Impedance transformation network for improved driver circuit performance

This disclosure provides systems, methods and apparatus for reducing harmonic emissions. One aspect of the disclosure provides a transmitter apparatus. The transmitter apparatus includes a driver circuit characterized by an efficiency and a power output level. The driver circuit further includes a filter circuit electrically connected to the driver circuit and configured to modify the impedance of the transmit circuit to maintain the efficiency of the driver circuit at a level that is within 20% of a maximum efficiency of the driver circuit when the impedance is within the complex impedance range. The filter circuit is further configured to maintain a substantially constant power output level irrespective of the reactive variations within the complex impedance range. The filter circuit is further configured to maintain a substantially linear relationship between the power output level and the resistive variations within the impedance range.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Provisional Application No. 61/769,152, entitled “IMPEDANCE TRANSFORMATION NETWORK FOR IMPROVED DRIVER CIRCUIT PERFORMANCE,” filed Feb. 25, 2013, which is hereby expressly incorporated by reference herein. This application is related to U.S. patent application Ser. No. 13/424,834 entitled “FILTER FOR IMPROVED DRIVER CIRCUIT EFFICIENCY AND METHOD OF OPERATION” filed Mar. 20, 2012; and U.S. patent application Ser. No. 13/625,813 entitled “FILTER FOR IMPROVED DRIVER CIRCUIT EFFICIENCY AND METHOD OF OPERATION” filed Sep. 24, 2012; the disclosures of both of which are hereby incorporated by reference in their entirety.

FIELD

The present invention relates generally to wireless power. More specifically, the disclosure is directed to improving the efficiency and power output of transmit circuit driving a load varying over a wide resistive and reactive range.

BACKGROUND

An increasing number and variety of electronic devices are powered via rechargeable batteries. Such devices include mobile phones, portable music players, laptop computers, tablet computers, computer peripheral devices, communication devices (e.g., Bluetooth devices), digital cameras, hearing aids, and the like. While battery technology has improved, battery-powered electronic devices increasingly require and consume greater amounts of power. As such, these devices constantly require recharging. Rechargeable devices are often charged via wired connections that require cables or other similar connectors that are physically connected to a power supply. Cables and similar connectors may sometimes be inconvenient or cumbersome and have other drawbacks. Wireless charging systems that are capable of transferring power in free space to be used to charge rechargeable electronic devices may overcome some of the deficiencies of wired charging solutions. As such, wireless charging systems and methods that efficiently and safely transfer power for charging rechargeable electronic devices are desirable.

SUMMARY

One aspect of the subject matter described in the disclosure provides a transmitter apparatus. The transmitter apparatus includes a driver circuit characterized by an efficiency and a power output level. The driver circuit is electrically connected to a transmit circuit having an impedance. The impedance of the transmit circuit is within a complex impedance range including resistive and reactive variations. The complex impedance range is defined by a minimum real impedance value, a maximum real impedance, a minimum imaginary impedance value, and a maximum imaginary impedance value. A ratio between the minimum and maximum real impedance value is at least two to one. A magnitude of the difference between the maximum and minimum imaginary impedance values being at least twice a magnitude of the difference between the minimum and maximum real impedance values. The transmitter apparatus further includes a filter circuit electrically connected to the driver circuit and configured to modify the impedance of the transmit circuit to maintain the efficiency of the driver circuit at a level that is within 20% of a maximum efficiency of the driver circuit when the impedance is within the complex impedance range. The filter circuit is further configured to maintain a substantially constant power output level irrespective of the reactive variations within the complex impedance range. The filter circuit is further configured to maintain a substantially linear relationship between the power output level and the resistive variations within the complex impedance range.

Another aspect of the subject matter described in the disclosure provides a transmitter apparatus. The transmitter apparatus includes a driver circuit including a switching amplifier circuit comprising a switch, a switch shunt capacitor, and a series inductor electrically connected to the output of the driver circuit. The transmitter apparatus further includes a transmit circuit including a coil having an inductance electrically connected in series to a capacitor to form a resonant circuit. The transmitter apparatus further includes a filter circuit electrically connected between the driver circuit and the transmit circuit, the filter circuit comprising solely of a single shunt capacitor network.

Yet another aspect of the subject matter described in the disclosure provides a method of selecting component values of one or more reactive components of a filter circuit for a wireless power transmitter device. The filter circuit is electrically connected between a driver circuit and a transmit circuit. The method includes determining a first set of complex impedance values for which efficiency of the driver circuit is above a threshold. The first set of complex impedance values substantially map to complex impedance values along a half circle path. The method further includes determining a second set of complex impedance values for which power output of the driver circuit is substantially constant. The second set of complex impedance values substantially map to values along a full circle path that is orthogonal to the half circle and which crosses the half circle at a maximum. The method further includes selecting the component values to provide an impedance transformation that modifies a variable complex impedance of the transmit circuit to complex impedance values derived from the first and second sent of complex impedance values.

Another aspect of the subject matter described in the disclosure provides a transmitter apparatus. The transmitter apparatus includes a driver circuit characterized by an efficiency and a power output level. The driver circuit is electrically connected to a transmit circuit having an impedance. The impedance of the transmit circuit is within a complex impedance range including resistive and reactive variations. The transmitter apparatus further includes a filter circuit electrically connected to the driver circuit and configured to modify the impedance of the transmit circuit. The filter circuit has one or more reactive components with values selected derived from a first value and a second value. The first value, Rd, corresponds to a radius of a half circle. The half circle is defined by a set of complex impedance values along the perimeter of the half circle that correspond to values for which efficiency of the driver circuit is at least within 20% of the maximum efficiency of the driver circuit. The second value R0, corresponds to a real impedance value at the load of the filter circuit that results in a desired transformed impedance being equal to Rd at an input of the filter circuit.

DETAILED DESCRIPTION

Wirelessly transferring power may refer to transferring any form of energy associated with electric fields, magnetic fields, electromagnetic fields, or otherwise from a transmitter to a receiver without the use of physical electrical conductors (e.g., power may be transferred through free space). The power output into a wireless field (e.g., a magnetic field) may be received, captured by, or coupled by a “receiving coil” to achieve power transfer.

FIG. 1is a functional block diagram of an exemplary wireless power transfer system100, in accordance with exemplary embodiments of the invention. Input power102may be provided to a transmitter104from a power source (not shown) for generating a field106for providing energy transfer. A receiver108may couple to the field106and generate output power110for storing or consumption by a device (not shown) coupled to the output power110. Both the transmitter104and the receiver108are separated by a distance112. In one exemplary embodiment, transmitter104and receiver108are configured according to a mutual resonant relationship. When the resonant frequency of receiver108and the resonant frequency of transmitter104are substantially the same or very close, transmission losses between the transmitter104and the receiver108are minimal. As such, wireless power transfer may be provided over larger distance in contrast to purely inductive solutions that may require large coils that require coils to be very close (e.g., mms). Resonant inductive coupling techniques may thus allow for improved efficiency and power transfer over various distances and with a variety of inductive coil configurations.

The receiver108may receive power when the receiver108is located in an energy field106produced by the transmitter104. The field106corresponds to a region where energy output by the transmitter104may be captured by a receiver106. In some cases, the field106may correspond to the “near-field” of the transmitter104as will be further described below.

The transmitter104may include a transmit coil114for outputting an energy transmission. The receiver108further includes a receive coil118for receiving or capturing energy from the energy transmission. The near-field may correspond to a region in which there are strong reactive fields resulting from the currents and charges in the transmit coil114that minimally radiate power away from the transmit coil114. In some cases the near-field may correspond to a region that is within about one wavelength (or a fraction thereof) of the transmit coil114. The transmit and receive coils114and118are sized according to applications and devices to be associated therewith. As described above, efficient energy transfer may occur by coupling a large portion of the energy in a field106of the transmit coil114to a receive coil118rather than propagating most of the energy in an electromagnetic wave to the far field. When positioned within the field106, a “coupling mode” may be developed between the transmit coil114and the receive coil118. The area around the transmit and receive coils114and118where this coupling may occur is referred to herein as a coupling-mode region.

FIG. 2is a functional block diagram of exemplary components that may be used in the wireless power transfer system100ofFIG. 1, in accordance with various exemplary embodiments of the invention. The transmitter204may include transmit circuitry206that may include an oscillator222, a driver circuit224, and a filter and impedance transforming circuit226. The oscillator222may be configured to generate a signal at a desired frequency, such as 468.75 KHz, 6.78 MHz or 13.56 MHz, that may be adjusted in response to a frequency control signal223. The oscillator signal may be provided to a driver circuit224configured to drive the transmit coil214at, for example, a resonant frequency of the transmit coil214. The driver circuit224may be a switching amplifier configured to receive a square wave from the oscillator222and output a sine wave. For example, the driver circuit224may be a class E amplifier. A filter and impedance transforming circuit226may be also included to filter out harmonics or other unwanted frequencies and match the impedance of the transmitter204to the transmit coil214. The filter and impedance transforming circuit226may be configured to perform a variety of impedance adjustments other than just matching the impedance of the transmitter204to the transmit coil214.

