Methods and circuits for mitigating saturation in wireless power systems

Abstract

The present disclosure provides methods and circuits for mitigating saturation in wireless power systems. The present disclosure provides a wireless charging system with a power transmitter that can wirelessly transfer power to a power receiver. Under some operating conditions, shield saturation may occur in the wireless power receiver, such as saturation of a ferrite structure. Saturation can cause destructive oscillations in power transfer. The power transmission may include control circuits for detecting and mitigating saturation.

Description

[0001] This application is a divisional application of an invention patent application with an application date of January 26, 2022, application number 202210093022.1, and name “Methods and circuits for reducing saturation in wireless power systems”.

[0002] This application claims priority to U.S. Patent Application No. 17 / 198,116, filed on March 10, 2021, and claims priority to U.S. Provisional Patent Application No. 63 / 143,704, filed on January 29, 2021, which are hereby incorporated by reference in their entirety. Technical Field

[0003] The present disclosure relates generally to power systems, and more particularly, to wireless power systems for charging electronic devices. Background Art

[0004] In a wireless charging system, a wireless power transmission device, such as a charging pad, wirelessly transmits power to a wireless power receiving device, such as a battery-powered portable electronic device. The wireless power transmission device has a coil that generates an electromagnetic flux. The wireless power receiving device has a coil and a rectifier circuit that uses the electromagnetic flux generated by the transmitter to generate DC power that can be used to power an electrical load in the battery-powered portable electronic device. Designing a wireless charging system can be challenging. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Figure 1A is a block diagram of an illustrative wireless charging system including a wireless power transmitting device and a wireless power receiving device according to some embodiments.

[0006] Figure 1B is an exploded view of an illustrative wireless power receiving device according to some embodiments.

[0007] Figure 2 is a circuit schematic of a wireless power transmission and reception circuit according to some embodiments.

[0008] Figure 3 is a graph illustrating the reduction in matching inductance due to saturation according to some embodiments.

[0009] Figure 4 is a timing diagram illustrating the behavior of the transmission coil current in a wireless power transmission device with and without saturation according to some embodiments.

[0010] Figure 5 is a flow chart of illustrative steps for performing a power ramp, according to some embodiments.

[0011] Figure 6 is a flow chart of illustrative steps for performing saturation detection and mitigation according to some embodiments.

[0012] Figure 7 is a circuit diagram of an inverter driving a resonant circuit according to some embodiments.

[0013] Fig. 8A is a timing diagram illustrating inverter output with 180° phase shift and symmetrical switching duty cycle according to some embodiments.

[0014] Figure 8B is a timing diagram illustrating inverter output with a 180° phase shift and an asymmetric duty cycle according to some embodiments.

[0015] Figure 8C is a timing diagram illustrating inverter output with a 90° phase shift and an asymmetric duty cycle according to some embodiments. DETAILED DESCRIPTION

[0016] The present invention discloses a wireless power system, which includes a wireless power transmission device. The wireless power transmission device transmits power wirelessly to one or more wireless power receiving devices. The wireless power receiving device may include an electronic device such as a watch, a cellular phone, a tablet computer, a laptop computer, an earbud, a battery box for earbuds and other devices, a tablet stylus (pen) and other input-output devices, a wearable device, or other electronic equipment. The wireless power transmission device may be an electronic device such as a wireless charging pad or puck, a tablet computer or other battery-powered electronic device with a wireless power transmission circuit, or other wireless power transmission device. The wireless power receiving device uses the power from the wireless power transmission device to power internal components and to charge an internal battery. Since the transmitted wireless power is typically used to charge an internal battery, the wireless power transmission operation is sometimes referred to as a wireless charging operation.

[0017] Figure 1A An exemplary wireless power system, sometimes referred to as a wireless charging system, is shown in FIG. Figure 1AAs shown, the wireless power system 8 includes a wireless power transmission device (such as wireless power transmission device 12), and includes a wireless power receiving device (such as wireless power receiving device 24). The wireless power transmission device 12 includes a control circuit 16. The wireless power receiving device 24 includes a control circuit 30. The control circuits in the system 8, such as the control circuit 16 and the control circuit 30, are used to control the operation of the system 8. This control circuit may include processing circuits associated with a microprocessor, a power management unit, a baseband processor, an application processor, a digital signal processor, a microcontroller, a battery charger, and / or an application-specific integrated circuit having a processing circuit. The processing circuit implements the desired control and communication features in the devices 12 and 24.

[0018] For example, the processing circuitry can be used to select a wireless power coil, determine a power transfer level, process sensor data and other data, process user input, handle negotiations between devices 12 and 24, send and receive in-band and out-of-band data, make measurements, and otherwise control the operation of system 8. As another example, the processing circuitry may include one or more processors, such as an application processor, which is used to run software, such as an Internet browsing application, Voice over Internet Protocol (VOIP), a phone call application, an email application, a media playback application, an operating system function, a power management function for controlling when one or more processors wake up, a gaming application, a map, an instant messaging application, a payment application, a calendar application, a notification / reminder application, and the like.

[0019] The control circuitry in the system 8 may be configured to perform operations in the system 8 using hardware (e.g., dedicated hardware or circuitry), firmware, and / or software. Software code for performing operations in the system 8 is stored on a non-transitory computer-readable storage medium (e.g., a tangible computer-readable storage medium) in the control circuitry 8. The software code may sometimes be referred to as software, data, program instructions, instructions, or code. The non-transitory computer-readable storage medium may include non-volatile memory such as non-volatile random access memory (NVRAM), one or more hard disk drives (e.g., a disk drive or a solid-state drive), one or more removable flash drives, or other removable media, etc. The software stored on the non-transitory computer-readable storage medium may be executed on the processing circuitry of the control circuitry 16 and / or 30. The processing circuitry may include a dedicated integrated circuit having a processing circuit, one or more microprocessors such as an application processor, a central processing unit (CPU), or other processing circuitry.

[0020] The wireless power transmission device 12 may be a stand-alone power adapter (e.g., a wireless charging pad or puck that includes power adapter circuitry), may be a wireless charging pad or puck that is coupled to a power adapter or other device via a cable, may be a battery-powered electronic device (a cell phone, tablet, laptop, removable case), may be equipment that has been incorporated into furniture, a vehicle, or other systems, or may be other wireless power transfer equipment. Exemplary configurations in which the wireless power transmission device 12 is a wireless charging pad or a battery-powered electronic device are sometimes described herein as examples.

