Abstract:
In one embodiment a wireless power transfer system comprises a transmitter including a power source configured to generate a time-varying current, a first coil configured to receive the first time-varying current from the power source, wherein the time-varying current flows in the first coil in a first direction, a second coil coupled to the first coil in such a way that the time-varying current flows in the second coil in a second direction, wherein the first direction is opposite from the second direction, and an underlying magnetic layer configured to magnetically couple the first coil with the second coil, and a wireless power receiver, a ferrite core and a receiver coil that share a longitudinal axis, and a receive circuit coupled to the receiver coil configured to convert a time varying current induced in the receiver coil into a voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 62/266,128, entitled “Wireless Power Transfer for Wearables,” filed on Dec. 11, 2015. This application is also related to U.S. patent application Ser. No. 15/082,533, entitled “Wireless Power Transfer Using Multiple Coil Arrays,” filed on Mar. 28, 2016. The subject matters of the related applications are hereby incorporated by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to inductive wireless power transfer and more specifically to a system for wireless power transfer for portable devices. 
       BACKGROUND 
       [0003]    Electronic devices typically require a connected (wired) power source to operate, for example, battery power or a wired connection to a direct current (“DC”) or alternating current (“AC”) power source. Similarly, rechargeable battery-powered electronic devices are typically charged using a wired power-supply that connects the electronic device to a DC or AC power source. The limitation of these devices is the need to directly connect the device to a power source using wires. 
         [0004]    Wireless power transfer (WPT) involves the use of time-varying magnetic fields to wirelessly transfer power from a source to a device. Faraday&#39;s law of magnetic induction provides that if a time-varying current is applied to one coil (e.g., a transmitter coil) a voltage will be induced in a nearby second coil (e.g., a receiver coil). The voltage induced in the receiver coil can then be rectified and filtered to generate a stable DC voltage for powering an electronic device or charging a battery. The receiver coil and associated circuitry for generating a DC voltage can be connected to or included within the electronic device itself such as a smartphone. 
         [0005]    The Wireless Power Consortium (WPC) was established in 2008 to develop the Qi inductive power standard for charging and powering electronic devices. Powermat is another well-known standard for WPT developed by the Power Matters Alliance (PMA). The Qi and Powermat near-field standards operate in the frequency band of 100-400 kHz. The problem with near-field WPT technology is that typically only 5 Watts of power can be transferred over the short distance of 2 to 5 millimeters between a power source and an electronic device, though there are ongoing efforts to increase the power. For example, some concurrently developing standards achieve this by operating at much higher frequencies, such as 6.78 MHz or 13.56 MHz. Though they are called magnetic resonance methods instead of magnetic induction, they are based on the same underlying physics of magnetic induction. There also have been some market consolidation efforts to unite into larger organizations, such as the AirFuel Alliance consisting of PMA and the Rezence standard from the Alliance For Wireless Power (A4WP), but the technical aspects have remained largely unchanged. 
         [0006]    Some techniques for WPT use two or more transmitter coils in an attempt to overcome the issue of low power transfer over short distances. Typically, two identical transmitter coils (e.g., both wound in the clockwise direction or both wound in the counter-clockwise direction and having the same number of turns and area) are coupled in series or parallel on a single magnetic layer to transfer power to a receiver coil. Alternatively, the coils can be placed in close proximity to one another without the use of a magnetic layer. This configuration results in the applied time-varying current flowing through both coils in the same direction at any point in time, generating an almost perpendicular combined magnetic field with flux lines that flow from both coils in the same direction (i.e., the magnetic field generated by either coil has the same polarity as the other coil). Magnetic flux lines tend to repel if they are in the same direction, which causes the flux lines to radiate through the air for great distances. When magnetic flux lines repel, the magnetic reluctance is high, resulting in a weak magnetic field that reduces the amount of magnetic coupling between the transmitter coils and a receiver coil placed in close proximity (i.e., 2-5 millimeters) to the transmitter coils. So although the coil area is larger than in a single-coil transmitter, the resulting magnetic flux available to transfer power is reduced. If the transmitter coils are placed on separate magnetic layers, an air gap exists between the magnetic layers resulting in an even weaker generated magnetic field as the air gap further increases the reluctance between the transmitter coils. 
