Patent Publication Number: US-2022224123-A1

Title: High Efficiency Power Converting Apparatus

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
     This application is a continuation of U.S. patent application Ser. No. 16/950,223, filed on Nov. 17, 2020, which is a divisional of U.S. patent application Ser. No. 15/985,227, filed on May 21, 2018, now U.S. Pat. No. 10,868,429 issued Dec. 15, 2020, each application is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a power converter, and, in particular embodiments, to a high efficiency power converter in a receiver of a wireless power transfer system. 
     BACKGROUND 
     As technologies further advance, wireless power transfer has emerged as an efficient and convenient mechanism for powering or charging battery based mobile devices such as mobile phones, tablet PCs, digital cameras, MP3 players and/or the like. A wireless power transfer system typically comprises a primary side transmitter and a secondary side receiver. The primary side transmitter is magnetically coupled to the secondary side receiver through a magnetic coupling. The magnetic coupling may be implemented as a loosely coupled transformer having a primary side coil formed in the primary side transmitter and a secondary side coil formed in the secondary side receiver. 
     The primary side transmitter may comprise a power conversion unit such as a primary side of a power converter. The power conversion unit is coupled to a power source and is capable of converting electrical power to wireless power signals. The secondary side receiver is able to receive the wireless power signals through the loosely coupled transformer and convert the received wireless power signals to electrical power suitable for a load. 
     As the power of the wireless power transfer system goes higher, there may be a need for achieving a high-efficiency wireless power transfer between the transmitter and the receiver. More particularly, achieving a high efficiency wireless power transfer under various input and output conditions (e.g., different load currents and/or different rated input voltages of the receiver) has become a significant issue, which presents challenges to the system design of the wireless power transfer system. 
     It would be desirable to have a high performance power receiver exhibiting good behaviors such as high efficiency under a variety of input and output conditions. 
     SUMMARY 
     These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present disclosure which provide a high efficiency power converter in a receiver of a wireless power transfer system. 
     In accordance with an embodiment, a system comprises a first coil configured to be magnetically coupled to a second coil, a rectifier coupled to the first coil through a capacitor, a first power stage connected between an output of the rectifier and an output voltage node, and a second power stage coupled between the output voltage node and a battery, wherein the first power stage is configured to charge the battery, and the second power stage is configured to provide isolation between the first power stage and the battery. 
     In accordance with another embodiment, a system comprises a receiver coil configured to be magnetically coupled to a transmitter coil, wherein the transmitter coil is coupled to an input power source through a power converter, a rectifier coupled to the receiver coil, and a high efficiency converter comprises a first power stage and a second power stage connected in cascade between an output of the rectifier and an output voltage node and a battery, wherein the first power stage is configured to charge the battery, and the second power stage is configured to provide isolation between the first power stage and the battery. 
     In accordance with yet another embodiment, an apparatus comprises a first power stage connected between an input power source and an output voltage node, and a second power stage coupled between the output voltage node and a battery, wherein the first power stage is configured to operate in various operating modes for charging the battery and the second power stage configured to provide isolation between the first power stage and the battery. 
     The foregoing has outlined rather broadly the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the claims of the disclosure. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the disclosure as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a block diagram of a wireless power transfer system in accordance with various embodiments of the present disclosure; 
         FIG. 2  illustrates a block diagram of the receiver shown in  FIG. 1  in accordance with various embodiments of the present disclosure; 
         FIG. 3  illustrates a schematic diagram of a first implementation of the receiver shown in  FIG. 1  in accordance with various embodiments of the present disclosure; 
         FIG. 4  illustrates a schematic diagram of a second implementation of the receiver shown in  FIG. 1  in accordance with various embodiments of the present disclosure; 
         FIG. 5  illustrates a schematic diagram of a hybrid converter in accordance with various embodiments of the present disclosure; 
         FIG. 6  illustrates a schematic diagram of a four-switch buck-boost converter in accordance with various embodiments of the present disclosure; 
         FIG. 7  illustrates a block diagram of a third implementation of the receiver shown in  FIG. 1  in accordance with various embodiments of the present disclosure; 
         FIG. 8  illustrates a block diagram of a fourth implementation of the receiver shown in  FIG. 1  in accordance with various embodiments of the present disclosure; 
         FIG. 9  illustrates a flow chart of applying a battery charging control mechanism to the high efficiency converter shown in  FIG. 3  in accordance with various embodiments of the present disclosure; and 
         FIG. 10  illustrates a flow chart of applying a low input voltage control mechanism to the high efficiency converter shown in  FIG. 2  in accordance with various embodiments of the present disclosure. 
