired types of antennas. 
     Device  12  and/or device  10  may communicate wirelessly over a wireless communications link established during operation of system  8 . Devices  10  and  12  may, for example, have wireless transceiver circuitry in control circuitry  50  and  20  (see, e.g., wireless communications circuitry such as circuitry  46  and  28  of  FIG. 1 ) that allows wireless transmission of signals (e.g., control signals or other wireless data) between devices  10  and  12  over wireless communications link  36 . Wireless communications link  36  may be bidirectional (i.e., wireless signals may be conveyed from device  12  to device  10  and from device  10  to device  12 ) or unidirectional (i.e., wireless signals may be conveyed from device  12  to device  10  or from device  12  to device  10 ). Wireless signals (data) conveyed over link  36  may be formatted according to a corresponding communications protocol (e.g., by baseband circuitry and transceiver in control circuitry  20  and  50 ). As examples, wireless signals conveyed over link  36  may be formatted according to a Wireless Personal Area Network (WPAN) protocol such as a Bluetooth® protocol, according to a Wireless Local Area Network (WLAN) signals such as WiFi® protocol, a cellular telephone communications protocol, or any other desired protocol. Antennas that are used in establishing link  36  may include antennas that are separate from capacitor electrodes  24  and  42 . In another suitable arrangement, capacitor electrodes  24  and/or  42  may be used in conveying signals over link  36 . 
     Wireless power transmitting device  12  may include input-output devices  30 . Input-output devices  30  may be used to allow data to be supplied to device  12  and to allow data to be provided from device  12  to external devices such as device  10 . Input-output devices  30  may include user interface devices, data port devices, and other input-output components. For example, input-output devices may include a touch screen (i.e., a display with touch sensors) such as display  32 , displays without touch sensor capabilities, and other input-output components such as components  34  (e.g., buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, cameras, speakers, status indicators, light sources, audio speakers, fingerprint sensors, light sensors, accelerometers, capacitance sensors, proximity sensors, infrared sensors, magnetic sensors, and other input-output components). 
     In the illustrative configuration of  FIG. 2 , which is sometimes described herein as an example, device  12  is a tablet computer or other device with a touch screen and device  10  is a computer stylus. A user can use stylus  10  to draw or write on tablet computer  12  and to provide other input to tablet computer  12 . 
     As shown in  FIG. 2 , tablet computer  12  may include a housing such as housing  64  in which display  32  is mounted. Input-output devices such as button  62  may be used to supply input to tablet computer  12 . Display  32  may be a capacitive touch screen display or a display that includes other types of touch sensor technology. The touch sensor of display  32  may be configured to receive input from stylus  10 . 
     Stylus  10  may have a cylindrical shape or other elongated body that extends along longitudinal axis  54 . The body of stylus  10  may be formed from metal and/or plastic tubes and other elongated structures. Wireless circuitry  28  in tablet  12  and wireless circuitry  46  in stylus  10  may support wireless communications via wireless communications link  36 . As an example, stylus  10  may supply wireless input to tablet  12  via link  36  (e.g., information on settings in a drawing program or other software running on tablet  12 , input to select a desired on-screen option, input to supply tablet  12  with a touch gesture such as a stylus flick, input to draw a line or other object on display  32 , input to move or otherwise manipulate images displayed on display  32 , etc.). 
     Stylus  10  may have a tip such as tip  56 . Tip  56  may contain a conductive elastomeric member that is detected by the capacitive touch sensor of display  32 . If desired, tip  56  may contain active electronics (e.g., circuitry that transmits signals that are capacitively coupled into the touch sensor of display  32  and that are detected as touch input on the touch sensor). 
     Shaft portion  58  of stylus  10  may couple tip  56  of stylus  10  to opposing end  60  of stylus  10 . End  60  may contain a conductive elastomeric member, active electronics (e.g., circuitry that transmits signals that are capacitively coupled into the touch sensor of display  32  and that are detected as touch input on the touch sensor), buttons, a metal connector that mates with an external plug, or other input-output components. 
     A force sensor may be incorporated into tip  56  and/or opposing end  60  of stylus  10 . A force sensor may be used to measure how forcefully a user is pressing stylus  10  against the outer surface of display  32 . Force data may then be wirelessly transmitted from stylus  10  to tablet  12  so that the thickness of a line that is being drawn on display  32  can be adjusted accordingly or so that tablet  12  may take other suitable action. 
     If desired, stylus  10  may be provided with a clip to help attach stylus  10  to a user&#39;s shirt pocket or other object, may be provided with a magnet to help attach stylus  10  to a magnetic attachment point in tablet  12  or other structure, or may be provided with other structures that help a user attach stylus  10  to external objects. End  60  may have a removable cap, a data port connector to receive a cable (e.g., a cable that supplies power signals for charging a battery in stylus  10  and/or that supplies digital data), input-output devices (e.g., a button and/or a light-emitting diode or other light-based output device), or other components (e.g., metal structures). Other components may be formed on stylus  10  (e.g., on shaft  58  or elsewhere) such as buttons, touch sensors, and other components for gathering input, light-emitting diodes or other components for producing output, etc. 
     Stylus  10  may include a metal tube or other conductive components in shaft portion  58 . The metal tube or other structures in stylus  10  may serve as an antenna ground for one or more antennas in stylus  10 . An antenna resonating element for the antenna may be formed from metal traces on a printed circuit or other dielectric support structure and/or from other conductive structures. An antenna resonating element may be located in end region  60 , along shaft  58 , in tip region  56 , or in other suitable portions of stylus  10 . The antenna may be used to support wireless link  36 . One or more wireless power receiving capacitor electrodes  42  may be formed along shaft  58 , in tip region  56 , in end region  60 , or in other suitable portions of stylus  10 . Capacitor electrodes  42  may be formed from metal traces on a printed circuit or other dielectric support structure, on a plastic tube that forms shaft  58 , or on other structures, as examples. 
     Housing  64  of tablet  12 , which may sometimes be referred to as an enclosure or case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. Housing  64  may be formed using a unibody configuration in which some or all of housing  64  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). In the example of  FIG. 2 , housing  64  includes a conductive peripheral sidewall structure  64 W that surrounds a periphery of tablet  12 . Housing  64  may, if desired, include a conductive rear wall structure  64 R that opposes display  32  (e.g., conductive rear wall structure  64 R may form the rear exterior face, side, or surface of tablet  12 ). If desired, rear wall  64 R and sidewalls  64 W may be formed from a continuous metal structure (e.g., in a unibody configuration) or from separate metal structures. Openings may be formed in housing  64  to form communications ports, holes for buttons, and other structures if desired. In another suitable arrangement, rear wall  64 R and/or sidewalls  64 W may be formed from dielectric materials such as ceramics, plastic, or glass. 
     Display  32  may be a touch screen display that incorporates a layer of conductive capacitive touch sensor electrodes or other touch sensor components (e.g., resistive touch sensor components, acoustic touch sensor components, force-based touch sensor components, light-based touch sensor components, etc.) or may be a display that is not touch-sensitive. Capacitive touch screen electrodes may be formed from an array of indium tin oxide pads or other transparent conductive structures. 
     Display  32  may have an active area that includes an array of display pixels. The array of pixels may be formed from liquid crystal display (LCD) components, an array of electrophoretic pixels, an array of plasma display pixels, an array of organic light-emitting diode display pixels or other light-emitting diode pixels, an array of electrowetting display pixels, or display pixels based on other display technologies. 
     Display  32  may be protected using a display cover layer such as a layer of transparent glass, clear plastic, transparent ceramic, sapphire, or other transparent crystalline material, or other optically transparent layer(s). The display cover layer may have a planar shape, a convex curved profile, a shape with planar and curved portions, a layout that includes a planar main area surrounded on one or more edges with a portion that is bent out of the plane of the planar main area, or other suitable shapes. The display cover layer may cover the entire front face of tablet  12  (e.g., extending across an entirety of a length dimension of tablet  12  parallel to the y-axis and a width dimension of tablet  12  parallel to the x-axis of  FIG. 2 ). Sidewalls  64 W may extend from a rear face of tablet  12  formed by rear wall  64 R to the display cover layer (e.g., extending across a height dimension of tablet  12  parallel to the z-axis of  FIG. 2 ). In another suitable arrangement, the display cover layer may cover substantially all of the front face of tablet  12  or only a portion of the front face of tablet  12 . Openings may be formed in the display cover layer. For example, an opening may be formed in the display cover layer to accommodate a button such as button  62 . An opening may also be formed in the display cover layer to accommodate ports such as a speaker port. One or more antennas for supporting wireless communications link  36  may be mounted within housing  64 . 
