Patent Publication Number: US-2022224137-A1

Title: Active receiver overvoltage and overpower protection in wireless power transfer

Description:
FIELD 
     The present invention relates generally to power charging systems, and in particular, wireless power charging systems for electronic devices. More particularly, the present invention relates to an apparatus, a method and a non-transitory computer readable medium storing a program for dynamically providing overvoltage and over power protection for a power receiver integrated circuit in electronic devices subject to a wireless transfer of power. 
     BACKGROUND 
     Wireless power charging devices and systems are becoming more prevalent and are appearing in varied forms. There are three basic categories of device chargers: desktop chargers, power banks, and embedded chargers. Desktop chargers may be in the form of a charging pad or stand. Power banks are similar but are designed for travel and contain batteries to provide power when it cannot be plugged in to an outlet. Embedded chargers may be built into objects, e.g., furniture, automobiles, other appliances, or provided in public locations. The largest demand for chargers is for home use, but the deployment of public chargers is abundant. 
     There are different standards currently in use for wireless transfer of power. An example of such standards is the Qi® (registered trademark) standard (hereinafter “Qi standard”) by the Wireless Power Consortium found at (www.wirelesspowerconsortium.com). The Qi standard defines an interface standard for wireless power transfer that ensures the interoperability of devices that conform to the Qi standard. 
     Typically, a wireless power transfer system uses magnetic induction to transfer power from a power transmitter of a wireless power charging device to a power receiver contained within the electric device being charged (e.g., a mobile device) when it is placed in proximity to the power transmitter. In the wireless power transfer system, an alternating current (AC) is applied to a power transmitting coil in a state where the power transmitting coil provided in the power transmitter is disposed proximate to a power receiving coil provided to the power receiver, and an alternating electromotive force (voltage) is induced in the power receiving coil of the power receiver to generate an alternating current in the power receiver. 
     One current technique to protect a power receiver of an electronic device being charged by wireless power transfer from over voltage and over power is based on an absorption technique. Such a technique requires adding a resistive load directly on the charging voltage (Vrect) over voltage protection (OVP) signal. The temporary excessive power can be absorbed by the resistive load, while communication is not affected. However, the size of the resistive load can be relatively large and the amount of excessive power varies across different brands of power transmitter. It is hard for one resistive load to cover all cases especially when minimizing layout area is a crucial requirement. 
     Another current technique to protect a power receiver of an electronic device being charged by wireless power transfer from over voltage and over power is based on power termination. In an example, such a technique requires the electronic device to send a command, e.g., an End of Power Transfer (EPT) packet, to the power transmitter, and the power transmitter terminates the power transmission actively. In another example, such a technique requires the electronic device to stop sending any packet to the power transmitter and wait for a timeout as a way to protect the power receiver. In this example, the communication time for a packet to be received is in tens of millisecond (ms) range, which means this technique can only respond to ms range dynamics. Further, there may be error in the decoding of the EPT packet at the power transmitter leading to continuation of power transfer even after the EPT packet is sent thereby resulting in failure to protect the power receiver from over voltage and over power. 
     SUMMARY 
     The following summary is merely intended to be exemplary. The summary is not intended to limit the scope of the claims. 
     According to an aspect, an apparatus for controlling power receiving operation is provided. The apparatus comprises: a controller configured to: compare a frequency of an electric current generated by a voltage induced in a power receiving circuit by a magnetic field generated by a power transmitting apparatus, against a frequency threshold to determine whether the frequency is equal to or below the frequency threshold; and in response to determining that the frequency is equal to or below the frequency threshold, control a communications circuit to communicate a control command message instructing the power transmitting apparatus to modify a power charge signal used to provide the magnetic field in a manner for protecting the apparatus. 
     According to a further aspect, a non-transitory computer-readable storage medium storing a program for controlling power receiving operation of an apparatus is provided. The program causes a processor to at least perform: comparing a frequency of an electric current generated by a voltage induced in a power receiving circuit by a magnetic field generated by a power transmitting apparatus, against a frequency threshold to determine whether the frequency is equal to or below the frequency threshold; and in response to determining that the frequency is equal to or below the frequency threshold, controlling a communications circuit to communicate a control command message instructing the power transmitting apparatus to modify a power charge signal used to provide the magnetic field in a manner for protecting the apparatus. 