     
    
    
     Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the disclosure, and do not limit the scope of the disclosure. 
     The present disclosure will be described with respect to preferred embodiments in a specific context, namely a high efficiency power converter operating in different operating modes for increasing efficiency and performance of a wireless power transfer system. The disclosure may also be applied, however, to a variety of power systems. Hereinafter, various embodiments will be explained in detail with reference to the accompanying drawings. 
       FIG. 1  illustrates a block diagram of a wireless power transfer system in accordance with various embodiments of the present disclosure. The wireless power transfer system  100  comprises a power converter  104  and a wireless power transfer device  101  connected in cascade between an input power source  102  and a load  114 . In some embodiments, the power converter  104  is employed to further improve the performance of the wireless power transfer system  100 . In alternative embodiments, the power converter  104  is an optional element. In other words, the wireless power transfer device  101  may be connected to the input power source  102  directly. 
     The wireless power transfer device  101  includes a power transmitter  110  and a power receiver  120 . As shown in  FIG. 1 , the power transmitter  110  comprises a transmitter circuit  107  and a transmitter coil L 1  connected in cascade. The input of the transmitter circuit  107  is coupled to an output of the power converter  104 . The power receiver  120  comprises a receiver coil L 2 , a resonant capacitor Cs, a rectifier  112  and a high efficiency power converter  113  connected in cascade. As shown in  FIG. 1 , the resonant capacitor Cs is connected in series with the receiver coil L 2  and further connected to the inputs of the rectifier  112 . The outputs of the rectifier  112  are connected to the inputs of the high efficiency power converter  113 . The outputs of the high efficiency power converter  113  are coupled to the load  114 . 
     The power transmitter  110  is magnetically coupled to the power receiver  120  through a magnetic field when the power receiver  120  is placed near the power transmitter  110 . A loosely coupled transformer  115  is formed by the transmitter coil L 1 , which is part of the power transmitter  110 , and the receiver coil L 2 , which is part of the power receiver  120 . As a result, electrical power may be transferred from the power transmitter  110  to the power receiver  120 . 
     In some embodiments, the power transmitter  110  may be inside a charging pad. The transmitter coil L 1  is placed underneath the top surface of the charging pad. The power receiver  120  may be embedded in a mobile phone. When the mobile phone is placed near the charging pad, a magnetic coupling may be established between the transmitter coil L 1  and the receiver coil L 2 . In other words, the transmitter coil L 1  and the receiver coil L 2  may form a loosely coupled transformer through which a power transfer occurs between the power transmitter  110  and the power receiver  120 . The strength of coupling between the transmitter coil L 1  and the receiver coil L 2  is quantified by the coupling coefficient k. In some embodiments, k is in a range from about 0.05 to about 0.9. 
     In some embodiments, after the magnetic coupling has been established between the transmitter coil L 1  and the receiver coil L 2 , the power transmitter  110  and the power receiver  120  may form a power system through which power is wirelessly transferred from the input power source  102  to the load  114 . 
     The input power source  102  may be a power adapter converting a utility line voltage to a direct-current (dc) voltage. Alternatively, the input power source  102  may be a renewable power source such as a solar panel array. Furthermore, the input power source  102  may be any suitable energy storage devices such as rechargeable batteries, fuel cells, any combinations thereof and/or the like. 
