Abstract:
Methods and apparatus are disclosed for wirelessly transmitting power. In one aspect, an apparatus for wireless transmitting power is provided. The apparatus comprises a first coil loop defining a first area, the first coil loop conducting current at a first current value for generating a first magnetic field. The apparatus further comprises a second coil loop surrounding the first coil loop and defining a second area, the second coil loop conducting current at a second current value generating a second magnetic field, wherein a ratio of the first current value to the first area is substantially equal to a ratio of the second current value to the second area.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 62/190,641 entitled “APPARATUS AND METHODS FOR WIRELESS POWER TRANSMITTER COIL CONFIGURATION,” filed on Jul. 9, 2015. 
     
    
     FIELD 
       [0002]    The present disclosure relates generally to a configuration of a wireless power transmitter. 
       BACKGROUND 
       [0003]    An increasing number and variety of electronic devices are powered via rechargeable batteries. Such devices include mobile phones, portable music players, laptop computers, tablet computers, computer peripheral devices, communication devices (e.g., Bluetooth devices), digital cameras, hearing aids, and the like. While battery technology has improved, battery-powered electronic devices increasingly require and consume greater amounts of power, thereby often requiring recharging. Rechargeable devices are often charged via wired connections through cables or other similar connectors that are physically connected to a power supply. Cables and similar connectors may sometimes be inconvenient or cumbersome and have other drawbacks. Wireless charging systems that are capable of transferring power in free space to be used to charge rechargeable electronic devices or provide power to electronic devices may overcome some of the deficiencies of wired charging solutions. As such, wireless power transfer systems and methods that efficiently and safely transfer power to electronic devices are desirable. 
       SUMMARY 
       [0004]    Various implementations of systems, methods and devices within the scope of the appended claims each have several aspects, no single one of which is solely responsible for the desirable attributes described herein. Without limiting the scope of the appended claims, some prominent features are described herein. 
         [0005]    Details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings, and the claims. Note that the relative dimensions of the following figures may not be drawn to scale. 
         [0006]    One aspect of the subject matter described in the disclosure provides an apparatus for wirelessly transmitting power. The apparatus includes a first coil loop defining a first area, the first coil loop conducting current at a first current value for generating a first magnetic field. The apparatus further includes a second coil loop surrounding the first coil loop and defining a second area, the second coil loop conducting current at a second current value generating a second magnetic field, wherein a ratio of the first current value to the first area is substantially equal to a ratio of the second current value to the second area. 
         [0007]    Another aspect of the subject matter described in the disclosure provides an implementation of a method of wirelessly transmitting power from a transmitter. The method generating a first magnetic field via a first coil loop. The first coil loop defines a first area and conducts current at a first current value. The method further includes generating a second magnetic field via a second coil loop. The second coil loop surrounds the first coil loop and defines a second area. The second coil loop further conducts current at a second current value, wherein a ratio of the first current value to the first area is substantially equal to a ratio of the second current value to the second area. 
         [0008]    Another aspect of the subject matter described in the disclosure provides an apparatus for wirelessly transmitting power. The apparatus includes a first means for generating a first magnetic field wound about a point to define a first area. The first means for generating the first magnetic field conducts time-varying electrical current at a first current value. The apparatus further includes a second means for generating a second magnetic wound about the point to define a second area. The second means for conducting electrical current conducts time-varying electrical current at a second current value, wherein a ratio of the first current value to the first area is substantially equal to a ratio of the second current value to the second area. 
         [0009]    Another aspect of the subject matter described in the disclosure provides an apparatus for wirelessly transmitting power. The apparatus includes a first coil loop defining a first area. The first coil loop conducts current having a first current value and generating a first magnetic field. The apparatus further includes a second coil loop separated from the first coil loop and defining a second area. The second coil loop conducts current having a second current value and generates a second magnetic field. The apparatus further includes at least one driver circuit configured to drive the first coil with a first current value and the second coil loop with a second current value. The first current value different from the second current value, wherein a ratio of the first current value to the first area is substantially equal to a ratio of the second current value to the second area. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a functional block diagram of an exemplary wireless power transfer system, in accordance with exemplary embodiments. 
