Patent Publication Number: US-2023163633-A1

Title: Hw and methods for improving safety protocol in wireless chargers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 17/374,840, filed on Jul. 13, 2021, which is a divisional application of U.S. application Ser. No. 16/413,283, filed on May 15, 2019, now issued as U.S. Pat. No. 11,095,146, which applications are hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to hardware (HW), and in particular embodiments, to an over-voltage protection circuit in a wireless power system. 
     BACKGROUND 
     Generally, a wireless power system uses mutual coupling to transfer power from a transmitting device to a receiving device. Myriad of causes can trigger the transmitting device to provide a voltage exceeding the operating capabilities of the various components of the receiving device. The excess power can cause hazardous conditions to the user, the receiving device, and the transmitting device. 
     Existing methods and systems use clamping of the AC voltage at the receiver side or de-tuning of the system impedance to counteract the excess voltage condition. As an example, a Zener clamping circuit is used to clamp the incoming AC voltage at the receiver side. As another example, de-tuning the receiving circuit may reduce the coupling factor between the primary winding and the secondary winding of the transformer and reduce the incoming power. Each solution can provide limited protection. However, the system remains susceptible to large amounts of incoming power. 
     Another method to counteract the excess power condition is hard over-voltage protection (HOVP). In hard over-voltage protection, when incoming voltage exceeds a threshold, the protection is triggered and the loading elements are shorted to ground. This solution disadvantageously causes the excess power to be dissipated solely in localized elements. Generally speaking, an excess of incoming power should be spread over several elements to avoid overstressing a single element. Additionally, the solution may cause in-band communication issues between the receiving device and the transmitting device, which may impact the response time for the transmitting device to reduce the excess power. 
     Thus, a need exists for an improved system and method to overcome these and other limitations in the existing solutions to counteract over-voltage conditions in wireless power systems. 
     SUMMARY 
     Technical advantages are generally achieved by embodiments of this disclosure, which describe an over-voltage protection circuit in a wireless power system. 
     A first aspect relates to an over-voltage protection circuit, the over-voltage protection circuit includes a first and second differential amplifier, a comparator, a switch, and a regulating circuit. A first input of the first differential amplifier is coupled to an output of a rectifier and a second input of the first differential amplifier is coupled to an output of a voltage regulator. A first input of the second differential amplifier is coupled to an output of the first differential amplifier and a second input of the second differential amplifier is coupled to a first reference voltage. A first input of the comparator is coupled to the output of the first differential amplifier and a second input of the comparator is coupled to a second reference voltage. An input of the switch is coupled to the output of the second differential amplifier and a control terminal of the switch is coupled to an output of the comparator. An input of the regulating circuit is coupled to the output of the rectifier, a control terminal of the regulating circuit is coupled to the output of the switch, and an output of the regulating circuit is coupled to reference ground. 
     In a first implementation form of the over-voltage protection circuit according to the first aspect, the first differential amplifier is configured to provide an output voltage corresponding to an amplified difference between a voltage at the output of the rectifier and a voltage at the output of the voltage regulator. The output voltage of the first differential amplifier is provided to each of the comparator and the second differential amplifier. 
     In a second implementation form of the over-voltage protection circuit according to the first aspect as such or any preceding implementation form of the first aspect, the second differential amplifier is configured to provide a control voltage to the control terminal of the regulating circuit. A value of the control voltage corresponds to an amplified voltage difference between a voltage at the output of the rectifier and a voltage at the output of the voltage regulator in reference to the first reference voltage. 
     In a third implementation form of the over-voltage protection circuit according to the first aspect as such or any preceding implementation form of the first aspect, the comparator is configured to provide a control signal to the switch. A value of the control signal corresponds to an amplified voltage difference between a voltage at the output of the rectifier and a voltage at the output of the voltage regulator in reference to the second reference voltage. 
     In a fourth implementation form of the over-voltage protection circuit according to the first aspect as such or any preceding implementation form of the first aspect, the switch is activated in response to a voltage difference between a voltage at the output of the rectifier and a voltage at the output of the voltage regulator exceeding a first threshold, the voltage at the output of the rectifier exceeding a second threshold, the voltage the output of the voltage regulator exceeding a third threshold, or a combination thereof. 
     In a fifth implementation form of the over-voltage protection circuit according to the first aspect as such or any preceding implementation form of the first aspect, the regulating circuit is configured to regulate a voltage at the output of the rectifier in accordance with a control voltage provided by the second differential amplifier. 
     In a sixth implementation form of the over-voltage protection circuit according to the first aspect as such or any preceding implementation form of the first aspect, the regulating circuit includes an active device and a dissipating element. 
     A second aspect relates to device that includes a rectifier, a voltage regulator, a monitoring circuit, and a regulating circuit. The rectifier is configured to receive an alternating current (AC) voltage and output a direct current (DC) voltage. The voltage regulator is configured to receive the DC voltage and output a regulated DC voltage. The monitoring circuit includes a first differential amplifier, a second differential amplifier, a comparator, and a switch. The monitoring circuit is configured to monitor the DC voltage, the regulated DC voltage, a difference between the DC voltage and the regulated DC voltage, or a combination thereof. The regulating circuit configured to regulate the DC voltage in response to the monitoring circuit determining that the AC voltage exceeds a steady-state operating condition. 
     In a first implementation form of the device according to the second aspect, the monitoring circuit further includes a multiplexer, a second comparator, and a third comparator. The regulating circuit is activated based on a combinational logic of the value of the DC voltage, the regulated DC voltage, and a voltage difference between the DC voltage and the regulated DC voltage. 
     In a second implementation form of the device according to the second aspect as such or any preceding implementation form of the second aspect, the device further includes a microcontroller configured to communicate an End Power Transfer (EPT) request to stop the AC voltage in response to the AC voltage exceeding the steady-state operating condition. 
     In a third implementation form of the device according to the second aspect as such or any preceding implementation form of the second aspect, the microcontroller is configured to communicate the EPT request after a delay period is elapsed from a time that the regulating circuit begins to regulate the DC voltage. 
     In a fourth implementation form of the device according to the second aspect as such or any preceding implementation form of the second aspect, the communicating the EPT request is communicated using an in-band communication path. 
     In a fifth implementation form of the device according to the second aspect as such or any preceding implementation form of the second aspect, the regulating circuit is further configured to stop regulating the DC voltage after an elapsing of a programmable time period at which time the rectifier is configured in a hard over-voltage protection mode to stop receiving the AC voltage. 
     In a sixth implementation form of the device according to the second aspect as such or any preceding implementation form of the second aspect, the device further includes a load configured to receive the regulated DC voltage. 
     A third aspect relates to a method for regulating an over-voltage condition in a wireless power system, the method includes monitoring a voltage at an output of a rectifier, a voltage at an output of a voltage regulator, or a combination thereof. The method further includes determining the over-voltage condition based on the monitoring; and in response to determining the over-voltage condition, regulating the voltage at the output of the rectifier in accordance with a voltage difference between the voltage at the output of the rectifier and the voltage at the output of the voltage regulator. 
