METALLIC BODY

TECHNICAL FIELD

[0001] The disclosure relates to wireless power transfer, and in particular to wireless power transfer in electronic devices having a tuned metallic body.

BACKGROUND

[0002] Wireless power transfer is an increasingly popular capability in portable electronic devices, such as mobile phones, computer tablets, etc. because such devices typically require long battery life and low battery weight. The ability to power an electronic device without the use of wires provides a convenient solution for users of portable electronic devices. Wireless power charging systems, for example, may allow users to charge and/or power electronic devices without physical, electrical connections, thus reducing the number of components required for operation of the electronic devices and simplifying the use of the electronic device.

[0003] Wireless power transfer allows manufacturers to develop creative solutions to problems due to having limited power sources in consumer electronic devices. Wireless power transfer may reduce overall cost (for both the user and the manufacturer) because conventional charging hardware such as power adapters and charging chords can be eliminated. There is flexibility in having different sizes and shapes in the components (e.g., magnetic coil, charging plate, etc.) that make up a wireless power transmitter and/or a wireless power receiver in terms of industrial design and support for a wide range of devices, from mobile handheld devices to computer laptops.

SUMMARY

[0004] In accordance with aspects of the disclosure, an electronic apparatus for wireless power transfer may include an electrically conductive body configured to magnetically couple to a first magnetic field. A first tuning element may be electrically connected to the electrically conductive body. An electrically conductive coil may be wound about an opening defined by the electrically conductive body; the electrically conductive coil may be configured to magnetically couple to a second magnetic field.

[0005] In some embodiments, the electronic apparatus may further include a second tuning element electrically connected to the electrically conductive coil. The first and second tuning elements each may comprise one or more capacitors. Either or both the first and second tuning elements may further comprise one or more inductors.

[0006] In some embodiments, the first magnetic field may be an externally generated magnetic field generated by a wireless power transmitter and the electrically conductive body generates the second magnetic field in response to being coupled to the externally generated magnetic field. The electronic apparatus may further include a rectifier connected to the electrically conductive coil. The rectifier may be configured to rectify current induced in the electrically conductive coil to provide power to electronics that comprise the apparatus

[0007] In some embodiments, the second magnetic field may be an externally generated magnetic field and the electrically conductive coil may generate the first magnetic field in response to being coupled to the externally generated magnetic field. The electronic apparatus may further comprise a rectifier connected to the electrically conductive body. The rectifier may be configured to rectify current induced in the electrically conductive body to provide power to electronics that comprise the apparatus. [0008] In some embodiments, the electronic apparatus may further include a printed circuit board having disposed thereon the first tuning element. A connector may be electrically connected to the first tuning element to the electrically conductive body.

[0009] In some embodiments, the electronic apparatus may further include a printed circuit board. The electrically conductive coil may be disposed between the electrically conductive body and the printed circuit board. The electronic apparatus may further include a ferrite material disposed between the electrically conductive coil and the printed circuit board.

[0010] In some embodiments, the first tuning element and the electrically conductive body may constitute a circuit having a resonant frequency defined by the first tuning element. The electronic apparatus may further include a second tuning element electrically connected to the electrically conductive coil to define a circuit having a resonant frequency substantially equal to the resonant frequency of the circuit comprising the first tuning element and the electrically conductive body. In some embodiments, the second tuning element may be electrically connected to the electrically conductive coil to define a circuit having a resonant frequency different from the resonant frequency of the circuit comprising the first tuning element and the electrically conductive body.

[0011] In some embodiments, the electronic apparatus may further include a metallic enclosure configured to house electronics. The metallic enclosure may comprise the electrically conductive body. [0012] In some embodiments, the electronic apparatus may further include a non-metallic enclosure configured to house electronics that comprise the apparatus. The electrically conductive body and the electrically conductive coil may be housed within the enclosure.

[0013] In some embodiments, the apparatus is a wearable electronic device. [0014] In accordance with aspects of the disclosure, a method for wireless power transfer to an electronic device may include magnetically coupling an externally generated magnetic field to an electrically conductive structure, that comprises a housing for the electronic device, to produce an induced magnetic field that emanates from the electrically conductive structure. The induced magnetic field may be magnetically coupled to a power receiving element to induce current in the power receiving element. The power receiving element may be electrically isolated from the electrically conductive structure. Power may be produced from the current induced in the power receiving element.

