Load impedance detection for static or dynamic adjustment of passive loads

This disclosure provides systems, methods and apparatus for detecting an impedance of a wireless power transmitter load. In one aspect, a method of determining a reactive condition of a wireless power transmitter apparatus is provided. The method comprises determining a value correlated to a voltage of a drain of a switching element of a driver circuit of the wireless power transmitter. The method further comprises determining a reactance load change based on the determined voltage.

FIELD

The present invention relates generally to wireless power. More specifically, the disclosure is directed to detecting the load impedance of a wireless power transmitter.

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 through 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 or provide power to electronic devices may overcome some of the deficiencies of wired charging solutions. As such, wireless power transfer systems and methods that efficiently and safely transfer power to electronic devices are desirable.

SUMMARY OF THE INVENTION

One aspect of the disclosure provides a method of operating a wireless power transmitter apparatus. The method includes determining a characteristic correlated to a voltage of a terminal of a switching element of a driver circuit of the wireless power transmitter. The method further includes determining a reactance of a load based on the determined characteristic.

Another aspect of the disclosure provides an apparatus for wireless power transmission. The apparatus includes means for determining a characteristic correlated to a voltage of a terminal of a switching element of a driver circuit of the wireless power transmitter. The apparatus further includes means for determining a reactance of a load based on the determined characteristic.

Another aspect of the disclosure provides a non-transitory computer-readable medium. The medium includes code that, when executed, causes an apparatus to determine a characteristic correlated to a voltage of a terminal of a switching element of a driver circuit of a wireless power transmitter. The medium further includes code that, when executed, causes the apparatus to determine a reactance of a load based on the determined characteristic.

Another aspect of the disclosure provides an apparatus configured to determine a reactive condition of a wireless power transmitter apparatus. The apparatus comprises a drain voltage input, a threshold voltage input, a gate drive voltage input, an output. The apparatus further comprises a comparator configured to compare the drain voltage input and the threshold voltage input. The comparator is further configured to output a digital signal indicative of whether the drain voltage input is greater than the threshold voltage input. The apparatus further comprises a flip-flop configured to receive the digital signal at a data input. The flip-flop is further configured to receive the gate drive voltage input at a clock input. The flip-flop is further configured to sample the digital signal on a rising or falling edge of the synchronized gate drive signal. The flip-flop is further configured to output an inverted or non-inverted version of the sampled voltage at the output.

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 do not 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 matching 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 oscillator22and output a sine wave. For example, the driver circuit224may be a class E amplifier. A filter and matching circuit226may be also included to filter out harmonics or other unwanted frequencies and match the impedance of the transmitter204to the transmit coil214.

The receiver108may include receive circuitry210that may include a matching circuit232and 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 coil352may be configured to include an air core or a physical core such as a ferrite core (not shown). Air core loop coils may be more tolerable to extraneous physical devices placed in the vicinity of the core. Furthermore, an air core loop coil352allows the placement of other components within the core area. In addition, an air core loop may more readily enable placement of the receive coil218(FIG. 2) within a plane of the transmit coil214(FIG. 2) where the coupled-mode region of the transmit coil214(FIG. 2) may be more powerful.

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, capacitor352and capacitor354may be added to the transmit circuitry350to create a resonant circuit that selects a signal356at 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 transmitter104may 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 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 13.56 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 power amplifier. Transmit circuitry406further includes a driver circuit424configured to drive an RF signal as determined by an oscillator423. 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 oscillator423during transmit phases (or duty cycles) for specific receivers, for adjusting the frequency or phase of the oscillator423, 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, especially when transitioning from one frequency to another.

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 oscillator423for 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 amplifier424in 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 transmitter. 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 (an 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 converter520and 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 amplifier 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 schematic diagram of a portion of transmit circuitry600that may be used in the transmit circuitry406ofFIG. 4. The transmit circuitry600may include a driver circuit624as described above inFIG. 4. As described above, the driver circuit624may be a switching amplifier that may be configured to receive a square wave and output a sine wave to be provided to the transmit circuit650. In some cases the driver circuit624may be referred to as an amplifier circuit. The driver circuit624is shown as a class E amplifier, however, any suitable driver circuit624may be used in accordance with embodiments of the invention. The driver circuit624may be driven by an input signal602from an oscillator423as shown inFIG. 4. The driver circuit624may also be provided with a drive voltage VDthat is configured to control the maximum power that may be delivered through a transmit circuit650. To eliminate or reduce harmonics, the transmit circuitry600may include a filter circuit626. The filter circuit626may be a three pole (C632, L630, C634) low pass filter circuit626.