     In a first implementation form of the method according to the third aspect, the method further includes communicating an End Power Transfer (EPT) request using an in-band communication path after determining the over-voltage condition. 
     In a second implementation form of the method according to the third aspect as such or any preceding implementation form of the third aspect, the communicating the EPT request is delayed for a set length of time after determining the over-voltage condition. 
     In a third implementation form of the method according to the third aspect as such or any preceding implementation form of the third aspect, monitoring the voltages includes continuously comparing one or more of the voltage at the output of the rectifier, the voltage at the output of the voltage regulator, or a voltage difference between the voltage at the output of the rectifier and the voltage at the output of the voltage regulator to a respective reference voltage. 
     In a fourth implementation form of the method according to the third aspect as such or any preceding implementation form of the third aspect, each respective reference voltage is a programmable value. 
     In a fifth implementation form of the method according to the third aspect as such or any preceding implementation form of the third aspect, determining the over-voltage condition based on the monitoring includes determining that one or more combinational logic of the output of the comparing satisfies the over-voltage condition. 
     Embodiments can be implemented in hardware, software, or in any combination thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a schematic diagram of a wireless power system; 
         FIG.  2    is a schematic diagram of an embodiment wireless power system having a regulation loop circuit; 
         FIG.  3    is a schematic diagram of another embodiment wireless power system having a regulation loop circuit; 
         FIG.  4    is a schematic of an embodiment regulating circuit for a wireless power system; 
         FIGS.  5 A-C  are plots of embodiment operational timing diagrams; 
         FIG.  6    is a schematic diagram of an embodiment wireless power system having a current sensing element; 
         FIG.  7 A  is a schematic diagram of a receiving device configured with Zener diodes in a clamping circuit; 
         FIG.  7 B  is a schematic diagram of a receiving device configured with a de-tuning circuit; 
         FIG.  7 C  is a schematic diagram of a receiving device configured with hard over-voltage protection; and 
         FIG.  8    is a flowchart of an embodiment method for monitoring and regulating voltages in a wireless power system to address an over-voltage condition. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     This disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments are merely illustrative of specific configurations and do not limit the scope of the claimed embodiments. Features from different embodiments may be combined to form further embodiments unless noted otherwise. 
     Variations or modifications described with respect to one of the embodiments may also be applicable to other embodiments. Further, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of this disclosure as defined by the appended claims. 
     While the inventive aspects are described primarily in the context of a wireless power system and inductive coupling, it should also be appreciated that these inventive aspects may also be applicable to any type of wireless energy transfer. For example, the embodiment methods and systems described in this disclosure can be similarly applied to capacitive coupling. Furthermore, the wireless power system may operate using resonant or non-resonant coupling. 
     In a typical wireless power system, mutual coupling is used to wirelessly transfer power from a transmitting device to one or more receiving devices. For instance, a transmitting device generates an alternating current (AC), using a power source. An alternating current at a first set of windings, or coils, of the transmitting device generates an alternating current at a second set of windings of the receiving device, using mutual inductance. The receiving device, using a rectifier, converts the alternating current at the second windings to a direct current (DC) voltage. The DC voltage may then be stored in a battery or consumed at a load of the receiving device. 
     A potential safety issue may arise if the received power, at the receiving device, exceeds the power need of the receiving device component(s) (e.g., receiving elements, load elements, etc.). This may lead to situations in which the receiving elements exceed their operating limits. 
     It is typical for the transmitting device to generate more power than may be required by the receiving device. For instance, the transmitting device may be used to wirelessly transfer power to more than one receiving device. In this example, the transmitting device may generate sufficient power to support both receiving devices. As another example, the transmitting device may increase the power at the transmitting end to compensate for losses between the transmitting and receiving devices due to, for example, a mismatch between the two devices. 
     For various reasons, the receiving device may receive an excess of incoming power that exceeds the operating limits of the receiving device. For instance, a load element at the receiving device may be disconnected and the event may not be properly communicated to the transmitting device in a request to reduce the power level. Or the receiving device may be slightly repositioned in reference to the transmitting device, which may cause a change in an impedance at each coil. It is possible that the coupling between the two devices is now improved and the receiving device suddenly receives a large amount of power. As another example, a defect at the transmitting device may lead to excess power to be received by the receiving device. 
     Regardless of the cause, an excess of incoming power at the receiving device may lead to a presence of an excess voltage (i.e., over-voltage) across, or an excess current (i.e., over-current) through, the receiving and loading elements of the receiving device. The excess of incoming power may instantly, or gradually, damage the components of the receiving device, or even the components of the transmitting device, which may be additionally harmful and hazardous to the end-user. Accordingly, system and methods to ensure the safe operation of the wireless power transfer are desired. 
     Amplitude shift keying (ASK) modulation is a form of modulation that can be used for communication between the transmitting device and the receiving device. In ASK modulation, the carrier frequency signal is multiplied by a binary digital to form a variable amplitude signal and information is communicated in the form of the carrier&#39;s amplitude. The receiving device uses backscatter modulation, by modulating the power drawn from the transmitting device, to communicate with the transmitting device. In an exemplary communication protocol, the ASK modulation allows the receiving device to communicate with the transmitting device. 
     In previous solutions, the ASK modulation signal becomes erratic and the signal communicated from the receiving device to the transmitting device, for example, the end power transfer (EPT) request, becomes corrupted. As a result, the transmitting device may not be able to properly receive the communication from the receiving device and the EPT request may be ignored. This in turn may cause continued damage to one or both of the devices. 
     Embodiments of this disclosure provide a system and method to monitor an over-voltage condition in a receiving device of the wireless power system. In various embodiments, a monitoring circuit determines the over-voltage condition at the output of a rectifier and/or at the output of a voltage regulator. Each voltage may be compared to a threshold voltage to determine the over-voltage condition. In some embodiments, the monitoring circuit is used to determine whether the voltage at the output of the rectifier exceeds the voltage at the output of the regulator by a threshold voltage. In some embodiments, the monitoring circuit may use a combinational logic, based on direct or differential monitoring of the various voltages, to determine the over-voltage condition. 
     In response to determining that an over-voltage condition has occurred, a regulating circuit is used to regulate the voltage at the output of the rectifier. In the event of an over-voltage condition, the monitoring circuit provides a control voltage to the regulating circuit. The regulating circuit includes an active device coupled to a dissipating element. The input of the active device is coupled to a node shared between the output of the rectifier and the input of the voltage regulator. The output of the active device is coupled to a dissipating element connected to the reference ground. The conductivity of the active device is controlled by the value of the control voltage. As the conductivity of the active device changes, the amount that the dissipating element dissipates varies. The control voltage is set such the dissipating element only dissipates the amount needed to regulate the output of the rectifier to the steady-state operational condition. Advantageously, the excess power is spread at the load and the regulating circuit, which minimizes damage to each device. 