[0015] In some embodiments, a resonant frequency of a circuit that includes the electrically conductive structure is substantially equal to a frequency of the externally generated magnetic field.

[0016] In some embodiments, a resonant frequency of a circuit that includes the power receiving element is substantially equal to a frequency of the externally generated magnetic field.

[0017] In some embodiments, magnetically coupling the externally generated magnetic field to the electrically conductive structure may include inducing current in a first circuit comprising the electrically conductive body electrically connected to a first tuning element. Magnetically coupling the induced magnetic field to the power receiving element may include inducing current in a second circuit comprising the electrically conductive coil electrically connected to a second tuning element. A resonant frequency of either or both the first circuit and the second circuit is substantially equal to a frequency of the externally generated magnetic field. [0018] In some embodiments, producing power may include rectifying the current induced in the power receiving element.

[0019] In accordance with aspects of the disclosure, an electronic apparatus may include a housing configured to enclose electronic components that comprise the apparatus. The housing may comprise a metallic portion. A first tuning element may be connected to the metallic portion of the housing. The metallic portion of the housing may have a shape that allows a flow of current to be induced therein in response to being magnetically coupled to an externally generated magnetic field. An induced magnetic field may emanate from the metallic portion in response to the flow of current. The apparatus may include an electrically conductive coil. A flow of current may be induced in the electrically conductive coil in response to being magnetically coupled to the induced magnetic field to produce a flow of current in the electrically conductive coil. A rectifier may be configured to rectify current induced in the electrically conductive structure to provide power to a load.

[0020] In some embodiments, the metallic portion of the housing may define an opening therethrough and define a slot from the opening to a periphery of the metallic portion.

[0021] In some embodiments, the rectifier may be electrically connected to the electrically conductive coil. [0022] In some embodiments, the first tuning element and the metallic portion of the housing may define a resonant frequency substantially equal to a resonant frequency defined by the second tuning element and the electrically conductive coil.

[0023] 27 In some embodiments, the first tuning element and the metallic portion of the housing may define a resonant frequency different from a resonant frequency defined by the second tuning element and the electrically conductive coil.

[0024] In accordance with aspects of the disclosure, an apparatus for wirelessly receiving power in an electronic device may include means for housing electronics of the electronic device. The means for housing may have a metallic portion including means for magnetically coupling to an externally generated magnetic field to produce an induced magnetic field that emanates from the means for magnetically coupling to the externally generated magnetic field. The means for magnetically coupling to the externally generated magnetic field is electrically connected to means for tuning the means for magnetically coupling to the externally generated magnetic field to resonate at a resonant frequency. The apparatus further includes means for magnetically coupling to the induced magnetic field to induce current. The means for magnetically coupling to the induced magnetic field is electrically isolated from the means for magnetically coupling to the externally generated magnetic field. Thea apparatus further includes means for producing power from the current induced in the second means for magnetically coupling to the induced magnetic field.

[0025] In some embodiments, either or both the means for magnetically coupling to the externally generated magnetic field and the means for magnetically coupling to the induced magnetic field have a resonant frequency substantially equal to a frequency of the externally generated magnetic field. [0026] In some embodiments, the means for magnetically coupling to the induced magnetic field comprises an electrically conductive coil.

[0027] The following detailed description and accompanying drawings provide a better understanding of the nature and advantages of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] With respect to the discussion to follow and in particular to the drawings, it is stressed that the particulars shown represent examples for purposes of illustrative discussion, and are presented in the cause of providing a description of principles and conceptual aspects of the disclosure. In this regard, no attempt is made to show implementation details beyond what is needed for a fundamental understanding of the disclosure. The discussion to follow, in conjunction with the drawings, makes apparent to those of skill in the art how embodiments in accordance with the disclosure may be practiced. Similar or same reference numbers may be used to identify or otherwise refer to similar or same elements in the various drawings and supporting descriptions. In the accompanying drawings: [0029] FIG. 1 is a functional block diagram of a wireless power transfer system in accordance with an illustrative embodiment.

[0030] FIG. 2 is a functional block diagram of a wireless power transfer system in accordance with an illustrative embodiment.

[0031] FIG. 3 is a schematic diagram of a portion of transmit circuitry or receive circuitry of FIG. 2 including a power transmitting or receiving element in accordance with an illustrative embodiment.

[0032] FIGS. 4A and 4B show a back enclosure in accordance with the disclosure.