         [0007]    Due to the short range of existing WPT technology, the transmitter coil must be centered with the receiver coil connected to a device and the coils cannot be more than 2-5 millimeters apart. This makes it difficult to implement wireless power transfer for devices that are not perfectly flat or do not have a large enough area for embedding a typical receiver coil (e.g., Android® wearable devices, Apple® watch, Fitbit® fitness tracker, etc.). The limitations of WPT also affect smartphones if the charging surface with the transmitter coil is not large enough to allow the smartphone device to sit flat on the surface (e.g., in vehicles, which typically do not have a large enough flat surface to accommodate a smartphone device). Thus, the current state of WPT technology is not suitable for many consumer or small industrial devices. 
       SUMMARY OF THE INVENTION 
       [0008]    In one embodiment a wireless power transfer system comprises a transmitter including a power source configured to generate a time-varying current, a first coil configured to receive the first time-varying current from the power source, wherein the time-varying current flows in the first coil in a first direction, a second coil coupled to the first coil in such a way that the time-varying current flows in the second coil in a second direction, wherein the first direction is opposite from the second direction, and an underlying magnetic layer configured to magnetically couple the first coil with the second coil, and a wireless power receiver including a ferrite core and a receiver coil that share a longitudinal axis, and a receive circuit coupled to the receiver coil configured to convert a time varying current induced in the receiver coil into a voltage. In one embodiment the wireless power receiver further includes a rechargeable battery. 
         [0009]    In one embodiment an electronic device comprises a wireless power receiver structure including a ferrite core, a receiver coil coupled to the ferrite core such that the ferrite core and the receiver coil share a longitudinal axis, a receive circuit coupled to the receiver coil configured to convert a time varying current induced in the receiver coil into a voltage, and a visible marking that indicates the longitudinal axis of the ferrite core and the receiver coil. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a diagram illustrating one embodiment of a wireless power transfer system according to the present invention. 
           [0011]      FIG. 2  is a diagram illustrating one embodiment of a wireless power transfer system according to the present invention. 
           [0012]      FIG. 3A  is a diagram illustrating one embodiment of a wearable device with a wireless power receiver coil structure according to the present invention. 
           [0013]      FIG. 3B  is a diagram illustrating one embodiment of a wearable device with a wireless power receiver coil structure according to the present invention. 
           [0014]      FIG. 4  is a diagram illustrating one embodiment of a mobile device with a wireless power receiver coil structure according to the present invention. 
           [0015]      FIG. 5  is a diagram illustrating one embodiment of a wearable device with a wireless power receiver coil structure according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  is a diagram illustrating one embodiment of a wireless power transfer system  100  including a transmitter  110  and a receiver  130 . Transmitter  110  includes, but is not limited to, a power circuit  112 , a coil structure  114 , and a capacitor  116 . Coil structure  114  includes, but is not limited to, a coil  122  and a coil  124  that are magnetically coupled together by a magnetic layer  126 . Magnetic layer  126  underlies both coil  122  and coil  124 . Magnetic layer  126  can be ferrite or any other magnetic layer known in the art. Coil  122  and coil  124  are preferably identical coils with the same number of turns and the same area. Power circuit  112  generates an AC signal having a voltage magnitude consistent with an input DC voltage applied to power circuit  112 . The generated AC signal can be, but is not limited to, a square wave, a sinusoidal wave, a triangular wave, or a sawtooth wave. The resonant frequency of transmitter  110  is determined by the capacitance of capacitor  116  and the total inductance of coil  122  and coil  124 . The AC signal causes current to flow from power circuit  112  to coil  122  via capacitor  116  and the flow of current through coil  122  generates a magnetic field. The current flows from coil  122  to coil  124 . When coils  122  and  124  are identical, the flow of current through coil  124  generates a magnetic field equivalent in magnitude to the magnetic field generated by coil  122 . Coils  122  and  124  can be formed of wire or traces on a printed circuit board using conductive material such as copper, gold, or any other conductive material known in the art. 
