Accessory devices to improve wireless power transfer efficiency

Abstract

To provide accessory devices for improving wireless power transfer efficiency.SOLUTION: An accessory for improving wireless power transfer efficiency in a wireless power transfer system can include a magnetic (e.g., ferrite) core dimensioned and positioned so as to reduce flux coupling from a power transmitter winding into friendly metal of a power receiver and / or to enhance flux coupling from the power transmitter winding to the power receiver winding. The core may define an aperture corresponding to an outer dimension (e.g., outer diameter) of the power receiver winding. The core may further have an outer dimension selected to intercept sufficient flux to reduce flux coupling into the friendly metal of the power receiver, such as an outer dimension corresponding to an outer dimension (e.g., outer diameter) of the power transmitter winding. The accessory may be a case for the power receiver device or may be configured to be affixed to a face of the power transmitter device and may include one or more alignment fixtures.SELECTED DRAWING: Figure 3A

Description

[Background technology]

[0001] An exemplary wireless (inductive) power transfer system is shown in FIG. 1. The efficiency of wireless (inductive) power transfer between a wireless power transmitter (PTx) 102 and a wireless power receiver (PRx) 104 depends on the degree of magnetic coupling between the PTx (winding 103) and PRx (winding 105) windings, respectively. The wireless (inductive) power transfer windings can take a variety of forms, including coils wound from suitable wires such as magnet wire or Litz wire, or traces suitably formed on a printed circuit board (PCB) or flexible printed circuit board (flex circuit). These windings may be considered generally circular and may, but need not, be substantially planar. Generally circular means that the windings form a closed loop, but the shape may be any closed form, including square, rectangular, circular, elliptical, polygonal, and other shapes, and may, but need not, have rounded corners. Additionally, windings often have a thickness that corresponds to the distance between the outermost turn of the winding and the innermost turn of the winding. For example, a circular winding may have an outer diameter (corresponding to the diameter of the outermost turn) and an inner diameter (corresponding to the diameter of the innermost turn of the winding). Depending on the context, "diameter" as used herein may refer to either the outer diameter, the inner diameter, or the average of the two. "Diameter" may also be understood to encompass similar inner or outer dimensions of windings that are not strictly circular in shape, such as the length of a side of a square winding or the length of the major or minor axis of a rectangular or elliptical winding. Summary of the Invention

[0002] One factor that can affect the degree of magnetic coupling between the windings is the relative size of those windings. If there is a size mismatch between the PTx and PRx coils, magnetic flux from the PTx winding can couple to metallic structures of the power receiving device other than the receiver windings. Such structures are sometimes known or referred to as "friendly metal." This coupling can be undesirable because it wastes energy from the transmitter and causes efficiency reductions. In some cases, such undesirable magnetic coupling can also lead to one or both of the PTx and PRx devices misunderstanding the amount of wireless power being transmitted, leading to feedback loop problems.

[0003] In a wireless power transfer system including a power transmitter (PTx) having a PTx winding and a power receiver (PRx) having a PRx winding, where the PTx winding and the PRx winding are dimensionally mismatched, an accessory for improving wireless power transfer efficiency may include a core formed from a material having selected magnetic properties. The core may be sized and positioned to perform at least one of reducing magnetic flux coupling from the PTx winding to a friendly metal of the PRx and enhancing magnetic flux coupling from the PTx winding to the PRx winding. The core may be a ferrite core. The dimensional mismatch between the PTx winding and the PRx winding may include a PTx winding that is larger than the PRx winding. In such a case, the core may define an opening corresponding to an outer dimension of the PRx winding. The core may have an outer dimension selected to block magnetic flux sufficient to reduce magnetic flux coupling to the friendly metal of the PRx. The opening may have an inner diameter corresponding to an outer diameter of the PRx winding. The accessory may be configured in a variety of forms, including a shim configured to attach to either the PTx or PRx by adhesive, magnets, mechanical features, etc. The accessory may be configured as a case for an electronic device such as a smartphone, smartwatch, etc.