     Aspects of this disclosure provide a regulated voltage in the event of an over-voltage condition, permitting the receiving device to communicate the abnormal condition to the transmitting device using, for example, ASK modulation. The monitoring circuit and the regulating circuit form a regulation loop circuit that ensures proper in-band communication from the receiving device to and from the transmitting device. In some embodiments, the regulation loop circuit conditions the voltage at the input of the voltage regulator to a stable value for a period of time such that the receiving device can communicate a request to the transmitting device for a reduction in the level of the power transference. Thus, the various receiving and loading elements of the receiving device remain properly biased to allow the receiving device an opportunity to communicate with the transmitting device. 
     In some embodiments, the receiving device may send an end power transfer (EPT) request to the transmitting device after an elapsing of a specific period of time from the activation of the regulating circuit. This advantageously provides the wireless power system a period of time to ignore any short surges of incoming power from the transmitting device to the receiving device, and to ignore any faulty information which would result, for example, from noise originating from the detection system (e.g., de-bouncing system). Thus, providing a smooth reaction response to an excess input power condition at the receiving device. These and other details are discussed in greater detail below. 
       FIG.  1    illustrates a schematic diagram of a wireless power system  100 . The wireless power system  100  includes a transmitting device  110  and a receiving device  120 . The wireless power system  100  is used to wirelessly transfer power  140  from the transmitting device  110  to the receiving device  120 . The transmitting device  110  may be a base station, such as a wireless charger, which provides inductive power. The receiving device  120  may be a mobile device, such as a personal computer (PC), a tablet, a cellular phone, a wearable communications device (e.g., a smartwatch, etc.), a toothbrush, an internet of things (IOTs) device, or others. The receiving device  120  consumes inductive power. 
     The transmitting device  110  may have a flat surface on which the receiving device  120  may be placed. The receiving device  120  may use guided positioning or free positioning to properly position the receiving device  120  with respect to the transmitting device  110 . 
     The transmitting device  110  includes a power source  112 , a supply-side capacitor  114 , and first windings  116 . The receiving device  120  includes second windings  122 , a resonant capacitor  124 , a rectifier  126 , a shunt capacitor  128 , a voltage regulator  130 , and a load  132 . 
     The power source  112  is any device that generates alternating current (AC) power supplied to the first windings  116 . Each of the first windings  116  and second windings  122  may be a loop antenna or a magnetic antenna. The windings may include a physical core (e.g., ferrite core) or an air core. The windings may be implemented as an antenna strip or using a Litz wire. The resonant frequency of the windings is based on the shape and size of the looping wire or coil. In some embodiments, additional capacitance and inductance may be added to each winding to create a resonant structure at the desired resonant operating frequency. 
     The rectifier  126  is a device that converts an alternating current (AC) voltage to a direct current (DC) voltage. The rectifier  126  may be any type of rectifier, such as a low-impedance synchronous rectifier having full-wave or half-wave rectification. The rectifier  126  shown in  FIG.  1    is a bridge rectifier, however, other types of rectifiers may also be contemplated. 
     The shunt capacitor  128  may be referred to as a smoothing or a reservoir capacitor. The shunt capacitor  128  is used to decrease any ripple at the input of the voltage regulator  130  from the rectifier  126 . A terminal of the shunt capacitor  128  is shared with the output terminal of the rectifier  126  and an input terminal of the voltage regulator  130  at the V RECT  node  220 . The second terminal of the shunt capacitor  128  is connected to the reference voltage V REF  at the V REF  node  160 . Additional filtering components may be contemplated. 
     The voltage regulator  130  is a device that maintains a constant output voltage for the load  132 . The voltage regulator  130  receives an input voltage from the rectifier  126 . The voltage regulator  130  may be any type of voltage regulator, such as a linear regulator (e.g., low drop-out (LDO) linear regulator). 
     In some embodiments, the rectifier  126  and voltage regulator  130  may be part of a switched-mode power supply (SMPS) circuit. 
     The transferred power is received at the load  132 . The load  132  may be a charge storage device, such as a battery. For instance, the load  132  may be a battery of a cellular phone. The transmitting device may be a charging pad and the cellular phone may be placed on the charging pad. The charging pad transfers wireless power to the battery of the cellular phone without the need for interconnecting cables between the two devices. 
     The power source  112  generates an alternating current (AC) at the first windings  116 , which induces a magnetic field at the second windings  122 , typically at a specified distance. The induced magnetic field in turn induces an alternating current at the second windings  122  through mutual coupling. The rectifier  126  converts the AC voltage at the second windings  122  to a direct current (DC) voltage. The voltage regulator  130  converts the DC voltage to match a desired DC voltage for the load  132 . 
     The receiving device  120  can communicate with the transmitting device  110 , for example, to properly adjust the power received at the receiving device  120 . A number of interface standards have been developed with the goal of standardizing wireless power transfer and related functions. For instance, Qi, which is promoted by the Wireless Power Consortium (WPC), and Rezence, which is promoted by the AirFuel Alliance are two competing standards that define interface standards for wireless power transfer. These and other similar types of protocols may be used to define the communication interface for adjusting the power supplied by the transmitting device  110  and demanded by the receiving device  120 . For instance, the receiving device  120  may request, from the transmitting device  110 , a change (e.g., an increase or decrease) in the amount of power that is being received. The receiving device  120  may communicate with the transmitting device using an in-band (e.g., Qi) or an out-of-band (e.g., AirFuel Alliance) communication path. In-band communication refers to signaling over the power path and/or at the same frequency as the power transfer. Out-of-band communication path refers to signaling over a different channel and/or at a different frequency than the power transfer. 
     The transmitting device  110  may change the amount of power that is being generated by the power source  112  as a result of an instruction received in a signal, or packet,  150  from the receiving device  120 . Thus, a feedback loop system may be utilized to adjust the amount of power transferred from the transmitting device  110  to the receiving device  120 . However, when the receiving device  120  becomes exposed to an excessive amount of power at the second windings  122 , a period of time may elapse before the transmitting device  110  receives the instructions and acts to reduce the power received at the receiving device  120 . Additionally, the transmitting device  110  may not properly receive an updated request to reduce the power level due to, for example, a corrupt signal exchange. 
     As an example, the transmitting device  110  may be capable of providing up to 30 Volts (V) at the rectifier  126 . The receiving device  120  may be operating stably and receiving more than 10 V at the rectifier  126 . Imagine that an end-user quickly substitutes the receiving device  120  with a new receiving device, and where the new receiving device does not have an initial load. In this scenario, the transmitting device  110  is still transmitting with the assumption that the device receiving the power is the old receiving device  120 . Immediately after this transition, the new receiving device, without the initial load, will be exposed at a high voltage at its corresponding rectifier. The new receiving device, to protect itself against over-voltage, may clamp this high voltage. As a result of the clamping, packets sent from the new receiving device may now be corrupted. A request to reduce the power to the transmitting device  110  using the corrupt packets may not be properly received at the transmitting device  110 . Accordingly, the new receiving device may be exposed to a large amount of power for an extended period of time, which may result in damage to various components in the new receiving device. 