         [0017]    A current  142  flows through coil  122  in the clockwise direction. The clockwise flow of current  142  through coil  122  generates a magnetic field represented by flux lines  152 . According to the “right-hand-rule,” the clockwise flow of current  142  through coil  122  causes flux lines  152  to flow in the downward direction. Current  142  flows from coil  122  to coil  124  through a connection  128  (i.e., coil  122  is coupled in series with coil  124 ). A current  144  flows through coil  124  in the counter-clockwise direction. The counter-clockwise flow of current  144  through coil  124  generates a magnetic field represented by flux lines  154 . According to the “right-hand-rule,” the counter-clockwise flow of current  144  through coil  124  causes flux lines  154  to flow in the upward direction. 
         [0018]    Current  142  is equivalent in magnitude to current  144  but flows in an opposite direction. If coil  142  and coil  144  are identical, the flow of current  142  through coil  122  generates a magnetic field equivalent in magnitude to the magnetic field generated by the flow of current  144  through coil  124 . Because current  142  and current  144  are flowing in opposite directions at any given point in time, the magnetic field generated by current  142  is in a different direction than the magnetic field generated by current  144  (i.e., the magnetic fields have different polarity). Further, because flux lines  152  and flux lines  154  are flowing in opposite directions, the magnetic reluctance between flux lines  152  and flux lines  154  is low, causing flux lines  152  and flux lines  154  to attract to each other. Flux lines  152  and flux lines  154  magnetically couple to form closed flux lines  156 . In another embodiment, coil  122  is coupled in parallel with coil  124  such that a current flowing in coil  122  is flowing in an opposite direction to a current flowing in coil  124  to form closed flux lines between coils  122  and  124 . 
         [0019]    Receiver  130  includes, but is not limited to, a receiver coil structure  132  and a receive circuit  134 . Receiver coil structure  132  includes a ferrite core  136  and a helical coil  138 . In the  FIG. 1  embodiment, ferrite core  136  is in the shape of a cylindrical rod and helical coil  138  is wrapped around ferrite core  136  such that ferrite core  136  and helical coil  138  have a common longitudinal axis. In other embodiments, ferrite core  136  may be a parallelepiped or other shape, or may be made of a flexible ferrite sheet. Helical coil  138  is preferably formed of wire made from a conductive material such as copper, gold, or any other conductive material known in the art. Receiver coil structure  132  is oriented in relation to transmitter coil structure  114  such that flux lines  156  of the magnetic field produced by transmitter  110  pass through ferrite core  136 . Receiver coil structure  132  is optimally oriented such that the longitudinal axis of ferrite core  136  is substantially parallel to a longitudinal axis  170  of transmitter coil structure  114 . In one embodiment, an outer surface of transmitter coil structure  114  includes a visible marking that indicates longitudinal axis  170 . Faraday&#39;s law provides that the time-varying current that flows in a receiver coil will oppose the magnetic field generated by a transmitter coil. Thus flux lines  156  passing through ferrite core  136  cause a time-varying current  162  to flow in helical coil  138 . Receiver coil structure  132  is coupled to receive circuit  134  such that current  162  is input to receive circuit  134 . Receive circuit  134  includes, but is not limited to, a rectifier to generate a DC voltage, a filter to reduce noise, and a voltage regulator to define a voltage magnitude and maintain the voltage under load. The voltage generated by receive circuit  134  as a result of the coupling of flux lines  156  to coil structure  132  can be used to charge a battery or power a device (e.g., a smart phone, laptop or any other electronic device). 
