Patent Publication Number: US-2021175749-A1

Title: Wireless Power Systems

Description:
This application is a continuation of patent application Ser. No. 16/357,040, filed Mar. 18, 2019, which claims benefit of provisional patent application No. 62/668,611, filed May 8, 2018, both of which are hereby incorporated by reference herein in their entireties. 
    
    
     FIELD 
     This relates generally to power systems, and, more particularly, to wireless power systems. 
     BACKGROUND 
     Portable electronic devices such as cellular telephones, wristwatch devices, tablet computers, wireless earbuds, and other portable devices use batteries. The batteries in these devices can be charged using a battery charging system. To enhance convenience for users, wireless power systems have been provided that allow batteries in portable electronic devices to be charged wirelessly. 
     SUMMARY 
     A power system has a wireless power transmitting device and a wireless power receiving device. Coils in the power transmitting and receiving devices are used to transmit and receive wireless power signals. Good coupling between transmitting and receiving coils promotes wireless power transfer efficiency. 
     Embodiments of power transmission coils in the transmitting and receiving devices may include pot core coils, multi-core coils such as figure eight coils having clockwise and counterclockwise windings around respective magnetic cores, solenoids, and other coils. 
     In some embodiments, a solenoid array may extend under a charging surface in a wireless power transmitting device such as a charging mat. Solenoids in the array may be separated from each other by small gaps. Solenoids may have rectangular outlines, hexagonal outlines, or other shapes. Clusters of solenoids that are overlapped by wireless power receiving coils may be driven together to produce wireless power signals. Adjacent solenoids can be driven in-phase or, in some configurations, can be drive out-of-phase with each other. 
     In some embodiments, pot core coils have a core of magnetic material with a groove. Wire windings are formed in the groove. The groove may have a rotationally symmetric shape such as a circular shape or may have other suitable shapes. 
     In some embodiments, magnets and other alignment structures in the transmitting and receiving devices help align coils in the transmitting and receiving devices. 
     In some embodiments, a receiving device may have a figure eight coil for receiving power from a corresponding figure eight coil in a wireless power transmitting device and a non-figure-eight coil formed from a single loop of wire turns that is used in receiving power from non-figure-eight coil(s) in a wireless power transmitting device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative wireless power system in accordance with an embodiment. 
         FIG. 2  is a schematic diagram of illustrative wireless power transmitting circuitry in accordance with an embodiment. 
         FIG. 3  is a diagram of illustrative wireless power receiving circuitry in accordance with an embodiment. 
         FIG. 4A  is a rear view of an illustrative wireless power receiving device with coils for receiving wireless power in accordance with an embodiment. 
         FIG. 4B  is a top view of an illustrative coil formed from a cluster of four cores in accordance with an embodiment. 
         FIG. 5  is a side view of an illustrative coil for a wireless power system in accordance with an embodiment. 
         FIG. 6  is a diagram of an illustrative wireless power system in which a wireless power transmitting device is electromagnetically coupled to a wireless power receiving device and in which the transmitting and receiving devices have respective coils with figure eight wire patterns in accordance with an embodiment. 
         FIGS. 7 and 8  are perspective views of portions of illustrative coil arrays that extend across planar charging surfaces in wireless power transmitting devices in accordance with embodiments. 
         FIGS. 9 and 10  are top views of portions of illustrative coil arrays in wireless power transmitting devices in accordance with embodiments. 
         FIG. 11  is a cross-sectional side view of portions of two adjacent coils in accordance with an embodiment. 
         FIG. 12  is a cross-sectional side view of an illustrative mated pair of coils with pot cores that are transferring wireless power in a wireless power transmitting system in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Portable electronic devices have batteries. Wired and wireless charging systems may be used in charging the batteries. For example, a user may place devices such as wristwatch devices and cellular telephones on a wireless charging mat to wirelessly charge these devices. 
     An illustrative wireless power system is shown in  FIG. 1 . Wireless power system  8  (sometimes referred to as a wireless charging system) has wireless power transmitting equipment that is used for supplying wireless power. The wireless power is used for charging batteries in electronic devices and in supplying power to other device components. 