     As another example, the receiving device  120 , with established receiving power transmitted from the transmitting device  110 , may replace a second receiving device, with higher established receiving power previously from a second transmitter in a second wireless power system. In this example, the receiving device  120  would be exposed to a higher power in the second wireless power system. 
       FIG.  2    is a schematic diagram of an embodiment wireless power system  200  having a regulation loop circuit  240 . The wireless power system  200  is used to provide a regulated voltage from the transmitting device  110  to the load  132 . The regulation loop circuit  240  provides protection against over-voltage conditions at the load or receiving elements of the receiving device  120 . Moreover, the regulation loop circuit  240  allows proper ASK modulation signaling to communicate the over-voltage condition from the receiving device  120  to the transmitting device  110 . 
     The receiving device  120  in the wireless power system  200 , in addition to the components previously discussed in  FIG.  1   , includes a first differential amplifier  202 , a second differential amplifier  204 , a comparator  206 , a switch  208 , a regulating circuit  210 , and a microcontroller  212 , which may (or may not) be arranged as shown in  FIG.  2   . The transmitting device  110  in the wireless power system  200 , in addition to the components previously discussed in  FIG.  1   , includes a microcontroller  214 , which may (or may not) be arranged as shown in  FIG.  2   . The receiving device  120  and the transmitting device  110  may include additional components not depicted in  FIG.  2   , such as long-term storage (e.g., non-volatile memory, etc.), a non-transitory computer-readable medium, one or more antenna elements, filter circuits, and impedance matching circuits. 
     The microcontroller  212  is coupled to the various components of the receiving device  120 . The microcontroller  214  is coupled to the various components of the transmitting device  110 . The microcontrollers  212  and  214  may be a part of a communications and control unit of each device. Microcontrollers  212  and  214  regulate the transferred power from the transmitting device  110  to the level requested by the receiving device  120 . Communication between the microcontrollers  212  and  214  may be unidirectional or bi-directional. Furthermore, the communication may be in-band or out-of-band to the wireless power transfer frequency. In various embodiments, the communication may be in accordance with a standard protocol interface, such as that promoted by the wireless power consortium (i.e., Qi). 
     The output of the rectifier  126 , the input of the voltage regulator  130 , and a first terminal of the shunt capacitor  128  share the rectified voltage (V RECT ) node  220 . The output of the voltage regulator  130  and the input of the load  132  share the regulated output voltage (V OUT ) node  230 . The voltage at the V RECT  node  220  is referred to as V RECT . The voltage at the V OUT  node  230  is referred to as V OUT . 
     Each of the differential amplifiers  202  and  204  receive a pair of input voltages at a corresponding pair of input terminals. Each differential amplifier provides an amplified difference of the two input voltages at its output. In some embodiments, one or both of the differential amplifiers  202  and  204  may be an operational amplifier (op-amp) configured for providing gain. 
     For example, the first differential amplifier  202  may be an op-amp configured for providing unity gain, while the second differential amplifier  204  may be another op-amp configured for providing a high gain (e.g., &gt;20 decibels (dB)). This configuration advantageously ensures that the regulation loop circuit  240  has enough open loop gain for proper and reliable operation. The regulation loop circuit  240  is discussed in further detail below. 
     A first (i.e., non-inverting) input of the first differential amplifier  202  is coupled to the V RECT  node  220 . A second (i.e., inverting) input of the first differential amplifier  202  is coupled to the V OUT  node  230 . It should be noted that in some embodiments the V RECT  node  220  may be coupled to the inverting input of the first differential amplifier  202  and the V OUT  node  230  may be coupled to the non-inverting input of the first differential amplifier  202 . It is typical for a person of ordinary skill in the art (POSITA) to configure the individual gains such that the overall feedback is a negative feedback. 
     An output of the first differential amplifier  202  provides an amplified difference between the voltages V RECT  and V OUT . In some embodiments, the output of the first differential amplifier  202  may be a non-amplified (i.e., gain of 0 dB) difference between the voltages V RECT  and V OUT . 
     A first (i.e., non-inverting) input of the second differential amplifier  204  is coupled to the output of the first differential amplifier  202 . A second (i.e., inverting) input of the second differential amplifier  204  is coupled to a first reference voltage (V REF1 ). Similar to the case of the first differential amplifier  202 , the input signals to the inverting and non-inverting inputs of the second differential amplifier  204  may be swapped while maintaining overall loop stability. 
     An output of the second differential amplifier  204  provides an amplified difference between the output voltage of the first differential amplifier  202  and the voltage V REF1 . In some embodiments, the output of the second differential amplifier  204  may be a non-amplified (i.e., gain of 0 dB) difference between the output voltage of the first differential amplifier  202  and the voltage V REF1 . The output of the second differential amplifier  204  may correspond to one or more of the gain of the first differential amplifier  202 , the gain of the second differential amplifier  204 , and the value of V REF1 . In other words, the output voltage of the second differential amplifier  204  is configurable using several controllable parameters. Generally speaking, the overall regulation loop gain is configured, such that by the distribution of the gain between the first differential amplifier  202 , the second differential amplifier  204 —and to an extent—the regulating circuit  210 , the overall regulation loop is provided sufficient gain to ensure proper regulation and stability. 
     The comparator  206  receives a pair of input voltages (or currents) and provides a digitized output (i.e., “0” or “1”), at a specific voltage, based on a comparison of the value at the two inputs. For example, a comparator may have a positive side input and a negative side input. In this example, the output voltage may indicate a digital representation of “1” if the voltage at the positive side is greater than the voltage at the negative side and indicate a digital representation of “0” if the voltage at the negative side is greater than the voltage at the positive side, or vice-versa. 
     A first (i.e., positive-side) input of the comparator  206  is coupled to the output of the first differential amplifier  202 . A second (i.e., negative-side) input of the comparator  206  is coupled to a second reference voltage (V REF2 ). It should be noted in some embodiments, the input voltages at the positive side and the negative side of the comparator  206  may be swapped. 
     Switch  208  can be, but is not limited to, a field-effect transistor (FET) or a bipolar junction transistor (BJT). A FET can be a metal-oxide-semiconductor FET (MOSFET), a junction FET (JFET), an insulated-gate bipolar transistor (IGBT), or any other semiconductor device used as a switch. The FET device may be a negative-type or a positive-type FET. For instance, the switch  208  may be an n-MOSFET, p-MOSFET, or the like. 
     An input of switch  208  is coupled to the output of the second differential amplifier  204 . Switch  208  includes a control terminal (e.g., gate terminal in a FET) that receives a control voltage from an output of the comparator  206 . An output of switch  208  is coupled to the regulating circuit  210 . 
     Switch  208  operates in either open or closed mode. In the close mode, switch  208  provides a voltage (V CONT ), at the V CONT  node  250 , from the second differential amplifier  204  to the regulating circuit  210 . In the open mode, switch  208  blocks the voltage V CONT  from the second differential amplifier  204  to be received at the regulating circuit  210 . In an embodiment, switch  208  may operate as normally open. In an embodiment, V CONT  may have a value from about −0.3 V to about 27 V. 