[0021] The wireless power receiving device 24 may be a portable electronic device, such as a wristwatch, a cellular phone, a laptop computer, a tablet computer, an accessory such as earbuds, a tablet computer input device such as a wireless tablet computer stylus (pen), a battery box, or other electronic equipment. The wireless power transmitting device 12 may include one or more input-output devices 62 (e.g., input devices and / or output devices described in conjunction with the input-output device 56) or the input-output device 62 may be omitted (e.g., to reduce device complexity). The wireless power transmitting device 12 may be coupled to a wall outlet (e.g., an AC power source), may have a battery for supplying power, and / or may have another power source. The device 12 may have an alternating current (AC)-to-direct current (DC) power converter, such as an AC-DC power converter 14, for converting AC power from a wall outlet or other power source into DC power.

[0022] In some configurations, the AC-DC power converter 14 may be disposed in a housing (e.g., a power brick housing) that is separate from the housing of the device 12 (e.g., a wireless charging puck housing or a battery-powered electronic device housing), and a cable may be used to couple DC power from the power converter to the device 12. The DC power may be used to power the control circuit 16. During operation, a controller in the control circuit 16 may use a power transmission circuit 52 to transmit wireless power to a power receiving circuit 54 of the device 24. The power transmission circuit 52 may have a switching circuit (e.g., an inverter circuit 60 formed by transistors) that is turned on or off based on a control signal provided by the control circuit 16 to form an AC current signal through one or more transmission coils 42. The coils 42 may be arranged in a planar coil array (e.g., in a configuration where the device 12 is a wireless charging pad) or may be arranged to form a coil cluster (e.g., in a configuration where the device 12 is a wireless charging pad). In some arrangements, the device 12 (e.g., a charging pad, puck, battery-powered device, etc.) may have only a single coil. In other arrangements, the wireless charging device 12 may have multiple coils (e.g., two or more coils, 5-10 coils, at least 10 coils, 10-30 coils, fewer than 35 coils, fewer than 25 coils, or another suitable number of coils).

[0023] When AC current passes through one or more coils 42, coils 42 generate electromagnetic field signals 44 in response to the AC current signal. The electromagnetic field signal (sometimes referred to as a wireless power signal) 44 may then induce a corresponding AC current to flow in one or more nearby receiver coils such as coil 48 in the power receiving device 24. When the alternating electromagnetic field is received by coil 48, a corresponding alternating current is induced in coil 48. A rectifier circuit such as rectifier 50 (which includes a rectifying component, such as a synchronous rectifier metal oxide semiconductor transistor arranged in a bridge network) converts the AC signal received from coil 48 (the received alternating current signal associated with the electromagnetic field 44) ​​into a DC voltage signal for providing power to a load in the device 24, such as providing power to an application processor and charging a battery in the device. This principle of wireless power transfer may be referred to as transmission and reception of wireless power signals.

[0024] The DC voltage generated by the rectifier 50 can be used to power an energy storage device such as a battery 58, and can be used to power other components in the device 24. For example, the device 24 may include input-output devices 56 such as a display, a touch sensor, communication circuits, audio components, sensors, components that generate electromagnetic signals sensed by a touch sensor in a tablet or other device with a touch sensor (e.g., for providing stylus input, etc.), and other components, and the other components and these components can be powered by the DC voltage generated by the rectifier 50 in combination with the DC voltage generated by other available energy sources such as the battery 58.

[0025] During wireless power transmission operation, the circuit 52 supplies an AC drive signal such as an AC current signal to one or more coils 42 at a given power transmission frequency. The power transmission frequency is sometimes referred to as a carrier frequency, a power carrier frequency, a drive frequency, or an inverter switching frequency Fs. The inverter switching frequency Fs may be, for example, a predetermined frequency of about 125kHz, about 128kHz, about 200kHz, about 326kHz, about 360kHz, at least 80kHz, at least 100kHz, less than 500kHz, less than 300kHz, or other suitable wireless power frequencies. Devices operating under the Qi wireless charging standard established by the Wireless Power Alliance typically operate between 110kHz-205kHz or 80kHz-300kHz. In some configurations, the switching frequency Fs is negotiated in the communication between the device 12 and the device 24. In other configurations, the power transmission frequency may be fixed.

[0026] Control circuitry 16 may also include external object measurement circuitry 41 configured to detect external objects on the charging surface of device 12 and make other desired measurements, such as current measurements, voltage measurements, power measurements, and / or energy measurements. Measurement circuitry 41 may detect an indication that an object is proximate to device 12. Measurement circuitry 41 may help detect whether a nearby object is compatible with wireless charging operations, or if the nearby object may be a foreign object, such as a coil, paper clip, coin, and other generally metallic objects that react to an inductive field but are not compatible with wireless charging.

[0027] Figure 1B 2 shows an exploded view of the power receiving device 24. Figure 1BAs shown, the exemplary power receiving device 24 includes a device housing such as a housing layer 300, a wireless power coil 48, shielding layers 302 and 304, a battery 58, a display 306, and a cover layer, such as a cover glass 308 disposed on the display 306. The device housing 300 and the cover glass 308 serve as a lower outer protective layer and an upper outer protective layer, respectively. Although not explicitly shown, additional components such as communication, storage, and processing components are included in the stack of the device 24. The arrangement of components in a device such as the device 24 may vary.

[0028] Electronic components within device 24 are subject to signal interference. Shielding layer 302 may be a metal shield configured to suppress electromagnetic interference. This type of shielding layer 302 may be formed of materials such as copper, nickel, silver, gold, other metals, combinations of these materials, or other suitable conductive materials that suppress radio frequency signals, and may sometimes be referred to as radio frequency shields or electronic shields.

[0029] Shielding layer 304 guides a relatively low frequency magnetic field to serve as a guide for electromagnetic flux received from a wireless power transmitter. Layer 304 can be a layer of magnetic material that can act as a magnetic shield (i.e., layer 304 can block magnetic flux and can have a relative permeability of 500 or greater, 1000 or greater, or other suitable values). An example of a material that can be used to form magnetic shielding layer 304 is ferrite. Another example of a material that can be used to form magnetic shielding layer 304 is a high permeability nickel-iron magnetic alloy, which is sometimes referred to as a special metal or permalloy. Another example of a material that can be used to form magnetic shielding layer 304 is an iron-based nanocrystalline material.

[0030] According to some embodiments, the power transmission device 12 may include one or more magnets that may facilitate certain characteristics in the shielding structure within the power receiving device 24. Figure 1B As shown, the power receiving device 24 may include a shielding layer 304. During wireless power transmission, the inverter 60 may drive an AC current signal through the coil 42. The AC current flowing through the coil 42 induces an AC magnetic flux, which may increase the DC magnetic flux associated with the magnet within the device 12. The combination of the AC magnetic flux and the DC magnetic flux at the transmitting device 12 may result in characteristic conditions, such as saturation at the shield 304. Saturation occurs when an increase in the applied magnetic field fails to further increase the magnetization of the material. Saturation may also occur at ferrite or nanocrystalline materials with high magnetic saturation or high AC flux. Saturation (e.g., magnetic saturation or magnetic flux saturation) may cause a reduction in the amount of mating inductance between the device 12 and the device 24, thereby affecting wireless charging performance. Figure 3 The reduction in matching inductance due to saturation is shown. Figure 3 Draw the matching inductance value LTX According to the current I flowing through the wireless power transmission coil 42 TX As shown in curve 110, the reduction in the matching inductance due to saturation is converted into a current I TX increase.