     According to a further aspect, a method for controlling power receiving operation of an apparatus is provided. The method comprises comparing a frequency of an electric current generated by a voltage induced in a power receiving circuit by a magnetic field generated by a power transmitting apparatus, against a frequency threshold to determine whether the frequency is equal to or below the frequency threshold; and in response to determining that the frequency is equal to or below the frequency threshold, controlling a communications circuit to communicate a control command message instructing the power transmitting apparatus to modify a power charge signal used to provide the magnetic field in a manner for protecting the apparatus. 
     Other objects and novel features will become apparent from the description of this specification and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  depicts an overview of a wireless power charging system in which the present embodiments are incorporated; 
         FIG. 2  shows a plot depicting the gain of a wireless power charging system versus a switching frequency of the power transmitter signal; 
         FIG. 3  depicts an exemplary wireless power transfer system in which an embodiment to control a transmitting power value so as to safely transmit power with high efficiency is employed; 
         FIG. 4  depicts a schematic diagram of a power transfer control loop for controlling power transfer according to an embodiment; and 
         FIG. 5  depicts a power transfer control method run by a controller at a power receiving apparatus in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts an overview of a wireless power charging system  10  in which the present embodiments are incorporated. As shown in  FIG. 1 , the wireless power charging system  10  can include a power transmitting/charging pad or stand  20 , a power cable  21  through which the power transmitting/charging pad or stand  20  receives electrical power, and an electronic device  11 , e.g., a mobile phone. The electronic device  11  can be physically located on or in proximity to the power transmitting/charging pad or stand  20  during a wireless charging process described below. 
     In the wireless power charging system  10 , electromagnetic induction is employed to wirelessly transfer power to a power receiver (PRx) subsystem contained within the electronic device  11  when the electronic device  11  is placed on top of or in proximity to a power transmitter (PTx) subsystem contained within the power transmitting/charging pad or stand  20 . 
     As further shown in  FIG. 1 , the power transmitter (PTx) subsystem at the power transmitting/charging pad or stand  20  can include a transmitting power signal coil  25  and the power receiver (PRx) subsystem at the electronic device  11  can include a suitably positioned charging coil  15 . As shown in  FIG. 1 , the basic physical principle that governs wireless power transfer specification in the wireless power charging system  10  is magnetic induction: the phenomenon that a time-varying magnetic field generates an electromotive force in a suitably positioned inductor. This electromotive force produces a voltage across the terminals of a coil-shaped inductor, and is used to drive the electronics of an appropriate load to which it is connected. 
     When charging begins, the power transmitter (PTx) subsystem runs an alternating current (AC) through the transmitting power signal coil  25  to generate a time-varying magnetic field  30  in accordance with Faraday&#39;s law. This time-varying magnetic field  30  is in turn picked up by the charging coil  15  of the power receiver (PRx) subsystem and induces an electromotive force (voltage) in the suitably positioned charging coil  15 . This electromotive force produces a voltage across the terminals of the charging coil  15  to drive current in the charging coil  15 . The power receiver (PRx) subsystem converts the power received back to AC electric current which is used by an electrical load. 
     The amount of power transmitted is based on the condition of the load, e.g., the state of the battery being charged. For example, the receiver communicates requests to the power charge transmitter subsystem to vary the power as the load changes. How the power transmitter (PTx) subsystem handles the request depends upon the transmitter, e.g., can decrease the input voltage or change the switching frequency. 
