PATENT DOCUMENT

Publication Number: US-10340711-B2
Application Number: US-201715699995-A
Country: US
Kind Code: B2

Title: Faraday cage for wireless charging devices

Abstract:
Embodiments describe a wireless charging device including: a housing having a planar charging surface and one or more walls that define an interior cavity; a transmitter coil arrangement positioned within the interior cavity; and a faraday cage enclosing the transmitter coil arrangement. The faraday cage includes: an electromagnetic shield positioned between the transmitter coil arrangement and the first shell; an interconnection structure positioned within the interior cavity below the transmitter coil arrangement, the interconnection structure including a plurality of packaged electrical components mounted on the interconnection structure; a ferromagnetic shield positioned between the transmitter coil arrangement and the interconnection structure; and a conductive grounding fence disposed around a perimeter of the interconnection structure and between the electromagnetic shield and the interconnection structure.

Claims:
What is claimed is: 
     
       1. A wireless charging device comprising:
 a housing having a planar charging surface and one or more walls that define an interior cavity; 
 a transmitter coil arrangement positioned within the interior cavity, the transmitter coil arrangement including a plurality of transmitter coils positioned within the interior cavity in an overlapping arrangement such that different coils in the plurality of coils are on different planes and each of the plurality of transmitter coils has a central axis positioned a lateral distance away from the central axes of all other transmitter coils of the plurality of transmitter coils; and 
 a faraday cage enclosing the transmitter coil arrangement, the faraday cage comprising: 
 an electromagnetic shield positioned between the transmitter coil arrangement and a first shell; 
 an interconnection structure positioned within the interior cavity below the transmitter coil arrangement, the interconnection structure including a plurality of packaged electrical components mounted on the interconnection structure; 
 a ferromagnetic shield positioned between the transmitter coil arrangement and the interconnection structure; and 
 a conductive grounding fence disposed around a perimeter of the interconnection structure and between the electromagnetic shield and the interconnection structure. 
 
     
     
       2. The wireless charging device of  claim 1 , wherein the conductive grounding fence is directly coupled to both the electromagnetic shield and the interconnection structure. 
     
     
       3. The wireless charging device of  claim 1 , wherein the transmitter coil arrangement is configured to generate a time-varying magnetic flux and an electric field;
 the electromagnetic shield is configured to allow the time-varying magnetic flux to pass through the electromagnetic shield while preventing the electric field from passing through the electromagnetic shield; 
 the plurality of packaged electrical components is configured to operate the plurality of transmitter coils during wireless power transfer; 
 the ferromagnetic shield is configured to redirect the time-varying magnetic flux to prevent the time-varying magnetic flux from propagating through to the interconnection structure; 
 the conductive grounding fence prevents the time-varying magnetic flux from propagating through the conductive grounding fence; 
 and the faraday cage allows the time-varying magnetic flux to propagate out of the faraday cage in one direction, while preventing the propagation of magnetic flux out of the faraday cage in all other directions. 
 
     
     
       4. The wireless charging device of  claim 1 , wherein the electromagnetic shield is laser welded onto a top surface of the conductive grounding fence. 
     
     
       5. The wireless charging device of  claim 3 , wherein the electromagnetic shield is configured to capture the electric field generated by the transmitter coil arrangement, which exists in the form of voltage on the electromagnetic shield. 
     
     
       6. The wireless charging device of  claim 5 , wherein the conductive grounding fence is configured to discharge the voltage to ground. 
     
     
       7. The wireless charging device of  claim 1 , wherein the conductive grounding fence is a length of wire. 
     
     
       8. The wireless charging device of  claim 1 , wherein the ferromagnetic shield is disposed on the interconnection structure and laterally from the conductive grounding fence. 
     
     
       9. The wireless charging device of  claim 1 , wherein the electromagnetic shield comprises:
 an electromagnetic body configured to capture electromagnetic fields generated by the transmitter coil arrangement; and 
 a copper border surrounding the electromagnetic body, wherein the copper border couples with the conductive grounding fence. 
 
     
     
       10. A wireless charging device comprising:
 a housing having a planar charging surface and one or more walls that define an interior cavity; 
 a transmitter coil arrangement positioned within the interior cavity, the transmitter coil arrangement including a plurality of transmitter coils positioned within the interior cavity in an overlapping arrangement such that different coils in the plurality of coils are on different planes and each of the plurality of transmitter coils has a central axis positioned a lateral distance away from the central axes of all other transmitter coils of the plurality of transmitter coils; and 
 a faraday cage enclosing the transmitter coil arrangement, the faraday cage comprising:
 an electromagnetic shield positioned between the transmitter coil arrangement and a first shell, wherein the electromagnetic shield comprises:
 an electromagnetic body configured to capture electromagnetic fields generated by the transmitter coil arrangement; and 
 a copper border surrounding the electromagnetic body; 
 
 an interconnection structure positioned within the interior cavity below the transmitter coil arrangement, the interconnection structure including a plurality of packaged electrical components mounted on the interconnection structure, wherein the interconnection structure is a printed circuit board (PCB); 
 a ferromagnetic shield positioned between the transmitter coil arrangement and the interconnection structure; and 
 a conductive grounding fence disposed around a perimeter of the interconnection structure and between the electromagnetic shield and the interconnection structure, wherein the conductive grounding fence is a length of wire formed of a conductive material, and wherein the copper border couples with the conductive grounding fence. 
 
 
     
     
       11. The wireless charging device of  claim 10 , wherein the conductive grounding fence is directly coupled to both the electromagnetic shield and the interconnection structure. 
     
     
       12. The wireless charging device of  claim 10 , wherein the transmitter coil arrangement is configured to generate a time-varying magnetic flux and an electric field;
 the electromagnetic shield is configured to allow the time-varying magnetic flux to pass through the electromagnetic shield while preventing the electric field from passing through the electromagnetic shield; 
 the plurality of packaged electrical components is configured to operate the plurality of transmitter coils during wireless power transfer; 
 the ferromagnetic shield is configured to redirect the time-varying magnetic flux to prevent the time-varying magnetic flux from propagating through to the interconnection structure; 
 the conductive grounding fence prevents the time-varying magnetic flux from propagating through the conductive grounding fence; 
 and the faraday cage allows the time-varying magnetic flux to propagate out of the faraday cage in one direction, while preventing the propagation of magnetic flux out of the faraday cage in all other directions. 
 
     
     
       13. A wireless charging system, comprising:
 an electrical device comprising a receiver coil configured to generate a current to charge a battery when exposed to a time-varying magnetic flux; and 
 a wireless charging mat configured to generate the time-varying magnetic flux to wirelessly charge the electrical device, the wireless charging mat comprising:
 a housing having a planar charging surface, the housing including first and second shells defining an interior cavity; 
 a transmitter coil arrangement, the transmitter coil arrangement including a plurality of transmitter coils positioned within the interior cavity in an overlapping arrangement such that different coils in the plurality of coils are on different planes and each of the plurality of transmitter coils has a central axis positioned a lateral distance away from the central axes of all other transmitter coils of the plurality of transmitter coils; and 
 a faraday cage enclosing the transmitter coil arrangement, the faraday cage comprising:
 an electromagnetic shield positioned between the transmitter coil arrangement and the first shell; 
 an interconnection structure positioned within the interior cavity below the transmitter coil arrangement, the interconnection structure including a plurality of packaged electrical components mounted on the interconnection structure; 
 a ferromagnetic shield positioned between the transmitter coil arrangement and the interconnection structure; and 
 a conductive grounding fence disposed around a perimeter of the interconnection structure and between the electromagnetic shield and the interconnection structure. 
 
 
 
     
     
       14. The wireless charging system of  claim 13 , wherein the conductive grounding fence is directly coupled to both the electromagnetic shield and the interconnection structure. 
     
     
       15. The wireless charging system of  claim 13 , wherein the transmitter coil arrangement is configured to generate a time-varying magnetic flux and an electric field;
 the electromagnetic shield is configured to allow the time-varying magnetic flux to pass through the electromagnetic shield while preventing the electric field from passing through the electromagnetic shield; 
 the plurality of packaged electrical components is configured to operate the plurality of transmitter coils during wireless power transfer; 
 the ferromagnetic shield is configured to redirect the time-varying magnetic flux to prevent the time-varying magnetic flux from propagating through to the interconnection structure; 
 the conductive grounding fence prevents the time-varying magnetic flux from propagating through the conductive grounding fence; 
 and the faraday cage allows the time-varying magnetic flux to propagate out of the faraday cage in one direction, while preventing the propagation of magnetic flux out of the faraday cage in all other directions. 
 
     
     
       16. The wireless charging system of  claim 13 , wherein the electromagnetic shield is configured to capture the electric field generated by the transmitter coil arrangement, which exists in the form of voltage on the electromagnetic shield. 
     
     
       17. The wireless charging system of  claim 16 , wherein the conductive grounding fence is configured to discharge the voltage to ground. 
     
     
       18. The wireless charging system of  claim 13 , wherein the conductive grounding fence is a length of wire. 
     
     
       19. The wireless charging system of  claim 13 , wherein the ferromagnetic shield is disposed on the interconnection structure and laterally from the conductive grounding fence. 
     
     
       20. The wireless charging system of  claim 13 , wherein the electromagnetic shield comprises:
 an electromagnetic body configured to capture electromagnetic fields generated by the transmitter coil arrangement; and 
 a copper border surrounding the electromagnetic body, wherein the copper border couples with the conductive grounding fence.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 62/399,243, filed on Sep. 23, 2016, U.S. Provisional Patent Application No. 62/399,245, filed on Sep. 23, 2016, U.S. Provisional Patent Application No. 62/399,248, filed on Sep. 23, 2016, U.S. Provisional Patent Application No. 62/399,255, filed on Sep. 23, 2016, U.S. Provisional Patent Application No. 62/399,259, filed on Sep. 23, 2016, U.S. Provisional Patent Application No. 62/399,263, filed on Sep. 23, 2016, U.S. Provisional Patent Application No. 62/399,269; filed on Sep. 23, 2016, U.S. Provisional Patent Application No. 62/399,273, filed on Sep. 23, 2016, U.S. Provisional Patent Application No. 62/399,276; filed on Sep. 23, 2016, and U.S. Provisional Patent Application No. 62/526,905; filed on Jun. 29, 2017, the disclosures of which are hereby incorporated by reference in their entirety and for all purposes. 
    
