Wireless charging system having measurement circuitry with foreign object detection capabilities

A wireless power transmitting device transmits wireless power signals to a wireless power receiving device using an output circuit that includes a wireless power transmitting coil. Measurement circuitry is coupled to the output circuit to help determine whether the wireless power receiving device is present and ready to accept transmission of wireless power. Oscillator circuitry for supplying signals to the output circuitry while making measurements with the measurement circuitry is coupled to the output circuit using an impedance injection network. The impedance injection network includes an inductor and a resistor coupled in series. Control circuitry opens a transistor in the output circuit when making measurements with the measurement circuitry and closes the transistor when transmitting the wireless power signals.

FIELD

This relates generally to wireless systems, and, more particularly, to systems in which devices are wirelessly charged.

BACKGROUND

In a wireless charging system, a wireless power transmitting device such as a device with a charging surface wirelessly transmits power to a portable electronic device. The portable electronic device receives the wirelessly transmitted power and uses this power to charge an internal battery or to power the device.

SUMMARY

In some situations, foreign objects may be accidentally placed on a charging surface. This can pose challenges when performing wireless power transmission operations. To address these challenges, a wireless power system is provided with foreign object detection capabilities.

In the system, a wireless power transmitting device transmits wireless power signals to a wireless power receiving device. The wireless power transmitting device has an inverter that supplies signals to an output circuit that includes a wireless power transmitting coil. The wireless power transmitting coil may be part of an array of wireless power transmitting coils that cover a wireless charging surface associated with the wireless power transmitting device.

Signal measurement circuitry is coupled to the output circuit to help determine whether the wireless power receiving device is present and ready to accept transmission of wireless power. The measurement circuitry includes a measurement circuit that is coupled to the output circuit and that measures signals while oscillator circuitry supplies the output circuit with signals at a probe frequency. Using measurements from this measurement circuitry at one or more probe frequencies, the wireless power transmitting device determines whether an external object is present on the coils. The oscillator circuitry is coupled to the output circuit with an impedance injection network having an inductor and resistor coupled in series.

Impulse response circuitry in the measurement circuitry is coupled to the output circuit and used to measure the response of the output circuit to an impulse signal supplied by an inverter in the wireless power transmitting device. The impulse response circuitry is used to make inductance and Q factor measurements.

During operation, information from the impulse response circuitry and measurements at the probe frequency can be used in determining whether a wireless receiving device is present over particular coils in wireless charging surface and can therefore be used in adjusting wireless power transmission with the wireless power transmitting device.

The measurement circuitry also includes a measurement circuit that is coupled to the output circuit and that measures signals while the oscillator circuitry sweeps an alternating-current output signal between a first frequency and a second frequency. Measurements resulting from frequency-sweeping operations are used to detect sensitive devices such as radio-frequency identification devices. If sensitive devices are detected, potentially damaging wireless power transmission operations can be avoided.

Switching circuitry is used to dynamically switch selected coils from the coil array that overlaps the charging surface into the output circuit, so that appropriate coils in the coil array can be probed for the presence of external objects and sensitive devices such as radio-frequency identification devices.

The output circuit has a transistor coupled to the wireless power transmission coil. The transistor is closed when the wireless power signals are transmitted with the wireless power transmission coil and is opened when the measurement circuitry makes measurements.

DETAILED DESCRIPTION

A wireless power system has a wireless power transmitting device that transmits power wirelessly to a wireless power receiving device. The wireless power transmitting device is a device such as a wireless charging mat, wireless charging puck, wireless charging stand, wireless charging table, or other wireless power transmitting equipment. The wireless power transmitting device has one or more coils that are used in transmitting wireless power to one or more wireless power receiving coils in the wireless power receiving device. The wireless power receiving device is a device such as a cellular telephone, watch, media player, tablet computer, pair of earbuds, remote control, laptop computer, other portable electronic device, or other wireless power receiving equipment.

During operation, the wireless power transmitting device supplies alternating-current signals to one or more wireless power transmitting coils. This causes the coils to transmit alternating-current electromagnetic signals (sometimes referred to as wireless power signals) to one or more corresponding coils in the wireless power receiving device. Rectifier circuitry in the wireless power receiving device converts received wireless power signals into direct-current (DC) power for powering the wireless power receiving device.

An illustrative wireless power system (wireless charging system) is shown inFIG. 1. As shown inFIG. 1, wireless power system8includes wireless power transmitting device12and one or more wireless power receiving devices such as wireless power receiving device10. Device12may be a stand-alone device such as a wireless charging mat, may be built into furniture, or may be other wireless charging equipment. Device10is a portable electronic device such as a wristwatch, a cellular telephone, a tablet computer, or other electronic equipment. Illustrative configurations in which device12is a mat or other equipment that forms a wireless charging surface and in which device10is a portable electronic device that rests on the wireless charging surface during wireless power transfer operations may sometimes be described herein as an example.

