Three-quarter bridge power converter for wireless power transfer applications and other applications

Abstract

A three-quarter bridge power converter (100, 300, 400, 500, 600, 800, 900) includes a first switch (102, 302, 402, 502, 602, 802, 902) configured to selectively couple a switch node (106, 306, 406, 506, 606, 806, 906) to a higher voltage. The power converter also includes a second switch (104, 304, 404, 504, 604, 804, 904) configured to selectively couple the switch node to a lower voltage. The power converter further includes a third switch (118, 318, 418, 518, 618, 818, 918) configured to selectively cause a third voltage to be provided to the switch node when the first and second switches do not couple the switch node to the higher and lower voltages. The third switch can be configured to selectively couple the switch node to an energy storage device or energy source, such as a capacitor (120, 320, 420, 820, 920). The third switch can also be configured to selectively couple an energy storage device or energy source (512, 612) to ground, where the energy storage device or energy source is coupled to the switch node.

Description

Technical Field

[0001] The present invention relates generally to power converters. More particularly, the present invention relates to a three-quarter bridge power converter for use in wireless power transfer applications and other applications. Background Art

[0002] Various power conversion architectures have been developed and used in a wide range of applications. Two common power conversion architectures are the half-bridge architecture and the full-bridge architecture.

[0003] A drawback of using a symmetrical drive half-bridge architecture is that at any duty cycle less than the maximum drive duty cycle, there is a "dead time" during which the switches in the half-bridge are inactive (non-conducting). This can cause current to flow through the body diodes of the switches, resulting in significant losses (typically approximately equal to or greater than I 2 R losses). Placing a Schottky diode in parallel with the switch can help, but can still result in significant Schottky losses during some operating conditions. While asymmetrical drive can address the dead time issue, the downside is that it typically results in poorer waveform quality, which may require much lower impedance in the matching network. This increases the half-bridge circulating RMS current, which in turn increases losses.

[0004] A full-bridge architecture with phase modulation solves the dead time problem because current always flows through both actuated switches. However, a full-bridge architecture requires driving the load differentially at high common-mode voltages, which can be insufficient in some applications. Summary of the Invention BRIEF DESCRIPTION OF THE DRAWINGS

[0005] For a more complete understanding of the present invention and its features, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

[0006] Figures 1 to 6 Describes an example three-quarter bridge power converter and related details according to the present invention;

[0007] Figure 7 An example control circuit for a three-quarter bridge power converter according to the present invention is described;

[0008] Figure 8 An example three-quarter bridge power converter with multiple transmit coils according to the present invention is described;

[0009] Figure 9 An example three-quarter bridge power converter with current and voltage sensing instrumentation according to the present invention is illustrated; and

[0010] Figure 10 An example power conversion method using a three-quarter bridge power converter according to the present invention is described. DETAILED DESCRIPTION

[0011] Discussed below Figures 1 to 10 The various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the present invention. Those skilled in the art will understand that the principles of the present invention can be implemented in any type of appropriately arranged device or system.

[0012] Figures 1 to 6 Example three-quarter bridge power converters and related details according to the present invention are described. These new three-quarter bridge power converter topologies can be used in a wide range of applications. The three-quarter bridge power converters can significantly reduce or substantially eliminate the rectifier diode conduction losses associated with symmetrical pulse width modulated (PWM) half-bridge power converters while maintaining many of the advantages of the basic half-bridge architecture.

[0013] An example of an area where this approach can be applied is in a magnetically coupled wireless power transfer environment. In these types of environments, the power "transmitter" (the primary side of the transformer) is physically separated from the power "receiver" (the secondary side of the transformer). A power transmitter represents any suitable structure for transmitting power, and a power receiver represents any suitable structure for receiving power. In practice, the primary side of the transformer resides in one physical device, and the secondary side of the transformer resides in a completely separate device. In addition, the receiver (secondary) coil can be of various shapes and sizes, and the transmitter and receiver can be manufactured by different companies. Overall, this environment presents several unique challenges compared to other isolated power transfer solutions:

[0014] The exact coupling coefficient k is unknown and lower than typical in isolated power converters, so the transformation ratio may not be easy to predict.

