Abstract:
In a wireless energy transfer system, an apparatus includes a coil and an adjustable alternating current power supply electrically connected to the coil. When a device is in the vicinity of the apparatus, the apparatus determines an operating parameter of the device and adjusts a power supply operating parameter based on the determined device operating parameter. The apparatus may be a transmitter wirelessly transmitting power to the device or a receiver wirelessly receiving power from the device. 
     The operating parameter may be frequency, and the apparatus adjusts the power supply frequency to be substantially equal to the device frequency, which may be the device resonant frequency. 
     The operating parameter may be phase, and the apparatus adjusts the power supply phase to be out of phase with the device. The phase of the power supply may be adjusted to ninety degrees out of phase with the device for maximum power transfer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims benefit to U.S. provisional application 61/428,143 filed Dec. 29, 2010 entitled Phase Shift Power Transfer, the contents of which is incorporated herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    A wireless energy transfer system may be designed for the maximum transfer of real power from a transmitter to one or more receivers. Energy transfer may be affected by the distance between the receiver and transmitter, the number of receivers attempting to receive energy from a transmitter, and the voltage at the transmitter and receiver, to name just a few limiting factors. Thus, it is desirable to be able to adjust one or more parameters of the receiver or transmitter to adjust the amount of transferred energy. 
     
    
     
       FIGURES 
         [0003]      FIG. 1A  illustrates a representative wireless energy transfer system including a transmitter and a receiver. 
           [0004]      FIG. 1B  illustrates a representative wireless energy transfer system in pi-model form. 
           [0005]      FIG. 2  illustrates a representative wireless energy transfer system in pi-model form with alternating current power sources. 
           [0006]      FIG. 3  illustrates a representative wireless energy transfer system with phase control. 
           [0007]      FIG. 4  illustrates a representative wireless energy transfer system with multiple receivers. 
           [0008]      FIG. 5  illustrates a representative mono-resonant wireless energy transfer system. 
           [0009]      FIG. 6  illustrates a phasor relationship between a transmitter and receiver in a wireless energy transfer system. 
           [0010]      FIG. 7  illustrates a relationship between load value and power transfer. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In a wireless energy transfer system it is desirable to transfer the maximum amount of real power from a primary coil to a secondary coil with the greatest efficiency possible. Real power transfer, referred to in shortened form as power transfer herein, is affected by multiple design and environmental factors. For example, in one exemplary implementation a secondary coil is included in a portable device such as a computer or a mobile phone or the like. In the exemplary portable device there is a rechargeable battery that is recharged wirelessly with energy received via the secondary coil from a charging device. One design factor affecting power transfer for the exemplary portable device is the voltage limitation of the rechargeable battery. One environment factor affecting power transfer for the exemplary portable device is the distance that the mobile device is from the charging device. The two mentioned factors affecting power transfer for the exemplary portable device are just two of the many design and environmental factors to consider. In other implementations the same or other factors may affect power transfer. 
         [0012]    A wireless energy transfer system may be designed to tolerate, compensate for, or otherwise adjust to changing environmental conditions. In some systems, frequency or phase of the transmitter or receiver may be adjusted to modify the maximum power transfer capability or to modify the efficiency of the power transfer. For example, a phase shift may be introduced between the transmitter and receiver to adjust the maximum power transfer capability. In another example, transmitter frequency may be adjusted to match the resonance frequency of the receiver to increase power transfer efficiency. 
         [0013]      FIG. 1A  illustrates a representative wireless energy transfer system  100  including a transmitter  110  with primary coil  120  having inductance L 1  and a receiver  130  with secondary coil  140  having inductance L 2 . A current I 1  flows through primary coil  120 . A current I 2  flows through secondary coil  140 . A power source  115  with voltage V 1  is applied across coil  120  and a power source  135  with voltage V 2  is applied across coil  140 . Coils  120  and  140  are coupled with coupling coefficient k. There is a mutual inductance M between primary coil  120  and secondary coil  140 . 
         [0014]      FIG. 1B  illustrates the system  100  of  FIG. 1A  represented by an equivalent pi (π) model. The n-model includes an inductor  150  having inductance L_a, an inductor  160  having inductance L_b, and an inductor  170  having an inductance L_c. In the π-model, current I 1  flows into node N 1  and current I 2  flows into node N 2 . Also in the n-model, a power source  175  with voltage V 1  located between nodes N 1  and N 3  and a power source  180  with voltage V 2  located between nodes N 2  and N 3 . 
         [0015]    The parameters of  FIGS. 1A and 1B  are related as shown in equations 1-4. 
         [0000]    
       
