Abstract:
A system for wireless energy transfer includes a circuit for wireless transmission of energy, including a first, tunable resonator circuit including a transmitter coil and a variable capacitance device connected in shunt across the transmitter coil. Also disclosed is a circuit for wireless reception of energy including a tunable second resonator circuit including a receiver coil inductively coupled to the transmitter coil and a variable capacitance device connected in shunt across the receiver coil. Also disclosed is an arrangement for wireless energy transmission and reception that foregoes the necessity for separate circuits for DC rectification at the reception end of the arrangement. Also disclosed a system for wireless energy transfer where the system includes a tunable resonator circuit embedded in a surface such as piece of furniture, counter, etc., e.g., a table.

Description:
BACKGROUND 
       [0001]    This invention relates to wireless power transfer and in particular to techniques that can be adapted for charging of batteries and the like. 
         [0002]    Wireless power transfer is known for over 100 years. With the boom of portable electronic devices in the last decade the interest in wireless power transfer for battery charging is growing rapidly. 
       SUMMARY 
       [0003]    To increase wireless power transfer system operating range, a high-Q factor system using self-resonating coils can be used. The problem with self-resonating coils is the difficulty to tune the two coils to the RF source frequency and between one another (this is done by changing the number of windings and distance between the windings and not suitable for automatic feedback control). 
         [0004]    For practical applications such as for use in charging household appliance applications, typical short range inductive chargers are limited to several centimeters or even millimeters. High-Q factor resonant systems partially offset the rapid decay of received power vs. distance between transmitter (transmitter) and receiver (receiver), at the cost of high operating voltages across the resonant coils, as the receiver coil voltage swings up wildly when the receiver device is moved closer to the transmitter. High-Q systems have to rectify and regulate voltages in the order of hundreds and even thousands of volts. This can pose a problem for typical low-voltage operated electronic devices operating in this type of environment. High input voltage regulators exist, although they are less efficient, bulky and costly. 
         [0005]    Inductive, e.g., battery chargers typically operate over a short spatial range, typically limited to several centimeters or even millimeters. High-Q resonant systems partially offset the rapid decay of received power vs. distance between transmitter (transmitter) and receiver (receiver). While a better Q-factor can occur in the megahertz range, e.g. 5 to 27 MHz, the performance of a typical rectifier circuit in such a range is limited by the maximum operating frequency of semiconductor devices. A conventional rectifier design may not perform satisfactorily at these high frequencies of operation. At such a high frequency, the output voltage waveform of a conventional rectifier may include ringing transients, due to parasitic inductance and capacitance resonances. These transients cause rectification efficiency losses and output signal noise. Moreover, input impedance characteristics of rectifiers change as a function of frequency and load. To reduce the impact, series resonant converters are used to regulate by varying the switching frequency and counteract the reactive impedance variations. 
         [0006]    According to an aspect, a circuit includes a resonator circuit for wireless reception of energy, including a receiver coil having a large number of windings N 1  and a variable capacitance device connect in shunt across the receiver coil with the receiver coil and the variable capacitor and intrinsic capacitance of the receiver coil forming the resonator circuit and an output coil with a low number of windings N 2  compared to the number of windings N 1  of the receiver coil, the output coil being inductively coupled to the receiver coil to provide power output from the circuit, with the resonator circuit having a resonant frequency in a range of about 100 kHz to 30 MHz. 
         [0007]    The following are embodiments. 
         [0008]    The resonator circuit is tuned by the variable capacitance device to a resonant frequency within the range of 100 kHz to 30 MHz. The resonator circuit oscillates at high voltage and produces a strong magnetic field that is coupled to the secondary coil. Inductive coupling of the two coils produces an output voltage at the output coil related to V L(coil) =V in *Q*N 2 /N 1 . The circuit further includes circuitry to convert an AC voltage across the output coil to a DC voltage. The circuit further includes circuitry to convert an AC voltage across the output coil to a DC voltage, the circuitry including a capacitor connected in shunt across the secondary coil and a DC/DC converter or battery charger coupled to the capacitor. The circuit further includes a rectifier circuit connected to the output coil to convert AC voltage across the output coil to a DC voltage, the circuitry further including a capacitor connected in shunt across the secondary coil and a DC/DC converter coupled in series with the capacitor. 