[0015] - To minimize electromagnetic interference (EMI) and enable the use of reactive impedance matching, the waveform should be as sinusoidal as possible;

[0016] - the primary and secondary sides are not on the same core, and the transmitter coil may be much larger than the receiver coil, so there may be magnetic flux lines that are not contained in the core; and

[0017] - To simplify instrumentation and facilitate switching between matrices of multiple transmit coils, it may be desirable to couple one side of the transmit (primary) coil to ground. The three-quarter bridge power converter shown and described below may be used in these or other environments to reduce power losses during operation.

[0018] Figure 1 A first example three-quarter bridge power converter 100 is described. Figure 1 As shown, power converter 100 includes switches 102-104, which may represent typical switches used in a half-bridge power converter architecture. Switch 102 is coupled to receive a supply voltage V+, and switch 104 is coupled to ground. V+ and ground represent the mains voltage. Switch 102 can selectively couple the supply voltage V+ to a switch node 106 under the control of a control signal G1. Switch 104 can selectively couple the switch node 106 to ground under the control of a control signal G2. Each of switches 102-104 includes any suitable switch structure, such as a MOSFET or other transistor device.

[0019] Switch node 106 is coupled to inductor 108, which in this example is coupled to output capacitor 110 and direct current (DC) blocking capacitor 112. Inductor 108 represents any suitable inductive structure having any suitable inductance. Each of capacitors 110-112 represents any suitable capacitive structure having any suitable capacitance.

[0020] Capacitor 112 is also coupled to a coil, which in this example represents the primary side of transformer 114. The secondary side of transformer 114 is coupled to load 116. Transformer 114 comprises any suitable structure for transmitting power in an isolated manner. Each side of transformer 114 can have any suitable structure, such as a coil with any number of turns. As described above, the primary side of transformer 114 can include a matrix of different transmit coils that can be switched into and out of power converter 100.

[0021] A third switch 118 is added to power converter 100 to form a three-quarter bridge architecture. Third switch 118 selectively couples switch node 106 to an energy storage device or source. In this case, the energy storage device or source is a power storage component formed by capacitor 120, but any other suitable energy source or storage component may be used. Capacitor 120 comprises any suitable capacitive structure having any suitable capacitance. In some embodiments, the ripple may be a strict function of the current during the on-time of switch 118. In practice, it may be beneficial to limit the ripple in capacitor 120 to a few hundred millivolts in order to reduce or minimize dielectric losses in capacitor 120. This can be achieved by increasing the size of capacitor 120.

[0022] Switch 118 can selectively couple switch node 106 to an energy storage device or energy source under the control of control signal G3. Switch 118 includes any suitable structure for coupling an energy storage device or energy source to a specified node. For example, switch 118 can represent a structure that provides bidirectional blocking capability. In some embodiments, switch 118 can be implemented using MOSFET transistors (or other types of transistors) coupled in series. For example, switch 118 can be formed using two MOSFETs whose source terminals are coupled together and whose gate terminals are configured to receive control signal G3. The drain terminal of the MOSFET can be coupled to the energy storage device or energy source and to switch node 106. In a particular embodiment, power converter 100 can use a total of four MOSFETs or other switches.

[0023] During operation, control signals G1 and G2 (used to control switches 102-104) can be the same as those used in a half-bridge architecture. Whenever both control signals G1 and G2 are deasserted (off), control signal G3 (used to control switch 118) can be asserted (on). Thus, when switches 102-104 are both off (non-conducting), switch 118 is on (conducting). The voltage on switch node 106 can thus be similar to that of a half-bridge, except that during the on-time of switch 118, the voltage is clamped to the voltage V stored on capacitor 120. 120 (or voltage from another energy storage device or energy source). Depending on the embodiment, the voltage V 120 It can be averaged to half of the power supply voltage V+.

[0024] In this way, the three-quarter bridge power converter 100 can reduce or eliminate "dead time" during its operation, so that the switch node 106 is always connected to the power rail, power storage component, or some other energy storage device or energy source. This can substantially eliminate rectifier diode conduction losses.

[0025] Load 116 represents any suitable structure for receiving power from power converter 100. Load 116 may, for example, include a wireless power receiver for wirelessly receiving power from power converter 100. The wireless power receiver may form part of a larger device, such as a mobile phone, portable computer, or other electronic device. Load 116 may also represent a motor, such as a reversible DC or AC motor. Any other suitable load 116 may be used, such as an RL load.