           [0011]      FIG. 2  is a functional block diagram of exemplary components that may be used in the wireless power transfer system of  FIG. 1 , in accordance with various exemplary embodiments. 
           [0012]      FIG. 3  is a schematic diagram of a portion of transmit circuitry or receive circuitry of  FIG. 2  including a transmit or receive antenna, in accordance with exemplary embodiments. 
           [0013]      FIG. 4  is a diagram of an exemplary power transmitting element antenna/coil structure in accordance with an embodiment. 
           [0014]      FIG. 5  is a diagram of another exemplary power transmitting element antenna/coil structure in accordance with an embodiment. 
           [0015]      FIG. 6  is a diagram of another exemplary power transmitting element antenna/coil structure in accordance with an embodiment. 
       
    
    
       [0016]    The various features illustrated in the drawings may not be drawn to scale. Accordingly, the dimensions of the various features may be arbitrarily expanded or reduced for clarity. In addition, some of the drawings may not depict all of the components of a given system, method or device. Finally, like reference numerals may be used to denote like features throughout the specification and figures. 
       DETAILED DESCRIPTION 
       [0017]    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 present disclosure. It will be evident, however, to one skilled in the art that the present 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. 
         [0018]    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. 
         [0019]      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 . 
         [0020]    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. 
         [0021]    In certain embodiments, the wireless field  105  may correspond to the “near field” of the transmitter  104  as will be further described below. 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 . 
         [0022]    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. 
         [0023]    In certain implementations, the transmitter  104  may output a time varying magnetic (or electromagnetic) field 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. 
         [0024]      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 transmitting unit, PTU) may include transmit circuitry  206  that may include an oscillator  222 , a driver circuit  224 , a front-end circuit  226 , and an impedance control module  227 . The oscillator  222  may be configured to generate a 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. 
         [0025]    The front-end circuit  226  may include a filter circuit to filter out harmonics or other unwanted frequencies. The front-end circuit  226  may include a matching circuit to match the impedance of the transmitter  204  to 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. The impedance control module  227  may control the front-end circuit  226 . 
         [0026]    The transmitter  204  may further include a controller  240  operably coupled to the transmit circuitry  206  configured to control one or 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. 
         [0027]    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 to match the impedance of the receive circuitry  210  to 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 . 
         [0028]    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 . 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 . 
         [0029]    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. 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. 
         [0030]    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 and the receiver. 
         [0031]      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). 
         [0032]    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 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  may be added to the transmit and/or receive circuitry  350  to create a resonant circuit. 
         [0033]    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 . 
         [0034]    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 . Embodiments and descriptions provided herein may be applied to resonant and non-resonant implementations (e.g., resonant and non-resonant circuits for power transmitting or receiving elements and resonant and non-resonant systems). 
         [0035]      FIG. 4  is a diagram of an exemplary power transmitting element structure  400  that may provide a more uniform magnetic field in accordance with aspects of embodiments of the present disclosure. As shown, the power transmitting element structure  400  comprises five concentric coil loops: the inner-most or smallest coil loop  401 , the next largest coil loop  402 , the next largest coil loop  403 , the next largest coil loop  404 , and the outer-most or largest coil loop  405 . The five coil loops  401 - 405  are exemplary and more or fewer coil loops are possible. The coil or coils used for coil loops  401 - 405  may include any conductive material (e.g., copper, silver, aluminum, etc.). 
         [0036]    In some aspects, it may be desirable for the power transmitting element  400 —also referred to as a transmitter resonator, transmitter antenna, or transmitter coil—of a particular design classification to generate a magnetic field that has uniform magnetic field distribution over its charging surface in order to get the substantially same charging efficiency irrespective of the power receiving unit (PRU) placement on the power transmitting element  400  or charging pad. 
         [0037]    In some aspects, the coil loops  401 - 405  are configured such that the electrical current amplitude of each coil loop is proportional to the area of the coil loop. The strength of the magnetic field at the center point (or at any other point of the power transmitting element  400 ) is inversely proportional to the distance from the coil and the current running through the coil, resulting in a lower magnetic field at the center point. In order to obtain uniform magnetic field distribution over the area covered by the outer-most coil  405 , several concentric coil loops  404 - 401  with decreasing radius and decreasing current amplitudes have been placed to strengthen the magnetic field in the inner regions (including the center point). The current amplitudes of the coil loops  401 - 405  are chosen to be proportional to the coil loop areas in order to achieve high uniformity in magnetic field distribution above the coil surface (i.e., charging pad). In some aspects, the current amplitude may be calculated as a peak value of an alternating current or as a root-mean-square value of a current. 