[0004] The accessory may be configured to improve wireless power transfer efficiency between the electronic device and the wireless charger when the wireless charging winding of the electronic device and the wireless charging winding of the wireless charger are dimensionally mismatched. The accessory may include a core formed from a material having selected magnetic properties, the core sized and positioned to at least one of reduce magnetic flux coupling from the wireless charging winding of the wireless charger to metal of the electronic device and enhance magnetic flux coupling from the wireless charging winding of the wireless charger to the wireless charging winding of the device. The accessory may further include one or more alignment fixtures. The one or more alignment fixtures may be configured to align or secure the accessory with respect to the electronic device. Additionally or alternatively, the one or more alignment fixtures may be configured to align or secure a case with respect to the wireless charger. The alignment fixture may include one or more magnets. The one or more magnets may form a ring. The core may be a ferrite core. The dimensional mismatch may include the wireless charging winding of the wireless charger being larger than the wireless charging winding of the electronic device. In such cases, the core can define an opening corresponding to an outer dimension of the wireless charging winding of the electronic device. The core can have an outer dimension selected to block magnetic flux sufficient to reduce magnetic flux coupling to friendly metal of the electronic device. The opening can have an inner diameter corresponding to an outer diameter of the wireless charging winding of the wireless charger.

[0005] The wireless charging accessory device can include means for reducing magnetic flux coupling from a wireless transmitting winding of a wireless power transmitter to a friendly metal of a wireless receiver or for enhancing magnetic flux coupling from the wireless transmitting winding to a dimensionally mismatched wireless receiving winding of the wireless receiver, and means for aligning the means for reducing magnetic flux coupling from the wireless transmitting winding of the wireless power transmitter to the friendly metal of the wireless receiver or for enhancing magnetic flux coupling from the wireless transmitting winding to the wireless receiving winding of the wireless receiver with respect to at least one of the wireless receiver or the wireless power transmitter. The means for reducing magnetic flux coupling from the wireless transmitting winding of the wireless power transmitter to the friendly metal of the wireless receiver or for enhancing magnetic flux coupling from the wireless transmitting winding to the wireless receiving winding of the wireless receiver can be a ferrite core. The dimensional mismatch between the wireless transmitting winding and the wireless receiving winding can include the wireless transmitting winding being larger than the wireless receiving winding, and the core defines an opening corresponding to an outer dimension of the wireless receiving winding. The core can have an outer dimension selected to block enough magnetic flux to reduce magnetic flux coupling to friendly metal of the wireless receiver.

[0006] The accessory may be configured to be attached to a surface of the wireless power transmitter to improve wireless power transfer efficiency between the electronic device and the wireless power transmitter, where the wireless charging winding of the electronic device and the wireless charging winding of the wireless power transmitter are dimensionally mismatched. The accessory may include a core formed from a material having selected magnetic properties, the core sized and positioned to at least one of reduce magnetic flux coupling from the wireless charging winding of the wireless power transmitter to a metal of the electronic device and enhance magnetic flux coupling from the wireless charging winding of the wireless power transmitter to a wireless charging winding of the electronic device. The accessory may be configured to be attached to the surface of the wireless power transmitter by adhesive or other methods, such as mechanical fasteners. The core may be a ferrite ring sized to shield the metal of the electronic device from magnetic flux induced by at least one wireless charging winding of the wireless power transmitter. The ferrite ring may include one or more tabs sized and dimensioned to be positioned near one or more wireless charging windings of the wireless power transmitter to provide a desired level of shielding or magnetic flux redirection. The core may further include additional ferrite bars sized and dimensioned to be positioned near one or more wireless charging windings of the wireless power transmitter to provide a desired level of shielding or magnetic flux redirection. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram illustrating a wireless power transmission system. [Figure 2A] 1A-1C illustrate wireless power transfer systems showing magnetic flux coupling in situations where the coils have different dimensions. [Figure 2B] 1A-1C illustrate wireless power transfer systems showing magnetic flux coupling in situations where the coils have different dimensions. [Figure 3A] FIG. 10 is a diagram showing a wireless power transmission system having an accessory interposed between PTx and PRx to improve wireless power transmission efficiency. [Figure 3B] FIG. 10 is a diagram showing a wireless power transmission system having an accessory interposed between PTx and PRx to improve wireless power transmission efficiency. [Figure 4A] 10A-10C illustrate various embodiments of accessories that improve wireless power transfer efficiency. [Figure 4B] 10A-10C illustrate various embodiments of accessories that improve wireless power transfer efficiency. [Figure 5A] 10A-10C illustrate various embodiments of accessories that improve wireless power transfer efficiency. [Figure 5B] 10A-10C illustrate various embodiments of accessories that improve wireless power transfer efficiency. [Figure 6A] 10A-10C illustrate various embodiments of accessories that improve wireless power transfer efficiency. [Figure 6B] 10A-10C illustrate various embodiments of accessories that improve wireless power transfer efficiency. [Figure 7A] 10A-10C illustrate various embodiments of accessories that improve wireless power transfer efficiency. [Figure 7B] 10A-10C illustrate various embodiments of accessories that improve wireless power transfer efficiency. [Figure 8] 1A-1C illustrate embodiments of a magnetic flux shielding / redirection accessory incorporating a ferrite core. [Figure 9] 9 shows further details of the magnetic flux shielding / redirecting accessory of FIG. 8. [Figure 10] 10A-10C illustrate an accessory for improving wireless power transfer efficiency, including a segmented ferrite core with a corresponding magnetic ring. DETAILED DESCRIPTION OF THE INVENTION

