(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2011/0199045 A1
`Hui et al.
`(43) Pub. Date:
`Aug. 18, 2011
`
`US 2011 0199045A1
`
`(54) POWER TRANSFER DEVICE AND METHOD
`
`Publication Classification
`
`(75) Inventors:
`
`(73) Assignee:
`
`Shu Yuen Ron Hui, Shatin (HK);
`Wing Choi Ho, Kowloon (HK)
`ConvenientPower HK Ltd, Shatin
`(HK)
`
`(21) Appl. No.:
`
`12/705,911
`
`(22) Filed:
`
`Feb. 15, 2010
`
`(51) Int. Cl.
`(2006.01)
`H02. 7/00
`(52) U.S. Cl. ......................................... 320/108:307/104
`(57)
`ABSTRACT
`The present invention provides a power transfer device that
`wirelessly transfers AC power for charging at least one load,
`and an associated method of wirelessly transferring power.
`The device and method of the invention use phase-shift con
`trol to control the wireless transfer of the AC power.
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`Ids(M4)
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`Aug. 18, 2011
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`POWER TRANSFER DEVICE AND METHOD
`
`RELATED APPLICATIONS
`
`0001. This application claims priority to U.S. patent appli
`cation Ser. No. 12/699,563, filed Feb. 3, 2010; and U.S. patent
`application Ser. No. 12/566.438, filed Sep. 24, 2009, which
`applications are incorporated herein by reference in their
`entirety and made a part hereof.
`
`FIELD OF THE INVENTION
`
`0002 The present invention relates to power transfer
`devices, particularly power transfer devices for wirelessly
`charging loads. The invention will be described in the context
`of power transfer devices that wirelessly charge the batteries
`of portable wireless communication devices. However, it will
`be appreciated that the invention is not limited to this particu
`lar use.
`
`BACKGROUND OF THE INVENTION
`
`0003 Traditional battery chargers transfer power to the
`batteries through electrical wires. Many switching control
`methods such as duty-cycle control, frequency control and
`phase-shift converter have been proposed for Voltage regula
`tion and soft-switching techniques to reduce the Switching
`losses and radiated electromagnetic interference, in order to
`increase the energy efficiency and comply with electromag
`netic compatibility requirements, respectively. Due to the
`small amount of radiated electromagnetic field involved (be
`cause the power transfer is carried out through wires), tradi
`tional power converters for battery charging applications do
`not cause significant interference with the signal transmission
`and reception in the antenna and other Sub-systems of loads
`being charged, such as mobile phones.
`0004. However, unlike the design objectives of switched
`mode power Supplies which focus mainly on energy effi
`ciency and Voltage regulation, power converters for wireless
`charging systems have to cope with not only the dynamic
`wireless power transfer, Voltage regulation and efficiency
`requirements but also, and more importantly, the radio-fre
`quency (RF) aspects of the systems. These RF aspects include
`the quality of the transmission and reception of RF signals in
`the electronic loads being charged by a wireless charging
`system and also the ability of bidirectional communication
`between the wireless charging system and the electronic
`loads being charged.
`0005. The AC electromagnetic flux generated by the
`power converter of a wireless charging system can cause
`interference with the signal transmission and reception in the
`antenna and other sub-systems of the electronic load being
`charged since energy is transferred through the AC magnetic
`flux to the load (the Applicant's previous U.S. patent appli
`cation Ser. No. 12/566.438 titled “Antenna Network for Pas
`sive and Active Signal Enhancement addressed other prob
`lems related to similar issues that are encountered in these
`wireless power transfer applications). The antenna and the
`sub-systems here form the entire electronic load. Therefore,
`the criteria for choosing the right control technique and
`Switching method for power converters for wireless charging
`
`systems are distinctly different from those of traditional
`power converters for wired charging systems.
`
`SUMMARY OF THE INVENTION
`0006. The present invention provides a power transfer
`device that wirelessly transfers AC powerfor charging at least
`one load, the power transfer device having a phase-shift con
`trol means to control the wireless transfer of the AC power.
`0007 Preferably, the power transfer device includes a
`power converter for generating the AC power, the phase-shift
`control means controlling the power converter.
`0008 Preferably, the power transfer device wirelessly
`transfers the AC power at a transfer frequency using a spread
`spectrum technique.
`0009. In another aspect, the present invention provides a
`method of wirelessly transferring AC power for charging at
`least one load, the method including controlling the wireless
`AC power transfer with phase-shift control.
