WO2018106762A1

WO2018106762A1 - Multi-transmitter wireless power systems

Info

Publication number: WO2018106762A1
Application number: PCT/US2017/064832
Authority: WO
Inventors: Izhar Bentov; Supratik Datta; Asif Hussain; Manisha P. Pandya
Original assignee: Apple Inc.
Priority date: 2016-12-11
Filing date: 2017-12-06
Publication date: 2018-06-14

Abstract

Systems and methods for wireless power transfer to an electronic device. In some embodiments, the disclosed devices includes one or more arrays of transmit coils within a charging surface on which an electronic device is placed for charging. A controller may apply a first alternating signal with a first phase to a first transmit coil, and a second alternating signal with a second phase to a second transmit coil. The offset of phases may produce increased magnetic flux through a receive coil of the electronic device to transfer power more efficiently.

Description

MULTI-TRANSMITTER WIRELESS POWER SYSTEMS CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This Patent Cooperation Treaty patent application claims priority to U.S Non- provisional Patent Application No. 15/467,998, filed March 23, 2017, and entitled "Multi- Transmitter Wireless Power Systems," and U.S. Provisional Patent Application No.

62/432,647, filed on December 1 1 , 2016, and entitled "Multi-Transmitter Wireless Power Systems," the contents of which are incorporated herein by reference in their entirety.

FIELD

[0002] Embodiments described herein generally relate to wireless power systems and, in particular, to wireless power systems configured to efficiently magnetically couple multiple transmit coils to a single receive coil.

BACKGROUND

[0003] A mobile device can recharge an internal battery by wirelessly coupling to a wireless power transmitter. Collectively, the mobile device and wireless power transmitter be referred to as a "wireless power system."

[0004] A conventional wireless power transmitter includes a set of transmit coils positioned below a charging surface. The transmit coils are independently operable to deliver power - by electromagnetic induction - to a mobile device placed on that charging surface. However, the system requires precise alignment of the mobile device with a single transmit coil to operate efficiently; if the mobile device is not aligned with one transmit coil of the set, received power is substantially reduced.

[0005] The system typically compensates for reduced received power by increasing power output from the transmit coil most closely aligned with the mobile device. However, the amount that power output may be increased is limited to prevent undesirable heating of the mobile device. Due to the power output limitation, charging time is increased when the mobile device is not precisely aligned with a transmit coil. In other words, positioning the mobile device at certain locations of the charging mat results in increased charging time, providing an inconsistent user experience.

SUMMARY

[0006] Embodiments described herein reference a wireless power system including a wireless power transmitter and an electronic device. The wireless power transmitter includes a charging surface configured to accommodate the electronic device. A first transmit coil and a second transmit coil are positioned below the charging surface. A controller, electrically connected to the first transmit coil and the second transmit coil, can be configured to apply a first waveform at a first phase to the first transmit coil and a second waveform (generally related to the first waveform) at a second phase to the second transmit coil. In these embodiments, the second phase is based, at least in part, on a location of the electronic device relative to the charging surface.

[0007] Other embodiments described herein reference method of operating a wireless power transmitter. The method includes the operations of: applying a first electrical signal at a first phase to a first transmit coil (the first electrical signal associated with a low-power mode of the wireless power transmitter); applying a second electrical signal at a second phase different from and relative to the first phase to a second transmit coil adjacent to the first transmit coil (the second electrical signal also associated with the low-power mode); monitoring a voltage across a receive coil of an electronic device adjacent to the first transmit coil and the second transmit coil; and modifying the second phase to increase the voltage beyond a threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements. [0009] FIG. 1 A illustrates electronic devices on a wireless power transmitter.

[0010] FIG. 1 B illustrates a side view of the wireless power transmitter of FIG. 1 A.

[0011] FIG. 2A illustrates internal transmit coils of the wireless power transmitter of FIG. 1 A, according to an embodiment.

[0012] FIG. 2B illustrates an alternate arrangement of transmit coils of the wireless power transmitter of FIG. 1 A, according to an embodiment.

[0013] FIG. 3 shows a representational cross-sectional view of a wireless power transmitter and magnetic fields of transmit coils, according to an embodiment.

[0014] FIG. 4A illustrates a top view of a charging surface having multiple layers of transmit coils, according to an embodiment. [0015] FIG. 4B illustrates a representational cross-sectional view of the charging surface of FIG. 4A, according to an embodiment. [0016] FIG. 5A illustrates a representational cross-sectional view of a charging surface with a transmit coils having parallel magnetic fields, according to an embodiment.

[0017] FIG. 5B illustrates a representational cross-sectional view of a charging surface with a transmit coils having opposite magnetic fields, according to an embodiment. [0018] FIG. 6 illustrates a top view of a charging surface having multiple layers, according to an embodiment.

[0019] FIG. 7 is flow chart of a method for charging an electronic device, according to an embodiment.

[0020] FIG. 8 is flow chart of a method for determining transmit coils and their phases, according to an embodiment.

[0021] FIG. 9 is flow chart of a method for determining phases for transmit coils, according to an embodiment.

[0022] FIG. 10 is flow chart of a method for determining power to apply to transmit coils, according to an embodiment. [0023] The use of cross-hatching or shading in the accompanying figures is generally provided to clarify the boundaries between adjacent elements and also to facilitate legibility of the figures. Accordingly, neither the presence nor the absence of cross-hatching or shading conveys or indicates any preference or requirement for particular materials, material properties, element proportions, element dimensions, commonalities of similarly illustrated elements, or any other characteristic, attribute, or property for any element illustrated in the accompanying figures.

[0024] Additionally, it should be understood that the proportions and dimensions (either relative or absolute) of the various features and elements (and collections and groupings thereof) and the boundaries, separations, and positional relationships presented

therebetween, are provided in the accompanying figures merely to facilitate an

understanding of the various embodiments described herein and, accordingly, may not necessarily be presented or illustrated to scale, and are not intended to indicate any preference or requirement for an illustrated embodiment to the exclusion of embodiments described with reference thereto. DETAILED DESCRIPTION

[0025] Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the disclosure to one preferred embodiment. To the contrary, it is intended that the following description covers alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims.

