Patent Publication Number: US-2021184503-A1

Title: Wireless power receiver with a transistor rectifier

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Non-Provisional Patent Application Ser. No. 15/803,476, filed on Nov. 3, 2017, entitled “Wireless Power Receiver With a Synchronous Rectifier,” which claims priority from U.S. Provisional Patent Application No. 62/417,115, filed Nov. 3, 2016 and entitled “Synchronous Rectifier for Wireless Power Receivers,” which are hereby incorporated by reference in its entirety. 
     This application is related to U.S. patent application Ser. No. 14/856,337 filed on Sep. 16, 2015; U.S. patent application Ser. No. 14/856,317, filed Sep. 16, 2015 and entitled “Receiver Devices Configured to Operate with a Wireless Charging System”; U.S. patent application Ser. No. 14/856,186, filed Sep. 16, 2015 and entitled “Systems and Methods for Transmitting Power to Receivers”; U.S. patent application Ser. No. 14/856,219, filed Sep. 16, 2015 and entitled “Systems and Methods for Tracking Movement of Receivers in a Transmission Field”; U.S. patent application Ser. No. 14/854,820, filed Sep. 15, 2015 and entitled “Identifying Receivers in a Wireless Charging Transmission Field”; U.S. patent application Ser. No. 14/860,824, filed Sep. 22, 2015 and entitled “Receiver Devices Configured to Determine Location Within a Transmission Field”; U.S. patent application Ser. No. 14/860,843, filed Sep. 22, 2015 and entitled “Systems and Methods for Identifying Receiver Locations Using Sensor Technologies”; U.S. patent application Ser. No. 14/860,858, filed Sep. 22, 2015 and entitled “Systems and Methods for Preconfiguring Transmission Devices for Power Wave Transmissions”; U.S. patent application Ser. No. 14/861,285, filed Sep. 22, 2015 and entitled “Systems and Methods for Identifying Sensitive Objects in a Wireless Charging Field”; U.S. patent application Ser. No. 14/861,350, filed Sep. 22, 2015 and entitled “Systems and Methods for Preconfiguring Sensor Data for Wireless Charging Systems”; U.S. patent application Ser. No. 14/860,991, filed Sep. 22, 2015 and entitled “Systems and Methods for Generating and Transmitting Wireless Power Transmission Waves”; U.S. patent application Ser. No. 14/860,963, filed Sep. 22, 2015 and entitled “Systems and Methods for Determining and Generating Waveform for Wireless Power Transmission Waves”; U.S. patent application Ser. No. 14/861,246, filed Sep. 22, 2015 and entitled “Systems and Methods for Nullifying Energy Levels for Wireless Power Transmission Waves”; U.S. patent application Ser. No. 14/861,397, filed Sep. 22, 2015 and entitled “Systems and Methods for Selecting Antennas to Generate and Transmit Power Transmission Waves”; U.S. patent application Ser. No. 14/861,443, filed Sep. 22, 2015 and entitled “Wireless Power Transmission Device Having Antennas Oriented in Three Dimensions,” all of which are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to wireless power receivers, and more specifically to synchronous rectifiers for wireless power receivers. 
     BACKGROUND 
     Portable electronic devices, such as laptop computers, mobile phones, tablets, and other electronic devices, require frequent charging of a power-storing component (e.g., a battery) to operate. Many electronic devices require charging one or more times per day. Often, charging an electronic device requires manually connecting an electronic device to an outlet or other power source using a wired charging cable. In some cases, the power-storing component is removed from an electronic device and inserted into charging equipment. Accordingly, charging is time consuming, burdensome, and inefficient because users must carry around multiple charging cables and/or other charging devices, and frequently must locate appropriate power sources to charge their electronic devices. Additionally, conventional charging techniques potentially deprive a user of the ability to use the device while it is charging, and/or require the user to remain next to a wall outlet or other power source to which their electronic device or other charging equipment is connected. 
     Some other conventional charging systems utilize inductive coils to generate a magnetic field that is used to charge a device. However, such inductive coupling has a limited short range, such as a few inches or less. Users typically must place the device at a specific position on a charging pad and are unable to move the device to different positions on the pad, without interrupting or terminating the charging of the device. This results in a frustrating experience for many users as they may be unable to locate the device at the exact right position on the pad to start charging their device. 
     Additionally, existing antennas used for transmission of power waves have relatively large cross-sectional areas, such as 6 inches by 6 inches for transmission of power waves at a frequency of 900 MHz. Due to these large cross-sectional areas, integrating these antennas with devices results in noticeable and undesired changes to the aesthetic appearance of the devices, thereby reducing the likelihood that users will be willing to install such devices in their homes or offices. 
     Other conventional systems use radio frequency (RF) waves to transmit power. These RF waves form pockets of energy by a controlled constructive interference of the RF waves at the location of a wireless power receiver. However, the power received through the controlled constructive interference must be rectified to direct current (DC) that can be supplied to the electronic device to be charged. Wireless power receivers with traditional diode based rectifiers are lossy and consume a significant portion of the received power to operate. 
     SUMMARY 
     Accordingly, there is a need for improved antenna designs that help to address the shortcomings of conventional charging systems described above. In particular, there is a need for antennas that have a form factor that is suitable for integration within devices. Also, there is a need for a more efficient wireless power receiver with a rectifier that enables the wireless power receiver to transfer most of the received power to an associated electronic device. 
     In an embodiment, a wireless power receiver comprises: a field effect transistor (FET) including a source terminal, a drain terminal, and a gate terminal, wherein: the source terminal is connected to the ground; the drain terminal is connected to an alternating current (AC) voltage source; and the gate terminal is configured to be driven by a feed-forward signal tapped from the AC voltage source. 
     In another embodiment, a method comprises: receiving, by a drain terminal of a field effect transistor (FET) of a wireless power receiver, an input alternating current (AC) voltage waveform from a AC voltage source; and receiving, by a gate terminal of the FET of the wireless power receiver, a feed-forward signal tapped from the input AC voltage waveform, wherein the feed-forward signal biases the FET to operate as an open switch such that a connected load receives a current only during a positive cycle of the AV voltage waveform received from the AC voltage source. 
     Some embodiments provide a wireless power receiver that includes at least one radio frequency (RF) antenna and a field effect transistor (FET). The at least one RF antenna is configured to generate an alternating current (AC) waveform from received RF waves. The FET includes a source terminal configured to be electrically coupled to ground, a drain terminal electrically coupled to the at least one antenna, and a gate terminal configured to be driven by a feed-forward signal tapped from the AC waveform generated by the at least one antenna. 
     In some embodiments, the wireless power receiver includes a tap-line configured to carry the feed-forward signal from the at least one antenna to the gate terminal. In some embodiments, the tap-line is a micro-strip of a predetermined length that is selected such that the feed-forward signal arrives at the gate terminal completely out of phase or nearly completely out of phase with the AC waveform arriving at the drain terminal. In some embodiments, a feed-forward controller inserts a phase delay into the feed-forward signal. 
     In some embodiments, the feed-forward controller is configured to insert the phase delay into the feed-forward signal such that the feed-forward signal arrives at the gate terminal completely out of phase or nearly completely out of phase with the AC waveform arriving at the drain terminal. In some embodiments, the feed-forward controller is configured to insert the phase delay into the feed-forward signal such that the feed-forward signal arrives at the gate terminal with a predetermined phase offset with the AC waveform arriving at the drain terminal. In some embodiments, the one or more capacitors are configured to store electric charge during a negative cycle of the AC waveform. In some embodiments, the drain terminal outputs a direct current waveform to a load. In some embodiments, the FET is selected from the group consisting of: complementary metal oxide semiconductor (CMOS) P-type FET, CMOS N-type FET, Gallium arsenide (GaAs) FET, Gallium nitride (GaN) FET, silicon on insulator (SOI) FET, and a floating gate metal oxide FET (MOSFET). In some embodiments, the FET is configured to implement a Class-E or a Class-F switch. 
     In some embodiments, a method of receiving wireless power is provided. Radio frequency (RF) waves are received at a RF antenna. The waves are converted to an alternating current (AC) waveform. A drain terminal of a field effect transistor (FET) receives the AC waveform. A gate terminal of the FET receives a feed-forward signal tapped from the AC waveform. The feed-forward signal biases the FET to operate as an open switch such that a connected load receives a direct current (DC) waveform during a positive cycle of the AC waveform. 