`0010 Preferably, the method includes generating the AC
`power with a power converter, and wherein controlling the
`wireless AC power transfer with phase-shift control includes
`controlling the power converter with phase-shift control.
`0011
`Preferably, the method includes using a spread
`spectrum technique to wirelessly transfer the AC power at a
`transfer frequency.
`0012. In both the aspects described above, the power con
`verter is preferably a DC-AC power converter, which is also
`known as an inverter.
`
`BRIEF DESCRIPTION OF THE FIGURES
`0013 Preferred embodiments in accordance with the best
`mode of the present invention will now be described, by way
`of example only, with reference to the accompanying figures,
`in which:
`0014 FIG. 1a is a schematic diagram of circuits of a
`wireless power transfer system incorporating a powertransfer
`device in accordance with an embodiment of the present
`invention;
`0015 FIG.1b is a schematic diagram of circuits of another
`wireless power transfer system;
`0016 FIG. 2a is a timing diagram showing the typical
`waveforms of an inverter operated under an embodiment of
`duty-cycle control;
`0017 FIG. 2b is a timing diagram showing the typical
`waveforms of the inverter of FIG. 2a operated under an
`embodiment of duty-cycle control where the duty cycle is
`large;
`0018 FIG. 2C is a timing diagram showing the typical
`waveforms of the inverter of FIG. 2a operated under an
`embodiment of duty-cycle control where the duty cycle is
`Small;
`0019 FIG. 3a is a timing diagram showing the typical
`waveforms of an inverter operated under an embodiment of
`frequency control;
`0020 FIG. 3b is a timing diagram showing the typical
`waveforms of the inverter of FIG. 3a operated under an
`embodiment of frequency control at low frequency;
`0021
`FIG. 3C is a timing diagram showing the typical
`waveforms of the inverter of FIG. 3a operated under an
`embodiment of frequency control at high frequency;
`0022 FIG. 4a is a timing diagram showing the typical
`waveforms of an inverter operated under phase-shift control
`in accordance with an embodiment of the present invention;
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`0023 FIG. 4b is a timing diagram showing the typical
`waveforms of the inverter of FIG. 4a operated under phase
`shift control with a small phase-shift angle in accordance with
`another embodiment of the present invention;
`0024 FIG. 4c is a timing diagram showing the typical
`waveforms of the inverter of FIG. 4a operated under phase
`shift control with a large phase-shift angle inaccordance with
`yet another embodiment of the present invention;
`0025 FIG. 5a is a timing diagram showing the typical
`waveforms of an inverter operated under phase-shift control
`in accordance with a further embodiment of the present
`invention;
`0026 FIG. 5b is a timing diagram showing the typical
`waveforms of the inverter of FIG. 5a operated under phase
`shift control with a small phase-shift angle in accordance with
`another embodiment of the present invention;
`0027 FIG. 5c is a timing diagram showing the typical
`waveforms of the inverter of FIG. 5a operated under phase
`shift control with a large phase-shift angle inaccordance with
`yet another embodiment of the present invention;
`0028 FIG. 6 is a schematic diagram of a circuit of a
`wireless power transfer system in which an inverter is oper
`ated under an embodiment of Voltage control;
`0029 FIG. 7a is a timing diagram showing the typical
`waveforms of an inverter operated under an embodiment of
`Voltage control;
`0030 FIG. 7b is a timing diagram showing the typical
`waveforms of the inverter of FIG. 7a operated under an
`embodiment of voltage control with high DC link inverter
`Voltage; and
`0031
`FIG. 7c is a timing diagram showing the typical
`waveforms of the inverter of FIG. 7a operated under an
`embodiment of voltage control with low DC link inverter
`Voltage.
`
`DETAILED DESCRIPTION OF THE BEST MODE
`OF THE INVENTION
`0032 Referring to the figures, there is provided a power
`transfer device 1 that wirelessly transfers AC powerfor charg
`ing at least one load 2, the power transfer device having a
`phase-shift control means 3 to control the wireless transfer of
`the AC power.
`0033. The power transfer device 1 includes a power con
`Verter 4 for generating the AC power, and the phase-shift
`control means 3 controls the power converter. In the present
`embodiment, the power converter 4 is a DC-AC power con
`Verter, which is also known as an inverter.
`0034. The power transfer device 1 includes a primary
`winding L, for inductively transferring the AC power to a
`secondary winding L, thereby wirelessly transferring the
`AC power. The secondary winding L, includes a series
`capacitor C, for reducing any leakage inductance. The sec
`ondary winding Lis also connected to a rectifier 5, which is
`preferably a synchronous rectifier.