[0031] Figure 4 is the transmission coil current I showing the presence and absence of saturation TX The waveform 120 represents the current I TX The behavior when saturation is not present, and waveform 122 represents the current I TX Behavior in the presence of saturation. Figure 4 As shown, waveform 120 switches back and forth at the inverter switching frequency Fs with a period Ts equal to the inverse of Fs (e.g., a duration Ts equal to the inverse of the power carrier frequency). Waveform 120 has relatively stable peaks and valleys from cycle to cycle, which produces the desired energy level at the basic switching frequency Fs.

[0032] In contrast, waveform 122 exhibits a much higher peak current level every other cycle (as shown by the increased peak 124) due to saturation and the reduction of the matching inductance. Waveform 122 returns to a relatively lower peak current level every other cycle (as shown by the decreased peak 126). Therefore, waveform 122 exhibits a significantly higher energy level at half of the switching frequency Fs / 2 with a period of 2*Ts. This phenomenon of higher energy levels at a certain fraction of the switching frequency Fs, especially at a subharmonic of Fs, indicates saturation.

[0033] Return to reference Figure 2 , the measurement circuit 41 may include a foreign object detection (FOD) circuit such as FOD circuit 100 and / or a saturation detection circuit such as saturation detection circuit 102. Saturation detection circuit 102 may include an energy measurement circuit that is configured to measure values ​​representing the energy level of the resonant tank at various frequency bands to determine whether saturation and, therefore, oscillation has occurred. Saturation detection circuit 102 may also be configured to measure the DC voltage on capacitor 70. A non-zero DC voltage on capacitor 70 does not necessarily imply saturation, but saturation will result in a non-zero DC bias voltage on capacitor 70.

[0034] Ferrite or other magnetic saturation and resulting oscillations that may occur within the power receiving device 24 may cause communication to fail between the device 12 and the device 24. As described above, oscillations occur when the transmitted electromagnetic flux becomes high enough to induce saturation in the power receiving device 24. In a typical wireless charging system, at startup, the transmitted power will begin to ramp up from a low power level to a target power level. Saturation (and characteristic oscillations) may occur as the transmitted power level ramps up. Saturation may occur or re-occur after the power ramp-up phase, for example, if the wireless power receiver moves relative to the wireless power transmitter during power transfer. This may also occur when certain environmental or operating conditions such as temperature change.

[0035] Figure 5 1 is a flow chart of illustrative steps for performing a power ramp operation. At step 130, the inverter power supply voltage Vin may be set to an initial voltage level. As an example, the voltage Vin may be initialized to 9V. This is merely illustrative. The inverter supply voltage Vin may be initialized to 4V, 5V, 6V, 7V, 8V, 10V, 11V, 1V-10V or other starting voltage levels.

[0036] At step 132, the phase of the AC drive signal output by the inverter 60 can be set to an initial phase amount. As an example, the phase of the inverter AC drive signal can be set to 90 degrees. The 90° phase can be converted to a 25% duty cycle. This is merely illustrative. The AC drive signal phase can be initialized to 45 degrees (e.g., 12.5% ​​duty cycle), 60 degrees (e.g., 16.7% duty cycle), 120 degrees (e.g., 33.3% duty cycle), 135 degrees (e.g., 37.5% duty cycle), 80 degrees-100 degrees, 70 degrees-110 degrees, 60 degrees-120 degrees, or other starting phase amounts.

[0037] At step 134, control circuitry, such as controller 1654, may determine whether a maximum phase has been reached. The control circuitry may compare the current phase level to the maximum phase level. As an example, the maximum phase level may be set to 180 degrees, which translates to a 50% duty cycle. This is illustrative only. The maximum phase may be set to 160 degrees, 170 degrees, 190 degrees, 200 degrees, less than 180 degrees, more than 180 degrees, 120 degrees-180 degrees, 180 degrees-360 degrees, 170 degrees-190 degrees, 160 degrees-200 degrees, 150 degrees-210 degrees, 140 degrees-220 degrees, or other maximum phase amounts.

[0038] If the maximum phase has not been reached (i.e., if the current phase is equal to the maximum phase limit), the control circuit will increase the phase of the AC drive signal by a phase step amount at box 136. The phase step amount can be 5 degrees, 10 degrees, 15 degrees, 20 degrees, or other phase deltas. The AC drive signal phase of the inverter can be increased by increasing the duty cycle of the AC drive signal. If the maximum value has been reached (i.e., if the current phase is equal to or greater than the maximum phase limit), the control circuit will increase the inverter supply voltage Vin by a voltage step amount at box 138. The voltage step amount can be 1V, 0.5V, 2V, 1.5V, 0.1V, 0.2V, 0.3V, 0.1V-2V, or other voltage deltas.

[0039] At step 140, the control circuit will determine whether the transmission power level has reached the target power level. The target power level can be 12V, 13V, 14V, 15V, 16V, 17V, 18V, 9V-18V, equal to or greater than 12V, equal to or greater than 18V, or other target power levels. If the target power level has not been reached, the process can return to step 134, as indicated by path 141. If the target power level has been reached, the power ramp is complete (step 142).

[0040] As described above, saturation may occur during or after the power ramp-up phase. According to some embodiments, control circuitry 16 (see, e.g., FIG. 1 ) within power delivery device 12 may be configured to perform saturation detection and mitigation during and / or after the power ramp-up phase. If no oscillation is detected during the power ramp-up phase, device 12 may continue to ramp its power level up. Figure 6 is a flow chart of illustrative steps for performing saturation detection and mitigation operations.

[0041] At step 200, the data receiver 40R may receive a control error packet (CEP), the communication between the device 12 and the device 24 may time out, or the saturation detection timer may expire. The Qi mechanism for controlling the transmission power level uses the power receiving device 24 to send a power adjustment request to the power transmission device 12, such as an ASK modulated packet sometimes referred to as a control error packet (CEP). The control circuit 16 may include a saturation detection timer, which expires to trigger a corresponding saturation detection operation. The saturation detection timer may be started periodically or in response to certain events such as the start of a power ramp-up phase.

