Patent Publication Number: US-2021194282-A1

Title: Wireless charging for devices with metal housings

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of and claims the benefit of International Application No. PCT/US2019/067057, filed Dec. 18, 2019. The disclosure of the foregoing application is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present specification relates to wireless charging for devices with metal housings. 
     BACKGROUND 
     Some modern electrical devices permit wireless charging for easier connectivity. In some cases, these devices may include a permanent magnet to align the devices with respect to a wireless charger. 
     Some devices have an exterior housing that includes metal. A metal exterior can provide appearance and durability. However, metal in the exterior of a device often interferes with wireless charging. 
     SUMMARY 
     In some implementations, a device includes a power receiving coil and a housing that has a metal exterior. The device can include features that enable efficient wireless charging despite the presence of the metal of the housing being within the region or footprint of a transmitting coil of a wireless charger. 
     A few examples of feature that help enable high power transfer efficiency are discussed briefly, and will be discussed further below. First, the metal housing can be formed of or broken into multiple electrically insulated segments to reduce eddy current formation. As shown in  FIG. 1 , the metal housing includes slits that separate pieces of the metal housing. Second, circuit boards or other metal elements within the electrical device can have cuts or slits to reduce the size of metal regions. Third, the electrical device can include a coil core for the power receiving coil that extends along the power receiving coil toward the exterior of the electrical device. This can enable the coil core to extend further toward the surface that is placed on the wireless charger, thus decreasing the effective distance between the coil core and the charger. Fourth, the electrical device can use a metal plate rather than a permanent magnet as an alignment feature to align with a wireless charger. In some implementations, the metal plate can be a laminated plate, such as one having electrically insulated metal strips, which can reduced eddy currents and power transfer loss in the metal plate. Fifth, the electrical device can include one or more layers of shielding to further reduce inefficiency in power transfer. Sixth, the dimensions and structure of the electrical device can be arranged to place the power receiving coil and an associated coil core close to the exterior surface of the electrical device  100 , thus allowing the power receiving coil and coil core to be near the power transfer coil of the charger. An electrical device can be made to enable efficient wireless charging by including any or all of these features, including any subcombination of the features. 
     As explained below, there are various aspects of the design that can improve charging efficiency and effectiveness. These include, for example, (1) a coil core defining a recess with at least a portion of the wireless power receiving coil disposed in the recess, (2) an alignment feature that includes a plurality of metal elements, and (3) a housing comprising a plurality metal portions (e.g., having slits cut to separate pieces of the housing). Each of these techniques, and others described below, can be used individually or in any appropriate combination. In other words, embodiments of the technology can include one or more of the techniques, in any combination or sub-combination. 
     One innovative aspect of the subject matter described in this specification can be embodied in a device comprising a wireless power receiving coil and an associated coil core, wherein the coil core defines a recess and at least a portion of the wireless power receiving coil is disposed in the recess; and a housing comprising one or more metal portions, wherein the housing is configured to receive at least a portion of the wireless power receiving coil and the associated coil core in an opening defined by the one or more metal portions. 
     In some implementations, the coil core has a substantially U-shaped cross-section. 
     In some implementations, the coil core has the U-shaped cross-section along at least a curved portion of the coil core. The recess is a curved channel defined along the curved portion. The power receiving coil is at least partially disposed in the curved channel. 
     In some implementations, the coil core is at least partially formed of ferrite. 
     In some implementations, the metal of the housing extends around a majority of the wireless power receiving coil and the associated coil core. 
     In some implementations, the metal of the housing extends around a majority of an outer perimeter of the wireless power receiving coil and the associated coil core. 
     In some implementations, the metal of the housing extends around substantially an entire outer perimeter of the wireless power receiving coil and the associated coil core. 
     In some implementations, the wireless power receiving coil has a substantially circular inner side and a substantially circular outer side, and the coil core extends (i) along a top side of the wireless power receiving coil and (ii) along the inner side of the wireless power receiving coil and/or the outer side of the wireless power receiving coil. 
     In some implementations, the wireless power receiving coil has top side and a bottom side opposite the top side, wherein the bottom side is configured to face toward a wireless charger when the device is in position to be receive power from the wireless charger. The wireless power receiving coil has a height from the top side to the bottom side, and wherein the coil core extends along a majority of the height of the wireless power receiving coil. 
     In some implementations, the coil core has an inner wall extending along an inner perimeter of the wireless power receiving coil. The coil core has an outer wall extending along an outer perimeter of the wireless power receiving coil. The wireless power receiving coil is at least partially disposed between the inner wall and the outer wall. 
     In some implementations, the device is a mobile device. 
     In some implementations, the device is a wearable device. 
     In some implementations, the device is a watch. 
     In some implementations, the device includes an alignment feature configured to interact with a magnet of a wireless charger to align the device with the wireless charger. 
     In some implementations, the alignment feature comprises a plurality of metal elements that are electrically insulated from each other. 
     In some implementations, the alignment feature is located at a center of the wireless power receiving coil. 
     In some implementations, the alignment feature is formed of steel strips that are electrically insulated from each other. 