       FIG. 2  shows a plot  50  depicting several gain curves  55  of different power transmitter (PTx) subsystems of different wireless power charging systems versus a switching frequency of power transmitter signals. In wireless power transfer, one effective way to prevent over voltage/power condition at the power receiver (PRx) subsystem is limiting the switching frequency of the alternating current to a power transmitting coil of the power transmitter (PTx) subsystem. In the plot  50  of  FIG. 2 , in certain operating modes, the power transmitter (PTx) subsystem uses both an input voltage (Y-axis) and switching frequency (X-axis) to control the gain of the wireless power charging signal and consequently the gain of the power charge signal received at the power receiver (PRx) subsystem. As shown in  FIG. 2 , the power transmitter (PTx) subsystem has an operating frequency  52  which is a switching frequency of the AC power charge signal set for nominal wireless charging operations such that the output gain of the wireless power charge signal received at the load is limited without possibility of reaching an over voltage/power condition at the power receiver (PRx) subsystem. However, different transmitters can be controlled to reach a different minimum operating AC power charge frequency  57 . That is, different types and brands of power transmitter (PTx) subsystems of power chargers can have different minimum operating AC power charge frequencies  57 . When an amount of power is transmitted that is greater than an amount of power consumed by a load of the power receiving side, heat caused by a difference power loss and a breakdown caused by an overvoltage can occur. That is, at the power transmitter (PTx) subsystem, a particular setting combination of high input voltage and a low or a minimum switching frequency poses a high risk of over voltage and over power at the power receiver (PRx) subsystem. 
     In one embodiment,  FIG. 3  depicts a wireless power transfer system  100  for controlling a transmitting power value so as to safely transmit power with high efficiency. The wireless power transfer system  100  can be compliant with the Qi standard, for example. 
     The wireless power transfer system  100  can include a base station  120  implemented as a power charger base or power charge stand that includes a power transmitter (PTx) subsystem  121 , and a power receiver (PRx) subsystem  110  implemented in an electronic device  101 . In embodiments, power receiver (PRx) subsystem  110  may be a system-on-chip receiver design and/or include circuitry integrating one or more integrated circuits (ICs). The power receiver (PRx) subsystem  110  can include, for example, wireless power receiver ICs that comply with the Qi standard, such as a P9222-R integrated single-chip wireless power receiver IC (PRx) available from Renesas Electronics Corporation. Other integrated circuit chips in compliance with the Qi standard that can be used include P9382, P9415 and P9418 products also available from Renesas Electronics Corporation. 
     As shown conceptually in  FIG. 3 , at the base station  120 , the power transmitter (PTx) subsystem  121  can include one or more power transmitting coils  125  that make power available to the electronic device  101 , a power conversion (modulation/demodulation) unit  135 , a communications transceiver unit  140  and a control (controller) unit  145 . The power conversion (modulation/demodulation) unit  135  employs DC-to-AC converter(s) and the power transmitting coil(s)  125  wirelessly transmits AC power signals at a specified amplitude and frequency to a receiver coil  115  in the power receiver (PRx) subsystem  110  of the electronic device  101 . The control (controller) unit  145  (e.g., a programmed microprocessor), receives and decodes messages from power receiver (PRx) subsystem  110 , configures the appropriate power transmitting coil  125 , executes the relevant power control algorithms and protocols, and drives the frequency of the AC waveform to control the power transfer. Control (controller) unit  145  also interfaces with other subsystems  99  of the base station  120  for example, for user interface purposes. 
     The power receiver (PRx) subsystem  110  of the electronic device  101  is configured to acquire near field inductive power  150  and to control its availability at its output. Power receiver (PRx) subsystem  110  can include receiver coil  115 , a power pick-up (circuit) unit  105 , a communications transceiver unit  108  and a control (controller) unit  122 . In an embodiment, control units  122  or  145  can be any suitable type of processor(s), such as one or more microprocessors, microcontrollers (MCUs), digital signal processors (DSPs), or the like, with supporting hardware and/or firmware (e.g., memory, programmable I/Os, etc.), as would be appreciated by one of ordinary skill in the art. 
     In operation, the receiver coil  115  receives the AC power signals  150  at a specific frequency transmitted from the power transmitting coil  125  of the power transmitter (PTx) subsystem  121 . Coils  115 ,  125  form a strongly-coupled inductor pair (magnetic induction transmission), or alternately form a loosely-coupled inductor pair (magnetic resonance transmission). The wireless power transfer system  100  employing near-field magnetic induction between coils  115 ,  125  can be a free-positioning or a magnetically-guided type of system. 