         [0038]    In some aspects of certain embodiments, in order to achieve a current amplitude in each coil loop that is proportional to the area of the coil, each coil loop may be coupled to a separate current source (as shown and described below with respect to  FIG. 6 ). Each of the separate current sources may be configured to provide a current amplitude in the coil loop that is proportional to the area of the coil. In some aspects, a current source may comprise a driver circuit (e.g., driver circuit  224 ) configured to drive a current and/or voltage through a coil. That is, I C =kA C , where I C  is the current for the coil loop, k is a proportionality constant, and A C  is the area for the coil loop. In some aspects, proportionality constant k may be any positive number. For example, the current sources for coil loops  401  and  402  may provide a current at a value such that I C1 /A 1 =I C2 /A 2 , where I C1  is the current in coil loop  401 , I C2  is the current in coil loop  402 , A 1  is the area of coil loop  401 , and A 2  is the area of coil loop  402 . 
         [0039]    In one aspect for one non-liming example and for purposes of illustration, coil loop  401  may have a radius of 0.2 meters (m), coil loop  402  may comprise a radius of 0.4 m, coil loop  403  may comprise a radius of 0.6 m, coil loop  404  may comprise a radius of 0.8 m, and coil loop  405  may comprise a radius of 1 m. In one geometric arrangement of the coil loops  401 - 405 , such as described above, the distance between each of the coil loops is the same (i.e., 0.2 m). Accordingly, the ratio of the radius for each coil loop to the radius of the outer coil loop  405  would be (r 1 /r 5 ) for coil loop  401 , (r 2 /r 5 ) for coil loop  402 , (r 3 /r 5 ) for coil loop  403 , (r 4 /r 5 ) for coil loop  404 , and (r 5 /r 5 ) for coil loop  405 , where r 1 , r 2 , r 3 , r 4 , r 5 , are the radius for the coil loops  401 ,  402 ,  403 ,  404 , and  405 , respectively. For the example above, the ratio of the radii for the coil loops  401 - 405  is 0.2:0.4:0.6:0.8:1.0 or 1:2:3:4:5. The area of a circle is given by the equation A=πr 2 , where A is the area and r is the radius of the circle. The area covered by the coil loops  401 - 405  then equals π(0.2)2, π(0.4) 2 , π(0.6) 2 , π(0.8) 2 , π(1) 2 , respectively. Therefore, the ratio of the area of the coil loops  401 - 405  to the area of coil loop  401  is A 1 /A 1  for coil loop  401 , A 2 /A 1  for coil loop  402 , A 3 /A 1  for coil loop  403 , A 4 /A 1  for coil loop  404 , and A 5 /A 1  for coil loop  405 , where A 3 , A 4 , A s  are the areas for coil loops  403 ,  404 , and  405 , respectively. For the example above, [π(0.2) 2 /π(0.2) 2 ]:[π(0.4) 2 /π(0.2) 2 ]:[π(0.6) 2 /π(0.2) 2 ]:[π(0.8) 2 /π(0.2) 2 ]:[π(1) 2 /π(0.2) 2 ] or 1:4:9:16:25. Stated another way, the area for each of the coil loops  401 - 405  would be A 1  for coil loop  401 , 4*(A 1 ) for coil loop  402 , 9*(A 1 ) for coil loop  403 , 16*(A 1 ) for coil loop  404 , and 25*(A 1 ) for coil loop  405 . 