         [0020]      FIG. 2  is a diagram illustrating one embodiment of a wireless power transfer system  200  including a transmitter  210  and a receiver  230 . Transmitter  210  includes, but is not limited to, a power circuit  212 , a transmitter coil structure  214 , and a capacitor  216 . Coil structure  214  includes, but is not limited to, a coil  222  and a coil  224  that are magnetically coupled together by a magnetic layer  226 . Magnetic layer  226  underlies both coil  222  and coil  224 . Magnetic layer  226  can be ferrite or any other magnetic layer known in the art. Coil  222  and coil  224  are preferably identical coils with the same number of turns and the same area. Coil  222  and coil  224  are both wound in the clockwise direction but both coils could alternatively be wound in the counter-clockwise direction. A power circuit  212  generates an AC signal having a voltage magnitude consistent with an input DC voltage applied to power circuit  112 . The AC signal can be, but is not limited to, a square wave, a sinusoidal wave, a triangular wave, or a sawtooth wave. The resonant frequency of transmitter  110  is determined by the capacitance of capacitor  116  and the total inductance of coil  122  and coil  124 . The AC signal causes current to flow from power circuit  212  to coil  222  via capacitor  216  and the flow of current through coil  222  generates a magnetic field. The current flows from coil  222  to coil  224 . When coils  222  and  224  are identical, the flow of current through coil  224  generates a magnetic field equivalent in magnitude to the magnetic field generated by coil  222 . Coils  222  and  224  can be formed of wire or traces on a printed circuit board using conductive material such as copper, gold, or any other conductive material known in the art. 
         [0021]    A current  242  flows through coil  222  in the clockwise direction. The clockwise flow of current  242  through coil  222  generates a magnetic field represented by flux lines  252 . According to the “right-hand-rule,” the clockwise flow of current  242  through coil  222  causes flux lines  252  to flow in the downward direction. Current  242  flows from coil  222  to coil  224  through a connection  228  (i.e., coil  222  is coupled in series with coil  224 ). A current  244  flows through coil  224  in the counter-clockwise direction. The counter-clockwise flow of current  244  through coil  224  generates a magnetic field represented by flux lines  254 . According to the “right-hand-rule,” the counter-clockwise flow of current  244  through coil  224  causes flux lines  254  to flow in the upward direction. 
         [0022]    Current  242  is equivalent in magnitude to current  244  but flows in an opposite direction. If coil  242  and coil  244  are identical, the flow of current  242  through coil  222  generates a magnetic field equivalent in magnitude to the magnetic field generated by the flow of current  244  through coil  224 . Because current  242  and current  244  are flowing in opposite directions at any given point in time, the magnetic field generated by current  242  is in a different direction than the magnetic field generated by current  244  (i.e., the magnetic fields have different polarity). Further, because flux lines  252  and flux lines  254  are flowing in opposite directions, the magnetic reluctance between flux lines  252  and flux lines  254  is low, causing flux lines  252  and flux lines  254  to attract to each other. Flux lines  252  and flux lines  254  magnetically couple to form closed flux lines  250 . In another embodiment, coil  222  is coupled in parallel with coil  224  such that a current flowing in coil  222  is flowing in an opposite direction to a current flowing in coil  224  to form closed flux lines between coils  222  and  224 . 
         [0023]    Receiver  230  includes, but is not limited to, a receive circuit  234  and a receiver coil structure  232 . Receiver coil structure  232  includes a ferrite core  236  and a helical coil  238 . In the  FIG. 2  embodiment, ferrite core  236  is in the shape of a cylindrical rod and helical coil  238  wraps around ferrite core  236  such that ferrite core  236  and helical coil  238  share a longitudinal axis. In other embodiments, ferrite core  236  may be a parallelepiped or other shape. Helical coil  238  is preferably formed of wire made from a conductive material such as copper, gold, or any other conductive material known in the art. Receiver coil structure  232  is oriented in relation to transmitter coil structure  214  such that flux lines  250  of the magnetic field produced by transmitter  210  pass through ferrite core  236  substantially parallel to the longitudinal axis of ferrite core  236  and helical coil  238 . Ferrite core  236  attracts and focuses flux lines  250  such that a substantial amount of the magnetic flux produced by transmitter  210  flows through receiver coil structure  232 . Receiver coil structure  232  is optimally oriented such that the longitudinal axis of ferrite core  236  is substantially parallel to a longitudinal axis  270  of transmitter coil structure  214 . In one embodiment, an outer surface of transmitter coil structure  214  includes a visible marking that indicates longitudinal axis  270 . Faraday&#39;s law provides that the time-varying current that flows in a receiver coil will oppose the magnetic field generated by a transmitter coil. Thus flux lines  250  passing through receiver coil structure  232  cause a time-varying current  246  to flow in helical coil  238 . Receiver coil structure  232  is coupled to receive circuit  234  such that current  242  is input to receive circuit  234 . Receive circuit  234  includes, but is not limited to, a rectifier to generate a DC voltage, a filter to reduce noise, and a voltage regulator to define a voltage magnitude and maintain the voltage under load. The voltage generated by receive circuit  234  as a result of the concentration of flux lines  250  through receiver coil structure  232  can be used to charge a battery or power a device. The presence of ferrite core  236  enhances the wireless transfer of power from transmitter  210  to receive circuit  234  by focusing a substantial amount of the magnetic flux generated by transmitter  210  through helical coil  238 . 