[0033] FIGS. 5 and 5A illustrate details of an embodiment in accordance with the disclosure.

[0034] FIG. 6 illustrates eddy currents in an embodiment in accordance with the disclosure. [0035] FIG. 6A illustrates the arrangement shown in FIG. 6 as a three-coil coupling configuration.

[0036] FIG. 7 shows a particular implementation of an embodiment in accordance with the disclosure.

[0037] FIGS. 8 and 8A illustrate alternative embodiments in accordance with the disclosure. [0038] FIGS. 9 and 9A illustrate details of a wearable embodiment in accordance with the disclosure.

[0039] FIG. 10 illustrates details of a portable computer embodiment in accordance with the disclosure. [0040] FIGS. 11 A and 1 IB show details of an embodiment in accordance with the disclosure.

DETAILED DESCRIPTION

[0041] In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of the disclosure. The disclosure as expressed in the claims may include some or all of the features in these examples, alone or in combination with other features described below, and may further include

modifications and equivalents of the features and concepts described herein.

[0042] Wireless power transfer may refer to transferring any form of energy associated with electric fields, magnetic fields, electromagnetic fields, or otherwise from a transmitter to a receiver without the use of physical electrical conductors (e.g., power may be transferred through free space). The power output into a wireless field (e.g., a magnetic field or an electromagnetic field) may be received, captured by, or coupled by a "power receiving element" to achieve power transfer.

[0043] FIG. 1 is a functional block diagram of a wireless power transfer system 100, in accordance with an illustrative embodiment. Input power 102 may be provided to a transmitter 104 from a power source (not shown in this figure) to generate a wireless (e.g., magnetic or electromagnetic) field 105 for performing energy transfer. A receiver 108 may couple to the wireless field 105 and generate output power 110 for storing or consumption by a device (not shown in this figure) coupled to the output power 110. The transmitter 104 and the receiver 108 may be separated by a distance 112. The transmitter 104 may include a power transmitting element 114 for transmitting/coupling energy to the receiver 108. The receiver 108 may include a power receiving element 118 for receiving or capturing/coupling energy transmitted from the transmitter 104.

[0044] In one illustrative embodiment, the transmitter 104 and the receiver 108 may be configured according to a mutual resonant relationship. When the resonant frequency of the receiver 108 and the resonant frequency of the transmitter 104 are substantially the same or very close, transmission losses between the transmitter 104 and the receiver 108 are reduced. As such, wireless power transfer may be provided over larger distances. Resonant inductive coupling techniques may thus allow for improved efficiency and power transfer over various distances and with a variety of inductive power transmitting and receiving element

configurations.

[0045] In certain embodiments, the wireless field 105 may correspond to the "near field" of the transmitter 104. The near-field may correspond to a region in which there are strong reactive fields resulting from the currents and charges in the power transmitting element 114 that minimally radiate power away from the power transmitting element 114. The near-field may correspond to a region that is within about one wavelength (or a fraction thereof) of the power transmitting element 114. [0046] In certain embodiments, efficient energy transfer may occur by coupling a large portion of the energy in the wireless field 105 to the power receiving element 118 rather than

propagating most of the energy in an electromagnetic wave to the far field.

[0047] In certain implementations, the transmitter 104 may output a time varying magnetic (or electromagnetic) field 105 with a frequency corresponding to the resonant frequency of the power transmitting element 114. When the receiver 108 is within the wireless field 105, the time varying magnetic (or electromagnetic) field may induce a current in the power receiving element 118. As described above, if the power receiving element 118 is configured as a resonant circuit to resonate at the frequency of the power transmitting element 114, energy may be efficiently transferred. An alternating current (AC) signal induced in the power receiving element 118 may be rectified to produce a direct current (DC) signal that may be provided to charge or to power a load.