     Housing  64  may have four peripheral edges (e.g., conductive sidewalls  64 W). One or more wireless power transmitting capacitor electrodes  24  may be mounted within housing  64  behind display  32 . If desired, one or more wireless power transmitting capacitor electrodes  24  may be mounted behind display  32  and adjacent to one of the four peripheral edges. For example, one or more capacitor electrodes  24  may be mounted behind display  32  within peripheral edge region  66 , within peripheral edge region  68 , within peripheral edge region  72 , and/or within peripheral edge region  70 . When mounted behind display  32 , capacitor electrodes  24  may wirelessly convey power (e.g., over path  26  of  FIG. 1 ) to stylus  10  through display  32  when stylus  10  is placed onto the surface of display  32 . 
     Consider an example in which a single wireless power transmitting capacitor electrode  24  is formed within region  70  of tablet  12 . In this scenario, when it is desired to charge stylus  10 , a user may place stylus  10  onto the surface of display  32  within region  70  (e.g., so that shaft  58  lies on the surface of display  32  and axis  54  is aligned with the y-axis of  FIG. 2 ). When stylus  10  is placed onto display  32  within region  70 , wireless power receiving capacitor electrode  42  on stylus  10  is aligned with the wireless power transmitting capacitor electrode  24  in region  70 . When aligned, a near field capacitive coupling may be established between transmitting capacitor electrode  24  and receiving capacitor electrode  42 . Transmitting capacitor electrode  24  may transmit wireless power over path  26  to receiving capacitor electrode  42  via the near field capacitive coupling (e.g., by driving capacitor electrode  24  with AC signals). The wireless power received by stylus  10  may be used to charge battery  52 . Once battery  52  has become sufficiently charged, the user may pick up stylus  10  and continue to use stylus  10  to provide user input to tablet  12 . 
     If desired, alignment structures may be formed within regions  66 ,  68 ,  72 , and/or  70  to help ensure that receiving capacitor electrodes  42  on stylus  10  are aligned with transmitting capacitor electrodes  24  on tablet  12  when stylus  10  is placed on the surface of display  32 . Examples of such alignment structures include magnetic alignment structures, indentations or grooves formed on the front face of display  32 , clip structures, adhesive structures, or any other desired alignment structures. In the example where transmitting capacitor electrodes  24  are located within region  70 , magnetic alignment structures may be formed within or adjacent to region  70  and under display  32  if desired. The magnetic alignment structures may attract conductive or magnetic structures on stylus  10  to snap and hold stylus  10  into a position at which capacitor electrodes  24  and  42  are aligned. 
     If desired, one or more wireless power transmitting capacitor electrodes  24  may be mounted within tablet  12  adjacent to housing sidewalls  64 W such as within region  74  of  FIG. 2 . In scenarios where housing sidewalls  64 W are formed from conductive material, a dielectric window may be formed within the sidewalls. Capacitor electrodes  24  may be mounted behind the dielectric windows to allow wireless power to be transferred to stylus  10  when stylus  10  is placed adjacent to the dielectric window. 
     Consider an example in which a single wireless power transmitting capacitor electrode  24  is formed within region  74  behind a given sidewall  64 W of tablet  12 . In this scenario, when it is desired to charge stylus  10 , a user may place stylus  10  adjacent to sidewall  64 W (e.g., onto a surface on which tablet  12  is resting). When stylus  10  is placed adjacent to region  74 , wireless power receiving capacitor electrode  42  on stylus  10  is aligned with the wireless power transmitting capacitor electrode  24  within region  74 . Transmitting capacitor electrode  24  may then transmit wireless power to receiving capacitor electrode  42  over path  26  for powering stylus  10 . If desired, alignment structures may be formed on tablet  12  within or adjacent to region  74  to help ensure that receiving capacitor electrodes  42  on stylus  10  are aligned with transmitting capacitor electrodes  24  when stylus  10  is placed adjacent to sidewall  64 W. For example, a magnetic alignment structure may attract magnetic structures on stylus  10  to snap and hold stylus  10  in place adjacent to region  74  of sidewall  64 W. Such magnetic alignment structures may, for example, hold stylus  10  to sidewall  64 W even if tablet  12  is not resting on a surface (e.g., stylus  10  may remain attached to sidewall  64 W even when a user picks up tablet  12 ). 
     The example of  FIG. 2  is merely illustrative. If desired, one or more wireless power transmitting capacitor electrodes  24  may be formed adjacent to rear housing wall  64 R for charging stylus  10  through rear wall  64 R. In scenarios where rear housing wall  64 R is formed from conductive materials, dielectric windows may be formed within rear housing wall  64 R and capacitor electrodes  24  may transmit wireless power to stylus  10  through the dielectric windows in rear housing wall  64 R. In another suitable arrangement, rear housing wall  64 R may be formed from dielectric (e.g., a dielectric cover layer that forms the rear face of the tablet). In general, wireless power transmitting capacitor electrodes  24  may be formed at any desired location along display  32 , along housing sidewalls  64 W, and/or along rear housing wall  64 R. Locating capacitor electrodes  24  along the periphery of display  32  such as in regions  66 ,  68 ,  70 , and  72  may allow stylus  10  to be placed on the surface of display  32  without blocking an excessive amount of the viewing region of display  32  (e.g., so that a user can still view images displayed using display  32  while stylus  10  is being charged). However, in general, wireless power transmitting capacitor electrodes  24  may be located at any desired location along the surface of display  32 . Wireless power transmitting electrodes  24  may be located along any of the four peripheral sidewalls  64 W of tablet  12 . 
     A circuit diagram of illustrative circuitry for wireless power transfer (wireless power charging) system  8  is shown in  FIG. 3 . As shown in  FIG. 3 , wireless power transmitting circuitry  22  of tablet  12  includes clocking circuitry such as clock circuitry  90 . As an example, clock circuitry  90  may include phase-locked loop (PLL) circuitry that outputs a clocking signal clk. This example is merely illustrative and, in general, clock circuitry  90  may include any desired clocking circuitry (e.g., a voltage controlled oscillator (VCO) circuit, an off-chip crystal oscillator, etc.). 
     Clock circuitry  90  may be controlled by control circuitry  20  ( FIG. 1 ) to generate clocking signal clk at a selected frequency (e.g., a square wave signal having the selected frequency). As examples, clock circuitry  20  may generate clocking signal clk at any desired frequency such as a frequency between 900 and 1000 MHz (e.g., 915 MHz), a frequency between 5.0 GHz and 6.0 GHz (e.g., 5.8 GHz), a frequency between 100 MHz and 900 MHz, a frequency less than 100 MHz, a frequency greater than 6.0 GHz, etc. In general, higher frequencies such as frequencies greater than 900 MHz may provide greater charging efficiency for system  8  than frequencies less than 900 MHz. Control circuitry  20  may control clocking circuitry  90  to change the selected frequency over time if desired. The example of  FIG. 3  is merely illustrative and, in general, clock circuitry  90  may include any desired oscillator circuitry that supplies an alternating-current (AC) drive signal at the selected frequency (e.g., 900 MHz or greater) to an input of amplifier circuitry  92  (e.g., a sinusoidal signal, square wave, saw-tooth signal, etc.). 
     Clock signal clk may be amplified by power amplifier circuitry  92 . The output of power amplifier  92  may be coupled to a corresponding wireless power transmitting capacitor electrode  24  via capacitor electrode terminal  94 . Terminal  94  may be formed at a first end of capacitor electrode  24  whereas the opposing second end  25  of electrode  24  may be floating (e.g., end  25  may not be connected to any conductive structures other than the conductive traces of electrode  24  and is not shorted to ground). Power amplifier circuitry  92  may amplify clock signal clk and drive capacitor electrode  24  using the amplified clock signal. In this way, the clock signal may be used to drive capacitor electrode  24  without encoding or modulating the clock signal (e.g., without formatting the clock signal according to a communications protocol). 
     As shown in  FIG. 3 , wireless power receiving capacitor electrode  42  on stylus  10  is brought into proximity of wireless power transmitting capacitor electrode  24  on tablet  12  for wirelessly charging stylus  10 . In general, the greater (stronger) the near field capacitive coupling between capacitor electrodes  24  and  42 , the greater the wireless power transfer efficiency between capacitor electrodes  24  and  42  over path  26  and the greater the overall wireless charging efficiency of system  8  (i.e., the ratio of power used to charge battery  52  on stylus  10  to power drawn from battery  16  on tablet  12  for charging stylus  10 ). The strength of the near field capacitive coupling between capacitor electrodes  24  and  42  (e.g., the capacitive coupling efficiency) is directly proportional to the degree of alignment between capacitor electrodes  24  and  42  (e.g., a maximal capacitive coupling may occur when capacitor electrodes  24  and  42  are perfectly aligned). Capacitor electrodes  24  and  42  may be perfectly aligned when capacitor electrode  42  completely overlaps capacitor electrode  24  without overlapping an area on tablet  12  that is not covered by capacitor electrode  24 , for example. In the example of  FIG. 3 , capacitor electrodes  24  and  42  have the same shape so as to maximize the degree of alignment and thus the capacitive coupling between the capacitor electrodes. This is, however, merely illustrative. If desired, electrodes  24  and  42  may have different shapes. 