     The regulating circuit  210  receives the voltage V CONT  from switch  208 . The regulating circuit  210  regulates the voltage at the V RECT  node  220  based on the value of the voltage V CONT . In an embodiment, excess power is dissipated from the V RECT  node  220  through the regulating circuit  210 . In one embodiment, the regulating circuit  210  may be a discrete component. In some embodiments, the regulating circuit  210  may include at least one active component. In some embodiments, the regulating circuit  210 , in addition to the at least one active component, may include one or more active or passive components. These and other details of the regulating circuit  210  are discussed further below in reference to  FIG.  4   . 
     The differential amplifiers  202  and  204 , the comparator  206 , the switch  208 , and the regulating circuit  210  form a regulation loop circuit  240 . The regulation loop circuit  240 , in response to determining that the value of V RECT  exceeds an operational limit of the receiving device  120 , utilizes the regulating circuit  210  to dissipate the excess power from the V RECT  node  220 . The regulation loop circuit  240  includes a 1) monitoring circuit, comprising the differential amplifiers  202  and  204 , the comparator  206 , and the switch  208 , and 2) the regulating circuit  210 . 
     In other words, the regulation loop circuit  240  is used to detect whether or not V RECT , which is the input voltage to the voltage regulator  130 , is causing the voltage across the voltage regulator  130  to exceed a threshold limit set for the system. In response to determining that this threshold has been exceeded, the regulating circuit  210  is activated. The controlled activation of the regulating circuit  210  allows the reduction of V RECT  to a value at which the voltage regulator  130  and load  132  can remain in the proper operational voltage. 
     In an original steady-state operation, or proper operational mode (i.e., when V RECT  is within normal operating conditions), the voltage regulator  130  provides a properly regulated voltage at the V OUT  node  230  to load  132 . For example, the value of V RECT  may be about 20.5 V while the value of V OUT  may be about 20 V. In an excess of incoming power condition, V RECT  increases, and the regulating circuit  210  in the regulation loop circuit  240  is activated. As a result, V RECT  is clamped at about 22 V. The wireless power system  200  is now operating in a new steady-state operational mode and the voltage regulator  130  continues to provide a properly regulated voltage V OUT . It is noted that the regulating circuit  210  absorbs the excess of power. 
     The value of the V REF2  voltage is set such that the switch  208  remains open as long as the amplified difference between V RECT  and V OUT  is less than an operational threshold of the receiving device  120 . If the amplified difference between V RECT  and V OUT  exceeds the operational threshold of the receiving device  120 , the switch  208  is closed and the output of the second differential amplifier  204  provides a control signal to the regulating circuit  210  relative to the desired amount of excess power that is to be dissipated from the V RECT  node  220  to reduce V RECT  in line with the operational parameters of the receiving device  120 . In an embodiment, the value of V REF2  may be set in about 0.5 V steps from about 2.5 V up to about 4 V. The gain of the first differential amplifier  202  may be set to about 0 dB. The value of V REF2  and the gain of the differential amplifier may be a programmable value that is configurable using, for example, the microcontroller  212 . 
     The regulation loop circuit  240  may continue to dissipate the excess power through the regulating circuit  210 , to provide sufficient time for the receiving device  120  to communicate a clear signal  150  to the transmitting device  110  and request a reduction of the transfer power  140 . 
     Advantageously, the embodiments of this disclosure allow the receiving device  120  to use in-band communication to communicate with the transmitting device  110  without the packet corruption that may occur as a result of, for example, with detuning or a hard over-voltage protection method. 
     Additionally, the regulation loop circuit  240  is advantageously capable of effectively clamping at any programmable voltage. This is in contrast to the clamping operation using Zener diodes, which is rigid and not configurable. 
     Furthermore, the effective clamping process in the embodiments of this disclosure is an active process. Thus, the regulation loop circuit  240  is able to release the clamping at any time, which again is different from a clamping operation using Zener diodes. 
     In some embodiments, the activation of switch  208  may trigger a signal to be received at the microcontroller  212 . In turn, the microcontroller may provide a packet to be transmitted to the transmitting device  110  to request a reduction in, or to end, the power transfer. 
     It should be noted that some excess power may be dissipated at the voltage regulator  130  due to the range of input voltages at which the voltage regulator  130  is capable of providing a stable output voltage to the load  132 . 
     In an exemplary embodiment, the steady-state value of V OUT  is 20 V, the gain of the first differential amplifier  202  and the second differential amplifier  204  is 0 dB, the value of V REF2  is set to 4 V, and the value of V REF1  is set to 2 V. In a first scenario of this exemplary embodiment, and in proper operational mode, V RECT  is at 20 V and the output of the first differential amplifier  202  is near 0 V. As the output of the first differential amplifier  202  is less than 4 V, the switch  208  remains open, the regulating circuit  210  is inactive, and V RECT  remains at 20V. 
     In a second scenario of the above exemplary embodiment, V RECT  exceeds 24 V and the output of the first differential amplifier  202  is now greater than 4 V. As the output of the first differential amplifier  202  exceeds 4 V, the switch  208  is now closed. The second differential amplifier  204  provides a control signal to the regulating circuit  210  such that V RECT  remains at about 24 V. The regulating circuit  210  dissipates a corresponding amount of power from the V RECT  node  220 , which stabilizes the value of V RECT . 
     In an embodiment, the rectifier  126 , the voltage regulator  130 , the microcontroller  212 , the differential amplifiers  202  and  204 , the comparator  206 , and the switch  208  may be formed in an integrated circuit (IC). Optionally, the integrated circuit may have additional components (not shown), such as a memory, a clock generator, a thermal protection circuit, an analog-to-digital converter (ADC), or the like. The memory may store an operating system, communication or configuration instructions. The clock generator may generate a clock signal to synchronize operations within the integrated circuit or the receiving device. The thermal protection circuit may be used to monitor, and protect against, over-heating conditions in the integrated circuit. 
     The integrated circuit may provide a highly efficient and low-power dissipating packaged circuit capable of integration in compact applications. The integrated circuit may have a plurality of terminals to connect to external discrete components, such as capacitors, resistors, inductors, or windings. In an embodiment, the integrated may be packaged in a flip-chip configuration. The integrated circuit may have terminals to interconnect the output of the switch  208  to the regulating circuit  210  and to conditionally provide the voltage V CONT . The integrated circuit may support a variety of standard or proprietary standards, such as the single wire protocol (SWP) interface, the serial peripheral interface (SPI), and the inter-integrated circuit (I2C) interface. 
     The IC provides a highly efficient and low-power dissipating circuit that is capable of integration in compact applications. The IC may comply with one or more wireless standard communication protocols, such as the Qi or AirFuel standards. The thermal protection circuit may detect that one or more of the components in the receiving device have exceeded a thermal threshold and trigger a signal to the microcontroller  212 . The microcontroller  212 , in response to receiving the trigger from the thermal protection circuit, and any other trigger, for example from the switch  308 , may transmit a packet to the transmitting device to reduce, or end, the power transfer. 