         [0024]      FIG. 3A  is a diagram illustrating one embodiment of a wearable device  310  with a wireless power receiver coil structure  312 . Wearable device  310  includes a strap  320 , an electronic device  322 , and receiver coil structure  312 . Electronic device  322  may be, for example, a fitness tracker, a pedometer, a heartrate monitor, a watch, a mobile telephone, or a computer and includes a rechargeable battery (not shown). Electronic device  322  also includes a receive circuit  324  coupled to receiver coil structure  312  to rectify and filter received energy into a voltage and charge the battery. Electronic device  322  has a housing that may be formed of plastic, metal, or a combination of materials. Receiver coil structure  312  includes a ferrite core  314  and a helical coil  316 . In the  FIG. 3A  embodiment, ferrite core  314  is in the shape of a cylindrical rod and helical coil  316  winds around ferrite core  314  such that ferrite core  314  and helical coil  316  share a longitudinal axis. Receiver coil structure  312  can be attached to an outer surface of strap  320  or embedded within strap  320 . Wearable device  310  can be placed on a surface of a transmitter such as transmitter  110  or  210  such that receiver coil structure  312  receives magnetic flux and generates a time varying current that is provided to receive circuit  324 . Wearable device  310  includes a visible marking  330  on the surface of strap  320  that indicates the longitudinal axis of receiver coil structure  312 . During power transfer, wearable device  310  is preferably oriented with respect to the transmitter such that the longitudinal axis of receiver coil structure  312  is substantially parallel to a longitudinal axis of the transmitter coil structure. In another embodiment, receiver coil structure  312  and receive circuit  324  are embodied in a receiver module that can be external to electronic device  322  or incorporated into electronic device  322 . 
         [0025]      FIG. 3B  is a diagram illustrating one embodiment of a wearable device  350  with a wireless power receiver coil structure  352 . Wearable device  350  includes a strap  360 , an electronic device  362 , and receiver coil structure  352 . Electronic device  362  may be, for example, a fitness tracker, a pedometer, a heartrate monitor, a watch, a mobile telephone, or a computer and includes a rechargeable battery (not shown). Electronic device  362  also includes a receive circuit  364  coupled to receiver coil structure  352  to rectify and filter received energy into a voltage and charge the battery. Electronic device  362  has a housing that may be formed of plastic, metal, or a combination of materials. Receiver coil structure  352  includes a ferrite core  354  and a coil  356 . In the  FIG. 3B  embodiment, ferrite core  354  is in the shape of a parallelepiped and coil  356  winds around ferrite core  354  such that ferrite core  354  and coil  356  share a longitudinal axis. Ferrite core  354  is formed of a flexible ferrite material that can flex in concert with strap  360 . Receiver coil structure  352  can be attached to an outer surface of strap  360  or embedded within strap  360 . Wearable device  350  can be placed on a surface of a transmitter such as transmitter  110  or  210  such that receiver coil structure  352  receives magnetic flux and generates a time varying current that is provided to receive circuit  364 . Wearable device  350  includes a visible marking  370  on the surface of strap  360  that indicates the longitudinal axis of receiver coil structure  352 . During power transfer, wearable device  350  is preferably oriented with respect to the transmitter such that the longitudinal axis of receiver coil structure  352  is substantially parallel to a longitudinal axis of the transmitter&#39;s coil structure. In another embodiment, receiver coil structure  352  and receive circuit  364  are embodied in a receiver module that can be external to electronic device  362  or incorporated into electronic device  362 . 