The receiver208may include receive circuitry210that may include a matching circuit232(or any other type of impedance adjustment circuit) and a rectifier and switching circuit234to generate a DC power output from an AC power input to charge a battery236as shown inFIG. 2or to power a device (not shown) coupled to the receiver108. The matching circuit232may be included to match the impedance of the receive circuitry210to the receive coil218. The receiver208and transmitter204may additionally communicate on a separate communication channel219(e.g., Bluetooth, zigbee, cellular, etc.). The receiver208and transmitter204may alternatively communicate via in-band signaling using characteristics of the wireless field206.

As described more fully below, receiver208, that may initially have a selectively disablable associated load (e.g., battery236), may be configured to determine whether an amount of power transmitted by transmitter204and receiver by receiver208is appropriate for charging a battery236. Further, receiver208may be configured to enable a load (e.g., battery236) upon determining that the amount of power is appropriate. In some embodiments, a receiver208may be configured to directly utilize power received from a wireless power transfer field without charging of a battery236. For example, a communication device, such as a near-field communication (NFC) or radio-frequency identification device (RFID) may be configured to receive power from a wireless power transfer field and communicate by interacting with the wireless power transfer field and/or utilize the received power to communicate with a transmitter204or other devices

FIG. 3is a schematic diagram of a portion of transmit circuitry or receive circuitry ofFIG. 2including a transmit or receive coil352, in accordance with exemplary embodiments of the invention. As illustrated inFIG. 3, transmit circuitry350used in exemplary embodiments may include a coil352. The coil may also be referred to or be configured as a “loop” antenna352. The coil352may also be referred to herein or configured as a “magnetic” antenna or an induction coil. The term “coil” is intended to refer to a component that may wirelessly output or receive energy for coupling to another “coil.” The coil may also be referred to as an “antenna” of a type that is configured to wirelessly output or receive power. The coil may also be referred to as a wireless power transfer component of a type that is configured to wirelessly transmit or receive power. The coil352may be configured to include an air core or a physical core such as a ferrite core (not shown).

As stated, efficient transfer of energy between the transmitter104and receiver108may occur during matched or nearly matched resonance between the transmitter104and the receiver108. However, even when resonance between the transmitter104and receiver108are not matched, energy may be transferred, although the efficiency may be affected. Transfer of energy occurs by coupling energy from the field106of the transmitting coil to the receiving coil residing in the neighborhood where this field106is established rather than propagating the energy from the transmitting coil into free space.

The resonant frequency of the loop or magnetic coils is based on the inductance and capacitance. Inductance may be simply the inductance created by the coil352, whereas, capacitance may be added to the coil's inductance to create a resonant structure at a desired resonant frequency. As a non-limiting example, capacitor354and capacitor356may be added to the transmit circuitry350to create a resonant circuit that selects a signal358at a resonant frequency. Accordingly, for larger diameter coils, the size of capacitance needed to sustain resonance may decrease as the diameter or inductance of the loop increases. Furthermore, as the diameter of the coil increases, the efficient energy transfer area of the near-field may increase. Other resonant circuits formed using other components are also possible. As another non-limiting example, a capacitor may be placed in parallel between the two terminals of the coil350. For transmit coils, a signal358with a frequency that substantially corresponds to the resonant frequency of the coil352may be an input to the coil352.

In one embodiment, the transmitter104(FIG. 1) may be configured to output a time varying magnetic field with a frequency corresponding to the resonant frequency of the transmit coil114. When the receiver is within the field106, the time varying magnetic field may induce a current in the receive coil118. As described above, if the receive coil118is configured to be resonant at the frequency of the transmit coil118, energy may be efficiently transferred. The AC signal induced in the receive coil118may be rectified as described above to produce a DC signal that may be provided to charge or to power a load.

FIG. 4is a functional block diagram of a transmitter404that may be used in the wireless power transfer system ofFIG. 1, in accordance with exemplary embodiments of the invention. The transmitter404may include transmit circuitry406and a transmit coil414. The transmit coil414may be the coil352as shown inFIG. 3. Transmit circuitry406may provide RF power to the transmit coil414by providing an oscillating signal resulting in generation of energy (e.g., magnetic flux) about the transmit coil414. Transmitter404may operate at any suitable frequency. By way of example, transmitter404may operate at the 6.78 MHz ISM band.

Transmit circuitry406may include a fixed impedance matching circuit406for matching the impedance of the transmit circuitry406(e.g., 50 ohms) to the transmit coil414and a low pass filter (LPF)408configured to reduce harmonic emissions to levels to prevent self-jamming of devices coupled to receivers108(FIG. 1). Other exemplary embodiments may include different filter topologies, including but not limited to, notch filters that attenuate specific frequencies while passing others and may include an adaptive impedance match, that may be varied based on measurable transmit metrics, such as output power to the coil414or DC current drawn by the driver circuit424. Transmit circuitry406further includes a driver circuit424configured to drive an RF signal as determined by an oscillator422. The transmit circuitry406may be comprised of discrete devices or circuits, or alternately, may be comprised of an integrated assembly. An exemplary RF power output from transmit coil414may be on the order of 2.5 Watts.

Transmit circuitry406may further include a controller410for selectively enabling the oscillator422during transmit phases (or duty cycles), for adjusting the frequency or phase of the oscillator422, and for adjusting the output power level for implementing a communication protocol for interacting with neighboring devices through their attached receivers. It is noted that the controller410may also be referred to herein as processor410. Adjustment of oscillator phase and related circuitry in the transmission path may allow for reduction of out of band emissions.

The transmit circuitry406may further include a load sensing circuit416for detecting the presence or absence of active receivers in the vicinity of the near-field generated by transmit coil414. By way of example, a load sensing circuit416monitors the current flowing to the driver circuit424, that may be affected by the presence or absence of active receivers in the vicinity of the field generated by transmit coil414as will be further described below. Detection of changes to the loading on the driver circuit424are monitored by controller410for use in determining whether to enable the oscillator422for transmitting energy and to communicate with an active receiver. As described more fully below, a current measured at the driver circuit424may be used to determine whether an invalid device is positioned within a wireless power transfer region of the transmitter404.

The transmit coil414may be implemented with a Litz wire or as an antenna strip with the thickness, width and metal type selected to keep resistive losses low. In a one implementation, the transmit coil414may generally be configured for association with a larger structure such as a table, mat, lamp or other less portable configuration. Accordingly, the transmit coil414generally may not need “turns” in order to be of a practical dimension. An exemplary implementation of a transmit coil414may be “electrically small” (i.e., fraction of the wavelength) and tuned to resonate at lower usable frequencies by using capacitors to define the resonant frequency.

The transmitter404may gather and track information about the whereabouts and status of receiver devices that may be associated with the transmitter404. Thus, the transmitter circuitry404may include a presence detector480, an enclosed detector460, or a combination thereof, connected to the controller410(also referred to as a processor herein). The controller410may adjust an amount of power delivered by the driver circuit424in response to presence signals from the presence detector480and the enclosed detector460. The transmitter404may receive power through a number of power sources, such as, for example, an AC-DC converter (not shown) to convert conventional AC power present in a building, a DC-DC converter (not shown) to convert a conventional DC power source to a voltage suitable for the transmitter404, or directly from a conventional DC power source (not shown).

As a non-limiting example, the presence detector480may be a motion detector utilized to sense the initial presence of a device to be charged that is inserted into the coverage area of the transmitter404. After detection, the transmitter404may be turned on and the RF power received by the device may be used to toggle a switch on the Rx device in a pre-determined manner, which in turn results in changes to the driving point impedance of the transmitter404.

As another non-limiting example, the presence detector480may be a detector capable of detecting a human, for example, by infrared detection, motion detection, or other suitable means. In some exemplary embodiments, there may be regulations limiting the amount of power that a transmit coil414may transmit at a specific frequency. In some cases, these regulations are meant to protect humans from electromagnetic radiation. However, there may be environments where a transmit coil414is placed in areas not occupied by humans, or occupied infrequently by humans, such as, for example, garages, factory floors, shops, and the like. If these environments are free from humans, it may be permissible to increase the power output of the transmit coil414above the normal power restrictions regulations. In other words, the controller410may adjust the power output of the transmit coil414to a regulatory level or lower in response to human presence and adjust the power output of the transmit coil414to a level above the regulatory level when a human is outside a regulatory distance from the electromagnetic field of the transmit coil414.

As a non-limiting example, the enclosed detector460(may also be referred to herein as an enclosed compartment detector or an enclosed space detector) may be a device such as a sense switch for determining when an enclosure is in a closed or open state. When a transmitter is in an enclosure that is in an enclosed state, a power level of the transmitter may be increased.