During operation of system8, a user places one or more devices10on the charging surface of device12. Power transmitting device12is coupled to a source of alternating-current voltage such as alternating-current power source50(e.g., a wall outlet that supplies line power or other source of mains electricity), has a battery such as battery38for supplying power, and/or is coupled to another source of power. A power converter such as AC-DC power converter40can convert power from a mains power source or other AC power source into DC power that is used to power control circuitry42and other circuitry in device12. During operation, control circuitry42uses wireless power transmitting circuitry34and one or more coils36coupled to circuitry34to transmit alternating-current electromagnetic signals48to device10and thereby convey wireless power to wireless power receiving circuitry46of device10.

Power transmitting circuitry34has switching circuitry (e.g., transistors in an inverter circuit) that are turned on and off based on control signals provided by control circuitry42to create AC current signals through appropriate coils36. As the AC currents pass through a coil36that is being driven by the inverter circuit, alternating-current electromagnetic fields (wireless power signals48) are produced that are received by one or more corresponding coils14coupled to wireless power receiving circuitry46in receiving device10. When the alternating-current electromagnetic fields are received by coil14, corresponding alternating-current currents and voltages are induced in coil14. Rectifier circuitry in circuitry46converts received AC signals (received alternating-current currents and voltages associated with wireless power signals) from one or more coils14into DC voltage signals for powering device10. The DC voltages are used in powering components in device10such as display52, touch sensor components and other sensors54(e.g., accelerometers, force sensors, temperature sensors, light sensors, pressure sensors, gas sensors, moisture sensors, magnetic sensors, etc.), wireless communications circuits56for communicating wirelessly with control circuitry42of device12and/or other equipment, audio components, and other components (e.g., input-output devices22and/or control circuitry20) and are used in charging an internal battery in device10such as battery18.

Devices12and10include control circuitry42and20. Control circuitry42and20includes storage and processing circuitry such as microprocessors, power management units, baseband processors, digital signal processors, microcontrollers, and/or application-specific integrated circuits with processing circuits. Control circuitry42and20is configured to execute instructions for implementing desired control and communications features in system8. For example, control circuitry42and/or20may be used in determining power transmission levels, processing sensor data, processing user input, processing other information such as information on wireless coupling efficiency from transmitting circuitry34, processing information from receiving circuitry46, using information from circuitry34and/or46such as signal measurements on output circuitry in circuitry34and other information from circuitry34and/or46to determine when to start and stop wireless charging operations, adjusting charging parameters such as charging frequencies, coil assignments in a multi-coil array, and wireless power transmission levels, and performing other control functions. Control circuitry42and/or20may be configured to perform these operations using hardware (e.g., dedicated hardware or circuitry) and/or software (e.g., code that runs on the hardware of system8). Software code for performing these operations is stored on non-transitory computer readable storage media (e.g., tangible computer readable storage media). The software code may sometimes be referred to as software, data, program instructions, instructions, or code. The non-transitory computer readable storage media may include non-volatile memory such as non-volatile random-access memory (NVRAM), one or more hard drives (e.g., magnetic drives or solid state drives), one or more removable flash drives or other removable media, other computer readable media, or combinations of these computer readable media or other storage. Software stored on the non-transitory computer readable storage media may be executed on the processing circuitry of control circuitry42and/or20. The processing circuitry may include application-specific integrated circuits with processing circuitry, one or more microprocessors, or other processing circuitry.

Device12and/or device10may communicate wirelessly. Devices10and12may, for example, have wireless transceiver circuitry in control circuitry42and20(and/or wireless communications circuitry such as circuitry56ofFIG. 1) that allows wireless transmission of signals between devices10and12(e.g., using antennas that are separate from coils36and14to transmit and receive unidirectional or bidirectional wireless signals, using coils36and14to transmit and receive unidirectional or bidirectional wireless signals, etc.).

With one illustrative configuration, wireless transmitting device12is a wireless charging mat or other wireless power transmitting equipment that has an array of coils36that supply wireless power over a wireless charging surface. This type of arrangement is shown inFIG. 2. In the example ofFIG. 2, device12has an array of coils36that lie in the X-Y plane. Coils36of device12are covered by a planar dielectric structure such as a plastic member or other structure forming charging surface60. The lateral dimensions (X and Y dimensions) of the array of coils36in device36may be 1-1000 cm, 5-50 cm, more than 5 cm, more than 20 cm, less than 200 cm, less than 75 cm, or other suitable size. Coils36may overlap or may be arranged in a non-overlapping configuration. Coils36can be placed in a rectangular array having rows and columns and/or may be tiled using a hexagonal tile pattern or other pattern.