         [0040]    For the geometric arrangement described above, and to maintain proportionality of the current in each coil loop to the area of each of the coil loops  401 - 405 , the current in each of the coil loops  401 - 405  would be I C1  for coil loop  401 , 4*(I C1 ) for coil loop  402 , 9*(I C1 ) for coil loop  403 , 16*(I C1 ) for coil loop  404 , and 25*(I C1 ) for coil loop  405 . Accordingly, the current sources for coil loops  401 - 405  may be configured to provide a current at a value proportional to the area of the coil loops  401 - 405 , I C =kA C . For the example above, assuming the proportionality constant k equals 1, the current source of coil loop  401  would provide a current of substantially π(0.2) 2  amperes, the current source of coil loop  402  would provide a current of substantially π(0.4) 2  amperes, the current source of coil loop  403  would provide a current of substantially π(0.6) 2  amperes, the current source of coil loop  404  would provide a current of substantially π(0.8) 2  amperes, and the current source of coil loop  405  would provide a current of substantially π(1) 2  amperes. Similar to the ratio of the area of the coil loops described above, the ratio of the current value in each of the coil loops  401 - 405  to the current value in the coil loop  401  would be [π(0.2) 2 /π(0.2) 2 ]: [π(0.4) 2 /π(0.2) 2 ]:[π(0.6) 2 /π(0.2) 2 ]:[π(0.8) 2 /π(0.2) 2 ]:[π(1) 2 /π(0.2) 2 ] or 1:4:9:16:25 for each coil from the inner coil loop  401  to the outer coil loop  405 , respectively. In some embodiments, rather than using a separate current source for each of the coil loops  401 - 405 , the power transmitting element structure  400  may comprise a capacitor network, or some other circuit structure, to adjust the current amplitude in each coil to achieve the same ratio. 
         [0041]    In some embodiments, the power transmitting element structure  400  and/or each of the coil loops may be coupled to more or fewer current sources to provide a current at a current value or amplitude in each coil loop  401 - 405  that is proportional to the area of the coil loop. For example, in some aspects, the power transmitting element structure  400  may comprise a single coil winding where the coil loops  401 - 405  are formed from the single coil winding. In this embodiment, the single coil may be coupled to a single current source. While circular coil loops are shown, in certain embodiments, the coil loops  401 - 405  (and coil loops  501 - 503  described below) may have non-circular geometries (e.g., oval, semi-rectangular, etc.) but equivalent areas could be calculated and current values for the coil loops could be further determined for generating uniformity in the magnetic field in accordance with the principles described herein. 
         [0042]      FIG. 5  is a diagram of another exemplary power transmitting element structure  500  that may provide a uniform magnetic field. As shown, power transmitting element structure  500  comprises three concentric coil loops, an inner coil loop  501 , a middle coil loop  502 , and an outer coil loop  503 , and a current source  510  coupled to the coil loops  501 - 503 . As shown, the coil loops  501 - 503  are formed from a single coil winding, coil loop  501  comprises a single turn with a radius r 1  and an area A 1 , coil loop  502  comprises two turns with a radius r 2  and an area A 2 , and coil loop  503  comprises three turns with a radius r 3  and an area A 3 . That is, the number of turns in each of the coil loops  501 - 503  would be N 1  for coil loop  501 , 2*(N 1 ) for coil loop  502 , and 3*(N 1 ) for coil loop  503 . In order to achieve a current amplitude in each coil loop that is proportional to the area of the coil, the number of turns in each of the coil loops  501 - 503  should be proportional to the area of the coil loop. That is, N C =kA C , where N C  is the number of turns in a coil loop. For example, in one aspect, the coil loop  501  may have an area of 1 m 2 , the coil loop  502  may have an area of 2 m 2 , the coil loop  501  may have an area of 3 m 2 . Additionally, the ratio of the area of the coil loops  501 - 503  to the area of the coil loop  501  is A 1 /A 1  for coil loop  501 , A 2 /A 1  for coil loop  501 , and A 3 /A 1  for coil loop  501 , or 1:2:3 for coil loops  501 ,  502 , and  503 , respectively. For the geometric arrangement shown in  FIG. 5 , the area for each of the coil loops  501 - 503  would be A 1  for coil loop  501 , 2*(A 1 ) for coil loop  502 , 3*(A 1 ) for coil loop  503 . From the equation I C =kA C  above, I 1 /A 1 =I 2 /A 2 =I 3 /A 3 =k, where I 1 ,  1   2 , and  1   3  are the currents in the coil loops  501 ,  502 , and  503 , respectively. Similarly, from the equation N C =kA C  above, N 1 /A 1 =N 2 /A 2 =N 3 /A 3 =k, where N 1 , N 2 , and N 3  are the number of turns in the coil loops  501 ,  502 , and  503 , respectively. Accordingly, solving for the current, and assuming the proportionality constant k equals 1, the current in each coil loop  501 - 503  would be 1 ampere for coil loop  501 , 2 amperes for coil loop  502 , and 3 amperes for coil loop  503 , in order to maintain the proportional relationship. 