    
     BACKGROUND 
     Electronic devices (e.g., mobile phones, media players, electronic watches, and the like) operate when there is charge stored in their batteries. Some electronic devices include a rechargeable battery that can be recharged by coupling the electronic device to a power source through a physical connection, such as through a charging cord. Using a charging cord to charge a battery in an electronic device, however, requires the electronic device to be physically tethered to a power outlet. Additionally, using a charging cord requires the mobile device to have a connector, typically a receptacle connector, configured to mate with a connector, typically a plug connector, of the charging cord. The receptacle connector typically includes a cavity in the electronic device that provides an avenue within which dust and moisture can intrude and damage the device. Furthermore, a user of the electronic device has to physically connect the charging cable to the receptacle connector in order to charge the battery. 
     To avoid such shortcomings, wireless charging devices have been developed to wirelessly charge electronic devices without the need for a charging cord. For example, some electronic devices can be recharged by merely resting the device on a charging surface of a wireless charging device. A transmitter coil disposed below the charging surface may produce a time-varying magnetic field that induces a current in a corresponding receiving coil in the electronic device. The induced current can be used by the electronic device to charge its internal battery. 
     Some existing wireless charging devices have a number of disadvantages. For instance, some wireless charging devices require an electronic device to be placed in a very confined charging region on the charging surface in order for the electronic device being charged to receive power. If an electronic device is placed outside of the charging region, the electronic device may not wirelessly charge or may charge inefficiently and waste power. This limits the ease at which an electronic device can be charged by the wireless charging device. 
     SUMMARY 
     Some embodiments of the disclosure provide a wireless charging device that includes a charging surface having a broad charging region upon which an electronic device can be placed to wirelessly receive power. In some embodiments the wireless charging device can be a wireless charging mat that includes an arrangement of wireless power transmitters beneath the charging surface defining a charging region. The wireless charging mat allows the electronic device to be charged at any location within the charging region, thereby increasing the ease at which electronic devices can be charged by the mat. 
     In some embodiments a wireless charging device includes: a housing having a planar charging surface and one or more walls that define an interior cavity; a transmitter coil arrangement positioned within the interior cavity, the transmitter coil arrangement including a plurality of transmitter coils positioned within the interior cavity in an overlapping arrangement such that different coils in the plurality of coils are on different planes and each of the plurality of transmitter coils has a central axis positioned a lateral distance away from the central axes of all other transmitter coils of the plurality of transmitter coils; and a faraday cage enclosing the transmitter coil arrangement. The faraday cage includes: an electromagnetic shield positioned between the transmitter coil arrangement and the first shell; an interconnection structure positioned within the interior cavity below the transmitter coil arrangement, the interconnection structure including a plurality of packaged electrical components mounted on the interconnection structure; a ferromagnetic shield positioned between the transmitter coil arrangement and the interconnection structure; and a conductive grounding fence disposed around a perimeter of the interconnection structure and between the electromagnetic shield and the interconnection structure. 
     In some additional embodiments, a wireless charging device includes: a housing having a planar charging surface and one or more walls that define an interior cavity; a transmitter coil arrangement positioned within the interior cavity, the transmitter coil arrangement including a plurality of transmitter coils positioned within the interior cavity in an overlapping arrangement such that different coils in the plurality of coils are on different planes and each of the plurality of transmitter coils has a central axis positioned a lateral distance away from the central axes of all other transmitter coils of the plurality of transmitter coils; and a faraday cage enclosing the transmitter coil arrangement. The faraday cage includes: an electromagnetic shield positioned between the transmitter coil arrangement and the first shell, where the electromagnetic shield comprises: an electromagnetic body configured to capture electromagnetic fields generated by the transmitter coil arrangement; and a copper border surrounding the electromagnetic body, where the copper border couples with the grounding fence; an interconnection structure positioned within the interior cavity below the transmitter coil arrangement, the interconnection structure including a plurality of packaged electrical components mounted on the interconnection structure, where the interconnection structure is a printed circuit board (PCB); a ferromagnetic shield positioned between the transmitter coil arrangement and the interconnection structure; and a conductive grounding fence disposed around a perimeter of the interconnection structure and between the electromagnetic shield and the interconnection structure, where the conductive grounding fence is a length of wire formed of a conductive material. 
     In some further embodiments, a wireless charging system includes: an electrical device comprising a receiver coil configured to generate a current to charge a battery when exposed to a time-varying magnetic flux; and a wireless charging mat configured to generate the time-varying magnetic flux to wirelessly charge the electronic device. The wireless charging mat includes: a housing having a planar charging surface, the housing including first and second shells defining an interior cavity; a transmitter coil arrangement, the transmitter coil arrangement including a plurality of transmitter coils positioned within the interior cavity in an overlapping arrangement such that different coils in the plurality of coils are on different planes and each of the plurality of transmitter coils has a central axis positioned a lateral distance away from the central axes of all other transmitter coils of the plurality of transmitter coils; and a faraday cage enclosing the transmitter coil arrangement. The faraday cage includes: an electromagnetic shield positioned between the transmitter coil arrangement and the first shell; an interconnection structure positioned within the interior cavity below the transmitter coil arrangement, the interconnection structure including a plurality of packaged electrical components mounted on the interconnection structure; a ferromagnetic shield positioned between the transmitter coil arrangement and the interconnection structure; and a conductive grounding fence disposed around a perimeter of the interconnection structure and between the electromagnetic shield and the interconnection structure. 
     A better understanding of the nature and advantages of embodiments of the present invention may be gained with reference to the following detailed description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified diagram illustrating an exemplary wireless charging mat and two devices positioned on the charging mat, according to some embodiments of the present disclosure. 
         FIG. 2  is a simplified diagram illustrating a transmitter coil arrangement embedded within a charging mat, according to some embodiments of the present disclosure. 
         FIG. 3  is a simplified diagram illustrating an exemplary base pattern having three transmitter coils, according to some embodiments of the present disclosure. 
         FIG. 4  is a simplified diagram illustrating an exemplary transmitter coil arrangement configured in a rosette pattern, according to some embodiments of the present disclosure. 
         FIGS. 5A-5C  are simplified diagrams illustrating the different layers of a transmitter coil arrangement configured in a rosette pattern, according to some embodiments of the present disclosure. 
         FIGS. 6A-6E  are simplified diagrams illustrating an expansion of a pattern of transmitter coils, according to some embodiments of the present disclosure. 
         FIGS. 7A-7C  are simplified diagrams and charts illustrating the formation of a continuous charging surface, according to embodiments of the present disclosure. 
         FIG. 8A  is a simplified diagram illustrating exemplary radial directions for two transmitter coils, according to some embodiments of the present disclosure. 
         FIG. 8B  is a simplified diagram illustrating an exemplary transmitter coil arrangement formed of three transmitter coil layers where the transmitter coils of each layer is arranged in a different radial direction than the other layers, according to some embodiments of the present disclosure. 
         FIGS. 9A-9C  are simplified diagrams illustrating different transmitter coil layers of the transmitter coil arrangement illustrated in  FIG. 8B , according to some embodiments of the present disclosure. 
         FIG. 10  is a simplified diagram illustrating an exemplary transmitter coil arrangement where transmitter coils are arranged in different radial directions based on their position in the transmitter coil arrangement, according to some embodiments of the present disclosure. 
         FIGS. 11A-11C  are simplified diagrams illustrating different transmitter coil layers of the transmitter coil arrangement illustrated in  FIG. 10 , according to some embodiments of the present disclosure. 
         FIG. 12A  is a simplified diagram illustrating an exemplary transmitter coil arrangement where all transmitter coils have substantially the same dimensions than other transmitter coils in the transmitter coil arrangement, according to some embodiments of the present disclosure. 
         FIG. 12B  is a simplified diagram illustrating an exemplary transmitter coil arrangement where one or more transmitter coils have different dimensions than other transmitter coils in the transmitter coil arrangement, according to some embodiments of the present disclosure. 
         FIG. 13A  is a simplified diagram illustrating an exemplary coil of wire formed of a plurality of thin wires, according to some embodiments of the present disclosure. 
         FIG. 13B  is a simplified diagram illustrating a cross-sectional view of a single turn of a coil of wire formed of a plurality of thin wires, according to some embodiments of the present disclosure. 
         FIG. 13C  is a simplified diagram illustrating an exemplary coil of wire formed of a single core of conductive wire, according to some embodiments of the present disclosure. 
         FIG. 13D  is a simplified diagram illustrating a cross-sectional view of a single turn of a coil of wire formed of a single core of conductive wire, according to some embodiments of the present disclosure. 
         FIG. 14A  is a simplified diagram illustrating a top perspective view of a coil of wire with termination ends positioned within an internal diameter of the coil of wire and arranged at an angle with respect to one another, according to some embodiments of the present disclosure. 
         FIG. 14B  is a simplified diagram illustrating a side view of the coil of wire illustrated in  FIG. 14A , according to some embodiments of the present disclosure. 
         FIG. 14C  is a simplified diagram illustrating a top perspective view of a coil of wire with termination ends positioned within an internal diameter of the coil of wire and arranged parallel to one another, according to some embodiments of the present disclosure. 
         FIGS. 15A-15D  are simplified diagrams illustrating top and side views of an exemplary bobbin, according to some embodiments of the present disclosure. 
         FIGS. 16A and 16B  are simplified diagrams illustrating top and bottom perspective views of an exemplary angle transmitter coil, according to some embodiments of the present disclosure. 
         FIG. 17A  is a simplified diagram illustrating an exemplary transmitter coil arrangement formed with angle transmitter coils, according to some embodiments of the present disclosure. 
         FIG. 17B  is a simplified diagram illustrating a zoomed-in, bottom perspective view of a portion of an exemplary transmitter coil arrangement formed with angle transmitter coils, according to some embodiments of the present disclosure. 
         FIGS. 18A-18B  are simplified diagrams illustrating top and bottom perspective views of an exemplary parallel transmitter coil, according to some embodiments of the present disclosure. 
         FIG. 19  is a simplified diagram illustrating an exemplary transmitter coil arrangement formed with parallel and angle transmitter coils, according to some embodiments of the present disclosure. 
         FIG. 20A  is a simplified diagram illustrating an exploded side-view perspective of a transmitter coil arrangement, according to some embodiments of the present disclosure. 
         FIG. 20B  is a simplified diagram illustrating side-view perspective of an assembled transmitter coil arrangement, according to some embodiments of the present disclosure. 
         FIG. 21  is a simplified diagram illustrating an exemplary transmitter coil without a bobbin, according to some embodiments of the present disclosure. 
         FIG. 22A  is a simplified diagram illustrating an exemplary transmitter coil arrangement formed of transmitter coils without bobbins, according to some embodiments of the present disclosure. 
         FIG. 22B  is a simplified diagram illustrating an exemplary transmitter coil arrangement formed of transmitter coils without bobbins and with similarly organized termination ends, according to some embodiments of the present disclosure. 
         FIGS. 22C-22E  are simplified diagrams illustrating individual layers of an exemplary transmitter coil arrangement shown in  FIGS. 22B and 22C , according to some embodiments of the present disclosure. 
         FIG. 23  is a simplified diagram illustrating an exploded view of an exemplary wireless charging mat having transmitter coils with bobbins, according to some embodiments of the present disclosure. 
         FIG. 24  is a simplified diagram illustrating an exploded view of an exemplary wireless charging mat having transmitter coils without bobbins, according to some embodiments of the present disclosure. 
         FIG. 25A  is a simplified diagram illustrating a top-view of an exemplary electromagnetic shield with a thin conductive border, according to some embodiments of the present disclosure. 
         FIG. 25B  is a simplified diagram illustrating a top-view of an exemplary electromagnetic shield with a conductive border that extends to edges of a transmitter coil arrangement, according to some embodiments of the present disclosure. 
         FIG. 26A  is a simplified diagram illustrating a cross-sectional view of a part of a faraday cage around a transmitter coil arrangement of a partially-formed wireless charging mat, according to some embodiments of the present disclosure. 
         FIG. 26B  is a simplified diagram illustrating a close-up cross-sectional view of an interface between a shielding body and a conductive border, according to some embodiments of the present disclosure. 
         FIGS. 27A and 27B  are simplified diagrams illustrating an exemplary standoff, according to some embodiments of the present disclosure. 
         FIGS. 28A and 28B  are simplified diagrams illustrating an exemplary standoff with hook structures, according to some embodiments of the present disclosure. 
         FIG. 29  is a simplified diagram illustrating an exemplary assembled transmitter coil arrangement attached to an underlying driver board, according to some embodiments of the present disclosure. 
         FIG. 30  is a simplified diagram illustrating a bottom-view of a drop frame coupled to a driver board, according to some embodiments of the present disclosure. 
         FIG. 31  is a simplified diagram illustrating a top-down view of an exemplary bottom shield, according to some embodiments of the present disclosure. 
         FIG. 32  is a simplified diagram illustrating an exploded view of an exemplary wireless charging mat including more than one transmitter coil arrangement, according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the disclosure describe a wireless charging mat where an electronic device can be efficiently charged across a vast majority, if not an entire area, of a charging surface of the wireless charging mat. Arrays of transmitter coils disposed below the charging surface may generate time-varying magnetic fields capable of inducing current in a receiver of the electronic device or of a docking station with which the electronic device is coupled. 
     The wireless charging mat may include multiple transmitter coil layers. Each layer can include an array of transmitter coils arranged in a grid pattern and configured to generate magnetic fields in a corresponding grid pattern. Spaces between each transmitter coil in the layer may be a “dead zone,” i.e., a region where a magnetic field is not generated. Thus, the multiple transmitter coil layers can be arranged so that there are minimal dead zones across a charging surface of the wireless charging mat. In some embodiments, the wireless charging mat includes three transmitter coil layers where each layer is arranged to fill dead zones in the other two layers. For instance, magnetic fields generated by coils in a first layer can fill in dead zones in the second and third layers. Likewise, magnetic fields generated by coils in the second layer may fill in dead zones in the first and third layers; and magnetic fields generated by coils in the third layer can fill in dead zones in the first and second layers. Accordingly, the three transmitter coil layers can collectively generate magnetic fields that span across the charging surface, thereby enabling an electronic device to be charged across a vast majority of the charging surface. Aspects and features of embodiments of such a wireless charging mat are discussed in further detail herein. 
     I. Wireless Charging Mat 
       FIG. 1  illustrates an exemplary wireless charging mat  100 , according to some embodiments of the present disclosure. Wireless charging mat  100  can include a charging surface  102  upon which a device having a wireless power receiver can be placed upon to wirelessly charge its battery. In some embodiments, charging surface  102  may be a region of a top surface  104  of wireless charging mat  100  that spans across a vast majority, if not the entire area, of top surface  104 . Time-varying magnetic fields generated by wireless charging mat  100  can propagate through regions of top surface  104  within charging surface  102  and form a continuous region within which devices can wirelessly receive power. 
     In some embodiments, devices can be placed in any location within charging surface  102  to receive power. For instance, a first device  106  can be positioned on a left side of wireless charging mat  100  within charging surface  102  and receive power from wireless charging mat  100 . And a second device, e.g., device  108 , can be positioned on a right side of wireless charging mat  100  within charging surface  102  to receive power from wireless charging mat  100 . It is to be appreciated that a device placed anywhere within charging surface  102  can receive power from wireless charging mat  100  according to embodiments of the present disclosure. In some embodiments, more than one device may be placed on wireless charging mat  100  to receive power. As an example, both devices  106  and  108  may be concurrently placed on wireless charging mat  100  and simultaneously receive power. 
     Devices  106  and  108  can be any suitable device configured to receive power from wireless charging mat  100 . For example, device  106  and/or device  108  can be a portable electronic device (e.g., a mobile phone, a media player, an electronic watch, and the like), a docking station, or an accessory electronic device, each having a receiver coil configured to receive power when exposed to magnetic fields produced by wireless charging mat  100 . 
     Wireless charging mat  100  can be shaped to provide a suitable surface upon which one or more devices can be charged. For instance, wireless charging mat  100  can be in the shape of a pill (a generally oval shape) as shown in  FIG. 1 , although other embodiments can have different shapes. Some embodiments can have a circular shape, rectangular shape, square shape, or any other suitable shape for providing a surface upon which a device can be wirelessly charged without departing from the spirit and scope of the present disclosure. 
     II. Arrangement of Transmitter Coils 
     Time-varying magnetic fields can be generated by multiple transmitter coils embedded within wireless charging mat  100 . For instance, wireless charging mat  100  can include a transmitter coil arrangement as shown in  FIG. 2 .  FIG. 2  illustrates transmitter coil arrangement  200  embedded within charging mat  100 , according to some embodiments of the present disclosure. The illustration of  FIG. 2  shows wireless charging mat  100  with top surface  104  removed so that the embedded transmitter coil arrangement  200  may be seen. Transmitter coil arrangement  200  can include multiple arrays of transmitter coils arranged in different layers and in a non-concentric fashion so that when all of the transmitter coils are operating, an array of magnetic fields can be generated across charging surface  102 . 
