WO2014125392A1

WO2014125392A1 - Dynamic resonant matching circuit for wireless power receivers

Info

Publication number: WO2014125392A1
Application number: PCT/IB2014/058663
Authority: WO
Inventors: Klaas Jacob Lulofs
Original assignee: Koninklijke Philips N.V.
Priority date: 2013-02-13
Filing date: 2014-01-30
Publication date: 2014-08-21

Abstract

A resonant matching circuit (320) for matching a resonant frequency of a wireless power transfer system to an operational frequency of a power signal. The system comprises a tuning capacitor (321) connected in series with a resonant element (320) of the wireless power transfer system; a diode (324) connected in parallel with the tuning capacitor; a switch (322) connected in parallel with the diode and tuning capacitor; and a controller (323) connected to the switch (322) and configured to control a duty cycle of the switch, causing an increase or a decrease of the resonant frequency of the wireless power transfer system.

Description

Dynamic resonant matching circuit for wireless power receivers

FIELD OF THE INVENTION

The invention generally relates to wireless power transfer systems, and more particularly to techniques for dynamically adjusting the resonant frequency of such systems. BACKGROUND OF THE INVENTION

A wireless power transfer refers to supplying electrical power without any wires or contacts. Thus, the powering of electronic devices is performed through a wireless medium. One popular application for a wireless power transfer is for the charging of portable electronic devices, e.g., mobile phones, laptop computers, and the like.

SUMMARY OF THE INVENTION

One technique for a wireless power transfer is by an inductive powering system. In such a system, the electromagnetic inductance between a power source

(transmitter) and a device (receiver) enables contactless power transfers. Both the transmitter and receiver are fitted with electrical coils, and when the coils are brought into physical proximity, an electrical signal flows from the transmitter to the receiver.

In inductive powering systems, the generated magnetic field is concentrated within the coils. As a result, the power transfer to the receiver pick-up field is very concentrated in space. This phenomenon creates hot-spots in the system which limits the efficiency of the system. To improve the efficiency of the power transfer, a high quality factor for each coil is needed. To this end, the coil should be characterized with an optimal inductance to resistance ratio, be composed of materials with low resistance, and fabricated using a Litze-wire process to reduce skin-effect. The coils should also be designed to meet complicated geometries to avoid Eddy-currents. Therefore, expensive coils are required for efficient inductive powering systems. A design for an inductive wireless power transfer system to be used over a large area would need many expensive coils.

Capacitive coupling is another technique for transferring power wirelessly. This technique is predominantly utilized in data transfers and sensing applications. A car- radio antenna glued on a window with a pick-up element inside the car is an example of a capacitive coupling. The capacitive coupling technique is also utilized for contactless charging of electronic devices. For such applications, the charging unit (implementing the capacitive coupling) operates at frequencies outside the inherent resonance frequency of the device.

A capacitive power transfer system can also be utilized to transfer power over large areas having a flat structure, e.g., windows, walls, and the like. An example of such a capacitive power transfer system is system 100, depicted in Fig. 1. As illustrated in Fig. 1, a typical arrangement of such a system includes a pair of receiver electrodes 111, 112 connected to a load 120 and an inductor 130. The system 100 also includes a pair of transmitter electrodes 141, 142 connected to a power driver 150, and an insulating layer 160.

The pair of transmitter electrodes 141, 142 is located on one side of the insulating layer 160, and the receiver electrodes 111, 112 are located on the other side of the insulating layer 160. This arrangement forms capacitive impedance between the pair of transmitter electrodes 141, 142 and the receiver electrodes 111, 112.

The power driver 150 generates a power signal that can be wirelessly transferred from the transmitter electrodes 141, 142 to the receiver electrodes 111, 112 to power the load 120. The efficiency of the wireless power transfer improves when a frequency of the power signal matches a series-resonance frequency of the system 100. The series-resonance frequency of the system 100 is a function of the inductive value of the inductor 130 and/or inductor 131, as well as of the capacitive impedance between the pair of transmitter electrodes 141, 142 and the receiver electrodes 111, 112 (see CI and C2 in Fig. 1). The capacitive impedance and the inductor(s) cancel each other out at the resonance frequency, resulting in a low-ohmic circuit. The load 120 may be, for example, a LED, a LED string, a lamp, a computer, loud speakers, and the like.