[0042] In response to the power transmission device 12 receiving a control error packet from the power receiving device 24, in response to a communication timeout event, or in response to the expiration of the saturation detection timer, the saturation detection circuit 102 (see, e.g., Figure 2) may be configured to perform a saturation detection operation at step 202. Various saturation detection schemes may be used.

[0043] As an example, the saturation detection circuit 102 may include a measurement circuit configured to measure an energy level or a value representing an energy level, such as a measured current level or a measured voltage level, of the resonant tank at a measurement frequency that is equal to half the inverter switching frequency (e.g., the measurement frequency may be equal to Fs / 2). Therefore, the measurement circuit 102 is sometimes referred to as an energy measurement circuit. The energy measurement circuit may be a frequency selective energy calculation block having a bandpass filter followed by an energy integrator (as an example). As another example, the energy measurement circuit may include a fast Fourier transform (FFT) block. The saturation detection circuit 102 may compare the measured value to a threshold value.

[0044] The threshold value may be equal to a percentage of the energy level or another value representing the energy level of the resonant tank at a switching frequency Fs. The energy level at frequency Fs may be the expected amount of energy generated by the AC drive signal at the output of the inverter 60 in the absence of saturation (e.g., Figure 4 saturation. The energy level at the switching frequency Fs may also be measured in real time using the measurement circuit 102 (e.g., by tuning a bandpass filter to Fs). This 1% threshold is merely illustrative. In other embodiments, the threshold may be equal to 0.1% of the expected / measured energy level at Fs, 0.1%-1.0% of the expected / measured energy level at Fs, 2% of the expected / measured energy level at Fs, 1%-5% of the expected / measured energy level at Fs, 1%-10% of the expected / measured energy level at Fs, less than 1% of the expected / measured energy level at Fs, greater than 1% of the expected / measured energy level at Fs, or other expected fraction of the expected / measured energy level at Fs. If the measured value exceeds the threshold, saturation has been detected. If the measured value does not exceed the threshold, saturation has not been detected and saturation detection terminates (at step 204).

[0045] The above example of a measurement circuit measuring an energy level (or a value representing an energy level) at Fs / 2 is merely illustrative. For another example, the measurement circuit may measure an energy representative value at Fs / 3. For another example, the measurement circuit may measure an energy representative value at 2*Fs / 3. For another example, the measurement circuit may measure an energy representative value at Fs / 4. For another example, the measurement circuit may measure an energy representative value at 3*Fs / 4. In general, the saturation detection circuit 102 may be configured to measure an energy representative value (e.g., a measured current value or a measured voltage value) at any suitable subharmonic range or fraction of the switching frequency Fs.

[0046] The above example in which the energy measurement circuit measures a value representing the energy level at a certain fraction of the switching frequency Fs is merely illustrative. Figure 3 As shown, the lower peak 126 of the waveform 122 showing saturation may excite energy at even harmonics. Thus, the measurement circuit may measure energy levels at 2*Fs, 4*Fs, 6*Fs, and so on, and compare the measured energy levels to a threshold value that is a fraction of the expected energy level at Fs. If desired, the saturation detection circuit 102 may be configured to measure energy levels at odd harmonics of the switching frequency (e.g., 3*Fs, 5*Fs, 7*Fs, etc.).

[0047] The above examples of saturation detection circuit 102 measuring energy levels in various frequency sub-bands are merely illustrative. In other embodiments, circuit 102 may perform saturation detection in the time domain. For example, saturation detection circuit 102 may measure the peak-to-peak variation over N≥2 cycles and compare the peak measured during one cycle to the peak measured during a subsequent cycle (e.g., by calculating the ratio of peak values ​​measured from at least two consecutive cycles). Saturation detection circuit 102 may monitor peak-to-peak current, peak-to-peak voltage, and / or peak-to-peak power levels.

[0048] like Figure 4 As shown, when saturation is present, the peak-to-valley variation of waveform 122 from one cycle to another can be quite large. For example, a first Δ value can be obtained by calculating the difference between the peak and the valley during a first cycle, while a second Δ value can be obtained by calculating the difference between the peak and the valley during a second cycle after the first cycle. If the maximum Δ value or if the variance of two Δ values ​​within N cycles exceeds the Δ threshold level, saturation is detected. If the maximum Δ value or if the variance of two Δ values ​​within N consecutive cycles does not exceed the Δ threshold level, saturation has not been detected and saturation detection is terminated (at step 204). This time domain peak-to-peak variation can also be accounted for by applying a smoothing filter (e.g., using a sliding average window). The threshold level used during time domain saturation detection can be a deterministic threshold identified experimentally or via simulation.

[0049] The above examples of saturation detection circuit 102 measuring energy levels at one or more frequencies are merely illustrative. As another example, measurement circuit 41 may use a separate energy loss indicator as a proxy for saturation. Saturation may result in excessive energy loss, which may inadvertently trigger FOD and may result in a shutdown. To prevent inadvertent FOD triggering, a blanking timer may be used to temporarily deactivate FOD circuit 100 during a power ramp-up phase or during a saturation detection operation. As another example, measurement circuit 41 may be configured to measure a series capacitor (see Figure 2 When saturation occurs, a non-zero bias voltage is seen on the series capacitors.

[0050] exist Figure 6 In the example of , M consecutive positive saturation detections may be required at step 202 before continuing the saturation mitigation operation. M may be equal to one, two, three, four, five, 1-5, more than one, more than five, 5-10, or other integers. A higher value of M can help filter out potential noisy saturation measurements and prevent false positive saturation detections.

[0051] If saturation is detected, various saturation mitigation operations may be performed (see e.g. Figure 6 206, 208, 210, and / or 212 in step 206). At step 206, the control circuit may reduce the phase (e.g., duty cycle) of the AC drive signal until a minimum phase is reached or until saturation is no longer detected. The minimum phase may be equal to 70 degrees, less than 70 degrees, more than 70 degrees, 60 degrees-80 degrees, 50 degrees-90 degrees, or other phase amounts. For example, the control circuit may reduce the phase by 5° and re-perform saturation detection to check whether saturation has been alleviated. The step size of 5° is merely illustrative. If desired, a phase step size of less than 5°, more than 5°, 1-5°, 5-10°, 1-10°, or other phase deltas may be used. If desired, the phase may be reduced more rapidly at higher phase levels and reduced more gradually at lower phase levels. Once saturation is no longer detected, the saturation mitigation operation is complete.

[0052] If the minimum phase has been reached but saturation still exists, the control circuit can reduce the inverter supply voltage Vin until saturation is no longer detected (step 208). For example, the control circuit can reduce the voltage Vin by 200mV and re-perform the saturation detection to check whether the saturation has been eliminated. The step size of 200mV is merely exemplary. If desired, a voltage step size of 10mV, 50mV, 100mV, 300mV, 10mV-300mV, 190mV-210mV, 180mV-220mV, 150mV-250mV, 100mV-300mV or other voltage deltas can be used. Once saturation is no longer detected, the saturation mitigation operation can be terminated.