     A. Transmitter Coil Patterns 
     According to some embodiments of the present disclosure, the specific arrangement of transmitter coils  200  enables wireless charging mat  100  to generate an array of magnetic fields that forms a continuous charging surface upon which an electronic device can be charged. The continuous charging surface allows an electronic device to be efficiently charged at any location within the charging surface. The charging surface can span across a vast majority, if not an entire area, of wireless charging mat  100 . In some embodiments, transmitter coil arrangement  200  can be arranged according to a base pattern that enables transmitter coil arrangement  200  to generate magnetic fields that form the continuous charging surface. The base pattern can be expanded to form more complex patterns that form a larger continuous charging surface. 
     1. Base Pattern 
       FIG. 3  illustrates an exemplary base pattern  300  having three transmitter coils: first transmitter coil  302 , second transmitter coil  304 , and third transmitter coil  306 , according to some embodiments of the present disclosure. First, second, and third transmitter coils  302 ,  304 , and  306  can be arranged in three separate layers, thereby forming a transmitter coil stack. For example, first transmitter coil  302  can be positioned in a first layer, second transmitter coil  304  can be positioned in a second layer above the first layer, and third transmitter coil  306  can be positioned in a third layer above the first and second layers. Each transmitter coil can be formed of a single layer of wire that is wound from an outer radius to an inner radius so that it forms a flat, ring-like shape, as will be discussed in detail further herein. As shown in  FIG. 3 , each transmitter coil is shown without a central member (e.g., a “bobbin” as will also be discussed further herein) so that other transmitter coils located in layers below the transmitter coil can be seen for ease of understanding. 
     In some embodiments, first, second, and third transmitter coils  302 ,  304 , and  306  can each include a central termination zone. A central termination zone can be a region at the center of each transmitter coil that is reserved for interfacing with an interconnection layer, such as a printed circuit board (PCB). As shown in  FIG. 3 , first, second, and third transmitter coils  302 ,  304 , and  306  can have central termination zones  316 ,  318 , and  320 , respectively. Central termination zones  316 ,  318 , and  320  can be regions at the center of each transmitter coil reserved for interfacing with the interconnection layer, as will be discussed further herein. Accordingly, first, second, and third transmitter coils  302 ,  304 , and  306  can be positioned in locations where their respective central termination zones can interface with the interconnection layer without being blocked by a neighboring transmitter coil. For instance, central termination zone  316  of transmitter coil  302  is laterally positioned outside of the outer diameter of transmitter coil  304  and  306 . The same can be said for central termination zones  318  and  320 . Accordingly, central termination zones  316 ,  318 , and  320  can extend through the transmitter coil stack without intersecting another transmitter coil. In some embodiments, central termination zones  316 ,  318 , and  320  may be positioned equally spaced apart from one another such that the central termination zones  316 ,  318 , and  320  form an equilateral triangle  322 . 
     2. Rosette Pattern 
     As mentioned above, the base pattern can be expanded upon to form other patterns for different shapes and sizes of wireless charging mats. One of such patterns is a rosette pattern, which may be suitable for substantially circular wireless charging mats given its circular profile. The rosette pattern can be a pattern where the transmitter coils are arranged in an overlapping arrangement such that different coils in the plurality of coils are on different planes and are non-concentric with each other. In an expanded base pattern, one or more transmitter coil layers can include more than one transmitter coil. 
       FIG. 4  illustrates an exemplary transmitter coil arrangement  400  configured in a rosette pattern, according to some embodiments of the present disclosure. Transmitter coil arrangement  400  can include three separate transmitter coil layers where one or more of those layers include multiple transmitter coils. For example, a first transmitter coil layer can include transmitter coils  402   a - c , a second transmitter coil layer can include transmitter coils  404   a - c , and a third transmitter coil layer can include transmitter coil  406 . Each transmitter coil in transmitter coil arrangement  400  can have an opening defined by an inner diameter of the transmitter coil, where each opening includes a termination zone  418  (i.e. central portion) that is not overlapping any portion of an adjacent transmitter coil. Additionally, the transmitter coils are arranged such that no two coils in the plurality of coils are concentric with each other. 
     The base pattern may be pervasive throughout the rosette pattern such that every group of three transmitter coils, one in each transmitter coil layer, that are closest together is arranged in the base pattern. For instance, transmitter coils  402   a ,  404   a , and  406  are arranged in the base pattern. Likewise, transmitter coils  402   a ,  404   b , and  406  are arranged in the base pattern, transmitter coils  404   b ,  402   c , and  406  are arranged in the base pattern, and so on and so forth. By arranging transmitter coil arrangement  400  according to the base pattern, transmitter coil arrangement  400  can create a continuous charging region within which an electronic device can charge in any location. 
     To better understand the arrangement of an expanded base pattern,  FIGS. 5A-5C  illustrate the different layers of transmitter coil arrangement  400 . Specifically,  FIG. 5A  illustrates the first layer including transmitter coils  402   a - c ,  FIG. 5B  illustrates the second layer including transmitter coils  404   a - c , and  FIG. 5C  illustrates the third layer including transmitter coil  406 . According to embodiments, transmitter coils in the same layer can be equally spaced apart so that the generated magnetic fields can be arranged in an evenly spaced grid pattern. For example, transmitter coils  402   a - c  and  404   a - c  can be spaced apart by a distance D 1 . The distance D 1  may be selected to be wide enough for parts of transmitter coils in other layers to fit within it for stacking purposes, as will be discussed further herein. In other embodiments, the distance D 1  may be selected to be wide enough so that adjacent transmitter coils do not make contact with each other. For instance, distance D 1  may be less than 3 mm. In a particular embodiment, distance D 1  is less than 1 mm. 
     The center of each transmitter coil in the same layer can be separated by a distance D 2 . Distance D 2  can affect the uniformity of magnetic flux across the charging surface. Larger distances D 2  result in lower magnetic flux uniformity across the charging surface, whereas smaller distances D 2  result in higher magnetic flux uniformity across the charging surface. In some embodiments, distance D 2  is selected to be the smallest distance that allows for a suitable distance D 1  between transmitter coils while taking into consideration the outer diameter of each transmitter coil. In additional embodiments, distance D 2  is the same for all adjacent transmitter coils in the same layer. Thus, groups of three transmitter coils (e.g., transmitter coils  402   a - c  and  404   a - c  in each of the first and second layers, respectively) can be arranged according to the end points of an equilateral triangle  422 . 
     Although  FIGS. 5A and 5B  illustrate only three transmitter coils in a single transmitter coil layer, it is to be appreciated that embodiments are not limited to transmitter coil layers having only three coils. Instead, other embodiments can include transmitter coil layers having more than three transmitter coils. In such embodiments, the transmitter coils are arranged equally spaced apart and placed in positions corresponding to corners of equilateral triangles. 
     B. Expanding Transmitter Coil Patterns 
     Like the base pattern, the rosette pattern (or any other pattern formed from the base pattern) can be expanded to form larger sets of transmitter coils for different shapes and sizes of wireless charging mats.  FIGS. 6A-6C  illustrate an expansion of a pattern of transmitter coils according to some embodiments of the present disclosure.  FIG. 6A  illustrates an initial pattern  600 , and  FIGS. 6B and 6C  each illustrate the initial pattern after it has been expanded by an incremental transmitter coil layer. 
     Initial pattern  600  in  FIG. 6A  is shown as a transmitter coil arrangement  600  arranged in a rosette pattern, though one skilled in the art understands that any initial pattern formed from the base pattern can be used as the initial pattern. Initial pattern  600  includes three transmitter coil layers where a first layer includes transmitter coils  602   a - c , a second layer includes transmitter coils  604   a - c , and a third layer includes transmitter coil  606   a . The second layer can be disposed between the first and third layers. 
     The way in which a pattern of transmitter coils may be expanded can be based on its existing transmitter coil arrangement. For instance, adding a transmitter coil to the existing pattern can be based on the layers in which the closest transistor coils are positioned, where the transmitter coil added to the pattern is placed in the layer in which the closest transmitter coils are not positioned. As an example, if the closest transmitter coils are positioned in the first and second layers, then the next transmitter coil used to expand the pattern is positioned in the third layer. Likewise, if the closest transmitter coils are positioned in the first and third layers, then the next transmitter coil is placed in the second layer; and if the closest transmitter coils are positioned in the second and third layers, then the next transmitter coil is placed in the first layer. This approach may be used to expand the pattern each time an additional coil is added to the existing transmitter coil arrangement. Each transmitter coil added to the pattern is positioned according to the base pattern discussed herein with respect to  FIG. 3 . 
     In the particular example shown in  FIG. 6B , transmitter coils  606   b  and  606   c  are added to transmitter coil arrangement  600  to form transmitter coil arrangement  601 . Using the approach discussed herein, transmitter coils  606   b  and  606   c  are placed in the transmitter coil arrangement  601  according to the positions of the outermost transmitter coils. Since the outermost transmitter coils  602   b ,  602   c , and  604   b , are positioned in the first and second layers, transmitter coils  606   b  and  606   c  can be positioned in the third layer. Expanding the pattern by another transmitter coil layer follows the same approach. For instance, as shown in  FIG. 6C , transmitter coil  602   d  is added to transmitter coil arrangement  601  to form transmitter coil arrangement  603 . Since the outermost transmitter coils  606   b ,  606   c , and  604   b  are positioned in the second and third layers, transmitter coil  602   d  can be positioned in the first layer. 
     Transmitter coil arrangement  600  can be expanded to any degree according to any design. For instance, transmitter coil arrangement  600  can be expanded according to a 16-coil design.  FIG. 6D  illustrates exemplary transmitter coil arrangement  605  formed of 16 coils, according to some embodiments of the present disclosure. As shown, the transmitter coils in transmitter coil arrangement  605  can be organized in an overlapping arrangement such that different coils in the plurality of coils are on different planes and are non-concentric with each other. Transmitter coil arrangement  605  can be similar to the coil arrangement of transmitter coil arrangement  200  briefly discussed herein with respect to  FIG. 2 . Thus, each transmitter coil can be positioned to provide broad coverage across charging surface  102  of charging mat  100 . 
     In some embodiments, transmitter coil arrangement  600  can be expanded further according to a different design. As an example, transmitter coil arrangement  600  can be expanded according to a 22-coil design.  FIG. 6E  illustrates exemplary transmitter coil arrangement  607  formed of 22 coils, according to some embodiments of the present disclosure. As shown, six additional coils can be added to the 16-coil design according to the steps explained herein with respect to  FIGS. 6A-6C . Adding additional coils can alter the shape and coverage of charging surface  102 . Furthermore, adding additional coils can change the density of magnetic flux across charging surface  102 . More coils may result in a larger charging surface  102  and a greater density of magnetic flux across charging surface  102  than a transmitter coil arrangement with less coils. 
     C. Coverage of Transmitter Coil Patterns 
     According to embodiments of the present disclosure, transmitter coils arranged in patterns formed from the base pattern can generate magnetic fields that form a continuous charging surface. The continuous charging surface allows electronic devices resting upon the charging surface to receive power in any location within it, thereby enhancing the ease at which a user may charge his or her device. 
       FIGS. 7A-7C  illustrate how the pattern of the transmitter coils creates the continuous charging surface, according to embodiments of the present disclosure. Each figure illustrates a separate layer of a transmitter coil arrangement and shows a corresponding graph plotting the strength of a magnetic field across a distance. The graph plots the strength of one transmitter coil, but can be applied to all transmitter coils in the same layer. Each graph has a Y-axis representing strength of the magnetic field (which may be expressed by the unit H representing amperes per meter) increasing upward, and an X-axis representing horizontal distance across a charging surface increasing to the right. 
       FIG. 7A  illustrates an array of transmitter coils  700  and a graph  708  representing a strength-to-distance curve of a magnetic field generated by a transmitter coil  702 , according to some of the present disclosure. Array of transmitter coils  700  can be an array of transmitter coils positioned within a first layer of a transmitter coil arrangement. In some embodiments, transmitter coil  702  generates a magnetic field having a strength-to-distance curve  714  that peaks near the center of transmitter coil  702  and decreases as you move farther away from the center of curve  714 . 
     In order for the transmitter coil to perform wireless charging, the transmitter coil may need to generate a magnetic field that is strong enough to extend above a charging surface. The threshold at which wireless charging is enabled may be represented by a strength threshold  715  shown in graph  708 . Portion of curve  714  above strength threshold  715  may be sufficient for wireless charging, and those portions of curve  714  below strength threshold  715  may be insufficient for wireless charging. Portions of curve  714  below strength threshold  715  may be designated as “dead zones”  716  and  718  where the magnetic field is not strong enough to wirelessly charge an electronic device resting on the charging surface. 
     Thus, according to embodiments, additional layers can be incorporated in the transmitter coil arrangement to fill in the dead zones. As shown in  FIG. 7B , a second transmitter coil layer can be placed on top of the first transmitter coil layer in a manner congruent to the arrangement of the base pattern discussed herein to fill in at least some of the dead zones of the first layer, thereby resulting in transmitter coil arrangement  701 . The second transmitter coil layer can include a transmitter coil  704  that has a magnetic field strength-to-distance curve  720 . By including transmitter coil  704  in the second layer, the magnetic fields generated by transmitter coil  704  (and other transmitter coils in the second layer) can fill in dead zone  718  from the first layer. Thus, portions of the charging surface corresponding to the transmitter coils in the second layer may be able to perform wireless charging. 
     As can be appreciated from chart  720 , there may still be some dead zones even with the addition of the second layer. For instance, portions of dead zone  716  may still exist, thereby causing some regions of the charging surface to not be capable of performing wireless charging, and resulting in a discontinuous charging surface. Thus, according to some embodiments of the present disclosure, a third layer can be incorporated to fill in the remaining dead zones. 
       FIG. 7C  illustrates a third transmitter coil layer formed on top of the first and second transmitter coil layers to form transmitter coil arrangement  703 . In some embodiments, the second transmitter coil layer can be positioned between the first and third transmitter coil layers. Third transmitter coil layer can include a transmitter coil  706  that has a magnetic field strength-to-distance curve  722 . By including transmitter coil  706  in the third layer, the magnetic fields generated by transmitter coil  706  (and other transmitter coils in the second layer) can fill in dead zone  716  from the first and second layers. Accordingly, there may no longer be any dead zones within the charging surface, thereby creating a continuous charging surface within which an electronic device can be wirelessly charged when resting in any location. 
     Although  FIGS. 7A-7C  illustrate a transmitter coil arrangement that has only three layers to create a continuous charging surface, embodiments are not limited to such configurations. Other embodiments can have more or less than three layers to form a continuous charging surface, without departing from the spirit and scope of the present disclosure. 
     D. Rotational Arrangement of Transmitter Coils 
     Arranging the transmitter coils so that they overlap one another in different layers increases the z-height (e.g., thicknesses) of the transmitter coil arrangement when assembled as compared to an array of similar coils arranged in a single layer. According to some embodiments of the present disclosure, transmitter coils in a transmitter coil arrangement can be oriented in various radial directions to minimize the z-height of a transmitter coil arrangement as described in various embodiments discussed below. A radial direction is the angle at which a transmitter coil is radially aligned with respect to a reference direction, which may be any arbitrary angular direction such as true north. The radial direction of a transmitter coil may be defined by an angular difference between a reference location of the transmitter coil and the reference direction. 
       FIG. 8A  illustrates exemplary reference locations for transmitter coils  801   a - b  with respect to an exemplary reference direction  807 . Exemplary reference direction  807  may be an angular direction corresponding to true north as shown in  FIG. 8A . A reference location may be represented by any structural part of a transmitter coil that is common in all other transmitter coils. For instance, a reference location  803   a  of transmitter coil  801   a  can be represented by a termination end  805  of transmitter coil  801   a . Likewise, a reference location  803   b  of transmitter coil  801   b  can be represented by a corresponding termination end  805   b  of transmitter coil  801   b . The radial direction of transmitter coil  801   a  can be defined by the angle between reference location  807  and reference location  803   a , and the radial direction of transmitter coil  801   b  can be defined by the angle between reference direction  807  and reference location and  803   b . Thus, transmitter coil  801   a  may be arranged in a different radial direction than transmitter coil  801   b  as shown in  FIG. 8A . 
     The particular way these transmitter coils are arranged can be based on one or more factors. For instance, the structure of the transmitter coil can include protrusions that can fit in the spaces between transmitter coils in adjacent layers, thereby minimizing z-height. As an example, a transmitter coil in the first layer can have protrusions that fit in the space between adjacent transmitter coils in the second layer. Details of such structures will be discussed further herein. 
     Transmitter coils in different transmitter coil layers can be arranged in different radial directions.  FIG. 8B  illustrates an exemplary transmitter coil arrangement  800  formed of three transmitter coil layers: a first transmitter coil layer  802 , a second transmitter coil layer  804 , and a third transmitter coil layer  806 , where the transmitter coils of each layer is arranged in a different radial direction than the other layers. Transmitter coil arrangement  800  is shown in an arrangement suitable for a pill-shaped wireless charging mat, such as wireless charging mat  100  in  FIGS. 1 and 2 , though it is to be appreciated that embodiments are not limited to such arrangements, and that other embodiments can have transmitter coil arrangements suitable for other shapes of wireless charging mats without departing from the spirit and scope of the present disclosure. 