During operation, a user places one or more devices10on charging surface60. Foreign objects such as coils, paper clips, scraps of metal foil, and/or other foreign conductive objects may be accidentally placed on surface60. System8automatically detects whether conductive objects located on surface60correspond to devices10or incompatible foreign objects and takes suitable action. With one illustrative arrangement, system8checks whether objects located on surface60include sensitive items such as radio-frequency identification (RFID) devices or other potentially sensitive electronic equipment that could be potentially damaged upon exposure to large fields from coils36before system8allows wireless power to be transmitted to those objects.

As shown in the example ofFIG. 2, external objects such as external object62and object64may overlap one or more coils36. In some situations, objects62and64will be portable electronic devices10. In other situations, one or more of objects62and64will be incompatible external objects (e.g., conductive foreign objects such as metallic coins, sensitive devices such as RFID devices, etc.). Situations may also arise in which incompatible external objects and portable electronic devices overlap the same coil or coils36.

Illustrative wireless power transmitting circuitry34that includes circuitry to detect and characterize external objects on surface60is shown inFIG. 3. As shown inFIG. 3, circuitry34may include an inverter such as inverter72or other drive circuit that produces wireless power signals that are transmitted through an output circuit that includes one or more coils36. A single coil36is shown in the example ofFIG. 2. In general, device12may have any suitable number of coils36(1-100, more than 5, more than 10, fewer than 40, fewer than 30, 5-25, etc.). Switching circuitry MX (sometimes referred to as multiplexer circuitry) that is controlled by control circuitry42can be located before and/or after each coil36and/or before and/or after the other components of output circuit71and can be used to switch desired sets of one or more coils36(desired output circuits71) into or out of use. For example, if it is determined that object62ofFIG. 2is a wireless power receiving device10and object64is an incompatible foreign object such as a coin, the coils overlapping object62may be activated during wireless power transmission operations and the coils under object64may be deactivated so that these coils do not transmit wireless power. Other coils36(e.g., coils not overlapped by object64in this example) can also be turned off during wireless power transmission operations, if desired.

With continued reference toFIG. 3, during wireless power transmission operations, transistors74of inverter72are driven by time-varying control signals from control circuitry42. Control circuitry42may also use transistors74of inverter72to apply square wave pulses or other impulses to coil36(e.g., during impulse response measurements). If desired, a capacitor such as capacitor C2may be placed at the output of inverter72to smooth the square wave pulses. The value of C2may be, for example, 4.7 nF, more than 2 nF, less than 6 nF or other suitable smoothing capacitance value.

Coil36(e.g., a coil that has been selected using multiplexing circuitry MX) has an inductance L. Capacitor96has a capacitance C1that is coupled in series with inductance L in output circuit90. When supplied with alternating-current drive signals from inverter72while switch (transistor) TP is closed, the output circuit formed from coil36and capacitor96produces alternating-current electromagnetic fields that are received by one or more coils14in device10. The inductance L of each coil36is influenced by magnetic coupling with external objects, so measurements of inductance L for one or more of coils36in device12at various frequencies can reveal information on objects on charging surface60.

To conserve power, device12may be operated in a standby mode while awaiting use to supply wireless power to devices10. The signal measurement circuitry ofFIG. 3(sometimes referred to as output circuit signal measurement circuitry, external or foreign object detection circuitry, etc.) monitors for the presence of external objects during standby. The power consumption of the measurement circuitry in transmitter circuitry34during standby operations may be less than 50 mW, less than 200 mW, more than 1 mW, or other suitable value.

In standby mode, device12periodically scans coils36(e.g., device12scans each of coils36) for the presence of external objects (e.g., devices10, foreign objects such as coins, etc.). To probe a selected coil for changes in inductance L due to external objects, a probe signal is driven onto node N1with oscillator circuitry84while control circuitry42turns off inverter72(e.g., transistors74are not used to drive signals onto node N2). Control circuitry42uses, for example, oscillator circuitry84(e.g., one or more voltage controlled oscillators, one or more other adjustable oscillators, and/or other oscillatory circuitry) to produce an alternating-current probe signal (e.g., a sine wave, square wave, etc.) at a probe frequency fr (e.g., 4 MHz or other suitable frequency such as a frequency of at least 500 kHz, at least 1 MHz, at least 2 MHz, less than 10 MHz, between 1 MHz and 10 MHz, or other suitable frequency). The probe frequency (oscillator output frequency) fr that is used during standby mode is a frequency that differs from RFID frequencies such as 13.56 MHz and that differs from the normal alternating-current frequency supplied to output circuit71by inverter72during wireless charging operations, which may be, for example, 100-500 kHz, more than 50 kHz, more than 100 kHz, more than 200 kHz, less than 450 kHz, less than 400 kHz, less than 300 kHz, or other suitable wireless power alternating-current drive frequency.