The signal output by the filter circuit626may be provided to a transmit circuit650comprising a coil614. The transmit circuit650may include a series resonant circuit having a capacitance612and inductance (e.g., that may be due to the inductance or capacitance of the coil614) that may resonate at a frequency of the filtered signal provided by the driver circuit624. The load of the transmit circuit650may be represented by a variable impedance618. The variable impedance618may include, for example, any combination of one or more variable resistors, variable capacitors, variable inductors, or other electronic elements. The load may be a function of a wireless power receiver508that is positioned to receive power from the transmit circuit650.

The load presented the driver circuit624may have a reactance that varies due to, for example, a variable number of wireless power receivers that are positioned to receive power. The reactance of the load may vary widely for a loosely coupled wireless power transfer system100. The efficiency of a driver circuit624may be sensitive to and vary due to changes in the load reactance. For example, Class E amplifiers may be sensitive to the loads placed on them, and may be damaged if the load changes excessively in either real or imaginary impedance. The switching devices may be damaged by overvoltage, over-current, or over-temperature operation.

Over-temperature operation can be caused by several issues, including a change in load. A class E amplifier may be highly efficient at one set of complex impedances. At this set of impedances, the class E amplifier exhibits zero-voltage switching behavior with a simple 50% gate drive duty cycle. The class E may turn on at zero voltage and the voltage returns to zero at the moment it shuts off. This may allow for an efficient switching operation.

FIG. 7shows a plot of voltage values across a device with different load characteristics. InFIG. 7, curve B represents the voltage across the device at ideal loads (i.e., optimal switching), curve A represents an excessively capacitive load, and curve C represents an excessively inductive load. Outside this range of ideal impedances, zero voltage switching may not occur. When the load is excessively capacitive, for example, the ideal gate turn on point may occur sooner than the 50% point, and thus the device may be driven backwards into reverse conduction if a fixed gate drive is used. This results in inefficiencies since the switching devices are typically more lossy in the reverse direction. When the load is excessively inductive, the ideal gate turn on point may come after the 50% point, and thus the device may be forced to switch at a non-zero voltage. This increases switching losses.

In an embodiment, the efficiency of a class E power amplifier (PA), such as the driver circuit624(FIG. 6), depends primarily on the shape of the PA field effect transistor (FET)604drain voltage, for example, as shown inFIG. 7. When the pulse width of the drain voltage matches the duty cycle of the driver circuit625, the PA is operating at maximum efficiency (curve B). In an embodiment, the pulse starts when the FET604turns off, so changing the pulse width changes the timing of the falling edge of the pulse. When the pulse is thin, efficiency drops slowly (curve A). When the pulse it is too wide it gets truncated by the FET604turning on, causing losses and possible damage of the FET604(curve C).

When taken to extremes, these additional losses can cause excessive heating and device failure. According to one embodiment, to avoid excessive heating and device failure, and to allow the class E to operate into non-ideal loads with higher efficiency, the driver circuit624and an impedance transformation circuit (such as the filter circuit626) may be designed to run efficiently over a large range of resistance values at a particular reactance. The system may become inefficient, however, as the reactance shifts. As there may be a wide variety of reactance presented to the system, it may be desirable to detect a change in the load reactance in order to, for example, shift the load reactance into an acceptable range.

FIG. 8is a diagram showing a wireless power transfer system100operating range.FIG. 8shows the wireless power transfer system100operating over a range of reactance values with a driver circuit624efficient operating range that may be effectively shifted to cover the system operating range. As shown, the driver circuit624operating range (defined by the reactances for which the driver circuit624is adequately efficient) may be relatively small as compared to a desired system operating range. The transmitter404may have an impedance detection circuit that may be configured to detect an amount of reactance of the load or at least be able to determine whether the reactance falls within a particular range. To effectively increase the efficient operating range of the driver circuit624, a switching network may be used to adjust the reactance of the load to a value that falls within the range for which the driver circuit624is efficient. The switching network may provide reactance shifts based on the output of the impedance detection circuit.