[0048] FIG. 2 is a functional block diagram of a wireless power transfer system 200, in accordance with another illustrative embodiment. The system 200 may include a transmitter 204 and a receiver 208. The transmitter 204 (also referred to herein as power transfer unit, PTU) may include transmit circuitry 206 that may include an oscillator 222, a driver circuit 224, and a front-end circuit 226. The oscillator 222 may be configured to generate an oscillator signal at a desired frequency that may adjust in response to a frequency control signal 223. The oscillator 222 may provide the oscillator signal to the driver circuit 224. The driver circuit 224 may be configured to drive the power transmitting element 214 at, for example, a resonant frequency of the power transmitting element 214 based on an input voltage signal (VD) 225. The driver circuit 224 may be a switching amplifier configured to receive a square wave from the oscillator 222 and output a sine wave. [0049] The front-end circuit 226 may include a filter circuit configured to filter out harmonics or other unwanted frequencies. The front-end circuit 226 may include a matching circuit configured to match the impedance of the transmitter 204 to the impedance of the power transmitting element 214. As will be explained in more detail below, the front-end circuit 226 may include a tuning circuit to create a resonant circuit with the power transmitting element 214. As a result of driving the power transmitting element 214, the power transmitting element 214 may generate a wireless field 205 to wirelessly output power at a level sufficient for charging a battery 236, or otherwise powering a load.

[0050] The transmitter 204 may further include a controller 240 operably coupled to the transmit circuitry 206 and configured to control one or more aspects of the transmit circuitry 206, or accomplish other operations relevant to managing the transfer of power. The controller 240 may be a micro-controller or a processor. The controller 240 may be implemented as an application-specific integrated circuit (ASIC). The controller 240 may be operably connected, directly or indirectly, to each component of the transmit circuitry 206. The controller 240 may be further configured to receive information from each of the components of the transmit circuitry 206 and perform calculations based on the received information. The controller 240 may be configured to generate control signals (e.g., signal 223) for each of the components that may adjust the operation of that component. As such, the controller 240 may be configured to adjust or manage the power transfer based on a result of the operations performed by it. The transmitter 204 may further include a memory (not shown) configured to store data, for example, such as instructions for causing the controller 240 to perform particular functions, such as those related to management of wireless power transfer.

[0051] The receiver 208 (also referred to herein as power receiving unit, PRU) may include receive circuitry 210 that may include a front-end circuit 232 and a rectifier circuit 234. The front-end circuit 232 may include matching circuitry configured to match the impedance of the receive circuitry 210 to the impedance of the power receiving element 218. As will be explained below, the front-end circuit 232 may further include a tuning circuit to create a resonant circuit with the power receiving element 218. The rectifier circuit 234 may generate a DC power output from an AC power input to charge the battery 236, as shown in FIG. 2. The receiver 208 and the transmitter 204 may additionally communicate on a separate communication channel 219 (e.g., Bluetooth, Zigbee, cellular, etc.). The receiver 208 and the transmitter 204 may alternatively communicate via in-band signaling using characteristics of the wireless field 205.

[0052] The receiver 208 may be configured to determine whether an amount of power transmitted by the transmitter 204 and received by the receiver 208 is appropriate for charging the battery 236. In certain embodiments, the transmitter 204 may be configured to generate a predominantly non-radiative field with a direct field coupling coefficient (k) for providing energy transfer. Receiver 208 may directly couple to the wireless field 205 and may generate an output power for storing or consumption by a battery (or load) 236 coupled to the output or receive circuitry 210.

[0053] The receiver 208 may further include a controller 250 configured similarly to the transmit controller 240 as described above for managing one or more aspects of the wireless power receiver 208. The receiver 208 may further include a memory (not shown) configured to store data, for example, such as instructions for causing the controller 250 to perform particular functions, such as those related to management of wireless power transfer.

[0054] As discussed above, transmitter 204 and receiver 208 may be separated by a distance and may be configured according to a mutual resonant relationship to minimize transmission losses between the transmitter 204 and the receiver 208.

[0055] FIG. 3 is a schematic diagram of a portion of the transmit circuitry 206 or the receive circuitry 210 of FIG. 2, in accordance with illustrative embodiments. As illustrated in FIG. 3, transmit or receive circuitry 350 may include a power transmitting or receiving element 352 and a tuning circuit 360. The power transmitting or receiving element 352 may also be referred to or be configured as an antenna or a "loop" antenna. The term "antenna" generally refers to a component that may wirelessly output or receive energy for coupling to another antenna. The power transmitting or receiving element 352 may also be referred to herein or be configured as a "magnetic" antenna, or an induction coil, a resonator, or a portion of a resonator. The power transmitting or receiving element 352 may also be referred to as a coil or resonator of a type that is configured to wirelessly output or receive power. As used herein, the power transmitting or receiving element 352 is an example of a "power transfer component" of a type that is configured to wirelessly output and/or receive power. The power transmitting or receiving element 352 may include an air core or a physical core such as a ferrite core (not shown in this figure).