[0053] As another example, the control circuit can optionally adjust the switching frequency of the AC drive signal until saturation is no longer detected (step 210). Adjusting the switching frequency (e.g., increasing or decreasing Fs) can reduce the coupling gain and the half-cycle period, which can help limit the increase in the transmission coil current and thus prevent saturation. The power transfer level can also be changed to detect whether the power transfer watt level affects saturation. For example, the Qi standard allows for different power profiles. In some specific implementations, the wireless power transmitter can take saturation into account when determining whether to operate under, for example, a basic power profile or an extended power profile.

[0054] As another example, the control circuit may optionally operate inverter 60 using an asymmetric switching scheme to mitigate saturation (step 212). Figure 7 An inverter 60 is shown driving a resonant tank circuit 72 having a coil 42 connected in series with a capacitor 70. Figure 7 As shown, the inverter 60 (e.g., a full-bridge inverter) may include switches S1 and S2 coupled in series between the Vin supply voltage and the ground, and may include switches S3 and S4 coupled in series between the Vin supply voltage and the ground. The resonant tank has one terminal connected to a first switch node N1 inserted between the inverter switch S1 and the inverter switch S2, and has another terminal connected to a second switch node N2 inserted between the inverter switch S3 and the inverter switch S4.

[0055] Fig. 8A is a timing diagram showing the inverter output behavior with 180° phase shift and symmetrical switching duty cycle. Fig. 8A As shown, node N1 is driven high to the supply voltage Vin for a duration T1, which is equal to half of the inverter switching period Ts / 2. The time delay between the rising edge of N1 and the rising edge of N2 is defined as the phase shift, and is equal to 180° in this example. After the 180° phase delay, node N2 is driven high to the supply voltage Vin for a duration T2 equal to Ts / 2. The third waveform shows the result of N1 minus N2, which is the drive voltage applied to the resonant tank 72. The result is positive Vin for duration T1, and then negative Vin for duration T2. Fig. 8A It shows how N1 and N2 have equal durations, resulting in a symmetrical switching waveform (symmetrical duty cycle) across N1 minus N2, where the duration of +Vin is equal to the duration of –Vin.

[0056] Symmetrical excitation of the inverter generally results in a symmetrical resonant waveform. In particular, the voltage waveform on the series capacitor 70 will be symmetrical in the two half switching cycles, and the average value will be zero. However, if the ferrite structure or other magnetically permeable material in the device 24 is saturated by a nearby DC magnet, the magnetic permeability of such material will decrease as the resonant current moves in one direction, and increase as the resonant current moves in the other direction. This results in different resonant inductance values ​​in the two half switching cycles. If it is assumed that the voltage on the capacitor 70 initially remains symmetrical in the two half switching cycles, the voltage on the coil 42 will remain symmetrical in the two half switching cycles, so the changing inductance of the coil 42 will cause unequal currents in the two half switching cycles. Unequal currents will move the average voltage of the capacitor 70 away from zero. When the voltage on the capacitor 70 (sometimes referred to as Vctx in this article) reaches an average voltage level (sometimes referred to as DC bias in this article) that re-stores the charge balance condition for the capacitor 70, a new equilibrium state will be established. Therefore, even if the inverter excitation is symmetrical, magnetic saturation (e.g., ferrite saturation) will cause a DC bias in Vctx.

[0057] According to an embodiment, removing such DC offset in Vctx can help remove oscillations caused by saturation. Using measurement circuit 41 (see Figure 2 ) can trigger a feedback control mechanism that adjusts inverter 60 in a manner that drives average Vctx to zero. For example, in response to detecting a non-zero DC bias in Vctx using measurement circuit 41, control circuit 16M (see Figure 2 ) The inverter 60 may be adjusted to reduce the duty cycle of the AC drive signal output by the inverter 60. Reducing the duty cycle of the inverter output signal may help drive the average Vctx to zero to help alleviate saturation. Other ways of adjusting the inverter 60 to reduce the average Vctx may also be used.

[0058] For example, the unwanted DC bias in Vctx may be removed by applying opposite offsets to the duty cycles of nodes N1 and N2. Figure 8B is a timing diagram showing the inverter output with 180° phase shift and asymmetric switching duty cycle. Fig. 8AIn contrast, node N1 is driven high to the supply voltage Vin for a modified duration T1' extended by the offset Toffset, while node N2 is driven high (after a phase shift of Phase') to the supply voltage Vin for a modified duration T2' shortened by the offset Toffset. The phase shift time of node N2 (as represented by Phase') effectively becomes (Ts / 2+Toffset). Here, Toffset is shown as a positive value, but Toffset can also be a negative value. This results in different waveforms (e.g., N1 minus N2) with +Vin for duration T1' and -Vin for duration T2'. This behavior in which the duty cycles of nodes N1 and N2 are different is sometimes referred to herein as inverter switching operation with an asymmetric duty cycle.

[0059] Figure 8C is a timing diagram showing the inverter output with a 90° phase shift and an asymmetric switching duty cycle. Figure 8B In comparison, node N1 is driven high to the supply voltage Vin for a modified duration T1' extended by the offset Toffset, while node N2 is driven high (after a phase shift Phase") to the supply voltage Vin for a modified duration T2' shortened in turn by the offset Toffset. The phase shift time of node N2 (as represented by Phase") effectively becomes (Ts / 4+Toffset). Here, Toffset is shown as a positive value, but Toffset can also be a negative value. This results in different waveforms (e.g., N1 minus N2) with +Vin for duration T1" and -Vin for duration T2". This behavior in which the duty cycles of nodes N1 and N2 are different is sometimes referred to herein as inverter switching operation with an asymmetric duty cycle.

[0060] The offset Toffset can be calculated by the compensator block within the control circuit 16 (FIG. 1), using the Vctx DC offset value as a negative feedback input. Figure 7 When the DC bias is positive, the offset will be negative, which shortens T1 while lengthening T2. This compensator block can take various forms, such as a proportional-integral-derivative (PID) controller, a proportional-integral (PI) controller, or a simple integrator. The compensator block should include an integral component because the saturation mitigation loop needs to maintain the Toffset value even when the DC bias is driven to zero.

[0061] like Figure 6As shown in the example, the phase is reduced until a certain minimum phase amount, and then the voltage Vin is reduced. As another example, once saturation is detected, the control circuit can lock or fix the voltage level of Vin. This prevents the supply voltage Vin from increasing further, thereby reducing one of the causes of saturation. The examples herein used to reduce saturation in response to detecting saturation caused by saturation are merely illustrative. In general, various embodiments for reducing saturation can also be applied in response to detecting that a magnetically permeable material exceeds its magnetic saturation level.