     The load  114  represents the power consumed by the mobile device (e.g., a mobile phone) coupled to the power receiver  120 . Alternatively, the load  114  may refer to a rechargeable battery and/or batteries connected in series/parallel, and coupled to the output of the power receiver  120 . Furthermore, the load  114  may be a downstream power converter such as a battery charger. 
     The transmitter circuit  107  may comprise primary side switches of a full-bridge converter according to some embodiments. Alternatively, the transmitter circuit  107  may comprise the primary side switches of any other suitable power converters such as a half-bridge converter, a push-pull converter, any combinations thereof and/or the like. 
     It should be noted that the power converters described above are merely examples. One having ordinary skill in the art will recognize other suitable power converters such as class E topology based power converters (e.g., a class E amplifier), may alternatively be used depending on design needs and different applications. 
     The transmitter circuit  107  may further comprise a resonant capacitor (not shown). The resonant capacitor and the magnetic inductance of the transmitter coil may form a resonant tank. Depending on design needs and different applications, the resonant tank may further include a resonant inductor. In some embodiments, the resonant inductor may be implemented as an external inductor. In alternative embodiments, the resonant inductor may be implemented as a connection wire. 
     The power receiver  120  comprises the receiver coil L 2  magnetically coupled to the transmitter coil L 1  after the power receiver  120  is placed near the power transmitter  110 . As a result, power may be transferred to the receiver coil and further delivered to the load  114  through the rectifier  112 . The power receiver  120  may comprise a secondary resonant capacitor Cs as shown in  FIG. 1 . Throughout the description, the secondary resonant capacitor Cs may be alternatively referred to as a receiver resonant capacitor. 
     The rectifier  112  converts an alternating polarity waveform received from the output of the receiver coil L 2  to a single polarity waveform. In some embodiments, the rectifier  112  comprises a full-wave diode bridge and an output capacitor. In alternative embodiments, the full-wave diode bridge may be replaced by a full-wave bridge formed by switching elements such as n-type metal oxide semiconductor (NMOS) transistors. 
     Furthermore, the rectifier  112  may be formed by other types of controllable devices such as metal oxide semiconductor field effect transistor (MOSFET) devices, bipolar junction transistor (BJT) devices, super junction transistor (SJT) devices, insulated gate bipolar transistor (IGBT) devices, gallium nitride (GaN) based power devices and/or the like. The detailed operation and structure of the rectifier  112  are well known in the art, and hence are not discussed herein. 
     The high efficiency power converter  113  is coupled between the rectifier  112  and the load  114 . The high efficiency power converter  113  is a non-isolated power converter. The high efficiency power converter  113  comprises a first power stage and a second power stage connected in cascade. The first power stage is configured to operate in different modes for efficiently charging the load  114  (e.g., a rechargeable battery shown in  FIG. 3 ). The second power stage is configured as a voltage divider or an isolation switch. The block diagram of the high efficiency power converter  113  will be described below with respect to  FIG. 2 . The detailed configuration (e.g., different operating modes and their corresponding converter configurations) of the high efficiency power converter  113  will be described below with respect to  FIGS. 3-6 . 
     In some embodiments, the input voltage of the high efficiency power converter  113  is in a range from about 9 V to about 22 V. The output voltage of the high efficiency power converter  113  is in a range from about 5 V to about 10 V. One advantageous feature of having the high efficiency power converter  113  is that a higher output voltage (e.g., 22 V) can be achieved at the output of the rectifier  112 . Such a higher output voltage helps to lower down the current flowing through the receiver coil L 2 , thereby improving the efficiency of the power receiver  120 . For example, the efficiency of a receiver having the high efficiency power converter  113  can be improved by at least 7% compared to a conventional implementation of the receiver. 
       FIG. 2  illustrates a block diagram of the receiver shown in  FIG. 1  in accordance with various embodiments of the present disclosure. In some embodiments, the high efficiency power converter  113  comprises a first stage  202  and a second stage  204  connected in cascade. As shown in  FIG. 2 , the inputs of the first stage  202  are connected to the outputs of the rectifier  112 . The inputs of the second stage  204  are connected to the outputs of the first stage  202 . The outputs of the second stage  204  are connected to the load  114 . 