[0026] Figure 2 Description and Figure 1 Example simulation waveforms associated with the three-quarter bridge power converter 100. Figure 2As shown, control signals G1 and G2 contain pulses for turning on switches 102-104. If control signals G1 and G2 define a 50% duty cycle, there will be no dead time during which none of switches 102-104 is conducting. However, if Figure 2 As shown, at lower duty cycles there can be significant dead time between pulses in control signals G1 and G2. As described above, the pulses of control signal G3 are high when both control signals G1 and G2 are low, thereby activating switch 118 to couple node 106 to capacitor 120. This helps reduce or eliminate dead time in power converter 100.

[0027] Figure 2 The voltage on the switch node 106 (V 106 ), the current through the inductor 108 (I L ) and the voltage on capacitor 120 (V 120 ).in addition, Figure 2 The current (I 102 , I 104 and I 118 ).like Figure 2 As shown, when switches 102-104 alternately couple switch node 106 to power supply voltage V+ and ground, the voltage V 106 In between those times (which would otherwise be dead time), the switch node 106 is coupled to the capacitor 120 and receives a voltage that is approximately half the supply voltage V+ in this example.

[0028] The simulation waveforms here show the specific control signal and the inductor current I L Phase relationship. This is a function of the resonant frequency and operating frequency chosen for the simulation. Different choices may result in different phase relationships. The operation of the three-quarter bridge power converter 100 does not specifically require that all switching currents (I 102 , I 104 and I 118 ) in the resonant circuit under the condition of conduction current I L The full 360°, so theoretically the current I L In practice, some non-overlap time between switching operations is usually used to ensure that there is no current shoot-thru.

[0029] Figure 3 A second example three-quarter bridge power converter 300 is described. Figure 3As shown, power converter 300 includes switches 302-304, inductor 308, and output capacitor 310 coupled to switch node 306. Capacitor 310 is coupled to a coil, which in this example represents the primary side of transformer 314. Load 316 is coupled to the secondary side of transformer 314 and may represent a wireless power receiver. Switch 318 couples an energy storage device or energy source, such as capacitor 320, to switch node 306. Figure 3 , power converter 300 operates using a series resonance formed by inductor 308 , output capacitor 310 , and the primary side of transformer 314 .

[0030] Figure 4 A third example three-quarter bridge power converter 400 is described. Figure 4 As shown, power converter 400 includes switches 402-404 coupled to a switch node 406. An output capacitor 410 is coupled to switch node 406 and a coil, which in this example represents the primary side of a transformer 414. A load 416 is coupled to the secondary side of transformer 414 and may represent a wireless power receiver. Switch 418 couples an energy storage device or energy source, such as capacitor 420, to switch node 406. Figure 4 , power converter 400 operates using a series resonance formed by output capacitor 410 and the leakage inductance of the primary side of transformer 414 .

[0031] Figure 5 A fourth example three-quarter bridge power converter 500 is described. Figure 5 As shown, power converter 500 includes switches 502-504 coupled to a switching node 506. A DC blocking capacitor 512 is coupled to switching node 506 and to a coil, which here represents the primary side of a transformer 514. A load 516 is coupled to the secondary side of transformer 514. A switch 518 is coupled to a node 522 between DC blocking capacitor 512 and transformer 514. Switch 518 selectively couples node 522 to ground. Figure 5 , power converter 500 operates in a non-resonant manner. However, switch 518 may still be turned on during the non-conduction time of switches 502-504 to avoid issues associated with dead time. In this embodiment, capacitor 512 effectively acts as an energy storage device or energy source coupled to switch node 506, and switch 518 causes the voltage across capacitor 512 to be seen at switch node 506.

[0032] Figure 6 A fifth example three-quarter bridge power converter 600 is described. Figure 6As shown, power converter 600 includes switches 602-604 coupled to a switching node 606. A DC blocking capacitor 612 is coupled to switching node 606 and to a load 616, which in this example is represented by an inductor 614 and a resistor 615. A switch 618 is coupled to a node 622 between DC blocking capacitor 612 and load 616. Switch 618 selectively couples node 622 to ground. Figure 6 In FIG. 6 , power converter 600 operates in a non-resonant and non-isolated manner with an inductive load rather than a wireless power transfer system. However, switch 618 may still be turned on during the non-conduction time of switches 602-604 to avoid issues associated with dead time. In this embodiment, capacitor 612 effectively acts as an energy storage device or energy source coupled to switch node 606, and switch 618 causes the voltage across capacitor 612 to be seen at switch node 606.