     If desired, wireless power transmitting circuitry  22  on tablet  12  may include coupler circuitry such as directional coupler  96 . Directional coupler  96  may be used to tap the amplified clock signals being conveyed from power amplifier  92  to capacitor electrode  24 . Directional coupler  96  may also tap a reflected version of the amplified clock signals that have been reflected off of capacitor electrode  24  back towards power amplifier  92 . The tapped clock signals may be processed using receiver circuitry such as power measurement circuitry  100 . Power measurement circuitry  100  may gather phase and magnitude information from the tapped antenna signals on path  98  if desired. Control circuitry  20  may use the gathered phase and magnitude information to determine the impedance of capacitor electrode  24  during the operation of wireless power transmitting circuitry  22 . 
     For example, control circuitry  20  may convert the measured phase and magnitude values to complex impedance data points. The complex impedance data points may include, for example, scattering parameter (so-called “S-parameters”) values that are indicative of the complex impedance of capacitor electrode  24 . Measurements of the S-parameters may include measured reflection coefficient parameter values (S11 values) that are indicative of the amount of signal that is reflected back towards coupler  96  from capacitor electrode  24  during transmission of the clock signal. 
     Control circuitry  20  may use the impedance of capacitor electrode  24  (e.g., the complex impedance data points or S11 values measured for capacitor electrode  24 ) to determine whether capacitor electrode  24  is capacitively coupled to wireless power receiving capacitor electrode  42 . Circuitry  30  may use the complex impedance values to determine the extent to which the capacitor electrode  24  is capacitively coupled to wireless power receiving capacitor electrode  42 . 
     For example, as capacitor electrode  42  approaches capacitor electrode  24  40 L, the amount of transmitted power that is reflected back towards coupler  96  may change. This change in signal reflection may change the S11 values that are measured over coupler  96 . When capacitor electrodes  24  and  42  are aligned and there is a relatively strong capacitive coupling between electrodes  24  and  42 , the amount of signal reflection at capacitor electrode  24  may be relatively low (e.g., the value of S11 measured by circuitry  100  may be relatively low). When capacitor electrodes  24  and  42  are misaligned or capacitor electrode  42  is excessively far from capacitor electrode  24  (e.g., when there is relatively weak capacitive coupling between electrodes  24  and  42 ), the amount of signal reflection at capacitor electrode  24  may be relatively high (e.g., the value of S11 measured by circuitry  100  may be relatively high). 
     Control circuitry  20  may use this information gathered by circuitry  100  in performing wireless charging of stylus  10 . For example, when control circuitry  20  identifies that capacitor electrode  42  has become capacitively coupled to capacitor electrode  24  (e.g., when control circuitry  20  identifies that the near field capacitive coupling between electrodes  24  and  42  exceeds a threshold value or that the value of S11 drops below an S11 threshold value), control circuitry  20  may begin to transmit wireless power or may increase the gain provided by power amplifier  92  for charging stylus  10 . This example is merely illustrative and, if desired, other components such as sensors (e.g., capacitive proximity sensors, magnetic sensors, accelerometers, touch sensors, light sensors, etc.) may be used by tablet  12  to identify when capacitor electrode  42  has approached capacitor electrode  24 . Power measurement circuitry  100  and coupler  96  may be omitted from tablet  12  if desired. 
     When driven with the amplified clock signal, capacitor electrode  24  may transmit wireless power to receiving capacitor electrode  42  on stylus  10  via near field capacitive coupling (path  26 ). Stylus  10  has wireless power receiving circuitry  40 . Circuitry  40  includes rectifier circuitry such as rectifier  102  (e.g., a synchronous rectifier controlled by signals from control circuitry  50 ) coupled to capacitor electrode  42  via capacitor terminal  103 . Capacitor terminal  103  may be formed at a first end of capacitor electrode  42  whereas the opposing second end  43  of electrode  42  may be floating (e.g., end  43  may not be connected to any conductive structures other than the conductive traces of electrode  42  and is not shorted to ground). Rectifier  102  converts received alternating-current signals from capacitor electrode  42  (e.g., wireless power signals received by capacitor electrode  42  over path  26 ) into direct-current (DC) power signals for powering circuitry in stylus  10 . Power management circuitry  104  may convey the DC power signals to power load circuitry such as battery  52  within stylus  10 . Power management circuitry  104  may, for example, include a power circuit such as a battery charging integrated circuit or other power management integrated circuit(s) that receives power from rectifier circuitry  102  and regulates the flow of this power to battery  52 , and/or other input-output devices  48  ( FIG. 1 ). Load circuitry that is powered by DC power signals generated by rectifier  102  may include temperature sensors, accelerometers, pressure sensors, force sensors, compasses and gyroscopes, light-based proximity sensors and other proximity sensors, magnetic sensors, and/or other sensors, buttons, audio components such as speakers and microphones, integrated circuits for implementing control circuitry and communications circuitry (e.g., wireless communications circuitry), and/or other components on stylus  10 . 
     Capacitor electrodes  24  and  42  may each be formed using conductive traces (e.g., metal traces on a dielectric substrate such as a rigid or flexible printed circuit board substrate), metal foil, stamped sheet metal, or any other desired conductive structures. In the example of  FIG. 3 , capacitor electrode  24  has a footprint defined by a first rectangular dimension  108  and a second rectangular dimension  110 . Capacitor electrode  42  has a footprint defined by a first rectangular dimension  112  and a second rectangular dimension  114 . In general, the strength of capacitive coupling and the capacitive coupling efficiency of capacitor electrodes  24  and  42  is directly proportional to the perimeter of capacitor electrodes  24  and  42 . As space is at a premium in relatively small form-factor devices such as tablet  12  and stylus  10 , it may be desirable to limit the size of the footprint of capacitor electrodes  24  and  42  (e.g., the size of dimensions  108 ,  110 ,  112 , and  114 ). 
     In order to maximize the perimeter of capacitor electrodes  24  and  42  for a given footprint size, capacitor electrodes  24  and  42  may each have a meandering shape. For example, as shown in  FIG. 3 , the conductive traces that form capacitor electrode  24  include alternating vertical portions (e.g., portions extending parallel to dimension  110 ) and horizontal portions (e.g., portions extending parallel to dimension  108 ) that collectively follow a meandering or zig-zag path from electrode terminal  94  (e.g., capacitor electrode  24  may follow a meandering path from electrode  94  to floating end  25  and may be ungrounded along its length). Similarly, the conductive traces that form capacitor electrode  42  include alternating vertical portions (e.g., portions extending parallel to dimension  114 ) and horizontal portions (e.g., portions extending parallel to dimension  112 ) that collectively follow a meandering path from electrode terminal  103  to floating end  43  (e.g., capacitor electrode  42  may follow a meandering path from electrode  103  to floating end  43  and may be ungrounded along its length). Grounded structures may be formed within tablet  12  and stylus  10  if desired. Because electrodes  24  and  42  are driven by radio-frequency signals (either directly by circuitry  22  or indirectly via capacitive coupling from transmitting electrode  24 ) and are not grounded along their lengths, electrodes  24  and  42  are different from the grounded structures in tablet  12  and stylus  10 . 
     In the example of  FIG. 3 , capacitor electrodes  24  and  42  have the same shape so as to maximize the possible capacitive coupling between capacitor electrodes  24  and  42 . However, if desired, capacitor electrode  24  may have a different shape than capacitor electrode  42 . Because capacitor electrode  24  has a meandering shape, the total length of the conductive traces used to form capacitor electrode  24  (e.g., the total length of capacitor electrode  24  measured over the meandering path from terminal  94  to the opposing floating end of capacitor  24 ) is greater than dimension  108  and dimension  110 . Similarly, the total length of the conductive traces used to form capacitor electrode  42  is greater than dimensions  112  and  114 . 