[0008] In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed concepts. As part of this description, some of the drawings in the disclosure represent structures and devices in block diagram form for simplicity. In the interest of clarity, not all features of an actual implementation are described in this specification. Moreover, the language used in this specification has been chosen solely for purposes of readability and explanation, and not to limit or restrict the disclosed subject matter. Rather, the appended claims are intended to achieve such a purpose.

[0009] Various embodiments of the disclosed concepts are illustrated in the accompanying drawings, by way of example, and not by way of limitation, where like reference numerals indicate like elements. For simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among different drawings to indicate corresponding and / or similar elements. In addition, numerous specific details have been described to provide a thorough understanding of the implementations described herein. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the relevant functionality being described. References to "an," "one," or "another" embodiment in the present disclosure do not necessarily refer to the same or different embodiments, but rather to at least one. A given drawing may be used to illustrate multiple embodiments of the present disclosure or multiple species of the present disclosure, and not all elements in a drawing may be required for a given embodiment or species. Reference numerals, when provided in a given drawing, may refer to the same element throughout the drawings, but may not be repeated in all drawings. The drawings are not to scale unless otherwise indicated and the proportions of certain parts may be exaggerated to better show the details and features of the present disclosure.

[0010] 2A and 2B illustrate a wireless (inductive) power transfer configuration that includes a size mismatch between the PTx winding 103 and the PRx winding 105. In the illustrated configuration, the PTx winding 103 has a larger diameter than the PRx winding 105. In other situations not shown, the reverse may be true. In the side view of FIG. 2B, magnetic flux lines 202 illustrate an example magnetic flux path associated with driving the PTx winding 103 (shown in the plan view of FIG. 2A) in a counterclockwise direction. As seen in FIG. 2B, some of the magnetic flux lines 202 pass through the PRx winding 105. This portion of the magnetic flux may be considered to effectively couple to the PRx device. However, some of the magnetic flux lines 202 do not pass through the PRx winding 105, but rather reach the metal components of the PRx device 104 (i.e., the aforementioned "friendly metal"), as indicated by the "X" in FIG. 2B. To the extent that some of the magnetic flux from 103 interacts with the friendly metal rather than the PRx winding 105, the magnetic coupling between PTx and PRx is reduced. This is due in part to the different dimensions of the PTx and PRx windings 103 and 105, respectively.