     As shown in  FIG. 8 , wireless power system  8  includes electronic devices  10 . Electronic devices  10  include electronic devices that provide power (e.g., charging mats, charging pucks, charging stands, tablet computers and other portable electronic devices with wireless power transmitting capabilities, etc.). Electronic devices  10  also include electronic devices that receive power. These power receiving devices may include, for example, portable electronic devices such as cellular telephones, wireless earbuds, and wristwatch devices (as examples). 
     Power can be used for powering circuitry in a power receiving device other than a battery and can be used for charging a battery in a power receiving device. Because battery charging is a common use of received power, wireless power transfer operations in system  8  are sometimes referred to as battery charging operations. Power can also be provided to a receiving device to operate a display or other circuitry in the receiving device without battery charging, if desired. 
     Charging can be performed by transferring power from a power transmitting device such as device  12  to a power receiving device such as device  24 . Power may be transferred between device  12  and device  24  wirelessly (e.g., using inductive charging). In the example of  FIG. 1 , power is being transferred wirelessly using wireless power signals  44 . 
     During operation of system  8 , wireless power transmitting device  12  wirelessly transmits power to one or more wireless power receiving devices such as device  24 . The wireless power receiving devices may include electronic devices such as wristwatches, cellular telephones, tablet computers, laptop computers, ear buds, battery cases for ear buds and other devices, tablet computer pencils and other input-output devices (e.g., accessory devices), wearable devices, or other electronic equipment. The wireless power transmitting device may be an electronic device such as a wireless charging mat that has a charging surface (e.g., a planar charging surface) that receives portable devices to be charged, a tablet computer or other portable electronic device with wireless power transmitting circuitry (e.g., one of devices  24  that has wireless power transmitting circuitry), or other wireless power transmitting device. The wireless power receiving devices use power from the wireless power transmitting device for powering internal components and for charging internal batteries. 
     As shown in  FIG. 1 , wireless power transmitting device  12  includes control circuitry  16 . Wireless power receiving device  24  includes control circuitry  30 . Control circuitry in system  8  such as control circuitry  16  and control circuitry  30  (and/or control circuitry in other devices  10 ) is used in controlling the operation of system  8 . This control circuitry may include processing circuitry associated with microprocessors, power management units, baseband processors, digital signal processors, microcontrollers, and/or application-specific integrated circuits with processing circuits. The processing circuitry implements desired control and communications features in devices  12  and  24 . For example, the processing circuitry may be used in selecting coils, adjusting the phases and magnitudes of coil drive signals, determining power transmission levels, processing sensor data and other data, processing user input, handling negotiations between devices  12  and  24 , sending and receiving in-band and out-of-band data, making measurements, starting and stopping charging operations, turning devices  10  on and off, placing devices  10  in low-power sleep modes, and otherwise controlling the operation of system  8 . 
     Control circuitry in system  8  may be configured to perform operations in system  8  using hardware (e.g., dedicated hardware or circuitry), firmware and/or software. Software code for performing operations in system  8  is stored on non-transitory computer readable storage media (e.g., tangible computer readable storage media) in control circuitry  8 . 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, or the like. Software stored on the non-transitory computer readable storage media may be executed on the processing circuitry of devices  10  (e.g., control circuitry  16  and/or  30 ). The processing circuitry may include application-specific integrated circuits with processing circuitry, one or more microprocessors, a central processing unit (CPU) or other processing circuitry. 
     Power transmitting device  12  may be a stand-alone power adapter (e.g., a wireless charging mat that includes power adapter circuitry), may be a wireless charging mat that is coupled to a power adapter or other equipment by a cable, may be a portable electronic device (cellular telephone, tablet computer, laptop computer, etc.), may be equipment that has been incorporated into furniture, a vehicle, or other system, or may be other wireless power transfer equipment. Illustrative configurations in which wireless power transmitting device  12  is a wireless charging mat or portable electronic device are sometimes described herein as an example. 