     At the power transmitter (PTx) subsystem  121 , signal processing circuitry of the power conversion unit  135  generates the AC power signals at a specified operating point, e.g., amplitude, frequency, and duty cycle, for transmission via the power transmitting coil  125 . The power pick-up (circuit) unit  105  of the power receiver (PRx) subsystem  110  receives the AC power signals and converts the AC power signals received into signals for directly charging an electrical device load  199 , e.g., storing charge in a local energy storage device such as one or more rechargeable batteries, at a constant current or constant voltage charging profile at the electronic device  101 . For example, at the power pick-up (circuit) unit  105 , rectifier circuits (e.g., a rectification circuit consisting of four diodes in a full bridge configuration) convert the received AC waveform to a DC power level for use in charging. 
     At the power transmitter (PTx) subsystem  121 , the communications transceiver unit  140  can perform two-way communication with the power receiver (PRx) subsystem  110  via the communications transceiver unit  108 . In embodiments, control (controller) unit  122  of the power receiver (PRx) subsystem  110  employs logic for controlling power transfer and communication of messages  155  with the power transmitter (PTx) subsystem  121 . For example, for controlling power transfer, communications transceiver unit  140  performs a demodulation of modulated control signals (packets) communicated by and received from the communications transceiver unit  108  at the power receiver (PRx) subsystem  110 . In an embodiment, control (controller) unit  145  can include a program module  146  including instructions for executing an application process based on data received by the communications transceiver unit  140 . 
     In inductive charging systems according to, for example, the Qi standard, throughout the power transfer phase, the power transmitter (PTx) subsystem  121  and the power receiver (PRx) subsystem  110  can form a closed-loop system to control the amount of power that is transferred. In an embodiment, the amount of power transferred to the electronic device  101  is controlled by the power receiver (PRx) subsystem  110 . That is, for wireless charging, an increase/decrease of the transmitting power is requested in a one-way communication from the power receiver (PRx) subsystem  110  to the power transmitter (PTx) subsystem  121  in order for the receiving side to have an optimal amount of power, as explained in more detail below. The power transmitting side controls the transmitted power value in accordance with the request received from the power receiver (PRx) subsystem  110 . In embodiments, the power receiver (PRx) subsystem  110  provides for power transfers up to 5 W, however in embodiments, this can be extended to power transfers greater than 5 W. 
       FIG. 4  illustrates a schematic diagram of a power transfer control loop  200  for controlling power transfer. In an embodiment, at block  205 , the power receiver (PRx) subsystem  110  selects a desired control point: a desired output current and/or voltage, a temperature measured somewhere in the electronic device  101 , etc. In addition, the power receiver (PRx) subsystem  110  determines at block  210  its actual control point. 
     The power receiver (PRx) subsystem  110  may use any approach to determine a control point. For example, the power receiver (PRx) subsystem  110  may receive a voltage or current value from the power pick-up (circuit) unit  105 . Moreover, the power receiver (PRx) subsystem  110  may change this approach at any time during the power transfer phase. Using the desired control point and actual control point, the power receiver (PRx) subsystem  110  at block  215  calculates a control error value. For example, the power receiver (PRx) subsystem  110  may calculate the control error value by taking the (relative) difference of the two output voltages or currents. The result of taking the (relative) difference being negative would indicate that the power receiver (PRx) subsystem  110  requires less power in order to reach its desired control point and the result of taking the (relative) difference being positive would indicate that the power receiver (PRx) subsystem  110  requires more power in order to reach its desired control point. That is, a negative control error value, when communicated by the power receiver (PRx) subsystem  110  to the power transmitter (PTx) subsystem  121  would direct the power transmitter (PTx) subsystem  121  to increase its operating frequency, or to decrease its voltage if the operating frequency has reached its maximum value, and a positive control error value, when communicated by the power receiver (PRx) subsystem  110  to the power transmitter (PTx) subsystem  121  would direct the power transmitter (PTx) subsystem  121  to increase its voltage, or to decrease its operating frequency if the voltage has reached its maximum value. 