     In some embodiments, a tap-line conducts the feed-forward signal from a source of the AC waveform (e.g., the at least one antenna) to the gate terminal. In some embodiments, the tap-line is a micro-strip described above. In some embodiments, a feed-forward controller inserts a phase delay in the feed-forward signal. In some embodiments, the feed-forward controller inserts the phase delay such that the feed-forward signal arrives at the gate terminal completely out of phase or nearly completely out of phase with the AC waveform arriving at the drain terminal. In some embodiments, the feed-forward controller inserts the phase delay such that the feed-forward signal arrives at the gate terminal with a predetermined phase offset with the AC waveform arriving at the drain terminal. In some embodiments, the one or more capacitors store electric charges from the current during a negative cycle of the AC waveform. In some embodiments, the FET is selected from the group consisting of: complementary metal oxide semiconductor (CMOS) P-type FET, CMOS N-type FET, Gallium arsenide (GaAs) FET, Gallium nitride (GaN) FET, silicon on insulator (SOI) FET, and a floating gate metal oxide FET (MOSFET). In some embodiments, the FET is configured to implement a Class-E or Class-F switch. 
     Note that the various embodiments described above can be combined with any other embodiments described herein. The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the present disclosure can be understood in greater detail, a more particular description may be had by reference to the features of various embodiments, some of which are illustrated in the appended drawings. The appended drawings, however, merely illustrate pertinent features of the present disclosure and are therefore not to be considered limiting, for the description may admit to other effective features. 
         FIG. 1  is a block diagram illustrating a representative wireless power transmission system in accordance with some embodiments. 
         FIG. 2A  is a block diagram illustrating a representative transmitter device in accordance with some embodiments. 
         FIG. 2B  is a block diagram illustrating a representative receiver device in accordance with some embodiments. 
         FIG. 3A  schematically illustrates a synchronized switching of a switching device in a rectifier, according an exemplary embodiment. 
         FIG. 3B  shows prior art diode characteristics. 
         FIG. 3C  shows a prior art half-wave rectifier with a diode. 
         FIG. 3D  shows a loss due to diode bias voltage in the prior art half-wave rectifier. 
         FIG. 4  is a circuit diagram illustrating a representative system for a synchronous rectification of an alternating current (AC) in accordance with some embodiments. 
         FIG. 5  is a circuit diagram illustrating a representative system for a synchronous rectification of an alternating current (AC) in accordance with some embodiments. 
         FIG. 6  is a block diagram illustrating a representative a wireless power receiver in accordance with some embodiments. 
     
    
    
     In accordance with common practice, the various features illustrated in the drawings may not be drawn to scale. Accordingly, the dimensions of the various features may be arbitrarily expanded or reduced for clarity. In addition, some of the drawings may not depict all of the components of a given system, method or device. Finally, like reference numerals may be used to denote like features throughout the specification and figures. 
     DETAILED DESCRIPTION 
     The present disclosure is here described in detail with reference to embodiments illustrated in the drawings, which form a part here. Other embodiments may be used and/or other changes may be made without departing from the spirit or scope of the present disclosure. The illustrative embodiments described in the detailed description are not meant to be limiting of the subject matter presented here. Furthermore, the various components and embodiments described herein may be combined to form additional embodiments not expressly described, without departing from the spirit or scope of the invention. 
     Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used here to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated here, and additional applications of the principles of the inventions as illustrated here, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention. 
       FIG. 1  is a block diagram illustrating a representative wireless power transmission system  100  in accordance with some embodiments. In accordance with some embodiments, the wireless power transmission system  100  includes transmitters  102  (e.g., transmitters  102   a,    102   b  . . .  102   n ) and a receiver  120 . In some embodiments, the wireless power transmission environment  100  includes multiple receivers  120 , each of which is associated with (e.g., coupled to) a respective electronic device  122 . 
     In accordance with some embodiments, the transmitter  102  (e.g., transmitter  102   a ) includes processor(s)  104 , memory  106 , one or more antenna array(s)  110 , communications component(s)  112 , and one or more transmitter sensor(s)  114  interconnected via a communications bus  108 . References to these components of transmitters  102  cover embodiments in which one or more than one of each of these components (and combinations thereof) are included. 
     In some embodiments, a single processor  104  (e.g., processor  104  of transmitter  102   a ) executes software modules for controlling multiple transmitters  102  (e.g., transmitters  102   b  . . .  102   n ). In some embodiments, a single transmitter  102  (e.g., transmitter  102   a ) includes multiple processors  104  such as one or more transmitter processors (e.g., configured to control transmission of signals  116  by antenna(s)  110 ), one or more communications component processors (e.g., configured to control communications transmitted by the communications component(s)  112  and/or receive communications via the communications component(s)  112 ), and/or one or more sensor processors (e.g., configured to control operation of the transmitter sensor(s)  114  and/or receive output from the transmitter sensor(s)  114 ). 
     The receiver  120  receives the power signals  116  and/or the communication signals  118  transmitted by the transmitters  102 . In some embodiments, the receiver  120  includes one or more rectennas  124  (e.g., an antenna array including multiple antenna elements and one or more rectifier elements), a power converter  126 , a receiver sensor  128 , and/or other components or circuitry (e.g., processor(s)  140 , memory  142 , and/or communications component(s)  144 ). References to these components of receiver  120  cover embodiments in which one or more than one of each of these components (and combinations thereof) are included. The receiver  120  converts energy from the received signals  116  (e.g., power waves) into electrical energy to power and/or charge the electronic device  122 . For example, the receiver  120  uses the power converter  126  to convert captured energy from the power waves  116  to alternating current (AC) electricity or to direct current (DC) electricity usable to power and/or charge the electronic device  122 . In some embodiments, the power converter  126  include at least one rectifier, while in some other embodiments, the antenna(s)  1254  and the power converter(s)  126  include at least one rectenna (i.e., a combination of an antenna and rectifier). Non-limiting examples of the power converter  126  include rectifiers, rectifying circuits, and voltage conditioners, among other suitable circuitry and devices. 
     In some embodiments, receiver  120  is a standalone device that is detachably coupled to one or more electronic devices  122 . For example, electronic device  122  has processor(s)  133  for controlling one or more functions of electronic device  122  and receiver  120  has processor(s)  140  for controlling one or more functions of receiver  120 . 
     In some embodiments, as shown, receiver  120  is a component of electronic device  122 . For example, processor(s)  133  controls functions of electronic device  122  and receiver  120 . 
     In some embodiments, electronic device  122  includes processor(s)  133 , memory  134 , communications component(s)  136 , and/or battery/batteries  130 . In some embodiments, these components are interconnected by way of a communications bus  138 . In some embodiments, communications between electronic device  122  and receiver  120  occur via communications component(s)  136  and/or  144 . In some embodiments, communications between electronic device  122  and receiver  120  occur via a wired connection between communications bus  138  and communications bus  146 . In some embodiments, electronic device  122  and receiver  120  share a single communications bus. 
     In some embodiments, the receiver  120  receives one or more power waves  116  directly from the transmitter  102 . In some embodiments, the receiver  120  harvests energy from one or more pockets of energy created by one or more power waves  116  transmitted by the transmitter  102 . 
     In some embodiments, after the power waves  116  are received and/or energy is harvested from a pocket of energy, circuitry (e.g., integrated circuits, amplifiers, rectifiers, and/or voltage conditioner) of the receiver  120  converts the energy of the power waves (e.g., radio frequency electromagnetic radiation) to a suitable form (e.g., electricity) for powering the electronic device  122  and/or storage in a battery  130  of the electronic device  122 . In some embodiments, a rectifying circuit of the receiver  120  translates the electrical energy from AC to DC for use by the electronic device  122 . In some embodiments, a voltage conditioning circuit increases or decreases the voltage of the electrical energy as required by the electronic device  122 . In some embodiments, an electrical relay conveys electrical energy from the receiver  120  to the electronic device  122 . 
     In some embodiments, the receiver  120  is a component of the electronic device  122 . In some embodiments, a receiver  120  is coupled (e.g., detachably coupled) to the electronic device  122 . In some embodiments, the electronic device  122  is a peripheral device of receiver  120 . In some embodiments, the electronic device  122  obtains power from multiple transmitters  102 . In some embodiments, the electronic device  122  using (e.g., is coupled to) multiple receivers  120 . In some embodiments, the wireless power transmission system  100  includes a plurality of electronic devices  122 , each having at least one respective receiver  120  that is used to harvest power waves from the transmitters  102  and provide usable power for charging/powering the electronic devices  122 . 