     Power receiving device  24  may be a portable electronic device such as a wristwatch, a cellular telephone, a laptop computer, a tablet computer, an accessory such as an earbud, a tablet computer input device such as a wireless tablet computer pencil, a battery case, or other electronic equipment. Power transmitting device  12  may be coupled to a wall outlet (e.g., an alternating current power source), may have a battery for supplying power, and/or may have another source of power. Power transmitting device  12  may have an alternating-current (AC) to direct-current (DC) power converter such as AC-DC power converter  14  for converting AC power from a wall outlet or other power source into DC power. In some configurations, AC-DC power converter  14  may be provided in an enclosure (e.g., a power brick enclosure) that is separate from the enclosure of device  12  (e.g., a wireless charging mat enclosure or portable electronic device enclosure) and a cable may be used to couple DC power from the power converter to device  12 . DC power may be used to power control circuitry  16 . 
     During operation, a controller in control circuitry  16  may use power transmitting circuitry  52  to transmit wireless power to power receiving circuitry  54  of device  24 . Power transmitting circuitry  52  may have switching circuitry (e.g., inverter circuitry  60  formed from transistors) that is turned on and off based on control signals provided by control circuitry  16  to create AC current signals through one or more transmit coils  42 . Coils  42  may be arranged in a planar coil array (e.g., in configurations in which device  12  is a wireless charging mat) or may be arranged in other configurations. In some arrangements, device  12  may have only a single coil. In arrangements in which device  12  has multiple coils, the coils may be arranged in one or more layers. Coils in different layers may or may not overlap with each other. 
     In some configurations, coils  42  are formed from solenoids that help direct magnetic fields vertically (e.g., parallel to the surface normal of a charging mat). Coils  48  can also be formed from solenoids. The solenoids in a charging mat may be formed in an array that lies under the charging surface of the charging mat and that extends across the charging surface of the charging mat. 
     Coils  42  and/or  48  can also be formed using figure eight winding patterns (e.g., wires wrapped around a pair of adjacent cores so that a first of the cores produces upwardly directed magnetic fields and a second of the cores produces downwardly directed magnetic fields. 
     In some configurations, coils  48  may be implemented using pot cores formed of magnetic material with circular grooves or grooves of other shapes. 
     As the AC currents pass through one or more coils  42 , a time varying electromagnetic (e.g., magnetic) field (signals  44 ) is produced that is received by one or more corresponding receiver coils such as coil  48  in power receiving device  24 . When the time varying electromagnetic field is received by coil  48 , corresponding alternating-current currents are induced in coil  48 . Rectifier circuitry such as rectifier  50 , which contains rectifying components such as synchronous rectification metal-oxide-semiconductor transistors arranged in a bridge network, converts received AC signals (received alternating-current signals associated with electromagnetic signals  44 ) from coil  48  into DC voltage signals for powering device  24 . 
     The DC voltages produced by rectifier  50  can be used in powering (charging) an energy storage device such as battery  58  and can be used in powering other components in device  24 . For example, device  24  may include input-output devices  56  such as a display, touch sensor, communications circuits, audio components, sensors, components that produce electromagnetic signals that are sensed by a touch sensor in tablet computer or other device with a touch sensor (e.g., to provide pencil input, etc.), and other components and these components may be powered by the DC voltages produced by rectifier  50  (and/or DC voltages produced by battery  58  or other energy storage device in device  24 ). 
     Device  12  and/or device  24  may communicate wirelessly (e.g., using in-band and out-of-band communications). Device  12  may, for example, have wireless transceiver circuitry  40  that wirelessly transmits out-of-band signals to device  24  using an antenna. Wireless transceiver circuitry  40  may be used to wirelessly receive out-of-band signals from device  24  using the antenna. Device  24  may have wireless transceiver circuitry  46  that transmits out-of-band signals to device  12 . Receiver circuitry in wireless transceiver  46  may use an antenna to receive out-of-band signals from device  12 . In some configurations, devices  10  can communicate through local area networks and/or wide area networks (e.g., the internet). 
     Wireless transceiver circuitry  40  can use one or more coils  42  to transmit in-band signals to wireless transceiver circuitry  46  that are received by wireless transceiver circuitry  46  using coil  48 . Any suitable modulation scheme may be used to support in-band communications between device  12  and device  24 . With one illustrative configuration, frequency-shift keying (FSK) is used to convey in-band data from device  12  to device  24  and amplitude-shift keying (ASK) is used to convey in-band data from device  24  to device  12 . Power may be conveyed wirelessly from device  12  to device  24  during these FSK and ASK transmissions. Other types of communications (e.g., other types of in-band communications) may be used, if desired. 