         
           
             
               
                 
                   L_a 
                   = 
                   
                     
                       
                         L 
                          
                         
                             
                         
                          
                         1 
                         * 
                         L 
                          
                         
                             
                         
                          
                         2 
                       
                       - 
                       
                         M 
                         2 
                       
                     
                     
                       
                         L 
                          
                         
                             
                         
                          
                         2 
                       
                       - 
                       M 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   L_b 
                   = 
                   
                     
                       
                         L 
                          
                         
                             
                         
                          
                         1 
                         * 
                         L 
                          
                         
                             
                         
                          
                         2 
                       
                       - 
                       
                         M 
                         2 
                       
                     
                     
                       
                         L 
                          
                         
                             
                         
                          
                         1 
                       
                       - 
                       M 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   L_c 
                   = 
                   
                     
                       
                         L 
                          
                         
                             
                         
                          
                         1 
                         * 
                         L 
                          
                         
                             
                         
                          
                         2 
                       
                       - 
                       
                         M 
                         2 
                       
                     
                     M 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   M 
                   = 
                   
                     k 
                      
                     
                       
                         L 
                          
                         
                             
                         
                          
                         1 
                         * 
                         L 
                          
                         
                             
                         
                          
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0016]    For small coupling coefficient k, the maximum power Pmax through the circuit is shown in equation 5, where the values for voltages V 1  and V 2  are in root-mean-square (rms) Volts. In equation 5, the term ω is equal to 2πf where f is the operating frequency of the system  100  power sources, for example power sources  175  and  180 . 
         [0000]    
       
         
           
             
               
                 
                   
                     P 
                      
                     
                         
                     
                      
                     max 
                   
                   = 
                   
                     k 
                     * 
                     
                       
                         V 
                          
                         
                             
                         
                          
                         1 
                         * 
                         V 
                          
                         
                             
                         
                          
                         2 
                       
                       
                         ω 
                          
                         
                           
                             L 
                              
                             
                                 
                             
                              
                             1 
                             * 
                             L 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0017]    From equation 5 it can be seen that maximum power increases with a decrease in frequency or coil inductance and that maximum power increases with an increase in coupling coefficient k or an increase in voltage V 1  or V 2 . Because a higher voltage V 1  or V 2  may also increase the efficiency of conversion, it may be desirable to operate system  100  with maximum coil voltages. 
         [0018]    Ideally the voltages V 1  and V 2 , across the primary and secondary coils respectively, are equal. However, in many implementations it is not practical to have equivalent primary and secondary coil voltages. For example, a portable device acting as a receiver may be limited by size, weight, and cost and the power source thus limited to lower voltages whereas the transmitter power source may be a line voltage of 110V or 220V or the like. Consequently, the primary and secondary coil voltages will not be equal. However, power transfer may still be relatively efficient with unequal primary and secondary coil voltages. In one implementation shown to have sixty percent (60%) efficiency with a maximum power of approximately three Watts (3 W), the primary coil voltage was 100V, secondary coil voltage was 10V, primary coil inductance was 100 μH, secondary coil inductance was 10 μH, and coupling coefficient was k=0.1. 
         [0019]      FIG. 1B  and equation 5 describe power sources  175  and  180  with voltage amplitude information only, respectively V 1  and V 2 . However, power sources  175  and  180  may be further described using phase information. Analyzing system  100  using both amplitude and phase information provides additional insight into design of the system for maximum power transfer. 
         [0020]      FIG. 2  illustrates the π-model of  FIG. 1B  with power sources  175  and  180  replaced by alternating current (AC) power sources  210  and  220 , respectively, each illustrated as having an amplitude and a phase. Power source  210  provides power at a magnitude V 3  and a phase angle θ 1 . Power source  220  provides power at a magnitude V 4  and a phase angle θ 2 . A phase Φ may be defined as the difference between phase angles θ 1  and θ 2  such that Φ is the phase angle between the primary and secondary coil voltages at any given time. Equation 5 describing the maximum power through the circuit of  FIG. 1A  may be modified to describe the maximum power through the circuit of  FIG. 2  by including a phase component as shown in equation 6. 
         [0000]    
       
         
           
             
               
                 
                   
                     P 
                      
                     
                         
                     
                      
                     max 
                   
                   = 
                   
                     k 
                     * 
                     
                       
                         V 
                          
                         
                             
                         
                          
                         3 
                         * 
                         V 
                          
                         
                             
                         
                          