[0056] When the power transmitting or receiving element 352 is configured as a resonant circuit or resonator with tuning circuit 360, the resonant frequency of the power transmitting or receiving element 352 may be based on the inductance and capacitance. Inductance may be simply the inductance created by a coil and/or other inductor forming the power transmitting or receiving element 352. Capacitance (e.g., a capacitor) may be provided by the tuning circuit 360 to create a resonant structure at a desired resonant frequency. As a non limiting example, the tuning circuit 360 may comprise a capacitor 354 and a capacitor 356, which may be added to the transmit and/or receive circuitry 350 to create a resonant circuit.

[0057] The tuning circuit 360 may include other components to form a resonant circuit with the power transmitting or receiving element 352. As another non-limiting example, the tuning circuit 360 may include a capacitor (not shown) placed in parallel between the two terminals of the circuitry 350. Still other designs are possible. In some embodiments, the tuning circuit in the front-end circuit 226 may have the same design (e.g., 360) as the tuning circuit in front-end circuit 232. In other embodiments, the front-end circuit 226 may use a tuning circuit design different than in the front-end circuit 232. [0058] For power transmitting elements, the signal 358, with a frequency that substantially corresponds to the resonant frequency of the power transmitting or receiving element 352, may be an input to the power transmitting or receiving element 352. For power receiving elements, the signal 358, with a frequency that substantially corresponds to the resonant frequency of the power transmitting or receiving element 352, may be an output from the power transmitting or receiving element 352. Although aspects disclosed herein may be generally directed to resonant wireless power transfer, aspects disclosed herein may be used in non-resonant implementations for wireless power transfer.

[0059] FIGS. 4A and 4B illustrate an example of an electronic device 40 (e.g., smartphone, computer tablet, laptop, etc.) in accordance with the disclosure. The electronic device 40 may include a back enclosure (housing) 400 to house electronics (not shown) that comprise the electronic device 40. In some embodiments, the back enclosure 400 may be metallic (metal back cover). In some embodiments, the back enclosure 400 may be segmented into several metallic portions 402a, 402, 402b. Portions 402a and 402 may be spaced apart to define a space or gap 404. Communication antennas (not shown) may be aligned relative to the gap 404 to enable the transmission and reception of communication signals. Similarly, portions 402 and 402b may be spaced apart to define a space or gap 406 for additional communication antennas (not shown).

[0060] The metallic portion 402 may comprise an electrically conductive body (structure) formed to define a portion of the back enclosure 400. In accordance with the disclosure, the metallic portion 402 may have the shape of an open loop 412 to define a power receiving element. FIG. 4B, for example, shows that metallic portion 402 may define an opening 408 formed through the metallic portion 402. The opening 408, for example, may be used to hold the lens (not shown) of a camera. A slot 410 is defined by and formed through the metallic portion 402 and extending between the opening 408 and an edge at the periphery of the metallic portion 402 may define the open loop 412. The current flow 422 represents a flow of eddy currents, which is explained below.

[0061] FIG. 5 is a schematic representation showing additional details of the metallic portion 402 in accordance with aspects of the disclosure. In some embodiments, for example, the metallic portion 402 may be electrically connected to a capacitor C i , thus creating a circuit 512. As depicted in FIG. 5, for example, since the metallic portion 402 is formed in the shape of an open loop 412 (FIG. 4B), the metallic portion 402 may be modeled as a resistance Rmodei connected in series with an inductance Lmodei . The capacitor C i completes the circuit 512.

[0062] In some embodiments, a power receiving element 502 may be defined by a multi-turn coil of electrically conductive material wound about the opening 408. The power receiving element 502 may be affixed to or otherwise disposed next to the inside surface of the metallic portion 402. In some embodiments, the power receiving element 502 may be connected to a capacitor C2. It will be appreciated, in various embodiments other circuits or circuit elements may replace capacitor C2 . In some embodiments (not shown), the capacitor C2 may be omitted. [0063] In some embodiments, means for rectifying may be connected to the power receiving element 502. For example, a rectifier 504 may be connected to the combination of power receiving element 502 and capacitor C2 to define a circuit that is electrically separate from circuit 512. The rectifier 504 may be any suitable design for rectifying alternating current (AC) signals in the power receiving element 502 to produce a direct current (DC) output voltage V _ou t (DC power). The output voltage V _ou t may be provided to a load (e.g., device electronics) 506 in the electronic device (40, FIG. 4A).