     During wireless power transmission operations, circuitry  52  supplies AC drive signals to one or more coils  42  at a given power transmission frequency. The power transmission frequency may be, for example, a predetermined frequency of about 125 kHz, at least 80 kHz, at least 100 kHz, less than 500 kHz, less than 300 kHz, or other suitable wireless power frequency. In some configurations, the power transmission frequency may be negotiated in communications between devices  12  and  24 . In other configurations, the power transmission frequency may be fixed. 
     During wireless power transfer operations, while power transmitting circuitry  52  is driving AC signals into one or more of coils  42  to produce signals  44  at the power transmission frequency, wireless transceiver circuitry  40  uses FSK modulation to modulate the power transmission frequency of the driving AC signals and thereby modulate the frequency of signals  44 . In device  24 , coil  48  is used to receive signals  44 . Power receiving circuitry  54  uses the received signals on coil  48  and rectifier  50  to produce DC power. At the same time, wireless transceiver circuitry  46  uses FSK demodulation to extract the transmitted in-band data from signals  44 . This approach allows FSK data (e.g., FSK data packets) to be transmitted in-band from device  12  to device  24  with coils  42  and  48  while power is simultaneously being wirelessly conveyed from device  12  to device  24  using coils  42  and  48 . Other types of in-band communications between device  12  and device  24  may be used, if desired. 
     In-band communications between device  24  and device  12  uses ASK modulation and demodulation techniques or other suitable in-band communications techniques. Wireless transceiver circuitry  46  transmits in-band data to device  12  by using a switch (e.g., one or more transistors in transceiver  46  that are coupled coil  48 ) to modulate the impedance of power receiving circuitry  54  (e.g., coil  48 ). This, in turn, modulates the amplitude of signal  44  and the amplitude of the AC signal passing through coil(s)  42 . Wireless transceiver circuitry  40  monitors the amplitude of the AC signal passing through coil(s)  42  and, using ASK demodulation, extracts the transmitted in-band data from these signals that was transmitted by wireless transceiver circuitry  46 . The use of ASK communications allows a stream of ASK data bits (e.g., a series of ASK data packets) to be transmitted in-band from device  24  to device  12  with coils  48  and  42  while power is simultaneously being wirelessly conveyed from device  12  to device  24  using coils  42  and  48 . 
     Control circuitry  16  has external object measurement circuitry  41  (sometimes referred to as foreign object detection circuitry or external object detection circuitry) that detects external objects on a charging surface associated with device  12 . Circuitry  41  can detect foreign objects such as coils, paper clips, and other metallic objects and can detect the presence of wireless power receiving devices  24 . During object detection and characterization operations, external object measurement circuitry  41  can be used to make measurements on coils  42  to determine whether any devices  24  are present on device  12  (e.g., whether devices  24  are suspected to be present on device  12 ). Measurement circuitry  43  in control circuitry  30  and/or in control circuitry  16  may also be used in making current and voltage measurements, and/or may be used in making other measurements on wireless power receiving circuitry  54 . 
     Illustrative wireless power transmitting circuitry  52  in a configuration in which wireless power transmitting device  12  has multiple coils  42  is shown in  FIG. 2 . With the illustrative arrangement of  FIG. 2 , circuitry  52  has inverter circuitry formed from multiple inverters  60 , each controlled by control circuitry  16  and each supplying drive signals to a corresponding wireless power transmitter circuit having a respective coil  42  and capacitance (e.g., capacitor  62 ). The phase and magnitude of the alternating-current drive signal supplied by each inverter  60  to its associated coil  42  can be adjusted independently by control circuitry  16 . As a result, one or more of coils  42  (e.g., coils in a cluster overlapped by coil  48  in device  24 ) can be activated while remaining coils are not driven and remain inactive. The phase of each active coil  42  can also be varied. For example, one coil may have a first phase and a second coil (e.g., an adjacent coil) may be driven with opposite phase (e.g., the second coil may have a second phase that is 180° out of phase with the first phase). Using arrangements such as these, control circuitry  16  can control the strength and orientation of the magnetic fields produced by coils  42 . 