In exemplary embodiments, a method by which the transmitter404does not remain on indefinitely may be used. In this case, the transmitter404may be programmed to shut off after a user-determined amount of time. This feature prevents the transmitter404, notably the driver circuit424, from running long after the wireless devices in its perimeter are fully charged. This event may be due to the failure of the circuit to detect the signal sent from either the repeater or the receive coil that a device is fully charged. To prevent the transmitter404from automatically shutting down if another device is placed in its perimeter, the transmitter404automatic shut off feature may be activated only after a set period of lack of motion detected in its perimeter. The user may be able to determine the inactivity time interval, and change it as desired. As a non-limiting example, the time interval may be longer than that needed to fully charge a specific type of wireless device under the assumption of the device being initially fully discharged.

FIG. 5is a functional block diagram of a receiver508that may be used in the wireless power transfer system ofFIG. 1, in accordance with exemplary embodiments of the invention. The receiver508includes receive circuitry510that may include a receive coil518. Receiver508further couples to device550for providing received power thereto. It should be noted that receiver508is illustrated as being external to device550but may be integrated into device550. Energy may be propagated wirelessly to receive coil518and then coupled through the rest of the receive circuitry510to device550. By way of example, the charging device may include devices such as mobile phones, portable music players, laptop computers, tablet computers, computer peripheral devices, communication devices (e.g., Bluetooth devices), digital cameras, hearing aids (and other medical devices), and the like.

Receive coil518may be tuned to resonate at the same frequency, or within a specified range of frequencies, as transmit coil414(FIG. 4). Receive coil518may be similarly dimensioned with transmit coil414or may be differently sized based upon the dimensions of the associated device550. By way of example, device550may be a portable electronic device having diametric or length dimension smaller that the diameter of length of transmit coil414. In such an example, receive coil518may be implemented as a multi-turn coil in order to reduce the capacitance value of a tuning capacitor (not shown) and increase the receive coil's impedance. By way of example, receive coil518may be placed around the substantial circumference of device550in order to maximize the coil diameter and reduce the number of loop turns (i.e., windings) of the receive coil518and the inter-winding capacitance.

Receive circuitry510may provide an impedance match to the receive coil518. Receive circuitry510includes power conversion circuitry506for converting a received RF energy source into charging power for use by the device550. Power conversion circuitry506includes an RF-to-DC converter508and may also in include a DC-to-DC converter510. RF-to-DC converter508rectifies the RF energy signal received at receive coil518into a non-alternating power with an output voltage represented by Vrect. The DC-to-DC converter510(or other power regulator) converts the rectified RF energy signal into an energy potential (e.g., voltage) that is compatible with device550with an output voltage and output current represented by Voutand Iout. Various RF-to-DC converters are contemplated, including partial and full rectifiers, regulators, bridges, doublers, as well as linear and switching converters.

Receive circuitry510may further include switching circuitry512for connecting receive coil518to the power conversion circuitry506or alternatively for disconnecting the power conversion circuitry506. Disconnecting receive coil518from power conversion circuitry506not only suspends charging of device550, but also changes the “load” as “seen” by the transmitter404(FIG. 2).

As disclosed above, transmitter404includes load sensing circuit416that may detect fluctuations in the bias current provided to transmitter power driver circuit410. Accordingly, transmitter404has a mechanism for determining when receivers are present in the transmitter's near-field.

When multiple receivers508are present in a transmitter's near-field, it may be desirable to time-multiplex the loading and unloading of one or more receivers to enable other receivers to more efficiently couple to the transmitter. A receiver508may also be cloaked in order to eliminate coupling to other nearby receivers or to reduce loading on nearby transmitters. This “unloading” of a receiver is also known herein as a “cloaking.” Furthermore, this switching between unloading and loading controlled by receiver508and detected by transmitter404may provide a communication mechanism from receiver508to transmitter404as is explained more fully below. Additionally, a protocol may be associated with the switching that enables the sending of a message from receiver508to transmitter404. By way of example, a switching speed may be on the order of 100 μsec.

In an exemplary embodiment, communication between the transmitter404and the receiver508refers to a device sensing and charging control mechanism, rather than conventional two-way communication (i.e., in band signaling using the coupling field). In other words, the transmitter404may use on/off keying of the transmitted signal to adjust whether energy is available in the near-field. The receiver may interpret these changes in energy as a message from the transmitter404. From the receiver side, the receiver508may use tuning and de-tuning of the receive coil518to adjust how much power is being accepted from the field. In some cases, the tuning and de-tuning may be accomplished via the switching circuitry512. The transmitter404may detect this difference in power used from the field and interpret these changes as a message from the receiver508. It is noted that other forms of modulation of the transmit power and the load behavior may be utilized.

Receive circuitry510may further include signaling detector and beacon circuitry514used to identify received energy fluctuations, that may correspond to informational signaling from the transmitter to the receiver. Furthermore, signaling and beacon circuitry514may also be used to detect the transmission of a reduced RF signal energy (i.e., a beacon signal) and to rectify the reduced RF signal energy into a nominal power for awakening either un-powered or power-depleted circuits within receive circuitry510in order to configure receive circuitry510for wireless charging.

Receive circuitry510further includes processor516for coordinating the processes of receiver508described herein including the control of switching circuitry512described herein. Cloaking of receiver508may also occur upon the occurrence of other events including detection of an external wired charging source (e.g., wall/USB power) providing charging power to device550. Processor516, in addition to controlling the cloaking of the receiver, may also monitor beacon circuitry514to determine a beacon state and extract messages sent from the transmitter404. Processor516may also adjust the DC-to-DC converter510for improved performance.

FIG. 6is a functional block diagram of an exemplary wireless power transfer system600as inFIG. 2, where a transmitter604may wirelessly provide power to multiple receivers608a,608b, and608c, in accordance with various exemplary embodiments of the invention. As shown inFIG. 6, a transmitter604may transmit power via a transmit coil614via a field606. Receiver devices608a,608b, and608cmay receive wireless power by coupling a portion of energy from the field606using receive coils618a,618b, and618cto charge or power respective loads636a,636b, and636c. Furthermore, the transmitter604may establish communication links619a,619b, and616cwith receivers618a,618b, and618crespectively. While three receivers608a,608b, and608care shown, additional receivers (not shown) may receive power from the transmitter604.

In a wireless power transfer system600the receivers608a,608b, or608cmay correspond to the load the transmitter drives while transferring power. As such, the load driven by the transmitter604may be a function of each receiver608a,608b, or608cthat is wirelessly receiving power from the field606. When receivers608a,608b, or608center the field606, leave the field, or disable or enable their capability to receive power from the field606, the complex load presented to the transmitter604is altered accordingly. Both resistive and reactive variations of the load are altered. The behavior of the transmitter604may be a function of characteristics of the variable complex load. For example, the efficiency at which the transmitter604may provide power to a receiver608a,608b, or608cmay vary as the complex load of the transmitter604varies. Furthermore, the amount of power that the transmitter604outputs may also vary as the complex load varies. Each of the receivers608a,608b, and608cmay form a portion of the load of the transmitter404when each receiver608a,608b, and608cis receiving power via the field606. The total impedance of the load seen by the transmit coil614may be a sum of impedances resulting from each receiver608a,608b, and608cas the impedances they present to the transmit circuit614may combine in series.

In one aspect, exemplary embodiments are directed to a transmitter604that is suitable for efficiently charging a dynamic number of receivers608a,608b, and608c. To efficiently allow for two receivers608aand608bto receive more power than when one receiver608ais positioned to receive power, the transmitter604may be preferably designed such that the load (characterized by its complex impedance) at which the maximum power may be delivered is lower than the load at which the maximum transmitter efficiency may be provided. Furthermore, the transmitter604may be preferably designed to provide power at high efficiency over a resistive and reactive range of complex load values as a variable number of receivers608a,608b, and608cwill result in a range of different loads being presented to the transmitter604. Otherwise, significant power losses may arise. Moreover, the transmitter604may be preferably designed such that the load at which maximum power is provided is greater than a total load presented by multiple receivers608a,608b, and608c. In this case, the transmitter604may have sufficient power to supply multiple devices simultaneously.

The transmit circuit may be driven by a driver circuit.FIG. 7is a schematic diagram of a driver circuit724that may be used in the transmitter604ofFIG. 6, in accordance with exemplary embodiments of the invention. As stated, the power output and efficiency of the driver circuit (e.g., a driver circuit424) varies as a function of the load presented to the driver circuit724. In some embodiments, the driver circuit724may be a switching amplifier. The driver circuit724may be configured to receive a square wave and output a sine wave to be provided to the transmit circuit750. The driver circuit724is shown as an ideal (i.e., no internal resistive losses) class E amplifier. The driver circuit724includes a switched shunt capacitor710and a series inductance708. VDis a DC source voltage applied to the driver circuit724that controls the maximum power that may be delivered into a series tuned load. The driver circuit724is driven by an oscillating input signal702to a switch704.