[0011] One way to address this size mismatch and improve the degree of magnetic coupling between the PTx winding 103 and the PRx winding 105 is to provide a flux shielding and / or redirecting accessory between the PTx and PRx windings. FIG. 3A shows a side view of such an arrangement, and FIG. 3B shows a top view of such an arrangement. The flux shielding and / or redirecting accessory may include a core 300 made from ferrite or other ferromagnetic material with suitable magnetic properties, such as sintered ferrite or nanocrystalline sheet. The core 300 may be sized to shield the friendly metal of the PRx winding 104 from the magnetic flux induced by the PTx winding 104 and / or to effectively redirect some or substantially all of that flux to the PRx winding 105.

[0012] In the illustrated example having a relatively large PTx coil 103 and a relatively small PRx coil 105, the ferrite core 300 may be ring-shaped with an inner diameter (ID) that generally corresponds to the inner diameter of the PRx coil and an outer diameter (OD) large enough to block magnetic flux from the PTx coil that would otherwise couple to friendly metal. By placing such a core 300 coaxially between, approximately parallel to, and overlapping each winding, magnetic coupling between the windings 103, 105 may be improved and magnetic flux coupling of the PRx device 104 to friendly metal may be reduced.

[0013] A substantially similar solution can be applied to the reverse situation, where the PRx coil is larger and the PTx coil winding is smaller. Also, as noted above, the shape of each winding and core need not be strictly circular but may be any closed or nearly closed shape, such that the aforementioned diameters may be the lengths of the inner sides, the lengths of the major and minor axes, etc. By "substantially closed" shape, it should be understood that the core may be comprised of two or more segments of magnetic material with a relatively small gap (compared to the core size) between the segments. What is important is that the core 300 is positioned and dimensioned to reduce undesired magnetic flux coupling from the PTx winding 103 to the friendly metal of the PRx device 104 and / or enhance magnetic flux coupling to the PRx winding 105. Additionally, while the described embodiment features a substantially planar coil and core 300, the same principles apply to non-planar windings and a non-planar intermediate core 300. In such an embodiment, the intermediate core 300 should be shaped, positioned, and sized to reduce unwanted magnetic flux coupling to so-called "friendly metals" and / or to increase the amount of magnetic flux that couples into the PRx windings.

[0014] In various other implementations, the core 300 dimensionally conforms to the shape of a PTx coil instead of a PRx coil. For example, when the phone is placed in the case, the ferrite core 300 can be aligned with the transmitter winding 103 of the charging pad 102 so that the outer diameter of the ferrite core / ring 300 aligns with the diameter of the transmitter winding 103, and the inner diameter of the ferrite core / ring 300 is sized so that the central opening of the structure 300 allows magnetic flux to flow to the receiving window 105 of the phone 104. That is, the inner diameter of the ferrite core / ring 300 is larger than the inner diameter of the receiver winding 105 of the phone 104. In some cases, the inner diameter of the ferrite core / ring 300 is larger than the inner diameter and smaller than the outer diameter of the receiver winding 105.

[0015] In various implementations of the PTx device 102 and the PRx device 104, the intermediate device incorporating the core 300 may take various forms. As an example, as shown in FIGS. 4A (top view) and 4B (side view), the PTx 102 may be a charging mat, the PRx 104 may be a mobile phone, a smart watch, a tablet PC, a wireless earphone charging case, or other device, and the intermediate device incorporating the ferrite core 300 may be an accessory 400 for the electronic device. The accessory 400 may be a mobile phone case, a "shim" that may be provided between the PTx and PRx, or another intermediate device incorporating the ferrite core 300. The ferrite core 300 may be provided within the accessory 400, which is designed and dimensioned to properly position the core 300 relative to the PRx windings of the PRx device. For example, when the phone is placed in the case, the ferrite core 300 can be suitably aligned with the receiver winding 105 of the phone 104 such that the inner diameter of the ferrite core / ring 300 aligns with the outer diameter of the receiver winding 105 and the annular shape of the ferrite core / ring 300 shields the friendly metal of the phone 104 from the magnetic flux induced by the PTx winding 103 of the charging pad 102.