     In one suitable arrangement, the first dimension  108  of transmitting capacitor electrode  24  may be greater than the first dimension  112  of receiving capacitor electrode  42  (whereas second dimension  110  of transmitting capacitor electrode  24  is the same as second dimension  114  of receiving capacitor electrode  42 ) and capacitor electrodes  24  and  42  may each be formed from repeating portions  106  of conductive traces. As an example, length  108  may be an integer multiple of length  112 . Repeating portions  106  may each include two consecutive vertical and horizontal portions of conductive traces. By forming capacitor electrodes  24  and  42  using repeating portions  106  in this example, the shape of capacitor electrodes  24  and  42  may exhibit a horizontal periodicity that allows capacitor plates  24  and  42  to retain a satisfactory amount of alignment even if capacitor electrode  42  is moved horizontally with respect to capacitor electrode  24  (e.g., even if capacitor electrode  42  or capacitor electrode  24  is shifted left or right as shown in  FIG. 3 ). This may allow for greater tolerance in the positioning of stylus  10  with respect to tablet  12  while still allowing for satisfactory wireless charging efficiency relative to scenarios without such periodicity or scenarios where dimension  108  is not greater than dimension  112  (e.g., capacitor electrode shapes having repeating portions  106  may have greater overall charging efficiency if the position of capacitor electrode  42  is shifted horizontally relative to the position of capacitor electrode  24  than in scenarios where the shape of electrodes  42  and  24  do not include any periodicity). The example of  FIG. 3  in which each repeating portion  106  of electrodes  24  and  42  includes two horizontal and two vertical portions of conductive traces is merely illustrative. In general, repeating portions  106  may include any desired number of conductive trace portions having any desired shapes and orientations. 
     In general, greater dimensions  108 ,  110 ,  112 , and  114  may allow for higher capacitive coupling between capacitor electrodes  24  and  42  but also occupy a greater amount of space on devices  10  and  12  than shorter dimensions  108 ,  110 ,  112 , and  114 . As one example, dimension  108  of transmitting capacitor electrode  24  may be between 1 cm and 5 cm whereas dimension  112  of receiving capacitor electrode  42  is between 0.5 cm and 1.5 cm. Second dimension  110  of transmitting capacitor electrode  24  and second dimension  114  of receiving capacitor electrode  42  may be between 1 mm and 1 cm, as an example. These examples are merely illustrative and, in as general, capacitor electrodes  24  and  42  may have any desired dimensions. Second dimension  110  may be greater than first dimension  108  and/or second dimension  114  may be greater than first dimension  112  if desired. 
     The example of  FIG. 3  is merely illustrative. In general, capacitor electrodes  24  and  42  may include conductive traces that follow any desired path and that have any desired shape. Capacitor electrodes  42  and  24  may have straight and/or curved edges. Capacitor electrodes  24  and  42  may be formed from conductive traces on a dielectric substrate such as a plastic support structure or a rigid or flexible printed circuit board substrate, from metal foil, stamped sheet metal, conductive adhesive, and/or from any other desired conductive structures. In scenarios where tablet  12  has multiple capacitor electrodes  24 , each capacitor electrode  24  may be coupled to respective clock circuitry  90  and amplifier circuitry  92 . Each capacitor electrode  24  may be selectively driven by the corresponding clock circuitry when it is desired to transmit wireless power with that capacitor electrode. In another suitable arrangement, the same clock circuitry  90  and amplifier circuitry  92  may be coupled to drive two or more capacitor electrodes  24 . In this scenario, each of the capacitor electrodes  24  may be driven at the same time by the amplified clock signal clk or one or more of the capacitor electrodes may be selectively switched into use when it is desired to transmit wireless power (e.g., using switching circuitry or other multiplexing circuitry interposed between power amplifier  92  and the capacitor electrodes). In scenarios where stylus  12  has multiple capacitor electrodes  42 , each capacitor electrode  42  may be coupled to a respective rectifier  102  and power management circuit  104  or two or more capacitor electrodes  42  may be coupled to the same rectifier  102  and/or power management circuit  104 . 
     If desired, other circuitry such as fixed or adjustable impedance matching circuitry, filter circuitry, switching circuitry, and/or other fixed or adjustable components may be interposed between the output of power amplifier  92  and electrode terminal  94  in tablet  12 . Other components such as filtering circuitry and/or switching circuitry may be interposed between clock circuitry  90  and the input of power amplifier  92 . Similarly, other components such as impedance matching circuitry, filter circuitry, and/or switching circuitry may be interposed between rectifier  102  and electrode  103  and/or between rectifier  102  and power management circuitry  104  of stylus  10  if desired. 
       FIG. 4  is a side-view of stylus  10  of  FIG. 2  having a wireless power receiving capacitor electrode  42  formed on shaft  58 . As shown in  FIG. 4 , wireless power receiving capacitor electrode  42  is formed from a meandering conductive trace on dielectric substrate  120 . Capacitor terminal  103  may be coupled to rectifier  102  within stylus  10  using a conductive via extending through substrate  110  or any other desired conductive interconnect structures. Dielectric substrate  120  may be, for example, a flexible printed circuit substrate. Dielectric substrate  120  may be formed on shaft  58  of stylus  10 . For example, the housing of stylus  10  may include an elongated plastic and/or metal tube (cylinder) extending from tip portion  56  to end portion  60 . Dielectric substrate  120  may be wrapped around at least some of the circumference of the tube (e.g., around longitudinal axis  54 ). In another suitable arrangement, substrate  120  may be omitted and capacitor plate  42  may be patterned directly onto the plastic tube of shaft  58 . If desired, magnetic structures such as magnetic alignment structures  121  may be formed on shaft  58 , end  60 , and/or tip  56 . Alignment structures  121  may include one or more magnets or other conductive structures such as metal. Alignment structures  121  may magnetically couple with magnetic or conductive structures on stylus  12  to hold or snap stylus  10  in place on tablet  12 . Alignment structures  121  may be formed on one or more sides of shaft  58 . In a scenario where alignment structures  121  are formed on one side of shaft  58 , alignment structures  121  may be formed on the same side of shaft  58  as electrode  42  so that structures  121  hold stylus  10  in place on tablet  12  at an orientation in which electrode  42  points towards (faces) electrode  24  on tablet  12 . If desired, structures  121  may include a ring formed around the circumference of shaft  58 . This may, for example, allow stylus  10  to be snapped to device  10  regardless of the rotational orientation of stylus  10  when placed into contact with tablet  12 . 
     The example of  FIG. 4  is merely illustrative. If desired, alignment structures  121  may include other alignment structures such as pins, adhesive, protruding housing portions, recessed housing portions, or any other desired structures for aligning capacitor electrode  42  with capacitor electrode  24  for performing wireless charging. Structures  121  may be omitted if desired. Substrate  120  and capacitor electrode  42  may be formed at any desired location along shaft  58 , on tip portion  56 , and/or on end portion  60  of stylus  10 . Capacitor terminal  103  may be interposed between floating end  43  of electrode  42  and end  60  of stylus  10  if desired. While  FIG. 4  only shows a single capacitor electrode  42 , any desired number of capacitor electrodes  42  may be formed along the length of shaft  58  and/or on other sides of shaft  58  (e.g., at other locations around axis  54 ). 
     A cross-sectional side view of shaft portion  58  of the elongated body of stylus  10  is shown in  FIG. 5 . As shown in  FIG. 5 , the housing of stylus  10  may surround interior cavity  122 . Components such as integrated circuits, battery,  52 , rectifier  102 , power management circuitry  104 , input-output devices  48 , control circuitry  50 , and other components may be mounted on one or more substrates (e.g., a dielectric support structure such as a rigid printed circuit formed from a rigid printed circuit board material such as fiberglass-filled epoxy or a flexible printed circuit formed from a flexible sheet of polyimide or other flexible polymer layer) within interior cavity  122 . 
     Interior cavity  122  may be surrounded by one or more layers of material such as layer  124  and optional layer  126 . These layers of material may form concentric cylindrical tubes and may be formed from metal, plastic, glass, ceramic, other materials, and/or two or more of these materials. As an example, outer layer  126  may form a plastic tube that serves as a cosmetic exterior for stylus  10  and inner layer  124  may form a metal tube that provides stylus  10  with structural support. In general, tube  126  may formed from metal, plastic, or other materials and tube  124  may be formed from metal, plastic, or other materials. If desired, optional outer tube  126  may be omitted. With another illustrative arrangement, inner tube  124  may be omitted and tube  126  may be formed from metal, plastic, or other materials. Configurations in which shaft  58  includes a single tube or includes solid portions without significant interior cavity portions may also be used. 