While the driver circuit724is shown as a class E amplifier, embodiments in accordance with the invention may use other types of driving circuits as may be known by those skilled in the art. A driver circuit724may be used to efficiently drive a load. The load may be a transmit circuit750configured to wirelessly transmit power. The transmit circuit750may include a series inductor714and capacitor716to form a resonant circuit as described above with reference toFIG. 3. While the load is shown as a transmit circuit750, embodiments in accordance with the invention may be applicable to other loads. As described above with reference toFIG. 6, the load presented to the transmit circuit750may be variable due to the number of wireless power receivers608a,608b, and608cand may be represented by a variable resistor712indicative of resistive variation of the load and a variable inductor712indicative of reactive variation of the load. The driver circuit724may be driven by an input signal702, such as from an oscillator222(FIG. 2). As the load presented to the transmit circuit750varies, for example, due to a dynamic number of wireless power receivers638a,638b, and638cas described above, a load presented to the driver circuit724may also vary according to a wide resistance and reactance range. For example, when an additional receiver638ais positioned to receive power from the transmit circuit750, the receiver638apicking up power increases the resistance presented to the transmit circuit750and therefore to the driver circuit724. In addition, adding a receiver638bthat includes certain material (e.g., metal) may result in a large reactance swing presented to the transmit circuit750and therefore to the driver circuit724. For example certain large receivers (e.g., a tablet) may present a negative reactance swing on the order of an excess of −j100. Power provided by the driver circuit724may not be flat across a load reactance range.

The load presented to the driver circuit724may be described by the impedance presented to the transmit circuit750including both resistive and reactive components and defined as Zin(TX)=Rin(TX)+jXin(TX). The value of Zin(TX) depends on various factors such as the transmit coil and receive coil structures, the type and number of devices to be charged, the power demanded by each receiver, and the like. The range of the load may be defined by four corner impedances:
RIN—TX—MIN≦Re{ZIN—TX}≦RIN—TX—MAX
XIN—TX—MIN≦Im{ZIN—TX}≦XIN—TX—MAX

FIG. 8Ais a diagram showing an exemplary range of impedances that may be presented to the transmit circuit750during operation.FIG. 8Ashows the corner impedances as described above. According to the type and number of devices to charge, the possible values for the corner impedances may vary widely. For example, for purposes of illustration, RIN—TX—MINmay be defined as 0Ω while RIN—TX—MAXmay be 75Ω. In addition, for purposes of illustration, XIN—TX—MINmay be defined as −50 jΩ while XIN—TX—MAXmay be +50 jΩ. In accordance with another embodiment, RIN—TX—MIN0Ω, RIN—TX—MAXis substantially 200Ω, XIN—TX—MINis substantially −200 jΩ, and XINT—TX—MAXis substantially +200 jΩ. The principles described herein may apply to these and other complex impedance ranges. According to another exemplary embodiment, in an operating mode the real load impedance (i.e., resistance) presented to the driver circuit724may fall between 1Ω and 40Ω. Additionally, in an operating mode, the imaginary load impedance (i.e., reactance) may be between 5j Ω and 48.7j (for example in the absence of multiple receivers). In another embodiment, impedances presented to the driver circuit724in an operating range may be from 4Ω to 40Ω and between −4jΩ and 50jΩ Due to, for example, a varying number of wireless power receivers or other factors, the driver circuit724may be presented loads with resistances in the 0 to 80Ω range and reactances from the −165jΩ to 95jΩ. It is desirable for the driver circuit724to operate efficiently and provide sufficient power to any load falling within this range. It is desirable to provide efficient and substantially constant power over all these ranges given various design considerations.

In one aspect, a range of impedance values presented to the driver circuit724may be defined by complex impedance values including real impedance values and imaginary impedance values. The real impedance values may be defined or characterized by a ratio between a first real impedance values to a second real impedance value. The ratio could be one of 2 to 1, 5 to 1, and 10 to 1. For example, the range of real impedance values presented to the driver circuit724could be between 8Ω and 80Ω (a ratio of 10:1). In another embodiment, the range could be between 4Ω and 40Ω (also a ratio of 10:1). In another embodiment, the range could be between substantially 1Ω and substantially 200Ω. In addition, the range of impedance values presented to the driver circuit724may be further defined by a range of imaginary impedance values. The range of the imaginary impedance values may be defined as a ratio of the magnitude of the imaginary impedance values (i.e., magnitude between minimum and maximum imaginary impedance values) to a magnitude of the real impedance values. For example, a magnitude of the real impedance values could be the magnitude of the difference between a first real impedance value and a second real impedance value. The ratio of the magnitude of the imaginary impedance values to the magnitude of the real impedance values may be at least one of 1:2, 2:1, 1:1, 2:3 etc. For example, if a real impedance range is between 8Ω and 80Ω, a magnitude may be 72Ω. As such, if the ratio of the magnitude of the imaginary impedance values to the magnitude of the real impedance values is 2 to 1, then the range of imaginary impedance values may be 144 (i.e., a range from −4jΩ to +140 jΩ). In any event, it is desirable to provide efficient and safe operation over a range of complex impedance values that may be defined according to various methods.

As described above, the power and efficiency of a driver circuit724are a function of the load the driver circuit724is driving.FIG. 8Bis a plot showing efficiency802and output power804of the driver circuit724ofFIG. 7as a function of the real impedance of a load (i.e., load resistance) of the driver circuit724. As shown inFIG. 8, 100% (or maximum) efficiency at a single real load impedance value may exist (e.g., 50Ω as shown inFIG. 8) for an ideal class E amplifier. The efficiency802decreases as the load impedance varies in either direction.FIG. 8also shows that the total output power804is similarly a function of the load impedance and which peaks at particular load impedance value (e.g., 20Ω). Similar results are described in Raab, “Effects of Circuit Variations on the class E Tuned Power Amplifier” (IEEE Journal of Solid State Circuits, Vol. SC-13, No. 2, 1978).

If the driver circuit724drives a load with a constant impedance, then the driver circuit724may be ideally designed (e.g., values of the capacitor710and inductor708, etc. may be chosen) such that the driver circuit724operates at maximum efficiency. For example, by using the values in the plot inFIG. 8B, if the driver circuit724is configured to drive a load with an unvarying impedance that is substantially equal to 50Ω, the driver circuit724may drive the load at a maximum efficiency level. However, if the load of the driver circuit724varies, then the average efficiency and power delivered by the driver circuit724may be significantly lower than its maximum efficiency or maximum power as shown inFIG. 8. Furthermore, as the impedance of the load increases, the power delivered may not increase.

As shown inFIG. 7and as described above, the load driven by the driver circuit724may be a wireless power transmit circuit750. The load presented to the transmit circuit750, given a varying number of wireless power receivers608a,608b,608c(FIG. 6), may thus vary the load seen by the driver circuit724. In this case, the total load impedance presented to the transmit circuit750may be the sum of each of the load impedances presented by each wireless power receiver608a,608b,608cas they may combine in series. Ideally, the driver circuit724would provide maximum efficiency over all loads while having the power increase linearly as the resistance of the load increases. Power would then be divided among the loads. However, as seen inFIG. 8B, maximum efficiency for the driver circuit724may occur for a single real load impedance value.

One aspect of exemplary embodiments are directed to achieving high efficiency of the driver circuit724as the real load impedance varies while also increasing power as the load resistance increases. In one aspect, this may allow for efficient wireless power transfer for a variable number of wireless power receivers608a,608b, and608c. To provide improved efficiency of a variety of loads, the efficiency of a class E amplifier724is analyzed over variations in both a real component of the load impedance (i.e., resistance) and the imaginary component of the load (i.e., reactance).FIG. 9is a contour plot showing the efficiency of a driver circuit724as inFIG. 7as a function of the real and imaginary components of the load impedance presented to the driver circuit724. The plot may correspond to a driver circuit724that is designed to have a maximum efficiency for a load with a resistance of 15Ω and a reactance of 0Ω In the illustrated embodiment, the drive voltage is 15 V. The complex load plot ofFIG. 9shows efficiency contours906a,906b, and906cat increments of 5%. For example, points along the contour906amay represent the combinations of resistance and reactance values that correspond to a load for which the class E amplifier is 95%. The contour902corresponds to load impedance values that correspond to an efficiency 100%.

The results of the plot shown inFIG. 8Bmay be seen inFIG. 9by holding the reactance at zero and varying the resistance from 0 to 40Ω as shown by the arrow908. The path908passes through the point904with a value of 15Ω+j0Ω where efficiency is 100%. The contour902shows that there is a path (e.g., a range of impedances) at which efficiency is 100%. As such, rather than just analyzing efficiency over real impedance values only, analyzing efficiency for both real and imaginary impedance values (i.e., a range of resistance and reactance values) shows that there is a range of complex impedance values for which efficiency of the driver circuit724is 100%.