     In some implementations, the alignment feature is a disc formed of laminated steel. 
     In some implementations, the housing includes a plurality of metal portions, wherein the housing is configured to receive at least a portion of the wireless power receiving coil and the associated coil core in an opening defined by the plurality of metal portions. 
     In some implementations, the plurality of metal portions are electrically insulated from each other. 
     In some implementations, the plurality of metal portions are placed at an exterior of the device and extend around substantially an outer perimeter of the device except for gaps between the metal portions, each of the gaps being 3 mm or less. 
     In some implementations each of the gaps are 1 mm or less. 
     In some implementations, the device is a watch that includes a first watch band segment coupled to the housing and a second watch band segment coupled to the housing. The housing has a first metal portion extending from the first watch band segment to the second watch band segment. The housing has a second metal portion extending from the first watch band segment to the second watch band segment. 
     In some implementations, the wireless power receiving coil and the associated coil core are part of an electronic assembly that is removable from the housing. 
     In some implementations, the device includes a circuit board having a ground plane, wherein the ground plane is divided into a plurality of separate sections. 
     In some implementations, the housing has a substantially circular outer perimeter and defines at least two slits that extend radially between metal portions of the housing. The device comprises a circuit board having a ground plane, wherein the circuit board has a substantially circular outer perimeter and defines at least two slits that extend radially and separate the ground plane into separate segments. 
     Another innovative aspect of the subject matter described in this specification can be embodied in a device comprising a wireless power receiving coil and an associated coil core; a housing comprising one or more metal portions, wherein the housing is configured to receive at least a portion of the wireless power receiving coil and the associated coil core in an opening defined by the one or more metal portions; and an alignment feature configured to interact with a magnet of a wireless charger to align the device with the wireless charger, wherein the alignment feature comprises a plurality of metal elements that are electrically insulated from each other. 
     In some implementations, the alignment feature is located at a center of the wireless power receiving coil. 
     In some implementations, the alignment feature is formed of steel strips that are electrically insulated from each other. 
     In some implementations, the alignment feature is a disc formed of laminated steel. 
     In some implementations, the device is a mobile device. 
     In some implementations, the device is a wearable device. 
     In some implementations, the device is a watch. 
     In some implementations, the coil core defines a recess and at least a portion of the wireless power receiving coil is disposed in the recess. 
     In some implementations, the coil core has a substantially U-shaped cross-section. 
     In some implementations, the coil core has the U-shaped cross-section along at least a curved portion of the coil core. The recess is a curved channel defined along the curved portion. The power receiving coil is at least partially disposed in the curved channel. 
     In some implementations, the coil core is at least partially formed of ferrite. 
     In some implementations, the metal of the housing extends around a majority of the wireless power receiving coil and the associated coil core. 
     In some implementations, the metal of the housing extends around a majority of an outer perimeter of the wireless power receiving coil and the associated coil core. 
     In some implementations, the metal of the housing extends around substantially an entire outer perimeter of the wireless power receiving coil and the associated coil core. 
     In some implementations, the wireless power receiving coil has a substantially circular inner side and a substantially circular outer side, and the coil core extends (i) along a top side of the wireless power receiving coil and (ii) along the inner side of the wireless power receiving coil and/or the outer side of the wireless power receiving coil. 
     In some implementations, the wireless power receiving coil has top side and a bottom side opposite the top side, wherein the bottom side is configured to face toward a wireless charger when the device is in position to be receive power from the wireless charger. The wireless power receiving coil has a height from the top side to the bottom side, and wherein the coil core extends along a majority of the height of the wireless power receiving coil. 
     In some implementations, the coil core has an inner wall extending along an inner perimeter of the wireless power receiving coil. The coil core has an outer wall extending along an outer perimeter of the wireless power receiving coil. The wireless power receiving coil is at least partially disposed between the inner wall and the outer wall. 
     In some implementations, the housing includes a plurality of metal portions, wherein the housing is configured to receive at least a portion of the wireless power receiving coil and the associated coil core in an opening defined by the plurality of metal portions. 
     In some implementations, the plurality of metal portions are electrically insulated from each other. 
     In some implementations, the plurality of metal portions are placed at an exterior of the device and extend around substantially an outer perimeter of the device except for gaps between the metal portions, each of the gaps being 3 mm or less. 
     In some implementations each of the gaps are 1 mm or less. 
     In some implementations, the device is a watch that includes a first watch band segment coupled to the housing and a second watch band segment coupled to the housing. The housing has a first metal portion extending from the first watch band segment to the second watch band segment. The housing has a second metal portion extending from the first watch band segment to the second watch band segment. 
     In some implementations, the wireless power receiving coil and the associated coil core are part of an electronic assembly that is removable from the housing. 
     In some implementations, the device includes a circuit board having a ground plane, wherein the ground plane is divided into a plurality of separate sections. 
     In some implementations, the housing has a substantially circular outer perimeter and defines at least two slits that extend radially between metal portions of the housing. The device comprises a circuit board having a ground plane, wherein the circuit board has a substantially circular outer perimeter and defines at least two slits that extend radially and separate the ground plane into separate segments. 