The signal at frequency fr is applied to node N1from oscillator circuitry84via impedance injection network132and capacitor86and is coupled to coil36via capacitor96. Inverter72may be on or may be held in an off state by control circuitry42. With one illustrative configuration, control circuitry42may help transistor T2on and transistor T1off. Impedance injection network132has an impedance that helps oscillator84effectively inject alternating-current signals (e.g., probe signals at one or more frequencies or a swept frequency signal) onto node N1for detection by measurement circuitry such as measurement circuit78and measurement circuit84ofFIG. 3. Control circuitry42controls multiplexer(s) MX to select the coil to which the signal at frequency fr is applied (e.g., coil36ofFIG. 3) from the array of coils36of device12shown inFIG. 2. Capacitance C1may have a value of 150 μF, more than 10 μF, less than 1000 μF, or other suitable value. Transistor TP may have a parasitic capacitance Cp (e.g., a parasitic capacitance of 85 pF, more than 10 pF, less than 800 pF, or other suitable value) when open.

With one illustrative configuration, direct-current voltage source130applies a 20 V direct-current bias voltage to node N1when transistor TP is open, which reduces parasitic capacitance Cp of transistor TP from about 150 pF to about 85 pF (as an example). For standby operations, control circuitry42opens transistor TP so that so that oscillator output signals from oscillator84are routed through coil36. As described more fully in connection with the equivalent circuit ofFIG. 7, when transistor TP is open, the presence of low parasitic capacitance Cp helps reduce the capacitance in parallel with coil36and reduces the potential negative impact of the potentially large parasitic capacitances of transistors T1and T2(e.g., 150 pF or more) and smoothing capacitor C2(e.g., 4.7 nF, more than 2 nF, less than 6 nF or other suitable smoothing capacitance value) on the detection sensitivity of measurement circuits78and94(e.g., signal attenuation on node N1will be avoided). This allows transistors T1and T2to be optimized for use in inverter74(e.g., the parasitic capacitances of field-effect transistors T1and T2can be large when lowering drain-source “on resistance” Rds-on to optimize field-effect transistors T1and T2for power transfer applications) and allows smoothing capacitor C2to be used at the output of inverter74.

With TP open, output circuit71(coil36in series with C1and Cp) will be characterized by a resonance at frequency fres of equation 1.
fres=1/(2π(LCp)1/2)  (1)

The expected measured signal at node N1(output voltage V1) as a function of applied signal frequency f in the absence of external objects on coil36is given by curve102ofFIG. 4. In the presence of an electronic device such as device10that contains one or more coils14overlapping coil36, curve102may shift to lower frequencies as shown by curve100. In the presence of a coin or other incompatible foreign object overlapping coil36, curve102may shift to higher frequencies as shown by curve104. Changes in load can be detected by monitoring the value of V1using measurement circuit78ofFIG. 3at one or more probe frequencies. For example, oscillator circuitry84may be used to apply a probe signal to node N1at a frequency fr that has been chosen to match resonant frequency fres of equation 1. If desired, multiple probe signals may be applied to output circuit72while using measurement circuitry to evaluate the resulting signal on node N1. For example, the direction of change in curve102(shifting higher or lower) can be detected by taking multiple measurements of V1at two or more frequencies near frequency fr ofFIG. 4).

To make measurements of V1, measurement circuit78includes peak detector80and analog-to-digital converter82. Circuit78measures the signal at node N1and supplies a corresponding digital version of this signal to control circuitry42. In the presence of an object overlapping coil36(whether from device10, a sensitive RFID device, or a coin or other incompatible foreign object), signal V1will drop. For example, the signal on node N1may drop from a value of P1(e.g., a peak value associated with curve102) when coil36is unloaded to a reduced value of P2when coil36is loaded due to the presence of an external object (e.g., a reduced value P2associated with shifted curve100from an overlapping wireless power receiving device with a coil or a reduced value P2associated with shifted curve102from an overlapping coin).

During standby operations, control circuitry42can scan through coils36by using multiplexer circuitry MX or other switching circuitry in circuitry34. In some embodiments, this sequentially couples each of coils36to node N1while circuitry78measures signal V1for each selected coil36. If no changes in signal V1are detected, control circuitry42can conclude that no objects are present on device12(e.g., no objects are resting on charging surface60). If a change in V1is detected, control circuitry42performs additional operations to confirm that device10is present rather than an incompatible foreign object such as a coin.