FIG. 10Ais a contour plot showing the power output of a driver circuit724as inFIG. 7as a function of real and imaginary components of the load impedance presented to the driver circuit724. The complex load plot ofFIG. 10Ashows power contours1006a,1006b, and1006cat 1 watt increments. For example, points along the contour1006bmay represent combinations of resistance values and reactance values that represent an impedance value at which 5 watts of power may be delivered. Points along the contour1006cmay represent combinations of resistance values and reactance values that represent an impedance value at which 10 watts of power may be delivered. The results of the plot shown inFIG. 8Bmay be seen by holding the reactance at zero and varying the resistance from 0Ω to 40Ω as shown by the arrow1008. The path1008passes through the point1004where efficiency (shown by the contour902fromFIG. 9) is 100% and power delivered is a little over 6 Watts. The 100% efficiency contour902ofFIG. 9placed in the plot ofFIG. 10shows that there is path902where efficiency is 100% and where the power continually increases as shown as the contours represent increasing power. As shown inFIGS. 9 and 10, the 100% efficiency path902starts at an impedance of j24Ω, passes through 15+j0Ω and continues to −j10Ω.

FIG. 10Bis another plot showing power output and efficiency of a driver circuit724as inFIG. 7as a function of real and imaginary components of the load impedance presented to the driver circuit724. The contour902represents the combinations of resistance and reactance values for which efficiency is maximum, where efficiency may be defined as the ratio of power delivered to the load divided by the DC power into the FET drain of the driver circuit724. The contour1006represents the resistance and reactance values for which power delivered to the load is constant for a particular drive voltage Vd. While power may depend on both the drive voltage Vdand the complex load, efficiency may depend on the load alone. The drive voltage Vdand power output may be chosen such that the power contour1006passes through the efficiency contour902at the peak value of the load to the FET of the driver circuit724. Accordingly, the contours1006and902are exemplary and indicate that there is a range of complex values for which the driver circuit724is efficient and for which power is constant. It is noted that the contours shown inFIG. 10Bmay reflect the load seen by the FET of the driver circuit724, Zload(FET) which may be offset from results that would be in terms of Zin(TX). The difference may be due to a series inductance. However, Zload(FET) and Zload may be used interchangeably.FIG. 10Bshows one particular results for when a drive voltage is on the order of 10 volts and resulting power is on the order of 2.45 Watts. In this case values were selected to result in the power contour1006passing through the efficiency circle at the peak value of Rload(FET) as shown. As a result when the Zload is 16.55+24.3j ohms, at 10 V dc, a single sided driver circuit724delivers 2.45 watts with maximum efficiency into the real part of the load. These values are merely exemplary and for purposes of illustration of values that may be found to define the maximum efficiency and constant power contours.

As indicated above, however, a wide range of reactive and resistive impedances may be presented to the driver circuit724from a transmit circuit750due to, for example, a variable number of receivers being positioned to receive power from the transmit circuit750. Impedance values presented to the driver circuit724as a result of the variation of impedance presented to the transmit circuit750may result in reduced efficiency and fluctuations in the amount of power delivered.

FIGS. 11A,11B,12A, and12B show corresponding measured results as compared toFIG. 10showing power output and efficiency of a driver circuit724as inFIG. 7as a function of real and imaginary components of the load impedance presented to the driver circuit724. The measured results show power and efficiency contours as described above and when taking into account various losses or other effects of the system. For example, the results may illustrate the effects of losses of the FET of the driver circuit (e.g., as compared to an ideal switch) and the effect of the resonant series LC circuit of the transmit circuit750between the switch and the load. As suchFIGS. 11A,11B,12A and12B show measured efficiency and power contours when all load values within the range defined by RIN—TX—MIN=0Ω, RIN—TX—MAX=75Ω, XIN—TX—MIN=−50 jΩ, and XIN—TX—MAX=+50 jΩ are presented to a driver circuit724. The results may reflect the output from a double-sided driver circuit724(i.e., including two driver circuits724ofFIG. 7) operating a particular driver voltage (e.g., both driven by a 5.5 V DC supply). In using the double-sided driver circuit724, load to each FET of the driver circuit724is ½ that seen inFIGS. 11A,11B,12A, and12B.

In accordance,FIG. 11Ashows measured efficiency contours for a tuned double-sided driver circuit724without an added series inductance708for the load range as noted above.FIG. 11Bshows measured efficiency contours for a tuned double-sided driver circuit724with an added series inductance708. In accordanceFIG. 12Ashows measured power contours for a tuned double-sided driver circuit724without an added series inductance708for a wide load range as described above.FIG. 11Bshows measured power contours for a tuned double-sided driver circuit724with an added series inductance708. Thus,FIGS. 11A-Bshown different powers for the same load, illustrating the effect of VDon power.FIGS. 12A-Bshow output power.

It is noted that the measured efficiency and power contours ofFIGS. 11A,11B,12A, and12B are merely exemplary for a particular configuration of driver circuit724and are used for purposes of illustration. More specifically,FIGS. 11A,11B,12A, and12B provide values illustrating the range of complex impedance values for which the driver circuit724is efficient and for which power is constant. It is noted that the contour plots ofFIGS. 11A,11B,12A, and12B may correspond to results from a double-sided driver circuit724configuration. To compare with a single sided driver circuit724, the values would be divided by two. As compared toFIG. 10B, it is noted that the constant power contours shown inFIGS. 12A and 12Bare still circular in form, although distorted due to the effect of body diodes inherent in FET704. Likewise, the efficiency contours shown inFIG. 11Aare similar toFIG. 10B, however efficiency values decrease as the load resistance to the FET decreases which may be due to losses in the FET704.

Based on the results ofFIGS. 9-12, certain aspects of exemplary embodiments are directed to an impedance transform circuit (also referred to herein as a filter circuit) located between the driver circuit724and the transmit circuit750that transforms a variable load impedance presented to the transmit circuit750into values for which the driver circuit724is highly efficient and where power is substantially constant. These values may be defined by the high efficiency and constant power contours as shown inFIGS. 9-12. The variable load presented to the transmit circuit750may vary widely over both reactance and resistance as further described above. As will be indicated below, the transform circuit is configured to transform the impedance to maintain the efficiency at a high level while keeping the power transfer as constant as possible over a wide range in reactive load. This may allow for a driver circuit724in a wireless power transmitter604to efficiently provide power as the load presented to the transmit circuit750varies reactively and resistively due to a dynamic number of wireless power receivers608a,608b, and608c(FIG. 6).

In one embodiment, a filter circuit is used to transform a variable load impedance presented to a transmit circuit750into complex load values for which the driver circuit724may be highly efficient and for which power is constant.FIG. 13is a schematic diagram of a driver circuit1324as inFIG. 7including a filter circuit1326, in accordance with exemplary embodiments of the invention. The filter circuit1326(i.e., a transform circuit) is positioned between the driver circuit1324and the transmit circuit1350. As shown the transmit circuit1350shows a variable load1312including both variations in resistance and variations in reactance. The filer circuit1326includes three reactive components, X11328, X21330, and X31332. In some embodiments, X11328and X31332are series inductors, while X21330is a shunt capacitor. The filter circuit1326is configured to transform the impedance Zin(TX) presented by the transmit circuit1350into an impedance Zload(XFRM) presented to the driver circuit1324including the series inductor1308. This impedance, Zload(XFRM) is then shifted by the series inductance1308so as to “best” fit the load lines for which the driver circuit1324is maximally efficient with a constant or steady power as the load varies. The full transformed impedance presented to the FET1304is defined by Zload(FET). The component values for the filter circuit1326and series inductance1308are configured to increase power linearly with the resistive portion of the load presented the transmit circuit1350and have as high efficiency as possible as the reactance varies. The values of the components are selected to provide a balance between maximizing efficiency versus variation in the resistive portion Rin(TX) and minimizing load power variation versus variation in the reactive portion Xin(TX).

While also operating as a low pass filter configured to reduce harmonics in the signal, the filter circuit1326is configured to convert linear variations in Zin(TX) to circular variations in Zin(XFRM). As noted above, the series reactance1308shifts the load into a particular range for the FET1304. The filter circuit1326is configured as a T network. While other configurations are also contemplated according to the embodiments described herein, the T network may reduce the number of components.