         [0043]    In order to achieve the current values in each coil loop  501 - 503  from the single current source  510 , the coil loops  501 - 503  may comprise multiple turns as shown. The current in a coil loop is directly proportional to the number of turns in the coil loop according to the equation I L =I S N, where I L  is the current of the coil loop, I S  is the current of the current source, and N is the number of turns. For example, if the current source  510  provides a current of 1 ampere, a coil loop of two turns would carry an equivalent current of 2 amperes, a coil loop of three turns would carry an equivalent current of 3 amperes, and so on. As shown in  FIG. 5 , the number of turns in each of the coil loops  501 - 503  is 1, 2, and 3, respectively, which is proportional to the area of the coil loops  501 - 503 . That is, the ratio of the number of turns in each of the coil loops  501 - 503  to the number of turns in the coil loop  501  is N 1 /N 1  for coil loop  501 , N 2 /N 1  for coil loop  502 , and N 3 /N 1  for coil loop  503 . Accordingly, the ratio of the number of turns is 1:2:3 for coil loops  501 ,  502 , and  503 , respectively. Thus, the ratio of the number of turns in each of the coil loops  501 - 503  is proportional to the ratio of the area of each of the coil loops  501 - 503  such that N 1 /A 1 =N 2 /A 2 =N 3 /A 3 . In the above example, the number of turns and the area of the coil loops each have the 1:2:3 ratio, respectively. That is, a ratio of the number of turns in the coil loop  501  to the area of the coil loop  501  is substantially equal to a ratio of the number of turns in the coil loop  502  to the area of the coil loop  502 , and is substantially equal to the same ratio for the coil loop  503 . 
         [0044]    Likewise, current amplitude for the coil loops  501 - 503  is proportional to the area of the coil loops  501 - 503  (A 1 -A 3 ), as shown above in the equation I C =kA C . In the example above, since the current source  510  is providing a current of a value of 1 ampere, the current in coil loop  501  would be 1 ampere, the current in coil loop  502  would be 2 amperes, and the current in coil loop  503  would be 3 amperes. Accordingly, the ratio of the current amplitude for the coil loops  501 - 503  is 1:2:3, which is proportional to both the area of the coil loops  501 - 503  and to the number of turns in each of the coil loop  501 - 503 . This configuration facilitates generating a substantially uniform magnetic field across the power transmitting element structure  500 . 
         [0045]    Similarly, in some embodiments, in order to achieve a uniform magnetic field distribution, the power transmitting element structure  500  may be configured such that the number of turns in each coil loop  501 - 503  is proportional to the current amplitude in the coil loop. For example, N C =kI C . In the embodiment described above, since the ratio of the current amplitude for each coil loop is 1:2:3 from the inner coil loop  501  to the outer coil loop  503 , the ratio of the number of turns in each coil loop will be 1:2:3 times the number of turns of the coil loop  501 . That is, a ratio of the number of turns in the coil loop  501  to the current amplitude in the coil loop  501  is substantially equal to a ratio of the number of turns in the coil loops  502  to the current amplitude in the coil loop  502 , and is substantially equal to the same ratio for the coil loop  503 . Stated another way N 1 / 1   1 =N 2 / 1   2 =N 3 /I 3 . 
         [0046]    Moreover, a distance between the coil loops  501 - 503  may be defined by the area of the coil loop, the number of turns of the coil loop or the current amplitude since each has a proportional relationship with the other. For example, since the area of the coil loop  501  to the area of the coil loop  502  is 1:2, the relationship between the radii of the coil loop  501  and coil loop  502  is 2πr 1   2 =πr 2   2 . Which results in r 2 =(√2)r 1 . The distance, D, between r 1  and r 2  is then r 2 −r 1 =(√2)r 1 −r 1 . Which results in (√2)−1 or 0.414 m. Using a similar calculation, the distance between r 1  and r 3  is then r 3 −r 1 =(√3)r 1 −r 1  or 0.732 m. 