     As shown in  FIG. 8B , transmitter coils of first transmitter coil layer  802  can be arranged in a first radial direction  808 , transmitter coils of second transmitter coil layer  804  can be arranged in a second radial direction  810 , and transmitter coils of third transmitter coil layer  806  can be arranged in a third radial direction  812 . First, second, and third radial directions  808 ,  810 , and  812  can be offset from one another by an angular offset  814 . The degree angular offset  814  may be determined to be an angle that enables transmitter coils of first, second, and third transmitter coil layers  802 ,  804 , and  806  to achieve minimal z-height when transmitter coil arrangement  800  is assembled, as will be discussed in detail further herein. In some embodiments, angular offset  814  ranges between 110 to 130 degrees, particularly around 120 degrees in certain embodiments. 
     While transmitter coils in different layers can be arranged in different radial directions, transmitter coils in the same coplanar layer can be arranged in the same radial direction. To better illustrate this concept,  FIGS. 9A-9C  each illustrate a different transmitter coil layer of transmitter coil arrangement  800  in  FIG. 8B . Specifically,  FIG. 9A  illustrates first transmitter coil layer  802 ,  FIG. 9B  illustrates second transmitter coil layer  804 , and  FIG. 9C  illustrates third transmitter coil layer  806 . 
     As can be seen from  FIGS. 9A-9C , transmitter coils  802  are all arranged in the same radial direction, e.g., first radial direction  808 . Likewise, transmitter coils  804  are all arranged in radial direction  810 , and transmitter coils  806  are all arranged in radial direction  812 . In some embodiments, the transmitter coils in the same layer are substantially coplanar. Additionally, adjacent transmitter coils in the same coplanar layer are positioned the same distance away from one another, as discussed herein with respect to  FIGS. 5A and 5B . Furthermore, each set of transmitter coils in a coplanar layer are symmetrical across a horizontal axis  900 . In some embodiments, only two out of the three layers of transmitter coils has the same number of transmitter coils. For instance, transmitter coils  802  in the first layer has the same number of transmitter coils as transmitter coils  804  in the second layer. Transmitter coils  806  in the third layer can have a different number of transmitter coils than the other two layers, such as two less transmitter coils than the other two layers. This phenomenon is an artifact of the expanded rosette pattern discussed herein above. 
       FIGS. 8B and 9A-9C  illustrate one exemplary transmitter coil arrangement; however, embodiments are not limited to such arrangements. Other embodiments can have different transmitter coil arrangements. As an example,  FIG. 10  illustrates an exemplary transmitter coil arrangement  1000  that includes sixteen individual coils where the transmitter coils are arranged in different radial directions based on their position in the transmitter coil arrangement, according to some embodiments of the present disclosure. For instance, transmitter coil arrangement  1000  can include twelve outer transmitter coils  1002  and four inner transmitter coils  1004 . Outer transmitter coils  1002  may be a set of transmitter coils positioned near the outermost regions of transmitter coil arrangement  1000 , while inner transmitter coils  1004  may be those transmitter coils surrounded by outer transmitter coils  1002 . As shown in  FIG. 10 , inner transmitter coils  1004  are indicated by bolded lines, and outer transmitter coils  1002  are indicated by non-bolded lines for ease of observation. 
     In some embodiments, outer transmitter coils  1002  are arranged in a different radial direction than inner transmitter coils  1004 . As shown in  FIG. 10 , outer transmitter coils  1002  can be arranged in a radial direction pointing toward the outer edges of transmitter coil arrangement  1000 , while inner transmitter coils  1004  can be arranged in various radial directions. Arranging outer transmitter coils  1002  in such a manner enables some portions of outer transmitter coils  1002  to be positioned away from an inner region of a charging surface. Such portions may be less efficient portions of the transmitter coils due to the structural configuration of the transmitter coil, as will be discussed further herein. 
     Given the multi-layered construction of transmitter coil arrangement  1000 , transmitter coils in the same coplanar layer can be arranged in different directions.  FIGS. 11A-11C  each illustrate a different transmitter coil layer of transmitter coil arrangement  1000  in  FIG. 10 . Specifically,  FIG. 11A  illustrates a first transmitter coil layer  1102 ,  FIG. 11B  illustrates a second transmitter coil layer  1104 , and  FIG. 11C  illustrates a third transmitter coil layer  1106  of transmitter coil arrangement  1000 . 
     As can be seen from  FIGS. 11A-11C , one or more transmitter coils in first transmitter coil layer  1102  are arranged in a different radial direction than other transmitter coils in the same layer. Transmitter coils that are part of outer transmitter coils  1002  in  FIG. 10 , e.g., transmitter coils  1108   a ,  1108   b ,  1108   d ,  1108   e , and  1108   f , can be arranged so that their radial direction face outward as discussed herein with respect to  FIG. 10  to achieve a more even charging surface across a wireless charging mat. Conversely, transmitter coils that are part of inner transmitter coils  1104  in  FIG. 10 , e.g., transmitter coil  1108   c , can be arranged so that its radial direction is an increment of between 110 to 130 degrees, such as 120 degrees discussed herein with respect to  FIG. 8B . Transmitter coils that are part of respective inner and outer transmitter coils in the second and third transmitter coil layers, as shown in  FIGS. 11B and 11C , can also be arranged based on the same principles. 
     Transmitter coils shown in  FIGS. 2-11C  in respective transmitter coil arrangements can have similar dimensions. For example, transmitter coils in each transmitter coil arrangement can have the same inner diameter and outer diameter.  FIG. 12A  illustrates an exemplary transmitter coil arrangement  1200  where all of the transmitter coils have substantially the same dimensions, e.g., the same inner and outer diameters. An inner diameter can be defined by the diameter of a perimeter formed by the turn of a transmitter coil that is closest to its center, and an outer diameter can be defined by the diameter of a perimeter formed by the turn of a transmitter coil that is farthest from its center. For instance, transmitter coil  1202   a  can have an inner diameter  1204  and an outer diameter  1206 . Even transmitter coils  1202   b  and  1202   c  positioned at the farthest left and right positions in transmitter coil arrangement  1200  can have substantially the same dimensions as all other transmitter coils. In some embodiments, transmitter coils have substantially the same dimensions when their respective inner and outer diameters differ by less than 10%, particularly less than 5% in certain embodiments. 
     Although transmitter coils arrangement discussed herein can have the substantially the same dimensions, some embodiments can have transmitter coil arrangements where some transmitter coils have different dimensions than other transmitter coils in the same transmitter coil arrangement, as will be discussed herein with respect to  FIG. 12B . 
       FIG. 12B  illustrates an exemplary transmitter coil arrangement  1201  where one or more transmitter coils have different dimensions than other transmitter coils in transmitter coil arrangement  1201 , according to some embodiments of the present disclosure. In some embodiments, all other transmitter coils in transmitter coil arrangement  1200  can have the same inner and outer diameters  1204  and  1206  except for the transmitter coils that are positioned at the farthest left and right positions of transmitter coil arrangement  1200 , such as transmitter coils  1202   b  and  1202   c.    
     Transmitter coils  1202   b  and  1202   c  can have a smaller inner diameter than all other transmitter coils in transmitter coil arrangement  1200  because of their positions. The farthest left and right positions of transmitter coil arrangement  1200  have the least density of transmitter coils by virtue of being at the very edge of the transmitter coil arrangement. Thus, magnetic flux generated in those positions may be less dense than magnetic flux generated at other areas of transmitter coil arrangement  1200 , such as magnetic flux generated near the center of transmitter coil arrangement  1200 . Accordingly, one or more transmitter coils located at the farthest left and right positions can have different coil dimensions to increase the magnetic flux density produced at those areas of the transmitter coil arrangement. For instance, transmitter coils  1202   b  and  1202   c  can have a smaller inner diameter  1208  but the same outer diameter  1210  when compared to other transmitter coils in transmitter coil arrangement  1200 , e.g., transmitter coil  1202   a . By having a smaller inner diameter, transmitter coils  1202   b  and  1202   c  can have more turns, thereby being capable of generating a greater amount of flux. In some embodiments, inner diameter  1208  is approximately three to five mm less than inner diameter  1204 . For instance, inner diameter  1208  can be approximately 4 mm less than inner diameter  1204  such that inner diameter  1208  is 13 mm and inner diameter  1204  is 17 mm. It is to be appreciated that other embodiments can modify the shape and/or geometry of the transmitter coils to achieve a smoother charging region. Additionally, the shape of one or more transmitter coils can be modified based on the geometry of the wireless charging mat and the location of the transmitter coils with respect to the wireless charging mat. For instance, if the wireless charging mat is in the general shape of a square or of another shape that has several straight edges, some transmitter coils disposed at the edges of the wireless charging mat can be in the shape of a “D” such that the straight edges of the transmitter coil can correspond to the straight edges of the wireless charging mat. 
     III. Transmitter Coil Structure 
     As illustrated in  FIGS. 2-12 , the transmitter coils are shown as circular “O”-shaped rings. It is to be appreciated that the circular “O”-shaped rings represent a coil of wire for generating time-varying magnetic fields capable of inducing a corresponding current in a receiver coil for performing wireless charging. In some embodiments, the coil of wire may be formed of a coil of wire where each turn of the wire includes a bundle of smaller coils of wire. In other embodiments, the coil of wire may be formed of a coil of wire where each turn of the wire includes a single core of conductive material. While  FIGS. 2-12  illustrate the transmitter coils as circular rings, in some embodiments each transmitter coil can an outer perimeter with a generally circular shape that is not a perfect circle due to the width of the wire and the spiraling nature of the wire as described in further detail below. As used herein, a “generally circular” coil refers to both a coil with a circular perimeter and a coil that has a perimeter that is close to being circular as discussed below. In other embodiments, transmitter coils may be non-circular, such as hexagonal so that the coils may maximize usage of the space between adjacent transmitter coils, or any other suitable shape, e.g., square, oval, rectangular, triangular, and the like. 
     A. Transmitter Coil Wiring 
       FIG. 13A  illustrates an exemplary coil of wire  1300  formed of a plurality of thin wires, according to some embodiments of the present disclosure. A single turn of wire can include a bundle  1302  of small conductive wires, as shown in  FIG. 13B .  FIG. 13B  illustrates a cross-sectional view  1301  of a single turn of wire of coil  1300 . The single turn of wire can include multiple thin wires  1305 , which can be arranged in sub-bundles, such as sub-bundles  1303   a ,  1303   b , and  1303   c . The overall width of bundle  1302  of wires may be determined by the thickness of each thin wire  1305  and the manner in which the bundle  1302  of thin wires are arranged (e.g., how many thin wires  1305  are stacked together in the z direction to define the height, H, of each sub-bundle). In some embodiments, the thickness of each thin wire  1305  may range between 110 and 120 microns, resulting in a bundle  1302  of thin wire having a width ranging between 1 to 2 mm and a height (H) ranging between 0.4 to 0.7 mm. Using a bundle of thin wire for each turn of the coil may be particularly useful for generating stronger magnetic fields given its ability to achieve a large number of turns in a limited amount of space. 
     Coil of wire  1300  may be formed of a coil of wire that winds between an inner radius  1304  to an outer radius  1306 . In some embodiments, coil of wire  1300  can be a flattened “O”-ring formed of single layer of wire that winds from inner radius  1304  to the outer radius  1306 , or vice versa. Inner radius  1304  can be a non-zero radius that allows coil of wire  1300  to have a vacant inner space. Having the coil of wire  1300  wind in a single layer of wire minimizes the overall height of the coil, which thereby decreases the overall height of the wireless charging mat once the coils are assembled. 
     In particular embodiments, each thin wire  1305  is an electrically insulated wire that is covered in one or more layers of dielectric material, such as polyurethane. The layer of electrical insulation prevents the thin wires from shorting with an adjacent thin wire when coiled. Additionally, coil of wire  1300  as a whole can be covered with another layer of insulating material, such as polyimide, to attach the wound wires together to form a single structure of coiled wire. Coil of wire  1300  can be attached to a bobbin, as will be discussed further herein, and can thus be easily picked up and placed (e.g., using a robot as part of a manufacturing process) in a transmitter coil arrangement. 
     In some embodiments, instead of using a bundle of smaller coils, a single core of conductive material may be used for each turn of wire, as shown in  FIG. 13C .  FIG. 13C  illustrates an exemplary coil of wire  1307  formed of a single core of conductive wire, according to some embodiments of the present disclosure.  FIG. 13D  illustrates a cross-sectional view  1309  of a single turn of wire of coil  1307 . As shown in cross-sectional view  1309 , the single turn of wire may be formed of a single core of conductive wire  1311  instead of a bundle of wires  1302  as shown in  FIG. 13B . Using a single core of conductive wire for each turn of the coil of wire may be particularly useful for applications where the transmitter coil is formed in a PCB, which can be printed with conductive lines having very small dimensions. In some embodiments, the single core of conductive wire can have a width between 0.9 and 1.3 mm, and a height between 0.08 to 0.18 mm. 
     With reference back to  FIG. 13A , coil of wire  1300  can have two termination ends: first termination end  1308  and second termination end  1310 . The termination ends may be the avenue through which current can enter and exit through coil of wire  1300 . In some embodiments, termination end  1310  can fold over coil  1300  to be positioned within an inner diameter of coil  1300  as shown in  FIGS. 14A and 14B . 
     Specifically,  FIG. 14A  illustrates a top perspective view of a coil of wire  1400  with termination ends  1402  and  1404  positioned within an internal diameter  1406  of coil of wire  1400 , according to some embodiments of the present disclosure, and  FIG. 14B  illustrates a side view of coil of wire  1400 . Positioning termination ends  1402  and  1404  within the internal diameter of coil of wire  1400  simplifies how coil  1400  is coupled to another structure, such as a driver board, because it enables the coupling to be performed at a single location, e.g., the center of coil of wire  1400 . 
     As shown in  FIG. 14A , termination end  1402  bends over coil  1400  so that it is positioned within internal diameter  1406 . Although termination end  1402  appears to bend over coil  1400  without folding over on itself, embodiments are not limited to such arrangements and that embodiments where termination end  1402  folds over itself to be positioned within internal diameter  1406  are envisioned herein as well. In some embodiments, a portion  1408  of the termination end  1402  rests on coil  1400  so that it protrudes above a plane of coil  1400 . For instance, with reference to  FIG. 14B , portion  1408  can extend above a plane  1410  of coil  1400  as defined by a surface formed by the winding of wire of coil  1400 . The protrusion may be positioned on only one side of coil  1400  so that the other side of coil  1400  may not have a protrusion. Unlike termination end  1402 , termination end  1404  may not protrude above plane  1410  as it may already be positioned within internal diameter  1406 . In some embodiments, termination end  1404  can merely bend toward the center of coil  1400  without folding over coil  1400 . 
     With reference back to  FIG. 14A , the directions at which termination ends  1402  and  1404  turn toward the center of coil  1400  can, in some embodiments, form an angle  1412  with respect to each other. Angle  1412  may be determined based on an offset angle, such as offset angle  814  discussed herein with respect to  FIG. 8B . Offset angle  814  may enable the overlapping portion  1408  of coil  1400  to be positioned in a gap between transmitter coils in an adjacent layer to minimize the z-height of a transmitter coil stack, as will be discussed further herein. 
     As can be appreciated from  FIG. 14A , a portion of coil of wire  1400  can have a different number of turns than other regions. For example, region  1414  of coil  1400  can have four turns of wire, while the rest of coil  1400  (e.g., regions of coil  1400  that is not part of region  1414 ) has five turns of wire as shown in  FIG. 14A . In another example (not shown in  FIG. 14A ), region  1414  of coil  1400  can have more turns than the rest of coil  1400 . It is to be appreciated that having more or less turns in region  1414  depends on the arrangement of termination ends  1402  and  1404  which define where the winding begins and ends. Accordingly, region  1414  may have different coupling characteristics with other transmitter coils when arranged in a transmitter coil arrangement than the other regions of coil  1400 . As an example, region  1414  may have more coupling with other transmitter coils in a transmitter coil arrangement. Having more coupling may reduce the efficiency of the transmitter coil. Thus, in some embodiments, region  1414  may be minimized to mitigate coupling with other transmitter coils by reducing the angle at which termination ends  1402  and  1404  are positioned. For example, termination ends  1402  and  1404  can be positioned parallel to one another, as shown in  FIG. 14C . In some embodiments, region  1414  is less than half of coil  1400  such that region  1414  is a smaller portion of coil  1400  than the rest of coil  1400 . 
       FIG. 14C  illustrates an exemplary coil of wire  1401  where termination ends  1402  and  1404  are arranged parallel to one another. By arranging termination ends  1402  and  1404  parallel to one another, region  1416  of coil  1401  having less turns than other regions of the coil may be minimized. For instance, region  1416  having only four turns of wire may be minimized to be the small distance between termination ends  1402  and  1404  shown in  FIG. 14C . In comparison, region  1416  may be substantially smaller than region  1414  in  FIG. 14A . Accordingly, by minimizing region  1416 , coil  1401  may operate in a more efficient manner. 
     In some embodiments, due to the width of each turn of the wire that makes up coils  1400  and  1401 , each coil can have a generally circular shape (as defined by the outer perimeter of the coil) that is not a true circle. That is, some regions of the outer perimeter of coils  1400  and  1401  may deviate from the outer perimeter of a true circle. For example, the outer perimeter of a true circle  1418 , represented by dashed and dotted lines, is superimposed over coil  1400  and  1401  in  FIGS. 14A and 14C . Portions of the outer perimeter of coils  1400  and  1401  having less turns, e.g., portions  1414  and  1416 , may deviate from the outer perimeter of a true circle  1418  by having a shorter radius. The non-circular shape of the transmitter coils can dictate the organization of a transmitter coil arrangement to ensure an even charging efficiency across the entire surface of the charging region, as will be discussed further below. 
     As will be appreciated further herein, the different ways the termination ends are arranged may affect the radial directions of the coils as discussed herein with respect to  FIGS. 8-11C . Details of this relationship will be discussed further herein with respect to  FIGS. 17A-19 . 
     B. Bobbin 
     According to some embodiments of the present disclosure, each coil of wire is wound around, and the termination ends of each coil are attached to, a central, disc-shaped support structure known as a “bobbin.” The structure formed by combining the coil of wire and the bobbin is sometimes referred to as “a transmitter coil” throughout the disclosure. The bobbin is a support structure that not only provides structural integrity for the coil of wire, but also provides a structure to which the termination ends can attach for coupling with a respective pair of contact pins. The contact pins can electrically couple the coil of wire to a driver board for operating the coil of wire as a transmitter coil for wireless charging. 