[0016] As shown in FIGS. 5A-5B, 6A-6B, and 7A-7B, each of the cases 500, 600, or 700 can optionally include one or more features to assist the user in aligning the accessory / phone combination with an external charging device. As an example, such an accessory, whether a case, shim, or otherwise, can include one or more magnets that can serve to align the accessory / device combination with a wireless power transmitter and / or receiver device having a complementary configured magnet. This alignment can assist in aligning the device's respective coils and / or ferrite core 300 with PTx winding 103 and / or PRx winding 105. In FIGS. 5A and 5B, the magnet ring 502 is shown below the ferrite ring 300; or, in other words, the ferrite ring 300 is disposed between the magnet and the device (e.g., a phone, not shown). 4A and 4B, the ferrite ring 300 prevents magnetic flux from the transmitter from coupling to the "friendly metal" of the device, reducing undesired power loss and associated heat generation. Additionally, the magnets of the magnet ring 502 may be segmented to reduce losses. In this embodiment, the accessory may be mechanically positioned and secured to the device (according to various known techniques), and the magnets may be used to assist in attaching and aligning the device / accessory combination to the power transmitting device (e.g., a charging pad, not shown). Additionally, as noted above and described in more detail below, the ferrite ring 300 may also be segmented, in which case the magnet ring segments and ferrite ring segments may be interspersed, as shown below with respect to FIG. 10.

[0017] In FIGS. 6A and 6B, the magnet ring 502 is shown above the ferrite ring 300; or, in other words, the ferrite ring 300 is disposed between the magnet and the power transmitting device (e.g., a charging pad, not shown). Similar to the embodiments of FIGS. 4A-4B and 5A-5B, the ferrite ring 300 prevents magnetic flux from the power transmitter from coupling to the "friendly metal" of the device, reducing unwanted power loss and associated heat generation. Additionally, the magnets of the magnet ring 502 may be segmented to reduce losses. In this embodiment, an accessory (e.g., a case or shim) may be magnetically secured to (and aligned with) the device (e.g., a phone), and the device / accessory combination may be freely positioned relative to the power transmitting device (e.g., a charging pad, not shown). As noted above and described in more detail below, the ferrite ring 300 may also be segmented, in which case the magnet ring segments and ferrite ring segments may be interspersed, as shown below with respect to FIG. 10.

[0018] In FIGS. 7A and 7B, magnet ring 502 is shown as being approximately in the same plane as ferrite ring 300. Exact coplanarity is not required and may not be possible depending on the respective heights of the magnets and ferrite cores in a given embodiment. As with the embodiments of FIGS. 4A-4B, 5A-5B, 6A-6B, and 7A-7B described above, ferrite ring 300 prevents magnetic flux from the transmitter from coupling to the "friendly metal" of the device, reducing undesirable power loss and associated heat generation. Additionally, the magnets of magnet ring 502 may be segmented to reduce losses. In this embodiment, magnets may be used to securely position the accessory relative to both the device and / or the power transmitting device (e.g., a charging pad).

[0019] In FIG. 10 , similar to FIGS. 7A and 7B , magnet ring 502 is shown as being approximately coplanar with ferrite ring 300. As noted above, precise coplanarity is not required, particularly given the potential for different heights of the magnets and ferrite cores for a given embodiment. However, for packaging purposes, it may be useful to configure the magnet components in ring 502 and the ferrite components in ring 300 so that at least a portion of these components are coplanar, thereby reducing the z-height of the resulting accessory. Similar to the embodiments of FIGS. 4A-4B , 5A-5B , 6A-6B , and 7A-7B described above, ferrite ring 300 prevents magnetic flux from the transmitter from coupling to the “friendly metal” of the device, reducing undesirable power loss and associated heat generation. Additionally, the magnets in magnet ring 502 are segmented into segments “M” to reduce losses and improve manufacturability. Similarly, ferrite ring 300 is segmented into segments "F" interposed between magnet segments "M." As described above, magnets may be used to secure and position an accessory relative to a device and / or power transmitting device (e.g., a charging pad). The relative sizes of the magnet and ferrite segments may be adjusted as needed to provide sufficient securing force, both radially and circumferentially, between accessory 1000 and a device (e.g., a phone) and / or power transmitting device (e.g., by increasing the size of the magnets) and / or to increase the magnetic flux shielding / coupling effect (by increasing the size of the ferrite segments). In some embodiments, the relative placement of the magnet and ferrite ring segments may be such that the various segments are approximately collinear along the circumference of the respective rings. Depending on the specific dimensions of the ferrite and magnet ring segments, precise collinearity may not be required or provided, as broad collinearity around the circumference may be appropriate for some applications.