     In some embodiments, the first stage  202  is implemented as a step-down power converter (known as buck converter). The step-down converter is configured to operate in either a voltage mode or a current mode depending different operating conditions and design needs. The detailed structure of the step-down converter will be described below with respect to  FIG. 3 . In alternative embodiments, the first stage  202  is implemented as a four-switch buck-boost power converter. The four-switch buck-boost power converter is configured to operate in either a buck converter mode or a boost converter mode depending different operating conditions and design needs. The detailed structure of the four-switch buck-boost power converter will be described below with respect to  FIG. 6 . Furthermore, the first stage  202  may be implemented as a hybrid power converter. The hybrid power converter is configured as a buck converter or a hybrid converter depending different operating conditions and design needs. The detailed structure and the operating principle of the hybrid power converter will be described below with respect to  FIG. 5 . 
     In some embodiments, the second stage  204  is implemented as a charge pump power converter. The charge pump power converter is configured as a high efficiency voltage divider. The detailed structure of the charge pump power converter will be described below with respect to  FIG. 3 . In alternative embodiments, the second stage  204  is implemented as an isolation switch. The isolation switch is formed by two back-to-back connected power switches. The detailed structure of the isolation switch will be described below with respect to  FIG. 4 . 
       FIG. 3  illustrates a schematic diagram of a first implementation of the receiver shown in  FIG. 1  in accordance with various embodiments of the present disclosure. The power receiver  120  comprises the receiver coil L 2 , the receiver resonant capacitor Cs, the rectifier  112  and the high efficiency power converter  113 . As shown in  FIG. 3 , the receiver resonant capacitor Cs, the rectifier  112  and the high efficiency power converter  113  are connected in cascade between the receiver coil L 2  and the load  114 . In some embodiments, the load  114  is a rechargeable battery. Throughout the description, the load  114  may be alternatively referred to as a battery. 
     In some embodiments, the rectifier  112  is implemented as a full-wave rectifier. The rectifier  112  includes four switching elements, namely MR 1 , MR 2 , MR 3  and MR 4 . As shown in  FIG. 3 , the switching elements MR 1  and MR 3  are connected in series between the output terminal of the rectifier  112  and ground. Likewise, the switching elements MR 2  and MR 4  are connected in series between the output terminal of the rectifier  112  and ground. As shown in  FIG. 3 , the common node AC 1  of the switching elements MR 1  and MR 3  is coupled to a first input terminal of the receiver coil L 2  through the receiver resonant capacitor Cs. The common node AC 2  of the switching elements MR 2  and MR 4  is coupled to a second input terminal of the receiver coil L 2 . 
     According to some embodiments, the switching elements MR 1 , MR 2 , MR 3  and MR 4  are implemented as MOSFET or MOSFETs connected in parallel, any combinations thereof and/or the like. According to alternative embodiments, the switching elements (e.g., switch MR 1 ) may be an insulated gate bipolar transistor (IGBT) device. Alternatively, the primary switches can be any controllable switches such as integrated gate commutated thyristor (IGCT) devices, gate turn-off thyristor (GTO) devices, silicon controlled rectifier (SCR) devices, junction gate field-effect transistor (JFET) devices, MOS controlled thyristor (MCT) devices, gallium nitride (GaN) based power devices and/or the like. 
     It should be noted that while the example throughout the description is based upon a full-wave rectifier (e.g., full-wave rectifier  112  shown in  FIG. 3 ), the implementation of the power receiver  120  shown in  FIG. 3  may have many variations, alternatives, and modifications. For example, half-wave rectifiers may be alternatively employed. 
     In sum, the full-wave rectifier  112  illustrated herein is limited solely for the purpose of clearly illustrating the inventive aspects of the various embodiments. The present disclosure is not limited to any particular power topology. 
     It should further be noted that while  FIG. 3  illustrates four switches MR  1 -MR  4 , various embodiments of the present disclosure may include other variations, modifications and alternatives. For example, additional switching elements may be connected in parallel with each switch of the full-wave rectifier  112 . The additional switching elements help to improve the efficiency of the rectifier  112 . 