An electric diagram 200 of the system 100 is provided in Fig. 2. The maximum power transfer is obtained when the frequency of the power signal U_gen is close to the series-resonance of the circuit. The circuit is comprised of the load R_L, the resistor Rs (represents the inductor resistance), capacitors Ci and C_2; and inductor Ls. The series- resonance is determined by the values of the capacitors Ci and C₂ and inductor Ls. The values of the capacitors Ci and C₂ and inductor Ls are selected such that they cancel each other out at the operating frequency of the signal U_gen. Therefore, only the series-resonance of the inductor Rs and the connectivity of the electrodes limit the power transfer. It should be appreciated that this allows transferring AC signals characterized by high amplitude and low frequencies. For both capacitive and inductive power transfer systems, power is efficiently transferred when the frequency of the AC power being input matches the resonant frequency at the receiver. For example, in the capacitive system that includes an inductive element such as the system shown in Figs. 1 and 2, the resonant frequency of the inductor(s) and the capacitive impedance should substantially match the frequency of the AC power signal.

One approach to match the resonant frequency of the receiver is to use variable resonant elements, e.g., variable inductors. However, such an approach may be bulky, expensive, or unavailable for an application. Another approach is to change the operational frequency of the power driver 150. However, this may not be a feasible solution in a system that includes multiple loads, because the frequency cannot be dynamically adjusted to ensure that all loads in the system will have the same resonant frequency. For example, changing the power signal's frequency to meet a resonance frequency of a first load may result in taking a second device out of its resonance state. Furthermore, there is only a limited range of frequencies that can be used, which are typically set by regulation organizations.

An additional solution for matching the resonant frequency includes a switch connected in parallel to an inductive element of the capacitive power transfer system. The switch is opened and closed for short time periods, thereby adding capacitive value to the resonant frequency of a wireless power transfer system. Controlling the switching time of the switch allows for control of the capacitive value added to the resonant elements. An example for such a solution is described in a PCT application No. PCT/1B2012/054006, filed on August 06 Aug 2012, assigned to the common assignee.

However, shorting the inductor of the capacitive power transfer system may cause a disruption of the energy transfer during the shorting of the inductor. As a result, less energy is transferred to the load and undesired Electromagnetic Interference may be generated.

Thus, an efficient solution is desired to match the resonance of a receiving circuit without changing the operational frequency of the power signal and without changing the resonant device's capacitive or inductive values.

Therefore, it would be advantageous to provide a solution for wireless power transfer systems that ensures an optimized power transfer by dynamically matching the resonance in such systems.

Certain embodiments disclosed herein include a resonant matching circuit for matching a resonant frequency of a wireless power transfer system to an operational frequency of a power signal. The circuit comprises a tuning capacitor connected in series to a resonant element of the wireless power transfer system; a diode connected in parallel to the tuning capacitor; a switch connected in parallel to the diode and tuning capacitor; and a controller connected to the switch and configured to control a duty cycle of the switch, causing an increase or a decrease of the resonant frequency of the wireless power transfer system.

Certain embodiments disclosed herein also include a resonant matching circuit for matching a resonant frequency of a capacitive power transfer system to a frequency of a power signal. The method comprises a tuning capacitor connected in series to an inductive element of the wireless power transfer system; a diode connected in parallel to the tuning capacitor; a switch connected in parallel to the diode and tuning capacitor; and a controller connected to the switch configured to control a duty cycle of the switch, causing an increase or a decrease of the resonant frequency of the wireless power transfer system, wherein the resonant frequency is a function of the inductive element, a capacitive impedance formed between receiver electrodes and transmitter electrodes of the capacitive power transfer system, and any one of the capacitive value of the tuning capacitor.