[0026] Embodiments described herein reference a wireless power system including a transmitter device and a receiver device. A number of transmit coils in the transmitter device each magnetically couple to a receive coil in the receiver device to transfer electrical power to the receiver device by cooperative electromagnetic induction. More specifically, a set of adjacent transmit coils in the transmitter device can be driven with specific waveforms having the same frequency at (optionally) different phases. The phases of these specific waveforms can be selected to maximize magnetic field coupling to the receive coil. More particularly, the phases of the waveforms can be selected based on the position of the receive coil relative to the set of transmit coils. As a result of driving adjacent transmit coils in the phase-shifted manner described herein, electrical power can be efficiently and consistently transferred from the transmitter device to the receiver device in a manner substantially independent of the relative alignment between the two devices. [0027] For simplicity of description, many embodiments that follow reference an example transmitter device as a "wireless power transmitter" and a receiver device as an "electronic device." However, it may be appreciated that other implementations and constructions are contemplated in view of the techniques and methods described herein. In particular, it may be appreciated that any suitable transmitter device construction can be driven in the phase- shifted manner described herein to cooperatively magnetically couple to any number of receiver devices that may be similarly, or uniquely, constructed.

[0028] Example suitable wireless power transmitters can incorporate multiple transmit coils of uniform or different size, distributed in a single layer or multiple layers, distributed in a pattern or shape, positioned relative to a single surface or multiple surfaces, constructed in the same or different manners, constructed with the same number of turns or a different number of turns, and so on. Example electronic devices can include one or more receive coils, distributed adjacent to, or separated from, one another, that may be larger or smaller than the transmit coils in an associated wireless power transmitter.

[0029] For simplicity of description, many embodiments that follow reference a wireless power system including a wireless power transmitter and an electronic device, although it may be appreciated that other configurations are possible in view of the systems, methods, and techniques described herein. In particular, in this example, the wireless power transmitter includes a charging surface that is configured to receive and support the electronic device. Below the charging surface, the wireless power transmitter includes multiple layers of transmit coils, each layer including multiple transmit coils distributed in a regular pattern laterally offset from coils of adjacent layers. In this example, the electronic device includes a single receive coil having a radius or apothem generally larger than that of the transmit coils of the wireless power transmitter. When the electronic device is placed on the charging surface, the wireless power transmitter selects a set of transmit coils that are most adjacent to the receive coil based on the position and orientation of the electronic device on the charging surface. Each transmit coil of the set of transmit coils is driven, simultaneously, with coil-specific waveforms, each waveform having the same frequency, but driven with (optionally) different phases and/or amplitudes so as to maximize magnetic coupling to the receive coil. [0030] Further embodiments described herein reference methods of selecting and/or determining appropriate phases for each waveform applied to each respective transmit coil of the set of transmit coils selected based on a particular position and/or orientation of an electronic device relative to the wireless power transmitter. Such methods include, but are not limited to: fixing a first waveform and varying a phase of a second waveform until a measured power received in the electronic device exceeds a threshold; fixing a first waveform and a phase of a second waveform while varying a phase of a third waveform until a measured power received in the electronic device exceeds a threshold; accessing a lookup table based on a position and/or orientation of the electronic device, the lookup table including one or more phase values; receiving a signal from the electronic device that received power is below a threshold and varying a phase of a waveform in response; fixing a first waveform and randomly varying a phase of a second waveform to determine a relationship between the second phase and power received at the electronic device (e.g., Monte Carlo simulation); and so on.

[0031] As used herein, the term "alternating signal" or, more generally, the term

"waveform" is used to refer to any one of a voltage, current or power signal that varies over time. Alternating signals include signals with sinusoidal variation as well as square waves, triangle waves, pulsed inputs, such as pulse width modulated signals, and the like.

[0032] These and other embodiments are discussed below in more detail with reference to FIGs. 1 A - 10. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting.

[0033] Generally and broadly, FIGs. 1 A - 1 B depict a wireless power system 100 including a wireless power transmitter 102, an electronic device 104, and an electronic device 106. In particular, FIG. 1 A depicts a plan view the wireless power system 100 and FIG. 1 B depicts a side view of the wireless power system 100, specifically illustrating an example embodiment in which a wireless power transmitter 102 is accommodated in a low- profile (e.g., thin) enclosure. As with other embodiments described the wireless power system 100 is configured to transfer power - via cooperative electromagnetic induction - from the wireless power transmitter 102 to one or more mobile devices, such as the electronic device 104 or the electronic device 106.

[0034] The wireless power transmitter 102 is disposed within an enclosure. The enclosure can accommodate: one or more transmit coils or transmit coil arrays or layers, a processor, memory, display, battery, network connections, sensors, input/output ports, acoustic elements, haptic elements, digital and/or analog circuits for performing and/or coordinating tasks of the wireless power system 100, and so on. For simplicity of illustration, the enclosure is depicted in FIG. 1 A without many of these elements, each of which may be included, partially and/or entirely, within the enclosure and may be operationally or functionally associated with the wireless power transmitter 102. In some embodiments, the wireless power transmitter 102 is fully-integrated; all components of the wireless power transmitter 102 are within the enclosure, apart from an electrical connection (e.g., cable) to mains voltage, which is not depicted.

[0035] Like the wireless power transmitter 102, the electronic device 104 is disposed within an enclosure. Typically, the enclosure of the electronic device 104 is smaller than the enclosure of the wireless power transmitter 102, but this may not be required. The enclosure of the electronic device 104 can accommodate: one or more receive coils, a processor, memory, display, battery, voltage-sensitive systems, capacitive or other sensors, network connections, input/output ports, acoustic elements, haptic elements, digital and/or analog circuits for performing and/or coordinating tasks of the wireless power system 100 or another electronic device, and so on. For simplicity of illustration, the enclosure of the electronic device 104 is depicted in FIG. 1 A without many of these elements, each of which may be included, partially and/or entirely, within the enclosure and may be operationally or functionally associated with the electronic device 104. The electronic device 106 may include similar or identical components. [0036] In some examples, either or both the electronic device 104 or the electronic device 106 is a mobile electronic device such as a cellular phone, a tablet computer, a wearable electronic device (e.g., watch, pendant, bracelet, necklace, anklet, ring, headphone, glasses, wearable display, and so on), a peripheral input device (e.g., keyboard, mouse, trackpad, remote control, stylus, gaming device, gesture input device, and so on), an authentication device or token (e.g., password token, vehicle key, and so on), an access card, an anti-theft or loss prevention device, a home automation device, a display, and so on.