     It should further be noted the rectifier structure shown in  FIG. 3  is merely an example. One person skilled in the art will recognize many alternatives, variations and modification. For example, the four switches MR 1 , MR 2 , MR 3  and MR 4  may be replaced by four diodes. 
     The output of the rectifier  112  is connected to a capacitor C 1 . The capacitor C 1  functions as an output capacitor of the rectifier  112  and an input capacitor of the high efficiency power converter  113 . The capacitor C 1  is employed to attenuate noise and provide a steady output voltage at the output of the rectifier  112 . 
     The high efficiency power converter  113  comprises the first stage  202  and the second stage  204  connected in cascade as shown in  FIG. 3 . The first stage  202  is a step-down power converter (also known as a buck converter). The first stage  202  includes a first switch MB 1 , a second switch MB 2 , an inductor Lo and an output capacitor C 2 . As shown in  FIG. 3 , the first switch MB 1  and the second switch MB 2  are connected in series between the output VRECT of the rectifier  112  and ground. The inductor Lo is connected between the common node of the first switch MB 1  and the second switch MB 2 , and the output capacitor C 2 . Throughout the description, the first switch MB 1  is alternatively referred to as a high-side switch of the first stage  202 . The second switch MB 2  is alternatively referred to as a low-side switch of the first stage  202 . 
     In some embodiments, both the first switch MB 1  is implemented and the second switch MB 2  are implemented as an n-type transistors as shown in  FIG. 3 . The gate of the first switch MB 1  and the gate of the second switch MB 2  are configured to receive gate drive signals generated by a controller (not shown). 
     It should be noted that the first stage  202  shown in  FIG. 3  is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the first switch MB 1  may be implemented as a p-type transistor. 
     In operation, the first stage  202  functions as a charging apparatus for charging the battery  114 . More particularly, during the process of charging the battery  114 , the first stage  202  may be configured to operate in a current control mode in which the resolution of the current regulation is equal to or less than about 400 mA. Furthermore, the first stage  202  may be configured to operate in a voltage control mode in which the resolution of the voltage regulation is equal to or less than about 40 mV. 
     The second stage  204  comprises an input capacitor C 3 , a first switch M 1 , a capacitor C CP , a second switch M 2 , a third switch M 3 , a fourth switch M 4  and an output capacitor C 4 . The first switch M 1 , the capacitor C CP  and the third switch M 3  are connected in series between the output terminal VOUT of the first stage  202  and the battery  114 . A common node of the first switch M 1  and the capacitor C CP  is denoted as CP+ as shown in  FIG. 3 . Likewise, a common node of the third switch M 3  and the capacitor C CP  is denoted as CP−. A common node of the second switch M 2  and the output capacitor C 4  is denoted as VBAT. As shown in  FIG. 3 , the second switch M 2  is connected between CP+ and VBAT. The fourth switch M 4  is connected between CP− and ground. 
     In some embodiments, the second stage  204  functions as a charge pump power converter. The charge pump power converter operates in two different phases. During the first phase of the charge pump mode, switches M 1  and M 3  are turned on, and switches M 2  and M 4  are turned off. Since switches M 1  and M 3  are turned on, a first conductive path is established between VOUT and VBAT. The first conductive path is formed by switch M 1 , the charge pump capacitor C CP  and switch M 3 . The current flows from VOUT to VBAT through the first conductive path. During the first phase of the charge pump mode, the charge pump capacitor C CP  is charged and energy is stored in the charge pump capacitor C CP  accordingly. 
     During the second phase of the charge pump mode, switches M 1  and M 3  are turned off, and switches M 2  and M 4  are turned on. Since switches M 2  and M 4  are turned on, a second conductive path is established. The second conductive path is formed by switch M 4 , the charge pump capacitor C CP  and switch M 2 . During the second phase of the charge pump mode, the current discharges the charge pump capacitor C CP  and the energy stored in the charge pump capacitor C CP  decreases accordingly. 