                         4 
                       
                       
                         ω 
                          
                         
                           
                             L 
                              
                             
                                 
                             
                              
                             1 
                             * 
                             L 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                     * 
                     
                       sin 
                        
                       
                         ( 
                         Φ 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0021]    From equation 6 it can be seen that maximum power transfer between the primary and secondary coils occurs when there is a phase difference of Φ=90 degrees between power sources  210  and  220 . 
         [0022]    Therefore, maximum power transfer may be achieved by controlling the phase difference Φ to be 90 degrees. 
         [0023]      FIG. 3  illustrates one exemplary implementation of a system  100  that allows the phase difference Φ to be controlled. The AC sources  210  and  220  of  FIG. 2  are implemented in  FIG. 3  using direct current (DC) power sources. DC source  310  having voltage V 5  is switched to primary coil  335  through switches  315 ,  320 ,  325 , and  330  at some frequency with phase angle θ 3 . The voltage generated by the switching appears to primary coil  335  as an AC source with magnitude V 7  and phase angle θ 3 . DC source  365  having voltage V 6  is switched to secondary coil  340  through switches  345 ,  350 ,  355 , and  360  at some frequency with phase angle θ 4 . The voltage generated by the switching appears to secondary coil  340  as an AC source with magnitude V 8  and phase angle θ 4 . Thus, the phase difference Φ=θ 3 -θ 4  may be controlled by changing the switching phase angle θ 3  of switches  315 ,  320 ,  325 , and  330  or by changing the switching phase angle θ 4  of switches  345 ,  350 ,  355 , and  360 . 
         [0024]    Among the advantages that phase angle control may provide is the advantage of fixed frequency operation, and the further advantage that tuning elements are not required. Additionally, the maximized voltage possible from phase angle control provides for power transfer even under low coupling conditions, for example, down to k=0.1. 
         [0025]    Maximum power transfer is not always desirable. Therefore, phase difference Φ may be controlled to be less than 90 degrees to meet the needs of the system  100 . 
         [0026]    Communication between a transmitter and receiver may be implemented to coordinate the switching and thereby achieve a desired phase difference Φ. However, many systems  100  do not include communication of phase information between the transmitter and receiver. Such systems may instead include a phase or frequency loop to automatically synchronize the transmitter and receiver. For example, a phase loop in the receiver may automatically lock on to the phase of the transmitter and the phase information may then be used to set the receiver phase as desired. 
         [0027]    In other implementations in which there is no communication of phase information between transmitter and receiver, the phase of either the transmitter or receiver may be swept to determine the phase at which maximum power transfer occurs. 
         [0028]    Multiple receivers can be accommodated easily using phase shift power transfer as described above. 
         [0029]      FIG. 4  illustrates an exemplary model of a wireless energy transfer system  400  with a transmitter, indicated as “Transmitter” and multiple receivers, indicated as ‘Receiver  1 ’, ‘Receiver  2 ’, and ‘Receiver n’. Three receivers are shown but system  400  may include more or less than three receivers. 
         [0030]    There will be negligible power transfer between multiple receivers when there is a phase difference of ninety degrees (90°) between the Transmitter and each Receiver. For example, a set of receivers such as the Receivers illustrated in  FIG. 4  have phase θ 1 =θ 2 =θ 3 =0 and Transmitter phase Φ=90° such that maximum power transfer is achieved between the Transmitter and each Receiver. Equation 6 is modified as shown in equation 7, where Pmax 12  is the maximum power transfer between Receiver  1  and Receiver  2  and L 1  and L 2  are the inductances of the coils of Receiver  1  and Receiver  2 , respectively. 
         [0000]    
       
         
           
             
               
                 
                   
                     P 
                      
                     
                         
                     
                      
                     max 
                      
                     
                         
                     
                      
                     12 
                   
                   = 
                   
                     k 
                     * 
                     
                       
                         V 
                          
                         
                             
                         
                          
                         2 
                         * 
                         V 
                          
                         
                             
                         
                          
                         3 
                       
                       
                         ω 
                          
                         
                           
                             L 
                              
                             
                                 
                             
                              
                             1 
                             * 
                             L 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                     * 
                     
                       sin 
                        
                       
                         ( 
                         
                           
                             θ 
                              
                             
                                 
                             
                              