     As shown in the cross-sectional side view of stylus  10  of  FIG. 5 , substrate  120  may be formed on inner tube  124 . Capacitor electrode  42  may be patterned onto a surface of substrate  120  (e.g., prior to placing substrate  120  onto tube  124 ). Outer tube  126  may serve to hide capacitor electrode  42  and substrate  120  from view and may serve to protect capacitor electrode  42  from damage if desired. In another suitable arrangement, capacitor electrode  42  may be patterned directly onto inner tube  124  and substrate  120  may be omitted (e.g., in scenarios where inner tube  124  is formed from a dielectric material). 
     Substrate  120  and capacitor electrode  42  may extend across an angle α around longitudinal axis  54  of stylus  10 . Larger angles α may allow for an increased probability of capacitor electrode aligning with transmission capacitor electrode  24  on tablet  12  when stylus  10  is brought into proximity of capacitor electrode  24  relative to smaller angles α (e.g., larger angles α may allow for an increased tolerance to rolling of stylus  10  about axis  54  without sacrificing capacitive coupling efficiency than smaller angles α when performing wireless charging). As examples, angle α may be equal to 30 degrees, 60 degrees, 45 degrees, an angle between 0 degrees and 180 degrees, or any other desired angle between 0 degrees and 360 degrees 
     In the example of  FIG. 5 , a single wireless power receiving electrode  42  is formed on shaft  58 . If desired, two or more capacitor electrodes  42  may be formed at different locations along the circumference of shaft  58 .  FIG. 6  is a cross-sectional side view of stylus  10  having two capacitor electrodes  42  and corresponding substrates  120  (e.g., a first capacitor electrode  42 - 1 , a second capacitor electrode  42 - 2 , a first substrate  120 - 1 , and a second substrate  120 - 2 ). 
     As shown in  FIG. 6 , first capacitor electrode  42 - 1  may be formed on first substrate  120 - 1  whereas second capacitor electrode  42 - 2  is formed on second substrate  120 - 2 . Substrates  120 - 1  and  120 - 2  may be formed on tube  124 . In another suitable arrangement, one or both of capacitor electrodes  42 - 1  and  42 - 2  may be patterned directly onto tube  124 . If desired, optional tube  126  may be formed over substrates  120 - 1  and  120 - 2 . 
     Substrate  120 - 1  and capacitor electrode  42 - 1  may extend across an angle α 1  around longitudinal axis  54  of stylus  10 . Substrate  120 - 2  and capacitor electrode  42 - 2  may extend across an angle α 2  around axis  54 . Substrate  120 - 1  and capacitor electrode  42 - 1  may be separated from substrate  120 - 2  and capacitor electrode  42 - 2  at tube  124  by separation angle β 1 . Angle α 1  may be equal to angle α 2  or may be different from angle α 2 . Separation angle β 1  may be less than angles α 1  and α 2 . As examples, angles α 1  and α 2  may each be equal to 60 degrees, 45 degrees, 30 degrees, an angle between 30 degrees and 90 degrees, or any other desired angle between zero and 179 degrees. Angle β 1  may be equal to 10 degrees, 20 degrees, 30 degrees, 45 degrees, an angle between 5 degrees and 90 degrees, any other desired angle that is less than α 1  or α 2 , or any other desired angle between 0 degrees and 179 degrees. If desired, one of angles α 1  or α 2  may between 179 and 330 degrees. 
     In this type of arrangement, capacitor electrodes  42 - 1  and  42 - 2  may be located at the side of stylus  10  that is intended to face tablet  12  during wireless charging. For example, magnets or other alignment structures in shaft  58  may be configured to snap the side of stylus  10  at which capacitor electrodes  42 - 1  and  42 - 2  are formed into alignment with corresponding wireless power transmission capacitor electrodes  24  on tablet  12 . This may help to ensure that capacitors electrodes  42  face transmission capacitor electrodes  24  on tablet  12  so that the capacitor electrodes may exhibit a strong capacitive coupling. Capacitor electrodes  42 - 1  and  42 - 2  may receive wireless power from a single transmission capacitor electrode  24  or may each receive wireless power from respective transmission capacitor electrodes  24  on tablet  12 . 
       FIG. 7  is a cross-sectional side view of stylus  10  having three capacitor electrodes  42  and corresponding substrates  120  (e.g., a first capacitor electrode  42 - 3 , a second capacitor electrode  42 - 4 , a third capacitor electrode  42 - 5 , a first substrate  120 - 3 , a second substrate  120 - 4 , and a third substrate  120 - 5 ). As shown in  FIG. 7 , first capacitor electrode  42 - 3  may be formed on first substrate  120 - 3 , second capacitor electrode  42 - 4  may be formed on second substrate  120 - 4 , and third capacitor electrode  42 - 5  may be formed on third substrate  120 - 5 . In another suitable arrangement, one or more of capacitor electrodes  42 - 3 ,  42 - 4 , and  42 - 5  may be patterned directly onto tube  124 . If desired, optional tube  126  may be formed over capacitor electrodes  42 . 
     Substrate  120 - 3  and capacitor electrode  42 - 3  may extend across angle α 3  about longitudinal axis  54  of stylus  10 . Substrate  120 - 4  and capacitor electrode  42 - 4  may extend across angle α 4  about axis  54 . Substrate  120 - 5  and capacitor electrode  42 - 5  may extend across angle as about axis  54 . Substrate  120 - 3  and capacitor electrode  42 - 3  may be separated from substrate  120 - 4  and capacitor electrode  42 - 4  at tube  124  by separation angle β 2 . Substrate  120 - 4  and capacitor electrode  42 - 4  may be separated from substrate  120 - 5  and capacitor electrode  42 - 5  by separation angle β 3 . Substrate  120 - 5  and capacitor electrode  42 - 5  may be separated from substrate  120 - 3  and capacitor electrode  42 - 3  by separation angle θ 4 . 
     Separation angles β 2 , β 3 , and β 4  may each be equal or one or more of separation angles β 2 , β 3 , and β 4  may be different. Angles α 3 , α 4 , and as may each be equal or one or more of angles α 3 , α 4 , and as may be different. Separation angles β 2 , β 3 , and β 4  may each be less than, equal to, or greater than angles α 3 , α 4 , and α 5 . As examples, one or more of angles α 3 , α 4 , and as may be equal to 60 degrees, 45 degrees, 30 degrees, an angle between 30 degrees and 90 degrees, or any other desired angle between zero and 119 degrees. One or more of angles β 2 , β 3 , and β 4  may be equal to 10 degrees, 20 degrees, 30 degrees, 45 degrees, 60 degrees, an angle between 5 degrees and 90 degrees, any other desired angle between 0 degrees and 119 degrees. If desired, one of angles α 3 , α 4 , or as may between 179 and 330 degrees. If desired, one of angles β 2 , β 3 , or β 4  may be between 179 and 330 degrees. 
     In this type of arrangement, capacitor electrodes  42 - 3 ,  42 - 4 , and  42 - 5  may be located along different sides of stylus  10 . Capacitor electrodes  42 - 3 ,  42 - 4 , and  42 - 5  may receive wireless power from a single transmission electrode  24  or may receive wireless power from two or more transmission electrodes  24  on tablet  12 . Forming capacitor electrodes  42  in this way may help to ensure that at least one capacitor electrode  42  is capacitively coupled to a corresponding transmission capacitor electrode  24  when placed in proximity to tablet  12  regardless of the rotational orientation of stylus  10  about axis  54 . Magnetic alignment structures  121  ( FIG. 4 ) may be formed from a ring extending 360 degrees around the circumference of shaft  58  or using two or more discrete magnets on different sides of shaft  58  in the example of  FIG. 7  if desired. 
     The examples of  FIGS. 4-7  in which stylus  10  has a cross-sectional shape are merely illustrative. In general, stylus  10  may have any desired cross-sectional shape. For example, shaft  58  may have a rectangular cross section, elliptical cross section, triangular cross section, hexagonal cross section or any other desired polygonal cross section (e.g., with each capacitor electrode  42  being formed on one or more sides of the polygonal cross section), combinations of these, etc. 
       FIG. 8  is a top-down view of tablet  12  of  FIG. 2  having a wireless power transmission capacitor electrode  24  adjacent to display  32  for charging stylus  10 . As shown in  FIG. 8 , wireless power transmission capacitor electrode  24  is formed within region  66  of display  32  ( FIG. 2 ). This example is merely illustrative and, if desired, capacitor electrode  24  may be formed within regions  68 ,  72 , and/or  70  of  FIG. 2  or elsewhere along the lateral area of display  32 . 