     In device  24 , wireless power receiver circuitry  54  may have one or more coils  48 . As shown in  FIG. 3 , for example, rectifier circuitry  50  can be used to receive wireless power from one or more, two or more, or three or more respective power receiving circuits, each of which includes a respective coil  48  and associated capacitance (see, e.g., each capacitor  64 ). Rectifier circuitry  50  may contain a single rectifier shared between each power receiving circuit using switches and/or may contain multiple rectifiers, each of which is coupled to a respective power receiving circuit. During operation, rectifier circuitry  50  receives wireless power signals  44  from device  12  using coil(s)  48  and supplies corresponding output power (e.g., DC power) at output  66  for powering the circuitry of device  24 . Coils  48  in circuitry  54  may be of the same type and/or may include coils  48  of different types. For example, one of coils  48  may be a single circular or rectangular loop with multiple turns and another of coils  48  may have a pair of cores and associated windings with a figure eight pattern that form a figure eight coil (as examples). Control circuitry  30  can use rectifier circuitry  50  to switch desired coil(s)  48  into use dynamically (e.g., upon detecting the type of wireless power signals  44  and/or wireless power protocols being used by device  12 , etc.). In some configurations, the power handling capability of the different coils  48  and the associated rectifier circuitry of device  24  may differ. For example, a first coil  48  may be formed from a single loop of one or more turns and may have a maximum power transfer capability of 7.5 W, whereas a second coil  48  may be formed from figure eight windings of one or more turns and may have a maximum power transfer capability of more than 7.5 W (e.g., 15 W). 
       FIG. 4A  is a rear view of an illustrative wireless power receiving device (e.g., a cellular telephone, tablet computer, wristwatch, etc.). Housing  68  of device  24 , 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. In the example of  FIG. 4A , device  24  has a rectangular housing  68  with a rear wall facing outwardly from the page. Housing  68  may have other shapes, if desired. For example, housing  68  may have a circular outline, may have a shape with one or more curved edges and/or one or more straight edges, and/or may have other suitable shapes. Housing  68  may be formed using a unibody configuration in which some or all of housing  68  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.). 
     Device  24  may have coils  48  for receiving wireless power. In the example of  FIG. 4A , one of coils  48  is formed from a single isolated loop of one or more turns of wire  72  and another of coils  48  has a figure eight pattern of wire windings with one or more turns (sometimes referred to as a figure-eight coil). During operation in system  8 , an appropriate coil is switched into use by control circuitry  30 . For example, if device  24  is located on a charging mat that has wireless power transmitting circuitry matched to a figure eight coil, the figure eight coil can be switched into use. In response to detecting that device  24  is located on a charging mat that is transmitting wireless power signals suitable for reception with the single isolated loop, the single loop coil can be switched into use. Configurations in which both coils are simultaneously used in receiving power can also be used. 
     A coil with figure eight windings (e.g., the lower of coils  48  in  FIG. 4A ) has a first core of magnetic material (e.g., iron, ferrite, etc.) with one or more turns of counterclockwise windings  74  and has a second core of magnetic material with one or more turns of clockwise windings  76 . Wire segment  78  is used to join the windings around the first core with the windings around the second core (e.g., a single continuous wire can be used in forming the windings on both cores). During operation, the core of coil  48  that is associated with windings  74  receives a magnetic field that is opposite in phase to the core of coil  48  that is associated with windings  76 . By aligning the figure eight coil of device  24  to a corresponding figure eight coil of device  12 , wireless power can be transferred efficiently. For example, a high coupling efficiency can be obtained (e.g., coupling coefficient k may be at least 0.8 or at least 0.9). If desired, the windings around the first and second cores can be used independently (e.g., first and second windings on first and second respective cores for coils  42  can be driven out of phase in device  12  or first and second windings on first and second respective cores for coils  48  can be used to rectify out of phase signals in device  24  without physically joining the first and second windings with a joining wire segment). Another possible arrangement for coils  42  and  48  involves creating a winding pattern for each coil that includes a pair of cores with counterclockwise windings and a pair of cores with clockwise windings. As shown in  FIG. 4B , for example, coil  48  (and coil  42 ) may be formed from a cluster of four cores in which two cores  77  at the 12:00 and 6:00 positions have clockwise windings and two cores  75  at the 3:00 and 9:00 positions have counterclockwise windings. A single continuous wire may be wrapped around all four cores in the coil. Illustrative configurations in which device  12  and/or device  24  has figure eight coils are sometimes described herein as an example. 