[0033] Compared to a conventional half-bridge architecture, a three-quarter bridge power converter can have improved efficiency without compromising waveform symmetry. Compared to a conventional full-bridge architecture, a three-quarter bridge power converter can have a transformer with its primary side connected to ground, which can facilitate easy switching between matrices of transmit coils and easy measurement of transformer voltage and current.

[0034] although Figures 1 to 6 The example and details of a three-quarter bridge power converter are given, but the Figures 1 to 6 Various changes may be made. For example, each component of the three-quarter bridge power converter described above may be implemented in any suitable manner. Figure 2 The waveforms shown are for illustrative purposes only, and the three-quarter bridge power converter may operate using different waveforms depending on the implementation. In addition, the use of capacitors as a mechanism for sourcing / sucking energy during operation of the power converter is for illustrative purposes only. Other energy storage elements or energy sources may also be used. For example, in other embodiments, the capacitor may be replaced by a bidirectional converter that outputs a voltage of approximately V+ / 2. The bidirectional converter may source energy to the third switch 118, 318, 418 by operating in synchronous buck mode, and drain energy from the switch 118, 318, 418 (and return it to V+) by operating in synchronous boost mode. In addition, Figures 1 to 6 Various components in the can be combined, omitted, or further subdivided, and additional components can be added according to specific needs. In addition, any of the power converters in this patent document can use multiple switches during normal half-bridge operation to selectively couple the switch node to different voltage rails, such as a higher voltage V+ and a lower voltage (not necessarily ground).

[0035] Figure 7An example control circuit 700 for a three-quarter bridge power converter according to the present invention is illustrated. The control circuit 700 can be used, for example, to generate control signals G1-G3 for any of the three-quarter bridge power converters described above or below. In this example, the control circuit 700 uses a mixed analog and digital approach to generate the control signals.

[0036] like Figure 7 As shown, control circuit 700 includes a frequency word unit 702 and a work word unit 704. These units 702-704 output values ​​representing the frequency and duty cycle of the control signal used to drive the power converter. The values ​​can represent 24-bit values. Phase accumulator 706 operates using the output of frequency word unit 702. The output of phase accumulator 706 is provided to adder 708, which adds the output of phase accumulator 706 to the output of work word unit 704.

[0037] A most significant bit (MSB) extraction unit 710 is used to identify and extract the high-order bits of the output of the phase accumulator. The high-order bits of the output of the phase accumulator are used as a reference phase. An MSB extraction unit 712 is used to identify and extract the high-order bits of the output of the adder. The high-order bits of the output of the adder are used as a variable phase.

[0038] The difference between the reference phase and the variable phase is used to generate control signals G1 and G2 (which are PWM signals in this case). Specifically, the output of the MSB extraction unit 710 is provided to an inverter 714 and an AND gate 720, and the output of the inverter 714 is provided to an AND gate 716. The output of the MSB extraction unit 712 is provided to an inverter 718 and an AND gate 716, and the output of the inverter 718 is provided to an AND gate 720. AND gates 716 and 720 output control signals G1 and G2, respectively. Control signal G3 is generated by performing a logical NOR operation on G1 and G2 using a NOR gate 722. Therefore, signal G3 is asserted whenever either G1 or G2 is not asserted. The duty cycle resolution in this example may be approximately 1.2x10 -7 , which is probably much better than what is needed.

[0039] The control signals G1, G2, and G3 in this example can have low phase jitter, for example, one clock period. For a 100 MHz clock, this results in a phase jitter of only 10 ns. This method (which is mathematically related to direct digital synthesis (DDS)) can inherently implement a dithering scheme that forces the duty cycle average value to be exactly equal to:

[0040] Work = (2 x Work Word) / 2 24

[0041] (Assuming 24-bit values ​​are used.) For a resonant converter, the jitter in the load can be reduced due to the high frequency roll-off of the resonant network.

[0042] although Figure 7 An example of a control circuit 700 for a three-quarter bridge power converter is described, but may be used for other Figure 7 Various changes may be made. For example, any other suitable combinational logic or other mechanism may be used to generate appropriate control signals. Figure 7 Various components in the may be combined, omitted, or further subdivided, and additional components may be added according to specific needs.

[0043] The power converter topologies described above can be used in a variety of applications. For example, a three-quarter bridge power converter can be used in any application where a half-bridge converter might be used. A three-quarter bridge converter has greater efficiency for any duty cycle (less than 50% duty cycle) that would produce dead time for a half-bridge converter.