         [0047]      FIG. 6  is a diagram of an exemplary power transmitting element structure  600  that may provide a more uniform magnetic field in accordance with aspects of embodiments of the present disclosure. The power transmitting element structure  600  is similar to and adapted from the power transmitting element structure  400  of  FIG. 4 . Only differences between the power transmitting element structures  400  and  600  are described for the sake of brevity. 
         [0048]    As shown in  FIG. 6 , the power transmitting element structure  600  comprises separate current sources  601 ,  602 ,  603 ,  604 , and  605  for each of the coil loops  401 ,  402 ,  403 ,  404 , and  405 , respectively. In some aspects, the current sources  601 ,  602 ,  603 ,  604 , and  605  may comprise any circuit configured to drive or output a current into the coil loops  401 - 405 . As described above with respect to  FIG. 4 , each of current sources  601 ,  602 ,  603 ,  604 , and  605  may be configured to drive a current in their respective coil loop such that the current is proportional to the area of the coil. 
         [0049]    In some aspects, the power transmitting element structures  400 ,  500 , and  600  provide a non-limiting benefit of a more uniform magnetic field distribution which increases efficiency and reduces the need for power receiving units with more complex circuitry to withstand high variances in applied voltages or to design power receiving units which averages the peaks and valleys of a magnetic field of the power transmitting element. For example, other power transmitting element structures that do not have good uniformity, may restrict the charging area and charging efficiency for the power transmitting element or coil. In some aspects, transmitting element structures can employ smaller coils in a M×N two dimensional matrix to increase the charging area, but this structure may result in weak charging zones on the charging surface where the efficiency is low when power receiving units are placed in those locations. In these aspects, there may be several spots on the power transmitting element where the field is much greater or much lower than the nominal field strength. This in turn may force a power receiving element  318  ( FIG. 3 ) designer to either build the receive circuitry of the power receiving element  350  ( FIG. 3 ) to withstand high variances in applied voltages or to design the power receiving element (e.g., antenna/coil configuration in combination with circuitry to create a resonate circuit) which averages the peaks and valleys of a magnetic field of the power transmitting element. 
         [0050]    In some embodiments, the proportionality of the current amplitude in a coil loop to the area of the coil loop and the proportionality of the number of turns in a coil loop to the area of the coil loop described herein may provide a more uniform magnetic field distribution than other power transmitting element designs using the same number of coil loops but not having the proportionality described herein. 
         [0051]    For example, in testing the uniformity of magnetic fields of a surface of a power transmitting element over various power transmitting element antenna/coil structure configurations, an optimal configuration included coil loops with a proportional relationship between the area of the coil loop and the current amplitude as described above. For one particular power transmitting element configuration with the coil structure having a proportional relationship between the area of the coil loop and the current amplitude, the power transmitting element configuration achieved up to a 6 decibel (dB) uniformity in the normal component of magnetic field over 64% of the surface of the power transmitting element at a height of one-fifth of outer coil radius. Other power transmitting element configurations not using the proportional relationship between the coil loop area and current amplitude or number of turns achieved less than 50% uniformity in the normal component of magnetic fields at the same height above the surface of the power transmitting element. In other aspects, the power transmitting element configurations described herein may provide greater variability of design dimensions to allow higher resolution of design tweaking. For example, while circular coil loops are shown herein, other coil shapes are possible. 
         [0052]    The various operations of methods described above may be performed by any suitable means capable of performing the operations, such as various hardware and/or software component(s), circuits, and/or module(s). Generally, any operations illustrated in the Figures may be performed by corresponding functional means capable of performing the operations. 
         [0053]    Information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
         [0054]    The various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. The described functionality may be implemented in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the embodiments of the invention. 
         [0055]    The various illustrative blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
         [0056]    The steps of a method or algorithm and functions described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a tangible, non-transitory computer-readable medium. A software module may reside in Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, a removable disk, a CD ROM, or any other form of storage medium known in the art. A storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer readable media. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal. 
         [0057]    For purposes of summarizing the disclosure, certain aspects, advantages and novel features of the inventions have been described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein. 
         [0058]    Various modifications of the above described embodiments will be readily apparent, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.