       FIG.  3    is a schematic diagram of an embodiment wireless power system  300  having a regulation loop circuit  310 . The regulation loop circuit  310 , similar to the regulation loop circuit  240 , provides protection against over-voltage conditions at the load or receiving elements of the receiving device  120 . However, switch  208  in the regulation loop circuit  310  is activated in response to a trigger from the multiplexer  306  based on any configured combinational logic of a) the difference between the voltages V RECT  and Vou, b) the difference between the voltages V RECT  and a first threshold (V TH1 ), and c) the difference between the voltages V OUT  and a second threshold (V TH2 ). 
     The regulation loop circuit  310 , in addition to the components previously discussed in  FIG.  2   , includes two additional comparators  302  and  304  and a multiplexer  306 , which may (or may not) be arranged as shown in  FIG.  3   . The receiving device  120  and the transmitting device  110  may include additional components not depicted in  FIG.  3   , such as long-term storage (e.g., non-volatile memory, etc.) or a non-transitory computer-readable medium. The regulation loop circuit  240  includes a 1) monitoring circuit, comprising the differential amplifiers  202  and  204 , the comparators  206 ,  302 , and  304 , the multiplexer  306 , and the switch  208 , and 2) the regulating circuit  210 . 
     Comparators  302  and  304  are functionally similar to comparator  206  of  FIG.  2   . A first (i.e., positive-side) input of the comparator  302  is coupled to V RECT  node  220 . A second (i.e., negative-side) input of the comparator  302  is coupled to V TH1 . A first (i.e., positive-side) input of the comparator  304  is coupled to V OUT  node  230 . A second (i.e., negative-side) input of the comparator  304  is coupled to V TH2 . It should be noted that in some embodiments, the input voltages at the positive side and the negative side of comparators  302  and  304  may be swapped. The values of the threshold voltages V TH1  and V TH2  are selected, respectively, in accordance with the steady-state operational mode values at the V RECT  node  220  and the V OUT  node  230 . In an embodiment, the values V TH1  and V TH2  may be programmable through the microcontroller  212  and set from 23 V up to 26.5 V, in 0.5 V increments. 
     The multiplexer  306  is coupled to the output of each comparator  206 ,  302 , and  304 . The multiplexer  306  receives an output signal from each comparator  206 ,  302 , and  304 , and provides an activation signal to switch  208 . Therefore, the multiplexer  306  may be configured to provide an activation signal to switch  208  based on the voltage V RECT  exceeding V OUT  by a value greater than V REF2 , the voltage V RECT  exceeding V TH1 , the voltage V OUT  exceeding V TH2 , or a combination thereof. The regulation loop circuit  310  advantageously provides over-voltage monitoring by directly monitoring the voltages V RECT  and V OUT  in addition to monitoring the voltage difference between V RECT  and V OUT . 
     The multiplexer  306  may be configured using the microcontroller  212 . The microcontroller  212  may include a memory component that stores various configuration settings. The various configuration settings may be selected such that the multiplexer  306  provides a trigger to activate switch  208  based on any logical combination of signals received from the comparators  206 ,  302 , and  304 . The various configuration settings may be pre-programmed at the multiplexer or programmed through, for example, a firmware update. 
     In an exemplary embodiment, the steady-state value of V OUT  is about 20V, the gain of the first differential amplifier  202  is about 0 dB and the second differential amplifier  204  is 20 dB, the value of V REF2  is set to about 4V, the value of V REF1  is set to about 4V, and the values of V TH1  and V TH2  are set to about 24 V. In a first scenario of this exemplary embodiment, and in proper operational mode, V RECT  is at about 20V and the output of the first differential amplifier  202  is near 0 V. The output of comparators  206 ,  302 , and  304  remains low. The output of the multiplexer  306  remains low and the switch  208  remains open, the regulating circuit  210  is inactive, and V RECT  remains at about 20V. 
     In a second scenario of the above exemplary embodiment, V RECT  exceeds 24V. The output of the first differential amplifier  202  is greater than 4 V. The output of the comparators  206 ,  302 , and  304  are high and switch  208  is closed. The second differential amplifier  204  provides a control signal to the regulating circuit  210 . The regulating circuit  210  is enabled and dissipates a corresponding amount of power from V RECT , which reduces the value of V RECT . 
     This solution advantageously provides a reliable and multi-angled approach to the monitoring of an over-voltage condition in the receiving device  120 . This solution may counteract a situation at which VOUT and VRECT may experience an uncorrelated behavior and the regulation loop circuit is configured to properly activate. 
     In an embodiment, the rectifier  126 , the voltage regulator  130 , the microcontroller  212 , the differential amplifiers  202  and  204 , the comparators  206 ,  302 , and  304 , the multiplexer  306 , and the switch  208  may be formed in an integrated circuit (IC). Optionally, the integrated circuit may have additional components (not shown), such as a memory, a clock generator, a thermal protection circuit, an analog-to-digital converter (ADC), or the like. The integrated circuit may have terminals to interconnect with the regulating circuit  210  and to conditionally provide the voltage V CONT . The integrated circuit may have a plurality of terminals to connect to external discrete components, such as capacitors, resistors, inductors, or windings. In an embodiment, the integrated may be packaged in a flip-chip configuration. The integrated circuit may have terminals to interconnect the output of switch  208  to the regulating circuit  210  and to conditionally provide the voltage V CONT . 
       FIG.  4    is a schematic of an embodiment regulating circuit  210  for a wireless power system. The regulating circuit  210  includes an active device  410  and a dissipating element  420 . The active device  410  receives a voltage V CONT  when switch  208  is activated. The value of V CONT  is set such that the active device  410  has only enough conductivity to dissipate a specific amount of excess energy from V RECT  and to set the voltage at the V RECT  node  220  to normal operating conditions. The excess energy is dissipated through the dissipating element  420  to reference ground at the V REF  node  160  and additionally through the voltage regulator  130 . 
     The active device  410  has an input terminal  412 , an output terminal  414 , and a control terminal  416 . The input terminal  412  is coupled to the V RECT  node  220 , the output terminal  414  is coupled to the dissipating element  420 , and the control terminal  416  is coupled to the V CONT  node  250 . In an embodiment, the active device  410  is a MOSFET. In some embodiments, the active device  410  may be a set of active devices. 
     The active device  410  is controlled through the voltage V CONT  at the control terminal  416 . The active device  410  operates linearly and the conductivity of the active device  410  depends on the value of the voltage V CONT . The voltage V CONT  is provided from the second differential amplifier  204 , as shown, for example, in the wireless power system  200  of  FIG.  2   . 
     In some embodiments, the dissipating element  420  may include one or more additional active devices. Optionally, in such embodiments, a source resistor may be coupled between the active device  410  and the one or more additional active devices to provide a smooth crossover between the active devices. As an example, the dissipating element  420  may include a silicon-controlled rectifier (SCR) or other similar types of semiconductor devices. 