       FIGS. 15A-15D  illustrate an exemplary bobbin  1500  according to some embodiments of the present disclosure. Specifically,  FIG. 15A  illustrates a top perspective view of bobbin  1500 ,  FIG. 15B  illustrates a side-view of bobbin  1500 , and  FIGS. 15C and 15D  illustrate side-views of exemplary bobbins  1520  and  1522 , respectively. Bobbins  1520  and  1522  may have similar features as bobbin  1500 , except that their contact housings and pins may be arranged differently, as discussed below. 
     Bobbin  1500  may be a generally flat and circular structure in the shape of a disc including substantially planar surfaces. For example, bobbin  1500  can have a substantially planar top surface  1502  and a substantially planar bottom surface  1504  as shown in  FIG. 15B . With reference back to  FIG. 15A , bobbin  1500  includes a contact housing  1506  positioned near the center of bobbin  1500 . A pair of contact pins  1508   a - b  can reside within contact housing  1502  for coupling with a respective pair of termination ends of a coil of wire. Contact pins  1508   a - b  may be contacts in the form of cantilever beams (or any other suitable form of contacts) that are configured to make contact with pads on a control board, e.g., a driver board formed as a PCB, for operating a coil of wire (not shown) wound around angular bobbin  1500 . 
     In some embodiments, contact housing  1506  can protrude past a planar surface of bobbin  1500 . As an example, contact housing  1506  can protrude past planar top surface  1502  as shown in  FIG. 15B . In another example, contact housing  1506  can protrude past both top and bottom surfaces  1502  and  1504 , respectively, as shown in  FIG. 15C , or can protrude past bottom surface  1504  as shown in  FIG. 15D . Contact housing  1506  protrudes past a plane of bobbin  1500  to provide additional vertical space for termination ends of a coil of wire to couple with contact pins  1508   a - b . For instance, contact housing  1506  may provide enough space for the termination ends to be soldered to bobbin  1500 . The resulting soldered structure may occupy more vertical space than the thickness of bobbin  1500  defined by top and bottom surfaces  1502  and  1504 . 
     Bobbin  1500  can also include a pair of contact pads  1512   a - b . Contact pads  1512   a - b  can provide a surface upon which termination ends of a coil of wire can attach to electrically couple with contact pins  1508   a - b . For instance, contact pads  1512   a - b  may be substantially flat surfaces that are electrically coupled to respective contact pins  1508   a - b . Bobbin  1500  may further include a pair of channels  1510   a - b  to allow termination ends to couple with contact pads  1512   a - b . In some embodiments, channels  1510   a - b  extend from outer rim  1516  toward contact housing  1506 , i.e., toward the center of bobbin  1500 . Channels  1510   a - b  can provide an avenue through which the termination ends traverse to make contact with contact pads  1512   a - b . As shown in  FIG. 15A , channels  1510   a - b  can be vacant regions in bobbin  1500  where termination ends can be positioned without substantially affecting the overall thickness of bobbin  1500 . 
     Bobbin  1500  can further include one or more openings  1516   a  and  1516   b . Each opening  1516   a  and  1516   b  can be a vacant space that extends through bobbin  1500  so that apparatuses can pass through from one side of bobbin  1500  to the other. In some embodiments, openings  1516   a  and  1516   b  are features that can be used to grab bobbin  1500  and to pick up and accurately place bobbin  1500  in specific locations, such as in a transmitter coil arrangement. Additionally, openings  1516   a  and  1516   b  provide avenues through which apparatuses may traverse to secure bobbin  1500  in a transmitter coil arrangement after being pick up and placed in its intended location. 
     In some embodiments, bobbin  1500  can include attachment pads  1514  for attaching the coil of wire to bobbin  1500 . Any suitable adhesive, such as an epoxy adhesive, may secure bobbin  1500  to the coil of wire by fixing the coil of wire to attachment pads  1514 . Although  FIG. 15A  shows three attachment pads  1514  disposed on only one side of bobbin  1500 , embodiments are not limited to such configurations. Other embodiments can have more or less attachment pads and the attachment pads can be disposed on either or both sides of the bobbin. 
     C. Angle Transmitter Coil 
     As shown in  FIG. 15A , channels  1510   a - b  of bobbin  1500  can be arranged at an angle  1518  with respect to one another. Angle  1518  can be a non-zero angle that is particularly suitable for allowing transmitter coils to be arranged in a stack with minimal z-height. For instance, angle  1518  may be between 110 to 130 degrees, such as 120 degrees in particular embodiments. Angle  1518  may correspond to angle  1412  between the termination ends of coil  1400  in  FIG. 14 . As such, a coil of wire wound about outer rim  1516  of bobbin  1500  may result in the formation of coil  1400 . According to some embodiments, winding a coil of wire about bobbin  1500  results in the formation of an angle transmitter coil as shown in  FIGS. 16A and 16B . 
       FIGS. 16A and 16B  illustrate top and bottom perspective views, respectively, of an exemplary angle transmitter coil  1600  formed of a coil of wire  1602  wound about bobbin  1604 , according to some embodiments of the present disclosure. As shown in  FIG. 16A , termination ends  1606  and  1608  can be attached to respective contact pads on bobbin  1604  at an angle, e.g., angle  1518  in  FIG. 15A . Once attached to the contact pads, termination ends  1606  and  1608  can be electrically coupled to respective contact pins  1610   a - b  in contact housing  1612 . Thus, when contact pads  1610   a - b  are coupled to a driver board (not shown), the driver board can be electrically coupled to coil  1602  to control the operation of angle transmitter coil  1600 . Additionally, once coil of wire  1602  is wound about bobbin  1604 , angle transmitter coil  1600  is formed and constructed as a single structure that can be picked up and placed on a driver board during assembly of a wireless charging mat. 
     As can be seen from the bottom perspective view of angle transmitter coil  1600  in  FIG. 16B , termination end  1608  can bend over coil  1602 . Thus, in addition to contact housing  1612 , termination end  1608  can also protrude from a plane of angle transmitter coil  1600 . In some embodiments, termination end  1608  and contact housing  1612  protrude from the same plane of angle transmitter coil  1600 . This protrusion may affect the way the transmitter coils are radially oriented when implemented in a transmitter coil arrangement, as will be further discussed with respect to  FIGS. 17A and 17B . 
       FIG. 17A  illustrates an exemplary transmitter coil arrangement  1700  formed with angle transmitter coils, according to some embodiments of the present disclosure. Each angle transmitter coil can be arranged in a radial direction suitable for minimizing the z-height of transmitter coil arrangement  1700  while also enabling contact pins from each transmitter coil to make contact with a driver board (not shown). Specifically, transmitter coil arrangement  1700  may be organized based on the transmitter coil arrangement shown in  FIGS. 8-9C . Similar to the discussion herein with respect to  FIGS. 8-9C , transmitter coils in different transmitter coil layers can be arranged in different radial directions, e.g., radial directions  1704 ,  1706 , and  1708  in the first, second, and third transmitter coil layers, to minimize the z-height of transmitter coil arrangement  1700 . Similar to radial directions  808 ,  810 , and  812  in  FIG. 8B , radial directions  1704 ,  1706 , and  1708  can be arranged in angular offsets of between 110 to 130 degrees, such as 120 degrees. The angular offset is selected so that the termination ends that protrude from a plane of the transmitter coil can be tucked between adjacent coils in another layer, thereby minimizing the z-height of transmitter coil arrangement  1700 . 
       FIG. 17B  illustrates a zoomed-in, bottom perspective view of a portion of transmitter coil arrangement  1700 . As shown, termination end  1710  of an angle transmitter coil in a first transmitter coil layer can be tucked in the space between adjacent transmitter coils  1712  and  1714  in a second transmitter coil layer. The space between adjacent transmitter coils may correspond to distance D 1  discussed herein with respect to  FIGS. 5A and 5B . Distance D 1  may be larger than the width of a termination end, i.e., the width of a wire of a transmitter coil. In some embodiments, distance D 1  ranges between 1.5 to 2 mm. 
     Contact housings of transmitter coils are positioned in locations where contact pins can interface with the driver board without being blocked by another transmitter coil. For instance, the contact housings can be positioned within central termination zones  1718  of the transmitter coils. Central termination zones  1718  may correspond to central termination zones  418  discussed herein with respect to  FIG. 4 . 
     D. Parallel Transmitter Coil 
       FIGS. 18A-18B  illustrate an exemplary parallel transmitter coil  1800  according to some embodiments of the present disclosure. Specifically,  FIG. 18A  illustrates a top perspective view of parallel transmitter coil  1800 , and  FIG. 18B  illustrates a bottom perspective view of parallel transmitter coil  1800 . 
     As shown in  FIG. 18A , parallel transmitter coil  1800  can include a coil of wire  1802  wound about a bobbin  1804 . Termination ends  1806  and  1808  can be attached to respective contact pads on bobbin  1804  and arranged parallel to one another. When termination ends  1806  and  1808  are arranged in parallel, a portion  1814  of coiled wire  1802  defined by the region between termination ends  1806  and  1808  may be smaller than portion  1614  of coiled wire  1602 . Thus, parallel transmitter coil  1800  may be more efficient than angle transmitter coil  1800 . Once attached to the contact pads, termination ends  1806  and  1808  may be electrically coupled to respective contact pins  1810   a - b  in contact housing  1812 . Thus, when contact pads  1810   a - b  are coupled to a driver board (not shown), the driver board may be electrically coupled to coil  1802  to control the operation of angle transmitter coil  1800 . 
     As can be seen from the bottom perspective view of parallel transmitter coil  1800  in  FIG. 18B , contact housing  1812  can protrude from a plane of parallel transmitter coil  1800 . Termination end  1808  can bend over coil  1802  and also protrude from a plane of parallel transmitter coil  1800 . In some embodiments, termination end  1808  and contact housing  1812  protrude from the same plane of angle transmitter coil  1800 . This protrusion may affect the way the transmitter coils are radially oriented when implemented in a transmitter coil arrangement. 
       FIG. 19  illustrates an exemplary transmitter coil arrangement  1900  formed with parallel and angle transmitter coils, according to some embodiments of the present disclosure. Each transmitter coil can be arranged in a radial direction suitable for maximizing efficiency of an interior region of transmitter coil arrangement  1900  while also minimizing z-height and enabling contact pins from each transmitter coil to make contact with a driver board (not shown). Specifically, the transmitter coil stack can be arranged according to the transmitter coil arrangement shown in  FIGS. 10-11C . Transmitter coil arrangement  1900  may similarly include outer transmitter coils  1902  and inner transmitter coils  1904 . Outer transmitter coils  1902  may be a single line of transmitter coils positioned near the outermost regions of transmitter coil arrangement  1900 , while inner transmitter coils  1904  may be those transmitter coils surrounded by outer transmitter coils  1902 . 
     In some embodiments, outer transmitter coils  1902  may be parallel transmitter coils arranged in radial directions pointing toward the outer edges of transmitter coil arrangement  1900 , e.g., the outer perimeter of the wireless charging mat within which transmitter coil arrangement  1900  is disposed. For instance, portions  1906  of outer transmitter coils  1902  that have less turns of wire, e.g., the less efficient portions of the coil of wire such as portion  1814  in  FIG. 18A , can be oriented toward the outer edges of transmitter coil arrangement  1900 . Accordingly, the rest of the portions of outer transmitter coils  1904  having more turns and better efficiency may be concentrated toward the interior of transmitter coil arrangement  1900 . This helps ensure that the wireless charging mat has a more consistent and efficient charging surface in the inner regions of the charging surface. 
     While outer transmitter coils  1902  may be formed of parallel transmitter coils, inner transmitter coils  1904  may be formed of angle transmitter coils because of the spatial constraints caused by the arrangement of outer transmitter coils  1902 . To fit inner transmitter coils  1904  within transmitter coil arrangement  1900  while minimizing z-height, inner transmitter coils  1904  may be arranged in various radial directions according to the principles discussed herein with respect to  FIG. 17A . That is, inner transmitter coils  1904  may be arranged in different radial directions according to an angular offset of between 110 to 130 degrees, such as 120 degrees, so that the termination ends that protrude from a plane of the angular transmitter coil can be tucked between adjacent coils in another layer, thereby minimizing the z-height of transmitter coil stack  1900 . 
     IV. Multi-Layer Arrangement of Transmitter Coils 
     To minimize the z-height of a transmitter coil arrangement, protrusions of transmitter coils caused by contact housings and the folding-over of termination ends of coils of wire may nest within the transmitter coil arrangement such that they do not protrude above or below the transmitter coil arrangement as a whole. To better understand this concept,  FIGS. 20A and 20B  illustrate side-views of an exemplary transmitter coil arrangement  2000  showing how the protrusion are positioned when assembled. The nesting of transmitter coil arrangement  2000  may be similar to, and a suitable representation of, how other transmitter coil arrangements are nested, such as transmitter coil arrangements  800 ,  1000 ,  1700 , and  1900  illustrated in  FIGS. 8, 10, 17A, and 19 . 
       FIG. 20A  illustrates an exploded view of transmitter coil arrangement  2000  to show how the protrusions are positioned. Transmitter coil arrangement  2000  can include a first transmitter coil  2002  in a first transmitter coil layer, a second transmitter coil  2004  in a second transmitter coil layer, and a third transmitter coil  2006  in a third transmitter coil layer. Each transmitter coil may be representative of other transmitter coils in the same layer. In some embodiments, first transmitter coil  2002  is positioned apart from third transmitter coil  2006  while second transmitter coil  2004  is positioned between first and third transmitter coils  2002  and  2006 , respectively. 
     First transmitter coil  2002  can have a protrusion  2008  extending past a planar surface  2010  of first transmitter coil  2002 . Surface  2010  can include corresponding planar surfaces of both the coil of wound wire and portions of the bobbin around which the coil of wire is wound. Accordingly, in some embodiments, the planar surfaces of the coil of wound wire and portions of the bobbin on corresponding sides of first transmitter coil  2002  may be substantially coplanar. Because other portions of the bobbin may not substantially protrude above surface  2010 , the other portions are not shown as they are hidden behind the coil of wire as perceived from the side-view perspective of  FIG. 20A . In some embodiments, protrusion  2008  can include the contact housing as well as the folded-over termination end of the coil of wire. Contact pins  2026  of first transmitter coil  2002  can be positioned to extend past a planar surface  2011  along a direction opposite of the protrusion. Contact pins  2026  protrude past planar surface  2011  to make contact with an underlying driver board, as will be discussed further herein. 
     Similar to first transmitter coil  2002 , third transmitter coil  2006  can have a protrusion  2012  extending past a planar surface  2014  of third transmitter coil  2002 . Protrusion  2012  can include a contact housing of a bobbin and a folded-over termination end of a coil of wire wound around the bobbin of third transmitter coil  2006 . Contact pins  2024  of third transmitter coil  2006  can be positioned to extend past an end of protrusion  2012  (e.g., past the contact housing of the bobbin) along a direction with the protrusion. Contact pins  2024  extends past an end of protrusion  2012  to make contact with the underlying driver board. 
     As further shown in  FIG. 20A , second transmitter coil  2004  can have a protrusion that includes two portions: a first portion  2016   a  and a second portion  2016   b . First and second portions  2016   a  and  2016   b  can include a contact housing of a bobbin and a folded-over termination end of a coil of wire wound around the bobbin of second transmitter coil  2004 . First portion  2016   a  can extend past a surface  2018  of second transmitter coil  2004 , and second portion  2016   b  can extend past a surface  2020  opposite of surface  2018 . Contact pins  2022  of second transmitter coil  2004  can be positioned to extend past an end of second portion  2016   b  along a direction with second portion  2016   b  to make contact with the underlying driver board. 
     According to some embodiments of the present disclosure, protrusions  2008  and  2012  of first and third transmitter coils  2002  and  2006 , respectively, can be positioned toward second transmitter coil  2004  so that when the transmitter coils are assembled into transmitter coil arrangement  2000 , the protrusions do not protrude above or below transmitter coil arrangement  2000  as a whole. Additionally, the position of the contact pads of the transmitter coils are arranged such that when the transmitter coils are assembled into transmitter coil arrangement  2000 , the contact pins can extend past a bottom surface of transmitter coil arrangement  2000  to make contact with an underlying driver board. The distance at which contact pins are positioned away from respective surfaces of the transmitter coils is configured such that they can make contact with the driver board even after being assembled as transmitter coil arrangement  2000 , as shown in  FIG. 20B . 
       FIG. 20B  illustrates assembled transmitter coil arrangement  2000  attached to an underlying driver board  2028 . When assembled, the protrusions from the transmitter coils can be nested within transmitter coil arrangement  2000  as shown by the dotted lines representing protrusions  2008 ,  2012 , and  2016   a - b . In some embodiments, no protrusions in transmitter coil arrangement  2000  extend above a top plane  2032  (i.e., surface  2013  of third transmitter coil  2006 ) or extend below a bottom plane  2030  (i.e., surface  2011  of first transmitter coil  2002 ) of transmitter coil arrangement  2000 . Accordingly, protrusion  2008  can extend a distance from surface  2010  that is less than the combined thickness of the coil of windings of second and third transmitter coils  2004  and  2006 ; likewise, protrusion  2012  can extend a distance from surface  2014  that is less than the combined thickness of the coil of windings of first and second transmitter coils  2002  and  2004 . Because second transmitter coil  2004  is positioned between first and third transmitter coils  2002  and  2006 , portion  2016   a  can extend a distance from surface  2018  that is less than the thickness of the coil of windings of third transmitter coil  2006 , and portion  2016   b  can extend a distance from surface  2020  that is less than the thickness of the coil of windings of first transmitter coil  2002 . 
     In some embodiments, contact pins may be arranged to make contact with driver board  2028  when transmitter coil arrangement  2000  is assembled. Thus, those transmitter coils that are positioned farthest away from driver board  2028  in the transmitter coil arrangement can have their contact pins positioned farthest away from its coil of wire. For instance, as shown in  FIG. 20B , third transmitter coil  2006  can be positioned farthest away from driver board  2028 . Thus, contact pins  2024  can be positioned farthest away from the coil of wire of third transmitter coil  2006  so that they can make contact with driver board  2028  when transmitter coil arrangement  2000  is assembled. In some embodiments, as shown in  FIG. 20A , contact pins  2024  are positioned a distance  2028  from surface  2014  of third transmitter coil  2006 , contact pins  2022  are positioned a distance  2030  from surface  2020  of second transmitter coil  2004 , and contact pins  2026  are positioned a distance  2032  from surface  2011  of first transmitter coil  2002 . Accordingly, distance  2028  may be greater than distance  2030  and  2032 , distance  2030  may be less than distance  2028  but greater than distance  2032 , and distance  2032  may be less than distances  2028  and  2030 . By arranging the contact pins of respective transmitter coils according to these distances, the contact pins can be positioned to make contact with underlying driver board  2028 , as shown in  FIG. 20B , even though the coils with which they are coupled are positioned at different distances away from driver board  2028 . 