     In some embodiments, the input voltage VRECT is in a range from about 9 V to about 22 V. The output voltage is about 3.8 V. The charge pump converter functions as a voltage divider. More particularly, by controlling the on/off time of the switches M 1 -M 4 , the output voltage VBAT of the charge pump power converter is equal to one half of the input voltage of the charge pump power converter. 
       FIG. 4  illustrates a schematic diagram of a second implementation of the receiver shown in  FIG. 1  in accordance with various embodiments of the present disclosure. The receiver  120  shown in  FIG. 4  is similar to that shown in  FIG. 3  except that the second stage  204  of the high efficiency power converter  113  is implemented as an isolation switch. As shown in  FIG. 4 , the isolation switch includes a first switch MS 1  and a second switch MS 2 . The first switch MS 1  and the second switch MS 2  are back-to-back connected, thereby providing isolation between the first stage  202  and the battery  114 . 
     One advantageous feature of having an isolation switch as the second stage is that the system configuration shown in  FIG. 4  is suitable for high voltage applications such as a rechargeable battery including two battery cells connected in series. 
     In some embodiments, the input voltage VRECT is in a range from about 9 V to about 22 V. The output voltage is in a range from about 7.6 V to about 7.7 V. The isolation switch provides a direct conduction path between the first power stage  202  and the battery  114 . The battery  114  may be formed by two battery cells connected in series. 
       FIG. 5  illustrates a schematic diagram of a hybrid converter in accordance with various embodiments of the present disclosure. In some embodiments, the first stage  202  shown in  FIG. 2  can be implemented as a hybrid converter as shown in  FIG. 5 . It should be noted that the hybrid converter shown in  FIG. 5  can be combined with any implementations of the second stage  204 . For example, when the first stage  202  is implemented as a hybrid converter, the second stage  204  can be any suitable implementations such as the charge pump power converter shown in  FIG. 3 , the isolation switch shown in  FIG. 4  and any combinations thereof. 
     As shown in  FIG. 5 , the hybrid converter comprises a first switch Q 1 , a capacitor C CP , a second switch Q 2 , a third switch Q 3 , a fourth switch Q 4 , an output inductor Lo and an output capacitor Co. As shown in  FIG. 5 , the output inductor Lo and the output capacitor Co form an output filter. The first switch Q 1 , the capacitor C CP  and the second switch Q 2  are connected in series between an input terminal VRECT and the output filter. A common node of the first switch Q 1  and the capacitor C CP  is denoted as CP+ as shown in  FIG. 5 . Likewise, a common node of the second switch Q 2  and the capacitor C CP  is denoted as CP−. A common node of the second switch Q 2  and the output filter is denoted as VX. As shown in  FIG. 5 , the third switch Q 3  is connected between CP+ and VX. The fourth switch Q 4  is connected between CP− and ground. 
     In some embodiments, the capacitor C CP  functions as a charge pump capacitor. Throughout the description, the capacitor C CP  is alternatively referred to as the charge pump capacitor C CP . 
     In accordance with an embodiment, the switches (e.g., switches Q 1 -Q 4 ) may be metal oxide semiconductor field-effect transistor (MOSFET) devices. Alternatively, the switching element can be any controllable switches such as insulated gate bipolar transistor (IGBT) devices, integrated gate commutated thyristor (IGCT) devices, gate turn-off thyristor (GTO) devices, silicon controlled rectifier (SCR) devices, junction gate field-effect transistor (JFET) devices, MOS controlled thyristor (MCT) devices and the like. 
     It should be noted while  FIG. 5  shows the switches Q 1 -Q 4  are implemented as single n-type transistors, a person skilled in the art would recognize there may be many variations, modifications and alternatives. For example, depending on different applications and design needs, the switches Q 1 -Q 4  may be implemented as p-type transistors. Furthermore, each switch shown in  FIG. 5  may be implemented as a plurality of switches connected in parallel. Moreover, a capacitor may be connected in parallel with one switch to achieve zero voltage switching (ZVS)/zero current switching (ZCS). 