     However, according to an embodiment, a negative control error value can direct the power transmitter (PTx) subsystem  121  to prevent the power transmitter (PTx) subsystem  121  from further lowering its power switching frequency regardless of the brand and make of the transmitter if an operating frequency condition at the power receiver (PRx) subsystem  110  has already reached a low frequency threshold value, as described in more detail below. 
     Subsequently, the power receiver (PRx) subsystem  110  transmits this control error value to the power transmitter (PTx) subsystem  121  over a signal bus or like communications channel for receipt at the communications transceiver unit  140  at the power transmitter (PTx) subsystem  121 . Such control error value provides input to control (controller) unit  145  at the power transmitter (PTx) subsystem  121 . 
     The control error value can be communicated as a byte word, i.e., a two&#39;s complement, signed integer value contained in a message field ranging between − 128  . . . + 127  (inclusive), in a message packet, referred to as Control Error Packet (CEP)  255 , that is sent to the power transmitter (PTx) subsystem  121  to control increasing power, decreasing power, or maintaining the power level charge being transferred to the load  199  via magnetic induction. In an embodiment, the CEP message can be transmitted over an PRx-to-PTx communication link as modulated signals on top of the power link that exists. In an embodiment, the bit rate for the PRx-to-PTx communication link is 2 kbps. 
     As further shown in  FIG. 4 , upon receiving the control error value in the Control Error Packet (CEP), the power transmitter (PTx) subsystem  121  adjusts its operating point, for example, within a pre-determined time window. At block  230 , the control (controller) unit  145  can run methods to determine the actual primary cell current value delivered to the power charge coil  125  for transfer by the power conversion unit  135  of the power transmitter (PTx) subsystem  1121 . Then, at block  235 , the control (controller) unit  145  uses the control error value from the Control Error Packet (CEP)  255  and the determined actual primary cell current to determine a new primary cell current. After the system stabilizes from the communications of the Control Error Packet (CEP)  255 , at block  240 , the power transmitter (PTx) subsystem  121  runs methods to control its actual primary cell current towards the determined new primary cell current within a short time window. Within this time window, at block  245 , the power transmitter (PTx) unit  110  reaches a new Operating Point: the amplitude, frequency, and duty cycle of the AC voltage that is applied to the power charge coil  125 . Subsequently, the power transmitter (PTx) subsystem  121  keeps its Operating Point fixed in order to enable the power receiver (PRx) subsystem  110  to communicate additional control and status information. 
     To set up power transfer and assist in its control, a power transmitter and power receiver execute a communication protocol with each other. According to the Qi standard, the power receiver uses amplitude shift keying to communicate requests and other information to the power transmitter by modulating its reflected impedance. The power transmitter (PTx) subsystem  121  communicates to the power receiver (PRx) subsystem  110  using frequency shift keying (FSK) in which the power transmitter modulates the operating frequency of the power signal transferred. Such communication provides synchronization and other information to the power receiver by modulating its operating frequency. 
       FIG. 5  depicts a power transfer control method  300  run by the control (controller) unit  122  at the power receiver (PRx) subsystem  110  for dynamically protecting the power receiver (PRx) subsystem  110  from overvoltage and overpower in accordance with an embodiment. 
     In the wireless power transfer system  100  illustrated in  FIG. 3 , power is transferred from the power transmitter (PTx) subsystem  121  contained in the base station  120  to the power receiver (PRx) subsystem  110  contained in the electronic device  101 . Before power transfer begins, the power receiver (PRx) subsystem  110  and the power transmitter (PTx) subsystem  121  communicate with each other to establish that the electronic device  101  is indeed capable of being charged, whether it needs to be charged, how much power is required, etc. In short, the communication ensures an appropriate power transfer from the power transmitter (PTx) subsystem  121  to the power receiver (PRx) subsystem  110 . 
     When power transfer begins, the power transmitter (PTx) subsystem  121  runs an alternating current through the power charge coil  125 , which generates an alternating magnetic field. This magnetic field is in turn picked up by the receiver coil  115  inside the power receiver (PRx) subsystem  110  and transformed by a power converter back into an alternating electrical current that can be used, for example, to charge a battery. 