     Another innovative aspect of the subject matter described in this specification can be embodied in a device comprising a wireless power receiving coil and an associated coil core; and a housing comprising a plurality metal portions, wherein the housing is configured to receive at least a portion of the wireless power receiving coil and the associated coil core in an opening defined by the plurality of metal portions. 
     In some implementations, the plurality of metal portions are electrically insulated from each other. 
     In some implementations, the plurality of metal portions are placed at an exterior of the device and extend around substantially an outer perimeter of the device except for gaps between the metal portions, each of the gaps being 3 mm or less. 
     In some implementations each of the gaps are 1 mm or less. 
     In some implementations, the device is a watch that includes a first watch band segment coupled to the housing and a second watch band segment coupled to the housing. The housing has a first metal portion extending from the first watch band segment to the second watch band segment. The housing has a second metal portion extending from the first watch band segment to the second watch band segment. 
     In some implementations, the wireless power receiving coil and the associated coil core are part of an electronic assembly that is removable from the housing. 
     In some implementations, the device includes a circuit board having a ground plane, wherein the ground plane is divided into a plurality of separate sections. 
     In some implementations, the housing has a substantially circular outer perimeter and defines at least two slits that extend radially between metal portions of the housing. The device comprises a circuit board having a ground plane, wherein the circuit board has a substantially circular outer perimeter and defines at least two slits that extend radially and separate the ground plane into separate segments. 
     In some implementations, the device is a mobile device. 
     In some implementations, the device is a wearable device. 
     In some implementations, the device is a watch. 
     In some implementations, the device includes an alignment feature configured to interact with a magnet of a wireless charger to align the device with the wireless charger. 
     In some implementations, the alignment feature comprises a plurality of metal elements that are electrically insulated from each other. 
     In some implementations, the alignment feature is located at a center of the wireless power receiving coil. 
     In some implementations, the alignment feature is formed of steel strips that are electrically insulated from each other. 
     In some implementations, the alignment feature is a disc formed of laminated steel. 
     In some implementations, the coil core defines a recess and at least a portion of the wireless power receiving coil is disposed in the recess. 
     In some implementations, the coil core has a substantially U-shaped cross-section. 
     In some implementations, the coil core has the U-shaped cross-section along at least a curved portion of the coil core. The recess is a curved channel defined along the curved portion. The power receiving coil is at least partially disposed in the curved channel. 
     In some implementations, the coil core is at least partially formed of ferrite. 
     In some implementations, the metal of the housing extends around a majority of the wireless power receiving coil and the associated coil core. 
     In some implementations, the metal of the housing extends around a majority of an outer perimeter of the wireless power receiving coil and the associated coil core. 
     In some implementations, the metal of the housing extends around substantially an entire outer perimeter of the wireless power receiving coil and the associated coil core. 
     In some implementations, the wireless power receiving coil has a substantially circular inner side and a substantially circular outer side, and the coil core extends (i) along a top side of the wireless power receiving coil and (ii) along the inner side of the wireless power receiving coil and/or the outer side of the wireless power receiving coil. 
     In some implementations, the wireless power receiving coil has top side and a bottom side opposite the top side, wherein the bottom side is configured to face toward a wireless charger when the device is in position to be receive power from the wireless charger. The wireless power receiving coil has a height from the top side to the bottom side, and wherein the coil core extends along a majority of the height of the wireless power receiving coil. 
     In some implementations, the coil core has an inner wall extending along an inner perimeter of the wireless power receiving coil. The coil core has an outer wall extending along an outer perimeter of the wireless power receiving coil. The wireless power receiving coil is at least partially disposed between the inner wall and the outer wall. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a backside of an electrical device. 
         FIG. 2A  is a backside view of an electrical assembly of the electrical device. 
         FIG. 2B  is a side view of the electrical assembly of the electrical device. 
         FIG. 2C  shows a cross-sectional view of a coil core and a coil winding of the electrical assembly. 
         FIG. 3A  is a cross-sectional view of portions of a device and a wireless charger. 
         FIG. 3B  is a cross-sectional view of portions of the electrical device of  FIG. 1  and a wireless charger. 
         FIG. 4A  is a perspective view of the electrical device of  FIG. 1 . 
         FIG. 4B  is a diagram that illustrates an example of simulated eddy current loops on the metal housing of the electrical device. 
         FIG. 5  is a backside view of a watch that has a metal housing and is configured for wireless charging. 
         FIG. 6A  is a perspective view of an example of a steel plate and permanent magnet. 
         FIG. 6B  is a table that indicates attraction strengths of various configurations of steel plates with respect to a permanent magnet, expressed as percentages of force compared to the attraction between two permanent magnets. 
         FIG. 7  is a diagram that illustrates an example of a laminate metal plate used in the electrical device of  FIG. 1 . 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view of a back side of an electrical device  100 . In some implementations, the electrical device  100  includes a power receiving coil to receive power transferred wirelessly from a power source, such as a wireless charger. The device  100  can also have a housing  110  that includes metal. To facilitate wireless charging, the electrical device  100  can include various features and enhancements that improve the efficiency of wireless charging in the presence of the metal housing  110 . 