[0020] FIG. 8 illustrates an alternative embodiment of a magnetic flux shielding / redirecting accessory 801 incorporating a ferrite core 300. The accessory 801 may be a shim device suitable for placement on and optionally fastening to the PTx device 102 itself. (As noted above, the PTx device 102 may be, for example, a charging mat, charging pad, or other suitable wireless / inductive charging device.) The accessory 801 may also include mechanical, adhesive, and / or magnetic features 802 for aligning and / or fastening the intermediate device 801 relative to the PTx 102 and associated PTx winding 103. As noted above, the core 300 should be sized and positioned to suitably reduce undesired magnetic flux coupling to the friendly metal of the PRx device (such as a cell phone or other electronic device or accessory) and / or enhance magnetic flux coupling to the PRx winding 105 of the PRx device. Thus, it needs to be positioned relative to the PTx winding 105 to block and / or redirect magnetic flux that would normally couple to the friendly metal of the PRx device.

[0021] FIG. 9 shows further details of embodiments 902, 904 of the magnetic flux shielding / redirection accessory 801 (FIG. 8). In embodiments 902 and 904, the PTx device 102 can include multiple transmission coils 103 configured as DDQ coils. The accessories 902, 904 can be positioned between the PTx device 102 and the PRx device 104 (e.g., a mobile phone). The accessories 902, 904 can include a ferrite ring 300, which can be implemented as a ferrite frame 300 that surrounds the top / Q coil 103 of the PTx device 102. As a result, the friendly metal of the PRx device 104 is effectively shielded from the magnetic flux induced by the coil 103. Similarly, the accessory 904 can include a ferrite frame that covers all or nearly all of the transmission area of ​​the PTx device 904. In such embodiments, additional ferrite bars 910 can be provided near specific coils as needed to provide the required level of shielding / flux redirection. Optional tabs 912 in the ferrite ring / frame 300 can also be used to achieve a similar effect.

[0022] The foregoing describes exemplary embodiments of accessories for wireless power transmission systems that provide improved coupling between wireless power transmitter (PTx) devices and wireless power receiver (PRx) devices. Such systems can be used in a variety of applications, but can be particularly advantageous when used with wireless charging devices (e.g., charging mats, charging pads, charging stands, etc.) configured for use with different devices, such as electronic devices, such as mobile computing devices (e.g., laptop computers, tablet computers, smartphones, etc.) and their accessories (e.g., wireless earbuds, styluses and other input devices, etc.). While numerous specific features and various embodiments have been described, it should be understood that, unless otherwise specified as mutually exclusive, the various features and embodiments may be combined in various permutations in a particular implementation. Accordingly, the various embodiments described above are provided by way of example only and should not be construed as limiting the scope of the present disclosure. Various modifications and variations can be made to the principles and embodiments herein without departing from the scope of the present disclosure and without departing from the scope of the claims.

[0023] The techniques presented and claimed herein refer to and apply material objects and practical examples of a practical nature that demonstrably improve the art, and are thus not abstract, intangible, or purely theoretical. Furthermore, when any claim at the end of this specification contains one or more elements recited as "means for [performing] _____ [function]" or "steps for _____ [function]," it is intended that such elements be construed under 35 U.S.C. 112(f). However, for claims containing elements recited in other ways, it is intended that such elements not be construed under 35 U.S.C. 112(f).

