Abstract:
A wireless power transmission system is presented. The wireless power circuit includes an adaptive transmitter coupled to a transmit coil; a receive coil in interaction with the transmit coil, the receive coil being thin and having an inner diameter large enough to accommodate sensors of a wearable device; and a receive circuit receiving power from the receive coil.

Description:
RELATED APPLICATION 
       [0001]    The present application claims priority to U.S. Provisional Patent Application 62/131,800, entitled “Adaptive Resonant Topology Application in Wearable Devices,” by Mehmet K. Nalbant, filed on Mar. 11, 2015, which is herein incorporated by reference its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    Embodiments of the present invention are related to adaptive resonant wireless charging in wearable devices. 
         [0004]    2. Discussion of Related Art 
         [0005]    Wearable devices, for example smart watches, have sensor technology that may be present in the center of any wireless receive coils that may be present. The complication of the sensor locations and the low efficiencies of wireless charging have made wireless charging technologies unattractive for wearable devices. 
         [0006]    Therefore, there is a need to develop better wireless charging technologies for wearable devices. 
       SUMMARY 
       [0007]    In accordance with aspects of the presents a wireless power transmission system. The wireless power circuit includes an adaptive transmitter coupled to a transmit coil; a receive coil in interaction with the transmit coil, the receive coil being thin and having an inner diameter large enough to accommodate sensors of a wearable device; and a receive circuit receiving power from the receive coil. 
         [0008]    A wireless power transmitter according to some embodiments of the present invention can include an adaptively controlled transmitter; and a transmit coil coupled to the adaptively controlled transmitter, the transmit coil being adapted to couple with a thin receiver coil with a large inner diameter to accommodate sensors. 
         [0009]    A wireless power receiver according to some embodiments of the present invention includes a receive coil, the receive coil being thin and having an inner diameter large enough to accommodate sensors; and a circuit coupled to the receive coil to receive power at a frequency adaptively chosen by an adaptive transmitter. 
         [0010]    These and other embodiments are further discussed below with respect to the following figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates a wearable device with a receive coil. 
           [0012]      FIG. 2  illustrates a transmit coil that can be used with the receive coil illustrated in  FIG. 1 . 
           [0013]      FIGS. 3A and 3B  illustrate in more detail an example of a transmitter and receiver pair according to some embodiments of the present invention. 
           [0014]      FIG. 4  illustrates the efficiency versus output power relationship for a transmit coil/receive coil combination according to some embodiments of the present invention. 
           [0015]      FIG. 5  illustrates the voltage and frequency versus output current for a transmit coil/receive coil combination according to some embodiments of the present invention. 
           [0016]      FIG. 6  illustrates a schematic drawing of a transmitter and receiver according to some embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    In the following description, specific details are set forth describing some embodiments of the present invention. It will be apparent, however, to one skilled in the art that some embodiments may be practiced without some or all of these specific details. The specific embodiments disclosed herein are meant to be illustrative but not limiting. One skilled in the art may realize other elements that, although not specifically described here, are within the scope and the spirit of this disclosure. 
         [0018]    This description and the accompanying drawings that illustrate inventive aspects and embodiments should not be taken as limiting—the claims define the protected invention. Various changes may be made without departing from the spirit and scope of this description and the claims. In some instances, well-known structures and techniques have not been shown or described in detail in order not to obscure the invention. 
         [0019]    Elements and their associated aspects that are described in detail with reference to one embodiment may, whenever practical, be included in other embodiments in which they are not specifically shown or described. For example, if an element is described in detail with reference to one embodiment and is not described with reference to a second embodiment, the element may nevertheless be claimed as included in the second embodiment. 
         [0020]    Some embodiments of the present invention present a high efficiency Adaptive Resonant (AR) topology application with receiver (RX) and transmitter (TX) coils specifically designed for smart watch applications where a number of sensors have to go through or be present at the center area of the RX coil. Smart watches and similar wearable devices often have sensors present in the center of the wireless RX coils. In addition to the sensor difficulty, current wireless power implementations suffer from low efficiency. Applying an AR wireless topology with RX and TX coils with large inner diameter area removed of magnetic shield material allows these sensors to ‘go through’ while at the same providing high efficiency power transfer. 
         [0021]    AR topology already allows for high efficiency wireless power transfer. Utilizing this topology with RX/TX Coils that have large center sections of their magnetic shield removed has the effect of redirecting the magnetic flux away from metal areas present in the sensors, thus preventing eddy current losses. This results in high efficiency operation. 
         [0022]    Previous efforts have utilized WPC qi like circuits and coils design. The best performance obtained thus far is about 50% efficiency. 