It is noted that according to some embodiments, where series inductors are used for reactance components X11328and X31332, high power requirements may increase the cost of the inductors. As such, it may be desirable to eliminate one or more of the reactance components. Using the T network may allow for eliminating the reactance components X11328and X31332which may be absorbed into the coil1314and/or the series inductor1308.FIG. 14is a schematic diagram of the circuit ofFIG. 13in accordance with an embodiment. As shown, reactance component X21330ofFIG. 13is shown as a shunt capacitor1430. Reactance components X1 and X3 ofFIG. 14are absorbed into the coil1414and series inductor1408. As such, the filter circuit1426includes only of a shunt capacitor. It is noted that the values of X1 and X3 are still selected based on the principles described herein to achieve the desired impedance transformation, and as such, the amount of the determined values of X1 and X3 is taken into account when being absorbed into the other series elements. It is noted that the shunt capacitor1430could be replaced by a single shunt capacitor network (e.g., some configuration of parallel or series connected capacitors). However, only a shunt network is needed. This may reduce the number of components and reduce other high cost inductors. The values of the reactive components1410,1408,1430,1416, and1414are selected such that the circuit1406is configured to transform the impedance into values that correspond to high efficiency for the driver circuit1424with constant power. For example, the shunt capacitor1430is selected based on an impedance transformation ratio which is proportional to output power or current at a fixed supply voltage. The series L value1408may then be selected from the shunt C value. As such the circuit1406may work efficiently across a wide reactance range while maintaining its output power, and reactance load switches to tune out the reactance swing may be unnecessary. As such, a single shunt capacitor network positioned between a class E amplifier and a resonant transmit circuit, when combined with tuning adjustments of the existing inductances, can provide a particular impedance transform as described herein. This includes delivering power linearly versus the real load over a wide reactance range, having reduced sensitivity to the reactive load changes and providing high efficiency for substantially maximum power. Furthermore, the value of the shunt capacitor1430may be selected to determine an impedance ratio of the filter transform that may allow trading the impedance ratio off against other factors such as mutual coupling between a source coil (i.e., transmitter) and the load coil (i.e., receiver).

With reference again toFIG. 13, values of the components of the filter circuit1326may be chosen such that the varying impedance of the transmit circuit1350(due to receivers608a,608b, and608c) is transformed by the filter circuit1326. The transformed impedance values may correspond to impedance values (such as those as shown inFIGS. 9 and 10) that provide highly efficient driver circuit1324operation while also increasing the flatness of power delivery given wide reactance swings. The component values of the filter circuit1326are chosen to perform an impedance transform that transforms the impedance of the load1312seen by the transmit coil1350into a complex impedance that fits as closely as possible to complex values that provide high efficiency and constant power as shown inFIGS. 9-12. In some embodiments as will be further described below, the selection of the series inductance1308of the driver circuit1324is used in conjunction with the filter circuit1326to shift the impedance transformation performed by the filter circuit1326to match as closely as possible to complex values that provide high efficiency and constant power.

In one exemplary embodiment, the filter circuit1326may be configured to modify the impedance presented to the filter circuit1326(e.g., the impedance of the transmit circuit1350due to a variable number of receivers608a,608b, and608c) to maintain the efficiency of a driver circuit1324at a level that is within 20% of a maximum efficiency of the driver circuit1324within some complex impedance range with real and reactive variations. In another embodiment, efficiency may be maintained at a level that is within 10% or lower of a maximum efficiency of the driver circuit1324. The filter circuit1326is further configured to maintain a substantially constant power output level irrespective of the reactive variations within the complex impedance range presented from the transmit circuit1350. Moreover, the filter circuit1326is configured to maintain a substantially linear relationship between the power output level and the resistive variations within the complex impedance range. The filter circuit1326may be referred to as or be configured as an impedance transformation network. The range of impedance values presented to the filter circuit1326that are transformed by the filter circuit1326may be characterized by a range of complex impedance values. The range of complex impedance values may be within a range defined by a first real impedance value and a second real impedance value, where a ratio between the first real impedance value to the second real impedance value is at least two to one. In addition, the range of reactive variation of the impedance may be related to the real impedance range. For example, the magnitude of the range of reactive variation for which the filter is configured to maintain efficiency at 20% of a maximum efficiency may be substantially twice the magnitude of the real impedance range. The center of the reactive impedance range may be substantially centered at the instantaneous real impedance value (i.e., resistive) presented to the transmit circuit. The filter circuit1326is further configured to maintain a substantially constant power level irrespective of the reactive variations within the impedance range. In addition, the filter circuit1326is configured to maintain a substantially linear relationship between the power level and the real variations within the impedance range. For example, the range of real (i.e., resistive) impedance values may be substantially between 8Ω and 80Ω or 4Ω and 40Ω having a ratio of 10 to 1. In this case, the range of imaginary (i.e., reactive) may have a magnitude on the order of 144 and could span anywhere from −74 jΩ to +152 jΩ depending on an instantaneous value of the real impedance. In another embodiment, the range of real impedance values may be between substantially 1Ω and substantially 200Ω. Furthermore, in an embodiment, the range of imaginary impedances may be between substantially −200 jΩ and +200 j jΩ. Within this range of real and imaginary impedances (i.e., resistive and reactive), the filter circuit1326and selection of series inductance1308is configured to maintain the efficiency of the driver circuit1324within 20% of a maximum efficiency of the driver circuit1324, maintain a substantially constant power level irrespective of the reactive variations, and/or maintain a substantially linear relationship between the power level and the real variations within the impedance range.

The imaginary impedance range may also be defined by a first imaginary impedance value and a second imaginary impedance value. The first imaginary impedance value and the second imaginary impedance value may define approximate minimum and maximum imaginary impedance values. The range of imaginary impedance values (i.e., the magnitude of the difference between the first imaginary impedance value and the second imaginary impedance value) may be defined by a ratio of the magnitude of the imaginary impedance value to the magnitude of the real impedance value (e.g., equal to a magnitude of the difference between the first real impedance value and the second real impedance value). The ratio may be at least one of 1:2, 2:1, 1:1, 2:3, 3:2, etc. For example, if the magnitude of the real impedance values is 72Ω, and the ratio is 2:1, the magnitude of the range of imaginary impedance values may be 144 jΩ (e.g., a range of a minimum to a maximum). In another example, in one embodiment, the first real impedance value may be substantially 4Ω, the second real impedance value may be substantially 40Ω, the first imaginary impedance value may be substantially −4 jΩ, and the second imaginary impedance value may be substantially j50Ω. A wide range of complex impedance values may be presented to the filter circuit1126given the design parameters and the potential number of receivers. As such, ranges and ratios contemplated by various exemplary embodiments described herein may substantially vary from the specific examples provided herein.

According to certain embodiments, a passive or fixed filter circuit1326(i.e., substantially all of the components of the filter circuit1326may be passive circuit elements) as shown inFIG. 13may be provided. The circuitry would have an absence of dynamic switching in of reactive elements. As such, the filter circuit1125may not require control signals or other dynamic logic to control or configure the circuit as the load changes during operation. This may reduce cost and complexity and may provide other benefits as will be appreciated by one/those skilled in the art.

In some embodiments, a driver circuit1324may generate harmonics of 6.78 MHz, when the operating frequency of the driver circuits1324is substantially 6.78 MHz. For various reasons, including for meeting regulatory requirements, the filter circuit1326may be further configured to reduce unwanted harmonics produced by the driver circuit1324. By using information derived from the plots such asFIGS. 9 and 10, the filter circuit1326may be designed (in various embodiments) to meet spectral emission mask requirements (via reducing harmonics), ensure that the load impedance at which maximum power may be delivered is above the load at which maximum efficiency is achieved, and/or expand the range of load impedance values for which the driver circuit1324is highly efficient.

Exemplary Filter Circuit Operation

FIG. 15is a plot showing the impedance transformed by the filter circuit1326versus the impedance presented to the transmit circuit1350as mapped to a high efficiency contour. For example, to illustrate the operation of the filter circuit1326,FIG. 15shows an exemplary result of a filter circuit1326configured to match the theoretical single sided driver circuit724and having the load contours shown inFIG. 10B. While specific values are described, it is noted that these values are merely exemplary and for purposes of illustration of the operation of the filter circuit for one particular driver circuit configuration, and a wide variety of other values are contemplated according to the principles described herein. Two circles1502and1504are shown inFIG. 15. One circle1502, centered at Rin(XFRM)=0+0 j, defines Zin(XFRM) for an Rin(TX) variation from 0 to 1000 ohms, in 5 ohm steps, with Xin(TX)=0. The path proceeds in a counter clockwise direction as Rin(TX) increases. The radius of this circle (defined as Rd) is 16.55 ohms, selected to match the 100% efficiency theoretical contour902ofFIG. 10B. Since the markers are for Rin(TX) 5 ohms apart, it is noted that Zin(XFRM) peaks when Zin(TX)=25+j0 Ohms. This defines the impedance transform ratio of the circuit, and can be set as desired. For variations in Rin(TX) given other values of Xin(TX), it is noted that the ratios result in circles that all pass through Zin(XFRM)=0, −16.55j.

The second circle1505, which is orthogonal to the first circle1502, defines Zin(XFRM) for an Xin(TX) variation from −1000 to 1000 in 5 ohm steps, when Rin(TX) is held constant at 25 Ohms. It is centered at Zin(XFRM)=16.55−16.55j Ohms which forces these two curves to intersect at Zin(XFRM)=16.55+0j ohms. For variations in Xin(TX) given other values of Rin(TX), it is noted that these result in circles that pass through Zin(XFRM)=0, −16.55j. This result is shown inFIG. 16, where Rin(TX) is varied over a wider range than specified.