                             1 
                           
                           - 
                           
                             θ 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0031]    Receiver  1  and Receiver  2  are in phase with each other, so that sin(θ 1 −θ 2 )=sin( 0 )=0. Therefore, Pmax 12  is substantially equal to zero, and power transfer between Receiver  1  and Receiver  2  is substantially equal to zero. Similarly, power transfer between Receiver  1  and Receiver  3 , and power transfer between Receiver  2  and Receiver  3 , will be substantially equal to zero. 
         [0032]    Therefore, phase shift power transfer not only provides better power transfer from a transmitter to a receiver, but also provides for minimized power transfer between multiple receivers. Additionally, the minimized power transfer between multiple receivers does not require additional computation or communication. 
         [0033]    Although the example above included θ 1 =θ 2 =θ 3 =0 for the receivers of  FIG. 4 , the example was not limiting. Two receivers may each be 90° out of phase with the transmitter but the two receivers may be 180° out of phase with each other. For a 180° phase difference, sin(180°)=0 and maximum power transfer is still zero. 
         [0034]    In some implementations, it may not be possible or alternatively may not be desirable for all receivers to be 90° out of phase with the transmitter. However, power transfer between receivers may still be low. For example, a first receiver is 80° out of phase with the transmitter and a second transmitter is 60° out of phase with the transmitter. For power transferred between the transmitter and first receiver, sin(80°)=0.98. For power transferred between the transmitter and second receiver, sin(60°)=0.87. For power transferred between the first receiver and the second receiver, sin(20°)=0.34. Thus, power transfer from transmitter to receivers is relatively high and power transfer between receivers is relatively low even when phase shift between transmitter and receivers is less than 90°. 
         [0035]    Further, if the receiver coil voltages are low, as may be the case if the receivers are portable devices, the product of the receiver coil voltages will be low and thus maximum power transfer from one receiver to another will be low relative to the maximum power transfer between transmitter and receiver. 
         [0036]    As multiple receivers are added, the representative Transmitter inductance LA in  FIG. 4  reduces causing higher circulating currents and hence higher losses in the primary. The number of receivers is therefore limited by transmitter power loss. 
         [0037]    Power transfer may be improved through the use of phase angle control as described above and as illustrated by equation 6. Equation 6 also illustrates that power transfer may be improved by increasing one or both of the primary and secondary coil voltages. Coil voltage may be increased through the use of resonance. For example, dual resonance may be employed wherein both the transmitter and receiver are maintained in a resonant mode. Dual mode resonance may boost the voltages at the coils for good power transfer capability even under very low coupling conditions, for example, for k=0.01. However, achieving resonance on both the transmitter side and the receiver side may require tuning elements and communication between the transmitter and receiver regarding phase and frequency information. Such communication may not be available. Therefore, in some implementations resonance is used only on one side of the power transfer, either the transmitter side or the receiver side. 
         [0038]      FIG. 5  illustrates an exemplary system  500  using mono-resonance wherein resonance is only on the receiver side. System  500  includes a transmitter  505  and a receiver  530 . Transmitter  505  includes power source  510  and primary coil  520  having inductance L 1 . Receiver  530  includes secondary coil  540  having inductance L 2 , capacitor  550  having capacitance Cs 2 , rectification circuit  560 , capacitor  570 , and load  580  represented by a resistor having resistance RL. 
         [0039]    Power source  510  provides one example of an AC source that receives power from a line input, for example at a building power outlet, rectifies the line input to direct current (DC), boosts the rectified line input, stores the energy in a capacitor, and converts the stored energy into an AC signal with controlled frequency and phase for application to primary coil  520 . 
         [0040]    Secondary coil  540  and capacitor  550  form a resonant circuit with resonance at w 2  as shown in equation 8. 
         [0000]    
       
         
           
             
               
                 
                   
                     ω 
                      
                     
                         
                     
                      
                     2 
                   
                   = 
                   
                     1 
                     
                       
                         L 
                          
                         
                             
                         
                          
                         2 
                         * 
                         Cs 
                          
                         
                             
                         
                          
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
         [0041]    Primary coil  520  is coupled to secondary coil  540  with coupling coefficient k. Power is wirelessly transferred from transmitter  505  to receiver  530  and an output voltage Vout is developed across capacitor  570  and delivered to load  580 . 
         [0042]    Load  580  is represented in  FIG. 5  by a resistor for simplicity and not by way of limitation. Load  580  may actually be many other types of loads, including a battery. 
         [0043]    The resonance frequency w 0  of system  500  is determined by the inductance of secondary coil  540 , the capacitance of capacitor  550 , and the coupling coefficient k, as shown in equation 9. 
         [0000]    
       
         
           
             
               
                 
                   
                     ω 
                      
                     
                         
                     
                      