         [0009]    According to an additional aspect, a circuit includes a resonator circuit for wireless transmission of energy, the circuit including a transmitter coil having a large number of windings N 1 , a variable capacitance device connected in shunt across the transmitter coil with the transmitter coil and the variable capacitor and intrinsic capacitance of the transmitter coil forming the resonator circuit, and an input coil configured to be coupled to an input RF source, with the input coil having a low number of windings N 2  compared to the number of windings N 1  of the transmitter coil, and the input coil being inductively coupled to the transmitter coil to provide power output from the circuit with the resonator circuit having a resonant frequency in a range of about 100 kHz to 30 MHz. 
         [0010]    The following are embodiments. 
         [0011]    The resonator circuit is tuned by the variable capacitance device. The resonator circuit oscillates at high voltage and produces a strong magnetic field at the transmitter coil. The circuit further includes an RF source connected to the input coil. Inductive coupling of the transmitter and input coils produces an output voltage at the transmitter coil related to V L(coil) =V in *Q*N 1 /N 2  where V in  is related to the voltage of the RF source connected to the input coil. 
         [0012]    According to an additional aspect, a system for wireless energy transfer includes a circuit for wireless transmission of energy, the circuit including a first resonator circuit including a transmitter coil having a large number of windings N 1 , a variable capacitance device connected in shunt across the transmitter coil with the transmitter coil and the variable capacitor and intrinsic capacitance of the transmitter coil forming the resonator circuit, an input coil configured to be connected to an input RF source, with the input coil having a low number of windings N 2  compared to the number of windings N 1  of the transmitter coil, and the input coil being inductively coupled to the transmitter coil to provide power output from the resonator circuit at a resonant frequency in a range of about 100 kHz to 30 MHz, a circuit for wireless reception of energy, the circuit includes a second resonator circuit includes a receiver coil inductively coupled to the transmitter coil, the receiver coil having a large number of windings N 3 , a variable capacitance device connected in shunt across the receiver coil with the receiver coil and the variable capacitor and intrinsic capacitance of the receiver coil forming the second resonator circuit, an output coil with a low number of windings N 2  compared to the number of windings N 1  of the receiver coil, the output coil being inductively coupled to the receiver coil to provide power output from the circuit, with the resonator circuit having a resonant frequency in a range of about 100 kHz to 30 MHz. 
         [0013]    The following are embodiments. 
         [0014]    The second resonator circuit is tuned by the variable capacitance device to a resonant frequency within the range of 100 kHz to 30 MHz. The circuit further includes circuitry to convert an AC voltage across the output coil to a DC voltage. 
         [0015]    According to an additional aspect, a system for wireless energy transfer includes a circuit for wireless transmission of energy, configured to be fed by an RF source the circuit for wireless transmission including a first resonator circuit, a circuit for wireless reception of energy, from the first resonator circuit, the circuit for wireless reception including a second resonator circuit, and a circuit disposed between the circuit for wireless transmission of energy and the circuit for wireless reception of energy to inductively couple with the first and second resonator circuits to effectively produce a rectified voltage having a DC component at the circuit for wireless reception of energy. 
         [0016]    The following are embodiments. 
         [0017]    The circuit to inductively couple includes a tunable passive resonator disposed between the first and second resonator circuits, the tunable passive resonator including a coil having a large number of windings N 1 , a variable capacitance device connected in shunt across the coil with the coil and the variable capacitor and intrinsic capacitance of the coil forming the tunable resonator circuit, with the first and second resonator circuits in combination with the passive tunable resonator circuit providing at an output of the second resonator circuit a voltage having a DC component. The circuit to inductively couple includes a magnetic field transmitter operating at the first harmonic or higher harmonic of the base transmitter frequency. The first resonator further includes a transmitter coil having a large number of windings N 1 , a variable capacitance device connected in shunt across the transmitter coil with the transmitter coil and the variable capacitor and intrinsic capacitance of the transmitter coil forming the resonator circuit, an input coil configured to be coupled to an input RF source, with the input coil having a low number of windings N 2  compared to the number of windings N 1  of the transmitter coil, and the input coil being inductively coupled to the transmitter coil to provide power output from the resonator circuit at a resonant frequency in a range of about 100 kHz to 30 MHz. The second resonator further includes a receiver coil inductively coupled to the transmitter coil, the receiver coil having a large number of windings N 3 , a variable capacitance device connected in shunt across the receiver coil with the receiver coil and the variable capacitor and intrinsic capacitance of the receiver coil forming the second resonator circuit, an output coil with a low number of windings N 2  compared to the number of windings N 1  of the receiver coil, the output coil being inductively coupled to the receiver coil to provide power output from the circuit, with the resonator circuit having a resonant frequency in a range of about 100 kHz to 30 MHz 
         [0018]    According to an additional aspect, a system for wireless energy transfer includes a table, a circuit embedded in the table for wireless transmission of energy, the circuit includes a first resonator circuit including a transmitter conductor and a variable capacitance device connected in shunt across the transmitter conductor with the transmitter coil and the variable capacitor and intrinsic capacitance of the transmitter coil forming the resonator circuit. 
