Abstract:
A resonant wireless power (RWP) system is provided that includes a signal generator that provides an input signal waveform. An amplifier structure amplifies signals for transmissions to a receiver that is powered from a fixed DC voltage supply. The amplifier structure is operated either using differential or single-ended amplifiers to provide two different output power levels, in burst mode to provide a range of output power levels, or using a capacitor in a matching network that is adjusted to provide a range of output power levels.

Description:
PRIORITY INFORMATION 
       [0001]    This application is a National Phase Application of PCT Application no. PCT/US2014/026952, filed on Mar. 14, 2014, which claims priority from provisional application Ser. No. 61/782604 filed Mar. 14, 2013, which are incorporated herein by reference in their entireties. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention is related to the field of resonant wireless power (RWP), and in particular to a RWP driver with adjustable power output. 
         [0003]    Wireless power (WP) transfer systems use the mutual inductance between two magnetic coils to transfer power through magnetic induction. These systems are commonly classified as either “inductive” or “resonant”. In a purely inductive wireless power transfer system, the source coil, which functions as the primary winding of a transformer, is driven by a voltage or current source. The receive coil, which functions as the secondary winding, is connected to a bridge rectifier, either directly or through an ac-coupling capacitor. The voltages and currents in the two windings can be determined by the relations commonly used to describe transformers. 
         [0004]    In a resonant wireless power (RWP) transfer system, the source and receiver coils are connected to capacitors to form electrical resonators. From a circuit-design standpoint, the function of the capacitors is to cancel some of the reactive impedance of the inductors, allowing more power to be transferred at a given voltage. The impedance of the inductors and capacitors varies in opposite directions with operating frequency, so the cancellation is only effective over a small range of frequencies. In other words, resonant wireless power systems utilize circuits tuned to a specific frequency at which power is to be transferred. They typically do not allow power transfer at other frequencies. 
         [0005]    In order to operate a RWP system at high efficiency over a wide range of loading conditions, the power output from the wireless power source must be adjustable. Operating the source at too low of a power level may result in insufficient power to supply the receiver devices. Operating it at too high of a power level may result in excess circulating currents, causing wasted energy. 
         [0006]    For MHz-frequency RWP sources, a Class-E amplifier is often used to convert dc power to ac. The Class-E amplifier is only efficient at one particular duty cycle, so it is not possible to use duty cycle control to modulate power. Nor is it typically possible to vary frequency, as the receiver devices are all tuned to a specific frequency. The best-known methodology for adjusting power level is to introduce a dc-dc converter between the input dc power supply and the Class-E amplifier. However, this dc-dc converter will introduce some efficiency loss as well as adding complexity to the design of the source electronics. 
       SUMMARY OF THE INVENTION 
       [0007]    According to one aspect of the invention, there is provided a resonant wireless power (RWP) system. The RWP system includes a signal generator that provides an input signal waveform; amplifier structure that amplifies signals for transmissions to a receiver. The amplifier structure is powered from a fixed DC voltage supply. The amplifier structure is operated either using differential or single-ended amplifiers to provide two different output power levels, in burst mode to provide a range of output power levels, or using a capacitor in a matching network that is adjusted to provide a range of output power levels. 
         [0008]    According to another aspect of the invention, there is provided a method of controlling the power of a resonant wireless power (RWP) system. The method includes providing an amplifier structure that is powered from a fixed DC voltage supply. The amplifier structure is operated either using differential or single-ended amplifiers to provide two different output power levels, in burst mode to provide a range of output power levels, or using a capacitor in a matching network that is adjusted to provide a range of output power levels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic block diagram illustrating a typical RWP transmitter; 
           [0010]      FIG. 2  is a schematic diagram illustrating one embodiment of the invention for varying the output power of the amplifier; 
           [0011]      FIG. 3  is a schematic diagram illustrating a second embodiment of the invention for varying the source output power; 
           [0012]      FIG. 4  is a graph illustrating properties of the burst mode; 
           [0013]      FIG. 5  is a schematic diagram illustrating a third embodiment of the invention for adjusting amplifier output power; 
           [0014]      FIG. 6  is a schematic diagram illustrating a simplified source circuit used in accordance with the invention; 
           [0015]      FIG. 7  is a schematic diagram illustrating a matched impedance control scheme used in accordance with the invention; and 
           [0016]      FIG. 8  is a schematic diagram illustrating a RWP system focusing on the differential Class-E amplifier used in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The invention relates to circuits and methods for controlling the power from the Class-E amplifier without using duty-cycle control, frequency variation or a dc-dc converter. 
         [0018]      FIG. 1  shows a block diagram of a typical RWP transmitter  2 . A fixed dc voltage  12  provides the power input to the transmitter. A dc/dc converter  4  transforms this fixed dc voltage  12  to the voltage level required by an amplifier  8 . A signal generator  6  provides the phase and frequency reference signals to the amplifier input. 
         [0019]    In a typical embodiment of an RWP transmitter  2 , these signals consist of two square waves of continuous fixed frequency with opposite phase. The amplifier  8  produces output power drawn from the dc/dc output at the frequency of the signal generator  6 . The output power of the amplifier  8  is conveyed through a matching network  10  into the source coil L 1 . The matching network  10  provides an impedance match between the amplifier  8  and source coil L 1 , ensuring an efficient delivery of power to the source coil L 1 . When a matched receiver coil is coupled to the source coil L 1 , it may receive power wirelessly through the magnetic coupling. 