[0062] According to an embodiment, a wireless power transmission device is provided, comprising: a resonant circuit having a wireless power transmission coil, the wireless power transmission coil being configured to transmit wireless power to a wireless power receiving device; an inverter, the inverter being configured to drive an alternating current signal onto the wireless power transmission coil; a measurement circuit, the measurement circuit being configured to measure a value representing an energy level in the resonant circuit; and a control circuit, the control circuit being configured to detect saturation in the resonant circuit by comparing the measured value with a threshold value, and to control wireless power transmission via the wireless power transmission coil in response to detecting the saturation.

[0063] According to another embodiment, the measured value comprises a measured current level.

[0064] According to another embodiment, the measured value comprises a measured voltage level.

[0065] According to another embodiment, the measuring circuit is configured to measure the value at a measuring frequency in a subharmonic range of the switching frequency.

[0066] According to another embodiment, the measuring frequency is half the switching frequency.

[0067] According to another embodiment, the measuring circuit is configured to measure the value at a measuring frequency, which is a harmonic of the switching frequency.

[0068] According to another embodiment, the measuring frequency is even a multiple of the switching frequency.

[0069] According to another embodiment, the measurement circuit is configured to measure an additional value representing the energy level in the resonance circuit at the switching frequency, and the threshold value is a fraction of the additional value.

[0070] According to another embodiment, the measured value comprises a measured current level or a measured voltage level, and the control circuit detects saturation by comparing a variance of the measured value to a threshold value.

[0071] According to another embodiment, the measurement comprises a peak-to-valley difference, and the control circuit detects saturation by comparing a variance of the peak-to-valley difference to a threshold value.

[0072] According to another embodiment, the measurement includes a peak-to-valley difference, and the control circuit detects saturation by comparing the peak-to-valley difference to a threshold value.

[0073] According to another embodiment, the measured value comprises a measured current level or a measured voltage level, and the control circuit detects saturation by comparing a ratio of the measured values ​​in two consecutive cycles to a threshold value.

[0074] According to another embodiment, the control circuit is configured to control wireless power transfer via the wireless power transfer coil by reducing a duty cycle of an alternating current signal driven onto the wireless power transfer coil in response to detecting saturation.

[0075] According to another embodiment, the control circuit is configured to control wireless power transfer via the wireless power transfer coil by reducing a power supply voltage providing power to the inverter in response to detecting saturation.

[0076] According to another embodiment, the control circuit is configured to control wireless power transfer via the wireless power transfer coil in response to detecting saturation by limiting the wireless power to a predetermined power level to alleviate the saturation.

[0077] According to another embodiment, the control circuit is configured to control wireless power transfer via the wireless power transfer coil by adjusting a switching frequency of the inverter in response to detecting saturation.

[0078] According to another embodiment, the resonant circuit also includes a capacitor coupled in series with the wireless power transmission coil, and the control circuit is configured to control wireless power transmission via the wireless power transmission coil by operating the inverter using an asymmetric duty cycle to remove the DC bias voltage on the capacitor in response to detecting saturation.

[0079] According to another embodiment, the control circuit is configured to increase the wireless power in response to not detecting saturation.

[0080] According to another embodiment, the wireless power transmission includes a foreign object detection circuit configured to detect the presence of a foreign object on the wireless power transmission device, wherein the foreign object detection circuit is deactivated when the control circuit increases the wireless power.

[0081] According to another embodiment, the control circuit is configured to receive a request from the wireless power receiving device to increase wireless power after saturation is detected and ignore the request until saturation is no longer detected.

[0082] According to an embodiment, a wireless power transmission device is provided, which includes a wireless power transmission coil configured to transmit wireless power to a wireless power receiving device, an inverter configured to drive an alternating current signal to the wireless power transmission coil, and a control circuit configured to detect a characteristic condition at a magnetically permeable structure of the wireless power receiving device, and adjust the inverter in response to detecting the characteristic condition at the magnetically permeable structure of the wireless power receiving device.

[0083] According to another embodiment, the magnetically permeable structure includes a ferrite structure, and the characteristic condition includes ferrite saturation at the ferrite structure.

[0084] According to another embodiment, the control circuit is configured to detect the characteristic condition by measuring a current or voltage level and comparing the measured current or voltage level to a threshold value.

[0085] According to another embodiment, the control circuit is configured to regulate the inverter by reducing a duty cycle of an alternating current signal driven onto the wireless power transfer coil in response to detecting the characteristic condition.

[0086] According to another embodiment, the inverter is configured to receive a power supply voltage, and the control circuit is configured to regulate the inverter by reducing the power supply voltage in response to detecting the characteristic condition.

[0087] According to another embodiment, the inverter is configured to receive a power supply voltage, and the control circuit is configured to fix the power supply voltage in response to detecting the characteristic condition.

[0088] According to another embodiment, the inverter is configured to drive the AC power signal at a switching frequency, and the control circuit is configured to regulate the inverter by adjusting the switching frequency.

[0089] According to another embodiment, the wireless power transmission device includes a foreign object detection circuit, which is configured to detect the presence of a foreign object on the wireless power transmission device, the inverter is configured to receive a power supply voltage, and when the control circuit is ramping the power supply voltage, the foreign object detection circuit is deactivated.

[0090] According to an embodiment, a wireless charging system is provided, which includes: a wireless power receiver having a magnetically permeable material; and a wireless power transmitter, the wireless power sensor being configured to transmit wireless power to the wireless power receiver, the wireless power transmitter having a slot circuit with a wireless power transmission coil, the wireless power transmission coil being configured to transmit wireless power to a wireless power receiving device; an inverter, the inverter being configured to drive an alternating current signal to the wireless power transmission coil at a switching frequency, and a control circuit, the control circuit being configured to detect characteristic conditions at the magnetically permeable material in the wireless power transmission device, and to control the wireless power transmission via the wireless power transmission coil in response to detecting the characteristic conditions.

[0091] According to another embodiment, the characteristic condition includes saturation at the magnetically transparent material.

[0092] According to another embodiment, the magnetically permeable material includes a ferrite, and the characteristic condition includes ferrite saturation at the ferrite.

[0093] According to another embodiment, the characteristic condition induces oscillations in the tank circuit.

[0094] According to another embodiment, the wireless power transmitter includes a measurement circuit configured to measure a value representing an energy level in a tank circuit at a measurement frequency that is a function of a switching circuit, and the control circuit is configured to detect a characteristic condition by comparing the measurement value to a threshold value.