[0044] Another application of a three-quarter bridge converter is as an alternative to a full-bridge converter in situations where it is necessary to connect one side of the load or one side of the transformer to ground. This can include, for example, applications where multiple transmit coils are used and one or more coils can be selectively coupled to the bridge. An example of this is in Figure 8 In the display, Figure 8 An example three-quarter bridge power converter 800 with multiple transmit coils in accordance with the present invention is illustrated.

[0045] like Figure 8 As shown, power converter 800 includes two switches 802-804 that selectively couple node 806 to a supply voltage V+ and ground, respectively. Node 806 is also coupled to switch 818, which selectively couples node 806 to an energy storage device or source (e.g., capacitor 820). In this example, node 806 is coupled to a plurality of strings, each of which includes coils 814a-814n (e.g., inductors) and transistors 815a-815n (e.g., MOSFETs) coupled in series. Coils 814a-814n represent a plurality of coils for transmitting power to a load 816. Coils 814a-814n may, for example, form part of a multi-coil power transmission pad, which allows for greater freedom in how a receiving coil 822 of load 816 is positioned relative to the pad.

[0046] The combination of series capacitors 810a-810n and the body diodes of transistors 815a-815n allows for a DC level shift in each string, which effectively separates the string from Figure 8 The rest of the circuit in is decoupled. A small bias current (e.g., FET leakage) flows to maintain the disconnection. This allows a single MOSFET to be used as transistor 815a-815n for selecting between coils 814a-814n.

[0047] although Figure 8 An example of a three-quarter bridge power converter 800 with multiple transmit coils is described, but may be used for Figure 8 Various changes may be made. For example, power converter 800 may include any number of transmit coils.

[0048] Another three-quarter bridge power converter may involve the use of current and voltage sensing instruments, an example of which is Figure 9 In the Figure 9 , a three-quarter bridge power converter 900 includes switches 902-904 coupled to a node 906, which is also coupled to a capacitor 910. Capacitor 910 is coupled to a coil 914, which may be part of an inductor or a transformer (e.g., a wireless power transmission coil). Switch 918 couples an energy storage device or energy source (e.g., capacitor 920) to node 906.

[0049] In this example, a voltage sensing unit 924 is coupled across coil 914, and a current sensing unit 926 is coupled in series with coil 914. Voltage sensing unit 924 includes any suitable structure for measuring voltage, and current sensing unit 926 includes any suitable structure for measuring current. In this example, the three-quarter bridge architecture facilitates the use of sensing units 924-926 in a single-ended manner, meaning that these units do not need to use differential signaling. This can help reduce or eliminate the need for high common-mode rejection in these units.

[0050] although Figure 9 One example of a three-quarter bridge power converter 900 with current and voltage sensing instruments is described, but may be used for Figure 9 Various changes may be made. For example, sensing units 924-926 may be used with any of the three-quarter bridge embodiments described above. Also, power converter 900 may include one of sensing units 924-926 while omitting the others.

[0051] Each component shown in the above circuits can be implemented using any suitable structure. In addition, these figures illustrate example implementations of the circuits. In these circuits, components can be added, omitted, combined, further subdivided, or moved according to specific needs. In addition, the waveforms shown above are for illustrative purposes only and represent possible or simulated behavior of a specific implementation of the circuit.

[0052] Figure 10 An example power conversion method 1000 using a three-quarter bridge power converter according to the present invention is described. Figure 10As shown, at step 1002, at least one drive signal for a three-quarter bridge power converter is received. This may include, for example, an external component providing one or more signals that identify a desired frequency and duty cycle for driving the three-quarter bridge power converter. The external component may represent any suitable source for controlling the power converter, such as an external processing device or controller.

[0053] At step 1004, control signals for switches in a three-quarter bridge power converter are generated. This may include, for example, a control circuit generating control signals G1-G3 for the power converter. As a specific example, this may include the control circuit generating G1 and G2 control signals to drive the power converter at a desired duty cycle. This may also include the control circuit generating a G3 control signal such that the G3 control signal is active (high) whenever the G1 and G2 control signals are inactive (low).

[0054] At step 1006, the first and second switches in the three-quarter bridge power converter are turned on and off. This is done to couple the switch node in the power converter to higher and lower rail voltages (e.g., V+ and ground). The ratio of the amount of time the switch node is coupled to the higher rail voltage to the lower rail voltage defines the duty cycle, and there may be some dead time when both the first and second switches are off. The first and second switches may be controlled by the G1 and G2 control signals.