     A few examples of feature that help enable high power transfer efficiency are discussed briefly, and will be discussed further below. First, the metal housing  110  can be formed of or broken into multiple electrically insulated segments to reduce eddy current formation. As shown in  FIG. 1 , the metal housing  110  includes slits  130  that separate pieces of the metal housing  110 . Second, circuit boards or other metal elements within the electrical device  100  can have cuts or slits to reduce the size of metal regions. Third, the electrical device  110  can include a coil core for the power receiving coil that extends along the power receiving coil toward the exterior of the electrical device  110 . This can enable the coil core to extend further toward the surface that is placed on the wireless charger, thus decreasing the effective distance between the coil core and the charger. Fourth, the electrical device  100  can use a metal plate  170  rather than a permanent magnet as an alignment feature to align with a wireless charger. In some implementations, the metal plate  170  can be a laminated plate, such as one having electrically insulated metal strips, which can reduced eddy currents and power transfer loss in the metal plate  170 . Fifth, the electrical device  100  can include one or more layers of shielding to further reduce inefficiency in power transfer. Sixth, the dimensions and structure of the electrical device  100  can be arranged to place the power receiving coil and an associated coil core close to the exterior surface of the electrical device  100 , thus allowing the power receiving coil and coil core to be near the power transfer coil of the charger. An electrical device can be made to enable efficient wireless charging by including any or all of these features, including any subcombination of the features. 
     In general, wireless charging can be used to transmit power to electric devices without any wired connection. A power source, e.g., a charger, can transmit energy in the form of varying magnetic fields to a receiving coil of the electrical device  100 . Wireless charging is attractive for a wide range of applications, especially for low-power electrical devices such as smart watches, mobile phones, and portable devices. For low-power electrical devices, wireless charging improves the user experience and can provide better durability, e.g., better water-proofing and dust-proofing due to the ability to avoid charging ports allowing ingress of dust and water. In some cases, various brands or models of electrical devices can all be charged by a same wireless charger. 
     Wireless charging of low-power electrical devices typically involves inductive coupling between a power receiving coil in the device to be charged and a power transmitting coil in the wireless charger. The power transmitting coil of the charger delivers magnetic flux to the power receiving coil of the electrical device. Power transfer happens varying magnetic fields from the power transmitter induce voltage and current in the receiving coil of the electrical device. In many cases, the power receiving coil is disposed on or near a receiving coil core, which is configured to enhance the inductive coupling and potentially limit the impact of nearby metallic objects on the charging process. For example, the receiving coil core can help direct magnetic flux to the receiving coil. The output of the receiving coil can be rectified and used to charge the electrical device&#39;s battery. The electrical device can communicate with a charger to specify desired charging levels and to stop charging when the battery is charged to capacity. 
     There are various wireless charging standards to specify interoperable wireless power transfer and data communication between the charger and the electrical device. One example is the Qi wireless standard. The devices and chargers discussed in this document can be configured to comply with the Qi standard or interoperate with devices that do. 
     Wireless charging efficiency depends on a number of factors, including how far apart the electrical device is from the wireless charger and how well the receiving coil and the transmitting coil are aligned with each other. One way to quantify the efficiency is with a coupling factor, which can indicate how much of the magnetic flux transmitted from a transmitting coil reaches the receiving coil. A high coupling factor indicates efficient energy transfer and a tightly coupled electrical device and the charger, while a low coupling factor indicates the coupling is loose with low energy transfer efficiency. 
     In some devices, a magnet is included to help align the receiving coil of the device with a transmitting coil of the charger. For example, the receiving coil and transmitting coil can each have a permanent magnet at the center. The magnets have opposite magnetic poles facing each other to cause attraction of the device toward the proper alignment for charging and to hold the device in place on the charger. However, a permanent magnet in the electrical device provides a static magnetic field that may saturate the coil core of a receiving coil and reduce efficiency. For example, a permanent magnet can provide a static magnetic field that saturates a ferrite coil core and reduces the ability of the coil core to respond to changing magnetic fields from the transmitting coil, and thus reduces the resulting efficiency of the reception by the receiving coil. As a result, an alternative material or design is preferable to allow for attraction and alignment of the electrical device to the charger, while also allowing higher charging efficiency. As discussed further below, this may be achieved using metal, such as steel, for the alignment feature in the device to be charged instead of using a permanent magnet. In addition, efficiency can be further enhanced by providing the metal alignment feature with small, separate metal segments, such as strips of steel to form a laminated steel element. In the presence of a magnet in a charger, the laminated steel element can provide a force of attraction that is similar or equivalent to the force of attraction for an integral steel element or even another magnet. The laminated steel element also provides greater efficiency, because the small steel segments develop much smaller eddy currents than an integral steel element would. 