In an embodiment, the detection circuitry can provide a binary feedback indicative of whether the load is within an acceptable range. For example, the detection circuitry may provide an output indicative of an acceptable load when the driver circuit624can achieve at least a threshold efficiency given the detected load. On the other hand, the detection circuitry may provide an output indicative of an unacceptable load when the driver circuit624will not achieve at least a threshold efficiency given the detected load. In another embodiment, the detection circuitry may provide an output indicative of whether the load value is too inductive, too capacitive or acceptable. In another embodiment, the detection circuitry may provide a continuous spectrum of feedback, the output indicating how inductive or capacitive the load is. A continuous output may allow a switching controller to can switch directly to the correct state, rather than iterating towards it.

FIG. 9is a schematic diagram of an impedance detection circuit900, according to an embodiment. As shown, the impedance detection circuit900includes a drain input910, a first voltage divider920, a threshold input930, a second voltage divider940, a comparator945, an output filter950, and an output960. In an embodiment, when the pulse width of the voltage at the FET604drain (FIG. 6) is thin, it can be detected by converting the drain voltage910pulse into a square wave using a fast comparator920to compare with a threshold voltage930, then averaging the resulting square wave with a filter950to get an analog value.

The drain input910serves to receive the voltage pulse at the drain of an amplifier. In an embodiment, the drain input910can receive the drain voltage from FET604(FIG. 6). In an embodiment, the drain input910can be filtered by the first voltage divider920. The first voltage divider920serves to divide the voltage of the drain input910, impedance match the input, and output the result into the comparator945.

The threshold input930serves to receive a threshold voltage indicative of a pulse at the drain input910. In various embodiments, the threshold input930can receive the threshold voltage from the input voltage602(FIG. 6), a rectified envelope of the drain voltage910, or a fixed voltage supply. In an embodiment, the second voltage divider940serves to divide the voltage of the threshold input930and to output the divided voltage into the comparator945.

The comparator945serves to measure the drain pulse width by comparing the voltage divided version of the drain input910with the threshold input930. The comparator945can be configured to output a square wave. For example, the comparator945can output a high voltage signal when the positive input is at or above the negative input, and can output a low voltage signal when the positive input is below the negative input.

The output filter950serves to average the square wave output of the comparator945. The resulting analog voltage can be fed into a microcontroller (not shown). Depending on the threshold input930voltage, the pulse width feedback may not increase monotonically. In many cases, the pulse width may go back down after the pulse width exceeds 50%. This may be due to a slower rising slope of the pulse making the threshold crossing point move while the falling slope is constrained by the FET604switching action. Moreover, this method may be less effective when the pulse width becomes greater than the drive duty cycle, because the pulse may become truncated as the FET604turns on.

FIG. 10is a schematic diagram of an impedance detection circuit1000, according to another embodiment. As shown, the impedance detection circuit1000includes a drain input1010, a voltage divider1020, a first operational amplifier1030, a gate drive input1040, a delay circuit1050, a sampling switch1060, a sampling capacitor1065, a second operational amplifier1070, and an output1080. In the illustrated embodiment, the drain voltage at the drain input1010can be sampled at the moment the FET604turns on. In various embodiments, the drain input1010can be sampled at substantially the same time as the transition time of the FET604, but it may precede or follow the transition. For example, the absolute difference between the sampling time and the transition time of the FET604can be less than about 10% of the oscillator423frequency, less than about 5% of the oscillator423frequency, or more particularly, less than about 1% of the oscillator423frequency. In various embodiments, it may not be feasible to directly measure the switching voltage without a potentially expensive high speed analog-to-digital converter (ADC). In the illustrated embodiment, the impedance detection circuit1000is a sample-hold circuit. The sample-hold impedance detection circuit1000can be synchronized with the FET drive signal602(FIG. 6) and can sample the drain input1010in a very short period of time. This sampled value can then be read by a low-speed ADC built into a microcontroller (not shown).

The drain input1010serves to receive the voltage pulse at the drain of an amplifier. In an embodiment, the drain input1010can receive the drain voltage from FET604(FIG. 6). In an embodiment, the drain input1010can be reduced by the voltage divider1020. The voltage divider1020serves to divide the voltage of the drain input1010, impedance match the input, and output the result into the first operational amplifier1030. The first operational amplifier1030can serve as a voltage buffer for the divided voltage of the drain input1010. In an embodiment, the first operational amplifier1030may receive the divided voltage of the drain input1010at a non-inverting input, and may feed back a first operational amplifier output to an inverting input.