     In some embodiments, the one or more transmitters  102  adjust one or more characteristics (e.g., phase, gain, direction, and/or frequency) of power waves  116 . For example, a particular transmitter  102  (e.g., transmitter  102   a ) selects a subset of one or more antenna elements of antenna(s)  110  to initiate transmission of the power waves  116 , cease transmission of the power waves  116 , and/or adjust one or more characteristics used to transmit the power waves  116 . In some embodiments, the one or more transmitters  102  adjust the power waves  116  such that trajectories of the power waves  116  converge at a predetermined location within a transmission field (e.g., a location or region in space), resulting in controlled constructive or destructive interference patterns. 
     In some embodiments, respective antenna array(s)  110  of the one or more transmitters  102  optionally include a set of one or more antennas configured to transmit the power waves  116  into respective transmission fields of the one or more transmitters  102 . In some embodiments, integrated circuits (not shown) of the respective transmitter  102 , such as a controller circuit and/or waveform generator, control the behavior of the antenna(s). For example, based on the information received from the receiver  120  via the communications signal  118 , a controller circuit determines a set of one or more characteristics or waveform characteristics (e.g., amplitude, frequency, trajectory, phase, among other characteristics) used for transmitting power waves that effectively provide power to the receiver  120 . In some embodiments, the controller circuit identifies a subset of antennas from the antenna(s)  110  that would be effective in transmitting the power waves  116 . As another example, a waveform generator circuit of the respective transmitter  102  (e.g., coupled to the processor  104 ) converts energy and generates the power waves  116  having the waveform characteristics identified by the controller, and then provides the power waves to the antenna array(s)  110  for transmission. 
     In some instances, constructive interference of power waves occurs when two or more power waves  116  are in phase with each other and converge into a combined wave such that an amplitude of the combined wave is greater than amplitude of a single one of the power waves. For example the positive and negative peaks of sinusoidal waveforms arriving at a location from multiple antennas “add together” to create larger positive and negative peaks. In some instances and embodiments, a pocket of energy is formed at a location in a transmission field where constructive interference of power waves occurs. 
     In some instances, destructive interference of power waves occurs when two or more power waves are out of phase and converge into a combined wave such that the amplitude of the combined wave is less than the amplitude of a single one of the power waves. For example, the power waves “cancel each other out,” thereby diminishing the amount of energy concentrated at a location in the transmission field. In some embodiments, destructive interference is used to generate a negligible amount of energy or “null” at a location within the transmission field where the power waves converge. 
     In some embodiments, the one or more transmitters  102  transmit power waves  116  that create two or more discrete transmission fields (e.g., overlapping and/or non-overlapping discrete transmission fields). In some embodiments, a first transmission field is managed by a first processor  104  of a first transmitter (e.g. transmitter  102   a ) and a second transmission field is managed by a second processor  104  of a second transmitter (e.g., transmitter  102   b ). In some embodiments, the two or more discrete transmission fields (e.g., overlapping and/or non-overlapping) are managed by the transmitter processors  104  as a single transmission field. 
     In some embodiments, the communications component(s)  112  transmit communication signals  118  via a wired and/or wireless communication connection to receiver  120 . In some embodiments, the communications component  112  generates communications signals  118  to determine positioning of the receiver  120  (e.g., via triangulation). In some embodiments, the communication signals  118  are used to convey information between the transmitter  102  and the receiver  120 . In some embodiments, the conveyed information is used for adjusting one or more characteristics used to transmit the power waves  116 . In some embodiments, the communications signals  118  relay information related to device status, efficiency, user data, power consumption, billing, geo-location, and/or other types of information. 
     In some embodiments, the receiver  120  includes a transmitter (not shown), or is a part of a transceiver, that transmits communications signals  118  to communications component  112  of transmitter  102 . 
     In some embodiments, the communications component  112  (e.g., communications component  112  of transmitter  102   a ) includes a communications component antenna for communicating with the receiver  120 , other transmitters  102  (e.g., transmitters  102   b  through  102   n ), and/or other remote devices. In some embodiments, these communications signals  118  represent a distinct channel of signals transmitted by transmitter  102 , independent from a channel of signals used for transmission of the power waves  116 . 
     In some embodiments, the receiver  120  includes a receiver-side communications component (not shown) configured to communicate various types of data with one or more of the transmitters  102 , through a respective communications signal  118  generated by the receiver-side communications component. The data optionally includes location indicators for the receiver  120  and/or the electronic device  122 , a power status of the device  122 , status information for the receiver  120 , status information for the electronic device  122 , status information regarding the power waves  116 , and/or status information for pocket(s) of energy. In some embodiments, the receiver  120  provides data to the transmitter  101 , via the communications signal  118 , regarding the current operation of the system  100 , including one or more of: information identifying a present location of the receiver  120  or the device  122 , an amount of energy received by the receiver  120 , and an amount of power received and/or used by the electronic device  122 , among other possible types of information. 
     In some embodiments, the data contained within communications signals  118  is used by electronic device  122 , receiver  120 , and/or transmitters  102  for determining adjustments of the one or more characteristics used by the antenna(s)  110  to transmit the power waves  116 . For example, using a communications signal  118 , the transmitter  102  communicates data that is used to identify the receiver(s)  120  within a transmission field, identify the electronic device(s)  122 , determine safe and effective waveform characteristics for power waves, and/or hone the placement of pockets of energy. In some embodiments, the receiver  120  uses a communications signal  118  to communicate data for alerting transmitters  102  that the receiver  120  has entered or is about to enter a transmission field, provide information about electronic device  122 , provide user information that corresponds to electronic device  122 , indicate the effectiveness of received power waves  116 , and/or provide updated characteristics or transmission parameters that the one or more transmitters  102  use to adjust transmission of the power waves  116 . 
     As an example, the communications component  112  of the transmitter  102  communicates (e.g., transmits and/or receives) one or more types of data (such as authentication data and/or transmission parameters) including various information such as a beacon message, a transmitter identifier, a device identifier for an electronic device  122 , a user identifier, a charge level for the electronic device  122 , a location of receiver  120  in a transmission field, and/or a location of the electronic device  122  in a transmission field. 
     In some embodiments, the transmitter sensor  114  and/or the receiver sensor  128  detect and/or identify conditions of the electronic device  122 , the receiver  120 , the transmitter  102 , and/or a transmission field. In some embodiments, data generated by the transmitter sensor  114  and/or the receiver sensor  128  is used by the transmitter  102  to determine appropriate adjustments to the one or more characteristics used to transmit the power waves  116 . In some embodiments, data from the transmitter sensor  114  and/or the receiver sensor  128  received by transmitter  102  includes raw sensor data and/or sensor data processed by a processor, such as a sensor processor. In some embodiments, the processed sensor data includes determinations based upon sensor data output. In some embodiments, sensor data is received from sensors that are external to the receiver  120  and the transmitters  102  (such as thermal imaging data, information from optical sensors, and others). 
     In some embodiments, the receiver sensor(s)  128  include a gyroscope that provides raw data such as orientation data (e.g., tri-axial orientation data). In some embodiments, processing this raw data includes determining a location of receiver  120  and/or a location of receiver antenna  124  using the orientation data. 
     In some embodiments, the receiver sensor(s)  128  include one or more infrared sensors (e.g., that output thermal imaging information), and processing this infrared sensor data includes identifying a person (e.g., indicating presence of the person and/or indicating an identification of the person) or other sensitive object based upon the thermal imaging information. 
     In some embodiments, the receiver sensor(s)  128  includes a gyroscope and/or an accelerometer that indicates an orientation of the receiver  120  and/or the electronic device  122 . As an example, transmitter(s)  102  receive orientation information from the receiver sensor  128  and the transmitter(s)  102  (or a component thereof, such as the processor  104 ) uses the received orientation information to determine whether the electronic device  122  is flat on a table, in motion, and/or in use (e.g., near to a user&#39;s ear). 
     In some embodiments, the receiver sensor  128  is a sensor of the electronic device  122  (e.g., an electronic device  122  that is remote from receiver  120 ). In some embodiments, the receiver  120  and/or the electronic device  122  includes a communication system for transmitting signals (e.g., sensor signals output by the receiver sensor  128 ) to the transmitter  102 . 
     Non-limiting examples of transmitter sensor  114  and/or receiver sensor  128  include infrared/pyro-electric sensors, ultrasonic sensors, laser sensors, optical sensors, Doppler sensors, gyro sensors, accelerometers, microwave sensors, millimeter sensors, RF standing-wave sensors, resonant LC sensors, capacitive sensors, and/or inductive sensors. In some embodiments, technologies for the transmitter sensor  114  and/or the receiver sensor  128  include binary sensors that acquire stereoscopic sensor data, such as the location of a human or other sensitive object. 