[0037] In the illustrated embodiment, the enclosure of wireless power transmitter 102 defines a charging surface for receiving the electronic device 104 and the electronic device 106. The charging surface can be substantially planar, although this is not required. For example, in some embodiments, the charging surface may be concave, convex, or may take another shape.

[0038] In many examples, the wireless power transmitter 102 includes multiple transmit coils distributed at different locations relative to the charging surface (see, e.g., FIG. 2A). In these embodiments, individual transmit coils can be associated with different portions of the charging surface. In this manner, the wireless power system 100 can selectively activate or deactivate transmit coils of the wireless power transmitter 102 independently.

[0039] Further, the wireless power system 100 can selectively control power output from each transmit coil by applying specific waveforms to each individual transmit coil. In many cases, the wireless power system 100 can selectively active a transmit coil (or more than one transmit coil) based on the position and/or orientation of the electronic device 104 (or the electronic device 106) relative to the charging surface and, in particular, relative to the location of the nearest transmit coil or coils. More specifically, the wireless power system 100 can selectively activate a transmit coil based on a coupling factor k that corresponds to the mutual coupling between that transmit coil and a receive coil disposed within the electronic device 104; the higher the coupling factor, the more likely the wireless power system 100 is to activate that transmit coil to effect power transfer from that transmit coil of the wireless power transmitter 102 to the receive coil within the electronic device 104.

[0040] FIG. 2A illustrates two transmit coils 200 and 202 of the wireless power transmitter 102. In the embodiment illustrated, the transmit coils 200 and 202 have a planar circular configuration, although this may not be required and other shapes and distributions of the transmit coils 200 and 202 may be possible. For example, although the transmit coils 200 and 202 are each depicted with three turns, this is merely one example. In other

embodiments, other turn counts are possible. In the illustrated embodiment, the transmit coil 200 is laterally offset from the transmit coil 202, but this may not be required; in some examples, the transmit coils 200 and 202 are either horizontally or vertically aligned.

[0041 ] In further examples, the transmit coils 200 and 202 can be formed onto the same layer or substrate below the charging surface of the wireless power transmitter 102. In these cases, the transmit coils 200 and 202 may be members of an array of transmit coils (see e.g., FIG. 2B). The array of transmit coils can be distributed in any suitable pattern, regular or otherwise.

[0042] In a typical example, the transmit coils 200 and 202 are operatively associated with or otherwise electrically coupled to an alternating signal source (not shown) or another waveform generator or controller.

[0043] As noted with respect to other embodiments described here, a waveform generator or controller can be configured to apply a waveform to each individual transmit coil. More specifically, the waveform generator can be configured to apply a first waveform to the transmit coil 200 and a second waveform to the transmit coil 202. In many cases, the first waveform and the second waveform have the same frequency, phase, and amplitude, but this may not be required. For example, in one embodiment, the phase of the second waveform may be different from that of the first waveform. As a result of the phase difference between the first waveform and the second waveform, the magnetic fields produced by the transmit coil 200 and the transmit coil 202 are also out of phase with one another. [0044] As may be known, adjacent magnetic fields interact with one another; the net flux produced by two adjacent magnetic field sources can be represented as a single magnetic flux field. In this manner, when the first waveform is applied to the transmit coil 200 at the same time the second waveform is applied to the transmit coil 202, the two generated magnetic fields interact with one another in a manner that can be mathematically

represented as a single magnetic flux field.

[0045] More specifically, the phase difference between the first waveform and the second waveform can change the interaction between the magnetic fields produced by the transmit coil 200 and the transmit coil 202, respectively. By adjusting the phase of the second waveform relative to the first waveform, the waveform generator of the wireless power transmitter 102 can effectively change the magnetic flux density measurable at any location of the charging surface of the wireless power transmitter 102.

[0046] For example, if the electronic device 104 (not shown in FIG. 2A, see FIGs. 1 A - 1 B) is positioned on the charging surface of the wireless power transmitter 102 between the transmit coil 200 and the transmit coil 202, the receive coil within the electronic device 104 may not strongly couple to either the transmit coil 200 or the transmit coil 202. However, if the transmit coil 200 and the transmit coil 202 are driven simultaneously, a phase difference can be selected between the first waveform and the second waveform that maximizes flux density at a location approximately in the center of the receive coil, thereby increasing power transferred to the electronic device 104.

[0047] In some embodiments, the phase difference between the waveforms applied to the transmit coil 200 and the transmit coil 202 is selected, at least in part, based on the geometry of the transmit coil 200 and the transmit coil 202. For example, as may be appreciated, the shape of a magnetic field generated by a circular electromagnetic coil may be different from that of a magnetic field generated by a rectangular electromagnetic coil. As a result, the magnetic field interaction pattern between coils may vary based on the shape of the coil.

[0048] FIG. 2B illustrates a top view of another example implementation of the wireless power transmitter 102 that includes multiple transmit coils, two of which are identified as the transmit coil 206 and the transmit coil 208. In this example, the transmit coils 206 and 208 have a rectilinear shape. The rectilinear shape allows for more space efficient packing of the transmit coils in the array. In other example, differently-spaced coils can be used. Suitable shapes include, but are not limited to: ovals, rectangles with rounded corners, octagons, symmetrical shapes, asymmetrical shapes, and so on. [0049] In the illustrated embodiment, the transmit coils 206 and 208 have the same shape and apothem as all other transmit coils of the array of transmit coils, but this may not be required. In some examples, different coils of the array of transmit coils can have a different shape or size.

[0050] As noted above with respect to the embodiment depicted in FIG. 2A, the shape of the transmit coil 206 and the transmit coil 208 may effect or otherwise influence the phase difference between a waveform applied to the transmit coil 206 and a waveform applied to the transmit coil 208. Further, it may be appreciated that the relative distance between the transmit coil 206 and the transmit coil 208 can inform or otherwise influence the phase difference between a first waveform applied to the transmit coil 206 and a second waveform applied to the transmit coil 208.

[0051] The foregoing embodiments depicted in FIGs. 2A - 2B and the various alternatives thereof and variations thereto are presented, generally, for purposes of explanation, and to facilitate an understanding of various possible coil configurations, distributions, and shapes that can be incorporated into, or be otherwise associated with, a wireless power transmitter, such as described herein. However, it will be apparent to one skilled in the art that some of the specific details presented herein may not be required in order to practice a particular described embodiment, or an equivalent thereof.