         [0019]    The following are embodiments. The transmitter conductor is coupled to an input RF source to provide power output from the resonator circuit at a resonant frequency in a range of about 100 kHz to 30 MHz. 
         [0020]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0021]      FIG. 1  is a schematic of an inductive power link employing separately tunable transmitter/receiver coils. 
           [0022]      FIG. 2  is a perspective view of an embodiment of a self-shielding coil pair. 
           [0023]      FIG. 3  is a schematic of an inductive power link employing separately tunable transmitter/receiver coils and an intermediate passive resonator. 
           [0024]      FIGS. 4A and 4B  are diagrammatical illustrations of an effect that can be produced by the arrangement of  FIG. 3 . 
           [0025]      FIG. 5  is diagram of a charging arrangement. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Referring now to  FIG. 1  a wireless energy transfer arrangement  10  including a transmitter  11  and receiver  21  is shown. The transmitter  11  and receiver  21  include respective ones of tunable resonators  12 ,  22 , respectively, for transmission and reception of wireless power. These tunable resonators  12 ,  22  are electrically isolated from driver electronics  16  on the transmitter side  11  and from receiver electronics  26 , e.g., a rectifier circuit  32  and step-down DC/DC  34  converter on the receiver side  21 . 
         [0027]    The tunable transmitter resonator  12  includes a coil  15  (a transmitter coil) having a high number of windings N 2  and a coil  14  (input coil) with a relatively low number of windings N 1  where N 2  is about 10 to 1000 times greater than N 1 . The tunable transmitter resonator  12  also includes a variable capacitance element (variable capacitor)  18  coupled in shunt across the coil  15 . The variable capacitance element  18  allows the resonator  12  to be tuned, via an electrically isolated LC circuit (coil  15  and capacitor  18 ). The coil  15  having the relatively large number of windings N 2  compared to coil  14  having windings N 1 . The tunable transmitter resonator  12  is fed via the input coil  14 . The input coil  14  is fed an RF signal from RF source  16 . 
         [0028]    Typical bandwidth ranges, voltage ranges and power ranges are:
       frequency range: 100 kHz to 30 MHz;   voltage range: 5V to 48V;   power range: 1 mW to 100 W       
 
         [0032]    Other ranges and sub-ranges within the above ranges, such as 5 to 27 MHz are possible. 
         [0033]    The ratio of windings N 2 /N 1  permits a relatively low voltage at the input coil  14  to transfer electrical energy by inductively coupling to the transmitter coil  15  and produce a very high magnetic field at transmitter coil  15 . This high magnetic field inductively couples to the receiver resonator  22 , as will be discussed shortly. The tunable transmitter  12  thus has a resonant structure operating at a high-voltage, producing a strong magnetic field. 
         [0034]    A resonator is provided by use of air-core coils with an air-gap variable capacitor and variable tuning. Other configurations are possible depending on the desired properties including the bandwidth, voltage and power requirements for the tunable transmitter resonator  12 . The tunable receiver resonator  22  includes a coil  25  (receiver coil) having a high number of windings N 2  and a coil  24  (output coil) having a relatively low number of windings N 1 , where N 2  is about 10 to 1000 times greater than N 1 . The tunable receiver resonator  22  also includes a variable capacitance element (variable capacitor)  28  coupled in shunt across the coil  25 . The variable capacitance element  28  allows the resonator  22  to be tuned, via an electrically isolated LC circuit (coil  25  and capacitor  28 ). The coil  25  having the relatively large number of windings N 2  compared to coil  24  having windings N 1  permits the receiver resonator (coil  26  and capacitor  28 ) to efficiently couple via inductive coupling for coil  25  into the relatively high magnetic field produced by the transmitter resonator  12 , producing a relatively high voltage at the coil  25 . The coil  25  is inductively coupled to the coil  24 . 