     In some embodiments, the transmitter sensor  114  and/or the receiver sensor  128  is configured for human recognition (e.g., is capable of distinguishing between a person and other objects, such as furniture) and/or human identification (e.g., is capable of distinguishing between two persons). Examples of sensor data output by human recognition-enabled sensors include: body temperature data, infrared range-finder data, motion data, activity recognition data, silhouette detection and recognition data, gesture data, heart rate data, portable devices data, and wearable device data (e.g., biometric readings and output, accelerometer data). 
     In some embodiments, the transmitters  102  adjust one or more characteristics used to transmit the power waves  116  to ensure compliance with electromagnetic field (EMF) exposure protection standards for human subjects. Maximum exposure limits are defined by US and European standards in terms of power density limits and electric field limits (as well as magnetic field limits). These include, for example, limits established by the Federal Communications Commission (FCC) for MPE, and limits established by European regulators for radiation exposure. Limits established by the FCC for MPE are codified at 47 CFR § 1.1310. For electromagnetic field (EMF) frequencies in the microwave range, power density can be used to express an intensity of exposure. Power density is defined as power per unit area. For example, power density can be commonly expressed in terms of watts per square meter (W/m 2 ), milliwatts per square centimeter (mW/cm 2 ), or microwatts per square centimeter (μW/cm 2 ). In some embodiments, output from transmitter sensor  114  and/or receiver sensor  128  is used by transmitter  102  to detect whether a person or other sensitive object enters a power transmission region (e.g., a location within a predetermined distance of a transmitter  102 , power waves generated by transmitter  102 , and/or a pocket of energy). In some embodiments, in response to detecting that a person or other sensitive object has entered the power transmission region, the transmitter  102  adjusts one or more power waves  116  (e.g., by ceasing power wave transmission, reducing power wave transmission, and/or adjusting the one or more characteristics of the power waves). In some embodiments, in response to detecting that a person or other sensitive object has entered the power transmission region, the transmitter  102  activates an alarm (e.g., by transmitting a signal to a loudspeaker that is a component of transmitter  102  or to an alarm device that is remote from transmitter  102 ). In some embodiments, in response to detecting that a person or other sensitive object has entered a power transmission region, the transmitter  102  transmits a digital message to a system log or administrative computing device. 
     In some embodiments, the antenna array(s)  110  includes multiple antenna elements (e.g., configurable “tiles”) collectively forming an antenna array. In various embodiments, the antenna(s)  110  generate RF power waves, ultrasonic power waves, infrared power waves, and/or magnetic resonance power waves. In some embodiments, the antenna(s)  110  (e.g., of a single transmitter, such as transmitter  102   a,  and/or of multiple transmitters, such as transmitters  102   a,    102   b,  . . . ,  102   n ) transmit two or more power waves that intersect at a defined location (e.g.,. a location corresponding to a detected location of a receiver  120 ), thereby forming a pocket of energy at the defined location. 
     In some embodiments, transmitter  102  assigns a first task to a first subset of antenna elements of antenna(s)  110 , a second task to a second subset of antenna elements of antenna(s)  110 , and so on, such that the constituent antennas of antenna(s)  110  perform different tasks (e.g., determining locations of previously undetected receivers  120  and/or transmitting power waves  116  to one or more receivers  120 ). As one example, the antenna(s)  110  includes ten antennas, nine antennas transmit power waves  116  that form a pocket of energy and the tenth antenna operates in conjunction with communications component  112  to identify new receivers in the transmission field. In another example, the antenna(s)  110  includes ten antenna elements is split into two groups of five antenna elements, each of which transmits power waves  116  to two different receivers  120  in the transmission field. 
     In some embodiments, a device for wirelessly charging a battery includes a transmitter unit having a transmitter and an antenna unit comprising the antenna array in communication with the transmitter. In some embodiments, the antenna unit is configured to transmit a wireless signal for use in charging a battery. The battery may be in a wearable device such as a smart watch, or a mobile device such as a smart phone. In some embodiments, the wireless signal is used for operating an electronic device. In some embodiments, the antenna unit is configured as a linear array. In some embodiments, the linear array is longer than 2 feet. In some embodiments, the linear array is formed from multiple linear subarrays with spaces between the multiple linear subarrays. The antenna unit comprising the array of antennas may be configured as a matrix. In some embodiments, the antenna elements are regularly spaced. In some embodiments, the antenna elements are variably or irregularly spaced. In some embodiments, the antenna elements are grouped into sub-arrays, and the sub-arrays are selectable for transmitting wireless power signals by the selected sub-arrays. In some embodiments, the antenna elements are individually selectable. In some embodiments, a processing unit (e.g., processor(s)  104 ) is configured to cause a transmitter to generate and transmit a wireless transmission signal via the antenna element(s). 
       FIG. 2A  is a block diagram illustrating a representative transmitter device  102  (also sometimes referred to herein as a transmitter) in accordance with some embodiments. In some embodiments, the transmitter device  102  includes one or more processing units (e.g., CPUs, ASICs, FPGAs, microprocessors, and the like)  104 , one or more communications components  112 , memory  106 , antenna(s)  110 , and one or more communication buses  108  for interconnecting these components (sometimes called a chipset). In some embodiments, the transmitter device  102  includes one or more sensor(s)  114  as described above with reference to  FIG. 1 . In some embodiments, the transmitter device  102  includes one or more output devices such as one or more indicator lights, a sound card, a speaker, a small display for displaying textual information and error codes, etc. In some embodiments, the transmitter device  102  includes a location detection device, such as a GPS (global positioning satellite) or other geo-location receiver, for determining the location of the transmitter device  102 . In some embodiments, the antenna(s)  110  include one or more multi-band antennas (such as those described below in reference to  FIGS. 3-5B ). 
     In various embodiments, the one or more sensor(s)  114  include one or more thermal radiation sensors, ambient temperature sensors, humidity sensors, IR sensors, occupancy sensors (e.g., RFID sensors), ambient light sensors, motion detectors, accelerometers, and/or gyroscopes. 
     The communications component(s)  112  enable communication between the transmitter  102  and one or more communication networks. In some embodiments, the communications component(s)  112  include, e.g., hardware capable of data communications using any of a variety of wireless protocols (e.g., IEEE 802.15.4, Wi-Fi, ZigBee, 6LoWPAN, Thread, Z-Wave, Bluetooth Smart, ISA100.11a, WirelessHART, MiWi, etc.) wired protocols (e.g., Ethernet, HomePlug, etc.), and/or any other suitable communication protocol, including communication protocols not yet developed as of the filing date of this document. 
     The memory  106  includes high-speed random access memory, such as DRAM, SRAM, DDR SRAM, or other random access solid state memory devices; and, optionally, includes non-volatile memory, such as one or more magnetic disk storage devices, one or more optical disk storage devices, one or more flash memory devices, or one or more other non-volatile solid state storage devices. The memory  106 , or alternatively the non-volatile memory within memory  106 , includes a non-transitory computer-readable storage medium. In some embodiments, the memory  106 , or the non-transitory computer-readable storage medium of the memory  106 , stores the following programs, modules, and data structures, or a subset or superset thereof:
         Operating logic  216  including procedures for handling various basic system services and for performing hardware dependent tasks;   Communication module  218  for coupling to and/or communicating with remote devices (e.g., remote sensors, transmitters, receivers, servers, mapping memories, etc.) in conjunction with communications component(s)  112 ;   Sensor module  220  for obtaining and processing sensor data (e.g., in conjunction with sensor(s)  114 ) to, for example, determine the presence, velocity, and/or positioning of object in the vicinity of the transmitter  102 ;   Power-wave generating module  222  for generating and transmitting (e.g., in conjunction with antenna(s)  110 ) power waves, including but not limited to, forming pocket(s) of energy at given locations; and   Database  224 , including but not limited to:
           Sensor information  226  for storing and managing data received, detected, and/or transmitted by one or more sensors (e.g., sensors  114  and/or one or more remote sensors);   Device settings  228  for storing operational settings for the transmitter  102  and/or one or more remote devices;   Communication protocol information  230  for storing and managing protocol information for one or more protocols (e.g., custom or standard wireless protocols, such as ZigBee, Z-Wave, etc., and/or custom or standard wired protocols, such as Ethernet); and   Mapping data  232  for storing and managing mapping data (e.g., mapping one or more transmission fields).   