[0064] In some embodiments, the coil of electrically conductive material that comprises the power receiving element 502 may wound about the opening 408 adjacent to the opening 408. In other embodiments, the power receiving element 502 may be around about a larger perimeter to encompass a larger area than the opening 408. FIG. 5 A, for example, shows the power receiving element 502 may be wound about a periphery of the metallic portion 402. It will be appreciated that in other embodiments, the perimeter of the power receiving element 502 may lie anywhere between the opening 408 and the perimeter of the metallic portion 402.

[0065] Referring to FIG. 6, in operation, when the metallic portion 402 is exposed to an externally generated magnetic field (e.g., the wireless field 105 of a wireless power transfer system 100 shown in FIG. 1), the metallic portion 402 can couple to the externally generated magnetic field and in response to the coupling, a flow of current (e.g., eddy currents) may be induced in the metallic portion 402. This flow of current is schematically represented in the figure as a flow of current 602 in circuit 512. For comparison purposes, FIG. 4B shows how induced current 422 may flow in the metallic portion 402 when capacitor Ci is omitted.

[0066] Recalling that the metallic portion 402 may be modeled as a series connection of a resistance Rmodei and an inductance L model, the capacitor Ci in series connection with the inductance Lmodei may cancel or at least significantly reduce the reactance presented by the inductance Lmodei . In principle, if the capacitance of capacitor Ci is properly selected then the reactance components of Lmodei and Ci , respectively y ^' co Lmodei and l//(oCi , would cancel each other, leaving a pure resistance component, namely Rmodei . Canceling or at least significantly reducing the reactance components in circuit 512 can increase the flow of current 602 induced in the metallic portion 402 and hence increase wireless power transfer.

[0067] The flow of current 602 induced in the metallic portion 402 may in turn create a magnetic field (induced magnetic field) that emanates from the metallic portion 402, which is schematically represented in FIG. 6 by the shaded area 604. The metallic portion 402 may therefore serve as a means for generating a magnetic field, namely the induced magnetic field 604. The power receiving element 502, in turn, can couple to the induced magnetic field 604, resulting in the flow of current 606 in power receiving element 502. The power receiving element 502 may therefore serve as a means for producing the flow of current 606.

[0068] The flow of current 606 induced in the power receiving element 502 may be rectified using a suitable rectifier (e.g., rectifier 504) to produce a DC voltage V _ou t, which can be used to power the load 506 (e.g., device electronics, battery, etc.). Accordingly, a properly selected capacitor Ci can maximize the induced flow of current 602 in metallic portion 402, which in turn can maximize the induced magnetic field 604 that can be coupled by power receiving element 502.

[0069] In accordance with the disclosure, a resonant frequency of the metallic portion 402 may be tuned by a suitable selection of capacitance for capacitor Ci ("tuning" the metallic portion

402) to set a resonant frequency of circuit 512. At resonance, the reactive components Lmodei and Ci substantially cancel at a particular frequency. Likewise, a resonant frequency of the power receiving element 502 may be tuned by a suitable selection of capacitance for capacitor C2 to set a resonant frequency of the circuit comprising the power receiving element 502. [0070] Referring to FIG. 6A, in accordance with the disclosure, power transfer (e.g., amount of power delivered and efficiency of delivery) from a transmit coil (e.g., in a power transmitting element 114, FIG. 1) to the power receiving element 502 may be controlled by varying a mutual inductance Ml (and hence the coupling) between the transmit coil and the metallic portion 402 and/or a mutual inductance M2 between the metallic portion 402 and the power receiving element 502. For example, the coupling between the transmit coil and the power receiving element 502 may be maximized for power transfer by maximizing both the mutual inductance Ml between the transmit coil and the metallic portion 402 and the mutual inductance M2 between the metallic portion 402 and the power receiving element 502. Maximum mutual inductance can be achieved by setting the resonant frequency of both the metallic portion 402 and the power receiving element 502 to be substantially equal to a frequency of the externally generated magnetic field generated by the transmit coil.