         [0020]    Depending on the conditions of operation of the wireless power system, the power required from the source may vary over some range. In the system of  FIG. 1 , the conventional method to vary the amplifier output power is to vary the voltage at the amplifier input. The dc/dc converter  4  is only required because of the need to vary amplifier output power. If output power can be varied using a fixed voltage supply  12 , the dc/dc converter  4  can be eliminated, thus improving efficiency and reducing system complexity. Although frequency and duty-cycle modulation have been used in inductive charging systems to vary source power, these techniques are not suitable to highly resonant systems. Note the amplifier  8  can include any class of amplifier, such Class-D, E, F amplifiers. 
         [0021]      FIG. 2  shows one embodiment  20  of the invention for varying the output power of the amplifier  8  without using input voltage, frequency or duty-cycle modulation. In this block diagram the amplifier  8  is powered directly from the fixed dc voltage  12 —there is no dc-dc converter. The amplifier  8  shown is typically operated as a differential amplifier, producing equal and opposite output voltages. In fact, the amplifier  8  is composed of two component amplifiers  22 ,  24 . If the signal generator  6  disables one of its frequency reference outputs, the corresponding component amplifier  24  can be made inactive. As a result, the output power from the amplifier  8  can be reduced by half. This provides two levels of output power adjustment, which may be sufficient for some basic wireless power systems. 
         [0022]      FIG. 3  shows another embodiment  30  for varying the source output power. As in the technique described in  FIG. 2 , the amplifier  8  is powered from a fixed dc voltage  12 . In this method, the frequency reference provided by the signal generator  6  is not a continuous square wave, but rather a sequence of bursts of pulses at the operation frequency, interleaved with periods of inactivity. An example of this burst-mode operation is shown in  FIG. 4 . The waveforms Vg 1  and Vg 2  represent the frequency references supplied to the top and bottom side of the differential amplifier, respectively. In the section of the figure labeled “4/8 Duty Cycle”, there is a burst of four pulses followed by an idle period. The overall pattern repeats with a period of eight pulse periods. During the burst, the inductor current IL ramps up as power is injected into the resonant circuit. During the idle period, power ramps down. The average current in the inductor IL is lower than it would be for a continuous pulse train. In the section of the figure labeled “2/8 Duty Cycle”, only two pulses are injected during a period of eight pulse periods. Since the ramp-up time of the inductor current is smaller, the average inductor current is lower than in the first case. In general, the higher the ratio of pulses to idle time, the higher the average inductor current. Thus the output power, which is proportional to inductor current, can be controlled by adjusting the pulse density. 
         [0023]      FIG. 5  shows another embodiment  34  of the invention for adjusting amplifier  8  output power. In this method, the amplifier input voltage is a fixed dc voltage  12  and the frequency reference is a continuous, fixed-frequency square wave. In this method, there are variable impedance elements  36  in the matching network  10 . Varying these elements allows the power to the source coil L 1  to be adjusted. In order to better understand this variable matching network technique,  FIG. 6  shows the amplifier abstracted to an ac voltage source  48 . 
         [0024]    In particular,  FIG. 6  also shows the components Rref and Xref, the reflected resistance and reactance. These elements are taken from transformer theory to model the effect of a receiver coil coupled to the source coil. Rref represents power delivered to the receiver coil, and Xref represents power reflected from the receiver coil. If the circuit  44  is perfectly matched, the impedance at the input of the matching network  50  will be exactly the real value of the equivalent source impedance with no imaginary (reactive) component. This ensures that the source can transfer maximum power to the load. However, since the input impedance to the matching network  50  is necessarily dependent on the reflected impedance, it is not possible to achieve perfect matching over all load conditions with a fixed matching network  50 . The matching network  50  can also transform the real part of the reflected impedance to a larger or smaller value, depending on the topology used. This effect is used in this invention to vary the power delivered to the load via the VAC  48 . The VAC is power by a DC source  46 . 
         [0025]      FIG. 7  illustrates a simplified block diagram of one such matched impedance control circuit  52 . Vac represents the amplifier output voltage. The matching network consists of the combination of a series reactance Xs and a parallel capacitance Cp. Under certain Q factor conditions, the combination of C p , L s  and R s  is transformed into an equivalent circuit that can be represented by L M  and R M . L M  and R M  can be calculated using equation 1 and 2 below, where co refers to the operating frequency and ω 0  is the resonant frequency of the parallel combination of C p  and L s . 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    From Eq. 3, one can calculate the value of C p  necessary to present an appropriate value of R M  at the fixed AC voltage VAC. From Eq. 1, one can calculate the value of X S  necessary to cancel the reactance presented by L M . 
         [0026]    It can be observed that the value of the equivalent resistance R M  and inductance Lm can be varied by changing Xs and Cp. If both are varied, the amplifier can deliver varying amounts of power in a perfectly matched condition. If only one or the other is varied, perfect matching may not be maintained, but the output power can be modulated. 
         [0027]      FIG. 8  shows a schematic of a RWP system  62 , in which a differential Class-E amplifier is used. The techniques described above are particularly well-suited to RWP systems including differential Class-E amplifiers. Class-E amplifiers are often used in RWP systems because they offer high efficiency and low equivalent output resistance. A description of the circuit is as follows: A signal generator  64  produces two frequency references with fixed frequency and duty cycle and opposite phase. Driver stages  66  and  68  amplify the frequency references with sufficient drive strength to control the gates of the power FETs M 1  and M 2 . The FETs are arranged with a conventional Class-E matching network consisting of choke inductors LS 1  and capacitors CS 1  and CS 2 . The amplifier produces a sinusoidal circulating current in L 1 , the source coil. This generates a magnetic field that induces a voltage in the receiver coil L 2 . This induced voltage can be used to transfer energy to the receiver load, represented as resistor R 1 . A matching network  70  provides maximum power transfer between the source or RF energy and its load. 
         [0028]    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.