               

     Each of the above-identified elements (e.g., modules stored in memory  106  of the transmitter  102 ) are optionally stored in one or more of the previously mentioned memory devices, and corresponds to a set of instructions for performing the function(s) described above. The above identified modules or programs (e.g., sets of instructions) need not be implemented as separate software programs, procedures, or modules, and thus various subsets of these modules are optionally combined or otherwise rearranged in various embodiments. In some embodiments, the memory  106 , optionally, stores a subset of the modules and data structures identified above. Furthermore, the memory  106 , optionally, stores additional modules and data structures not described above, such as a tracking module for tracking the movement and positioning of objects within a transmission field. 
     In some instances and embodiments, the transmitter(s)  102  are positioned in households and commercial settings, such as conference rooms, where it is necessary and/or preferable that the transmitter(s)  102  are sized in a manner that results in a small footprint and/or profile. Although the size of the footprint (e.g., width of overall antenna arrays) in some embodiments has to have a certain length for creating the energy pockets at various distances, the profiles (e.g., length of the antenna elements along the z-axis, which defines the distance that the transmitters  102  extend from a wall) is optionally reduced to be more viable for use by consumers and businesses. 
       FIG. 2B  is a block diagram illustrating a representative receiver device  120  (also sometimes called a receiver) in accordance with some embodiments. In some embodiments, the receiver device  120  includes one or more processing units (e.g., CPUs, ASICs, FPGAs, microprocessors, and the like)  140 , one or more communications components  144 , memory  256 , antenna(s)  124 , power converter(s)  126 , power harvesting circuitry  259 , and one or more communication buses  258  for interconnecting these components (sometimes called a chipset). In some embodiments, the receiver device  120  includes one or more sensor(s)  128  such as one or sensors described above with reference to  FIG. 1 . In some embodiments, the receiver device  120  includes an energy storage device  261  for storing energy harvested via the power harvesting circuitry  259 . In various embodiments, the energy storage device  261  includes one or more batteries (e.g., battery  130 ,  FIG. 1 ), one or more capacitors, one or more inductors, and the like. 
     As described above in reference to  FIG. 1 , in some embodiments, the receiver  120  is internally or externally connected to an electronic device (e.g., electronic device  122   a,    FIG. 1 ) via a connection (e.g., a bus)  138 . 
     In some embodiments, the power harvesting circuitry  259  includes one or more rectifying circuits and/or one or more power converters. In some embodiments, the power harvesting circuitry  259  includes one or more components (e.g., a power converter  126 ) configured to convert energy from power waves and/or energy pockets to electrical energy (e.g., electricity). In some embodiments, the power harvesting circuitry  259  is further configured to supply power to a coupled electronic device (e.g., an electronic device  122 ), such as a laptop or phone. In some embodiments, supplying power to a coupled electronic device include translating electrical energy from an AC form to a DC form (e.g., usable by the electronic device  122 ). In some embodiments, the antenna(s)  124  include one or more multi-band antennas (such as those described below in reference to  FIGS. 3-5B ). 
     In some embodiments, the receiver device  120  includes one or more output devices such as one or more indicator lights, a sound card, a speaker, a small display for displaying textual information and error codes, etc. (in some embodiments, the receiver device  120  sends information for display at an output device of an associated electronic device). In some embodiments, the receiver device  120  includes a location detection device, such as a GPS (global positioning satellite) or other geo-location receiver, for determining the location of the receiver device  120 . 
     In various embodiments, the one or more sensor(s)  128  include one or more thermal radiation sensors, ambient temperature sensors, humidity sensors, IR sensors, occupancy sensors (e.g., RFID sensors), ambient light sensors, motion detectors, accelerometers, and/or gyroscopes. 
     The communications component(s)  144  enable communication between the receiver  120  and one or more communication networks. In some embodiments, the communications component(s)  144  include, e.g., hardware capable of data communications using any of a variety of custom or standard wireless protocols (e.g., IEEE 802.15.4, Wi-Fi, ZigBee, 6LoWPAN, Thread, Z-Wave, Bluetooth Smart, ISA100.11a, WirelessHART, MiWi, etc.) custom or standard wired protocols (e.g., Ethernet, HomePlug, etc.), and/or any other suitable communication protocol, including communication protocols not yet developed as of the filing date of this document. 
     The memory  256  includes high-speed random access memory, such as DRAM, SRAM, DDR SRAM, or other random access solid state memory devices; and, optionally, includes non-volatile memory, such as one or more magnetic disk storage devices, one or more optical disk storage devices, one or more flash memory devices, or one or more other non-volatile solid state storage devices. The memory  256 , or alternatively the non-volatile memory within memory  256 , includes a non-transitory computer-readable storage medium. In some embodiments, the memory  256 , or the non-transitory computer-readable storage medium of the memory  256 , stores the following programs, modules, and data structures, or a subset or superset thereof:
         Operating logic  266  including procedures for handling various basic system services and for performing hardware dependent tasks;   Communication module  268  for coupling to and/or communicating with remote devices (e.g., remote sensors, transmitters, receivers, servers, mapping memories, etc.) in conjunction with communications component(s)  144 ;   Sensor module  270  for obtaining and processing sensor data (e.g., in conjunction with sensor(s)  128 ) to, for example, determine the presence, velocity, and/or positioning of the receiver  120 , a transmitter  102 , or an object in the vicinity of the receiver  120 ;   Wireless power-receiving module  272  for receiving (e.g., in conjunction with antenna(s)  124  and/or power harvesting circuitry  259 ) energy from power waves and/or energy pockets; optionally converting (e.g., in conjunction with power harvesting circuitry  259 ) the energy (e.g., to direct current); transferring the energy to a coupled electronic device (e.g., an electronic device  122 ); and optionally storing the energy (e.g., in conjunction with energy storage device  261 ); and   Database  274 , including but not limited to:
           Sensor information  276  for storing and managing data received, detected, and/or transmitted by one or more sensors (e.g., sensors  128  and/or one or more remote sensors);   Device settings  278  for storing operational settings for the receiver  120 , a coupled electronic device (e.g., an electronic device  122 ), and/or one or more remote devices; and   Communication protocol information  280  for storing and managing protocol information for one or more protocols (e.g., custom or standard wireless protocols, such as ZigBee, Z-Wave, etc., and/or custom or standard wired protocols, such as Ethernet).   
               

     Each of the above identified elements (e.g., modules stored in memory  256  of the receiver  120 ) are optionally stored in one or more of the previously mentioned memory devices, and corresponds to a set of instructions for performing the function(s) described above. The above identified modules or programs (e.g., sets of instructions) need not be implemented as separate software programs, procedures, or modules, and thus various subsets of these modules are optionally combined or otherwise rearranged in various embodiments. In some embodiments, the memory  256 , optionally, stores a subset of the modules and data structures identified above. Furthermore, the memory  256 , optionally, stores additional modules and data structures not described above, such as an identifying module for identifying a device type of a connected device (e.g., a device type for an electronic device  122 ). 
     In some embodiments, the antennas (e.g., antenna(s)  110  and/or antenna(s)  124 ) have orientations that cause the wireless power signals  116  to be transmitted at different polarizations depending on an orientation of the electronic device with respect to the antennas. For example, in accordance with some embodiments, the receiver  120  includes a monopole or a dipole antenna so that orientation of the receiver  120  with respect to the antenna(s)  110  has minimal impact on the amount of power that is received from the wireless power signals  116 . 
     A wireless power receiver may include a rectifier to rectify alternating current (AC) received at one or more antennas of the receiver to direct current (DC). Conventional rectifiers may use diodes for rectifying AC to DC. As such, a conventional rectifier may operate as follows: a diode may operate like a valve for electric current and allows the flow of the current in one direction and opposes the flow of the current in other direction. Therefore, for a load that is configured to operate using DC, a diode may be placed in between an AC source and the load. When the AC is flowing in a first direction, the diode may be forward biased (i.e. may operate as a closed switch) to allow the current to flow through the load; and when the AC is flowing in a second direction that is opposite to the first direction, the diode may be reverse biased (i.e. may operate as an open switch) such that no current flows through the load. Therefore, a unidirectional current without a change in polarity may flow through the load. 
     As mentioned above, in some embodiments the power converter(s)  126  of  FIGS. 1 and 2B , include one or more rectifiers or rectennas (i.e., antennas with rectifiers). Conventional rectifiers are often bridge rectifiers or the like that use a series of diodes arranged so that the output from the rectifier is always a positive signal (e.g., a DC signal), regardless of the incoming signal&#39;s polarity (e.g., an AC signal). Diodes, however, are lossy components in an electrical circuit, and may consume a significant amount of the power that they are being implemented to rectify. Generally, a typical diode may have a threshold voltage of 0.7 V, i.e., it may consume about 0.7 V to be forward biased. Consequently, there is a lossy voltage drop across a diode when a current is flowing through the diode of about 0.7 V. 