[0055] At step 1008, the switch node is coupled to a voltage using a third switch during the off-time of the first and second switches. This may include, for example, closing the third switch so that the voltage across the capacitor is received at the switch node. The third switch may be opened whenever the switch node is coupled to the rails via either the first or second switch. This substantially reduces or eliminates dead time in the three-quarter bridge power converter. The third switch may be controlled by the G3 control signal.

[0056] although Figure 10 An example power conversion method 1000 using a three-quarter bridge power converter is described, but may be used for Figure 10 For example, although shown as a series of steps, Figure 10 The various steps in may overlap, occur in parallel, occur multiple times, or occur in a different order.

[0057] It may be helpful to set forth definitions of certain words and phrases used within this patent document. The term "coupled" and its derivatives refer to any direct or indirect communication between components, regardless of whether those components are in physical contact with one another. The terms "include" and "comprises," and their derivatives, mean inclusion without limitation. The term "or" is inclusive, meaning and / or. The phrases "associated with" and "associated with," and their derivatives, may mean including, contained within, interconnected with, containing, contained within, connected to or connected with, coupled to or coupled with, communicable with, cooperating with, interleaved, juxtaposed, proximate to, coupled to or coupled with, having the characteristics of, having a relationship with, and the like.

[0058] Although the present invention has described certain embodiments and generally associated methods, those skilled in the art will appreciate variations and permutations of these embodiments and methods. Therefore, the above description of the example embodiments does not define or constrain the present invention. Other changes, substitutions, and modifications are possible without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims

1. A three-quarter bridge power converter circuit for providing a regulated voltage to a load, comprising: a first switch configured to selectively couple a switch node to a higher voltage; a second switch configured to selectively couple the switch node to a lower voltage; a clamping voltage element for storing a clamping voltage between the higher voltage and the lower voltage; a bidirectional clamp switch for selectively coupling the switch node to the voltage element when the first switch is not coupling the switch node to the higher voltage and the second switch is not coupling the switch node to the lower voltage, thereby clamping the switch node to the clamp voltage; as well as A control circuit for generating control signals G1, G2, and G3 to cooperatively switch the first switch, the second switch, and the bidirectional clamp switch, respectively, including generating G1 and G2 based on values ​​associated with a driving frequency and a driving duty cycle, the values ​​being converted to a reference phase value related to the driving frequency and a variable phase value related to both the driving frequency and the driving duty cycle, respectively, such that a difference between the reference phase value and the variable phase value is used to generate the control signals G1 and G2, and asserting the control signal G3 to clamp the switch node to the clamping voltage when neither G1 nor G2 is asserted, such that the control signals G1, G2, and G3 provide a desired duty cycle resolution. 2 . The three-quarter bridge power converter circuit of claim 1 , wherein the voltage element comprises a capacitor. 3 . The three-quarter bridge power converter circuit of claim 1 , further comprising a reactive element coupled between the switch node and an output node coupled to the load.

4. The three-quarter bridge power converter circuit according to claim 3, wherein: The reactive element comprises at least one coil or transformer winding.

5. The three-quarter bridge power converter circuit of claim 4 , the power converter further comprising at least one of the following: a single-ended voltage sensor configured to measure the voltage across the coil or transformer winding; and A single-ended current sensor is configured to measure the current through the coil or transformer winding.

6. The three-quarter bridge power converter circuit according to claim 3, wherein: The reactive element coupled between the switch node and the output node is a capacitor, and wherein the bidirectional clamp switch is coupled between the output node and ground for selectively coupling the capacitor coupled to the output node to ground when the first switch does not couple the switch node to the higher voltage and the second switch does not couple the switch node to the lower voltage.

7. The three-quarter bridge power converter circuit of claim 6 , wherein the control circuit is based on a direct digital synthesis configuration, the control circuit comprising: a phase accumulator configured to receive a value associated with the drive frequency; an adder configured to add an output of the phase accumulator to a value associated with the drive duty cycle; at least one extraction unit configured to identify a most significant bit in the output of the phase accumulator and a most significant bit in the output of the adder corresponding to the reference phase value and the variable phase value, respectively; as well as Combinational logic is configured to generate the control signals G1, G2, and G3 using the most significant bits.