     To help align figure eight coils in system  8 , magnets  80  (and/or alignment components formed from corresponding magnetic materials such as iron bars), or other alignment mechanisms (e.g., physical alignment structures having mating protrusions and recesses, etc.) can be included in device  12  and device  24 . Magnets  80  help a user align device  24  and its figure eight coil to a corresponding power transmitting figure eight coil in device  12 , thereby enhancing coupling efficiency. 
     In some configurations for device  24 , a display is formed on the front face of device  24  (e.g., on an opposing face of device  24  from the rear face of device  24  that is formed by the rear housing wall in housing  68 ). In the example of  FIG. 4A , display  70  has been formed on the front face of device  24  and overlaps coils  48 . Display  70  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  70  may have an active area that includes an array of pixels. Display  70  may be a liquid crystal display, a light-emitting diode display (e.g., an organic light-emitting diode display), an electrophoretic display, or a display formed using other display technologies. Display  70  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. In some configurations, the display cover layer covers the entire front face of device  24 . 
     In some embodiments, coils  42  and/or  48  are formed from cores of magnetic material wound with helical wire windings to form solenoids. An illustrative solenoid coil is shown in  FIG. 5 . Coil  82 , which may sometimes be referred to as a solenoid, has magnetic core  86  and wire windings  84 . Coils  42  and/or coils  48  of  FIG. 1  may be formed using coils such as illustrative coil  82  of  FIG. 5 . 
     Core  86  of coil  82  is formed from a magnetic material (e.g., ferrite or other material with a high permeability). Core  86  may have any suitable footprint (outline when viewed from above). The magnetic cores of the solenoids that are used for forming coils  48  and/or  42  may sometimes be referred to as posts. The height H and diameter D of each solenoid (e.g., the post of magnetic material forming the solenoid core) may have any suitable ratio R=H/D. For example, the value of R may be at least 0.1, at least 0.2, at least 0.5, at least 1, at least 3, less than 2, less than 1, less than 0.5, less than 0.3, less than 0.2, or other suitable value. The value of height H in a post may be, for example, at least 0.5 mm, at least 1 mm, at least 2 mm, at least 3 mm, at least 4 mm, at least 5 mm, less than 10 mm, less than 6 mm, less than 4 mm, or other suitable value. The maximum lateral dimension (e.g., the diameter of a circular post) of each post may be less than 5 cm, less than 3 cm, less than 2 cm, at least 1 cm, or other suitable size. 
     Conductive lines such as wires  84  (e.g., wires formed from insulated copper or other wire structures) are wound helically around core  86  for a number of turns N. The value of N may be at least 3, at least 7, at least 10, at least 20, less than 50, less than 15, less than 6, less than 4, or other suitable number. The number of turns in coil  42  and the number of turns in coil  48  may be the same or may differ. For example, coil  48  may have more turns than coil  42  to help raise the voltage of the DC power signals in device  24  and thereby lower FR losses in device  24 . As an example, if coil  42  has NT turns, coil  48  may have at least 1.2NT turns, at least 1.5NT turns, at least 2 NT turns, at least 3 NT turns, fewer than 5NT turns, etc. The wire used in forming coils  42  and  44  may be copper wire or other suitable wire (e.g., iron, iron-nickel, wire of other materials, multi-strand wire, etc.). The configuration of  FIG. 5  may be used to help produce magnetic fields B that are parallel to surface normal n of core  86 . In some arrangements, the use of vertically oriented magnetic fields and/or small maximum lateral coil dimensions may help enhance coupling efficiency and avoid situations in which eddy currents are induced in metal housing structures and other conductive structures in device  24 . 