     In light of this issue, there is a need a rectifier with a lower loss of voltage and that consumes significantly less power compared to the conventional diode-based rectifiers. Moreover, as diodes heat-up, their performance suffers. Other systems may use a transistor as a switching device, but these systems typically require a separate power source to bias the gate of the transistor.  FIG. 3B  shows the characteristics of a current vs. voltage relationship for a typical diode. A bias voltage level must be reached before current flows when the diode is positively biased. Below the bias voltage or when the diode is negatively biased, no current flows through it. While not a typical operational setup for a diode, there is a breakdown voltage when a diode is negatively biased beyond a voltage at which the diode characteristics described here break down and the current flows in the reverse direction. 
       FIG. 3C  shows the typical implementation of a half-wave rectifier using a diode.  FIG. 3D  shows the input output voltage relationship for a half-wave rectifier using the circuitry of  FIG. 3C . Due to the bias voltage required to turn on a diode (i.e., the voltage required to allow current to flow through the diode), and due to the voltage drop across the diode, there is a loss of energy incurred.  FIG. 3D  shows the loss due to the bias voltage as well as the voltage drop across the diode. The embodiments disclosed herein negate or reduce this loss by using field-effect transistors (FETs) rather than diodes in rectifier implementation. 
     In applications where a wirelessly received waveform is rectified, there may be a need to synchronize the switching points in the rectifier with the received waveform. To synchronize the switching with the voltage waveform generated by a receiving antenna, the received voltage is tapped-off by a tap-line and it is used to bias the FET. E.g., in a Class-E amplifier. In other words, the voltage waveform generated in the antenna is fed forward by the feed-forward tap-line such that the FET switches synchronously with the received AC voltage waveform. In some embodiments, the rectifiers described herein use a Class-E amplifier as a switching device. 
     In some embodiments, the tap-line is a micro-strip of a preselected length. The length is selected such that the voltage waveform biases the FET synchronous with the voltage waveform arriving at the drain of the FET. For example, in some embodiments, the voltage waveform arriving at the gate of the FET is 180 degrees phase shifted from the voltage waveform arriving at the drain of the FET. Therefore, during the positive cycle of the AC voltage, wherein the AC power source has a positive voltage, i.e. higher potential than the ground, the FET operates as an open switch, thereby having no conduction path through the FET. As the current cannot pass through the FET in the positive cycle, the current passes through the load. However, during the negative cycle of the AC voltage, wherein the AC power source has a negative voltage, i.e. lower potential than the ground, the FET operates as a closed switch, thereby providing a path of least resistance for a current from the ground to the AC voltage of the drain. Therefore, the current flows from the ground towards the power source, in other words the drain, through the FET. A resistive load may not provide the path of least resistance, and therefore the current does not flow through the load in the negative cycle of the AC voltage. IN some embodiments, during the negative cycle of the AC voltage, the charge from the current from the ground towards the power source is stored at one or more capacitors, and the stored charge is used during the positive cycle of the AC voltage. 
     In some embodiments, a feed-forward controller circuit is used to control the phase of the voltage waveform passing through the tap-line to the gate of the FET. For example, in some embodiments, the controller circuit is a LC circuit introducing the requisite amount of phase delay such that the zero-crossings of the feed-forward voltage waveforms at the gate of the FET match with the zero-crossings of the voltage waveform seen by the drain of the FET. In some embodiments, the feed-forward controller circuit is programmable. 
     In some embodiments, the phase of feed-forward voltage is controlled such that the transistor switches asynchronously with the received waveform. By doing so, a portion of the received power is rectified to be transferred to the load. The remaining power is reflected back to the antenna or sent to the ground such that the circuit components do not heat up due the excess amount of unused power. In other embodiments, the phase of the feed-forward voltage is controlled such that the switching of the transistor does not allow for any rectification of the received power. 
     Although the figures and description below describe an electrical ground for the sake of brevity, one ordinarily skilled in the art should appreciate that the ground may refer to an equipotential surface. 
       FIG. 3A  schematically illustrates a synchronized switching  300  of a switching device in a rectifier. An incoming alternating current (AC) or radio frequency (RF) wave  301  may reverse the polarity in every time period T. For example, for a 5.8 GHz wave has 5.8*109 cycles every second, and therefore the number of times the polarity switched per second is 2*5.8*109. In this case, the time period T is approximately 8.62*10-11 seconds, or 86.2 picoseconds. To be synchronous with the polarity reversal, the switching device may have to switch between the two states at the time the polarity of the RF wave  301  reverses. As shown in the timing graph  302 , for example a bias-voltage, for the switching device, may also switch the state every T time period. In the context of rectifying a wirelessly received waveform, for example an RF waveform, in addition to the time period T, the switching device may switch between its states at the same time the polarity of the RF wave  301  is reversed. For a load configured to operate using the positive polarity of the RF wave  301 , the switching device may close a circuit providing a current to the load, and may open the circuit such that no current flows to the load. The switching device may open the circuit or may close the circuit based on a bias voltage provided to the switching device. Therefore, to achieve an efficient switching, the bias voltage may have to be synchronized with the phase of the RF wave  301  such that the switching device switches exactly at the zero-crossings of the RF wave  301 , or as close to it as possible. 
       FIG. 4  illustrates a system  400  for a synchronous rectification of an alternating current (AC) signal, accordingly to an exemplary embodiment. The system  400  is configured to rectify AC generated by an AC power source  401  to supply a direct current (DC) to a load  406 . In the context of rectifying a wireless waveform received through an antenna, the AC waveform, for example the RF waveform received through one or more antennas may be the power source  401 , as explained further below. In some embodiments, the system  400  comprises one or more electrical components such as capacitors  402 , inductors  403 , one or more transistors  404 , resistors (not shown), and/or other various circuit components. In some embodiments, the system  400  uses the electrical components for electrical functions such as storing electrical charges temporarily, controlling the fluctuation of the voltage across various portions of the circuit, and/or controlling the fluctuation of current passing through various portions of the circuit. In some embodiments, the system  400  further includes control circuits to control various aspects of the system  400  and its operation. In some embodiments, the system  400  also includes one or more impedance matching circuits, resonance matching circuits, filtering circuits, and/or other kinds of circuits. 
     In some embodiments, AC power source  401  is a radio frequency (RF) antenna or any other type of energy harvesting device, which receives power from one or more power waves. For instance, in some embodiments, the AC power source  401  is a receiving antenna, where the electrons within the antenna are excited by electromagnetic waves to generate AC in a conductor connected to the antenna. In some embodiments, in addition to the antenna, RF receiving circuitry is also included in the receiver, for example to down-convert the received signal to baseband. In some embodiments, the load  406  is any device that is powered by the power harvested from one or more power RF waves. In some embodiments, the load  406  is configured to operate using DC power. Although the aforementioned embodiment describes receiving AC power from one or more power waves and rectifying the AC into DC power for the load  406 , it should be understood that the system  400  may be used for any type of rectification, regardless the source of AC power and the load device. 
     In some embodiments, the capacitors  402  are any suitable capacitors. For example, the capacitors  402  may have fixed capacitance values or may be have variable capacitance values. For capacitors  402  having variable capacitance, the capacitance is adjusted manually or by a control circuit (not shown). In some embodiments, the system  400  uses the capacitors  402  as a temporary storage of charges during the operation of the system  400 . In some embodiments, the system  400  temporarily stores the charge in the capacitors  402  and releases the stored charge to control the voltage fluctuations across various portions of the circuit. In some embodiments, the system  400  uses the capacitors  402 , for example capacitor  402   a,  to filter-out or convert to DC from the AC. The inductors  403  may be any kind of inductors known in the art. In some embodiments, the system  400  uses the inductors  403  to smooth out the fluctuation of the current passing through the inductors  403 . 