The gate drive input1040serves to receive the FET drive signal602(FIG. 6). The delay circuit1050may receive the FET drive signal603from the gate drive input1040and delay the FET drive signal603sufficient to synchronize the FET drive signal603and the output of the first operational amplifier1030at the sampling switch1060. In various embodiments, the delay circuit1050may include one or more buffers and/or inverters. The sampling switch1060serves to sample the output of the first operational amplifier1030when it receives the synchronized gate drive input1040from the delay circuit1050. In an embodiment, the sampling switch1060can include a pass-gate.

The sampling capacitor1065serves to store the voltage output by the first operational amplifier1030. When the sampling switch1060is closed, the sampling capacitor1065may receive the voltage output by the first operational amplifier1030. When the sampling switch1060is open, the sampling capacitor1065may continue to store the voltage output by the first operational amplifier1030at a non-inverting input of the second operational amplifier1070. The second operational amplifier1070can serve as a voltage buffer for the sampled drain input1010. In an embodiment, the second operational amplifier1070may receive the divided voltage of the drain input1010from the sampling capacitor1065at a non-inverting input, and may feed back a second operational amplifier output to an inverting input. The second operational amplifier1070can output the sampled voltage at the output1080.

In the illustrated embodiment, the further past the duty cycle the pulse continues, the higher the sampled voltage. One potential downside of this approach is the cost of analog sample and hold circuitry. In an embodiment, if continuous detection is not desired, the technique can be simplified by using digital sample and hold. In an embodiment where only the region of operation is determined, detection can be simplified by using a comparator and digital flip-flops to sample the state rather than an analog value.

FIG. 11is a schematic diagram of an impedance detection circuit1100, according to an embodiment. As shown, the impedance detection circuit1100includes a drain input1110, a drain filter1120, a threshold input1130, a voltage divider1140, a comparator1145, a gate drive input1150, a delay circuit1160, a flip-flop1170, and an output1150. In the illustrated embodiment, the pulse width of the voltage at the FET604drain (FIG. 6) can be converted to digital values using the comparator1145as described above with respect toFIG. 9. Instead of averaging the output, the digital value can be sampled by the flip-flop1170clocked with a specific delay. The illustrated embodiment may handle wide pulses because the FET604may not be a perfect switch.

The drain input1110serves to receive the voltage pulse at the drain of an amplifier. In an embodiment, the drain input1110can receive the drain voltage from FET604(FIG. 6). In an embodiment, the drain input1110can be filtered by the drain filter1120. The drain filter1120serves to divide the voltage of the drain input1110, filter the pulse, and output the result into the comparator1145.

The threshold input1130serves to receive a threshold voltage indicative of a pulse at the drain input1110. In various embodiments, the threshold input1130can receive the threshold voltage from the input voltage602(FIG. 6), a rectified envelope of the drain voltage1110, or a fixed voltage supply. In an embodiment, the voltage divider1140serves to divide the voltage of the threshold input1130and to output the divided voltage into the comparator1145.

The comparator1145serves to measure the drain pulse width by comparing the voltage divided version of the drain input1110with the threshold input1130. The comparator1145can be configured to output a square wave. For example, the comparator1145can output a high voltage signal when the positive input is at or above the negative input, and can output a low voltage signal when the positive input is below the negative input.

The gate drive input1150serves to receive the FET drive signal602(FIG. 6). The delay circuit1160may receive the FET drive signal603from the gate drive input1150and delay the FET drive signal603sufficient to synchronize the FET drive signal603and the output of the comparator1145at a data input of the flip-flop1170. In various embodiments, the delay circuit1160may include one or more buffers and/or inverters. In one embodiment, the delay circuit1160can be omitted.

The flip-flop1170serves to sample the output of the comparator1145at the data input. In an embodiment, the flip-flop1170can be a d-flip-flop. The flip-flop1170may sample the data input at a rising or falling edge on a clock input. The clock input of the flip-flop1170may receive the delayed gate drive input1150from the delay circuit1160. By using the envelope of the drain voltage as the reference voltage for the pulse width detector the resulting pulse width peaks then begins dropping after the clipping occurs. By sampling the digital value coming out of the comparator1145with some delay after the gate drive switches, the side of the pulse width curve that is being measured can be determined.