With one illustrative approach, control circuitry42uses impulse response measurement circuitry76(sometimes referred to as inductance measurement circuitry and/or Q factor measurement circuitry) to perform low-frequency measurements of inductance L and quality factor Q in response to detection of a load on one or more coils36during standby. During impulse response measurements, control circuitry42directs inverter72to supply one or more excitation pulses (impulses) to coil36while turning on transistor TP, so that L and C1in output circuit71form a resonant circuit (e.g., a circuit where resonant current passes through T2, which can be turned on). The impulses may be, for example, square wave pulses of 1 μs in duration. Longer or shorter pulses may be applied, if desired. The resonant circuit may resonate at a frequency near to the normal wireless charging frequency of coil36(e.g., about 320 kHz, 100-500 kHz, more than 50 kHz, more than 100 kHz, more than 200 kHz, less than 450 kHz, less than 400 kHz, less than 300 kHz, or other suitable wireless charging frequency).

The impulse response (voltage signal V2on node N1) of circuit71to the applied pulse(s) is as shown inFIG. 5. The frequency of the impulse response signal ofFIG. 5is proportional to 1/sqrt(LC), so L can be obtained from the known value of C1and the measured frequency of the impulse response signal. Q may be derived from L and the measured decay of the impulse response signal. As shown inFIG. 5, if signal V2decays slowly, Q is high (e.g., HQ) and if signal V2decays more rapidly, Q is low (e.g., SQ). Measurement of the decay envelope of V2and frequency of V2of the impulse response signal ofFIG. 5with circuitry76will therefore allow control circuitry42to determine Q and L.

If the measured value of L for a given coil matches the normal L value expected for each of coils36in the array of coils36overlapping surface60(e.g., when the measured L value is not influenced by the presence device10or other external object on surface60), control circuitry42can conclude that no external object suitable for wireless charging is present. If a given measured value of L is larger than that expected for an unloaded coil, control circuitry42can conclude that an external object is present that appears to be in an appropriate condition for wireless charging and can perform additional measurement operations. For example, control circuitry42can perform a swept-frequency measurement (sometimes referred to as an RFID checking measurement) on node N1to check whether a sensitive device such as an RFID device is present on surface60.

The measurements made by circuitry76are performed on one or more of coils36(e.g., these measurements may be performed on each of coils36in the array of coils in device12). Circuitry42uses these impulse response measurements to identify spatial patterns in measured L values (and/or Q factor values) across surface60. Analysis of a pattern of measured inductance (L) change can help determine whether a known type of device10is present on coils36. Analysis of the spatial patterns of measured inductance L (and, if desired, Q factor, which has an inverse relationship with respect to L), as a function of coil position in the X-Y plane of surface60may be used in determining when to transit wireless power from device12to device10. If, for example, the value of L for each of coils36is unchanged from its nominal state, circuitry42can conclude that no external device suitable for wireless charging is present. If the value of L for a given one of coils36is elevated or other suitable pattern of measured L values is detected, circuitry42can conclude that an external device that is suitable for wireless charging is present on that coil and can prepare to transmit wireless power using that coil.

Before transmitting wireless power, it may be desirable to check whether a sensitive device such as an RFID device is present on surface60. Sensitive devices can potentially be harmed by excessive wireless power levels, so checking for sensitive devices helps avoid damage to sensitive devices during subsequent wireless power transfer operations. In some scenarios, both portable device10and a sensitive device may be present over the same coil36in the array of coils36in device12. A sensitive device may, as an example, be present under a cellular telephone, watch, or other portable device10that includes a wireless power receiving coil14. Even though the presence of the portable device10can be detected by making inductance measurements with coils36, it is desirable to check whether a sensitive device is also present so as to avoid damaging the sensitive device by exposure to wireless power transmissions.

Radio-frequency identification (RFID) devices typically have RFID coil circuits that resonate at relatively high frequencies such as a frequency of 13.56 MHz. In some embodiments, to determine if an RFID is present on surface60, RFID checking measurements are performed by measuring a voltage signal V3on node N1using measurement circuit94(FIG. 3). During these checking measurements, control circuitry42directs oscillator circuitry84to sweep the frequency of the signal supplied to node N1between a first frequency f1and a second frequency f2covering the expected resonant frequencies of popular RFID coils. Transistor TP may remain open so that current from oscillator circuitry84flows through each coil36that has been selected during measurement operations. The value of f1may be, for example, 10 MHz, more than 5 MHz, less than 11 MHz, less than 12 MHz, less than 15 MHz, or other suitable value. The value of f2may be 30 MHz, more than 14 MHz, more than 15 MHz, more than 20 MHz, less than 45 MHz, or other suitable value.