     In some embodiments, the transistor  404  is a field effect transistor (FET). For example, in some embodiments, the transistor  404  is a complementary metal oxide semiconductor (CMOS) P-type FET or N-type FET. In some embodiments, the transistor  404  is a Gallium arsenide (GaAs) FET or a Gallium nitride (GaN) FET. In some embodiments, the transistor  404  is a 2.5 V silicon on insulator (SOI) CMOS NFET implementing a Class-E switch. In some embodiments, the transistor may be a floating gate FET, for example floating gate MOSFET. In some embodiments, the Class-E switch is used to construct a Class-E power amplifier. In some embodiments, a Class-F switch is used to construct a synchronous Class-F rectifier. In some embodiments, a Class-E switch is used to construct a synchronous Class-E rectifier. In some embodiments, the transistor  404  comprises a gate  405 , a drain  210 , and a source  211 . It should be understood that one or more types of the transistors  404  described herein are merely exemplary, and other transistors and/or switching devices may be used without deviating from the scope of this disclosure. 
     In operation, the AC power source  401  may generate an AC voltage. In some implementations, the frequency of the generated AC voltage may be fixed, for example at 5.8 GHz. In other implementations, the frequency of the AC voltage may be variable. In these implementations, the frequency of the AC voltage may fluctuate within a frequency band. One ordinarily skilled in the art appreciates that the AC voltage may reverse its polarity at every cycle. Furthermore, as the AC power source  401  reverses the polarity of the AC voltage, the direction of the flow of current through one or more portions of the system  400  reverses as well. For example, during the positive polarity (“positive cycle”) of the AC voltage generated by the AC power source  401 , a current may be moving in a direction  412   a.  During the negative polarity (“negative cycle”) of the AC voltage generated by the AC power source  401 , the current may be moving in an opposite direction  412   b.  While reversing the polarity of the voltage, that is, transitioning from the positive cycle to the negative cycle and vice-versa, the AC voltage generated by the power source  401  may pass through a zero-crossing. A zero-crossing may be defined as a point where the value of the AC voltage generated by the AC power source is zero. When the current and the voltage are in phase, the zero crossing for the voltage may coincide with the zero-crossing of the current generated by the voltage. A person ordinarily skilled in the art understands that, for an efficient rectification of the AC to DC, the transistor  404  may have to switch at every zero-crossing of the voltage or current. In other words, the transistor  404  may have to be synchronized with the alternating current or voltage. Failure to switch at the zero-crossing, i.e. lack of synchronous switching of the transistor  404 , may result in the loss of power. Although the following description applies to both synchronization with the phases of each of alternating voltages and currents in a circuit, the following description uses synchronization based on the phases of the voltage for the sake of brevity. 
     To synchronize the switching of the transistor  404  with the AC voltage generated by the AC power source, the system  400  includes a tap-line  407  connected at a position  408  to a power line coming out of the AC power source. In some embodiments, the tap-line  407  may comprise a micro-strip of a preselected length. The length of the micro-strip may be selected such that the phase of the waveform seen by the drain  410  of the transistor  404  is out of phase of the waveform seen by the gate  405  of the transistor  404 . In some embodiments, the length of the micro-strip is selected to be different for different frequencies of the AC voltage being rectified. For instance, for any point in time, if the reference phase at the voltage source is φREF and the phase of the waveform seen by the drain  410  is φREF+φOFFSET, the length of the tap-line  407  may be selected such that the phase of the waveform seen by gate  405  is φREF+φOFFSET+π or φREF+φOFFSET−π. Therefore, during the positive cycle of the AC when the current is moving in the direction  412   a,  the voltage waveform seen by the drain  410  may have a positive polarity while the voltage waveform seen by the gate  405  may have a negative polarity, and vice versa. The voltage waveform with negative polarity may bias the gate  405  such that the transistor  404  operates as an open switch, that is, there may be no conduction path for the current through the transistor  404 . Therefore, the current may pass through the inductor  403   b  and onto the load  406  in the direction  409   a.    
     During the negative cycle of the AC, when the current is moving in the direction  412   b,  the voltage waveform seen by the drain  410  may have a negative polarity while the voltage seen by the gate  405  may have a positive polarity due to the predetermined length of the tap-line  407 , as detailed above. The voltage waveform with the positive polarity at the gate  405  may bias the gate  405  such that the transistor  404  operates as a closed switch, that is, there may be a conduction path from the ground through the transistor  404  towards the power source  401 . Therefore, as the power source  401  is at a lower potential that the ground, the transistor  404  may provide a path of least resistance between the ground and the power source  401 . As a result, the current may flow from the ground and through the transistor in the direction  409   b.  On the contrary, the load  406  may not provide a path of least resistance between the ground and the power source  401 , and therefore no current may flow from the load  406  during the negative cycle of the AC. 
     During the negative cycle of the AC, the system  400  may store charges from the current flowing in the direction  409   b,    412   b  in one or more charge storage devices, such as the capacitor  402   b.  The system  400  may transfer the stored charges from the capacitor  402   b  to the load during one or more subsequent positive cycles of the AC when current is allowed to pass through the load  406 . The system  400  may use the stored the charge to smooth out the current passing through the load  406  during the positive cycle of the AC. 
       FIG. 5  illustrates a system  500  for a synchronous rectification of an alternating current (AC), accordingly to an exemplary embodiment. The system  500  may be configured to rectify AC generated by an AC power source  501  to supply a direct current (DC) to a load  506 . The system  500  may comprise electrical components such as capacitors  502 , inductors  503 , one or more transistors  504 , resistors (not shown), and/or other various circuit components. The system  500  may use the electrical components for electrical functions such as storing electrical charges temporarily, controlling the fluctuation of the voltage across various portions of the circuit, and/or controlling the fluctuation of current passing through various portions of the circuit. The system  500  may further include control circuits to control various aspects of the system  500  and its operation. The system  500  may also include impedance matching circuits, resonance matching circuits, filtering circuits, and/or other kinds of circuits. 
     In some embodiments, the AC power source  501  is a radio frequency (RF) antenna or any other type of energy harvesting device, which receives power from one or more power waves. In some embodiments, it may also include additional receiver circuitry, for instance RF receiving circuitry, down-converter. For instance, in some embodiments, the AC power source  501  is a receiving antenna, wherein the electrons within the antenna may be excited by the electrical component of electromagnetic waves thereby generating an AC in a wire connected to the antenna. In some embodiments, the load  506  is any device that is powered by the power harvested from one or more RF power waves, i.e., RF waves that transmit power. In some instances, the load  506  is configured to operate using DC power. Although the aforementioned embodiment describes receiving AC power from one or more power waves and rectifying the AC into DC power for the load  506 , it should be understood that the system  500  may be used for any type of rectification, regardless the source of AC power and the load device. 
     The capacitors  502  may be any kind of capacitors. For example, in some embodiments, the capacitors  502  have fixed capacitance values or may be have variable capacitance values. For capacitors  502  having variable capacitance, the capacitance may be adjusted manually or by a control circuit (not shown). The system  500  may use the capacitors  502  as a temporary storage of charges during the operation of the system  500 . In some embodiments, the system  500  temporarily stores the charge in the capacitors  502  and release the stored charge to control the voltage fluctuations across various portions of the circuit. In some embodiments, the system  500  uses the capacitors  502 , for example the capacitor  502   a,  to convert or filter-out DC from the AC. The inductors  503  may be any kind of inductors known in the art. In some embodiments, the system  500  uses the inductors  503  to smooth the fluctuation of the current passing through the inductors  503 . 
     In some embodiments, the transistor  504  is a field effect transistor (FET). For example, in some embodiments, the transistor  504  is a complementary metal oxide semiconductor (CMOS) P-type FET or N-type FET. In some embodiments, the transistor  504  is a Gallium arsenide (GaAs) FET or a Gallium nitride (GaN) FET. In some embodiments, the transistor  504  is a 2.5 V silicon on insulator (SOI) CMOS NFET implementing a class-E switch. In some embodiments, the transistor is a floating gate MOSFET. In some embodiments, the Class-E switch may be used to construct a Class-E power amplifier. In some embodiments, a Class-F switch is used to construct a synchronous Class-F rectifier. In some embodiments, a Class-E switch is used to construct a synchronous class-E rectifier. The transistor  504  may comprise a gate  505 , a drain  310 , and a source  311 . It should be understood that one or more types of the transistors  504  described herein are merely exemplary, and other transistors and/or switching devices may be used without deviating from the scope of this disclosure. 