     Metal exteriors can provide a premium appearance for devices and improved product durability. However, metals can cause inductive heating and reduce wireless charging efficiency. During wireless charging, magnetic flux generates eddy currents in metal that can lead to additional heating in the electrical device and consume the energy intended for wireless charging. One technique that can reduce induced currents in a metal housing an improve efficiency is to form the metal housing of separate metal segments, e.g., electrically isolated segments, that present smaller regions of continuous metal. For example, the metal housing  110  can have slits  130  cut to separate the metal housing  110  into multiple sections. Limiting the size of metal elements can also limit the size of eddy currents developed in response to wireless charging energy. As shown in  FIG. 1 , the metal housing  110  may still extend around a majority of or substantially all of the electrical device  100 , but may do so with separate metal segments. 
     Referring still to  FIG. 1 , the electrical device  100  can represent a user device such as a watch. Other components of the watch, such as a watch face (e.g., a display screen), a watch band, a top-side housing, etc. are not shown in  FIG. 1 . Although various examples in this document describe a watch, the techniques for improving wireless charging efficiency in devices with a metal housing  100  can be use with other devices, such as phones, tablet computers, laptop computers, wearable devices, and other devices. 
     In the example of  FIG. 1 , the metal housing  110  provides an exterior for at least a portion of the electronic device  100 . The metal housing  110  is configured to receive and contain the internal components of the electrical device  100 , such as the electrical assembly  120 . 
     The metal housing  110  has a window defined through it, shown as center window  140 . This window  140  can be filled with a non-metallic material, such as glass, plastic, ceramic, etc. to allow magnetic flux to reach the power receiving coil through the window  140 . In some implementations, the window  140  is translucent, transparent, or substantially transparent to facilitate the operation of sensors (e.g., that may send and/or receive light or other signals through the window  140 , such as PPG sensors). 
     In  FIG. 1 , the metal housing  110  has a substantially round perimeter. The exterior surface of the metal housing  110  is curved and convex, with the largest outer diameter curving inward toward the center window  140  at the bottom of the housing  110 . The metal housing  110  can have a non-metallic material fixed to the center to provide the center window  140 . The metal housing  110 , the center window  140 , and their connections to other components of the electrical device  100  can be fitted to be water-proof and dust-proof. The metal housing  110  may be fabricated into any appropriate shape such as round, square, oval, rectangular, etc., including much more complex shapes. In some other implementations, the metal housing  110  may be in a rectangular shape, or other shapes. 
     The electrical assembly  120  includes various electrical components of the electrical device  100 . For example, the electrical assembly  120  includes a power receiving coil, sensors, and one or more printed circuit board (PCB). The electrical assembly  120  is located in the metal housing  110  and is protected by the metal housing  110 . Details of the electrical assembly  120  will be shown in later figures. 
       FIG. 2A  is a bottom view of the electrical assembly  120  of the electrical device  100 .  FIG. 2B  is a side view of the electrical assembly  120  of the electrical device  100 .  FIG. 2C  shows a cross-sectional view of a coil core  230  and a coil winding  240  of the electrical assembly  120 . 
     The electrical assembly  120  includes a power receiving coil that includes a coil winding  240  and an associated coil core  230 . The coil core  230  and the coil winding  240  operate as a power receiving unit to inductively couple with the power transmitting coil of a wireless charger and receive power transmitted through varying magnetic fields. The current developed in the coil winding  240  can be provided to a rectifier and other electronics of the electrical device  100  to generate power to charge a battery of the electrical device  100  and to operate the electrical device  100 . 
     The coil core  230  may be made of ferrite or nanocrystalline magnetic materials. The coil core  230  can direct and confine magnetic flux from the power transmitting coil to the region where the coil winding  240  is located, to enhance coupling of the coil winding  240  with the power transmitting coil of a charger. The ferrite can increase the effective inductance of the coil winding  240  and in turn enhance mutual inductance between the receiving unit and a charger. Furthermore, the ferrite coil core  230  can help magnetically shield the coil winding  240  from nearby metallic objects in the electrical device  100 . 
     In the example of  FIGS. 2A-2C , the coil core  230  has a generally annular shape as shown in the bottom view of the electrical device  100  in  FIG. 2A . The coil core  230  defines an annular recess or opening to receive at least a portion of the coil core  230 . This recess  232  is shown in the cross-section of  FIG. 2C . The coil core  230  can have a substantially U-shaped cross-section. For example, the coil core  230  can have an annular inner wall  233  and an annular outer wall  234  that both extend downward, e.g., from a top wall  235  along axis  205 , toward the bottom surface of the electrical device  100 . The side walls  233 ,  234  can extend along a majority of the height of the coil winding  240  along the axis  205 , or along all or substantially all of the height of the coil winding  240  along the axis  205 . In some implementations, the bottom surface of the side walls  233 ,  234  of the coil core  230  may extend to or beyond a bottom surface of the coil winding  240 . 
     The coil core  230 , and thus the coil winding  240 , can be sized and positioned to be located at the region of the window  140  (see  FIG. 1 ), which provides the core  230  and winding  240  at a metal-free region of the exterior of the electrical device  100 . The annular outer wall  234  can extend to or nearly to the outer perimeter of the window  140  to maximize the size of the coil core  230  and coil winding  240  given the size of the window  140 . The side walls  233 ,  234  of the coil core  230  and the coil winding  240  can also extend downward along axis  205  to the window  140 , so that the core  230  and winding  240  are as close to the charger as possible during wireless charging. 