     The hybrid converter may operate in three different operating modes, namely a hybrid mode, a charge pump mode and a buck mode. When the hybrid converter is employed as the first stage  202 . The hybrid converter may only operate in the charge pump mode or the buck mode. 
     In the hybrid mode, the hybrid converter operates in four different phases. In each phase, the current flowing through the output inductor Lo may ramp up or down depending on different combinations of the input voltage VRECT, the voltage across the charge pump capacitor C CP  and the output voltage VOUT. In the hybrid mode, the voltage of the hybrid converter can be regulated to a predetermined voltage. 
     In the buck mode, the hybrid converter operates in two different phases. The second switch Q 2  and the third switch Q 3  are always-on. As a result, the charge pump capacitor C CP  is shorted and not part of the operation of the buck mode. In each phase, the current flowing through the output inductor Lo may ramp up or down depending on different combinations of the input voltage VRECT and the output voltage VOUT. 
       FIG. 6  illustrates a schematic diagram of a four-switch buck-boost converter in accordance with various embodiments of the present disclosure. In some embodiments, the first stage  202  shown in  FIG. 2  can be implemented as a four-switch buck-boost converter as shown in  FIG. 6 . It should be noted that the four-switch buck-boost converter shown in  FIG. 6  can be combined with any implementations of the second stage  204 . For example, when the first stage  202  is implemented as a four-switch buck-boost converter, the second stage  204  can be any suitable implementations such as the charge pump power converter shown in  FIG. 3 , the isolation switch shown in  FIG. 4  and any combinations thereof. 
     As shown in  FIG. 6 , the buck-boost converter comprises a first high-side switch Q 1 , a first low-side switch Q 2 , a second low-side switch Q 3 , a second high-side switch Q 4  and an inductor Lo. The first high-side switch Q 1  and the first low-side switch Q 2  are connected in series between VRECT and ground. The second high-side switch Q 4  and the second low-side switch Q 3  are connected in series between VOUT and ground. The inductor Lo is coupled between the common node of the first high-side switch Q 1  and the first low-side switch Q 2 , and the common node of the second high-side switch Q 4  and the second low-side switch Q 3  as shown in  FIG. 6 . 
     The buck-boost converter may be divided into two portions, namely a buck converter portion and a boost converter portion. The buck converter portion may comprise the first high-side switch Q 1  and the first low-side switch Q 2 . The buck converter portion and the inductor Lo may function as a step-down converter when the second high-side switch Q 4  is always on and the second low-side switch Q 3  is always off. Under such a configuration, the buck-boost converter operates in a buck mode. 
     The boost converter portion of the buck-boost converter may comprise the second high-side switch Q 4  and second low-side switch Q 3 . The boost converter portion and the inductor Lo may function as a step-up converter when the first high-side switch Q 1  is always on and the first low-side switch Q 2  is always off. Under such a configuration, the buck-boost converter operates in a boost mode. Furthermore, the buck-boost converter operates in a pass-through mode when the high-side switches Q 1  and Q 4  are always on, and the low-side switches Q 2  and Q 3  are always off. 
     The switches (e.g., the first high-side switch Q 1 ) shown in  FIG. 6  may be implemented as n-type metal oxide semiconductor (NMOS) transistors. Alternatively, the switches may be implemented as other suitable controllable devices such as metal oxide semiconductor field effect transistor (MOSFET) devices, bipolar junction transistor (BJT) devices, super junction transistor (SJT) devices, insulated gate bipolar transistor (IGBT) devices, gallium nitride (GaN) based power devices and/or the like. 
     It should further be noted that while  FIG. 6  illustrates four switches Q 1 , Q 2 , Q 3 , and Q 4 , various embodiments of the present disclosure may include other variations, modifications and alternatives. For example, the first low-side switch Q 2  may be replaced by a freewheeling diode and/or the like. The second high-side switch Q 4  may be replaced by a rectifier diode and/or the like. 
     Based upon different application needs, the buck-boost converter may be configured to operate in three different operating modes, namely the buck mode, the boost mode and the pass-through mode. 