The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings:

Fig. 1 is a diagram of a capacitive power transfer system;

Fig. 2 is an electric diagram of the capacitive power transfer system;

Fig. 3 is an electrical diagram illustrating capacitive power transfer systems with a resonant matching circuit implemented according to one embodiment;

Figs. 4 and 5 are graphs illustrating the operation of the resonant matching circuit;

Fig. 6 is an electrical diagram illustrating the resonant matching circuit implemented according to another embodiment; and

Fig. 7 is an electrical diagram illustrating an inductive power transfer receiver with a resonant matching circuit implemented according to another embodiment; DETAILED DESCRIPTION OF EMBODIMENTS

It is important to note that the embodiments disclosed are merely examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in plural and vice versa with no loss of generality. In the drawings, like numerals refer to like parts through several views.

Various embodiments disclosed herein include a resonant matching circuit designed to dynamically match the resonant frequency of a wireless power transfer system to the resonant frequency of an AC power signal, without changing the AC power signal's frequency. The disclosed matching resonant circuit is based in part on a tuning capacitor and a simple switch designed to reduce the energy losses and undesired electromagnetic interferences in wireless power transfer systems.

Fig. 3 shows an exemplary and non-limiting electrical diagram of a capacitive power transfer system 300 designed with a resonant matching circuit according to one embodiment. The system 300 includes a transmitter 305 and a receiver 310 wirelessly coupled to each other as discussed above. The capacitive impedance between the electrodes of the transmitter 305 and receiver 310 is illustrated as CI and C2 in Fig. 3. As discussed above, the resonance frequency of the capacitive power transfer system is a function of the capacitive impedance formed between the transmitter and receiver electrodes and the inductive elements of such a system.

The receiver 310 includes an inductor (LI) 311 connected in series with one of the receiver's electrodes and a load 312. Also connected within the receiver 310 is a resonant matching circuit 320 designed to match the resonance frequency of the receiver 310 to the operating frequency of the power signal generated by a power driver 307 in the transmitter 305.

According to one embodiment, the resonant matching circuit 320 is connected in series with the inductor 311 and load 312. In a preferred embodiment, the resonant matching circuit 320 includes a tuning capacitor 321, a switch 322, a controller 323, and a diode 324. The controller 323 controls the operation of the switch 322. Specifically, in one embodiment, the controller 323 detects a zero-voltage crossing, i.e., a transition from a positive potential to a negative potential, or vice versa, across the diode 324 and tuning capacitor 321. In another embodiment, switching the switch from a closed to an open state is timed by the detection of the zero-voltage crossing. Upon transition from an open state to a closed state, the switch remains closed for a predefined period of time. This period of time is a portion of time that the capacitor is shorted. The controller 323 controls the switching time based on the received power, a phase error between a current signal and voltage signal, or combination thereof. As noted above, when the operational frequency matches the resonance frequency, a maximum power is received and the phase error is zero. It should be noted that the switching need not occur immediately after detection of a zero crossing, but rather can be performed any time after the zero voltage crossing but before the signal flowing through the diode 324 changes its polarity.

In yet another embodiment, the switch is closed for one or more complete resonant cycles and stays opens for one more complete resonant cycles. The embodiments for controlling the switch 322 are described below with reference to Figs. 4 and 5.

The duration of time that the switch 322 is closed determines the effective capacitance of the receiver 310, hence the resonance frequency of the receiver 310. In one embodiment, when the switch 322 is closed, the effective capacitance of the receiver 310 is determined by the capacitive impedance of CI and C2. When the switch 322 is open, the effective capacitance of the receiver 310 is determined by the capacitive impedance of CI and C2 and the value of the tuning capacitor 321.