     The electrical assembly  120  includes various layers and components. From top to bottom, as shown in  FIGS. 2B and 2C , the electrical assembly includes a ground plane  215 , a PCB  210 , electromagnetic shielding  250 , a second ground plane  225 , additional electromagnetic shielding  260 , the coil core  230 , and the coil winding  240 . As shown in  FIGS. 2A and 2C , one or more sensors  280  and a wireless charging alignment feature, such as a metal plate  170  or a magnet, can be located in an opening defined by the inner surface of the annular inner wall  233  of the coil core  230 . 
     As illustrated in  FIGS. 2B and 2C , the coil winding  240  is disposed in or on the coil core  230 , and the coil core  230  is disposed on a magnetic shielding layer  250 . The magnetic shielding layer  250  is configured to reduce or block magnetic flux. The magnetic shielding layer  260  is also configured to reduce or block magnetic flux. In some implementations, the PCB shielding layers  250  and  260  are formed of ferrite. In some other implementations, the PCB shielding layers  250  and  260  are formed of nanocrystalline materials, which may provide a higher amount of shielding than ferrite for a given thickness. The shielding layers  560 ,  260  and the ground plane  225  can be printed circuit board layers. The ground plane  225  located between the shielding layers  250 ,  260  can be a ground plane for the one or more sensors  280 . The ground plane  225  can be one layer of a flex printed circuit board (PCB) for the one or more sensors. The flex PCB may have multiple layers, such as 4 layers, with top and bottom layers for signal routing and the center two layers for PCB ground plans. Here, only a ground plane  225  is shown for clarity in illustration, but the ground plane  225  shown may represent a multilayer circuit board with multiple ground planes and/or other layers. 
     The one or more sensors can include one or more photoplethysmography (PPG) sensors and the ground plane  225  can be a PPG ground plane used by these sensors. In a watch, the PPG sensors can face toward a user&#39;s skin and perform physiological measurements through the window  140 . For example, a PPG sensor may include light emitting diodes and a receiver, at least some of which may be arranged in cross-shaped profile, to detect a user&#39;s pulse. 
     In some implementations, the PCB  210  is a multiplayer PCB that permits high component density in the electrical device  100 . The PCB  210  includes a ground plane  215 . Various electronics, such as a processor, memory, voltage regulators, a rectifier, a battery, a battery charging circuit, a display screen, a camera, a microphone, user input devices, and so on can be coupled to the PCB  210 , including potentially on layers above the ground plane  215 . As shown in  FIG. 2A , the ground plane  215  can include slits  290  that break up the ground plane and reduce the size of contiguous metal regions present. The slits  290  can separate the ground plane into several separate regions, to disrupt the formation of eddy currents in the ground plane  215  due to the application of wireless power. This can reduce eddy currents to smaller loops on each isolated ground plane regions, to reduce the induction, with resulting heating and power transfer loss, due to eddy currents flowing in the ground plane  215 . The ground plane  225  may similarly have slits or be otherwise be formed with separate regions to reduce eddy current formation. 
     The electrical device  100  also includes a metal plate  170  or other wireless charging alignment feature configured to cooperate with a magnet in a wireless charger to position and/or hold the electrical device  100  in the correct position for wireless charging. The metal plate  170  may be located at or near the center of the electrical assembly  120  and/or the electrical device  100 , such as in within an opening defined by the annular inner wall  233  of the coil core  230 . The metal plate  170  may be designed with any appropriate shape, such as round, oval, and square. 
     Generally, the metal plate  170  is configured to align the receiving coil winding  240  of the electrical device  100  to a transmitting coil of a charger, by a magnetic force generated between the metal plate  170  and a magnet (e.g., a permanent magnet or electromagnet) of the charger. For example, the magnetic attraction can align the electrical device  100  laterally along the plane of the surface of the wireless charger. The generated magnetic force can also help attract the electrical device  100  to the charger and reduce an air gap between the bottom of the electrical device  110  and a top surface of the charger. In some implementations, the diameter of the metal plate  170  is between 2 mm and 6 mm, or between 3 mm and 5 mm, or approximately 4 mm. In some implementations, the thickness of the metal plate  170  is between 0.25 mm and 3 mm, or between 0.5 mm and 2 mm, or approximately 1 mm. In some implementations, the metal plate  170  is formed as a laminated metal plate with a dielectric material between metal strips or segments to provide electrical insulation. More details regarding the laminated metal plate are presented in  FIGS. 5 and 6 . 
       FIG. 3A  is a cross-sectional view of portions of a device  300  and a wireless charger  305 . The device  300  includes a receiving coil core  330  and a receiving coil winding  340 . At the exterior of the device  300  there is a metal housing  303  and a non-metallic window  307 . The receiving coil winding  340 , as shown in  FIG. 2A , has a rectangular cross-section and, unlike the coil core  230  of  FIG. 2A-2C , does not extend downward along the sides of the coil winding  340 . The receiving coil winding  340  has a width of Wi. The material of the window  307  and the upper surface of the charger  305  provides a gap  350  between the receiving coil core  330  and the transmitting coil  310 . 