[0052] FIG. 3 illustrates a cross-sectional view of an example receive coil, identified as the receive coil 300a, of an electronic device positioned relative to a transmit coil 302 of a wireless power transmitter, through a charging surface 304. For simplicity of illustration, the electronic device and the wireless power transmitter are omitted from FIG. 3.

[0053] As with other embodiments described herein, the wireless power transmitter may be configured to apply different waveforms to different transmit coils at (optionally) different phases to increase power transfer to the receive coil. However, in certain cases, driving more than one coil may not be required. For example, in the illustrated embodiment, the receive coil 300a is axially aligned with the transmit coil 302; the receive coil 300 is highly coupled to the transmit coil 302 and inductive power transfer between them may be highly efficient. More specifically, flux density through the center of the receive coil 300 may be substantially maximized.

[0054] In other cases, a receive coil may not be axially aligned with any single transmit coil. For example, the receive coil 300b may be laterally offset from each of the transmit coils 306 and 308. In this example, as with other embodiments described herein, the wireless power transmitter may be configured to apply a first waveform to the transmit coils 306 and a second waveform to the transmit coil 308. In many cases, the phase of the second waveform can be the same as the phase of the first waveform, but this may not be required. For example, as illustrated, the phase of the second waveform is lagging the phase of the first waveform. As a result of the phase difference, such as illustrated, flux density through the center of the receive coil 300b can be increased, despite that the receive coil 300b is not highly coupled to either the transmit coil 306 nor the transmit coil 308.

[0055] As with other embodiments described herein, the phase difference between the first waveform and the second waveform can depend on, or can be based on, a number of factors. For example, the phase difference between the first waveform and the second waveform can vary based on the location of the receive coil 300b relative to the transmit coil 306 and the transmit coil 308. In one example, the closer the receive coil 300b is to alignment with the transmit coil 306, the greater the phase difference between may be between the first waveform and the second waveform. In another example, the closer the receive coil 300b is to a center point between the transmit coil 306 and the transmit coil 308, the smaller the phase difference between the first waveform and the second waveform may be. In other cases, the phase difference can vary differently; the phase difference may increase as the receive coil 300b approaches the center point between the transmit coil 306 and the transmit coil 308.

[0056] In other embodiments, the phase difference between the first waveform and the second waveform can vary based on the geometry of the receive coil 300b and/or the transmit coil 306 and the transmit coil 308. In other examples, the phase difference can be based on an amount of power required by the electronic device. In yet other examples, the phase difference can be based on a size (e.g., radius, apothem, and so on) of the receive coil 300b relative to one or both of the transmit coil 306 and the transmit coil 308.

[0057] In many cases, such as illustrated, the transmit coil 306 and the transmit coil 308 may have a smaller size than the receive coil 300b, although this may not be required. In many cases, the transmit coil 306 and the transmit coil 308 may have a smaller radius or apothem than the receive coil 300b in order to support a wider range of relative positions of the electronic device on the charging mat 302.

[0058] In further embodiments, more than three transmit coils can be included. For example, an array of transmit coils, such as shown in FIG. 2B, may be incorporated into a wireless power transmitter, such as the wireless power transmitter described with respect to FIG. 3. In these embodiments, two or more transmit coils can be simultaneously-driven at (optionally) different phases in order to increase net flux density through a central portion of a receive coil. [0059] In still further cases, a wireless power transmitter can include more than one layer of transmit coils. In these cases, a first layer of transmit coils can be positioned over a second layer of transmit coils. Typically, the transmit coils of the second layer of transmit coils are offset from the transmit coils of the first layer. In some embodiments, the offset can be referred to as a "lateral offset." [0060] In these cases, the wireless power transmitter can be configured to select a set of transmit coils that are adjacent to or nearby the receive coil of an electronic device. In some cases, the set of transmit coils can be selected from a single layer of transmit coils, whereas in other cases, the set of transmit coils can be selected from multiple layers of transmit coils.

[0061 ] When a set of transmit coils has been selected by the wireless power transmitter, whether the transmit coils of the set are within a single layer or multiple layers, different waveforms for each transmit coil of the set of transmit coils can be selected. In some cases, the coil-specific waveforms each have a different phase relative to a first waveform, but this may not be required of all embodiments. For example, a first subset of the set of transmit coils can be driven at the same phase when a second subset of the transmit coils can be driven at a different phase. As with other embodiments described herein, the phase of an individual waveform associated with a particular transmit coil is selected to maximize magnetic coupling to the receive coil (e.g., flux density generally through the center of the receive coil). [0062] In still further embodiments, the wireless power transmitter can be configured to select a particular amplitude for each waveform associated with each transmit coil of the set of selected transmit coils. In some cases, the amplitude of waveforms associated with transmit coils farther away from the receive coil can be greater than the amplitude of waveforms associated with transmit coils closer to the receive coil. In other cases, the amplitude of waveforms associated with the transmit coils tightly coupled to the receive coil can be greater than the amplitude of waveforms associated with transmit coils loosely coupled to the receive coil. In still further examples, other aspects or characteristics of the various waveforms can be changed, varied, or otherwise customized for a particular transmit coil. [0063] The foregoing embodiment depicted in FIG. 3 and the various alternatives thereof and variations thereto is presented, generally, for purposes of explanation, and to facilitate an understanding of various possible factors that can influence a phase difference between simultaneously-driven transmit coils of a wireless power transmitter such as described herein. However, it will be apparent to one skilled in the art that some of the specific details presented herein may not be required in order to practice a particular described

embodiment, or an equivalent thereof.

[0064] For example, FIGs. 4A - 4B depict a wireless power transmitter 400 in which there are two arrays of transmit coils, arranged in two layers, and positioned below a charging surface 402. The transmit coils shown with solid outline, such as the transmit coil 404, are members of a first layer of transmit coils and transmit coils with dashed outline, such as the transmit coil 406, are members of a second layer of transmit coils. The second layer of transmit coils is located below the first layer of transmit coils and is positioned such that the coils of the second layer are each offset from the coils of the first layer.