As shown inFIG. 3, swept-frequency measurement circuit94includes a peak detector such as peak detector88that measures the voltage on node N1, band pass filter90, and analog-to-digital converter circuitry92. Analog-to-digital converter circuitry92supplies a digital version of its input to control circuitry42.

When no RFID device is present on charging surface60of device12, peak detector88will detect a signal such as the signal of curve108inFIG. 6. When an RFID device overlaps charging surface60, signal V3(see, e.g., curve110) will exhibit a resonance signal such as signal112in as frequency f is swept between f1and f2. Resonance signal112may, for example, correspond to a resonance frequency such as an RFID resonant frequency of 13.56 MHz.

Frequency f is swept between f1and f2at a predetermined speed. For example, control circuitry42may sweep frequency from f1to f2in an interval of 2 ms, at least 1 ms, less than 3 ms, or other suitable time period. The pass frequency of band pass filter90is selected so that resonance signal112will pass through band pass filter90as band pass filtered signal112′ of band pass output curve114when frequency f is changed between f1and f2at the predetermined speed (e.g., when the full sweep range is covered in an interval of 2 ms, etc.). The use of band pass filter90helps remove non-resonant signal fluctuations from curve110(e.g., signal tilt and slowly varying increases and/or decreases of the type shown by illustrative curve110ofFIG. 6). The resulting band-pass-filtered signal (curve114and filtered signal resonance112′) can be processed by control circuitry42to confirm that an RFID resonance at a particular frequency has been detected. Control circuitry42can then take appropriate action. For example, if no RFID signature is detected, control circuitry42can conclude that the detected external object on surface60is likely a portable device (device10with coil14) without any intervening (overlapping) sensitive RFID device. If an RFID signature (e.g., resonant signal112′ at an RFID frequency such as 13.56 MHz) is detected, control circuitry42can reduce the level of wireless power transmitted by coils36or can prevent wireless power from being transmitted by coils36(or at least the coils that are overlapped by the sensitive RFID device) so as to mitigate damage to the RFID device. Optionally, control circuit42can issue an alert to a user.

FIG. 7is an equivalent circuit for the circuitry ofFIG. 3during signal measurement operations with circuits78and94. In this situation, transistor TP is open and exhibits parasitic capacitance Cp. Due to the presence of the 20 V bias from bias circuit130, transistor TP exhibits a relatively low value of parasitic capacitance (e.g., Cp may be about 85 pF). Cpinv represents the capacitance of smoothing capacitor C2(e.g., 4.7 nF, more than 2 nF, less than 6 nF or other suitable smoothing capacitance value) in parallel with the parasitic capacitances of transistors T1and T2(e.g., 150 pF each) and may have a value of about 5 nF (as an example). The presence of parasitic capacitance Cp lowers the capacitance in parallel with coil36, which is being used to detect objects on surface60and thereby enhances detection sensitivity.

Impedance injection network132and capacitor86are coupled between oscillator84and coil36. Impedance injection network132includes resistor RI in series with inductor LI. Inductor LI provides network132with inductance that helps enhance measurement sensitivity. The impedance of circuitry150is mostly capacitive, so impedance injection network132is mostly inductive to effectively match network132to the impedance of circuitry150. This amplifies the response detected by detection circuits94and78at node N1. Network132flattens the response of node N1as oscillator84sweeps frequency f between frequencies f1and f2while gathering response curve114ofFIG. 6, so that signal114can be uniformly amplified at frequencies between f1and f2. Resistor RI helps balance the impedance of network132over the range of frequencies f1to f2. With one illustrative configuration, resistor RI may have a value of 25 ohms (e.g., at least 10 ohms, less than 75 ohms, etc.), inductor LI may have an inductance of 100 nH (at least 10 nH, less than 1000 nH), and capacitor86may have a value of 2.2 nF (e.g., at least 0.2 nF, less than 200 nF, etc.).

Oscillator84includes oscillator circuitry such as one or more voltage controlled oscillators. During frequency sweeping operations with oscillator84, control circuitry42supplies oscillator84with control signal VCOCNT (FIG. 3). The magnitude of control signal VCOCNT controls the output frequency from oscillator84that is supplied to impedance injection network. As shown in the upper trace ofFIG. 8, the signal VCOCNT produced by control circuitry42has a sawtooth shape. The resulting magnitude of the output OUT of oscillator84as a function of time is shown in the lower trace ofFIG. 8. Each time there is a knee in signal VCOCNT, an artifact (rise in output magnitude OUT) is produced at the output of oscillator84. For example, when sawtooth signal VCOCNT reaches a peak (see, e.g., knee KP) because the output frequency f of oscillator84has reached a peak and is being decreased, an artifact such as artifact AL is produced. When sawtooth signal VCOCNT reaches a valley (see, e.g., knee KV) because the output frequency f of oscillator84has reached a valley and is being increased, an artifact such as artifact AH is produced, which can potentially decrease measurement accuracy.