         [0026]      FIG. 4  is a diagram illustrating one embodiment of a mobile device  410  with a wireless power receiver coil structure  412 . Mobile device  410  may be any type of electronic device powered by a battery (not shown), for example a smartphone, a tablet, a camera, or a toy. Mobile device  410  includes, but is not limited to, a receiver coil structure  412  and a receive circuit  422  coupled to receiver coil structure  412  to rectify and filter received energy into a voltage and charge the battery. Mobile device  410  has a housing that may be formed of plastic, metal, or a combination of materials. Receiver coil structure  412  is preferably located within the housing of mobile device  412 . Receiver coil structure  412  includes a ferrite core  414  and a coil  416 . In the  FIG. 4  embodiment, ferrite core  414  is in the shape of a parallelepiped and coil  416  winds around ferrite core  414  such that ferrite core  414  and coil  416  share a longitudinal axis. Coil  416  is preferably formed of wire made from a conductive material such as copper, gold, or any other conductive material known in the art. Mobile device  410  can be placed on a surface of a transmitter such as transmitter  110  or  210  such that receiver coil structure  412  receives magnetic flux and generates a time varying current that is provided to receive circuit  422 . Mobile device  410  may include a visible marking  450  that indicates the longitudinal axis of receiver coil structure  412 . During power transfer, mobile device  410  is preferably oriented with respect to the transmitter such that the longitudinal axis of receiver coil structure  412  is substantially parallel to a longitudinal axis of the transmitter&#39;s coil structure. 
         [0027]      FIG. 5  is a diagram illustrating one embodiment of a wearable device  510  with a wireless power receiver coil structure  512 . Wearable device  510  includes an eyepiece  522 , temple pieces  520 , and receiver coil structure  512 . Wearable device  510  may be, for example, 3-D glasses, a virtual reality (VR) headset, sunglasses with built-in Bluetooth speakers, or other type of electronic device intended to be worn on the head, and includes a rechargeable battery (not shown). Wearable device  510  also includes a receive circuit (not shown) coupled to receiver coil structure  512  to rectify and filter received energy into a voltage and charge the battery. Receiver coil structure  512  includes a ferrite core  514  and a coil  516 . In the  FIG. 5  embodiment, ferrite core  514  is in the shape of a parallelepiped and coil  516  winds around ferrite core  514  such that ferrite core  514  and coil  516  share a longitudinal axis. Ferrite core  514  is formed of a flexible ferrite material that can flex in concert with temple piece  520 . Coil  516  is preferably formed of wire made from a conductive material such as copper, gold, or any other conductive material known in the art. Receiver coil structure  512  can be attached to an outer surface of temple piece  520  or embedded within temple piece  520 . Wearable device  510  can be placed on a surface of a transmitter such as transmitter  110  or  210  such that receiver coil structure  512  receives magnetic flux and generates a time varying current that is provided to receive circuit. Wearable device  510  may include a visible marking  550  on temple piece  520  that indicates the longitudinal axis of receiver coil structure  512 . During power transfer, wearable device  510  is preferably oriented with respect to the transmitter such that the longitudinal axis of receiver coil structure  512  is substantially parallel to a longitudinal axis of the transmitter&#39;s coil structure. In another embodiment, receiver coil structure  512  is incorporated into eyepiece  522 . 
         [0028]    Receiver coil structures  312 ,  352 ,  412 , and  512  may also be used to provide power to other types of devices with or without a rechargeable battery including, but not limited to, medical implants, medical point-of-care equipment, vacuum cleaners, tablets, laptops, smartphones, two-way radios, toys, virtual reality glasses, cameras, portable tools, lighting, remote controls, emergency lamps, gaming stations, electric vehicle charging, in-cabin car charging, robots, unmanned aerial vehicles (drones). 
         [0029]    The invention has been described above with reference to specific embodiments. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The foregoing description and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.