         [0023]      FIG. 1  illustrates a wearable device  100 . As shown in the example of  FIG. 1 , wearable device  100  includes a RX coil  102  and sensors  104  located in the center of RX coil  102 . Sensors  104  can be any sensors that are used by wearable device  100 .  FIG. 2  illustrates a TX coil  200  that can be used to supply power to RX coil  102 . In embodiments of the present invention, TX coil  200  is driven with an adaptive resonant driver that optimizes power transfer between TX coil  200  and RX coil  102 . 
         [0024]    RX coil  102  and TX coil  200  are specifically adapted to wearable device  100 . RX coil  102  can absent of magnetic shielding material at its center and is of large enough diameter to accommodate sensors  104 . Additionally, RX coil  102  may be relatively thin (e.g., thinner than sensors  104 ) so as to not interfere with sensors  104 , some of which may be contact a user directly when device  100  is worn. 
         [0025]      FIGS. 3A and 3B  illustrate in more detail a system  300  with an example transmit circuit  302  an example receive circuit  304  according to some embodiments. As shown in  FIG. 3A , transmitter  302  receives an input voltage Vin from power source  306 , which is coupled through an inductor  308  (L1) to a center tap  310  of TX coil  200 . In some embodiments, transmit coil  200  may not include a center tap  310  and other configurations may be derived for transmitter  302 . A capacitor  318  (Crt) is connected across TX coil  200  and FETs  312  (Q1) and  314  (Q2) are coupled between each side of TX coil  200  and ground. The gates of FETs  312  and  314  are coupled to TX switching  316 , which alternates grounding of each side of TX coil  200  in order to activate TX coil  200 . 
         [0026]      FIG. 3B  illustrates voltage vs. time curves at points A, B, and C in the transmitter circuit  302 . Point A is the voltage at the center tap  310  of TX coil  200 , which as shown alternates between 0 and Vin. Point B is the Drain of transistor  314  (Q2) and point C is the drain of transistor  312  (Q1). As is illustrated, transistors  312  and  314  are turned on (the drain is then ground) by TX switching  316  as the voltage at the drain of that transistor decreases to 0 and is turned off when the drain of the other transistor decreases to 0. Consequently, the frequency at which TX switching  316  switches transistors  312  (Q1) and  314  (Q2) is determined by the dynamics of the transmit/receive circuit itself. This arrangement further provides for an operating frequency that maximizes the power transferred between the TX coil  200  and RX coil  102 . Such an arrangement is referred to herein as an adaptive transmit arrangement. 
         [0027]    The receive circuit illustrated in  FIG. 3A  receives power through RX coil  102  in a similar fashion. Transistors  322  (Q1r) and  324  (Q2r), which rectify the power signal received by RX coil  102 , are switched by RX switching  328  similarly as that described above with respect to TX switching  316  for transmit transistors  312  (Q1) and  314  (Q2). Consequently, transistors  322  (Q1r) and  314  (Q2r) are switched at a frequency that matches the switching frequency of transistors  312  (Q1) and  314  (Q2). 
         [0028]    Transistors  322  (Q1r) and  324  (Q2r) allow for a half-duplex rectification of the signal Vout taken from center tap  320  of coil  102 . Capacitor  326  (Cout) filters the signal Vout to a DC voltage. 
         [0029]    As a particular example of a system  300 , RX coil  102  can have an outer diameter of 29 mm, an inner diameter of 19 mm, and a thickness of less than 0.5 mm. Correspondingly, TX coil  200  can have an OD of 29 mm, an ID of 18 mm, and a thickness of less than 1.5 mm. Using these dimensions, the efficiency of power transfer between TX coil  200  and RX coil  120 , operating as described with respect to system  300  of  FIG. 3A , is illustrated in  FIG. 4 . As is illustrated in  FIG. 4 , the efficiency at power levels above about 1 W of transmit power is greater than 75%. 
         [0030]      FIG. 5  illustrates the output voltage Vout of a receiver and frequency as a function of output current for the TX/RX pair described above. In the illustration of  FIG. 4 , Vin is 5.0 V. Curve  502  illustrates the output voltage Vout as a function of output current Iout while curve  504  illustrates the operating frequency as a function of output current Iout. 
         [0031]      FIG. 6  illustrates a TX/RX schematic illustrating the adaptive transmitter and receiver pair of system  300  as illustrated in  FIG. 3A , according to some embodiments of the present invention. As is shown in  FIG. 5 , transmitter circuit  302  includes TX coil  200  is driven by IC  502 . IC  502  receives an input voltage Vin and provides switching at a frequency that minimizes the impedance of the interacting TX coil  200  and RX coil  102  pair, thus maximizing the power transferred between TX coil  200  and RX coil  102 . Receiver circuit  304  includes IC  504  that matches the switching frequency provided by IC  502  in order to efficiently receive the power from RX coil  102  and provide an output voltage Vout that can be used by wearable device  100 . 
         [0032]    The above detailed description is provided to illustrate specific embodiments of the present invention and is not intended to be limiting. Numerous variations and modifications within the scope of the present invention are possible. The present invention is set forth in the following claims.