[0065] As with other embodiments described herein, the wireless power transmitter 400 may be configured to detect when an electronic device (e.g., such as the electronic device 102 depicted in FIGs. 1 A - 1 B) is placed on the charging surface. In some cases, the wireless power transmitter 400 can be configured to authenticate or verify the presence of the electronic device prior to initiating a power transfer operation (e.g., foreign object detection), but this may not be required. [0066] Once the wireless power transmitter 400 has determined that an electronic device is positioned on the charging surface 402, the wireless power transmitter 400 can determine a location of a receive coil within the electronic device using any suitable technique or combination of techniques such as, but not limited to: communicating with the electronic device over a communications channel; iteratively activating each transmit coil of the first or second layer of transmit coils to determine which coil is most coupled to the receive coil; obtaining input from sensors such as, but not limited to, pressure sensors, capacitive sensors, inductive casing sensors, and so on; and so on.

[0067] Once the wireless power transmitter 400 has determined the position of the receive coil (or receive coils) within the electronic device, the wireless power transmitter 400 can select a set of transmit coils that are adjacent to the receive coil. A set of transmit coils typically includes two or more transmit coils.

[0068] In some cases the selected set of transmit coils can include coils from the first array of transmit coils and coils from the second array of transmit coils. For example, if the receive coil is determined to be adjacent to, or overlapping, each of the transmit coils 404, 406, and 408, these three coils may be selected by the wireless power transmitter 400 as the set of transmit coils.

[0069] Once the set of transmit coils most adjacent to the receive coil is selected, the wireless power transmitter 400 can determine an appropriate coil-specific waveform to apply to each transmit coil of the set of transmit coils. In one example, the wireless power transmitter 400 can be configured to apply a first waveform to the transmit coil 404, apply a second waveform to the transmit coil 406, and apply a third waveform to the transmit coil 408. In this example, the wireless power transmitter 400 can be configured to iteratively vary the phases of the second waveform and the third waveform until an electronic device positioned adjacent to the transmit coils 404, 406, and 408 reports to the transmit coils 404, 406, and 408 that power received by the electronic device has increased beyond a threshold. In some cases, the electronic device can enter a low-power mode while the phase(s) of the coil-specific waveforms are determined. Thereafter, once appropriate phases are determined, the electronic device can signal the wireless power transmitter 400 that the electronic device is prepared to enter a high power mode.

[0070] In some cases, the phase of the first waveform and the second waveform are varied linearly, but this may not be required; any suitable means of determining the phase of the second and third waveforms relative to the first waveform can be used by the wireless power transmitter 400. [0071] In another example, the transmit coils 404, 406, and 408 can be configured to access a database that includes phase information. The database can be a relational database, stored in a non-transitory memory that is accessible to the wireless power transmitter 400, either remotely or locally. In this example, the relational database can include phase and/or transmit coil information that is related to the position of a receive coil relative to the charging surface of the wireless power transmitter 400. For example, the wireless power transmitter 400 can query the relational database with a position (in any suitable coordinate space or system) of the receive coil of the electronic device. In response, the relational database can return a list of transmit coils and a phase associated with coil- specific waveforms to be applied to each transmit coil of the list of transmit coils.

[0072] In other cases, the relational database can be configured to provide different phase information depending upon the electronic device itself. For example, different electronic devices may include different receive coils which may necessitate different phase information. In still further embodiments, the relational database can be configured to provide different phase information depending upon the requested or instantaneous power requirements of the electronic device.

[0073] In further embodiments, the wireless power transmitter 400 can be configured to determine the phase of the coil-specific waveforms in another manner. It may be appreciated that any number of suitable techniques may be applied or exercised by the wireless power transmitter 400 to determine the phase of one or more waveforms associated with one or more transmit coils based on the location of a particular electronic device on a particular charging surface.

[0074] As may be appreciated, and as noted with respect to other embodiments described herein, the relative position of transmit coils in the selected set of transmit coils can influence the phase selected for coil-specific waveforms applied to the transmit coils. For example, as shown in FIGs. 4A - 4B, individual coils in layers are offset from one another such that individual coil in one layer overlap at least one coil in another layer.

[0075] In another example, such as shown in FIGs. 5A - 5B, a wireless power transmitter 500 can include two layers of transmit coils (for simplicity, only one transmit coil per layer is shown, identified as the transmit coil 502 and the transmit 504 respectively). In this example, if the transmit coil 502 and the transmit coil 504 are driven with waveforms that are in-phase with one another (see, e.g., FIG. 5A), a reduction in net flux at the overlap between the coils can be achieved. Alternatively, if the transmit coil 502 and the transmit coil 504 are driven with waveforms that are out of phase with one another (e.g., 180 degrees separated), such as shown in FIG. 5B, an increase in net flux at the overlap between the coils can be achieved. Depending upon the implementation, an increase or a decrease in flux may be desired by the wireless power transmitter 500.

[0076] For example, if a foreign object is placed nearby the overlap between the coils, the wireless power transmitter 500 may be configured to reduce net flux at that location.

Alternatively, if an electronic device is placed nearby the overlap between the coils, the wireless power transmitter 500 may be configured to increase net flux at that location. As with other embodiments described herein, it may be appreciated that the wireless power transmitter 500 can be configured in any implementation specific or suitable manner to effect an increase in net flux or a decrease in net flux at any suitable location. [0077] In another example configuration, FIG. 6 depicts an example wireless power transmitter 600 that includes three layers of transmit coils. More specifically, the wireless power transmitter 600 includes a transmit coil 602. The transmit coil 602 a member of a first layer of transmit coils closest to the surface of wireless power transmitter 600. The wireless power transmitter 600 also includes a transmit coil 604. The transmit coil 604 is a member of a second layer of transmit coils positioned below the first layer of transmit coils. As illustrated, the second layer is disposed such that each transmit coil of the second layer is laterally offset from each transmit coil of the first layer. The wireless power transmitter 600 includes a transmit coil 606. The transmit coil 606 is a member of a third layer of transmit coils positioned below the first layer of transmit coils and the second layer of transmit coils. As illustrated, the third layer is disposed such that each transmit coil of the third layer is laterally offset from each transmit coil of the first layer and laterally offset from each transmit coil of the second layer.

[0078] The transmit coils of any of the layers may be similarly shaped transmit coils and arranged in a close-pack configuration. In other embodiments the transmit coils may have wider separation, or have different shapes. Embodiments having three layers of transmit coils may also make use of methods, to be described below, of applying alternating signals, with possibly distinct phases, to two or more transmit coils to compensate for an imperfect alignment of an axis of a receive coil with any axis of the transmit coils.