`0035. The secondary winding L, forms part of the load
`2. Preferably, the power transfer device 1 wirelessly transfers
`AC powerfor charging a plurality of loads 2. Also, these loads
`2 can be of different types. For example, they can include
`mobile phones, laptop computers, or any other portable elec
`tronic devices, which may or may not be capable of wireless
`communication.
`0036. In further detail, the DC-AC power converter 4
`includes two pairs of switches M1, M2, M3, and M4. The
`off-diagonal Switches work as a pair, that is, Switches M1 and
`
`M4 are one pair and switches M2 and M3 are the other pair.
`The phase-shift control means 3 varies the AC power by
`adjusting a phase angle C. between gating signals of each pair
`of switches. Each switch M1, M2, M3, and M4 is operated at
`a constant frequency and a constant duty-cycle.
`0037. As will be described in greater detail below, not only
`does the use of phase-shift control result in better efficiency,
`lower cost, as well as addressing the Voltage floating problem,
`but it also reduces or minimizes RF interference. For
`example, where one of the loads 2 is a wireless communica
`tion device (Such as a mobile phone) having a communication
`bandwidth, the use of the phase-shift control means 3 reduces
`or minimizes interference signals within the communication
`bandwidth. The use of the phase-shift control means 3 also
`reduces or minimizes interference signals within the power
`transfer device 1 itself.
`0038 Also, in another preferred embodiment, the power
`transfer device 1 wirelessly transfers the AC power at a trans
`fer frequency using a spread-spectrum technique. The spread
`spectrum technique is at least one of dithering, pseudo-ran
`dom, random, chaotic, and modulated type, and thereby
`varies the transfer frequency. Generally, the spread-spectrum
`technique varies the transfer frequency within a transfer
`bandwidth that maximizes the energy efficiency of the AC
`power transfer by the power transfer device 1.
`0039. As mentioned above, the power transfer device 1
`utilizes Switching to generate the AC power. The spread
`spectrum technique varies at least one of the characteristics of
`the Switching. In particular, the spread-spectrum technique
`varies at least one of Switching frequency, Switching pulse
`width, and Switching pulse position.
`0040. In one embodiment, the spread-spectrum technique
`utilizes a direct sequence spread-spectrum method.
`0041. Where one of the loads 2 is a wireless communica
`tion device (Such as a mobile phone) having a communication
`bandwidth, the spread-spectrum technique reduces or mini
`mizes interference signals within the communication band
`width. The spread-spectrum technique also reduces or mini
`mizes interference signals within the power transfer device 1
`itself.
`0042. The use of a spread-spectrum technique in wireless
`power transfer applications such as that presently contem
`plated is described in further detail in the Applicant's previous
`U.S. patent application Ser. No. 12/699,563, which is incor
`porated herein by reference in its entirety. It will be appreci
`ated that the spread-spectrum techniques and other features of
`the invention disclosed in U.S. patent application Ser. No.
`12/699,563 can be combined with embodiments of the
`present invention.
`0043. In order to demonstrate the surprising and unex
`pected Suitability of using phase-shift control in wireless
`powertransfer applications, such as those contemplated in the
`present invention, the following analysis is provided. Control
`methods for power converters are analyzed in the context of
`wireless battery charging systems with an emphasis on
`energy efficiency and interference between the charging flux
`of the charging system (such as those including a charging
`pad) and the antenna and other Sub-systems of a load being
`charged.
`0044) More specifically, analysis is carried out for the
`following methods for controlling the wireless power trans
`fer:
`0045 (i) duty-cycle control;
`0046 (ii) frequency control;
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`0047 (iii) phase-shift control (two versions, referred to as
`Schemes I and II); and
`(iv) Voltage control.