       FIG. 3B  is a cross-sectional view of portions of the electrical device  100  of  FIG. 1  and a wireless charger  305 . The electrical device  100  has several differences from the device  300  that provide higher wireless power transfer efficiency. As shown in  FIGS. 2A-2C , the receiving coil winding  240  is at least partially disposed in the recess (e.g., a groove or channel) of the receiving coil core  230 . The receiving coil core  230  has a width, w 2 , that is wider than the receiving coil core  330  of  FIG. 3A . This can enable the receiving coil  230  to receive larger amounts of magnetic flux and provide better inductive coupling and higher reception efficiency than the coil  340 . 
     In  FIG. 3B , inner and outer walls of the receiving coil core  230  are extended toward the bottom side of the device  100 , so that the bottom surface of the coil core  230  is closer to the transmitting coil  310  of the charger  305 . In this example, the bottom surface of the coil core  230  extends to the bottom surface of the receiving coil winding  240 . The gap  320  between the receiving coil  240  and the transmitting coil  310  is shorter than the gap  350  of  FIG. 3A . These changes allow the coil core  230  of the electrical device  100  to better direct and localize the transmitted magnetic fields from the transmitting coil  310 , allowing a greater amount of the magnetic flux to be received by the receiving coil  240  compared to the receiving coil  340  of  FIG. 3A . The reduced gap  320 , resulting from the extended ends of the U-shaped coil core  230 , leads to an increased magnetic field density at the receiving coil  240  and improved charging efficiency. 
       FIG. 4A  is a perspective view of the electrical device  100  of  FIG. 1 . The view shows two slits  130  cut through the metal of the metal housing  110 . The slits  130  extend through the metal to separate the metal into separate sections. It is generally desirable to be able to charge the device  100  using a charger that has a transmitting coil larger than the receiving coil  240  of the electrical device  100 . For example, it may be desirable to charge a watch with a general-purpose charger that has coils meant for phones, tablet computers, or other devices. As a result the transmitting coil will often be larger than the window  140  and the coil core  230  and receiving coil  240 . At least a portion of the transmitted magnetic field passes through the metal housing  110  and can induce currents in the metal housing  110 . For example, eddy currents can flow in closed loops in planes perpendicular to the magnetic field and cause heating. By including the slits  130 , or by otherwise breaking up the metal of the metal housing  120  into two or more separate sections, the amount of contiguous metal surface in the presence of the transmitted magnetic field is reduced and the resulting heating and current generation is reduced as well. 
     As shown in  FIG. 4A , two slits  130  are introduced to break the metal housing  110  radially through the metal housing ring. Where large eddy currents would have been possible in an unbroken ring, the current loops that occur in the smaller, separate metal housing segments are smaller. In some implementations, the slits  130  may have a width (e.g., a distance of separation between two adjacent sections) of 3 mm or less. In some implementations, the slits  130  each have a width that is 1 mm or less. The gaps between the metal housing segments that the slits  130  provide may be filled in by electrically insulating materials. In some implementations, multiple metal segments may be combined to form a substantially continuous metal housing  110 , e.g., a housing that extends around an entire perimeter of the device  100  except in the regions of the slits  130 . 
       FIG. 4B  is a diagram that illustrates an example of simulated eddy current loops on the metal housing of the electrical device. The simulation results reveals local current loops in the metal housing segments. For example, as shown in  FIG. 4B , the eddy current loops are isolated in high current density paths that are separated by the slits  130  at 11 o&#39;clock and 5 o&#39;clock of the metal housing  110 . The slits  130  provide greater coupling efficiency and thus higher charging efficiency than would be achieved with an integral metal housing. For example, simply adding two slits  130  to separate the metal housing  110  into two metal sections can improve the coupling coefficient (e.g., coupling efficiency) by 5% or more. Additional slits  130  may be provided in some implementations, to break the metal of the housing  110  into a larger number of sections. The metal housing  110  and the electrical assembly  120  are isolated by a layer of dielectric materials, for example, a layer of diamond-like carbon. The different metal sections of the housing  110  can be electrically insulated from each other. The metal sections of the housing  110   
       FIG. 5  is a diagram that illustrates an example of a backside view of the device  100  configures as a watch  500  with slits  130  on the metal housing  110 . In this example, two slits  130  are aligned along a length direction of the watch band  510 . As shown in  FIG. 5 , the band attachments  520  connect the watch band  510  to the metal housing  110 . The slits located close to the center of the band attachments  420  on the metal housing  110 . As a result, forces that pull on the two portions of the watchband will pull on both metal segments of the metal housing  110 , without pulling the metal segments away from each other. In other words, forces on the watch band are transmitted to both metal segments in a direction that is along the two integral metal elements rather than pulling the two metal segments in opposing directions from each other (e.g. in a direction that would tend to separate the two pieces of the metal housing  110 ). As discussed earlier, the slits  130  extend entirely through the metal of the metal housing  110 . 