     Display  32  may have an active area AA that includes an array of display pixels. The array of pixels may be formed from liquid crystal display (LCD) components, an array of electrophoretic pixels, an array of plasma display pixels, an array of organic light-emitting diode display pixels or other light-emitting diode pixels, an array of electrowetting display pixels, or display pixels based on other display technologies. Display  32  may be protected using a display cover layer such as a layer of transparent glass, clear plastic, transparent ceramic, sapphire, or other transparent crystalline material, or other transparent layer(s). The display cover layer may extend across active area AA and inactive area IA (e.g., across the entire length and width of tablet  12 ) and may cover capacitor electrode  24 . 
     Display  32  may have an inactive border region that runs along one or more of the edges of active area AA. Inactive area IA may be free of pixels for displaying images and may overlap circuitry and other internal device structures in tablet  12 . To block these structures from view by a user of tablet  12 , the underside of the display cover layer or other layer in display  32  that overlaps inactive area IA may be coated with an opaque masking layer in inactive area IA. The opaque masking layer may have any suitable color. This example is merely illustrative and, if desired, capacitor electrode  24  may be formed within active area AA. 
     As shown in  FIG. 8 , capacitor electrode  24  is formed on substrate  130 . Substrate  130  may be, for example, a rigid or flexible printed circuit, a plastic support structure, or any other desired support structure. Electrode terminal  94  may be coupled to power amplifier  92  ( FIG. 3 ) using a conductive via that extends through substrate  130  or using any other desired conductive interconnect structures (e.g., conductive pins or clips, conductive springs, conductive foam or adhesive, welds, solder, etc.). Capacitor electrode  24  may be formed from conductive traces that are patterned onto substrate  130 . Capacitor terminal  94  may be formed at any desired location along the conductive traces forming capacitor electrode  24 . Alignment structures such as alignment structures  132  may be formed under or on the display cover layer. 
     As one example, alignment structures  132  may include an indentation or groove in the exterior surface of the display cover layer. When a user wishes to wirelessly power (charge) stylus  10 , the user may place stylus  10  within the groove. The groove may hold stylus  10  in place on the surface of the display cover layer to keep stylus  10  from rolling off of tablet  12  and/or to ensure that transmission capacitor electrode  24  on tablet  12  is aligned with receiving capacitor electrode  42  on stylus  10  for performing wireless power transfer operations (e.g., to ensure sufficient capacitive coupling for wireless power transfer). 
     As another example, alignment structures  132  may be magnetic structures mounted under the display cover layer. When a user wishes to wirelessly power stylus  10 , the user may place stylus  10  on the display cover layer over within region  66  (e.g., with the longitudinal axis  54  of stylus  10  parallel to the top edge of tablet  12 ). Magnetic structures  132  may snap stylus  10  in place over capacitor electrode  24  (e.g., in such a way so as to align transmit capacitor electrode  42  with capacitor electrode  42  on stylus  10  for performing wireless power transfer operations). These examples are merely illustrative. If desired, alignment structures  132  may include pin structures, adhesive structures, clip structures, or may be omitted. 
     By forming transmission capacitor electrode  24  with a greater dimension  108  than dimension  112  of receiving electrode  42  ( FIG. 3 ), stylus  10  may be moved horizontally on the surface of display  32  (as shown by arrows  140 ) without impacting the near field capacitive coupling between transmit capacitor electrode  24  and receive capacitor electrode  42 . Transmission capacitor electrode  24  may extend across some or all of the width of tablet  12 . In one suitable arrangement, transmission capacitor electrode  24  may extend across at least half of the width of tablet  12 . If desired, capacitor electrode  24  of  FIG. 8  may be replaced by two or more separate, discrete capacitor electrodes  24 . 
       FIG. 9  is a cross-sectional side view of tablet  12  having a display-adjacent wireless power transmitting capacitor electrode  24  (e.g., as taken along line AA′ of  FIG. 8 ). As shown in  FIG. 9 , tablet computer  12  includes conductive housing  64 . Conductive housing  64  includes conductive rear wall  64 R and conductive sidewalls  64 W. Display  32  includes an associated display module  134  and display cover layer  136 . Display module  134  may be a liquid crystal display module, an organic light-emitting diode display, or other display for producing images for a user. Display module  134  may include touch sensitive components. Display cover layer  136  may be a clear sheet of glass, a transparent layer of plastic, or other transparent member. Display cover layer  136  may be formed from dielectric. If desired, display cover layer  136  may form a portion of display module  134 . 
     In active area AA, an array of display pixels associated with display structures such as display module  134  may present images to a user of tablet  12 . In inactive display border region IA, the inner surface of display cover layer  136  may be coated with a layer of black ink or other opaque masking layer  140  to hide internal device structures from view by a user. Wireless power transmission capacitor electrode  24  may be mounted within housing  64  under opaque masking layer  140 . Forming capacitor electrode  24  under inactive region IA of display  14  may allow capacitor electrode  24  to transmit wireless power over path  26  through display cover layer  136  without the wireless power being blocked or otherwise impeded by the active circuitry in display module  134 . Other components  142  may be formed within housing  64  (e.g., components such as portions of wireless power transmitting circuitry  22 , control circuitry  20 , battery  16 , converter  18 , and some of input-output devices  30  of  FIG. 1 ). Components  142  may be mounted to one or more substrates such as printed circuit board  144  (e.g., a main logic board). 
     In the example of  FIG. 9 , substrate  130  is placed on a top surface of dielectric support structure  138 . Support structure  138  may be a plastic support structure, foam support structure, or any other desired support structure. If desired, support structure  138  may mechanically bias substrate  130  and capacitor electrode  24  towards display cover layer  136  (e.g., capacitor electrode  24  may be placed into contact with ink layer  140  and/or display cover layer  136 ). In another suitable arrangement, substrate  130  may be omitted and capacitor electrode  24  may be formed from sheet metal or metal foil placed over support structure  138  or may be formed from conductive traces patterned directly onto support structure  138 . Dielectric support structure  138  may be hollow or solid or may include hollow and solid portions. If desired, dielectric support structure  138  may form a cavity for a speaker on tablet  12 . 
     As shown in  FIG. 9 , stylus  10  may be placed on the exterior surface of display cover layer  136  within region  66  for wirelessly powering stylus  10 . When stylus  10  is placed on display cover layer  136 , wireless power receiving capacitor electrode  42  is capacitively coupled with wireless power transmitting electrode  24 . The capacitive coupling may be relatively unaffected by the vertical separation between the edge of capacitor electrode  42  and the surface of display cover layer  136 . This is in contrast to scenarios in which inductive coils are used for wirelessly charging stylus  10 . In such inductive charging arrangements, the inductive coils are significantly larger than the capacitor electrodes and are unable to maintain satisfactory coupling due to the radius of curvature of stylus  10  excessively separating transmit and receive coils. In addition, by extending capacitor electrode  42  across a suitably large angle α, stylus may roll/rotate around axis  54  as shown by arrow  146  without significantly reducing the capacitive coupling between electrodes  42  and  24 . Wireless charging operations performed over a capacitive coupling link between electrodes  42  and  24  may thereby be rotationally invariant about axis  54 . 
     The example of  FIG. 9  in which capacitor electrode  24  is placed on dielectric support structure  138  is merely illustrative. In another suitable arrangement, capacitor electrode  24  may be placed on a portion of conductive sidewall  64 W, as shown in  FIG. 10 . 
     As shown in  FIG. 10 , conductive housing sidewall  64 W may include a ledge portion  150  that extends towards the interior of tablet  12 . Display cover layer  136  may be placed on ledge  150 . Ledge  150  may provide structural support for display cover layer  136 . If desired, adhesive may be used to adhere display cover layer  136  to ledge  150 . Substrate  130  and capacitor electrode  24  may be formed on ledge  150 . Capacitor electrode  24  may transmit wireless power to stylus  10  over path  26  via near field capacitive coupling (e.g., through opaque masking layer  140  and display cover layer  136 ). Adhesive may be interposed between capacitor electrode  24  and display cover layer  136  if desired. 
       FIG. 11  is a side-view of tablet  12  having wireless power transmission capacitor electrode  24  formed adjacent to a corresponding housing sidewall  64 W for charging stylus  10 . As shown in  FIG. 11 , wireless power transmission capacitor electrode  24  is formed within region  74  of housing sidewall  64 W ( FIG. 2 ). Sidewall  64 W may be formed from conductive material such as metal. A dielectric window  162  may be cut into sidewall  64 W. Capacitor electrode  24  and substrate  130  may be aligned with window  162 . 