[0079] Applying coil-specific waveforms having (optionally) different phases to two or more transmit coils may allow transmit coils to have a smaller size than a receive coil in an electronic device placed on the wireless power transmitter 600. By adjusting the coil-specific waveforms to the multiple transmit coils, the magnetic fluxes produced by the transmit coils may be arranged to produce a magnetic flux through a receive coil greater than could be possible using only one transmit coil or multiple transmit coils with the same signal. [0080] Further, and as described with respect to other embodiments herein, adjusting the phases or power levels in alternating signals applied to multiple transmit coils may allow for the generation of useful levels of magnetic flux through a receive coil that is arbitrarily positioned. [0081] Some or all of the various actions needed to apply different alternating signals or coil-specific waveforms to transmit coils may be performed by the controller 608. The controller 608 may be an internal part of a charging surface that contains the transmit coils, or it may be exterior to the charging surface. In still further cases, the controller 608 may be a device that is communicably coupled to the wireless power transmitter 600 (e.g., wired communications, wireless communications, and so on).

[0082] The controller 608 may connect to an exterior power supply, or include internal power storage, such as internal batteries. The controller 608 may include circuitry to convert a power source's output into alternating signals to be applied to transmit coils at particular or selected phases and/or amplitudes. The controller 608 may include logic circuitry, such as processors and memory, to implement the methods for choosing which transmit coil or transmit coils to use to charge an electronic device, for determining phases for the coil- specific waveforms to use, for determining power levels for the alternating signals, and for other operations. For example, the controller 608 may produce a pulse width modulated output signal, such as may be used to drive resonant circuit, and the controller 608 may be modify the duty cycle or amplitude.

[0083] Other operations that may be performed by the controller 608 are foreign object detection or authentication of devices placed on the charging surface. The controller 608 may contain communication circuitry that can form a wireless link with an electronic device. The wireless link may be used for authentication (e.g., to determine that the device is compatible for charging with the wireless power transmitter) or for receiving information from the electronic device regarding power transfer and charging.

[0084] FIG. 7 is a flow chart of a method 700 for use with any of the previously described wireless power transmitters. The method 700 begins at operation 702 at which a location of a receive coil on the charging surface is determined. As noted with respect to other embodiments described herein, the operation of determining the location of a receive coil can be performed using any suitable technique including, but not limited to: receiving input from a sensor system within the wireless power transmitter that corresponds to the location of a receive coil within the electronic device or that corresponds to a location of the electronic device; receiving input from the electronic device corresponding to a location of a receive coil within the electronic device; and so on. [0085] One specific example of determining the location of the electronic device involves iteratively applying low-power signals, such as pulse width modulated signals having a low duty cycle, to one or more transmit coils and detecting the power received through the receive coil of the electronic device. The received power can be detected by measuring a load impedance on the transmit coil. When the receive coil is separated from the currently- active transmit coil, the observed load to a circuit coupled to the currently-active transmit coil is minimal. Alternatively, if the receive coil is located adjacent the currently-active transmit coil, an increased load can be detected.

[0086] In some cases, the operation 702 can occur occurs after a foreign object detection operation has determined that an electronic device is on the charging surface and, if needed, that the electronic device is authenticated and capable of receiving power. Such preliminary actions may occur, for example, after a user places an electronic device on the charging surface. The preliminary actions can include, but are not limited to: establishment of a communication link between the electronic device and the wireless power transmitter. The electronic device may communicate, for example, a model number, authentication certificate, passcode, authorized user, and so on to the wireless power transmitter.

[0087] At operation 704, the location of the receive coil with respect to the various transmit coils is used to determine a set of transmit coils to use for power transfer. A first aspect of selecting the transmit coils can involve excluding transmit coils that are too far from the axis of the receive coil to produce magnetic flux through the receive coil. One selection criterion may be that location of a transmit coil must be within a fixed distance of the receive coil. The determination of which transmit coils to use for charging may also be based on a stored look-up table based on the detected location of the receive coil.

[0088] Further selection operations may then be applied to determine which transmit coils to use, such as selecting a fixed number of transmit coils that are closest to the receive coil. Still further selection operations can involve further reducing the number of transmit coils to use in the event that one transmit coil is near optimally positioned to transfer power to the receive coil (e.g., one transmit coil is coaxially aligned with the receive coil).

[0089] As noted with respect to other embodiments described herein, the set of transmit coils can include transmit coils from one or more layers.

[0090] At operation 706, in the event that two or more transmit coils are selected to use to charge the electronic device, a respective phase for each signal to be applied to the transmit coils is determined. The determination can be of a phase offset from the phase of a first alternating signal. The bases for the phase determination can include maximizing magnetic flux through the receive coil.

[0091] A look-up table or relational database can be queried to determining appropriate phases. If further refinement of the phase values is desired, a search, optimization, and/or minimization algorithm may be applied using the phases obtained from the look-up table or database as an initial phase value. In other cases, phases can be determined in another manner such as by simulated annealing, Monte Carlo simulations, guess-and-check techniques, and so on.

[0092] FIG. 8 is a flow chart corresponding to operations of a method 800 that may be used to determine which transmit coils of a wireless power transmitter to use for transferring power to an electronic device. The method 800 can be used in conjunction with or as an alternative to the method 700. The method 800 may use or incorporate, but does not necessarily require, the location of the electronic device or the receive coil.

[0093] At operation 802, a low-power alternating signal is iteratively applied to individual transmit coils of the wireless power transmitter. Examples of such low-power transmit signals include a pulse width modulated signal having a low duty cycle, and/or a reduced amplitude. In some cases, one or more transmit coils can be driven simultaneously, although this is not required and coils may be driven one at a time in certain embodiments. The iteration over the transmit coils may be nested, with an outer loop of the iteration over each of the layers (for wireless power transmitters that include more than one layer of transmit coils) and an inner loop of the iteration over the transmit coils within each layer. The iteration may be over a subset of the transmit coils, the subset being based on a location of the electronic device or a receive coil within the electronic device. The iteration may be performed by the controller 608, such as shown in FIG. 6. [0094] As the low-power alternating signal is applied to each transmit coil, a loading on a circuit of the wireless power transmitter due to a presence of a receive coil of the electronic device can be detected. When the loading is detected, and the loading exceeds a threshold, the corresponding transmit coil can be selected for possible inclusion with the transmit coils to be used for power transfer. [0095] In another example, an initial coil can be tested first and a set of neighboring coils can be selected. The set of neighboring coils can include nearby coils within the same layer, coils of different layers having axes adjacent to the axis of the initial coil, and so on. A low- power alternating signal is iteratively applied to each neighboring coil of the set of neighboring coils. When a loading on a circuit of the wireless power transmitter is detected, and the loading exceeds a threshold, the corresponding neighboring transmit coil can be selected for possible inclusion with the set of transmit coils to be used for power transfer.