     As indicated at step  302 , in a power transfer control phase, the power transmitter (PTx) subsystem  121  continues to provide power to the power receiver (PRx) subsystem  110 , adjusting its primary cell current in response to control data that the power transmitter (PTx) subsystem  121  receives from the power receiver (PRx) subsystem  110 . In an embodiment, the power receiver (PRx) subsystem continuously sends the control error packets (CEPs) to the power transmitter (PTx) subsystems  121  regardless of the relationship between charge voltage used to control the power signal transmitted and a target charge value. Throughout this phase, the power receiver (PRx) subsystem  110  controls the power transfer from the power transmitter (PTx) subsystem  121 , by means of control error data value in the Control Error Packet (CEP)  255  that the power receiver (PRx) subsystem  110  transmits to the power transmitter (PTx) subsystem  121 . For the Control Error Packet (CEP) having control error values greater than 0, the power transmitter (PTx) subsystem  121  responsively increases its voltage to increase the power transferred. However, if the voltage has reached its maximum value, the power transmitter (PTx) subsystem  121  decreases its operating frequency to increase the power transferred. 
     The power receiver (PRx) subsystem  110  receives the magnetically induced power and continuously monitors the state of the power at the power receiver (PRx) subsystem  110 . For example, a charge voltage and the current through a rectifier circuit can be sampled periodically and digitized by an analog-to-digital converter  109  as shown in  FIG. 3 . The digital equivalents of the voltage and current are supplied to a microprocessor control unit  122  as shown in  FIG. 3 , and by running internal control logic embodied as an application  130  in firmware, the microprocessor control unit  122  decides whether the loading conditions indicate that a change in the operating point is required. If the load is heavy enough to bring the charge voltage below a target, the power transmitter (PTx) subsystem  121  is instructed to increase its voltage or move its frequency lower, e.g., closer to resonance. If the charge voltage is higher than its target, the power transmitter (PTx) subsystem  121  is instructed to increase its frequency. In an embodiment, the control (controller) unit  122  measures the rectifier voltage and sends Control Error Packets (CEPs) to the power transmitter (PTx) subsystem  121  to adjust the rectifier voltage to the level required to maximize the efficiency of the power receiver (PRx) subsystem  121 . 
     At step  306 , a first determination is made as to whether the charge voltage obtained from the transmitted PTx power signal is below an over power threshold. If the transmitted PTx power signal reaches or exceeds an over power threshold, the power receiver (PRx) subsystem  110 , at step  308 , generates and transmits an End Power Transfer Packet containing an End Power Transfer Code, which is used to instruct the power transmitter (PTx) subsystem  121  to terminate power transfer operations. On receipt of the End Power Transfer Packet containing this value, the power transmitter (PTx) subsystem  121  removes the power signal and effectively terminates the power charge transfer operations with the electronic device  101 . Otherwise, returning to step  306 , while the charge voltage obtained from the transmitted PTx power signal is below the over power threshold, e.g., is at a normal operating point, the process proceeds to step  310  where a further determination is made as to whether the charge voltage obtained from the transmitted PTx power signal is less than a target charge threshold value. If at step  310 , it is determined that the charge voltage obtained from the transmitted PTx power signal is less than the target charge threshold value, then at step  314  the power receiver (PRx) subsystem  110  sets and periodically transmits a Control Error Packet (CEP) with a control error value greater than zero. The power transmitter (PTx) subsystem  121  responsively increases its voltage to increase the power transferred, or the power transmitter (PTx) subsystem  121  decreases its operating frequency thereby decreasing the frequency of the AC power signal transferred depending upon the received control error value. In either event, power receiver (PRx) subsystem  110  continuously transmits Control Error Packets (CEPs) with a control error value greater than zero. Otherwise, at step  310 , if it is determined that the charge voltage obtained from the transmitted PTx power signal is equal to or greater than a target charge threshold value, then at step  318  the power receiver (PRx) subsystem  110  sets and periodically transmits a Control Error Packet (CEP) with a control error value less than or equal to zero. The power transmitter (PTx) subsystem  121  responsively decreases its voltage to decrease the power transferred, or the power transmitter (PTx) subsystem  121  increases its operating frequency thereby increasing the frequency of the AC power signal transferred depending upon the received control error value. In either event, power receiver (PRx) subsystem  110  continuously transmits Control Error Packets (CEPs) with a control error value less than or equal to zero. 