     Capacitor electrode  24  may transmit wireless power through window  162  for wirelessly charging stylus  10 . If desired, alignment structures  160  may be formed behind or on window  162 . As an example, alignment structures  160  may include magnetic structures mounted under window  162  (e.g., magnetic structures mounted to an interior surface of window  162  or mounted to a substrate aligned behind window  162 ). When a user wishes to wirelessly power (charge) stylus  10 , the user may place stylus  10  adjacent to window  162  (e.g., either directly onto housing sidewall  64 W and window  162  or adjacent to window  162  on a surface on which tablet  12  is resting). Magnetic structures  160  may magnetically couple with magnetic structures on stylus  10  (e.g., one or more magnets or metal structures on stylus  10 ) and may snap stylus  10  into place in such a way so as to align transmit capacitor electrode  24  with receive capacitor electrode  42  on stylus  10  for performing wireless power transfer operations. If desired, magnetic structures  160  may affix stylus  10  to housing sidewall  64 W so that tablet  12  and stylus  10  can be picked up off of a surface without disrupting wireless power transfer operations to stylus  10 . These examples are merely illustrative. If desired, alignment structures  160  may include pin structures, adhesive structures, clip structures, or may be omitted. Alignment structures  160  may be mounted to conductive housing sidewall  64 W if desired. 
       FIG. 12  is a cross-sectional side view of tablet  12  having a sidewall-adjacent wireless power transmitting capacitor electrode  24  (e.g., as taken along line BB′ of  FIG. 11 ). As shown in  FIG. 12 , tablet computer  12  includes conductive housing  64 . Dielectric window  162  is formed within sidewall  64 W of conductive housing  64 . Capacitor electrode  24  may be formed on dielectric support structure  164  and aligned with window  162 . Dielectric support structure  164  may include plastic, foam, ceramic, or any other desired materials. Capacitor electrode  24  may be patterned onto a surface of support structure  164  or may be formed on another substrate (e.g., a flexible printed circuit board) that is placed onto or adhered to support structure  164 . Capacitor electrode  24  may be separated from window  162  or may be placed into contact with window  162 . If desired, support structure  164  may bias capacitor electrode  24  against window  162 . 
     In the example of  FIG. 12 , capacitor electrode  24  is formed on a given side/surface of support structure  164  (e.g., the side of structure  164  facing window  162 ). The remaining sides of substrate  164  may be covered by conductive structures  166 . Conductive structures  166  may be coupled to a ground plane or other ground structures within tablet  12  (e.g., using conductive foam, conductive connectors such as screws or clips, solder, welds, wires, conductive pins or contact pads, conductive adhesive, conductive tape, or using any other desired conductive interconnect structures) and may sometimes be referred to herein as grounded conductive structures  166 . Grounded conductive structures  166  may include sheet metal structures, conductive traces, metal foil, conductive portions of electronic components within tablet  12 , conductive housing portions, or any other desired conductive structures. If desired, conductive structures  166  may be shorted to conductive housing  64  (e.g., along the peripheral edges of window  162 ). Grounded conductive structures  166  may form a grounded cavity that backs capacitor electrode  24  and that shields capacitor electrode  24  from other components within tablet  12 . The grounded cavity formed by structures  166  may surround substrate  164  (e.g., substrate  164  and electrode  24  may be enclosed within and completely surrounded by housing wall  64 W, window  162 , and conductive structures  166 ). If desired, one or more sides of substrate  164  may be free of structures  166  (e.g., structures  166  need not be formed on all remaining sides of substrate  164 ). In one suitable arrangement, a grounded conductive structure such as a conductive trace, sheet metal layer, metal foil layer, or conductive portion of an electrical component within tablet  12  is formed on the side of dielectric support structure  164  that opposes the side of support structure on which capacitor electrode  24  is formed. In this scenario, the grounded conductive structure on the side of substrate  164  opposing electrode  24  may be shorted to housing wall  64 W and/or housing wall  64 R using any desired conductive interconnect structures such as conductive foam, conductive screws, conductive clips, solder, welds, wires, conductive pins, conductive adhesive, conductive tape, metal traces on substrate  164 , stamped sheet metal, metal foil, contact pads, other conductive housing portions, or any other desired conductive structures. 
     When stylus  10  is placed adjacent to window  162 , wireless power receiving capacitor electrode  42  on stylus  10  is placed into a relatively strong capacitive coupling with wireless power transmitting electrode  24 . In the example of  FIG. 12 , tablet  12  is placed onto surface  172  (e.g., a table or desktop). Stylus  10  may be placed onto surface  172  adjacent to window  162  for wirelessly charging stylus  10 . If desired, alignment structures  160  ( FIG. 11 ) may interact with magnetic components on stylus  10  to hold stylus  10  against sidewall  64 W with capacitor electrode  42  aligned with capacitor electrode  24 . Capacitor electrode  24  may subsequently transmit wireless power to capacitor electrode  42  via near field capacitive coupling (e.g., over path  26 ). 
     In general, capacitor electrodes  24  and  42  may have any desired shapes. In the example of  FIGS. 3, 4, 8, and 11 , capacitor electrodes  24  and  42  are formed from conductive traces that follow a meandering path and that have a uniform width across their length. In general, the conductive traces that form capacitor electrodes  24  and  42  need not have a uniform width across their lengths.  FIG. 13  is a diagram showing how the conductive traces may have non-uniform widths. 
     As shown in  FIG. 13 , conductive trace  180  may include multiple repeating units (segments)  106  that follow a meandering path. Conductive trace  180  may be used in forming transmit capacitor electrode  24  and/or receive capacitor electrode  42 . Capacitor electrode  24  and capacitor electrode  42  may each include any desired number of repeating units  106  (e.g., between one and ten repeating units  106 , thirteen repeating units  106 , sixteen repeating units  106 , between ten and twenty repeating units  106 , more than twenty repeating units  106 , etc.). Each repeating unit  106  of conductive trace  180  includes a thick portion  182  and a thin portion  184  extending from a first edge  185  of the thick portion. Thick portion  182  has a width  186  that is greater than the width  188  of thin portion  184 . 
     Thin portions  184  of trace  180  may each include a first segment  184 - 1  extending from first edge  185  of the corresponding thick portion  182 , a second segment  184 - 2  extending from an end of first segment  184 - 1 , a third segment  184 - 3  extending from an end of second segment  184 - 2 , and a fourth segment extending from an end of third segment  184 - 3  to second edge  189  of the thick portion  182  in the next repeating unit  106 . In the example of  FIG. 13 , segment  184 - 2  extends parallel to segment  184 - 4  whereas segment  184 - 1  extends parallel to segment  184 - 3 . Segments  184 - 4  and  184 - 2  extend perpendicular to segments  184 - 1  and  184 - 3 . This example is merely illustrative. In general, segments  184 - 1 ,  184 - 2 ,  184 - 3  and  184 - 4  may extend at any desired angles in any desired shape. Conductive trace  180  may have any desired shape. The edges of conductive trace  180  may be curved and/or straight. Thick portions  182  of conductive trace  180  may serve to increase the overall capacitance of capacitor electrodes  24  and  42  given the constraints imposed by the relatively small footprint defined by dimensions  108 ,  110 ,  112 , and  114  of  FIG. 3 . Thin portions  184  of conductive trace  180  may be used to increase the overall perimeter and inductance of capacitor electrodes  24  and  42  given the constraints imposed by the relatively small footprint defined by dimensions  108 ,  110 ,  112 , and  114  of  FIG. 3 . 
     In some scenarios, the diameter of stylus  10  is greater than the height (thickness)  170  of tablet  12  ( FIG. 12 ). In other words, the radius of stylus  10  may be greater than half the height  170  of tablet  12 . In the example of  FIG. 12 , stylus  10  has a radius  168  that is greater than half of height  170  of tablet  12 . In this scenario, when both tablet  12  and stylus  10  are placed on surface  172  for wirelessly charging stylus  10 , capacitor electrode  42  may be slightly misaligned with respect to wireless power transmission capacitor electrode  24  (e.g., portions of capacitor electrode  42  may extend beyond the height of tablet  12 ). If desired, conductive traces  180  that are used to form capacitor electrodes  24  and/or  42  may be provided with a short circuit path that serves to redistribute current on the capacitor electrodes to compensate for these misalignments. 