     The electrical device  100 , as shown in  FIG. 2A , includes an alignment feature, such as a metal plate  170 . The metal plate  170  is disposed at the center of the coil core  230  and on the PPG ground plane  225 . The metal plate  170  is configured to help attract and align the electrical device  100  on a charger, through a magnetic force between the metal plate and a permanent magnet in the charger. 
       FIG. 6A  is a diagram that illustrates an example of a Steel—Permanent Magnet model  600  for magnetic force simulation. This model  600  includes a Permanent Magnet  610  and a steel plate  620 . The steel plate  620  and the permanent magnet  610  are attracted to each other by the magnetic force, with an interface layer that represents the interface, e.g., an air gap, in the wireless charging. The dimensions of the steel plate  620  are the key parameters to achieve a comparable magnetic force in the model  600  compare to a Permanent Magnet—Permanent Magnet model. In the model  600 , the diameter of the permanent magnet  610  is 0.5 inch and the thickness of the permanent magnet  610  is 0.125 inch. In this illustration, the dimensions of the steel plate  620  are varied to simulate the magnetic forces as functions of dimensions of the steel plate  620 . The reference magnetic force comes from the Permanent Magnet—Permanent Magnet model wherein the permanent magnets are in same size as permanent magnet  610 . In this simulation, the diameter of the steel plate  620  is changing from 0.5 inch up to 1 inch, and the thickness of the steel plate  620  is varied from 0.012 inch to 0.194 inch. 
       FIG. 6B  is a diagram that illustrates a simulation result of the Steel—Permanent Magnet model  600  compared to the Permanent Magnet—Permanent Magnet model.  FIG. 6B  reveals that, with a fixed steel plate thickness, e.g. 0.06 inch, the magnetic force of model  600  increases as the diameter of the steel plate  620  increases. Similarly, with a fixed steel plate diameter, e.g., 0.6 inch, the magnetic force of the model  600  increases as the thickness of the steel plate  620  increases. The trends observed in the simulation results recommend a steel plate with diameter equal to or up to 20% more than that of the diameter of the permanent magnet  610 , and a thickness roughly same as the thickness of the permanent magnet  610 , in order to achieve a comparable magnetic force compare to the Permanent Magnet—Permanent Magnet model. 
       FIG. 7  is a diagram that illustrates an example of a laminated metal plate  710  in the electrical device  100 . As discussed earlier, a metal plate  170  in the electrical device  100  is configured to align the electrical device  100  to a charger through the magnetic force the metal plate  170  and a permanent magnet in the charger. The electromagnetic field passes through the metal plate and generates eddy current loops along the surface of the metal plate  170 . The eddy current loops will cause induction heating and consume the wireless charging energy. 
     In this illustration, the metal plate  170  is broken into a plurality of metal strips to reduce the induction heating and loss when in the presence of a varying magnetic field. As shown in  FIG. 7 , a laminated steel plate  710  can be used in the electrical device  100  and can be located at the center of the receiving coil windings  240 . The laminated steel plate  710  includes a plurality of steel strips or layers, and the steel strips are connected by electrically insulating material in between. The plurality of steel strips and the insulating material form the laminate steel plate  710 . The laminated steel plate  710  allows for an attractive force to a magnet of the charger that is near to or equivalent to the amount of magnetic force that would occur using a solid steel disc, but with significantly lower loss and heating. In this example, the gaps between the neighboring metal strips may small, for example, in a range from 1 mm to lum. 
     In the laminated metal plate  710 , the small metal strips provide a much higher impedance compared with that of a solid steel disc. This lowers eddy currents and contributes to a reduction of the induction heating during the wireless charging. In this illustration, the laminated metal plate  710  maintains a same size and shape as those of the metal plate  170 , and thus leads to a same magnetic force between the laminated metal plate  710  and the permanent magnet of the charger. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, various forms of the flows shown above may be used, with steps re-ordered, added, or removed. 
     Embodiments of the invention and all of the functional operations described in this specification can be implemented in electronic circuitry, or in computer software, firmware, and/or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. A circuit board can be implemented as a mixed-signal chip (e.g., a CMOS integrated circuit) that includes analog, digital, and mixed-signal circuits, as well as potentially firmware or embedded software. For example, operations of the control circuitry may be implemented using digital circuitry, an FPGA (field programmable gate array) or other programmable logic device, a processor and corresponding software, and so on. 
     The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). 
     While this specification contains many specifics, these should not be construed as limitations on the scope of the invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     In each instance where an HTML file is mentioned, other file types or formats may be substituted. For instance, an HTML file may be replaced by an XML, JSON, plain text, or other types of files. Moreover, where a table or hash table is mentioned, other data structures (such as spreadsheets, relational databases, or structured files) may be used. 
     Particular embodiments of the invention have been described. Other embodiments are within the scope of the following claims. For example, the steps recited in the claims can be performed in a different order and still achieve desirable results.