In one embodiment, the tuning capacitor 321 and diode 324 are configured to decrease the resonance frequency of the receiver 310 to match the operational frequency of the power driver 307. Upon a zero-voltage-level crossing across the diode 324 of a negative to a positive polarity, the diode 324 starts to conduct, and the switch is then closed. The switch 322 remains closed when the voltage across the diode 324 changes from a positive polarity to a negative polarity. Consequently, the capacitor 321 is shorted, thereby increasing the effective capacitance of the receiver 310, causing a decrease of the resonance frequency of the receiver 310. Thus, controlling the duty cycle of the switch 322 allows for controlling the effective capacitance of the receiver 310 and the resonance frequency of the receiver 310. In an exemplary embodiment, the effective capacitance of the receiver 310 can be determined using the following equation:

Ceff =—^— ;

(l -DS) where C₃ is the capacitance value of the tuning capacitor 321, and DS is the duty switch of the switch 322. When DS = 1, the switch 322 is fully closed and the effective capacitance (C_eff) approaches infinity. When DS = 0, the switch 322 is open, and the effective capacitance is determined by the tuning capacitor 321. Because the capacitance of CI and C2 is in series with this tuning capacitor 321, the resonance when the switch is fully closed is solely determined by the capacitance of CI and C2, whereby the value of C3 is of no influence. When DS = 0 the resonance is determined by a capacitor having a capacitive value which is a series connection of C3 and capacitance CI and C2.

The controller 323 can be realized using one or more analog comparators and a timer for measuring the time period in which the switch 322 should remain closed. The controller 323, in one embodiment, detects when the voltage level across the tuning capacitor 321 changes from a negative to a positive plurality and when the voltage-level across the diode 324 changes from a positive to a negative plurality.

When the voltage-level across the diode 324 changes from a positive to a negative plurality, the diode starts to conduct and charges the tuning capacitor 321 with a DC voltage. This allows for using a simple switch such as the switch 322. In an embodiment disclosed herein, a simple switch comprises a single semiconductor element including, for example, a metal-oxide-semiconductor field-effect transistor (MOSFET) or a field-effect transistor (FET). One of ordinary skill in art should appreciate that a conventional AC switch includes two semiconductor elements. The diode 324 may be a diode element connected in the circuit or a body diode of a FET.

Fig. 4 illustrates the operation of the resonant matching circuit 320 according to one embodiment. The simulation shown in Fig. 4 is a time domain simulation of two complete resonant cycles. The simulation starts with the switch 322 being closed. At Tl, the switch 322 is opened. As a result, illustrated by the curve 401 the AC current is redirected through the tuning capacitor 321 and the voltage signal across this capacitor resonates (see curve 402). At time T2, the voltage-level across the tuning capacitor 321 and diode 324 changes polarity and the diode 324 starts to conduct (see curve 403). At any time between time periods T2 and T3, the switch 322 closes (during these time periods the diode 324 conducts). At T3, the polarity of the diode 324 changes, thus the switch 322 is closed prior to T3. Closing the switch 322 shorts the tuning capacitor 321, the effective capacitance of the receiver 310 is increased, and thus the resonant frequency of the receiver is also decreased. At T4, once the predefined period of time for closing the switch has elapsed, the switch 322 is opened again. The predefined period of time that the switch 322 remains closed is determined by the controller 323 as mentioned above.

Fig. 5 illustrates the operation of the resonant matching circuit 320 according to another embodiment. The simulation shown in Fig. 5 is a time domain simulation of two complete resonant cycles. According to this embodiment, the control of the switch 322 is performed in complete resonant cycles. That is, for one or more complete resonant cycles the switch 322 is open, and in the subsequent resonant cycles the switch 322 is closed. In Fig. 5, curve 501 represents the current through switch 322, curve 502 represents the current through the diode 324, curve 503 represents the current through the tuning capacitor 321, and curve 504 represents the voltage signal across the tuning capacitor 321.

The simulation starts with the switch 322 closed, and the AC current flows through the switch 322 as shown in curve 503. At time Tl, the voltage signal across the diode 324 changes from a positive polarity to negative polarity. When the voltage across the diode 324 changes from a positive to negative polarity, the diode 324 starts to conduct (see curve 502). During this time, the switch 322 is opened and the capacitor 321 is shorted resulting in a decrease in resonant frequency.