     In operation, the AC power source  501  may generate an AC voltage. In some implementations, the frequency of the generated AC voltage may be fixed, for example at 5.8 GHz. In other implementations, the frequency of the AC voltage may be variable. In these implementations, the frequency of the AC voltage may fluctuate within a frequency band. One ordinarily skilled in the art appreciates that the AC voltage may reverse its polarity at every cycle. Furthermore, as the AC power source  501  reverses the polarity of the AC voltage, the direction of the flow of current through one or more portions of the system  500  reverses as well. For example, during the positive polarity (“positive cycle”) of the AC voltage generated by the AC power source  501 , a current may be moving in a direction  512   a.  During the negative polarity (“negative cycle”) of the AC voltage generated by the AC power source  501 , the current may be moving in an opposite direction  5312   b.  While reversing the polarity of the voltage, that is, transitioning from the positive cycle to the negative cycle and vice-versa, the AC voltage generated by the power source  501  may pass through a zero-crossing. A zero-crossing may be defined as a point where the value of the AC voltage generated by the AC power source is zero. When the current and the voltage are in phase, the zero crossing for the voltage may coincide with the zero-crossing of the current generated by the voltage. A person ordinarily skilled in the art understands that, for an efficient rectification of the AC to DC, the transistor  504  may have to switch at every zero-crossing of the voltage or current. In other words, the transistor  504  may have to be synchronized with the alternating current or voltage. Failure to switch the zero-crossing, i.e. lack of synchronous switching of the transistor  504 , may result in the loss of power. Although the following description applies to both synchronization with the phases of each of alternating voltages and currents in a circuit, the following description uses synchronization based on the phases of the voltage for the sake of brevity. 
     To synchronize the switching of the transistor  504  with the AC voltage generated by the AC power source, the system  500  includes a tap-line  507  connected at a position  508  to a power line coming out of the AC power source. In some embodiments, a feed-forward controller  513  may be connected to the tap-line  507 . In some embodiments, the feed-forward controller  513  may be a series LC circuit to shift the phase of a voltage waveform. The feed-forward controller  513  may control the phase of the voltage, for example by inserting a delay, in the tap-line  507  to synchronize and control the rectification of the AC voltage generated by the AC power source  501 . For example, the feed-forward controller  513  may select the phase of the waveform passing through the tap-line  507  to the gate  505  of the transistor such the phase of the waveform seen by the drain  310  of the transistor  504  is out of phase of the waveform seen by the gate  505 . For instance, for any point in time, if the reference phase at the voltage source is φREF and the phase of the waveform seen by the drain  510  is φ REF +φ OFFSET , the feed-forward controller may select the that the phase of the waveform seen by gate  505  to be φ REF +φ OFFSET +π or φ REF +φ OFFSET −π. Therefore, during the positive cycle of the AC when the current is moving in the direction  512   a,  the voltage waveform seen by the drain  510  may have a positive polarity while the voltage waveform seen by the gate  505  may have a negative polarity. The voltage waveform with negative polarity may bias the gate  505  such that the transistor  504  operates as an open switch, that is, there may be no conduction path for the current through the transistor  504 . Therefore, the current may pass through the inductor  503   b  and onto the load  506  in the direction  509   a.    
     During the negative cycle of the AC, when the current is moving in the direction  512   b,  the voltage waveform seen by the drain  510  may have a negative polarity while the voltage seen by the gate  505  may have a positive polarity due to the phase selection by the feed-forward controller  513 , as detailed above. The voltage waveform with the positive polarity at the gate  505  may bias the gate  505  such that the transistor  504  operates as a closed switch, that is, there may be a conduction path from the ground through the transistor  504  towards the power source  501 . Therefore, as the power source  501  is at a lower potential that the ground, the transistor  504  may provide a path of least resistance between the ground and the power source  501 . As a result, the current may flow from the ground and through the transistor in the direction  509   b.  On the contrary, the load  506  may not provide a path of least resistance between the ground and the power source  501 , and therefore no current may flow from the load  506  during the negative cycle of the AC. 
     During the negative cycle of the AC, the system  500  may store charges from the current flowing in the direction  509   b,    512   b  in one or more charge storage devices, such as the capacitor  502   b.  The system  500  may transfer the stored charges from the capacitor  502   b  to the load during one or more subsequent positive cycles of the AC when current is allowed to pass through the load  506 . The system  500  may use the stored the charge to smooth out the current passing through the load  506  during the positive cycle of the AC. 
     In some embodiments, the feed-forward controller  513  may control the phase of the waveform through the tap-line such that the tap-line biases one or more switching devices (for example transistor  504 ) such that the switching may occur out of phase with the voltage waveform. Such out of phase switching may regulate the amount of power being rectified by the system  500 . In some embodiments, the switching is completely out of phase by one or more switching devices such that no power is being rectified by the system  500 , and the incoming power may be bounced back to the power source. Therefore, in these embodiments, the unwanted power may be kept out of various components of the system  500 , thereby avoiding unnecessary heating of the components of the system  500 . 
       FIG. 6  shows an exemplary wireless power receiver  600 , according to an exemplary embodiment. In some embodiments, the wireless power receiver  600  comprises an antenna  601 , a rectifier  602 , and a load  603 . In some embodiments, the antenna  601  receives radio frequency (RF) power waves transmitted from one or more transmitters and generate an alternating current (AC) voltage from the RF power waves. In some embodiments, the antenna  601  is any type of antenna such a patch antenna, a loop antenna, and/or a spiral antenna. In some embodiments, the AC voltage generated by the antenna  601  is conducted to the rectifier  602 . In some embodiments, the rectifier  602  is a synchronous rectifier that uses a field effect transistor (FET) as a Class-E switch. In some embodiments, the rectifier  602  converts the received AC voltage to DC voltage and supply the DC voltage to the load  603 . 
     In some embodiments, the receiver  600  comprises electrical components such as capacitors, inductors, resistors, and/or other various circuit components. In some embodiments, the receiver  600  uses the electrical components for electrical functions such as storing electrical charges temporarily, controlling the fluctuation of the voltage across various portions of the circuit, and/or controlling the fluctuation of current passing through various portions of the circuit. In some embodiments, the receiver  600  further includes control circuits to control various aspects of the receiver  600  and its operation. In some embodiments, the receiver  600  also includes impedance matching circuits, resonance matching circuits, filtering circuits, and/or other kinds of circuits. In some embodiments, the receiver  600  also comprises one or more power converters to step up or step down voltages at various portions of the receiver  600 . 
     The capacitors may be any kind of capacitors. For example, the capacitors may have fixed capacitance values or may be have variable capacitance values. For capacitors having variable capacitance, the capacitance may be adjusted manually or by a control circuit. The receiver  600  may use the capacitors as a temporary storage of charges during the operation of the receiver  600 . A person ordinarily skilled in the art appreciates that, the receiver  600  may temporarily store the charge in the capacitors and release the stored charge to control the voltage fluctuations across various portions of the receiver  600 . The receiver  600  may also use the capacitors to filter out DC component in an AC. The inductors may be any kind of inductors known in the art. The receiver  600  may use the inductors to smooth the fluctuation of the current passing through the inductors. 
     The foregoing method descriptions and the process flow diagrams are provided merely as illustrative examples and are not intended to require or imply that the steps of the various embodiments must be performed in the order presented. As will be appreciated by one of skill in the art the steps in the foregoing embodiments may be performed in any order. Words such as “then,” “next,” and the like, are not intended to limit the order of the steps; these words are simply used to guide the reader through the description of the methods. Although process flow diagrams may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination may correspond to a return of the function to the calling function or the main function. 
     The various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention. 
     Embodiments implemented in computer software may be implemented in software, firmware, middleware, microcode, hardware description languages, or any combination thereof. A code segment or machine-executable instructions may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc. 
     The actual software code or specialized control hardware used to implement these systems and methods is not limiting of the invention. Thus, the operation and behavior of the systems and methods were described without reference to the specific software code being understood that software and control hardware can be designed to implement the systems and methods based on the description herein. 
     When implemented in software, the functions may be stored as one or more instructions or code on a non-transitory computer-readable or processor-readable storage medium. The steps of a method or algorithm disclosed herein may be embodied in a processor-executable software module that may reside on a computer-readable or processor-readable storage medium. A non-transitory computer-readable or processor-readable media includes both computer storage media and tangible storage media that facilitate transfer of a computer program from one place to another. A non-transitory processor-readable storage media may be any available media that may be accessed by a computer. By way of example, and not limitation, such non-transitory processor-readable media may comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other tangible storage medium that may be used to store desired program code in the form of instructions or data structures and that may be accessed by a computer or processor. Disk and disc, as used herein, include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. Additionally, the operations of a method or algorithm may reside as one or any combination or set of codes and/or instructions on a non-transitory processor-readable medium and/or computer-readable medium, which may be incorporated into a computer program product. The preceding description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the following claims and the principles and novel features disclosed herein. While various aspects and embodiments have been disclosed, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.