`0048
`0049 FIGS. 1a, 1b, and 6 show typical circuits for a wire
`less power transfer system. FIG. 1a includes the power trans
`fer device 1, which was broadly described earlier, that incor
`porates method (iii). FIGS. 1b and 6 show a similar system
`that includes a power transfer device 6 that can incorporate
`one of the methods (i), (ii), and (iv), and more specifically,
`includes control means 7 that can implement one of those
`methods. For a given constant input DC voltage source, a
`primary side of the system includes one of the power transfer
`devices 1 and 6, which in turn, includes the DC-AC power
`converter 4 (also called a power inverter) driving the primary
`winding L, or a group of primary windings (preferably
`through a matching network). A secondary side of the system
`includes a secondary module (in the form of the load 2),
`which in turn, includes the secondary winding L, prefer
`ably with a series capacitor Such as C, and the rectifier circuit
`5, which can be a synchronous rectifier. The series capacitor
`Cin the secondary winding Lis preferred because it allows
`the effect of the leakage inductance in this loosely coupled
`system to be cancelled so that the power transfer can be
`maximized. In general, the off-diagonal Switches of the
`power inverter 4 work as a pair (i.e. M1 and M4 as a pair and
`M2 and M3 as another pair, as mentioned earlier).
`0050 A. Duty-Cycle Control
`0051. The cycle control is carried out by controlling the
`duty cycle D of the switches M1, M2, M3, and M4. FIG. 2a
`shows typical waveforms of the gate signals of the four
`Switches of the inverter 4 and also the AC output Voltage V
`of the power inverter 4. Usually the inverter 4 is operated at
`constant Switching frequency. There is a constant 90-degree
`phase shift between the Switching patterns of the diagonal
`Switch pairs. Because of the constant phase shift, the output
`Voltage magnitude is controlled by varying the duty cycle
`from 0 to 0.5. The output voltage increases with increasing
`duty cycle. It is important to note that the duty-cycle control
`scheme is easy to design. The duty cycle of a diagonal pair of
`switches (e.g. M1 and M4) are identical. Its duty cycle is
`varied and this duty cycle control method can control the
`magnitude of the output voltage V without requiring a front
`DC-DC converter stage to vary the DC link voltage for the
`power inverter 4.
`0052 FIG.2b and FIG.2c show the simulated waveforms
`of the duty-cycle control method when the duty cycle is large
`and Small, respectively. Controlling the duty cycle can control
`the power flow. However, for wireless energy transfer sys
`tems, duty-cycle control has the following disadvantages:
`0053 (i) The primary current is distorted and not sinusoi
`dal, regardless of whether the duty cycle is large or small. The
`distorted current indicates the presence of current harmonics
`and harmonic losses and thus poor energy efficiency. The
`harmonic currents will cause harmonic heating in the primary
`winding, resulting in high conduction loss and poor energy
`efficiency.
`0054 (ii) When the duty cycle is small, sharp voltage
`ringing occurs across V. The sharp Voltage pulses (V4)
`and its high-frequency harmonics would be a source of elec
`tromagnetic interference (EMI) to the load, and therefore
`causing RF signal jamming to the antenna of the load.
`0055 (iii) Bi-directional communication (such as fre
`quency or amplitude modulation and demodulation methods)
`
`between the primary charging system and the load on the
`secondary side cannot be easily achieved in the duty-cycle
`control scheme.
`0056 (iv) When the duty cycle is very small, the current
`could become discontinuous. Consequently, there are fre
`quent moments that all the four switches are turned off simul
`taneously, resulting in the primary winding floating. With
`unpredictable floating Voltage in the primary winding, the
`bidirectional communication signals can be affected.
`0057 B. Frequency Control Scheme
`0.058 A frequency controlled inverter 4 usually uses a
`resonant circuit consisting of an inductor and a capacitor as
`the matching network. By changing the frequency at constant
`duty cycle, the inverter 4 can vary the output Voltage accord
`ing to the Voltage gain profile of the LC resonant circuit. FIG.
`3a shows the timing diagram of the gating signals of the
`frequency control scheme. The simulated waveforms of the
`frequency-control scheme at low and high frequency opera
`tions are included in FIG.3b and FIG. 3c, respectively. It can
`be seen that frequency control can vary the power flow. If the
`frequency is reduced, the current and therefore power
`increases, and vice versa.
`0059. The frequency control scheme is easy to implement
`and has been commonly adopted in dimmable electronic bal
`lasts for lighting applications. It can vary the output Voltage of
`the inverter 4 without using a front power stage to vary the DC
`link voltage of the inverter. However, for wireless energy
`transfer systems, frequency control has the following disad
`Vantages:
`0060 (i) The frequency-dependent voltage gain of the LC
`resonant circuit does not change linearly with frequency,
`making the power control nonlinear.
`0061
`(ii) For a secondary module 2 with a fixed inductor
`and series capacitor (i.e. secondary resonant circuit), only
`when the inverter frequency matches the secondary resonant
`frequency does the operation achieve optimal operating fre
`quency. All other frequencies do not match the secondary
`resonant frequency and energy efficiency cannot be maxi
`mized.