     As shown in  FIG. 14 , a short path  190  may be coupled between third edge  187  of thick portion  182  and segment  184 - 3  of thin portion  184  in each repeating unit  106  of conductive trace  180 . In the example of  FIG. 14 , short path  190  is coupled to third edge  187  of thick portion  182  adjacent to where segment  184 - 1  is coupled to thick portion  182  at edge  185 . However, in general, short path  190  may be located at any desired distance  192  with respect to lower edge  191  of trace  180 . Edges  189 ,  185 ,  187 , and  191  of thick portion  182  may sometimes be referred to herein as sides or ends of thick portion  182 . In the example of  FIGS. 13 and 14 , edges  189 ,  185 ,  187 , and  191  are straight, where edge  189  extends parallel to edge  187  and edge  185  extends parallel to edge  191  and perpendicular to edges  189  and  187 . This is merely illustrative and, in general, edges  189 ,  185 ,  187 , and/or  191  may have any desired shape and may extend at any desired angles with respect to each other. Distance  192  may be, in one example, between 1 mm and 2 mm. Short path  190  may extend parallel to segment  184 - 2  or may extend at a non-zero angle with respect to segment  184 - 2 . Segment  190  may have the same thickness  188  as segment  184 - 2  or may have a different thickness. Short path  190  may have straight edges and/or curved edges. If desired, two or more short paths  190  may be coupled between segment  184 - 3  and thick portion  182 . Short path  190  may be coupled between segment  184 - 3  and  184 - 1  if desired. In another suitable arrangement, short path  190  may extend from segment  184 - 3  to edge  189  of thick portion  182  in the next repeating unit  106  (e.g., in parallel with segment  184 - 4 ). Short path  190  may serve to redistribute current flow across traces  180  relative to the arrangement of  FIG. 13 . The redistributed current flow may increase the capacitive coupling between capacitor electrodes  24  and  42  and thus the overall wireless charging efficiency of system  8  in scenarios where radius  168  of stylus  10  is greater than half the height of tablet  12 , for example. 
     If desired, tablet  12  may be provided with a wireless power transmitting structure having a half-cylindrical shape for receiving stylus  10 .  FIG. 15  is a perspective view of a half-cylindrical wireless power transmitting structure  200 . Structure  200  may be formed on rear wall  64 R, on sidewall  64 W, or on the surface of display  32  of tablet  12 , if desired. In another suitable arrangement, structure  200  may be incorporated within a cover or case for tablet  12 . 
     As shown in  FIG. 15 , wireless power transmitting structure  200  has a dielectric carrier  202  with a half-cylindrical interior surface  206 . The example in which interior surface  206  is half-cylindrical is merely illustrative and, if desired, surface  206  may have any curved or polygonal shape (e.g., a shape that mates with the shape of stylus  10 ). Structure  200  may receive stylus  10  for wireless charging. When it is desired to wirelessly charge stylus  10 , stylus  10  is placed onto interior surface  206 . One or more substrates  130  and corresponding wireless power transmission capacitor electrodes  24  may be formed on interior surface  206 . If desired, capacitor electrodes  24  may be patterned directly onto interior surface  206 . A protective dielectric layer may be formed over capacitor electrodes  24  at interior surface  206  if desired. 
     Interior surface  206  may serve to hold stylus  10  in place during wireless charging operations. If desired, the one or more capacitor electrodes  24  may extend 120 degrees or greater about longitudinal axis  54  of stylus  10  when placed on surface  206 . This may allow for satisfactory capacitive coupling between electrodes  24  and  42  regardless of how stylus  10  is oriented or rotated about axis  54  on surface  206 . This may, for example, allow a user to charge stylus  10  without having to focus on how stylus  10  is physically placed within structure  200 . In one suitable arrangement, providing stylus  10  with three capacitor electrodes such as electrodes  42 - 3 ,  42 - 4 , and  42 - 5  of  FIG. 7  may ensure that stylus  10  has a strong capacitive coupling to electrodes  24  through a full 360 degrees of rotation about axis  54 . However, in general, any desired number of capacitor electrodes  42  may be formed on stylus  10 . If desired, conductive layer  204  may be formed over the exterior surface of dielectric carrier  202 . Conductive layer  204  may provide structural support for structure  200  and may shield capacitor electrodes  24  and  42  from exterior interference, for example. 
     If desired, tablet  12  may be provided with a wireless power transmitting structure having a cylindrical or tubular shape for receiving stylus  10 .  FIG. 16  is a perspective view of a cylindrical wireless power transmitting structure  210 . Structure  210  may be formed on rear wall  64 R, within tablet  12  (e.g., so that stylus  10  is inserted into structure  210  through an opening in a sidewall  64 W), or on the surface of display  32  of tablet  12 , if desired. In another suitable arrangement, structure  210  may be incorporated within a cover or case for tablet  12 . 
     As shown in  FIG. 16 , wireless power transmitting structure  210  has a dielectric carrier  212  with a cylindrical or tubular interior surface  212 . The example in which interior surface  212  is cylindrical is merely illustrative and, if desired, surface  212  may have any curved or polygonal shape (e.g., a shape that mates with the shape of stylus  10 ). Structure  210  may receive stylus  10  for wireless charging. When it is desired to wirelessly charge stylus  10 , stylus  10  is placed into structure  210  and on interior surface  212 . One or more substrates  130  and corresponding wireless power transmission capacitor electrodes  24  may be formed on interior surface  206 . If desired, capacitor electrodes  24  may be patterned directly onto interior surface  206 . A protective dielectric layer may be formed over capacitor electrodes  24  at interior surface  206  if desired. 
     In one exemplary arrangement three capacitor electrodes  24  are provided on interior surface  212 . The capacitor electrodes  24  may be evenly spaced about longitudinal axis  54 . The capacitor electrodes may, for example, extend at least 120 degrees about axis  54 . As an example, each capacitor electrode  24  extends 60 degrees about axis  54  (with 60 degrees separating each electrode). As another example, each capacitor electrode  24  extends 40 degrees about axis  54  (with 80 degrees separating each electrode). This is merely illustrative and, in general, any desired number of electrodes having any desired size may be used. Forming electrodes  24  at different locations around longitudinal axis  54  may allow satisfactory capacitive coupling between electrodes  24  and  42  regardless of how stylus  10  is oriented or rotated about axis  54  on surface  206 . This may, for example, allow a user to charge stylus  10  without having to focus on how stylus  10  is physically placed within structure  210 . If desired, conductive layer  214  may be formed over the exterior surface of dielectric carrier  202 . Conductive layer  214  may provide structural support for structure  210  and may shield capacitor electrodes  24  and  42  from exterior interference, for example. 
       FIG. 17  is a diagram showing how wireless power transmission structures such as half-cylindrical structure  200  of  FIG. 15  or cylindrical structure  210  of  FIG. 16  may be integrated within a cover for tablet computer  12 . As shown in  FIG. 17 , tablet computer  12  may be mounted to cover (case)  220 . Case  220  may include a first portion  230  to which tablet computer  12  is mounted and a second portion  232  that rotates with respect to first portion  230  about fold axis  224  (as shown by arrow  226 ). When in a closed position, second portion  232  may serve to protect screen  32  of tablet  12  from damage. When in an open position, a user may view content displayed on screen  32 . 
     In one suitable arrangement, wireless power transmission structure  200  of  FIG. 15 or 210  of  FIG. 16  may be formed on cover  220  within region  222  adjacent to axis  224 . In another suitable arrangement, wireless power transmission structure  200  or  210  may be formed within region  228  on cover portion  232  (e.g., adjacent to an edge of cover portion  232  opposite to axis  224 ). In general, structures  200  and  210  may be formed at any desired location on cover  220 . Cover  220  may include conductive lines and/or other circuitry for conveying power from tablet  12  to capacitor electrodes  24  in structures  200 / 210 . 
     The examples of  FIGS. 2-17  in which device  10  is a computer stylus and device  12  is a tablet computer are merely illustrative. In general, wireless power receiving device  10  and wireless power transmitting device  12  may be any desired devices (e.g., a mobile telephone, a computer mouse, a desktop computer, a laptop computer, a peripheral device, a wireless keyboard, etc.). The arrangement of  FIGS. 1-17  may provide a sufficiently high end-to-end (overall) charging efficiency (e.g., 80% or higher) while also requiring less space within devices  10  and  12  than in scenarios where larger components such as inductive coils are used for performing wireless charging. Capacitor electrodes  24  and  42  may exhibit a sufficiently high capacitive coupling efficiency for wirelessly powering device  10  (e.g., a capacitive coupling efficiency of 80-95% or higher). 
     The foregoing is illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20200626
Publication Date: 20210713
Grant Date: 20210713
Priority Date: 20170427
Inventors: JIANG, BING
MARSHALL, BLAKE R.
SEN, INDRANIL S.
TAN, LIQUAN
NASIRI MAHALATI, REZA
JIANG, YI
NARANG, MOHIT
Assignee: APPLE INC
CPC Classifications: [{"code": "H02J7/0044", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02J50/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/03545", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/03545", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02J7/0044", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02J50/05", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J50/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "Y02B40/00", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02J50/05", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B5/0037", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J50/05", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/03545", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04B5/79", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B5/79", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 63916625