It has been determined that the size of artifact AH is generally larger than the size of artifact AL, so control circuitry can minimize inaccuracies by gathering measurements with measurement circuit94while frequency f is being swept from high (e.g., a frequency near frequency f2when VCOCNT is at knee KP) to low (e.g., a frequency near frequency f1, which is lower than f2, when VCOCNT is at knee KV). Accuracy is further improved by providing buffer ranges BF1and BF2at opposing ends of the swept frequency range. To provide buffers BF1and BF2, control circuitry42sweeps than the size of VCOCNT over a wider range than needed to change frequency f from f2to f1. In particular, VCOCNT is varied from a value above that necessary to produce frequency f2to a value below that necessary to produce frequency f1and measurement circuit94is only used to gather data during central time period RNG (e.g., when frequency f varies from f2to f1). By gathering output from circuit94only when oscillator84is operated in range RNG (frequency f2to f1), artifacts AL and AH are avoided and accuracy is enhanced.

FIG. 9is a flow chart of illustrative operations involved in using system8. During the operations of block120, system8performs standby measurements. For example, device12may use circuitry such as circuit78ofFIG. 3to monitor one or more of coils36(e.g., each coil36in the array of coils36in device12) for the presence of an external object such as one of devices10which is potentially compatible for wireless power transfer or an incompatible object such as a coin or badge. A single measurement at frequency fr may be made to determine whether V1is lower than expected for any coils36or, if desired, multiple measurements at different frequencies near fr may be made (e.g., to determine which direction the coil resonance has shifted due to an external object and thereby help determine whether the object is an electronic device or is a coin or other incompatible foreign object). The standby operations of block120consume a low amount of power (e.g., 50 mW or less, 100 mW or less, more than 1 mW, or other suitable amount). During standby operations, transistor TP is opened to help decrease the capacitance in parallel with coil36and thereby enhance measurement sensitivity, as described in connection withFIG. 7.

In response to detection of an external object with control circuitry42during the operations of block120, control circuitry42performs additional detection operations such as low-frequency impulse response measurements (block122). During the operations of block122, control circuitry42may, for example, use inverter72or other resonant circuit drive circuitry to apply a stimulus (e.g., a square wave or other signal impulse) to the circuit formed from one or more of coils36(e.g., to each coil36in the array of coils36in device12, a subset of these coils such as those for which foreign object presence has been detected during the operations of block120, and/or other suitable sets of one or more of coils36), thereby causing that circuit (and that coil36) to resonate while using a measurement circuit such as impulse response measurement circuitry76ofFIG. 3to measure the response of the resonant circuit. As described in connection withFIG. 5, the characteristics of the resulting circuit resonance may then be measured and analyzed. For example, control circuitry42may use information on the measured resonant frequency to measure inductance and may use information on the decay of the signal resonance to determine resistance R and Q factor. If desired, the measurements of blocks120and/or122can be mapped in dimensions X and Y across surface60to help identify devices10and foreign objects.

If the operations of block122reveal that a compatible electronic device10is present, additional checking operations may be performed during block124to detect whether foreign objects such as radio-frequency identification devices are present. In particular, frequency sweep measurements with circuitry such as oscillator circuitry84and swept-frequency measurement circuit94ofFIG. 3may be performed to check for the presence of a sensitive RFID device, as described in connection withFIG. 6. While making frequency sweeping measurements, control circuitry42gathers measurements with circuit94during periods of sawtooth control signal VCOCNT in which VCOCNT is decreasing (frequency is decreasing) but not increasing and gathers measurements in middle range RNG but not during buffer periods BF1and BF2at the beginning and end of each of these periods. Impedance injection network132helps flatten the frequency response of node N1during frequency sweep measurements with oscillator84and measurement circuit94.

Appropriate action are taken during the operations of block126based on the results of measurements such as the measurements of blocks120,122, and/or124. If, as an example, a sensitive RFID device is detected during the operations of block124or if a foreign object is detected, wireless charging operations with all of coils36or an appropriate subset of coils36can be blocked. In response to detection of an electronic device10having a known characteristic L response (and/or Q response) and in response to determining that no RFID device is present after checking one or more of coils36, as appropriate, with circuit94(e.g., the coils36for which L and/or Q measurements and/or other measurements indicate may be overlapped by an object or all of coils36), control circuitry42can use wireless power transmitting circuitry34to transmit wireless power to wireless power receiving circuitry46.