[0096] In some embodiments, the low-power alternating signal may be generated by applying a low duty cycle pulse width modulated waveform into a resonant circuit to generate an alternating signal for input to the transmit coil. The generation of such a low duty cycle pulse waveform, or of the alternating signal directly, may be controlled by the controller 608 shown in FIG. 6.

[0097] At operation 804, a determination is made of the transmit coils to be used to charge or power the electronic device. The determination criteria can include an observed load on transmit coil's circuit under the alternating signal, a feedback from the electronic device itself regarding a signal (such as voltage, current, power, etc.) in the receive coil while the low-power signal was applied to a transmit coil, a known location of the receive coil, a maximum number of transmit coils to use, or other criteria.

[0098] At operation 806, determination is made of the phases for the alternating signals to be applied to the transmit coils selected at operation 804. The phase determination may make use of a look-up table or any other suitable method, including other methods describe herein.

[0099] FIG. 9 is a flow chart corresponding to operations of a method 900 of determining phases to apply alternating signals to be applied to the transmit coils chosen to be used to charge or power and electronic device. The transmit coils chosen for use may have been chosen using either of method 700 or method 800.

[0100] At operation 902 an initial low-power signal, such as described above, is applied to a first transmit coil of the chosen transmit coils. The selection of the transmit coil chosen as first may be based on which of the chosen transmit coils were observed to have greatest power transfer efficiency to the receive coil, or on other criteria. Once the initial low-power alternating signal is applied to the first transmit coil, it is maintained for the following operations.

[0101] At operation 904, an outer iterative procedure is begun. The outer iteration loops over the remaining transmit coils of those transmit coils chosen for use in charging the electronic device.

[0102] At operation 906, an inner iteration loop over offset phases is performed. The offset phases are used to create low-power signals to be iteratively applied to the chosen transmit coil from the outer iteration loop. The created low-power signal may be the low- power signal applied to the first transmit coil, but with the phase shift given by the offset phase of the current step of the inner iteration loop. For a given offset phase of the inner iteration loop, a corresponding low-power signal with that offset phase is applied to the chosen transmit coil of the outer iteration loop while simultaneously the initial low-power signal is still applied to the first transmit coil.

[0103] The inner iteration loop over offset phases may increment the offset phase in uniformly spaced steps, for example, by steps of 5 degrees. Alternatively, the inner iteration loop may use non-uniform step sizes. A set of non-uniform step sizes may be selected based on the location of the chosen transmit coil in relation to the first transmit coil, the location of the receive coil, or other criteria.

[0104] Additionally and/or alternatively, the number of steps and/or step sizes of the inner iteration over offset phases may be recursively and dynamically updated to further refine the offset phase to be selected. For example, the inner iteration over the offset phases may use an initial coarse step size of 30 degrees. If it is determined that the best observed power transfer to the receive coil occurs at offset phases of 60 degrees and 90 degrees, the inner iteration may then iteratively apply offset phases in step size of 5 degrees from of 60 degrees to 90 degrees to determine an improved value of the offset phase to be applied to the alternating signal for the current transmit coil of the outer iteration loop.

[0105] At operation 908, a determination is made of the net power (or other electrical parameter) transferred to the receive coil by the combination of the first transmit coil with the initial low-power signal and the current transmit coil with the low-power signal shifted by the current offset phase of the inner iteration loop. The determination of the net power transferred may be made as described above for method 800, or by another method.

[0106] At operation 910, the determined net power transferred to the receive coil is compared to a first threshold. The first threshold may reflect that sufficient power is being transferred by the transmit coils presently in operation with their respective low-power signals. The first threshold may be based on anticipated net power transfer to the receive coil when the power is increased to the transmit coils selected for use in charging the electronic device. [0107] If, at operation 910, the net power transferred to the receive coil does not exceed the first threshold, the inner iteration proceeds to the next offset phase to be used.

[0108] If, alternatively, at operation 910 the net power transferred to the receive coil does exceed the first threshold, in one alternative the inner iteration may continue over the remaining offset phases of the inner iteration loop, and may also dynamically update the offset phases of the inner iteration loop, as described above. In this alternative, once an offset phase is determined based on optimization criteria, the flow passes out the inner iteration loop to the YES point below decision operation 910.

[0109] In a second alternative for when the net power transferred to the receive coil does exceed the first threshold, the flow may immediately exit the inner iteration loop to the YES point below decision operation 910.

[0110] At the YES point below decision operation 910, the method 900 may exit the outer iteration loop and end. This can occur if sufficiently many transmit coils, and corresponding alternating signals, have been determined so that the electronic device can be charged with those sufficiently many transmit coils.

[0111] Alternatively, at the YES point below 910, the method 900 may continue the outer iteration by selecting the next transmit coil of the outer iteration loop, and flow passes into the inner iteration loop of operation 906.

[0112] Two variations of method 900 may be used when the outer iteration loop over the transmit coils proceeds to a third (or subsequent) transmit coil. In the first variation, the low- power alternating signal (with respective chosen offset phase) that was applied to the second transmit coil (i.e., the first transmit coil of the outer iteration loop) is maintained in the second transmit coil while the inner iteration loop proceeds over offset phases for the low- power signal applied to the third transmit coil. In another variation, only the initial low-power signal to the first transmit coil is maintained while the inner iteration loop proceeds over offset phases for the low-power signal applied only to the third transmit coil.

[0113] A result of method 900 is determination of transmit coils and their respective low- power signals to be used for charging the electronic device. Once the determination is made, the methods described below can be used to increase the power in the alternating signals of the transmit coils.