At time T2, the voltage polarity across the diode 324 is reversed, and the diode starts blocking the current. The load current is redirected through the tuning capacitor 321. As a result, the voltage across the capacitor 321 is increased as shown in curve 504. During this resonance cycle, the tuning capacitor 321 is placed in series with the resonant element resulting in an increase of resonance.

At time T3, the voltage across the diode 324 changes from a positive to negative polarity causing the diode 324 to conduct and switch 322 is closed, causing an increase in the resonant frequency. Again, at time T4, the diode 324 starts to conduct and switch 322 can be opened. Up to time T6, the tuning capacitor 321 is shorted resulting in a decrease in the resonant frequency. It should be noted that the resonant cycles for shorting the capacitor 321 can be randomly altered, resulting on average variable resonant frequency.

According to one embodiment, the resonant frequency matching circuit can be constructed to include a plurality of tuning capacitors and switches. Such a configuration allows for a reduction of voltage stress across the switch and increasing the control range of a DC voltage across the tuning capacitors. An exemplary and non-limiting diagram of a resonant matching circuit 620 according to this embodiment is shown in Fig. 6.

The matching circuit 620 is connected in series with the electrodes of the receiver 610 being wirelessly coupled to a transmitter 605 having a power driver 307. The exemplary circuit 620 comprises a pair of tuning capacitors 621, 622 respectively connected to MOSFETs 623, 624 and controlled by the controllers 625, 626. Each of the MOSFETs 623, 624 serves as a switch and diode, for example, as described with reference to Fig. 3. As illustrated in Fig. 6, the tuning capacitors 621 and 622 are connected in series. Each branch of the circuit 620 comprises a tuning capacitor, a MOSFET, and a controller that operates as the circuit 320 described above. That is, the switching of the MOSFET shorts the tuning capacitor such that the effective capacitance of the receiver 610 is increased, thus the resonant frequency is increased as well. In another embodiment, the DC power to the electronics of the controllers 625, 626 (a boot circuit) includes a diode and a capacitor connected to an output of a load 611. The boot circuit connected to the controller 625 comprises capacitor "Cboot" and diode "D5", while the boot circuit connected to the controller 626 includes capacitor "Cboot2" and diode "D6" which supply the other polarity of the DC voltage.

Fig. 7 shows an exemplary non-limiting electric circuit diagram of a receiver 700, which may be implemented in an inductive power transfer system. According to this embodiment the receiver 700 includes a resonant matching circuit 710, an inductor 701, a capacitor 702, and a load 703. The resonant matching circuit 710 performs dynamic resonant frequency matching and includes a switch and a diode realized as a MOSFET 711, a tuning capacitor 712, and a controller 713 that controls the operation of the switch. An alternating magnetic field from a transmitter side (not shown) induces voltage in the inductor 701. The frequency signal generated by the voltage source is equal to the operating frequency. The resonant frequency of the inductor 701 and capacitor 702 does not match the operating frequency of the signal provided by the transmitter.

According to this embodiment, the resonant matching circuit 710 is connected in series between the capacitor 702 and the load 703. The controller 713 controls the opening and closing of the switch to add capacitance to the receiver 700, and hence to increase the resonant frequency as discussed in detail above.

It should be noted that resonant matching circuit 710 may include a boot circuit to supply DC power to the controller. In addition, the resonant matching circuit 710 may also be implemented by using a plurality of tuning capacitors as discussed above.

In the embodiments illustrated in Figs. 3, 6 and 7, the load of the wireless power transfer system typically includes a rectifier, a smoothing capacitor, and the electrical element (e.g., a LED, a lamp, etc.) to which electrical power is delivered. Typically, a rectifier is utilized to convert an AC signal to a DC signal and is implemented using a diode-bridge.

It should be noted that the resonant matching circuit, in any of the embodiments disclosed in detail above, can be connected in a capacitive power transfer system or in an inductive power transfer system that includes a plurality of receivers.