     Regardless of the value of the Control Error Packets (CEPs) being sent to control transmitter operations at the power transmitter (PTx) subsystem  121 , the process continues to step  325  where the control (controller) unit  122  at the power receiver (PRx) subsystem  110  makes a determination as to a frequency characteristic of the received AC power signal transferred, i.e., the switched frequency at the transmitting apparatus (TX freq SW ). In an embodiment, the power receiver (PRx) subsystem  110  can include a frequency detector (circuit) unit  107 , as shown in  FIG. 3 , that can detect the received frequency of the transferred power signal at the synchronous rectifier of the power pick-up (circuit) unit  105 . In another example, the frequency detection can be determined by the power receiver (PRx) subsystem  110  obtaining information from a communication packet transmitted from the power transmitter (PTx) subsystem  121 . The value of the frequency of the transferred power signal is converted to a digital value TX freq SW  and is compared to a lowest frequency threshold value (thresh freq LOW ) corresponding to a lowest frequency of any transferred power signal that ensures protection of the power receiver (PRx) subsystem  110  against an over voltage/over power condition. The lowest frequency threshold value is independent of the power transmitter (PTx) subsystem  121 . Further the lowest frequency threshold value can be a pre-determined, fixed value or a variable that is calculated by the power receiver (PRx) subsystem  110 . 
     At step  325 , if it is determined that the value of TX freq SW  is greater than the value of the lowest frequency threshold freq LOW , the process continues to step  328  where the control error value of the control error packet (CEP) is maintained and, at step  335 , a Control Error Packet (CEP) is transmitted to the power transmitter (PTx) subsystem  121  and the process returns to step  306  to repeat the monitoring steps  306  and  310  while continuing to generate Control Error Packets (CEPs) having control error values greater than zero. Otherwise, if it is determined at step  325  that the value of TX freq SW  is less than or equal to the value of the lowest frequency threshold freq LOW  indicating the receiver coil  115  current frequency is lower than the threshold set in power receiver (PRx) subsystem  110 , then at step  330  the control (controller) unit  122  of the power receiver (PRx) subsystem  110  sets the control error value of the CEP packet to a non-positive number and at step  335  generates and initiates sending a Control Error Packet (CEP) to the power transmitter (PTx) subsystem  121  with the non-positive control error value in order to prevent the power transmitter (PTx) subsystem  121  from further lowering its switching frequency regardless of the brand and make of the base station  120 . In this way, at step  330 , the power receiver (PRx) subsystem  110  actively prevents asking the power transmitter (PTx) subsystem  121  for more power. 
     The elements illustrated in the drawings and described above as functional blocks for performing various processes can be implemented hardware-wise by a CPU, a memory, and other circuits, and software-wise by a memory or other non-transitory computer-readable storage medium having a program stored thereon or the like. Accordingly, it is to be understood by those skilled in the art that these functional blocks can be implemented in various forms including being implemented by hardware alone, software alone, or a combination of hardware and software. 
     Besides mobile devices, wireless charging according to embodiments herein can be used in many other consumer product categories—smart watches, power banks, Bluetooth headsets, cameras, electric shavers, etc. Virtually any device that uses a rechargeable battery can be designed to use wireless power charging technique described herein. Further, wireless power transfer technique described herein is not limited to charging batteries. It can also be used to power devices that require electric current and will remain stationary while in use, such as desktop lamps or speakers. 
     Moreover, further examples of a standalone wireless charger can include, but is not limited to: charging pads, which lie flat on a table or desktop; charging stands, which are designed to hold a smart phone upright in a viewing position while charging; and power banks, which are similar to charging pads, but contain internal batteries as a portable power source 
     Although the present invention made by the inventors have been specifically described based on the embodiments, the present invention is not limited to the embodiment described above, and various modifications can be made without departing from the gist thereof.