[0095] According to another embodiment, the wireless power transmitter includes a measurement circuit configured to measure a peak value, and the control circuit is configured to detect the characteristic condition by comparing a measure as a function of the measured peak value to a threshold value.

[0096] According to another embodiment, the wireless power transmitter includes a foreign object detection circuit configured to detect the presence of a foreign object on the wireless power transfer device, and when the control circuit detects the characteristic condition, the foreign object detection circuit is deactivated.

[0097] The foregoing is merely exemplary and various modifications may be made to the embodiments described. The foregoing embodiments may be implemented independently or in any combination.

Claims

1. A control circuit for a wireless power transmitter of a wireless power transmission device, the wireless power transmitter comprising: a communication circuit configured to be coupled to a wireless power transfer coil of the wireless power transmitter, the communication circuit further configured to receive communications from the wireless power receiver via modulation of one or more parameters of the wirelessly transferred power by a wireless power receiver of a wireless power receiving device; a measurement circuit configured to be coupled to the wireless power transmission coil and configured to detect saturation of a magnetic core of the wireless power reception coil, wherein the measurement circuit is configured to detect saturation of the magnetic core of the wireless power reception coil by detecting rising current peaks in alternating cycles of current passing through the wireless power transmission coil at a wireless power transmission frequency; as well as An inverter drive circuit is configured to operate an inverter of the wireless power transmitter to mitigate the detected saturation of the magnetic core in response to the measurement circuit.

2. The control circuit of claim 1, wherein the inverter drive circuit is further configured to operate the inverter to deliver a power level requested by the wireless power receiver in response to the communication circuit. 3 . The control circuit of claim 1 , wherein the inverter drive circuit is configured to operate the inverter in response to the measurement circuit to mitigate the detected saturation of the magnetic core by reducing the output power of the inverter.

4. The control circuit of claim 3, wherein reducing the output power of the inverter comprises at least one of the following: reducing the phase of a drive signal provided to the inverter; Reduce the inverter supply voltage; or The inverter is operated with an asymmetric switching scheme.

5. The control circuit of claim 4, wherein the asymmetric switching scheme comprises reducing the on-time of the positive half cycle.

6. The control circuit of claim 1 , wherein the measurement circuit is configured to detect saturation of the magnetic core in the frequency domain by measuring energy in a resonant tank including the wireless power transmission coil and a capacitor in a frequency band corresponding to a portion of the wireless power transmission frequency. 7 . The control circuit of claim 6 , wherein the portion of the wireless power transfer frequency corresponds to a subharmonic of the wireless power transfer frequency.

8. The control circuit of claim 6, wherein the portion is half of the wireless power transfer frequency.

9. The control circuit of claim 1, wherein the measurement circuit comprises a frequency selective energy calculation block having a band pass filter followed by an energy integrator.

10. The control circuit of claim 1, wherein the measurement circuit comprises a Fast Fourier Transform block.

11. The control circuit of claim 6 , wherein the measurement circuit compares the measured energy to a threshold value, the threshold value being a percentage of the energy level of the resonant tank in a frequency band corresponding to the wireless power transmission frequency, wherein saturation is detected if the measured energy exceeds the threshold value, and saturation is not detected if the measured energy does not exceed the threshold value. 12 . The control circuit of claim 11 , wherein the threshold is a percentage between 1% and 10% of the energy level of the resonant tank in the frequency band corresponding to the wireless power transmission frequency. 13 . The control circuit of claim 12 , wherein the threshold is a percentage between 1% and 5% of the energy level of the resonant tank in the frequency band corresponding to the wireless power transmission frequency. 14 . The control circuit of claim 13 , wherein the threshold is 2% of the energy level of the resonant tank in the frequency band corresponding to the wireless power transmission frequency.

15. The control circuit of claim 1, wherein the measurement circuit detects saturation of the magnetic core in the time domain by measuring a peak-to-valley inverter current over two or more cycles of the wireless power transfer frequency.

16. The control circuit of claim 15, wherein measuring the peak-to-valley inverter current over two or more cycles of the wireless power transfer frequency comprises: calculating a first delta value as a difference between a peak and a valley of the inverter current over a first cycle; calculating a second delta value as a difference between a peak and a valley of the inverter current over a second cycle; as well as saturation is detected if a maximum Δ value of the first Δ value and the second Δ value or a variance of the first Δ value and the second Δ value over two or more cycles exceeds a threshold; or If the maximum delta value or the variance of the first delta value and the second delta value over two or more cycles does not exceed the threshold, saturation is not detected. 17 . The control circuit of claim 6 , wherein the measurement circuit is further configured to detect saturation of the magnetic core by using a DC voltage on the capacitor.

18. The control circuit of claim 1, wherein the measurement circuit detects saturation of the magnetic core only in response to a plurality of consecutive saturation detection events.

19. A method of operating a wireless power transmitter of a wireless power transmission device, the wireless power transmitter having a resonant tank including a wireless power transmission coil and a capacitor, an inverter driving the resonant tank at a wireless power transfer frequency to transfer power to a wireless power receiver of a wireless power receiving device via a wireless power receiving coil of the wireless power receiver, and a control circuit, the method being performed by the control circuit and comprising: receiving a communication from the wireless power receiver via a communication circuit coupled to the wireless power transfer coil, the wireless power transfer coil detecting modulation of one or more parameters of wirelessly transferred power by the wireless power receiver; detecting saturation of a magnetic core of the wireless power receiving coil using a measurement circuit coupled to the wireless power transmitting coil, wherein the measurement circuit is configured to detect saturation of the magnetic core of the wireless power receiving coil by detecting increasing current peaks in alternating cycles of current passing through the wireless power transmitting coil at a wireless power transmission frequency; as well as The inverter is operated using an inverter drive circuit to mitigate the detected saturation of the magnetic core in response to the measurement circuit.

20. The method of claim 19, further comprising operating the inverter using inverter drive circuitry in response to the communication circuitry to deliver a power level requested by the wireless power receiver.

21. The method of claim 19, wherein operating an inverter drive circuit in response to the measurement circuit to mitigate the detected saturation of the magnetic core comprises reducing an output power of the inverter.

22. The method of claim 21, wherein reducing the output power of the inverter further comprises at least one of the following: reducing the phase of a drive signal provided to the inverter; Reduce the inverter supply voltage; or The inverter is operated with an asymmetric switching scheme.

23. The method of claim 22, wherein operating the inverter with the asymmetric switching scheme comprises reducing an on-time of a positive half cycle.

24. The method of claim 19, wherein detecting elevated current peaks in alternating cycles of current through the wireless power transfer coil at the wireless power transfer frequency further comprises measuring energy in the resonant tank in a frequency band corresponding to a portion of the wireless power transfer frequency.