     It is to be appreciated that contact pins  2022 ,  2024 , and  2026  extend toward driver board  2028  regardless of which direction protrusions  2016   a - b ,  2012 , and  2008  extend. As an example, contact pins  2026  of first transmitter coil  2002  extend downward toward driver board  2028  even though its protrusion  2008  extends upward. Contact pins  2022 ,  2024 , and  2026  extend toward driver board  2028  to make contact with driver board  2028  so that control board  2028  can operate the transmitter coils to perform wireless charging. 
     V. Transmitter Coils without Bobbins 
     Aforementioned embodiments discussed herein are directed to transmitter coil arrangements formed of transmitter coils with bobbins. However, it is to be appreciated that transmitter coil arrangements according to embodiments of the present disclosure are not required to be formed of transmitter coils with bobbins. In some embodiments, the transmitter coil arrangements may be formed of transmitter coils without bobbins and yet still achieve the same coverage, performance, and efficiency of transmitter coil arrangements formed of transmitter coils with bobbins. 
       FIG. 21  illustrates an exemplary transmitter coil  2100  without a bobbin, according to some embodiments of the present disclosure. Transmitter coil  2100  can include a coil of wire  2102  wound between an inner radius  2104  and an outer radius  2106 . Coil of wire  2102  can be formed of a plurality of thin wires, similar to coil of wire  1200  in  FIG. 12A , or formed of a single core of conductive material, similar to coil of wire  1300  in  FIG. 13A . 
     In some embodiments, coil of wire  2102  may wind from an initial location  2108  to a termination location  2110 . Initial location  2108  may be a position along coil of wire  2102  where the wire initiates winding, and termination location  2110  may be a position along coil of wire  2102  where the wire terminates winding. The windings of wire may not substantially diverge from one another between initial location  2108  and termination location  2110 . In some embodiments, termination location  2108  can be positioned based on initial location  2108  to achieve a substantially even winding profile. For instance, termination location  2108  can be positioned directly across coil of winding  2102  from initial location  2108 . By positioning initial location  2108  and termination location  2110  this way, the number of windings may be as close to a whole integer as possible, thereby achieving a substantially even winding profile. The substantially even winding profile can minimize the size of a portion  2116  that has a different number of turns than the rest of transmitter coil  2100 , as discussed herein with respect to FIGS.  14 A and  14 C. Furthermore, the number of turns may be determined according to a target inductance value determined by design. As more turns are formed in a transmitter coil, the inductance of the transmitter coil increases. Having too much inductance in a transmitter coil may create inefficient power delivery. In particular embodiments, the number of turns may range between six to eight turns, such as seven turns in some embodiments. 
     Transmitter coil  2100  can also include a first termination end  2112  and a second termination end  2114 . Each termination end  2122  and  2214  can be a point at which coil of wire  2102  physically ends. Unlike transmitter coils with bobbins, second termination end  2114  may not fold over coil of wire  2102  to be positioned within the inner diameter of transmitter coil  2100 . Instead, second termination end  2114  may begin to diverge away from coil of wire  2102  at termination location  2110  and stop outside of coil of wire  2102 . First and second termination ends  2112  and  2114  can couple with first and second termination zones  2118  and  2120  to make contact with an underlying driver board. First termination zone  2118  may be positioned within the inner diameter of transmitter coil  2100 , but second termination zone  2120  may be positioned outside of the inner diameter of transmitter coil  2100 . In some embodiments, second termination zone  2120  may be positioned within another transmitter coil when transmitter coil  2100  is assembled in a transmitter coil arrangement, as discussed herein with respect to  FIG. 22 . 
       FIG. 22A  illustrates an exemplary transmitter coil arrangement  2200  formed of transmitter coils without bobbins, according to some embodiments of the present disclosure. Positions of the transmitter coils in transmitter coil arrangement  2200  can be controlled by carriers that define the positions of the transmitter coils according to the respective positions shown in  FIG. 22A  during assembly. Each carrier can include an array of bosses that define the location of the transmitter coils. The bosses can protrude from the carrier surface and provide a structure around which the transmitter coils may be positioned. In some embodiments, each carrier temporarily holds the transmitter coils in place until they are secured to contacts on a driver board. Each carrier may be specific to a different layer of the transmitter coil arrangement. Once the transmitter coils are secured to the driver board, the carrier may be removed, thereby leaving the transmitter coils in their respective positions according to the transmitter coil arrangement. Each layer is assembled, one-by-one, until all the layers are assembled to form the transmitter coil arrangement shown in  FIG. 22 . As will be discussed further herein, each layer of transmitter coils in transmitter coil arrangement  2200  can be fixed in position by a cowling. 
     Each transmitter coil can be arranged in a radial direction suitable for minimizing coupling within an interior region of transmitter coil arrangement  2200 , while also enabling termination ends of each transmitter coil to make contact with a driver board (not shown). Similar to transmitter coil arrangement  1900  in  FIG. 19 , transmitter coil arrangement  2200  can be arranged in three transmitter coil layers according to the transmitter coil arrangement shown in  FIGS. 10-11C . Thus, transmitter coil arrangement  2200  can include outer transmitter coils  2202  and inner transmitter coils  2204 . Outer transmitter coils  2202  may be a single line of transmitter coils positioned near the outermost regions of transmitter coil arrangement  2200 , while inner transmitter coils  2204  may be those transmitter coils surrounded by outer transmitter coils  2202 . 
     In some embodiments, outer transmitter coils  2202  may be arranged in a first radial arrangement where its radial directions point toward the outer edges of transmitter coil arrangement  2200  so that their regions that have a different number of turns, e.g., region  2116  in  FIG. 21 , are oriented toward the outer edges of transmitter coil arrangement  2200 . Thus, the portions of outer transmitter coils  2204  having more turns and lower coupling tendencies may be concentrated toward the interior of transmitter coil arrangement  2200 . This helps ensure that the wireless charging mat has a more consistent and efficient charging surface in the inner regions of the charging surface. Additionally, inner transmitter coils  2204  may be arranged in a second radial arrangement different than the first radial arrangement. The second radial arrangement can be where inner transmitter coils  2204  are arranged according to different angular offsets with respect to one another as shown in  FIG. 22 . For instance, inner transmitter coils  2204  can be arranged in angular offsets between 50-70 degrees, particularly 60 degrees in some embodiments. Arranging inner transmitter coils  2204  according to the second radial arrangement allows their termination ends to reach an underlying interconnection structure by terminating in the inner diameters of adjacent transmitter coils. 
     Given that transmitter coils without bobbins do not have a folding-over portion nor a bobbin that protrudes from a plane of a winding of coil, inner transmitter coils  2204  do not need to be arranged in a radial direction that nests the protrusions in adjacent layers to minimize z-height. Instead, inner transmitter coils  2204  may only need to be arranged so that their second termination ends can make contact with an underlying driver board (not shown). The second termination ends of the transmitter coils can make contact with the underlying driver board when the second termination zones are positioned so that they are not blocked by another transmitter coil. Accordingly, the second termination zones for the inner transmitter coils  2204  can be positioned within an inner diameter of an adjacent transmitter coil. As shown in  FIG. 22 , inner transmitter coils  2204  can be arranged in various radial directions offset from one another at an angular offset of between 50 and 70 degrees, such as approximately 60 degrees in some embodiments. Arranging inner transmitter coils  2204  in this way allows their second termination zones to be positioned within the inner diameter of neighboring transmitter coils so that their second termination ends can make contact with the underlying driver board. 
     As can be seen in  FIG. 22A , each transmitter coil can have an outer termination zone  2208  and an inner termination zone  2206  where respective termination ends reside. As mentioned herein, each termination zone may be a region where a termination end is positioned. The termination end can be a point at which a winding of the respective transmitter coil physically ends, but whose electrical connection can continue if it is coupled with a standoff for connecting with an underlying driver board. Outer termination zone  2208  can be a termination zone that is positioned outside of an outer diameter of its respective transmitter coil, e.g., transmitter coil  2210 . Inner termination zone  2208  can be a termination zone that is positioned inside an inner diameter of its respective transmitter coil. Thus, outer transmitter coils can have an outer termination zone that is positioned near an outer perimeter of the transmitter coil arrangement, and inner transmitter coils can have outer termination zones that are positioned within an inner diameter of an adjacent transmitter coil. For instance, transmitter coil  2212  can be positioned as an inner transmitter coil and have an outer termination zone  2214  that is positioned in an inner diameter of adjacent transmitter coil  2218 . 
     Given that each transmitter coil has two termination zones, it can be appreciated that a transmitter coil arrangement can have numerous termination zones for coupling with an underlying driver board. In many cases, the positions of these termination zones can affect the efficiency at which the transmitter coil arrangement operates. Thus, in some embodiments, termination zones of a transmitter coil arrangement can be arranged to have a degree of similarity to improve simplicity in design and improvement in operating efficiency, as discussed herein with respect to  FIGS. 22B-22E . 
       FIG. 22B  is a simplified diagram illustrating an exemplary transmitter coil arrangement  2201  formed of transmitter coils without bobbins and with similarly organized termination ends, according to some embodiments of the present disclosure. Transmitter coil arrangement  2201  is formed of 22 transmitter coils arranged in an overlapping arrangement such that different coils in the plurality of coils are on different planes and each transmitter coil of the transmitter coil arrangement has a central axis that is positioned a lateral distance away from central axes of all other transmitter coils, as discussed herein with respect to  FIGS. 3-7C . According to some embodiments of the present disclosure, the organization of termination zones can be derived according to a base pattern of termination zones that is repeated substantially throughout the transmitter coil arrangement. As an example, transmitter coil arrangement  2201  can have termination zones that are substantially positioned according to a base pattern  2220 . In some embodiments, base pattern  2220  is established by the termination zones of five transmitter coils shown with bolded lines in  FIG. 22B . The termination zones of base pattern  2220  can be repeated throughout a majority of transmitter coil arrangement  2201  except for the termination zones of the farthest left and right transmitter coils. The termination zones of those farthest left and right transmitter coils can be positioned such that one termination zone is outside of the coil and the other termination zone is inside of the coil. 
     A more detailed view of the transmitter coils in transmitter coil arrangement  2201  can be seen in  FIGS. 22C-E .  FIGS. 22C-E  are simplified diagrams of sets of transmitter coils in each layer of transmitter coil arrangement  2201 .  FIG. 22C  illustrates the angular orientations of a first set of transmitter coils  2222 . As shown in  FIG. 22C , transmitter coils  2228   b ,  2228   c ,  2228   e  and  2228   f  that are positioned amongst the outer transmitter coils can have angular orientations that are either vertically upward or downward, except for the farthest right transmitter coil  2228   g  when arranged in transmitter coil arrangement  2201 . Transmitter coils  2228   a  and  2228   d  that are positioned amongst the inner transmitter coils can have angular orientations that are vertically upward. As shown in  FIG. 22D , transmitter coils  2230   a ,  2230   b ,  2230   d ,  2230   e ,  2230   g , and  2230   h  that are positioned amongst the outer transmitter coils can have angular orientations that are either vertically upward or downward. Transmitter coils  2230   c  and  2230   f  that are positioned amongst the inner transmitter coils can have angular orientations that are vertically upward. And, as shown in  FIG. 22E , transmitter coils  2232   c ,  2232   b ,  2232   e , and  2232   f  that are positioned amongst the outer transmitter coils can have angular orientations that are either vertically upward or downward, except for the farthest right transmitter coil  2232   a  when arranged in transmitter coil arrangement  2201 . Transmitter coils  2232   d  and  2232   g  that are positioned amongst the inner transmitter coils can have angular orientations that are vertically upward. 
     As can be appreciated by  FIGS. 22C-E , the outer transmitter coils of a transmitter coil arrangement can be arranged in an angular direction that are either facing vertically upward or downward, except for the farthest left and right transmitter coils. And, the inner transmitter coils of a transmitter coil arrangement can be arranged in an angular direction that is facing in the same direction, e.g., vertically upward. In this manner, the position of termination zones for the transmitter coils in transmitter coil arrangement  2201  can be substantially similar to each other, thereby simplifying design and enhancing charging efficiency. 
     Unlike transmitter coils with bobbins that have contact pins that extend below a plane of the coil of wire to make contact with the underlying driver board, transmitter coils without bobbins can make contact with the underlying driver board by making contact with surface-mounted standoffs having contact pads that are elevated from the underlying driver board once installed on a driver board. Thus, the contact pads can be positioned in the same plane as the respective transmitter coils to which they are coupled. Details of such standoffs will be discussed further herein. 
     VI. Wireless Charging Mat Assembly 
       FIG. 23  illustrates an exploded view of an exemplary wireless charging mat  2300  having transmitter coils with bobbins, according to some embodiments of the present disclosure. Transmitter coils with bobbins can correspond to transmitter coils discussed herein with respect to  FIGS. 16A-20B . Wireless charging mat  2300  can include a housing formed of two shells: a first shell  2302  and a second shell  2304 . First shell  2302  can mate with second shell  2304  to form an interior cavity within which internal components may be positioned. Specifically, surfaces of first and second shells  2302  and  2304  can form walls that define the internal cavity. For instance, first shell  2302  can have a bottom surface that forms a first wall defining a top boundary of the internal cavity. Further, second shell  2304  can have a top surface that forms a second wall defining a bottom boundary of the internal cavity. Side surfaces of both first and second shells  2302  and  2304  can have sidewalls that form the lateral boundaries of the internal cavity. First and second shells  2302  and  2304  can also include notches  2306   a  and  2306   b , respectively, that form an opening within the housing when first and second shells  2302  and  2304  are mated. An electrical connector  2308 , such as a receptacle connector, can be positioned within the opening so that wireless charging mat  2300  can receive power from an external power source through a cable connected to electrical connector  2308 . In some embodiments, electrical connector  2308  may include a plurality of contact pins and a plurality of terminals electrically coupled to the contact pins so that power can be routed from the external power source to the wireless charging mat  2300  to provide power for wireless power transfer. 
     First and second shells  2302  and  2304  may each be formed of more than one layer. For instance, first shell  2302  can include a top covering  2310 , a compliant layer  2312 , and a stiffening layer  2314 . In some embodiments, compliant layer  2312  can be disposed between top covering  2310  and stiffening layer  2314 . Top covering  2310  may be a cosmetic layer that is exposed when wireless charging mat  2300  is assembled. According to some embodiments, a top surface of top covering  2310  includes a charging surface  2316  upon which a device  2340  having a wireless power receiver coil  2342  may be placed to receive power from wireless charging mat  2300 . The size and dimensions of charging surface  2316  can be defined by one or more transmitter coil arrangements (e.g., any transmitter coil arrangement discussed herein) encased between first and second shells  2302  and  2304 . 
     Stiffening layer  2314  can be a rigid structure that gives wireless charging mat  2300  structural integrity. Any suitable stiff material may be used to form stiffening layer  2314  such as fiberglass. Compliant layer  2312  can be positioned under top covering  2310  to provide a soft, pillow-like texture for devices to rest on when contacting with top covering  2310  to receive power. Compliant layer  2312  can be formed of any suitable compliant material, such as a foam or any other porous material. 
     Second shell  2304  can include a bottom covering  2318 , a bottom chassis  2320 , and a drop frame  2322 . In some embodiments, bottom chassis  2320  can be positioned between bottom covering  2318  and drop frame  2322 . Bottom covering  2318  may be an outer covering that is exposed when wireless charging mat  2300  is assembled. Bottom chassis  2320  can be a stiff structure for providing structural rigidity for wireless charging mat  2300 . In some embodiments, bottom chassis  2320  can be formed of any suitable stiff materials, such as fiberglass or carbon fiber. Drop frame  2322  may be a structural support layer that forms the backbone of wireless charging mat  2300 . In some embodiments, drop frame  2322  is a stiff layer of plastic within which a plurality of openings  2348  are formed. Each opening  2348  can be formed to have dimensions corresponding to an electronic device, such as an inverter for operating one or more transmitter coils, as will be discussed further herein. 
     As mentioned above, top and bottom shells  2302  and  2304  can mate to form an inner cavity. Several internal components as shown in  FIG. 23  can be positioned within the inner cavity. The internal components may include detection coils  2324  positioned below first shell  2302 . Detection coils  2324  can be an arrangement of coils designed to operate at a predetermined frequency that enables detection coils  2324  to detect the presence of a device positioned on top covering  2310  within charging surface  2316 . 
     In some embodiments, the internal components can also include a transmitter coil arrangement  2326  disposed below detection coils  2324 . According to some embodiments of the present disclosure, transmitter coil arrangement  2326  can be formed of a plurality of generally planar transmitter coils arranged in multiple layers and in an overlapping and non-concentric arrangement where no two coils are concentric with each other. In other words, each transmitter coil can have a central axis that is positioned a lateral distance away from central axes of all other transmitter coils in the plurality of transmitter coils. For instance, transmitter coil arrangement  2326  can include three layers of transmitter coils (e.g., first layer  2328 , second layer  2330 , and third layer  2332 ) where each layer includes a plurality of transmitter coils that are arranged coplanar with one another. Some exemplary transmitter coil arrangements include transmitter coil arrangements  800 ,  1000 ,  1900 , and  2200  in  FIGS. 8, 10, 19, and 22  discussed herein above. Transmitter coil arrangement  2326  can be formed of stranded transmitter coils as discussed herein with respect to  FIGS. 16A-16B and 18A-18B . In some other embodiments, transmitter coil arrangement  2326  can be formed as an array of patterned conductive wires in a PCB. 