     In some embodiments, the dissipating element  420  may be formed as one or more discrete components. In other embodiments, the dissipating element  420  may be formed in one or more integrated circuits (ICs). In one embodiment, the dissipating element  420  includes one or more resistors. 
     The active device  410  is biased such that as the active device  410  becomes conductive, the dissipating element  420  begins to sink current to ground. The value of the voltage V CONT  is set such that only as much current as is necessary is sunk to ground to provide a regulated voltage at the V RECT  node  220 . 
     When the wireless power system is operating at a steady-state mode (e.g., V RECT  is close to V OUT ), the value of the voltage V CONT  is set such that the active device  410  is not conducting and the regulating circuit  210  is not dissipating any current through the dissipating element  420 . 
       FIGS.  5 A-C  are plots of embodiment operational timing diagrams, as may be performed by a receiving device in accordance with the embodiments of this disclosure. In each timing diagram, at time T 0 , an event occurs where the output power of the transmitting device begins to exceed the steady-state operational voltage of the receiving device. At time T 1 , the difference in value between the two voltages V RECT  and V OUT  exceeds the voltage V REF2  (see plot  502 ), at which time the switch  208  is activated and the voltage V CONT  is provided to the regulating circuit  210  (see plot  504 ). 
       FIG.  5 A  is a plot of an embodiment operational timing diagram  500  in a first operational scenario. In the first operational scenario, the receiving device  120  at time T 2 , shortly after the activation of the switch  208  and the activation of the regulating circuit  210  at time T 1 , transmits an end power transfer (EPT) request to the transmitting device (see plot  506 ). Between time T 2  and time T 3 , the transmitting device receives the EPT request and the power transmission is successfully halted. In response, at time T 3 , the difference in value between the voltages V RECT  and V OUT  falls below V REF2  (see plot  502 ), at which time the switch  208  is deactivated and the voltage V CONT  is no longer provided to the regulating circuit  210  (see plot  504 ). Shortly thereafter, at time T 4 , the receiving device stops the EPT request transmission (see plot  506 ). 
       FIG.  5 B  is a plot of an embodiment operational timing diagram  520  in a second operational scenario. In the second operational scenario, the receiving device  120  at time T 2 , shortly after the activation of the switch  208  and the activation of the regulating circuit  210  at time T 1  (see plot  522 ), transmits an end power transfer (EPT) request to the transmitting device (see plot  526 ). However, unlike the first operational scenario, the EPT request is unsuccessful and the power transmission is not halted (see plot  526 ). 
     The switch  208  remains activated and the voltage V CONT  continues to be provided to the regulating circuit  210 . Until at time T 5 , at which time the switch  208  is deactivated and the amplified difference in value between the two voltages V RECT  and V OUT  begins to increase (see plot  522 ). Until at time T 6  when, for example, a more drastic protection scheme like hard over-voltage protection is applied at the receiving device  120  (see plot  528 ). This results in the shutting down of the receiving device at risk of deterioration of the receiving device and/or the transmitting device, but that may be used as an ultimate countermeasure. The time period between time T 5  and time T 1  may be set as a configurable (i.e., programmable) option using the microcontroller  212  or a similar component. In some embodiments, the time period may be set anywhere from as little as 50 milliseconds (ms), up to 300 ms. 
     The wireless power system advantageously allows for the system to return to steady-state mode within the time period T 1  and T 5 . If the wireless power system, however, does not return to steady-state mode (i.e., the original steady-state mode before the surge of incoming power) within that period of time, the hard over-voltage protection is applied at time T 6 , to protect the wireless power system from further damage. The difference in time between time T 6  and T 1 , or time T 6  and T 5 , may be a configurable option. 
       FIG.  5 C  is a plot of an embodiment operational timing diagram  540  in a third operational scenario. In the third operational scenario, the receiving device waits until time T′ 2  to transmit the end power transfer (EPT) request to the transmitting device (see plot  544 ). The delay, advantageously allows the wireless power system to remain operational in the event that a surge in power occurs for a short period of time (i.e., less than T′ 2 -T 1 ). For instance, if the surge in power occurs for a period of time that is less than the delay, the receiving device does not send the EPT request. The delay may be a configurable option using the microcontroller  212 . In some embodiments, the delay may be from about 1 microseconds (μs) to about 10 μs. 
       FIG.  6    is a schematic diagram of an embodiment wireless power system  600  having a current sensing element  602 . The wireless power system  600 , similar to the wireless power systems  200  and  300 , provides over-voltage protection at the load and receiving elements of the receiving device  120 . The wireless power system  600 , in additional to the components previously disclosed in  FIG.  2   , includes the current sensing element  602 , which may (or may not) be arranged as shown. As shown, the current sensing element  602  is a resistor. However, other components may be contemplated to serve a similar function. 
     The current sensing element  602  is coupled in series between the output of the rectifier  126  and the input of the voltage regulator  130 . The input node of the regulating circuit  210  is coupled between the output of the rectifier  126  and the current sensing element  602 . 
     A current-sensing amplifier, for example in the microcontroller  212 , can be used to monitor and accurately measure the voltages across the current sensing element  602  and the power at the receiving device  120 . The receiving device  120  may communicate the level of power at the current sensing element  602  to the transmitting device  110 , for example, in response to a foreign objection detection (FOD) check by the transmitting device  110 . 
     In one embodiment, the transmitting device  110  can compare the power received at the receiving device with the power being transmitted. The transmitting device  110  may then determine that a power loss condition has occurred at the receiving device  120  to shut down the power transfer. 
     Indeed, as the regulating loop circuit  240 —and in particular, the regulating circuit  210 —is placed before the current sensing element, any power absorbed by the regulating circuit  210  is not taken into account in the power reported by the microcontroller  212 . Thus, in the event that the microcontroller  214  of the transmitting device  110  prompts the receiving device  120  to report power, and make a comparison, the microcontroller  214  may resolve that some power is lost at the receiving device  120  and determine that a foreign object device (FOD) condition is present. 
       FIG.  7 A  is a schematic diagram of a receiving device  700  configured with Zener diodes (i.e., Z 1   702 , Z 2   704 , Z 3   706 , and Z 4   708 ) in a clamping circuit. The clamping circuit, or limiter, is used to limit or cut-off portions of the incoming voltage at the rectifier  126 . The values of the Zener diodes Z 1   702 , Z 2   704 , Z 3   706 , and Z 4  are selected such as to clip an AC voltage that exceeds a threshold voltage at the rectifier  126 . This effectively regulates the incoming AC voltage at the rectifier  126 . 
     As shown, Zener diode Z 1   702  and Zener diode Z 2   704  are arranged as back-to-back connected Zener diodes. The cathode terminal of Z 1   702  is coupled to the first terminal of the second windings  122  through the resonant capacitor  124 . The anode terminal of Z  702  is connected to the anode terminal of Z 2   704 . The cathode terminal of Z 2  is connected to reference ground. The Zener diodes Z 3   706  and Z 4   708  are also arranged as back-to-back connected Zener diodes. The cathode terminal of Z 3   706  is connected to the second terminal of the second windings  122 . The anode terminal of Z 3   706  is connected to the anode terminal of Z 4   708 . The cathode terminal of Z 8  is connected to reference ground. 