[0114] FIG. 10 is a flow chart for a method 1000 of selecting transmit coils of a wireless power transmitter to use as part of a process for charging an electronic device. Method 1000 also involves adjusting phases and power levels of alternating signals applied to those transmit coils. [0115] At operation 1002 a determination is made of the transmit coils of the wireless power transmitter to be used for charging an electronic device. This may use any of methods 700, 800, or 900, or other appropriate method. [0116] At operation 1004, phases are determined for signals to be applied to each to the determined transmit coils. This may use any of methods 700, 800, or 900, or other appropriate method.

[0117] At operation 1006, power levels are determined for the signals to be applied to each to the determined transmit coils. In some embodiments, this may be performed by another iterative process that increments the power applied in the signals in steps. The steps and step sizes may be uniform, or non-uniform. The initial power level may be the low-power level of the low-power signals described with respect to method 900. In those embodiments in which the alternating signals are pulse width modulated signals, the power levels may be incrementally increased by increasing the duty cycles.

[0118] The iterative process may proceed until a power level is reached at which the power transfer from the wireless power transmitter to the electronic device exceeds a second threshold. The second threshold may be based on the amount of received power in the electronic device that can be effectively used to charge a battery, such as for a battery that can only accept a maximum amount of recharging current. The amount of power (or current or voltage) received at the electronic device may be determined by a communication link between the wireless power transmitter, by determination from the loads on circuits of the transmit coils, or by other methods.

[0119] The iterative process may use an inner iterative process that loops over the determined transmit coils. That is, given a next power level to be used, that power level is applied sequentially to each transmit coil, and the amount of received power in the electronic device is detected. Once the power transfer from the wireless power transmitter to the electronic device exceeds the second threshold, the process may exit both iterative processes. This can result in less total power being applied by the wireless power transmitter to the transmit coils to achieve the desired recharging of the electronic device.

[0120] At operation 1008, the determined power levels are applied to the respective signals of the transmit coils. The signals may then be maintained by the wireless power transmitter until it is determined that electronic device is fully charged, or has been removed from the charging surface. [0121] It is to be noted that when a wireless power transmitter, having any the arrangements of transmit coils described above, has more than one electronic device present on the charging surface, the various methods just described may be applied to each electronic device separately. [0122] The foregoing description, for purposes of explanation, used specific

nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not targeted to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.

Claims

CLAIMS What is claimed is:

1 . A wireless power transmitter, comprising:

a charging surface configured to accommodate an electronic device;

a first transmit coil and a second transmit coil positioned below the charging surface; and

a controller electrically connected to the first transmit coil and the second transmit coil and configured to:

apply a first waveform at a first phase to the first transmit coil; and apply the first waveform at a second phase to the second transmit coil;

wherein

the second phase is based, at least in part, on a location of the electronic device relative to the charging surface.

2. The wireless power transmitter of claim 1 , wherein:

the first transmit coil generates a first magnetic flux;

the second transmit coil generates a second magnetic flux; and

the second phase causes the second magnetic flux to constructively interfere with the first magnetic flux within a receive coil of the electronic device.

3. The wireless power transmitter of claim 2, wherein a radius or apothem of the receive coil is larger than a radius or apothem of the first transmit coil and the second transmit coil.

4. The wireless power transmitter of claim 1 , wherein:

the first transmit coil is associated with a first layer of transmit coils; and

the second transmit coil is associated with a second layer of transmit coils positioned below the first layer.

5. The wireless power transmitter of claim 4, wherein the second transmit coil is laterally offset from the first transmit coil.

6. The wireless power transmitter of claim 5, further comprising a third transmit coil that is associated with a third layer of transmit coils, wherein:

the third layer is positioned below the second layer; and

the third transmit coil is laterally offset from both the first transmit coil and the second transmit coil.

7. The wireless power transmitter of claim 6, wherein the third transmit coil has a radius or apothem smaller a radius or apothem of the first transmit coil.

8. The wireless power transmitter of claim 6, wherein the controller is configured to apply the first waveform at a third phase to the third transmit coil, the third phase based, at least in part, on the location of the electronic device relative to the charging surface.

9. A method of operating a wireless power transmitter, the method comprising:

applying a first electrical signal at a first phase to a first transmit coil, the first electrical signal associated with a low-power mode of the wireless power transmitter;

applying a second electrical signal at a second phase different from and relative to the first phase to a second transmit coil adjacent to the first transmit coil, the second electrical signal associated with the low-power mode;

monitoring a voltage across a receive coil of an electronic device adjacent to the first transmit coil and the second transmit coil; and

modifying the second phase to increase the voltage beyond a threshold.

10. The method of claim 9, wherein:

modifying the second phase comprises incrementally increasing an angle of the second phase relative to the first phase.

1 1 . The method of claim 9, wherein modifying the second phase comprises accessing a look-up table.

12. The method of claim 9, further comprising:

after the voltage exceeds the threshold:

applying a third electrical signal at the first phase to the first transmit coil, the third electrical signal associated with a high-power mode of the wireless power transmitter; and

applying a fourth electrical signal at the modified second phase to the second transmit coil, the fourth electrical signal associated with the high-power mode.

13. The method of claim 12, wherein:

the threshold is a first threshold; and

the method further comprises increasing a power output of the first transmit coil and the second transmit coil until the voltage exceeds a second threshold.

14. The method of claim 9, further comprising: applying a third electrical signal having a third phase relative to and different from the first phase to a third transmit coil adjacent to the first transmit coil and the second transmit coil, the third electrical signal associated with the low-power mode.

15. The method of claim 14, wherein:

the threshold is a first threshold; and

further comprising modifying the third phase until the voltage is greater than a second threshold.

16. A wireless power transmitter comprising:

a charging surface;

a first transmit coil in a first layer below the charging surface;

a second transmit coil in a second layer below the first layer and laterally offset from the first transmit coil;

a controller coupled to the first transmit coil and the second transmit coil, the controller configured to:

apply a first signal to the first transmit coil; and

apply a second signal having a phase different from the first alternating signal to the second transmit coil, the phase based at least in part on a position of the second coil relative to the first coil.

17. The wireless power transmitter of claim 16, wherein the phase of the second signal increases magnetic flux through a receive coil of an electronic device.

18. The wireless power transmitter of claim 16, wherein the phase of the second signal is determined by an iterative search operation.

19. The wireless power transmitter of claim 16, wherein the controller is further configured to increase a power level of the first signal or the second signal.

20. The wireless power transmitter of claim 16, wherein the controller is configured to generate at least one of the first signal and the second signal using a pulse width modulated signal.