     Transmitter coil arrangement  2326  can be operated to generate time-varying magnetic fields that propagate above the top surface of first shell  2302  to induce a current in receiver coil  2342  in electronic device  2340 . Coverage of the time-varying magnetic fields generated by transmitter coil arrangement  2326  may coincide with the dimensions of charging surface  2316 . In some embodiments, every transmitter coil in transmitter coil arrangement  2326  includes a coil of wire that is wound in the same direction. Receiver coil  2342 , on the other hand, can include a coil of wire that is wound in the opposite direction as the transmitter coils. For instance, every coil of wire in transmitter coil arrangement  2326  is wound in a clockwise direction, while the coil of wire of receiver coil  2342  is wound in a counter-clockwise direction. 
     In some embodiments, a ferrite layer  2334  can be disposed below transmitter coil arrangement  2326 . Ferrite layer  2334  may be a layer of ferromagnetic material configured to prevent magnetic fields generated by transmitter coil arrangement  2326  from disrupting components disposed below transmitter coil arrangement  2326 . Ferrite layer  2334  can be sized and shaped to correspond to charging surface  2316  and/or to transmitter coil arrangement  2326 . In certain embodiments, ferrite layer  2334  can be positioned directly below first transmitter coil layer  2328 . In such embodiments, first transmitter coil layer  2328  can include coils of wire that have less turns than the coils of wire in second and third transmitter coil layers  2330  and  2332 . Ferrite layer  2334  can include a plurality of openings corresponding to the positions of contacts pins of transmitter coils in transmitter coil arrangement  2326 . The plurality of openings allow the transmitter coils to make contact with components disposed below ferrite layer  2334 . For instance, the plurality of openings can allow the transmitter coils to make contact with a driver board  2336  disposed below ferrite layer  2334 . 
     Driver board  2336  may be an electrical interconnection structure, such as a PCB, flex circuit, patterned ceramic board, patterned silicon substrate, and the like, configured to route signals and power for operating transmitter coil arrangement  2326 . In some embodiments, driver board  2336  includes plurality of contacts  2346  positioned to make contact with corresponding contact pins of transmitter coils in transmitter coil arrangement  2326 . A plurality of inverters can be mounted on an underside of driver PCB  2336  for operating the transmitter coils in transmitter coil arrangement  2326 . Each inverter can be positioned at locations corresponding to respective transmitter coils with which the inverter makes contact. In some embodiments, the plurality of inverters can be surface mounted to the bottom surface of driver PCB  2336  such that they extend below driver PCB  2336 . Accordingly, the plurality of inverters can insert into respective openings  2348  in drop frame  2322 . Openings  2348  can be positioned at locations corresponding to respective inverters mounted on driver PCB  2336 . As shown in  FIG. 23 , notches  2350  may be formed in ferrite layer  2334  and driver PCB  2336  for receptacle connector  2308  to be positioned within wireless charging mat  2300  when assembled. 
     In some embodiments, a ground ring  2338  can be wound along at least a portion of the outer perimeter of driver PCB  2336 . Ground ring  2338  may be a conductive wire wound along the outer perimeter of driver PCB  2336  except for a location where receptacle connector  2308  is coupled to driver PCB  2336 . 
       FIG. 24  illustrates an exploded view of an exemplary wireless charging mat  2400  having transmitter coils without bobbins, according to some embodiments of the present disclosure. Transmitter coils without bobbins can correspond to transmitter coils discussed herein with respect to  FIGS. 21 and 22 . Like wireless charging mat  2300 , wireless charging mat  2400  can include a housing formed of two shells: a first shell  2402  and a second shell  2404 . First shell  2402  can mate with second shell  2404  to form an interior cavity within which internal components may be positioned. Similar to wireless charging mat  2300 , first and second shells  2402  and  2404  can also include notches  2406   a  and  2406   b , respectively, that form an opening within the housing when first and second shells  2402  and  2404  are mated. An electrical connector  2408 , such as a receptacle connector, can be positioned within the opening so that wireless charging mat  2400  can receive power from an external power source through a cable connected to electrical connector  2408 . In some embodiments, electrical connector  2408  may include a plurality of contact pins and a plurality of terminals electrically coupled to the contact pins so that power can be routed from the external power source to the wireless charging mat  2400  to provide power for wireless power transfer. 
     First and second shells  2402  and  2404  can each be formed of more than one layer. For instance, first shell  2402  can include a top covering  2410  and a stiffening layer  2412 . Top covering  2410  can be a cosmetic layer that is exposed when wireless charging mat  2400  is assembled. According to some embodiments, a top surface of top covering  2410  includes a charging surface  2414  upon which a device  2416  having a wireless power receiver coil  2415  may be placed to receive power from wireless charging mat  2400 . The size and dimensions of charging surface  2416  can be defined by one or more transmitter coil arrangements (e.g., any transmitter coil arrangement discussed herein) encased between first and second shells  2402  and  2404 . 
     In some embodiments, top covering  2410  can include a compliant layer (not shown) disposed below charging surface  2414 . The compliant layer can be configured to provide a soft, pillow-like texture for devices to rest on when contacting with top covering  2410  to receive power. The compliant layer can be formed of any suitable compliant material, such as a foam or any other porous material. Stiffening layer  2414  can be positioned below top covering  2410 , and be composed of a rigid structure that gives wireless charging mat  2400  structural integrity. Any suitable stiff material may be used to form stiffening layer  2414  such as fiberglass or a stiff polymer (e.g., molded Kalix). 
     Second shell  2404  can include a bottom covering  2418  and a bottom chassis  2420 . In some embodiments, bottom chassis  2420  can be positioned against bottom covering  2418  such that bottom chassis  2420  is not shown when wireless charging mat  2400  is assembled. Bottom covering  2418  may be an outer covering that is exposed when wireless charging mat  2400  is assembled. Bottom chassis  2420  can be a stiff structure for providing structural rigidity for wireless charging mat  2400 . In some embodiments, bottom chassis  2420  can be formed of any suitable stiff materials, such as fiberglass, carbon fiber, or stainless steel. 
     As mentioned above, top and bottom shells  2402  and  2404  can mate to form an inner cavity. As shown in  FIG. 24 , various internal components can be positioned within the inner cavity. For instance, the internal components can include a transmitter coil arrangement  2429 . According to some embodiments of the present disclosure, transmitter coil arrangement  2429  can be formed of a plurality of generally planar transmitter coils arranged in multiple layers and in an overlapping and non-concentric arrangement where no two coils are concentric with each other. For instance, transmitter coil arrangement  2429  can include three layers of transmitter coils (e.g., first layer  2428   a , second layer  2428   b , and third layer  2428   c ) where each layer includes a plurality of transmitter coils that are arranged coplanar with one another. Some exemplary transmitter coil arrangements include those that have transmitter coils wound about a bobbin, such as transmitter coil arrangements  800 ,  1000 , and  1900  in  FIGS. 8, 10, and 19  discussed herein, and those that include transmitter coils that are not wound about a bobbin, such as transmitter coil arrangement  2200  shown in  FIG. 22 , discussed herein. Furthermore, transmitter coil arrangement  2429  can have any suitable number of transmitter coils. For instance, transmitter coil arrangement  2429  can have a total of 16 coils, such as transmitter coil arrangement  605  in  FIG. 6D , or a total of 22 coils, such as transmitter coil arrangement  607  in  FIG. 6E . 
     Transmitter coil arrangement  2429  can be operated to generate time-varying magnetic fields that propagate above the top surface of first shell  2402  to induce a current in receiver coil  2415  in electronic device  2416 . Coverage of the time-varying magnetic fields generated by transmitter coil arrangement  2429  may coincide with the dimensions of charging surface  2416 . 
     Wireless charging mat  2400  can also include a plurality of cowlings  2431  for housing transmitter coil arrangement  2429 . For instance, plurality of cowlings  2431  can include a first cowling  2430   a , a second cowling  2430   b , and a third cowling  2430   c . Each cowling can be a substantially planar structure that has openings  2431   a - c  within which transmitter coils can reside. For instance, first cowling  2430   a  can house first transmitter coil layer  2428   a , second cowling  2430   b  can house second transmitter coil layer  2428   b , and third cowling  2430   c  can house third transmitter coil layer  2428   c . When the transmitter coils are housed within the cowling, the cowling can confine the transmitter coils to their respective positions and prevent them from shifting in any lateral direction. Some parts of each cowling can also reside within an inner diameter of transmitter coils to avoid any vacant space within the layer. Vacant space can allow deflection of structures in adjacent layers, which can cause physical stress upon one or more components and lead to excessive wear and tear. In some embodiments, the thickness of each cowling  2430   a - c  is equal to the thickness of a transmitter coil. Thus, when transmitter coils are housed within a respective cowling, the cowling and transmitter coils combine to form a substantially planar structure that does not have large open spaces within it. 
     In some embodiments, wireless charging mat  2400  can also include one or more spacers for separating each layer of transmitter coils and cowlings. For instance, wireless charging mat  2400  can include a first spacer  2444   a , a second spacer  2444   b , and a third spacer  2444   c . First spacer  2444   a  can be positioned between first transmitter coil layer  2428   a  and second transmitter coil layer  2428   b  to separate the two transmitter coil layers  2428   a  and  2428   b  by a set distance defined by the thickness of first spacer  2444   a . Similarly, second spacer  2444   b  can be positioned between second transmitter coil layer  2428   b  and third transmitter coil layer  2428   c  to separate the two transmitter coil layers  2428   b  and  2428   c  by a set distance defined by the thickness of second spacer  2444   b . Furthermore, third spacer  2444   c  can be positioned between third transmitter coil layer  2428   c  and electromagnetic shield  2422  to separate them by a set distance defined by the thickness of third spacer  2444   a . In some embodiments, the thickness of spacers  2444   a - c  are equal such that each transmitter coil layer  2428   a - c  and electromagnetic shield  2422  are separated from each other by the same distance. One purpose of spacers  2444   a - c  is to define a degree of parasitic capacitance between adjacent conductive layers (e.g., transmitter coil layers  2428   a - c  and electromagnetic shield  2422 ). By defining the space between the conductive layers to be equal, it provides an increase of sensitivity to detection of foreign objects on charging surface  2414 , specifically in the high frequency range. 
     During wireless power transfer, transmitter coil arrangement  2429  can generate time-varying magnetic fields for inducing a corresponding current in receiver coil  2415 . These generated magnetic fields, if not controlled, can generate noise and detrimentally affect surrounding components. Thus, transmitter coil arrangement  2429  can be surrounded by several components to confine the magnetic fields such that they are generated in one direction and do not disturb neighboring components. In some embodiments, the components include a ferromagnetic shield  2432 , an electromagnetic shield  2422 , a grounding fence  2424 , and a driver board  2426  as will be discussed further herein. 
     Ferromagnetic shield  2432  can be a layer of ferromagnetic material that is disposed below transmitter coil arrangement  2429  and configured to prevent magnetic fields generated by transmitter coil arrangement  2429  from disrupting components disposed below ferromagnetic shield  2432 . Ferromagnetic shield  2432  can be sized and shaped according to charging surface  2416  and/or to transmitter coil arrangement  2429 . In certain embodiments, ferromagnetic shield  2432  can be positioned directly below first transmitter coil layer  2428   a . In such embodiments, first transmitter coil layer  2428   a  can include coils of wire that have less turns than the coils of wire in second and third transmitter coil layers  2428   b  and  2428   c . Ferromagnetic shield  2432  can include a plurality of openings corresponding to the positions of contacts pins of transmitter coils in transmitter coil arrangement  2429 . The plurality of openings allow the transmitter coils to make contact with components disposed below ferromagnetic shield  2432 , such as driver board  2426 . 
     As mentioned herein, electromagnetic shield  2422  can also be included with wireless charging mat  2400 . Electromagnetic shield  2422  can be positioned below first shell  2402  and can be configured to prevent the generation of detrimental voltages on a receiver coil during wireless power transfer. Particularly, electromagnetic shield  2422  can be configured to intercept electric fields generated by transmitter coils within wireless charging mat  2400  during wireless power transfer so that detrimental voltages are prevented from being generated on a receiver coil, e.g., receiver coil  2415 . The structure and material composition of electromagnetic shield  2422  is discussed further herein with respect to  FIGS. 25A and 25B . 
       FIG. 25A  is a top-view illustration of an exemplary electromagnetic shield  2500 , according to some embodiments of the present disclosure. Electromagnetic shield  2500  can include a shielding body  2502  and a conductive border  2504  around a perimeter of shielding body  2502 . Shielding body  2502  can intercept electric fields generated by one or more transmitter coils in wireless charging mat  2400  and discharge the voltage generated by the intercepted electric fields to ground through conductive border  2504 . In some embodiments, shielding body  2502  is constructed of a material having properties that enable magnetic flux to pass through the shielding body but prevent electric fields from passing through. For instance, shielding body  2502  can be formed of silver laminated on a layer of pressure sensitive adhesive (PSA). The silver layer can have a thickness of approximately 30-40 μm, particularly 35 μm in one embodiment. As further shown in  FIG. 25A , conductive border  2504  can be constructed as a thin conductive region around shielding body  2502 ; however, embodiments are not so limited. Other embodiments can have different configurations of conductive border  2504 , as shown in  FIG. 25B . 
       FIG. 25B  is a top-view illustration of another exemplary electromagnetic shield  2501 , according to some embodiments of the present disclosure. Electromagnetic shield  2501  can include shield body  2502  and a conductive border  2506  that extends to edges of a transmitter coil arrangement, such as any transmitter coil arrangement discussed herein. By extending conductive border  2506  to edges of the transmitter coil arrangement, transmission efficiency of magnetic fields thorough electromagnetic shield  2501  can be improved over the transmission efficiency of electromagnetic shield  2500 . 
     Conductive border  2504  and  2506  can be formed of a conductive material, such as copper. The conductive border  2504  and  2506  can be a thin sheet of copper that is adhered onto the surface of shielding body  2502 . The conductive properties of conductive border  2504  and  2506  allows voltage generated by intercepted electric fields to be routed to ground. In some embodiments, conductive border  2504  can route voltage to a grounding fence, such as grounding fence  2424  shown in  FIG. 24 . 
     Referring back to  FIG. 24  and as aforementioned herein, wireless charging mat  2400  can include grounding fence  2424 , according to some embodiments of the present disclosure. Grounding fence  2424  can be wound along at least a portion of the outer perimeter of driver board  2426  and attach to at least a portion of the outer perimeter of electromagnetic shield  2422 . Grounding fence  2424  can be formed of a length of wire having conductive properties, as well as shielding properties to inhibit propagation of magnetic fields through grounding fence  2422 . For instance, grounding fence  2422  can be formed of a metal, e.g. steel, or a coated metal, e.g., nickel plated steel. 
     Driver board  2426  can be a PCB configured to route signals and power for operating transmitter coil arrangement  2429 . In some embodiments, driver board  2426  can include a plurality of bonding pads  2442  for routing power to transmitter coil arrangement  2429  via a plurality of standoffs, as will be discussed further herein. Electrical connector  2408  can be mounted on driver board  2426  so that driver board  2426  can receive power from an external source to operate transmitter coil arrangement  2429 . The combination of driver board  2426 , grounding fence  2424 , ferromagnetic shield  2432  and electromagnetic shield  2422  can form a faraday cage that encloses transmitter coil arrangement  2429  to control the emission of time-varying magnetic fields generated by transmitter coil arrangement  2429 . For instance, the faraday cage can direct magnetic flux out of the faraday cage in a single direction while substantially preventing the propagation of magnetic flux in all other directions out of the faraday cage. A better understanding and a different perspective of this faraday cage is discussed with respect to and shown in  FIGS. 26A and 26B . 
       FIG. 26A  is a cross-sectional view of a part of the faraday cage around transmitter coil arrangement  2429  (not shown) of a partially-formed wireless charging mat, according to some embodiments of the present disclosure. It is to be appreciated that transmitter coil arrangement  2429  is not shown because only an edge of the faraday cage is shown and that transmitter coil arrangement  2429  is positioned away from the edges of the faraday cage, but edges of plurality of cowlings  2431  can be seen. Furthermore, it is to be appreciated that the part of the faraday cage shown in  FIG. 26A  is only for one side of the wireless charging mat and that one skilled in the art understands that this illustration is representative of substantially all edges of a wireless charging mat. As shown in  FIG. 26A , plurality of cowlings  2431  (and transmitter coil arrangement  2429 ) are enclosed by a faraday cage formed of electromagnetic shield  2422 , grounding fence  2424 , ferromagnetic shield  2432 , and driver board  2426 . 
     According to some embodiments, the faraday cage can be configured to allow magnetic flux to propagate in one direction. For instance, grounding fence  2424  can be configured to substantially resist propagation of magnetic flux from transmitter coil arrangement  2429  through grounding fence  2424  so that magnetic fields are contained within the faraday cage in a lateral direction. Additionally, ferromagnetic shield  2432  can be configured to redirect magnetic flux to substantially mitigate the propagation of magnetic flux into driver board  2426  from transmitter coil arrangement  2429 , i.e., downward and out of the faraday cage. However, electromagnetic shield  2422  can be configured to allow magnetic flux to propagate through so that the magnetic flux is directed out of the faraday cage in a single direction, e.g., upwards toward a receiver coil in an electronic device. By configuring the faraday cage to allow the propagation of magnetic flux in one direction, the faraday cage can prevent the generated magnetic flux from creating noise in other electrical systems in the wireless charging mat while purposefully allowing magnetic flux to propagate in a direction toward a receiver coil to perform wireless charging. 
     In some embodiments, electromagnetic shield  2422  is attached to grounding fence  2424  so that voltages generated on electromagnetic shield  2422  during wireless charging can be discharged to ground. In some instances, conductive border  2506  of electromagnetic shield  2422  is attached to grounding fence  2424  via laser welding to achieve a robust electrical and physical connection. Furthermore, ferromagnetic shield  2432  can be positioned on a surface of driver board  2426  to mitigate the propagation of magnetic flux into driver board  2426 . In some embodiments, ferromagnetic shield  2432  is positioned on driver board  2426  and laterally from grounding fence  2424  such that ferromagnetic shield  2432  is not positioned between grounding fence  2424  and driver board  2426 . By not attaching ferromagnetic shield  2432 , its brittle structure will not be exposed to physical stresses at the interface between grounding fence  2424  and driver board  2426 , thereby minimizing damage to ferromagnetic shield  2432 . 