As the foregoing demonstrates, a series switch TP and a matching network (impedance injection132plus capacitor86ofFIG. 3) may be used to allow oscillator84to drive a signal into the drive circuitry (circuitry34) at a much higher frequency than it is tuned for. The placement of switch TP in series with the coil36and its tuning network allows the tuning network to be effectively broken when seeking to drive higher frequency oscillator signals into the coil. So when it is desired to drive coil36with inverter72, the switch is shorted and when it is desired to drive a high frequency signal into coil36, the switch is opened. As a result, tuning capacitor C1does not interfere with high frequency signals from oscillator84. The injection impedance inductance allows more current to be driven into the coil at frequencies of interest (e.g., higher frequencies). This increase in current results in additional signal response during measurements. The probe frequency (e.g., a single frequency from oscillator84) may be used to determine whether an object has been placed on charging surface60by measuring V1at N1(or other suitable node) to detect impedance changes in the system.

During RFID detection operations, the frequency may be swept across a band of interest (e.g., a frequency band associated with potential RFID resonant objects), thereby generating an impedance profile (signature) to compare with known profiles to whether unexpected objects are present on charging surface60. Voltage V3may be measured at node N1or other suitable node to generate the profile. The swept frequency may be swept up or down depending on circuit parameters in order to minimize knee response artifacts. Bandpass filter90may have its pass band tailored to the speed of the frequency sweep in order to remove non-resonant responses in the measurement circuitry.

In general, impulse response measurements and RFID detection measurements can be performed in any suitable order. With one illustrative configuration, the RFID detection system is used before the impulse response measurements are made to prevent the impulse detection system from damaging RFID devices.

The RFID detection circuitry (84,132,88,90,92,78, etc.) can be connected at other nodes, such as N2or MX. Similarly, impulse response circuitry76can be connected at different nodes. If desired, the RFID detection circuitry and impulse response circuitry be connected to the same node, as these circuits do not interfere with each other.

In the example ofFIG. 3, inverter74of circuitry wireless power transmitting circuitry34has two transistors (T1and T2) in a half bridge inverter configuration. If desired, invertor74of wireless power transmitting circuitry34may have four transistors in a full bridge inverter configuration. This type of arrangement is shown inFIG. 10. As shown inFIG. 10, inverter74is a full bridge inverter that includes transistors T1, T2, TP, and TP′. By using a full bridge arrangement rather than a half bridge arrangement, the peak-to-peak voltage of the drive signals for coil36can be increased for a given DC power supply voltage. For example, if the DC power supply of circuitry34is 20V, a half bridge inverter can drive signals of 20V peak-to-peak into coil36, whereas a full bridge inverter can drive signals of 40V peak-to-peak into coil36. Full bridge arrangements for inverter74can be used in embodiments in which circuitry34include multiple coils36or in embodiments in which multiplexer MX is omitted and circuitry34includes only a single coil36.

To avoid shunting current through transistors TP′ and TP during wireless power transmission operations, transistors TP′ and TP can be provided with direct-current bias from direct-current voltage sources130′ and130, respectively. If, as an example, the DC power supply of circuitry34is 20V, source130′ can supply a 20V output and source130can supply a 10V output. In this scenario, there will be a 10 V bias across transistor TP′ and a 10V bias across transistor TP. In general, any suitable levels of bias may be supplied to transistors TP′ and TP. The use of 10V biases is illustrative.

Transistors TP′ and TP may be metal-oxide-semiconductor field-effect transistors (MOSFETs). By DC biasing transistors TP′ and TP, the parasitic capacitances of transistors TP′ and TP can be reduced. This reduces the potential for drive signals to shunt through transistors TP′ and TP rather than flowing through coil36and thereby helps to enhance power transmission efficiency during wireless power transmission operations.

When it is desired to transmit power in a wireless power transmission mode with circuitry34ofFIG. 10, control circuitry42supplies control signals to the gates of transistors T1, T2, TP′, and TP to create the drive signal through coil36. When it is desired to use output circuit signal measurement circuitry to monitor for foreign objects, RFID devices, coil impedances, etc. (e.g., when it is desired to make measurements with circuitry84,132,86,94,78,76, etc. in a measurement mode), control circuitry42opens transistors TP′, TP, and T1(and optionally transistor T2).

Consider, as an example, a scenario in which it is desired to make RFID measurements using oscillator84and measurement circuitry94. As described in connection withFIG. 3, turning off transistor TP (and, in the current scenario, turning off transistors T1, T2, and TP′) during these measurements ensures that oscillator output signals from oscillator84are routed through coil36. As a result, tuning capacitor C1does not interfere with high frequency signals from oscillator84and measurements may be make satisfactorily to determine whether an RFID device is present.