25. The method of claim 24, wherein the portion of the wireless power transfer frequency corresponds to a subharmonic of the wireless power transfer frequency.

26. The method of claim 24, wherein the portion is half of the wireless power transfer frequency.

27. The method of claim 19, wherein detecting elevated current peaks of current passing through the wireless power transfer coil at the wireless power transfer frequency in alternating cycles further comprises: comparing the measured energy to a threshold value, the threshold value being a percentage of an energy level of the resonant tank in a frequency band corresponding to the wireless power transmission frequency; as well as Saturation is detected if the measured energy exceeds the threshold, and saturation is not detected if the measured energy does not exceed the threshold.

28. The method of claim 27, wherein the threshold is a percentage between 1% and 10% of the energy level of the resonant tank in the frequency band corresponding to the wireless power transmission frequency.

29. The method of claim 28, wherein the threshold is a percentage between 1% and 5% of the energy level of the resonant tank in the frequency band corresponding to the wireless power transmission frequency.

30. The method of claim 29, wherein the threshold is 2% of the energy level of the resonant tank in the frequency band corresponding to the wireless power transmission frequency.

31. The method of claim 19, wherein detecting rising current peaks of current passing through the wireless power transfer coil at the wireless power transfer frequency in alternating cycles further comprises detecting saturation of the magnetic core in the time domain by measuring peak-to-valley inverter current over two or more cycles of the wireless power transfer frequency.

32. The method of claim 31 , wherein measuring the peak-to-valley inverter current over two or more cycles of the wireless power transfer frequency further comprises: calculating a first delta value as a difference between a peak and a valley of the inverter current over a first cycle; calculating a second delta value as a difference between a peak and a valley of the inverter current over a second cycle; as well as saturation is detected if a maximum Δ value of the first Δ value and the second Δ value or a variance of the first Δ value and the second Δ value over two or more cycles exceeds a threshold; or If the maximum delta value or the variance of the first delta value and the second delta value over two or more cycles does not exceed the threshold, saturation is not detected.

33. The method of claim 19, wherein measuring saturation of a magnetic core of the wireless power transmission coil using a measurement circuit coupled to the wireless power transmission coil further comprises detecting saturation of the magnetic core by using a DC voltage on the capacitor.

34. The method of claim 19, further comprising detecting saturation of the magnetic core only in response to a plurality of consecutive saturation detection events.

35. A wireless power transmitter of a wireless power transmission device, the wireless power transmitter comprising: a resonant tank comprising a wireless power transmission coil and a capacitor; an inverter that drives the resonant tank at a wireless power transmission frequency to transmit power to the wireless power receiver via a wireless power receiving coil of a wireless power receiver of a wireless power receiving device; The control circuit further comprises: a communication circuit coupled to the wireless power transmission coil, configured to receive communications from the wireless power receiver via modulation of one or more parameters of wirelessly transmitted power by the wireless power receiver; a measurement circuit coupled to the wireless power transmission coil, the measurement circuit detecting saturation of a magnetic core of the wireless power reception coil, wherein the measurement circuit detects saturation of the magnetic core of the wireless power reception coil by detecting rising current peaks in alternating cycles of current passing through the wireless power transmission coil at the wireless power transmission frequency; and An inverter drive circuit operates the inverter to mitigate the detected saturation of the magnetic core in response to the measurement circuit.

36. The wireless power transmitter of claim 35, wherein the inverter drive circuit is further responsive to the communication circuit to operate the inverter to deliver a power level requested by the wireless power receiver.

37. The wireless power transmitter of claim 35, wherein the inverter drive circuit operates the inverter in response to the measurement circuit to mitigate detected saturation of the magnetic core by reducing output power of the inverter.

38. The wireless power transmitter of claim 37, wherein reducing the output power of the inverter comprises at least one of: reducing the phase of a drive signal provided to the inverter; Reduce the inverter supply voltage; or The inverter is operated with an asymmetric switching scheme.

39. The wireless power transmitter of claim 38, wherein the asymmetric switching scheme comprises reducing the on-time of a positive half cycle.

40. The wireless power transmitter of claim 35, wherein the measurement circuit detects saturation of the magnetic core in the frequency domain by measuring energy in the resonant tank in a frequency band corresponding to a portion of the wireless power transmission frequency.

41. The wireless power transmitter of claim 40, wherein the portion of the wireless power transfer frequency corresponds to a subharmonic of the wireless power transfer frequency.

42. The wireless power transmitter of claim 40, wherein the portion is half of the wireless power transfer frequency.

43. The wireless power transmitter of claim 35, wherein the measurement circuit comprises a frequency selective energy calculation block having a bandpass filter followed by an energy integrator.

44. The wireless power transmitter of claim 35, wherein the measurement circuit comprises a fast Fourier transform block.

45. The wireless power transmitter of claim 35 , wherein the measurement circuit compares the measured energy to a threshold value, the threshold value being a percentage of the energy level of the resonant tank in a frequency band corresponding to the wireless power transmission frequency, wherein saturation is detected if the measured energy exceeds the threshold value, and saturation is not detected if the measured energy does not exceed the threshold value.

46. ​​The wireless power transmitter of claim 45, wherein the threshold is a percentage between 1% and 10% of the energy level of the resonant tank in the frequency band corresponding to the wireless power transmission frequency.

47. The wireless power transmitter of claim 46, wherein the threshold is a percentage between 1% and 5% of the energy level of the resonant tank in the frequency band corresponding to the wireless power transmission frequency.

48. The wireless power transmitter of claim 47, wherein the threshold is 2% of the energy level of the resonant tank in the frequency band corresponding to the wireless power transmission frequency.

49. The wireless power transmitter of claim 35, wherein the measurement circuit detects saturation of the magnetic core in the time domain by measuring peak-to-valley inverter current over two or more cycles of the wireless power transfer frequency.

50. The wireless power transmitter of claim 49, wherein measuring the peak-to-valley inverter current over two or more cycles of the wireless power transfer frequency comprises: calculating a first delta value as a difference between a peak and a valley of the inverter current over a first cycle; calculating a second delta value as a difference between a peak and a valley of the inverter current over a second cycle; as well as saturation is detected if a maximum Δ value of the first Δ value and the second Δ value or a variance of the first Δ value and the second Δ value over two or more cycles exceeds a threshold; or If the maximum delta value or the variance of the first delta value and the second delta value over two or more cycles does not exceed the threshold, saturation is not detected.

51. The wireless power transmitter of claim 35, wherein the measurement circuit detects saturation of the magnetic core by using a DC voltage on the capacitor.

52. The wireless power transmitter of claim 35, wherein the measurement circuit detects saturation of the magnetic core only in response to a plurality of consecutive saturation detection events.

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