8. The three-quarter bridge power converter circuit of claim 1 , wherein: The higher voltage includes a power supply voltage; The lower voltage includes ground; and The clamping voltage is half of the power supply voltage.

9. A system for power conversion, comprising: load; as well as a three-quarter bridge power converter configured to provide power to the load, the power converter comprising: a first switch configured to selectively couple a switch node to a higher voltage; a second switch configured to selectively couple the switch node to a lower voltage; a clamping voltage element for storing a clamping voltage between the higher voltage and the lower voltage; a bidirectional clamp switch for selectively coupling the switch node to the voltage element when the first switch is not coupling the switch node to the higher voltage and the second switch is not coupling the switch node to the lower voltage, thereby clamping the switch node to the clamp voltage; and A control circuit for generating control signals G1, G2, and G3 to cooperatively switch the first switch, the second switch, and the bidirectional clamp switch, respectively. The control circuit includes generating G1 and G2 based on values ​​associated with a driving frequency and a driving duty cycle, the values ​​being converted to a reference phase value related to the driving frequency and a variable phase value related to both the driving frequency and the driving duty cycle, respectively. The difference between the reference phase value and the variable phase value is used to generate the control signals G1 and G2. The control signal G3 is asserted to clamp the switch node to the clamping voltage when neither G1 nor G2 is asserted, so that the control signals G1, G2, and G3 provide a desired duty cycle resolution.

10. The system of claim 9, wherein the voltage element comprises a capacitor.

11. The system of claim 9, further comprising a reactive element coupled between the switch node and an output node coupled to the load.

12. The system according to claim 11, wherein The reactive element comprises at least one coil or transformer winding.

13. The system of claim 12, wherein the power converter further comprises at least one of: a single-ended voltage sensor configured to measure the voltage across the coil or transformer winding; and A single-ended current sensor is configured to measure the current through the coil or transformer winding.

14. The system according to claim 11, wherein: The reactive element coupled between the switch node and an output node is a capacitor, and wherein the bidirectional clamp switch is coupled between the output node and ground for selectively coupling the capacitor coupled to the output node to ground when the first switch does not couple the switch node to the higher voltage and the second switch does not couple the switch node to the lower voltage.

15. The system of claim 14, wherein the control circuit is configured based on direct digital synthesis, the control circuit comprising: a phase accumulator configured to receive a value associated with the drive frequency; an adder configured to add an output of the phase accumulator to a value associated with the drive duty cycle; at least one extraction unit configured to identify a most significant bit in the output of the phase accumulator and a most significant bit in the output of the adder corresponding to the reference phase value and the variable phase value, respectively; as well as Combinational logic is configured to generate the control signals G1, G2, and G3 using the most significant bits.

16. The system of claim 9, wherein: The three-quarter bridge power converter comprises a portion of a wireless power transmitter including a matrix comprising a plurality of transmit coils or transformer windings; and The load includes a wireless power receiver.

17. The system according to claim 16, wherein: The higher voltage includes a power supply voltage, The lower voltage includes ground, The clamping voltage is half of the power supply voltage.

18. A method for power conversion, comprising: repeatedly coupling the switch node to a higher voltage and a lower voltage using first and second switches, respectively; as well as selectively coupling the switch node to a clamp voltage between the upper voltage and the lower voltage using a bidirectional clamp switch when the first and second switches are not coupling the switch node to the upper and lower voltages, thereby clamping the switch node to the clamp voltage by: Generate control signals G1, G2, and G3 to cooperatively switch the first switch, the second switch, and the bidirectional clamp switch, respectively, including generating G1 and G2 based on values ​​associated with a driving frequency and a driving duty cycle, the values ​​being converted to a reference phase value related to the driving frequency and a variable phase value related to both the driving frequency and the driving duty cycle, respectively, the difference between the reference phase value and the variable phase value being used to generate the control signals G1 and G2, and asserting the control signal G3 to clamp the switch node to the clamping voltage when neither G1 nor G2 is asserted, such that the control signals G1, G2, and G3 provide a desired duty cycle resolution.

19. The method of claim 18, further comprising a capacitor coupled between the switch node and an output node coupled to a load, and wherein selectively coupling the switch node to the clamp voltage is accomplished by selectively coupling the output node to ground such that a voltage across the capacitor comprises a clamp voltage.

20. The method of claim 18, adapted for use in a wireless power transmitter comprising a matrix including a plurality of transmit coils or transformer windings to supply the wireless power to a load comprising a wireless power receiver.

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