         [0035]    In some embodiments, the coil  24  is magnetically shielded from the coil  15  so as not to allow significant inductive coupling between coil  15  in the transmitter and coil  24  in the receiver. Such magnetic shielding could be accomplished in various ways including placing the secondary coil within a region defined by and confined by the primary coil  25  on the receiver  21 . One such arrangement to magnetically shield the coil  24  in the receiver is illustrated in  FIG. 2 . 
         [0036]    Referring now to  FIG. 2  a structure  40  carrying two coils that can be used as part of the tunable resonators  12 ,  22  ( FIG. 1 ) is shown. The structure  40  includes a band  42  or a disc having a first coil (illustratively  25  in  FIG. 1 ) comprised of N 2  windings of a continuous, electrically isolated conductor  44  disposed on an outer surface  42   a  of the band  42 . The structure  40  also includes a second coil (illustratively  24  in  FIG. 1 ) comprised of N 1  windings (where N 2 &gt;&gt;N 1 ) of a continuous, electrically isolated conductor  46  disposed on an inner surface  42   b  of the band  42 . This structure  40  permits, e.g., tunable resonator  22  ( FIG. 1 ) having the first coil  25  disposed on the outer surface  42   a  of the band  42  to at least partially shield the second coil  24  from a field produced by other coils that might couple to the structure  40 , such as coil  15  from tunable resonators  12  ( FIG. 1 ) while permitting such a field to couple to coil  25  of the structure  40 . 
         [0037]    The tunable receiver resonator  22  receives energy via inductive coupling from the tunable transmitter resonator  12  and the tunable receiver resonator  22  inductively couples that energy to the coil  24 . An output voltage at the coil proportional to the ratio N 1 /N 2  is produced. This voltage is much lower than the voltage induced across coil  25  from tunable resonator  12 . This lower AC voltage at the coil  24  is rectified by a full wave rectifier  33  to produce a DC voltage. The capacitor  30  smoothes/filters this DC voltage and the DC/DC converter  32 , converts the DC voltage to a desired value according to input voltage requirements of a subsequent device such as a load  34 . As the load  34  draws current from the DC/DC converter circuit  32 , the voltage at the output of the coil  25  does not drop, but causes the DC/DC converter to increase current drawn through the coil  25 . The separately tunable resonators for both transmission and reception increase the operating range of the arrangement and improve the manufacturability of the arrangement in comparison to self-resonator coils that rely in intrinsic coil capacitance. The inclusion of a variable capacitor permits precise tuning and thus selection of the resonant frequency of the resonator. The inclusion of the secondary coils  14 ,  24  for transmission and reception respectively, reduces the voltage requirement at the RF source  16 , as well as the voltage requirements of the processing circuits, e.g., capacitor  30 , rectifier  33  and DC/DC converter  32  at the receiver. 
         [0038]    Because the power output is connected to the coil  24  having a low number of windings N 2 , the ratio between output voltage and resonator voltage is controlled by the ratio of the number of windings of the two coils, as: 
         [0000]        VL   (coil)   =V in* Q*N 2 /N 1 
         [0039]    Where Q is the quality factor of the resonator and N 2  and N 1  are respectively the number of windings for the coils  25  and  24 . For example, if Q=10, N 1 =100 and N 2 =10, the coil voltage will be the same as the input voltage, convenient to rectify and regulate. Another advantage of this arrangement is the ability to tune transmitter and receiver resonators  12 ,  22  separately to the oscillator/driver RF power source frequency rather than tuning the source to the resonator. This can enable tuning multiple receivers separately while powered by a single transmitter. Tuning can be performed manually with a variable capacitor, or automatically using a voltage-controlled capacitor, such as reverse-biased semiconductor junction. 