The impedance seen by the FET1308may be the same plot ofFIG. 16shifted to the right by the added series inductance1308. The results shown inFIG. 16further shows that the curves that make up Zin of the transform are independent of the transform selected. Only the mapping of Zin(XFRM) to Zin(TX) changes.

Given the observation fromFIG. 12, to select the value of the series inductance1308, it is decided where to locate the point where all the curves, as seen by the FET1304, meet. For example,FIG. 10Bshows impedances at FET1304to achieve 100% efficiency. In this case, Rd was 16.55 Ohms, and the theoretical intercept point is at Zin(FET)=0+7.74j Ohms.

FIG. 17shows the path for an Rd equal to twice 16.55 Ohms, which would reduce the shunt capacitor1310on the FET1304also by a factor of 2. In this case the intercept is at Zload(FET)=0+15.5j Ohms.

To improve the match to the FET1304and thus achieve maximum efficiency as Rin(TX) varies (but only for Xin(TX)=0), a series reactance1308is added between the filter circuit1602and the FET1304.FIG. 18shows a final result, superimposed on the theoretical power and efficiency contours ofFIG. 10B.

In an ideal result, the efficiency is 100% for all Rin(TX) when Xin(TX)=0, but the constant power contour is not a perfect fit. For Vd=10 Vdc, the power is predicted to be at a constant amount along the dotted path, and where the curves cross, but will increase for non-zero Xin(TX), since higher power regions are ever smaller circles located inside one another.

As such,FIGS. 15-18illustrate the operation of the impedance transformation by the filter circuit1326having components with configured values in accordance with exemplary embodiments.

Method of Selecting Filter Circuit Components

As indicated, the components of the filter circuit1326are selected to perform the desired impedance transform as described above. In accordance with an embodiment, a method is provided for determining values of the components of the filter circuit1326that provides the described impedance transform. In one aspect, the method is derived from the relationship that defines the transform from Zin(TX) to Zin(XFRM). Zin(TX) may be defined as R+jX and Zin(XFRM) as Zin(R,X). In this form Zin(R,X) is:

According to the method, two variables are used to determine the filter circuit reactances, X11328, X21330, and X31332. The first, variable Rd that best fits Zin(R,X) to a measured efficiency contour as R varies while X is fixed at 0 ohms. The half circle defining these values is centered at Zin(R,X)=0+j*0 and reaches a maximum of Rd at some Zin(TX)=Rin0+j*0. The variable R0, which is defined as the value of R, when X=0, that results in Zin(R0, 0)=Rd. This defines the impedance transform ratio of the filter circuit1326, which may be done at one point, since this transform may be non-linear.

The method includes determining the efficiency and power contours vs. complex load at the FET1304. This may be using a direct measurement to account for losses as described above. For example, exemplary contours are shown inFIGS. 11A,11B,12A, and12B that could be used according to this method. Note the test results to according to the method include the capacitor1310that shunts the FET1304. The total capacitance at this node may determine the maximum efficiency region.

The method further includes determining if increasing efficiency vs. real load or holding power constant vs. imaginary load is more preferable according to the particular desired operation. This sets the placement of the circles generated by the transform including X11328, X21330and X31332. The series reactance1308needed for input shifting may be determined subsequently. The series reactance needed for output (load) shifting can be accomplished by detuning the TX coil1314.

The method further includes selecting the half circle path radius, Rd as described above, for Zin(R,0) that will best fit the measured efficiency contours. As noted, this half circle is centered at Zin(R,X)=0+j*0 and reaches a maximum of Rd at some Zin(TX)=Rin0+j*0.

The method further includes determining the full circle path that crosses this maximum value Rd of the half circle path. This circle is orthogonal to the constant efficiency contour, and lies approximately along constant power contours. For Zin=Rin0+j*Xin, where Rin0 is fixed and Xin is variable, the resulting Zin(Rin0,Xin) will lie along this circle.

The method further includes selecting the value for R0 of the impedance where Zload will peak and is equal to Rd+0j. From Rd and R0, the values for X11328, X21330and X31332may be determined according to the following equations.
X1(R0):=(2·Rd·R0)0.5−Rd
X2(R0):=−(2·Rd·R0)0.5
X3(R0):=(2·Rd·R0)0.5−R0

In accordance, given Rd and R0, Zin(R,X) equals:

Zin⁡(R,X):=-j·Rd·[R+j·(R0+X)R-j·(R0+X)]
Example of Filter Circuit Configuration

FIGS. 19-21show results for on exemplary filter circuit1326configuration in accordance with an embodiment. It is noted while specific numerical values are provided, the values are to illustrate on example of a selection of components values for a filter circuit1326, and other values for the components may be used in accordance with the principles described herein. The values of the filter circuit1326are configured to provide substantially constant power over a wide range in the reactive load, X. The values are indicated as 2 times the values used for each side of the driver circuit724for a double-sided configuration. The values for Rd and R0 for this exemplary filter were found to be 17.6 and 23.3 respectively.

For the FET1308, the value for Cshunt1310is selected as 2*287 pf. This includes each FET capacitance, estimated at 80 pf. For the filter circuit1326it was determined to maximize the fit to the constant power curves, for example as inFIGS. 12A and 12B. This results in an offset from the maximum efficiency path and this fit comes by adding a series inductance1308. FromFIGS. 11A and 11B, the choice for the circle radius is 35.2 ohms. The resulting FET load paths peaks at Rin(TX)=46.6 ohms, as desired.

The resulting transformed impedance for a double sided filter circuit1326over the specified Zin(TX) load range is shown inFIG. 19. The transformed impedance is then shifted to align with the desired FET load, by Lseries1308of 2*516 nH. The double sided filter circuit1326includes of a 2*820 nH series inductance, an 820/2 pf shunt capacitor and a 2*25 nH series inductance. This final series inductor can be realized simply by off-tuning the TX coil1314by +10.6 ohms.

It is noted that “straight line” impedances corresponding to variations in the real impedance for a particular reactance are transformed into orthogonal circles. This will be true for any filter circuit1326as shown inFIG. 13. Furthermore, it is noted that Zin(XFRM) given Zin(TX)=Rin(TX)+0*j. For this result, Rin(TX) varies as 0, 5, . . . 75 ohms. Ideally Zin(ZFRM) reaches a peak of 35.2 ohms, with X=0 when Rin(TX)=45 ohms. In addition, the peak value and location of Rin(XFRM) changes as Xin(TX) changes. This is why the transform is designed with Xin(TX)=0. Furthermore, the transform with Zin(TX)=45 ohms +j Xin(TX). Xin(TX) varies as −50, −40 . . . 50 ohms. Other points ofFIG. 19show all possible points that can be outcomes given the specified range of Zin(TX).

In order to overlay the transform prediction on the data ofFIGS. 11A,11B,11C, and11D, the transform ofFIG. 19may be shifted to match the measured results from the load box. There are two reactive shifts involved. The first shift is to add two series reactances between the FETs and their filter circuits (for double-sided configuration). This added reactance (Xshift) is combined with X11328of the filter circuit1326so that the total series inductance on each side is about 820 nH.

The second shift is to remove the two 600 nH series inductances used to test the driver circuit without any transform. After these shifts, the transformed paths can be overlaid on the power and efficiency contours measured for the FETs alone.

FIG. 20shows the paths of the filter circuit1326for this example overlaid on the measured data ofFIG. 12B. As before, the half circle is for fixed X=0, and variable R, and the full circle is with fixed Rin(TX)=45 ohms and variable X. As can be seen the power vs. X contour is expected to be rather constant, while the power vs. variable R keeps increasing with R. It is noted that the power where the lines cross can be seen to be about 1.3 Watts, delivered into a Rload(FET) of 35 ohms with Vd=5.5 Volts. By increasing Vd to 10 Volts, this power should increase to about 4.3 Watts.

FIG. 21shows the paths of the filter circuit1326for this example overlaid on the measured data ofFIG. 11B. As before, the half circle is for fixed X, and variable R, and the full circle is with fixed R and variable X. As can be seen, the efficiency is not at the maximum for Rload=46, Xload=0. The maximum occurs around Rload=30, Xload=−20, which does lie on the path of increasing Rin(TX).

It is noted that in addition to fitting the power and efficiency contours, the methods described herein may allow to set the impedance ratio of the transformer.

In addition to the systems and methods disclosed herein above, attached is one appendix: Appendix A. Appendix A is in 20 (twenty) pages describing various configurations of a filter circuit1326that may be used by one or more of the methods and systems disclosed herein. While the appendix may illustrate filter circuits having particular component values, it is noted that the values are provided for illustration of the operation and performance of the impedance transform and that the component values for other embodiments selected according to the principles described herein may vary widely.