Accordingly, the resonant frequency of each receiver is controlled by its resonant matching circuit. As a result, the resonant frequencies of the plurality of receivers can be independently matched to the operational frequency of the power signal.

While various embodiments have been described at some length and with some particularity, the invention should not be limited to any such particulars or

embodiments or any particular embodiment, but it is to be construed with references to the appended claims so as to provide the broadest possible interpretation of such claims in view of the prior art and, therefore, to effectively encompass the intended scope of the invention. Furthermore, the foregoing describes the invention in terms of embodiments foreseen by the inventor for which an enabling description was available, notwithstanding that insubstantial modifications of the invention, not presently foreseen, may nonetheless represent equivalents thereto.

Claims

CLAIMS:

1. A resonant matching circuit (320) for matching a resonant frequency of a wireless power transfer system to an operational frequency of a power signal, comprising:

a tuning capacitor (321) connected in series with a resonant element (320) of the wireless power transfer system;

a diode (324) connected in parallel with the tuning capacitor; a switch (322) connected in parallel with the diode and tuning capacitor; and a controller (323) connected to the switch (322) and configured to control a duty cycle of the switch, causing an increase or a decrease of the resonant frequency of the wireless power transfer system.

2. The circuit of claim 1, wherein the controller is further configured to close the switch for a predefined period of time after the diode starts to conduct and prior to a change in a voltage polarity of the diode.

3. The circuit of claim 2, wherein closing the switch causes a short of the tuning capacitor and a decrease of the resonant frequency.

4. The circuit of claim 2, wherein opening the switch causes an increase in the resonant frequency.

5. The circuit of claim 2, wherein the predefined period of time for closing the switch is at least one complete resonant cycle, and wherein a period of time for opening the switch is at least one complete resonant cycle.

6. The circuit of claim 3, wherein the predefined period of time is determined based on at least one of: a power of a received power signal and a phase error.

7. The circuit of claim 1, wherein the switch is at least any one of: a metal- oxide-semiconductor field-effect transistor (MOSFET) and a field-effect transistor (FET).

8. The circuit of claim 7, wherein the diode is a body diode of any one of the

MOSFET and the FET.

9. The circuit of claim 1, wherein the wireless power transfer system is any one of a capacitive power transfer system and an inductive power transfer system.

10. The circuit of claim 1, wherein the wireless power transfer system includes a plurality of receivers, wherein a resonant matching circuit is connected to each receiver of the plurality of receivers to independently match the resonant frequency of the receiver.

11. The circuit of claim 1, wherein the tuning capacitor includes a plurality of tuning capacitors (621, 622) connected in series to each other, wherein each of the plurality of tuning capacitors (621, 622) is connected to a respective switch (623, 624) controlled by a respective controller (625, 626).

12. A resonant matching circuit (320) for matching a resonant frequency of a capacitive power transfer system (300) to a frequency of a power signal (307), comprising:

a tuning capacitor (321) connected in series with an inductive element of the wireless power transfer system;

a diode (324) connected in parallel with the tuning capacitor;

a switch (322) connected in parallel with the diode and tuning capacitor; and a controller (323) connected to the switch (322) configured to control a duty cycle of the switch, causing an increase or a decrease of the resonant frequency of the wireless power transfer system, wherein the resonant frequency is a function of the inductive element (311), a capacitive impedance formed between receiver electrodes and transmitter electrodes of the capacitive power transfer system, and any one of the capacitive value of the tuning capacitor.

13. The circuit of claim 12, wherein the controller is further configured to close the switch for a predefined period of time after the diode starts to conduct and prior to a change in a voltage polarity of the diode.

14. The circuit of claim 12, wherein closing the switch causes a short of the tuning capacitor and a decrease of the resonant frequency, and wherein opening the switch causes an increase in the resonant frequency.

15. The circuit of claim 12, wherein the switch and diode are realized by at least any one of: a metal-oxide-semiconductor field-effect transistor (MOSFET) and a field-effect transistor (FET).