                     0 
                   
                   = 
                   
                     1 
                     
                       
                         L 
                          
                         
                             
                         
                          
                         2 
                         * 
                         Cs 
                          
                         
                             
                         
                          
                         2 
                         * 
                         
                           ( 
                           
                             1 
                             - 
                             
                               k 
                               2 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0044]    For small values of k, equation 7 reduces to equation 10. 
         [0000]    
       
         
           
             
               
                 
                   
                     ω 
                      
                     
                         
                     
                      
                     0 
                   
                   = 
                   
                     
                       1 
                       
                         
                           L 
                            
                           
                               
                           
                            
                           2 
                           * 
                           Cs 
                            
                           
                               
                           
                            
                           2 
                         
                       
                     
                     = 
                     
                       ω 
                        
                       
                           
                       
                        
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0045]    Thus, for small values of k, the resonance frequency of system  500  is substantially equal to the resonance frequency of receiver  530 . Based on this result, a frequency sweep may be made in power source  510  to find the system  500  resonance frequency. Power source  510  may then be operated at the resonance frequency for maximum power transfer. 
         [0046]    Among the advantages of the mono-resonant system illustrated in  FIG. 5  is the advantage of not needing resonance on the transmitter, thereby eliminating the need for resonance tuning elements. A further advantage is that because resonance is only on the receiver side there is no splitting of resonance frequencies from the interaction of two resonant circuits, thus voltages are predictable and, correspondingly, lower voltage circuit elements may be selected. 
         [0047]      FIG. 5  is a phasor diagram illustrating a portion of the relationship between the transmitter and receiver of  FIG. 5 . As seen in the phasor diagram, the system of  FIG. 5  automatically adjusts to load transients. In the phasor diagram the phasor E 2  represents the primary coil voltage reflected to the secondary coil as an independent source. Equation 11 describes E 2 . 
         [0000]    
       
         
           
             
               
                 
                   
                     E 
                      
                     
                         
                     
                      
                     2 
                   
                   = 
                   
                     k 
                     * 
                     
                       
                         
                           L 
                            
                           
                               
                           
                            
                           2 
                         
                         
                           L 
                            
                           
                               
                           
                            
                           1 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0048]    E 2 =I 2 *RL when the receiver is tuned, where I 2  is the current through capacitor  550  and RL is the value representing load  580 . 
         [0049]    E 2  is a constant value. Power source voltage V 9  and output voltage Vout are also substantially constant values. For higher loading, meaning smaller values of RL, the voltage V 10  across the secondary coil must increase to maintain the output voltage. Correspondingly, as seen in  FIG. 5 , the phase difference D between the primary and secondary coils increases. 
         [0050]    Equation 6 is reproduced as equation 12 using the nomenclature of  FIG. 5 . 
         [0000]    
       
         
           
             
               
                 
                   
                     P 
                      
                     
                         
                     
                      
                     max 
                   
                   = 
                   
                     k 
                     * 
                     
                       
                         V 
                          
                         
                             
                         
                          
                         9 
                         * 
                         V 
                          
                         
                             
                         
                          
                         10 
                       
                       
                         ω 
                          
                         
                           
                             L 
                              
                             
                                 
                             
                              
                             1 
                             * 
                             L 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                     * 
                     
                       sin 
                        
                       
                         ( 
                         Φ 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
         [0051]    As phase difference Φ increases, the value of sin(Φ) increases. Thus, as seen in equation 12, because secondary coil voltage V 10  and sin(Φ) are increasing, the power transferred increases. Therefore higher load automatically increases the power transfer across the coupled inductor and as such the circuit is stable for load transients. 
         [0052]    In addition because of increased phase difference Φ the AC efficiency increases. Thus both power transfer and efficiency increase with higher load. For very high load currents the secondary conduction losses dominate resulting in diminished efficiency. 
         [0053]      FIG. 6  is a graph that shows the change in power transfer and efficiency versus a change in load. The graph is the product of a simulation on a system substantially similar to the system of  FIG. 5 , using coupling coefficient k=0.1. 
       CONCLUSION 
       [0054]    The power transferred in a wireless energy transfer system including a transmitter with a primary coil and a receiver with a secondary coil may be adjusted by adjusting the phase difference between the primary and secondary coils. Maximum power transfer may be achieved when the transmitter and receiver coils are operated ninety degrees out of phase with each other. 
         [0055]    The power transferred may be increased by setting the frequency on a first side of the system to the resonant frequency of the other side of the system, thereby increasing the voltage on the first side for increased power transferred.