[0071] In some situations, less than maximum power transfer may be desirable. The degree of power transfer may be controlled by lowering the mutual inductance. For example, the resonant frequency of the metallic portion 402 may be set to a frequency different from frequency of the externally generated magnetic field (referred to as being "off resonance") to reduce the mutual inductance Ml between the transmit coil and the metallic portion 402, while leaving the resonant frequency of the power receiving element 502 substantially equal to the frequency of the externally generated magnetic field. Reducing the mutual inductance Ml between the transmit coil and the metallic portion 402 may have an overall effect of reducing the power transfer from the transmit coil to the power receiving element 502. Conversely, the resonant frequency of the metallic portion 402 may remain substantially equal to the frequency of the externally generated magnetic field, while the resonant frequency of the power receiving element 502 may be set to a frequency different from the frequency of the externally generated magnetic field to reduce the mutual inductance M2 between the metallic portion 402 and the power receiving element 502. In some embodiments, both the mutual inductance M2 between the transmit coil and the metallic portion 402 and the mutual inductance between the metallic portion 402 and the power receiving element 502 may be reduced, for example, by tuning both the metallic portion 402 and the power receiving element 502 to be off resonance with respect to the frequency of the externally generated magnetic field.

[0072] Referring back to FIGS. 5, 5A, and 6, the schematic representations depict capacitor Ci electrically connected to the metallic portion 402. In some embodiments, the capacitor CI may be directly connected (e.g., soldered) to the metallic portion 402. In other embodiments, directly attaching the capacitor Ci to the metallic portion 402 may not be practical. FIG. 7, therefore, illustrates an example of a particular implementation of tuning the metallic portion 402 using capacitor Ci in accordance with other embodiments of the disclosure. A tuning element 712 may be disposed on a printed circuit board (PCB) 702 comprising the device electronics of an electronic device (e.g., 40, FIG. 4A). Connectors 714 (e.g., pogo pins) attached to the PCB 702 and connected to the tuning element 712 may extend from the tuning element 712 to make electrical contact with contact points 716 formed on the metallic portion 402, thus electrically connecting the tuning element 712 to the metallic portion 402.

[0073] In various embodiments, the tuning element 712 may be any suitable circuitry or circuit element. In some embodiments, for example, the tuning element 712 may be a capacitor, such as capacitor Ci (FIG. 5). In other embodiments, the tuning 712 element may include a variable capacitor, a network of capacitors including series-connected capacitors, parallel-connected capacitors, and so on. As mentioned above, the metallic portion 402 has an inductance L model (FIG. 5) associated with it by virtue of its loop shape 412 (FIG. 4B). In some embodiments, the inductance of the metallic portion 402 may be changed. Accordingly, in some embodiments, the tuning element 712 may include one or more inductive elements to increase or decrease the total inductance presented by the metallic portion 402 and tuning element 712.

[0074] In accordance with the disclosure, the roles of the metallic portion 402 and the power receiving element 502 may be reversed. Referring to FIG. 8, for example, in some embodiments the metallic portion 402 may serve as a power receiving element 802. A rectifier 814 may be connected to the metallic portion 402 to define a circuit 812. In some embodiments, a tuning capacitor Ci (or other tuning circuit) may be added to the circuit 812, for example, to tune a resonant frequency of the circuit 812. A means for generating a magnetic field may include a coil 804 of electrically conductive material wound about the opening 408. A capacitor C2 may be connected to the coil 804 to tune a resonant frequency of the circuit defined by the coil 804 and capacitor C2.

[0075] As described above, power transfer from a transmit coil (e.g., power transmitting element 114, FIG. 1) to the power receiving element 802 may be controlled by controlling the mutual inductance between the transmit coil and the power receiving element 802 and/or the mutual inductance between the power receiving element 802 and the coil 804. For example, the power receiving element 802 may be tuned (e.g., by tuning Ci) to be resonant or off-resonance with respect to the frequency of an external magnetic field generated by the transmit coil in order to alter the mutual inductance between the transmit coil and the power receiving element 802. Likewise, the coil 804 may be tuned (e.g., by tuning C2) to be resonant or off-resonance with respect to the frequency of the external magnetic field in order to alter the mutual inductance between the power receiving element 802 and the coil 804. [0076] Referring to FIG. 8A, in other embodiments, the coil 804 may be omitted, leaving only the resonant circuit 812 comprising the power receiving element 802 (realized using the metallic portion 402), and the capacitor CI and rectifier 814.

[0077] Referring to FIGS. 9 and 9A, embodiments in accordance with the disclosure may include a wearable electronic device. In some embodiments, for example, a wearable electronic device 90 may include a device body 92 connected to a fastener 94. The wearable electronic device 90 may be a smartwatch, a fitness monitoring device, and so on.