Further design characteristics of the driver circuit1324that may be used may include the driver circuit characteristic impedance, input voltage, and series reactance. In accordance with some embodiments, a variety of characteristics of the filter circuit1326may be chosen to arrive at a desired impedance transformation that may be correlated with the high efficiency curve902and power contour. Characteristics of the filter circuit1326may include the number of desired poles, the type of filter circuit, or a number of stacked filter circuits. The filter circuit1326may be a ladder network of reactive elements that may take a variety of forms. For example, the ladder network may comprise multiple reactive stages (i.e., reactive circuits) each including a combination of reactive components. Any of single value or multiple values of the ladder network may be adjusted based on a desired response. Some filter circuits may be less desirable such as a filter circuit1326that creates a simple reactance shift regardless of the characteristic impedance chosen. The ladder network may also include—more than three reactive elements, in which case all these elements may be varied using a common parameter. Using multiple elements can greatly reduce harmonics. However, it some cases it may be desirable to reduce the number of components for the filter circuit, for example, as described above with reference toFIG. 14.

Furthermore, as described above, the prototype class, the type of filter, the cutoff frequency, and the characteristic impedance of the filter circuit1326may also be configured to achieve a desired impedance response that can be used. The prototype class may indicate how the component values are chosen based on the other parameters. The type of filter circuit1326can be a low pass, high pass, band pass, notch, or combination thereof. The cutoff frequency may be a 3 dB attenuation point, although the cutoff frequency may vary depending on the prototype class. The characteristic impedance may be the target real impedance of the filter circuit1326if this were being used in a single impedance circuit (e.g., a 50Ω RF circuit).

According to one embodiment, given a set of several of these characteristics (e.g., selecting the driver circuit design and driver circuit filter series reactance1108), non-selected characteristics (e.g., a filter circuit1326design) may be derived that allows the system to perform an impedance transformation of a range of real and reactive load impedances that transforms the real and reactive load impedance to a value for which the driver circuit1324is highly efficient and for which power is constant.

Another method may include applying the reverse transformation of the series reactance and ladder network (i.e., filter circuit1326) to the high efficiency curve902and constant power.

As indicated herein, the driver circuit1324may behave as an ideal AC current source (with a source impedance above some ratio of the maximum real load) that supplies a constant current for a range of impedances regardless of the impedance in that range presented to the driver circuit1324. The particular constant current may be chosen based on the combinations of characteristics used. As such, the wireless power transmitter404may be able to source more power as the resistance (i.e., real impedance) increases.

Once the component values of the filter circuit1326has been determined, if the elements of the filter circuits are altered with different impedance values, the impedance transformation might not result in the desired impedances for which the driver circuit1326is efficient and for achieving constant power. For example, an “undesirable” transformation may result from using low tolerance components, for example, components which could vary 20 percent. In this filter design, while using similar components, a change in impedance value can fail to result in impedance transformation that allows for the driver circuit1324to be efficient when a variety of complex impedances are presented to the driver circuit1324by the transmit circuit1350.

According to the configuration of the filter circuit, the values of the impedances of the various elements are particularly chosen in such a manner to achieve the desired transformation as described above. Altering these impedance values for any given element, even by 5%, results in a significantly different impedance transformation. As such, the given impedance transformation as described above that results in maintaining the driver circuit1124at a high efficiency (e.g., within 20% of the maximum efficiency) is achieved after careful selection of impedance values for the filter elements according to the principles described herein.

FIG. 22is a flowchart of an exemplary method for designing a highly efficient transmit circuit. The transmit circuitry may be configured for wirelessly outputting power to charge or power a receiver device. In block2202, a driver circuit1324may be selected that is configured to operate at an efficiency threshold over a first range of complex impedance values presented by a load to the driver circuit1324. Based on the characteristics chosen, in block2204, a filter circuit1326may be selected that is configured to perform an impedance transformation to transform an impedance presented to the filter circuit1326to a second range of complex impedance values that is correlated to the first range of complex impedance values. In block2206, an impedance adjustment element1308may be selected that is configured to shift the second range of complex impedance values such that the impedance presented to the driver circuit1324is substantially equivalent to impedance values of the first range of complex impedance values.

It should be further appreciated that the filter circuit1326may be configured to transform impedance values for other types of loads other than a transmit circuit1350and thus principles of various embodiments may be practice with a wide variety of loads. As such, embodiments described herein are not limited to providing wireless power, and exemplary embodiments in accordance with the invention may be applied in other situations where a driver circuit1324may drive a variable load of any type having a range of impedance values. In some embodiments, the transmit circuit1350may include a transmit coil (or loop antenna) configured to resonate at a frequency of the signal provided by the driver circuit1350. The transmit circuit1350may be configured to wirelessly output power to charge or power a receiver608a,608b, and/or608cas described above. The transmit circuit1350may further be configured to wirelessly transmit power to a plurality of receivers608a,608b, and608c. Each of the receivers608a,608b, and608cmay alter the impedance seen by the transmit circuit1350such that the transmit circuit1350may include a wide range of complex impedance values that may be transformed by the filter circuit1326. The filter circuit1326may transform the impedance value into a value that has a non-zero reactance such that it is a complex impedance value with a real portion corresponding to resistance and an imaginary portion corresponding to a reactance.

In some embodiments, the filter circuit1326may be a passive circuit and may not require added logic or control signals to operate. The filter circuit1326may be a low pass filter circuit1324. It should be appreciated that a wide variety of filter circuit configurations may be used in accordance with exemplary embodiments and may be selected as described according to the principles herein.

The amount of power provided by the driver circuit1324may be configured to increase as an amount of the resistive portion of the impedance seen by the driver circuit1324increases. This may allow for continually delivering higher power while maintaining efficiency as more wireless power receivers608a,608b, and608creceive power from the transmit circuit1350. Furthermore, the filter circuit1326may allow such that a magnitude of the impedance seen by driver circuit1324at which maximum power may be provided is higher than the magnitude of the impedance seen by the driver circuit1324at which maximum efficiency of the driver circuit1324is provided. As such, the driver circuit1324may perform as a constant current source over a range of resistances (i.e., real impedance values). As described above, the driver circuit1324may be a class E amplifier or other amplifier such as switching amplifier. The driver circuit1324may include other types of amplifiers as described above. Of note, however, in certain embodiments a class D circuit would act like a voltage source.

It should be further appreciated that while shown as a filter circuit1326, other types of circuits, components, or modules may be used to perform the type of impedance transformation as described above to transform a range of impedance values into a complex value for which a driver circuit1324is highly efficient, in accordance with the principles described herein.

As described with reference toFIG. 13, one of the functions of the filter circuit1326is to remove unwanted harmonics produced by the driver circuit1324. In one aspect, the harmonics may result in undesired emissions from the transmit circuit1350. As such, the filter circuit1326may be configured to reduce emissions from the transmit circuit1350to reduce emissions to meet spectral emission requirements as well as perform the impedance transformation as described above. For example, as described above, the filter circuit1326may be a low pass filter that is seventh order capable to reject radiated emissions and conducted emissions of the transmit circuit1350and reduce coupling from a receiver to the transmitter. In one embodiment, the filter circuit1326may be configured to reduce/reject radiated emissions and conducted emissions of the transmitter between substantially 20-250 MHz. It should be appreciated that the filter circuit1326may further be configured to reject emissions in other frequency ranges according to different applications and power requirements or different operating frequencies.

FIG. 23is another schematic diagram of portion of transmit circuitry2300, in accordance with an exemplary embodiment. The transmit circuitry2300shows an example comprises dual driver circuits2324in addition to other components for reduction of emissions. As shown, a first filter circuit2360and a bypass capacitor2370configured to reduce emissions from the driver circuit2324and the transmit circuit2350to the power source2302are included. In some embodiments, the transmit circuitry2300can include a shunt capacitor on VDD. The inductors at2360may be a common mode choke. The driver circuit2324includes dual class E amplifiers2324which drive the transmit circuit2350via a third filter circuit2323as shown. The third filter circuit2323(dual filter circuits) is configured perform the impedance transformation as described above. The third filter circuit2323can be further configured to reduce emissions of the transmit circuit2350, as described above.

FIG. 24shows measured results for power output a driver circuit1324as inFIG. 13as a function of real and imaginary components of the load impedance presented to the driver circuit1324. The measured results show power contours as described above and when the impedance Zload(XFRM) is presented to the driver circuit1324. As shown, the power output is approximately independent of Xload, while increasing vs. Rload at any Xload.

The various operations of methods described above may be performed by any suitable means capable of performing the operations, such as various hardware and/or software component(s), circuits, and/or module(s). Generally, any operations illustrated in the Figures may be performed by corresponding functional means capable of performing the operations. For example, means of transmitting may include a transmit circuit. Means for driving may include a driver circuit. Means for filtering may comprise a filter circuit.

In one or more exemplary embodiments, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer readable medium. Computer readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A storage media may be any available media that may be accessed by a computer. By way of example, and not limitation, such computer readable media may comprise RAM, ROM, EEPROM, CD ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that may be used to carry or store desired program code in the form of instructions or data structures and that may be accessed by a computer. Also, any connection is properly termed a computer readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer readable media.