FIG. 12is a flowchart of an exemplary method1200of detecting a load impedance of a wireless power transmitter. Although the method of flowchart1200is described herein with reference to the circuit1100discussed above with respect toFIG. 11, a person having ordinary skill in the art will appreciate that the method of flowchart1200may be implemented by the transmitter104discussed above with respect toFIG. 1, the transmitter204discussed above with respect toFIG. 2, and/or any other suitable device. In an embodiment, the steps in flowchart1200may be performed by a processor or controller in conjunction with one or more of the comparator1145, the delay circuit1160, and the flip-flop1170. Although the method of flowchart1200is described herein with reference to a particular order, in various embodiments, blocks herein may be performed in a different order, or omitted, and additional blocks may be added.

In block1202, a detector determines a characteristic correlated to a voltage of a terminal of a switching element of a driver circuit of the wireless power transmitter. In various embodiments, the detector can include the impedance detection circuit900,1000, and/or1100, described above with respect toFIGS. 9,10, and11respectively.

In an embodiment, the characteristic is a digital characteristic indicating a voltage level above or below a limit, based on the reactance of the load. For example, the characteristic can be the output of the comparator945or1145, described above with respect toFIGS. 9 and 11respectively. The flip-flop1170can sample the digital characteristic using a flip-flop clocked after the switching element turns on. In an embodiment, the flip-flop1170is clocked with a delay. In various embodiments, the flip-flop1170can sample the digital characteristic at substantially the same time as the transition time of the switching element, but it may precede or follow the transition. For example, the absolute difference between the sampling time and the transition time of the FET604can be less than about 10% of the oscillator423frequency, less than about 5% of the oscillator423frequency, or more particularly, less than about 1% of the oscillator423frequency.

In another embodiment, the characteristic is an analog characteristic indicating the reactance of the load. For example, the characteristic can be the output of the op-amp1030, described above with respect toFIG. 10. The switch1060(FIG. 10) and the capacitor1065can sample and hold the analog characteristic for a period of time.

In another embodiment, determining the characteristic includes comparing the voltage to a threshold voltage.

In another embodiment, the characteristic can be the voltage at the drain of the transistor604(FIG. 6). Determining the characteristic can include comparing the characteristic to a threshold characteristic. For example, the comparator945or1145, described above with respect toFIGS. 9 and 11respectively, can compare the drain voltage to a threshold voltage.

In various embodiments, the load can include a transmit circuit and/or resonator. In an embodiment, the terminal is a source terminal. In another embodiment, the terminal is a drain terminal.

In block1204, a controller may determine a reactance load change based on the determined voltage. For example, a change in the characteristic over time can indicate a change in reactance load. In an embodiment, the controller may adjust an impedance of a transmit circuit based on the determined reactance load change.

FIG. 13is a functional block diagram of a load impedance detector1300, in accordance with an exemplary embodiment of the invention. The load impedance detector1300includes means1302for determining a characteristic correlated to a voltage of a terminal of a switching element of a driver circuit of the wireless power transmitter. In an embodiment, means1302for determining a characteristic correlated to a voltage of a terminal of a switching element of a driver circuit of the wireless power transmitter may be configured to perform one or more of the functions discussed above with respect to the block1202. The load impedance detector1300further includes means1304for determining a reactance of a load based on the determined characteristic. In an embodiment, means1304for determining a reactance of a load based on the determined characteristic may be configured to perform one or more of the functions discussed above with respect to the block1204.

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. Means for driving may be provided including driver circuits such as a switching amplifier. The means for driving may be a class E amplifier. Means for adjusting impedance may include an impedance adjustment circuit comprising one or more reactive elements that may be selectively switched into the circuit. Means for switching may be provided including electric switches such as solid state switches, reed relays, armature type relays, and the like. Means for wirelessly transmitter power may include the wireless power transmitter as described above.

The steps of a method or algorithm and functions described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a tangible, non-transitory computer-readable medium. A software module may reside in Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, a removable disk, a CD ROM, or any other form of storage medium known in the art. A storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. 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. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal.

Various modifications of the above described embodiments will be readily apparent, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.