     Although the solution is functional, the receiving device  700  is unaware that the clipping has occurred and that the voltage from a transmitting device exceeds the operational limits of the receiving device  700 . Therefore, disadvantageously the receiving device  700  does not communicate a request to reduce the power level to the transmitting device. The solution is purely a safety precaution and disadvantageously does not provide a corrective action. Furthermore, excess power is continuously being dissipated through the receiving device  700 , which may damage one or both of the receiving device  700  and the transmitting device. Also, the voltage clamping at the output of the rectifier  126  is fixed in voltage in respect to the reference. This may not adequately address various over-voltage scenarios, especially if the voltage at the output of the voltage regulator  130  is programmable. This may lead to a large range of operating voltage at the output of the rectifiers  126 . 
       FIG.  7 B  is a schematic diagram of a receiving device  750  configured with a de-tuning circuit. The de-tuning circuit includes a pair of active devices  752  and  756  and a pair of capacitors  754  and  758 . Each capacitor  754  and  758  is coupled to an input terminal of the rectifier  126 . The active device  752  is activated in accordance with the control signal  760  and the active device  756  is activated in accordance with the control signal  762 . The control signals  760  and  762  may be provided by a microcontroller or activated automatically, for example, through a regulating monitoring circuit. 
     The receiving device  750 , in response to determining an excess AC voltage at the input of the rectifier  126 , provides control signals  760  and  762  to the active devices  752  and  756 . The capacitors  754  and  758  de-tune the impedance between the second windings  122  and the rectifier  126 . This effectively reduces the resonance factor at the second windings  122 . 
     Similar to  FIG.  7 A , this is a functional solution to reduce the voltage at the rectifier  126 . The level of de-tuning, however, is limited and can only protect the receiving device  750  against a limited amount of excess power. Therefore, disadvantageously the voltage at the input of the voltage regulator  130  may continue to increase despite the activation of the soft over-voltage-protection provided by the de-tuning. 
       FIG.  7 C  is a schematic diagram of a receiving device  780  configured with a hard over-voltage protection circuit. The hard over-voltage protection circuit may be implemented in any of the embodiments of this disclosure. 
     The hard over-voltage protection circuit includes a first active device  782  and second active device  784 . The first active device  782  is coupled between a first input terminal  786  of the rectifier  126  and reference ground. The second active device  784  is coupled between a second input terminal  788  of the rectifier  126  and reference ground. In an embodiment, each of the first and second active device  782  and  784  is a MOSFET. 
     When the AC voltage at the two input terminals  786  and  788  of the rectifier  126  exceeds a predetermined voltage, the hard over-voltage protection may kick in. The hard over-voltage protection consists of shorting each of the two input terminals  786  and  788  to the reference ground through activating the first and second active device  782  and  784 , respectively. 
     Disadvantageously, the solution blocks incoming wireless power transfer. Therefore, the receiving device  780  may not be able to properly communicate with the transmitting device  110 , specifically in the case of in-band communication. Additionally, the excess power is dissipated in a limited number of components, which can cause damage to the various components. 
     These and other issues of the prior art are addressed with the embodiments of this disclosure, for example using the embodiments in  FIGS.  2 - 4   . 
       FIG.  8    is a flowchart of an embodiment method  800  for monitoring an over-voltage condition and regulating the input voltage to a voltage regulator  130  using a regulating circuit, as may be performed by a receiving device. At step  810 , the receiving device monitors the voltage across the input and output terminals of the voltage regulator  130  using a monitoring circuit. In some embodiments, the monitoring may include direct voltage monitoring at each terminal. In some embodiments, the monitoring may include a difference in voltage across the voltage regulator  130 . In other embodiments, the monitoring may include direct voltage monitoring at each terminal and monitoring the difference in voltage across the voltage regulator  130 . 
     At step  820 , the monitoring circuit upon determining that an over-voltage condition is present at the receiving device, activates a regulating circuit  210 . The regulating circuit  210  regulates the voltage at the output of the rectifier  126  to return the voltage to a steady-state value. In one embodiment, the monitoring circuit may determine the over-voltage condition after detecting that the voltage V RECT  exceeds a threshold voltage V TH1 . In one embodiment, the monitoring circuit may determine the over-voltage condition after detecting that the voltage V OUT  exceeds a threshold voltage V TH2 . In one embodiment, the monitoring circuit may determine the over-voltage condition after detecting that the difference in voltage between the voltage V RECT  and V OUT  exceeds a reference voltage V REF2 . In other embodiments, the monitoring circuit may determine the over-voltage condition after detecting any logical combination of any of the previous conditions. 
     The regulating circuit receives a voltage V CONT  from the monitoring circuit. The value of V CONT  is set such that the regulating circuit  210  reduces the value of the voltage V RECT  to a value near the steady-state voltage. In practical terms, the wireless power system reaches a new steady-state mode in which the value of the V RECT  voltage is higher than the former steady-state mode (i.e., steady-state mode before the incoming power surge). The new steady-state mode is still within the steady-state parameters for which voltage is determined by the wireless power system parameters. 
     Optionally, at step  830 , the receiving device may communicate to the transmitting device an end power transfer (EPT) request. In some embodiments, the EPT request is transmitted immediately after step  820  and the activation of the regulating circuit  210 . In other embodiments, the transmission of the EPT request is delayed until a period of time has elapsed. In these embodiments, the receiving device checks if the regulating circuit  210  is still activated after the delay period, before transmitting the EPT request. 
     Optionally, at step  840 , the receiving device may activate hard over-voltage protection and stop the receiving of the power at the receiving device. The receiving device may delay the activation of the hard over-voltage protection for a period of time after the regulating circuit  210  is activated to allow for the EPT request to be received and the transmitting device to shut down the power transfer. 
     It is noted that the receiving device  120  in the various embodiments of this disclosure may additionally include over-current, over-voltage, and over-temperature sensing circuits. The various sensing circuits may work in combination with the regulation loop circuit in the various embodiments to provide additional safety protection to the receiving device  120 . 
     Although the description has been described in detail, it should be understood that various changes, substitutions, and alterations may be made without departing from the spirit and scope of this disclosure as defined by the appended claims. The same elements are designated with the same reference numbers in the various figures. Moreover, the scope of the disclosure is not intended to be limited to the particular embodiments described herein, as one of ordinary skill in the art will readily appreciate from this disclosure that processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, may perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. Regardless of the standard or type of wireless power transfer used, embodiments of this disclosure provide solutions to address over-voltage at the receiving elements and/or loading elements of the receiving device. 
     The specification and drawings are, accordingly, to be regarded simply as an illustration of the disclosure as defined by the appended claims, and are contemplated to cover any and all modifications, variations, combinations, or equivalents that fall within the scope of the present disclosure.