Claims

1. An accessory for improving wireless power transmission efficiency in a wireless power transmission system including a power transmitter (PTx) having a PTx winding and a power receiver (PRx) having a PRx winding, wherein the PTx winding and the PRx winding are dimensionally mismatched, the accessory comprising: a core formed from a material having selected magnetic properties, said core comprising: reducing magnetic flux coupling from the PTx winding to the friendly metal of the PRx; enhancing magnetic flux coupling from the PTx winding to the PRx winding; an accessory sized and arranged to perform at least one of the following:

2. The accessory of claim 1 , wherein the core is a ferrite core.

3. 2. The accessory of claim 1, wherein the dimensional mismatch between the PTx winding and the PRx winding includes a PTx winding that is larger than the PRx winding, and wherein the core defines an opening corresponding to an outer dimension of the PRx winding.

4. 4. The accessory of claim 3, wherein the core has outer dimensions selected to block sufficient magnetic flux to reduce magnetic flux coupling of the PRx to the friendly metal.

5. The accessory of claim 4 , wherein the opening has an inner diameter corresponding to an outer diameter of the PRx winding.

6. 1. An accessory configured to improve wireless power transfer efficiency between an electronic device and a wireless charger, wherein a wireless charging winding of the device and a wireless charging winding of the wireless charger are dimensionally mismatched, the accessory comprising: a core formed from a material having selected magnetic properties, said core comprising: reducing magnetic flux coupling from the wireless charging winding of the charger to metal of the device; enhancing magnetic flux coupling from the wireless charging winding of the charger to the wireless charging winding of the phone; an accessory sized and arranged to perform at least one of the following:

7. The accessory of claim 6 , further comprising one or more alignment fixtures.

8. The accessory of claim 7 , wherein the one or more alignment fixtures are configured to align or secure the accessory relative to the electronic device.

9. The accessory of claim 7 , wherein the one or more alignment fixtures are configured to align or secure the accessory relative to the wireless charger.

10. The accessory of claim 7 , wherein the one or more alignment fixtures are configured to align or secure the accessory relative to both the device and the wireless charger.

11. The accessory of claim 7 , wherein the alignment fixture comprises one or more magnets.

12. The accessory of claim 11 , wherein the one or more magnets form a ring.

13. The accessory of claim 6 , wherein the core is a ferrite core.

14. 7. The accessory of claim 6, wherein the dimensional mismatch includes the wireless charging winding of the wireless charger being larger than the wireless charging winding of the electronic device, and the core defines an opening corresponding to an outer dimension of the wireless charging winding of the electronic device.

15. 15. The accessory of claim 14, wherein the core has outer dimensions selected to block sufficient magnetic flux to reduce magnetic flux coupling of the electronic device to the friendly metal.

16. The accessory of claim 15 , wherein the opening has an inner diameter corresponding to an outer diameter of the wireless charging winding of the wireless charger.

17. The accessory of claim 6 , wherein the accessory is a case for the electronic device.

18. The accessory of claim 6 , wherein the accessory is a shim configured to be disposed between the device and the wireless charger.

19. 1. An accessory configured to be attached to a surface of a wireless power transmitter to improve wireless power transfer efficiency between an electronic device and a wireless power transmitter, wherein a wireless charging winding of the electronic device and a wireless charging winding of the wireless power transmitter are dimensionally mismatched, the accessory comprising: a core formed from a material having selected magnetic properties, reducing magnetic flux coupling from the wireless charging winding of the wireless power transmitter to metal of the electronic device; enhancing magnetic flux coupling from the wireless charging winding of the wireless power transmitter to the wireless charging winding of the electronic device; a core sized and arranged to perform at least one of an adhesive configured to secure the accessory to the surface of the wireless power transmitter; Included, accessories.

20. 20. The accessory of claim 19, wherein the core is a ferrite ring sized to shield the metal of the electronic device from magnetic flux induced by at least one wireless charging winding of the wireless power transmitter.

21. 21. The accessory of claim 20, wherein the ferrite ring comprises one or more tabs sized and dimensioned to be positioned near one or more wireless charging windings of the wireless power transmitter to provide a desired level of shielding or magnetic flux redirection.

22. 21. The accessory of claim 20, wherein the ferrite core further comprises additional ferrite bars sized and dimensioned to be positioned near one or more wireless charging windings of the wireless power transmitter to provide a desired level of shielding or magnetic flux redirection.

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