         [0040]    In an example, tunable AM-radio antenna resonators usable in a range of 550 kHz to 1600 kHz are used for the coil arrangement of  FIG. 1 . The transmitter coil is powered by an RF oscillator set at 1 MHz sinusoidal signal via 10 W Power Amplifier driver. The receiver coil can be displaced, e.g., 24″ away. The two resonators are tuned to the oscillator frequency and feature Q-factors of 10 each. 
         [0041]    Referring now to  FIG. 3 , an alternative enhancement uses a tunable passive resonator  62  comprising coil  64  and variable capacitor  68  with the resonator  62  disposed between the transmitter  11  of  FIG. 1 , and a receiver  21 ′ similar to that of receiver  21  of  FIG. 1  without the rectifier circuit. Here the tunable passive resonator  62  boosts received power and range by tuning the passive resonator  62  around the resonant frequency and providing phase control to bias a waveform on the receiver  11 ′ side to effectively rectify the RF signal coupled to the receiver  11 ′. The tunable passive resonator  62  enables resonant wireless power transfer systems to operate in the high radio frequency range and provide AC to DC power conversion without a rectifier circuit. The technique is applicable to alternating current to direct current conversion, as well as RF to DC (radio frequency to direct current) conversion using resonant circuits for transmission, conditioning and reception of wireless power. 
         [0042]    The resonator  62  is a LC tank circuit with resonant frequency f, driven by rectangular pulse oscillator and a power switch or amplifier. A passive LC repeater is tuned to the first harmonic  2   f  of the resonant frequency f. The two frequencies are mixed in the receiver to produce an essentially asymmetrical DC component in the output without a rectifier circuit. DC power is produced from AC source by mixing the two signals f and  2   f  to produce a signal having a sum output voltage along with a DC component. The two voltages with different frequency can be produced from the same AC or RF power source, using the base frequency f and the first harmonic  2   f.    
         [0043]    By controlling the phase shift between the two voltages, a higher amplitude positive or negative DC component can be produced at the output. Signals at the two frequencies are mixed at the power source or transmitted separately and mixed at the receiver. By using resonant circuits for transmitter and receiver, amplification of the output voltage can be achieved. That is, by mixing the two sinusoidal waveforms, f and  2   f  (as illustrated in  FIGS. 4A ,  4 B) an asymmetrical waveform with a DC component is produced. For example, mixing a frequency f with the first harmonic  2   f  with equal amplitudes can produce a sum with positive ( FIG. 4A ) or negative ( FIG. 4B ) high-amplitude, half-waves, depending on the phase shift between the two voltages. For instance, in  FIG. 4A , a 270° phase shift between the two source voltages results in positive peaks  69   a  whereas in  FIG. 4B  a 90° phase shift between the two source voltages results in negative peaks  69   b . Either the preponderance of the positive or negative peaks  69   a ,  69   b  can be used to directly charge a battery without the user of traditional rectifier and filter circuitry. 
         [0044]    Referring now to  FIG. 5 , in a practical application, a single-wire transmitter conductor  70  forms a loop disposed about a periphery of a structure such as a table  72 , as shown. The transmitter inductor  70  (corresponding to coil  15  in  FIG. 1 ) is coupled in parallel with a tuning capacitor  74  (corresponding to capacitor  18  in  FIG. 1 ) at two ends of the conductor. The receiver can be positioned within the area enclosed by the conductor  72  and about 1 m up or down from the surface of the table  72 . Such a single wire loop is fed via an RF source, (not shown) as discussed above, and could deliver power over a relatively large area providing sufficient power for charging of low-rate intermittent-usage devices  78 , such as household care appliances, e.g., cell phones, electric shavers, electric toothbrushes, etc. or other appliances with low average charging rate. 
         [0045]    This arrangement permits RF transmission that can charge multiple devices (and the emissions can be under acceptable limits, such as those established by for example the International Commission on Non-Ionizing Radiation Protection). With this arrangement, e.g., around 100 mW each (typically 30 mW needed) could be received by multiple devices over several cubic meters of space and resonant repeaters enhance power for small devices. In this arrangement the loop around the table provides a large surface area where devices having tunable receiver resonators ( 22  or  22 ′) can be positioned freely. 
         [0046]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, instead of the passive resonator a magnetic field transmitter operating at the first harmonic or higher harmonic of the base transmitter frequency can be used to provide effective rectification, as discussed above. 
         [0047]    Accordingly, other embodiments are within the scope of the following claims.