       FIG. 7  illustrates a block diagram of a third implementation of the receiver shown in  FIG. 1  in accordance with various embodiments of the present disclosure. The receiver  120  shown in  FIG. 7  is similar to that shown in  FIG. 2  except that the receiver resonant capacitor Cs is connected in parallel with the receiver coil L 2 . The structures and the operating principles of the rectifier  112  and the high efficiency power converter  113  have been described above in detail with respect to  FIGS. 3-6 , and hence are not discussed herein again to avoid unnecessary repetition. 
       FIG. 8  illustrates a block diagram of a fourth implementation of the receiver shown in  FIG. 1  in accordance with various embodiments of the present disclosure. The receiver  120  shown in  FIG. 8  is similar to that shown in  FIG. 2  except that the receiver coil L 2  has been replaced by two coils L 21  and L 22 . Furthermore, the switches of the rectifier  112  have been replaced by two diodes D 1  and D 2  as shown in  FIG. 8 . 
     It should be noted the rectifier structure shown in  FIG. 8  is merely an example. One person skilled in the art will recognize many alternatives, variations and modification. For example, the two diodes D 1  and D 2  may be replaced by two switching elements. 
       FIG. 9  illustrates a flow chart of applying a battery charging control mechanism to the high efficiency converter shown in  FIG. 3  in accordance with various embodiments of the present disclosure. This flowchart shown in  FIG. 9  is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, various steps illustrated in  FIG. 9  may be added, removed, replaced, rearranged and repeated. 
     At step  902 , an output voltage of a wireless power system is detected by a suitable sensing apparatus or a plurality of sensing devices. The detected voltage is processed by a controller. In particular, the detected voltage is compared with a predetermined voltage threshold. 
     The receiver of the wireless power system comprises a high efficiency power converter. The high efficiency power converter comprises a first stage  202  and a second stage  204  connected in cascade. The first stage  202  is employed to charge a battery and the second stage  204  is employed to provide isolation between the first stage  202  and the battery. 
     At step  904 , the first stage  202  of the high efficiency power converter is configured to operate in a current control mode to charge the battery when the output voltage of the wireless power system is less than the predetermined voltage threshold. 
     At step  906 , the first stage  202  of the high efficiency power converter is configured to operate in a voltage control mode to charge the battery when the output voltage of the wireless power system is greater than the predetermined voltage threshold. 
       FIG. 10  illustrates a flow chart of applying a low input voltage control mechanism to the high efficiency converter shown in  FIG. 2  in accordance with various embodiments of the present disclosure. This flowchart shown in  FIG. 10  is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, various steps illustrated in  FIG. 10  may be added, removed, replaced, rearranged and repeated. 
     A receiver of a wireless power system comprises a high efficiency power converter. The high efficiency power converter comprises a first stage  202  and a second stage  204  connected in cascade. The first stage  202  is employed to charge a battery and the second stage  204  is employed to provide isolation between the first stage  202  and the battery. 
     In some embodiments, the first stage  202  is implemented as a four-switch buck-boost converter as shown in  FIG. 7 . The second stage  204  is implemented as a charge pump converter as shown in  FIG. 3 . 
     At step  1002 , an input voltage of the receiver of the wireless power system is detected by a suitable sensing apparatus or a plurality of sensing devices. The detected voltage is processed by a controller. In particular, the detected voltage is compared with predetermined voltage thresholds. 
     At step  1004 , the first stage  202  is configured to operate in a buck converter mode when the input voltage of the wireless power system is greater than a first predetermined voltage threshold. 
     At step  1006 , when the input voltage is greater than a second predetermined voltage threshold and less than the first predetermined voltage threshold, the charge pump power converter does not function as a voltage divider. Instead, the charge pump power converter provides a direct conduction path between the first stage  202  and the battery. 
     At step  1008 , the first stage  202  is configured to operate in a boost converter mode when the input voltage of the wireless power system is less than the second predetermined voltage threshold. 
     Although embodiments of the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.