     As mentioned herein with respect to electromagnetic shield  2501  shown in  FIG. 25B , conductive border  2506  is adhered to shielding body  2502 . In some embodiments, one or more adhesives can be used to attach conductive border  2506  to an edge of shielding body  2502 .  FIG. 26B  is a close-up cross-sectional view of an interface between shielding body  2502  and conductive border  2506 . As shown, conductive border  2506  can be attached to shielding body  2502  by adhesive layers  2602  and  2604 . Adhesive layers  2602  and  2604  can be any suitable conductive adhesive, such as a single or double sided copper tape. In some embodiments, adhesive layer  2602  is a layer of double-sided copper tape and adhesive layer  2604  is a layer of single-sided adhesive tape. Using a conductive adhesive allows voltage captured on electromagnetic shield  2502  to be routed to grounding fence  2424  through conductive border  2506 . Although  FIG. 26B  illustrates conductive border  2506 , it is to be appreciated that disclosures herein also apply to embodiments where conductive border  2504  is used instead. In some embodiments, shield  2422  can be secured to third cowling layer  2430   c  with an adhesive so that it does not substantially move in place during use. For instance, shield  2422  can be secured via an adhesive  2606 , such as PSA. 
     As discussed herein, a driver board can be a PCB configured to operate a transmitter coil arrangement. Thus, with reference back to  FIG. 24 , driver board  2426  can be electrically coupled to the transmitter coils in transmitter coil arrangement  2429  via a plurality of standoffs  2434 , according to some embodiments of the present disclosure. In some embodiments, each standoff is coupled to a respective bonding pad  2442  for enabling power transfer from driver board  2426  to transmitter coil arrangement  2429 . Standoffs  2434  can be configured to route power between driver board  2426  and each layer of transmitter coil arrangement  2429 . For instance, standoffs  2434  can be composed of a plurality of conductive posts that can route power from one end of the post to an opposite end of the post, as discussed herein with respect to  FIGS. 27A-B  and  28 A-B. 
       FIGS. 27A and 27B  illustrate an exemplary standoff  2700 , according to some embodiments of the present disclosure. Standoff  2700  can include a first contact  2702  on one end and a second contact  2704  on an opposite end. A connector  2706  can electrically couple first contact  2702  to second contact  2704  so that power can be routed between contacts  2702  and  2704 . In some embodiments, first contact  2702 , second contact  2704 , and connector  2706  form one monolithic structure that is shaped like the letter “U” tilted on its side. This monolithic structure can have a degree of mechanical compliance when pressure is applied in the vertical direction. Thus, in some embodiments, first contact  2702 , second contact  2704 , and connector  2706  can be formed of a substantially stiff material that is highly conductive, such as a copper alloy with a conductivity of approximately 60%-90% of the conductivity of copper. Some exemplary copper alloys include, but are not limited to, NKC4419, NKE 010, and C19210. 
     In addition to using mechanically strong conductive materials for forming the monolithic structure, separate support structures can be used to strengthen standoff  2700  as well. For example, a support component  2708  can be positioned between first and second contacts  2702  and  2704  to provide structural support for standoff  2700 . Support component  2708  can also extend over sidewalls of first and second contacts  2702  so that only the top surface of first contact  2702  and the bottom surface of second contact  2704  are exposed. To strengthen the structural coupling between support component  2708  and the monolithic structure, one or more hook structures can be implemented in first and/or second contacts  2702  and  2704 , as shown in  FIG. 28A . 
       FIGS. 28A and 28B  illustrate an exemplary standoff  2800  with hook structures  2810 , according to some embodiments of the present disclosure. Like standoff  2700 , standoff  2800  can include a first contact  2802  and a second contact  2804  that are coupled together via connector  2806 . First connect  2802 , second contact  2804 , and connector  2806  can form a monolithic structure that is similar to standoff  2700 . In some embodiments, standoff  2800  includes hook structures  2810  that extend from first contact  2802  and/or second contact  2804 . As shown in  FIG. 28A , hook structures  2810  extend from first contact  2802  and also form part of the monolithic structure. Hook structures  2810  provide additional surface area for making contact with a support structure  2808  shown in  FIG. 28B  to enhance the mechanical coupling with support structure  2808 . 
     As discussed herein, standoffs  2434  can be configured to couple driver board  2426  with each transmitter coil of transmitter coil arrangement  2429 . Accordingly, standoffs  2434  can be configured to have different heights to couple driver board  2426  with transmitter coils in different layers. 
       FIG. 29  illustrates an exemplary assembled transmitter coil arrangement  2900  attached to an underlying driver board (e.g., driver board  2426 ) with standoffs  2902 ,  2904 , and  2906 , according to some embodiments of the present disclosure. Transmitter coil arrangement  2900  can include transmitter coil  2908  in a first transmitter coil layer, transmitter coil  2910  in a second transmitter coil layer, and transmitter coil  2912  in a third transmitter coil layer. Only one transmitter coil from each layer of transmitter coil arrangement  2429  is shown in  FIG. 29  for clarity purposes. 
     When assembled, standoffs  2902 ,  2904 , and  2906  can be nested within transmitter coil arrangement  2900  as shown by the dotted lines. Each standoff  2902 ,  2904 , and  2906  can be configured to have a different height that corresponds to the respective layer of a transmitter coil to which it is coupled. For instance, standoff  2902  can have a first height suitable for coupling driver board  2426  with transmitter coil  2908  in the first transmitter coil layer, standoff  2904  can have a second height suitable for coupling driver board  2426  with transmitter coil  2910  in the second transmitter coil layer, and standoff  2906  can have a third height suitable for coupling driver board  2426  with transmitter coil  2912  in the third transmitter coil layer. Accordingly, standoff  2906  can be taller than both standoffs  2902  and  2904 , and standoff  2904  can be taller than standoff  2902  but shorter than standoff  2906 . Once the three layers of transmitter coils are assembled, adjacent transmitter coils can rest against each other yet still couple with driver board  2426 , thereby minimizing the z-height of transmitter coil arrangement  2900 . 
     With reference back to  FIG. 24 , wireless charging mat  2400  can also include a drop frame  2436  and a bottom shield  2438  for drop frame  2436 , according to some embodiments of the present disclosure. When assembled in wireless charging mat  2400 , bottom shield  2438  can be adhered to drop frame  2436 . Drop frame  2436  can be a structural support layer that forms the backbone of wireless charging mat  2300 . In some embodiments, drop frame  2322  is a stiff layer of plastic within which a plurality of openings  2440  are formed. Each opening  2440  can be formed to have dimensions and a position corresponding to one or more electronic devices, such as a plurality of inverters for operating one or more transmitter coils, as will be discussed further herein. 
       FIG. 30  is a bottom-view illustration of drop frame  2436  coupled to driver board  2426 , according to some embodiments of the present disclosure. The illustration shows drop frame  2436  and driver board  2426  without a bottom shield so that the placement of a plurality of packaged electrical components  3002  can be seen with respect to drop frame  2436 . Thus, openings  2440  of drop frame  2436  can allow driver board  2426  to be seen through each opening  2440  when viewed from the bottom-view perspective. In some embodiments, packaged electrical components  3002 , shown as a plurality of black components of various sizes and shapes, can be disposed on driver board  2426  within openings  2440 . Electrical components  3002  can be any suitable electrical component utilized for operating wireless charging mat  2400 . For instance, electrical components  3002  can be power electronics, microcontrollers, capacitors, resistors, and the like. In some embodiments, electrical components  3002  include a plurality of inverters that can be mounted on a corresponding underside region of driver board  2426  for operating the transmitter coils in transmitter coil arrangement  2429 . 
     In particular embodiments, some of openings  2440  can provide space within which packaged inverters are disposed to operate an arrangement of transmitter coils, such as arrangement of transmitter coils  605  or  607  shown in  FIGS. 6D and 6E . For instance, inverter openings  2442  can be used to provide space in which the packaged inverters are positioned. Inverter openings  2442  are shown with bolded lines so that they are easier to be seen. In some embodiments, the number of inverter openings  2442  for the packaged inverters correspond with the number of transmitter coils used in the arrangement of transmitter coils. For instance, if wireless charging mat incorporates an arrangement of transmitter coils that is composed of 22 coils, then drop frame  2436  can include 22 inverter openings  2442 , where each inverter opening provides corresponds with a respective inverter for supporting a respective transmitter coil. In some embodiments, inverter openings  2442  are disposed such that the packaged inverters can be positioned directly below respective transmitter coils that they support. In other embodiments, one or more inverter openings  242  may not be positioned to allow a packaged inverter to be disposed directly below its respective transmitter coil. However, these inverter openings nevertheless can allow the packaged inverter to be placed very close to its respective transmitter coil and not at an edge of the wireless charging mat where it is far from its respective transmitter coil. By allowing the packaged inverters to be positioned close to, if not directly below, their respective transmitter coils, timing delays and losses caused by high resistances from long trace lengths (as experienced by conventional charging mats where inverters are placed at the perimeter of a charging mat and need to be routed to transmitter coils in the center of the charging mat) can be minimized. 
     According to some embodiments of the present disclosure, bottom shield  2438  (not shown in  FIG. 30 ) can be laminated on a side of drop frame  2436  opposite of the side to which driver board  2426  is coupled. Bottom shield  2438  thus encloses electrical components  3002  within respective openings  2440  so that not only are the electrical components protected from outside electrical disturbances, but that components of wireless charging mat  2400  outside of openings  3002  are not disturbed by noises generated from electrical components  3002 . In some embodiments, bottom shield  2438  is formed of shielding layer and a plurality of insulating layers as shown in  FIG. 31 . 
       FIG. 31  is a top-down view of an exemplary bottom shield  3100 , according to some embodiments of the present disclosure. Bottom shield  3100  can include a shielding layer  3102  and a plurality of insulating layers  3104  attached to shielding layer  3102 . In some embodiments, insulating layers  3104  correspond to one or more openings of a drop frame, such as openings  2440  in  FIG. 30 . For instance, insulating layers  3104  can be configured as strips that correspond to more than one opening  2440 / 2442 , as shown in  FIG. 31 . When constructed in the wireless charging mat, insulating layer  3104  can be attached to drop frame  2436  and positioned between drop frame  2436  (along with its one or more openings) and shielding layer  3102 . Insulating layers  3104  can prevent electrical coupling of electrical components  3002  with the shielding layer  3102 . In some embodiments, shielding layer  3102  is a thin material that is flexible. Thus, areas of shielding layer  3102  directly above openings  2440  can deflect into openings  2440  and make contact with one or more electrical components  3002 . Accordingly, insulating layers  3104  can prevent electrical coupling between shielding layer  3102  and one or more electrical components  3002 . 
     Shielding layer  3102  can be formed of any material suitable for shielding against electrical emissions to and from electrical components  3002 . For instance, shielding layer  3102  can be formed of copper. Insulating layers  3104  can be formed of any electrically insulating material, such as polyimide. 
     In some embodiments, a plurality of posts can be disposed within openings  2440  to mitigate the degree of travel when shielding layer  3102  is depressed into openings  2440 . For instance, with reference back to  FIG. 30 , posts  3004  can be positioned on driver board  2426  in areas where there are open spaces to mitigate deflection of bottom shield  2438 . Additionally, posts  3004  can also prevent electrical components  3002  from damage caused by external objects pressing to openings  2440 . 
     VII. Hybrid PCB and Stranded Coil Wireless Charging Mat 
     According to some embodiments of the present disclosure, a wireless charging mat can be configured to provide power to more than one different device. For instance, one device can be a larger device with larger receiving coils, e.g., a smart phone, table, laptop, and the like, while the other device can be a smaller device with smaller receiving coils, e.g., a smart watch, a small portable music player, and the like. In such embodiments, the wireless charging mat can include more than one transmitter coil arrangement where each transmitter coil arrangement is optimized for charging a different electronic device. Accordingly, the wireless charging mat can advantageously charge more than one different device at a time and/or be equally efficient at charging multiple different devices. 
       FIG. 30  illustrates an exploded view of an exemplary wireless charging mat  3200  including more than one transmitter coil arrangement, according to some embodiments of the present disclosure. Wireless charging mat  3200  can include a first shell  3202  and a second shell  3204 , each of which may be constructed similar to first and second shells  2302  and  2304  in  FIG. 23 . First and second shells  3202  and  3204  can mate to form an inner cavity within which internal components can be housed. In some embodiments, the inner cavity can include more than one transmitter coil arrangement. For instance, the inner cavity can include two transmitter coil arrangements: first transmitter coil arrangement  3206  and second transmitter coil arrangement  3208 . It is to be appreciated that wireless charging mat  3200  can further include other internal components similar to wireless charging mat  2320  in  FIG. 23  but are not shown for clarity purposes. 
     First transmitter coil arrangement  3206  may be optimized to charge a first device  3212  including a first receiver coil  3214  and second transmitter coil arrangement  3208  may be optimized to charge a second device  3216  including a second receiver coil  3218  that has a different size and shape, and thus different electrical characteristics, than the first receiving coil. For example, first device  3212  can be a larger device than second device  3216 , and first receiver coil  3214  can be larger than second receiver coil  3218 . Although each transmitter coil arrangement  3206  and  3208  can be optimized to charge a different device, each transmitter coil arrangement may still charge other devices that they are not optimized to charge but in a less efficient manner. It is to be appreciated that even though  FIG. 30  illustrates only two devices, embodiments discussed herein may be configured to charge more than two devices, each having different sizes and shapes than those shown in  FIG. 30 . Furthermore, it is to be appreciated that each transmitter coil arrangement can charge an electronic device across the entire charging surface. It is not the case where one transmitter coil arrangement can only charge devices in a sub-region of the charging surface, and that the other transmitter coil arrangement can only charge devices in another sub-region of the charging surface. 
     In some embodiments, first and second transmitter coil arrangements  3206  and  3208  can be formed of transmitter coils where the sizes of the transmitter coils are optimized for a different electrical device. As an example, first transmitter coil arrangement  3206  can be formed of transmitter coils of a first size, while second transmitter coil arrangement  3208  is formed of transmitter coils of a second size. The first size can correspond to the size of receiver coil  3214  in first electronic device  3212 , whereas the second size can correspond to the size of receiver coil  3218  in second electronic device  3216 . Accordingly, first transmitter coil arrangement  3206  may be particularly efficient at inducing a current in receiver coil  3214  of first device  3212  but less efficient at inducing a current in receiver coil  3218  of second device  3216 . Conversely, second transmitter coil arrangement  3208  may be particularly efficient at inducing a current in receiver coil  3218  of second device  3216  but less efficient at inducing a current in receiver coil  3214  of first device  3212 . It is to be appreciated that each transmitter coil arrangement can charge an electronic device across the entire charging surface. 
     In additional embodiments, first and second transmitter coil arrangements  3206  and  3208  can be arranged in different patterns where each pattern is optimized for a different electrical device. As an example, first transmitter coil arrangement  3206  can be arranged in a single row of transmitter coils, while second transmitter coil arrangement  3208  is arranged according to any of transmitter coil arrangements  800 ,  1000 ,  1900 , and  2200  in  FIGS. 8, 10, 19 , and  22  discussed herein above. Accordingly, first transmitter coil arrangement  3206  may be particularly efficient at inducing a current in receiver coil  3214  of first device  3212  but less efficient at inducing a current in receiver coil  3218  of second device  3216 , and vice versa. 
     As discussed herein, transmitter coil arrangements can generate time-varying magnetic fields. Thus, first and second transmitter coil arrangements  3206  and  3208  can be operated at various frequencies to generate the time-varying magnetic fields. In some embodiments, first transmitter coil arrangement  3206  can operate at a first frequency while second transmitter coil arrangement  3208  operates at a second frequency. The first and second frequencies may be the same or different when first and second transmitter coil arrangements  3206  and  3208  are arranged in different patterns. However, the first and second frequencies are different when first and second transmitter coil arrangements  3206  and  3208  are arranged in the same pattern. For instance, first and second transmitter coil arrangements  3206  and  3208  can both be arranged according to any of transmitter coil arrangements  800 ,  1000 ,  1900 , and  2200  in  FIGS. 8, 10, 19 , and  22  discussed herein above, or any other transmitter coil arrangement. In such cases, first transmitter coil arrangement  3206  may operate at a frequency that is particularly efficient at inducing a current in receiver coil  3214  of first device  3212  but less efficient at inducing a current in receiver coil  3218  of second device  3216 . Conversely, second transmitter coil arrangement  3208  can operate at a frequency that is particularly efficient at inducing a current in receiver coil  3218  of second device  3216  but less efficient at inducing a current in receiver coil  3214  of first device  3212 . The difference in operating frequencies may depend on the particular operating frequencies of the respective receiver coils. In some embodiments, the difference can range between orders of magnitudes. As an example, the first frequency can be an order of one or two magnitudes higher than the second frequency. In particular embodiments, the first device  3212  is a smart watch and second device  3216  is a smart phone. 
     Furthermore, in some embodiments, first and second transmitter coil arrangements  3206  and  3208  can be formed from the same or different transmitter coils. That is, first transmitter coil arrangement  3206  can be formed from transmitter coils having stranded coiled wire with or without bobbins, e.g., transmitter coils  1600 ,  1800 , and  2100  in  FIGS. 16, 18, and 21 , while second transmitter coil arrangement  3208  can be formed within a PCB. Each form of construction can be tailored to efficiently induce power in a respective device. For instance, the stranded coil construction of second transmitter coil arrangement  3208  may be particularly efficient at inducing a current in receiver coil  3218  of second device  3216  but less efficient at inducing a current in receiver coil  3214  of first device  3212 . 
     Although the invention has been described with respect to specific embodiments, it will be appreciated that the invention is intended to cover all modifications and equivalents within the scope of the following claims.

Metadata:
Filing Date: 20170908
Publication Date: 20190702
Grant Date: 20190702
Priority Date: 20160923
Inventors: GRAHAM, Christopher S.
KARANIKOS, DEMETRIOS B.
KASAR, DARSHAN R.
THOMPSON, PAUL J.
JOL, ERIC S.
HAUG, Grant S.
LARSSON, KARL RUBEN F.
Assignee: APPLE INC
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Family ID: 61685785