[0078] The device body 92 may include a metallic portion 902. The metallic portion 902 may have an open loop shape defining a central opening 908 and a slot 910 that connects the opening 908 and a periphery of the metallic portion 902. A capacitor Ci may be connected to the metallic portion 902 to define a circuit, tuned by the capacitor Ci . The device body 92 may include a power receiving element (e.g., coil) 912 wound about the opening 908. The power receiving element 912 may be connected to a capacitor C2 (e.g., to tune the power receiving element 912), and to a rectifier 904 to produce a DC voltage V _ou t using the flow of current induced in the power receiving element 912 when exposed to an externally generated magnetic field.

[0079] In some embodiments, the metallic portion 902 may serve as a housing to house the device electronics (not shown) that comprise the wearable electronic device 90. Referring to FIG. 9 A, in other embodiments, a wearable electronic device 91 may include a device body 93 that may include a non-metallic housing 96 to house the metallic portion 902 and the power receiving element 912.

[0080] As described above, power transfer from a transmit coil (e.g., power transmitting element 114, FIG. 1) to the power receiving element 912 may be controlled by controlling the mutual inductance between the transmit coil and the power receiving element 912 and/or the mutual inductance between the power receiving element 912 and the metallic portion 902. For example, the power receiving element 912 may be tuned (e.g., by tuning C2) to be resonant or off-resonance with respect to the frequency of an external magnetic field generated by the transmit coil in order to alter the mutual inductance between the transmit coil and the power receiving element 912. Likewise, the metallic portion 902 may be tuned (e.g., by tuning Ci ) to be resonant or off-resonance with respect to the frequency of the external magnetic field in order to alter the mutual inductance between the power receiving element 912 and the metallic portion 902.

[0081] Referring to FIG. 10, embodiments in accordance with the disclosure may include portable computers; e.g., laptop computers, computer tablets, and the like. In some embodiments, for example, a portable computer 10 may comprise a front enclosure 1002 and a back enclosure 1008 to house a display 1004 and device electronics (e.g., circuitry, battery, etc.) 1006, and a wireless power receiver 1010. The back enclosure 1008 may be a non-metallic material in order not to interfere with the wireless power receiving function. Details of the wireless power receiver 1010 in accordance with the disclosure will now be described.

[0082] FIGS. 11 A and 1 IB are schematic representations showing details of a wireless power receiver 1010 in accordance with embodiments of the disclosure. The wireless power receiver 1010 may include a metallic portion 1102 having the shape of an open loop defining an opening 1108 and a slot 1110 that extends between the opening 1108 and a periphery of the metallic portion 1102. A power receiving element (e.g., coil) 1112 may be wound about the opening

1108. A rectifier 1104 may be connected to the power receiving element 11 12 to produce a DC voltage Vout from a flow of current that can arise in the power receiving element 1112 in response to an externally generated magnetic field.

[0083] A ferrite layer 1122 may be disposed between the power receiving element 1112 and the device electronics 1006, in order to prevent magnetic fields that can be generated by the power receiver 1010 from coupling to the device electronics 1006. The ferrite layer 1122 is omitted in FIG. 1 IB to more clearly illustrate details of the metallic portion 1102.

[0084] A capacitor Ci may be used to tune the metallic portion 1102 to be in resonance with an externally generated magnetic field or out of resonance with the externally generated magnetic field in order to alter the mutual inductance between the metallic portion 1102 and the transmit coil. Similarly, a capacitor C2 may be used to tune the power receiving element 1112 to be in resonance or out of resonance with the externally generated magnetic field in order to alter the mutual inductance between the power receiving element 1112 and the metallic portion 1102.

[0085] In accordance with embodiments of the disclosure, the power receiver 1010 may have an area that is smaller than the area of the portable computer 10 shown in FIG. 10. In some embodiments, the area of the power receiver 1010 may be less than 50% of the area of the portable computer 10. Due to the amplifying effect that of the tuned metallic portion 1102, the coil 1112 may couple more strongly to an externally generated magnetic field and thus achieve a greater power transfer than in a wireless power transfer system that does not use an amplifying element such as the tuned metallic portion 1112. Accordingly, the power receiver 1010 may be smaller and still achieve a similar power transfer as compared to larger wireless power transfer systems. [0086] The above description illustrates various embodiments of the disclosure along with examples of how aspects of the particular embodiments may be implemented. The above examples should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the particular embodiments as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents may be employed without departing from the scope of the disclosure as defined by the claims.