       FIG. 6  is a cross-sectional side view of portions of devices  12  and  24  in system  8  showing how figure eight coils can be used in conveying wireless power signals  44  between devices  12  and  24 . In the example of  FIG. 6 , device  12  is a wireless charging mat having a planar housing that lies in the X-Y plane of  FIG. 6  (e.g., a housing  90  with planar opposing upper and lower surfaces). Housing  90  and housing  68  of device  24  has portions formed from dielectric, metal, or other materials. For example, housing  90  of device  12  may have a polymer upper wall covering magnets  80  and each coil  42  in an array of coils  42  extending laterally across the charging surface. The outer (upwardly facing) surface of the polymer (or other dielectric) that forms the upper wall defines a charging surface for device  12 . 
     During operation, alignment magnets  80  (e.g., permanent magnets configured to mate with opposing permanent magnets or with opposing magnetic material such as bars of magnetic material) are used to ensure that a first figure eight coil in device  12  (e.g., coil  42 ) is aligned with a second figure eight coil in device  24  (e.g., coil  48 ). Each figure eight coil has a pair of cores that are wound with wires in a figure eight pattern, as described in connection with the figure eight coil of  FIG. 4A . The cores and windings may have any suitable shapes (e.g., a solenoid configuration of the type described in connection with  FIG. 3  or other suitable shape). 
     Coil  42  includes first portion  42 - 1  with a first core and first wire windings  94  and a second portion  42 - 1  with a second core and second wire windings  96 . A layer of magnetic material  92  magnetically joins the respective cores in portions  42 - 1  and  42 - 2  to form a U-shaped magnetic core structure for figure eight coil  42 . Coil  48  includes first portion  48 - 1  with a first core and first wire windings  74  and a second core with second wire windings  76 . A layer of magnetic material  88  magnetically joins the respective cores in portions  48 - 1  and  48 - 2  to form a U-shaped magnetic core structure for figure eight coil  48 . During operation, coil  42  is driven with a current that produces magnetic field B. Due to the figure eight arrangement of the windings in coil  42 , magnetic field B is driven upwardly (in the positive Z direction of  FIG. 6 ) in coil portion  42 - 1  and is driven downwardly (in the negative Z direction of  FIG. 6 ) in coil portion  42 - 2 . Because coil  48  is aligned with coil  42 , magnetic field B flows in a loop through coils  48  and  42 , as shown in  FIG. 6 . In particular, magnetic field B flows upwardly in portion  42 - 1  and portion  48 - 1 , is conveyed horizontally through layer  88  to portion  48 - 2 , passes downwardly through portions  48 - 2  and  42 - 2 , and is conveyed horizontally back to portion  42 - 1  through layer  92 . 
     Layers  88  and  92  may be formed from ferrite or other magnetic material. With one illustrative configuration, layers  88  and/or  92  are formed from a crystalline foil of magnetic material having a thickness of 50-200 microns, at least 40 microns, at least 75 microns, less than 500 microns, less than 400 microns, less than 300 microns, or other suitable thickness. Layers  88  and/or  92  and/or the magnetic material forming the cores of coils  42  and  48  may have a relatively high permeability (e.g., at least 500, at least 600, at least 800, at least 1000, at least 1400, less than 2000, or other suitable value) and a high magnetic saturation value (e.g., a saturation flux density B sat  of 1.0 to 1.2 T, at least 0.5 T, at least 0.8 T, etc.). In some arrangements, layers  88  and/or  92  may be formed from M sublayers (where M is at least 2, at least 4, 5, less than 8, etc.). Magnetic material layers with a cracked structure may be used to help break up eddy currents. During operation, magnetic field B oscillates (because signals  44  are alternating current signals) and conveys power wirelessly from device  12  to device  24 . There is a high magnetic coupling with the configuration of  FIG. 6 , so wireless power transfer operations are efficient. 
