Abstract:
A resonant wireless power (RWP) receiver is provided that includes an inductor element that couples with a resonant wireless power source. A capacitor arrangement is coupled to the inductor element altering the open-circuit impedance of the RWP receiver to reduce the ac voltage under certain defined situations. The capacitor arrangement includes a plurality capacitors tuned to a control ac voltage in the RWP receiver.

Description:
PRIORITY INFORMATION 
       [0001]    This application claims priority from provisional application Ser. No. 61/706,932 filed Sep. 28, 2012, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention is related to the field of resonant wireless power (RWP), and in particular to an open-circuit impedance control of a RWP receiver for voltage limiting. 
         [0003]    A resonant wireless power receiver includes a matching network designed to resonate at the frequency of power transfer. This feeds into a semiconductor rectifier, then a dc/dc converter in order to convert the AC power to regulated DC. If the magnitude of the AC voltage exceeds the operating voltage of the rectifier, or if the rectified voltage exceeds the operating voltage of the dc/dc converter, these circuits can be damaged. 
       SUMMARY OF THE INVENTION 
       [0004]    According to one aspect of the invention, there is provided a resonant wireless power (RWP) receiver. The RWP receiver includes an inductor element that couples with a resonant wireless power source. A capacitor arrangement is coupled to the inductor element altering the open-circuit impedance of the RWP receiver to reduce the ac voltage under certain defined situations. The capacitor arrangement includes a plurality capacitors tuned to a control ac voltage in the RWP receiver. 
         [0005]    According to another aspect of the invention, there is provided a resonant wireless power (RWP) system. The RWP system includes a source circuit having a reflected impedance in series with a source coil. A receiver circuit is coupled to the source circuit using an inductor element, and includes a capacitor arrangement coupled to the inductor element altering the open-circuit impedance of the receiver circuit to reduce the ac voltage under certain defined situations. The capacitor arrangement includes a plurality capacitors tuned to a control ac voltage in the receiver circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a schematic diagram illustrating a resonant wireless power (RWP) system used in accordance with the invention; 
           [0007]      FIG. 2  is a circuit diagram illustrating a series-parallel receiver circuit with Zoc adjustment capacitor; 
           [0008]      FIG. 3  is a circuit diagram illustrating a parallel-series receiver circuit with Zoc-adjustment capacitor; 
           [0009]      FIG. 4  is a circuit diagram illustrating a RWP receiver with a diode rectifier; and 
           [0010]      FIG. 5  is a circuit diagram illustrating a series-parallel receiver with modulation capacitors switched to ground. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    The invention provides novel techniques for adaptively altering the open-circuit impedance of the resonant receiver to reduce the ac voltage under certain defined situations. 
         [0012]      FIG. 1  shows a resonant wireless power (RWP) system. There are two complex impedances that can be used to describe most of the important aspects of a RWP system: the open-circuit impedance Zoc and the reflected impedance Zref. Mutual inductance, or coupling, between the source coil L 1  and receiver coil L 2  can be modeled in a number of different ways. 
         [0013]    In this case, the coupling k is modeled as a current-controlled voltage source Voc in series with the receiver coil L 2 . The impedance seen by this voltage source Voc, which includes the coil L 2 , matching network  6  and load (e.g., rectifier, dc/dc, load current) R 1 , is Zoc. On the source side, when coupling is present, one can model the effect of the coupling as an impedance in series with the source coil L 1  called Zref, the reflected impedance. Both the open-circuit impedance and the reflected impedance are complex quantities—they have real (resistive) and imaginary (reactive) components. For a 1:1 RWP system, the reflected impedance is related to the open-circuit impedance by this formula: 
         [0000]    
       
         
           
             
               
                 
                   
                     Z 
                     ref 
                   
                   = 
                   
                     
                       
                         ( 
                         
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                         ) 
                       
                       2 
                     
                     
                       Z 
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                   Eq 
                   . 
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0014]    In the series-parallel resonant circuit of  FIG. 2 , the capacitor Cd can be switched in or out of the circuit. The open-circuit impedance is altered as a result. If the switch is open, the open-circuit impedance Zoc is given by 
         [0000]    
       
         
           
             
               
                 
                   
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                   2 
                 
               
             
           
         
       
     
         [0015]    If the switch is closed, Zoc is given by 
         [0000]    
       
         
           
             
               
                 
                   
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                   Eq 
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                   3 
                 
               
             
           
         
       
     
         [0016]    A capacitor that can be switched in to adjust the open-circuit impedance Zoc, such as Cd in this example, is defined as a Zoc-adjustment capacitor. 
         [0017]    The two open-circuit impedances can be represented as the “X” and “O” symbols on the complex impedance plane  10 , as shown in  FIG. 2 . The magnitude of the change depends on the relative values of the inductors L 2  and capacitor s C 2   a,  C 2   b,  Cd in the resonant circuit  8 . Changing the open-circuit impedance also changes the reflected impedance seen by the source. Depending on the tuning of the system, either one may give a higher ac voltage at the rectifier input. Thus it is possible to design a RWP receiver in one of two ways: 
         [0018]    1. Tune the receiver for normal operation with the switch open, and close the switch in an excess power condition to reduce the ac voltage at the rectifier input. 
         [0019]    2. Tune the receiver for normal operation with the switch closed, and open the switch in an excess power condition to reduce the ac voltage at the rectifier input. 
         [0020]    If the capacitance values of C 2   a  and C 2   b  are comparable, the circuit of  FIG. 2  has the particular advantage that the voltage across the switch  9  is reduced compared to the coil voltage. This is advantageous for the implementation of the switch in a semiconductor process that may have a limited voltage tolerance. 
         [0021]      FIG. 3  shows a parallel-series topology of a receiver circuit  12  that includes a Zoc-adjustment capacitor. As in the series-parallel receiver, the Zoc-adjustment capacitor Cd can be either normally in-circuit or normally out-of-circuit. The circuit  12  of  FIG. 3  also has the advantageous property that the voltage across the switch  13  can be substantially lower than the coil voltage, provided that C 2   b  is not much larger than C 2   a.    
         [0022]      FIG. 4  shows an implementation of a receiver circuit  14  in which the ac power is rectified using a diode rectifier  16  to produce a dc voltage supply VRECT. This circuit can be represented for AC analysis by the circuit of  FIG. 2 . Any design of a rectifier can be used, including a synchronous rectifier. Note the modulation capacitor Cm is similar to the capacitor Cd, as shown in  FIG. 2 . 
         [0023]      FIG. 5  shows an alternative implementation of the modulation circuit  18  that is electrically equivalent to  FIG. 4 . The modulation capacitance is split into two capacitors of value 2 Cm. The series combination of these two capacitors has a capacitance Cm. Switching the capacitors to ground instead of across the AC terminals reduces the voltage stress on the switches  20 ,  22 , making this implementation more suitable for an integrated circuit implementation. 
         [0024]    The techniques described herein adaptively alters the open-circuit impedance of any resonant receiver to reduce the ac voltage. The invention allows one to change the open-circuit impedance that also changes the reflected impedance seen by a source by tuning the system to produce a higher ac voltage at a rectifier input. Moreover, the invention demonstrate several designs of a RWP receiver by tuning the receiver for normal operation with the switch open, and close the switch in an excess power condition to reduce the ac voltage at the rectifier input. Also, a receiver can be tuned for normal operation with the switch closed, and open the switch in an excess power condition to reduce the ac voltage at the rectifier input. 
         [0025]    Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.