     In some configurations for system  8 , device  12  has an array of coils  42 . Coils  42  may be solenoids (see, e.g.,  FIG. 5 ) or other suitable coils. In the arrangements of  FIG. 7 , coils  42  have rectangular outlines (e.g., the footprint of coils  42  is square when viewed from above). In the arrangement of  FIG. 8 , coils  42  have circular shapes. Other coil shapes may be used, if desired (e.g., triangular coils as shown in  FIG. 9 , hexagonal coils as shown in  FIG. 10 , etc.). These coils may be solenoids or other coils. The lateral dimensions (e.g., maximum width D of  FIG. 8 ) of the coils  42  in device  10  may have any suitable value (e.g., at least 0.5 cm, at least 1 cm, at least 2 cm, at least 4 cm, at least 8 cm, less than 20 cm, less than 10 cm, less than 5 cm, less than 3 cm, less than 2.5 cm, less than 1.5 cm, less than 2 cm, less than 0.8 cm, less than 0.4 cm, or other suitable value). 
     With one illustrative configuration, the core of coil  42  may have a maximum lateral dimension of 0.5-2 cm. The use of coils with relatively small lateral dimensions may help concentrate magnetic fields and enhance wireless charging efficiency (e.g., by avoiding scenarios in which magnetic fields induce unwanted eddy currents in conductive housing structures, etc.). If desired, multiple coils  42  may be driven in phase (in effect producing a larger single coil) when such coils are overlapped by a single larger coil  48  or are otherwise in a configuration in which each of the multiple coils  42  is well coupled to the wireless power receiving circuitry of device  24 . In general, any suitable pattern of coils  42  may be actively driven to produce signals  44  and these coils may be driven in phase or with any suitable set of out-of-phase drive signals. As an example, a cluster of at least 2, at least 3 at least 4, or other suitable number of coils  42  (e.g., a cluster that fits within a relatively small area such as a circle with a diameter of about 2-3 cm, etc.) may be driven in phase to provide magnetic field to a coil  48  that overlaps each of the coils  42  in the cluster. In another illustrative configuration, one or more adjacent coils may be driven 180° out of phase or with other suitable phase relative to one or more other adjacent coils. 
     To help reduce coupling inefficiency, coils in device  12  and/or device  24  can be packed tightly. As shown in  FIG. 11 , for example, the gap G between adjacent coils may be close to twice the diameter D of the wire used in forming coil windings (e.g., G may be at least 2D, at least 2.1 D, at least 2.2D, at least 2.5D, less than 5D, less than 4D, less than 3D, less than 2.5D, less than 2.2D, etc.). In the example of  FIG. 11 , a left-hand one of cores  86  has been wound with a first set of wires  84  and an adjacent right-hand one of cores  86  has been wound with a second set of wires  84  in close proximity to the first set of wires. The sidewall surfaces of cores  86  (surfaces  96 ) are separated by a relatively small gap G to enhance wireless power transfer efficiency. The value of G may be, for example, 100-600 microns, at least 25 microns, at least 50 microns, at least 200 microns, less than 2000 microns, less than 1000 microns, or less than 500 microns (as examples). 
     Pot cores can be used in forming the magnetic core structures in coils  42  and/or  48 . Consider, as an example, the pot core coils of  FIG. 12  (e.g., coil  42  in device  12  and coil  48  in device  24 ). As shown in  FIG. 12 , coil  42  has core  86 A and coil  48  has core  86 B. Cores  86 A and  86 B are formed from magnetic material. Recesses such as groove  100 A in core  86 A and groove  100 B in core  86 B may be configured to receive windings  84 A and  84 B, respectively. The grooves may have circular outlines or outlines of other suitable shapes (rectangular, hexagonal, triangular, square, etc.). For example, groove  100 A may be a circular groove and groove  100 B may be a circular groove when viewed in direction  102  (e.g., the Z axis). The use of a circular shape for grooves  100 A and  100 B may allow device  24  to rotate (about the Z axis) relative to device  14  (e.g., the circular shape of the pot core grooves may provide angular orientation independence). The alignment structures used in system  8  in this type of configuration may also exhibit angular independence. For example, each pot coil core may have a circular periphery that is surrounded by a circular alignment magnet  80  or sets of alignment magnets  80  may be used around the pot core coil that allow device  24  to be placed in multiple different angular orientations with respect to device  12  while still ensuring that the pot cores of coils  42  and  48  are aligned satisfactorily. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.