`0062 (iii) Frequency control is not suitable for common
`secondary circuit design (which has a single resonant fre
`quency as explained in (ii)).
`0063 (iv) The wide frequency range of the inverter 4 also
`means that the interference between the AC flux of this vary
`ing frequency and the antenna signal will be complicated. The
`noise induced will spread over a wider spectrum, making it
`difficult to reduce the signal mixing andjamming effects due
`to this interference.
`0064 C. Phase-Shift Control Schemes
`0065 (a) Phase-Shift Control Scheme I
`0066. The phase-shift control Scheme I operates the
`inverter 4 at constant frequency and constant duty-cycle, with
`each Switch operated at half the duty-cycle. Thus, this means
`that each diagonal pair of the switches operates for half of the
`cycle. The output Voltage magnitude is controlled by varying
`the phase shift of the switching patterns of the two sets of
`diagonal Switch pairs. That is to say, the control scheme varies
`the output Voltage V by adjusting a phase angle C. between
`the gating signals of each diagonal pair of the Switches (M1
`and M4 as one pair, and M2 and M3 as another pair). In actual
`operation, each pair of the Switches Switch at a duty cycle of
`0.5 minus the dead time for transition from one pair of
`Switches to the other pair, that is, their duty cycles remain at
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`or close to 0.5. In this way, an AC Voltage can be generated in
`the output of the phase-shift inverter 4.
`0067. The timing diagram of the gating signals and the
`inverter output voltage is shown in FIG. 4a. Although the
`inverter output voltage waveform looks like that of the duty
`cycle control in FIG. 2a, there are several major differences
`that make this scheme have different features from those of
`the duty-cycle control. Firstly, the gating signals of the diago
`nal pair of switches are not identical. There exists the phase
`angle C. between them as shown in FIG. 4a. Increasing a can
`reduce the primary Voltage and current and therefore power.
`Since power control can be carried out in one power stage,
`high efficiency can be achieved. Secondly, each switch M1,
`M2, M3, and M4 is operated at a respective constant duty
`cycle. In this particular scheme, each Switch is operated at
`half duty-cycle. The continuous conduction states of the
`respective Switches allows the current in the primary winding
`L., and the matching network to flow continuously and
`remain in a sinusoidal manner, therefore reducing current
`harmonics, harmonic heating loss in the winding and electro
`magnetic interference (EMI) emitted from the electromag
`netic flux generated in the primary winding. By contrast,
`under a duty-cycle control Scheme, there is a constant 90-de
`gree phase shift between the Switching patterns of the diago
`nal Switch pairs. Because of the constant phase shift, the
`output Voltage magnitude is controlled by varying the duty
`cycle from 0 to 0.5.
`0068. The phase-shift Scheme I is easy to implement.
`Because of the large duty cycle, the harmonics can be mini
`mized. Since the output Voltage can be controlled by adjust
`ing the phase angle, there is no need to use a front stage
`DC-DC converter to vary the DC link voltage of the inverter
`4. Thus, the energy efficiency can be high. As the current in
`the primary winding can flow continuously, there is no volt
`age floating” problem in the primary winding L,
`0069. The only disadvantage is that this method is only
`applicable for a full-bridge inverter (and not a half-bridge
`inverter). However, a full-bridge is acceptable in the wireless
`charging application because the DC link Voltage of the
`inverter 4 is usually low and typically between 10V to 20V.
`Using a full-bridge in Such a low-voltage environment is
`useful in full utilization of the limited voltage range.
`0070 (b) Phase-Shift Control Scheme II
`0071. The phase-shift control Scheme II is a modified
`version of Scheme I. This is also a constant-frequency
`method. The gate signals Gate 1 for M1 and Gate 2 for M2 are
`kept out of phase. The pulse width of Gate 4 for M4 (of the
`diagonal pair M1 and M4) is controlled with a phase angle C.
`with respective to Gate 1 as shown in timing diagram of FIG.
`5a. The simulated waveforms of such a scheme with small
`and large phase shift angles are shown in FIG.5b and FIG.5c,
`respectively. Similar to Scheme I, increasing the phase shift
`angle can reduce the power. With only one power stage, this
`
`scheme can achieve high efficiency. Good sinusoidal current
`waveforms are observed in both cases, implying good RF
`performance.
`(0072 D. Voltage Control Method
`0073. Unlike the previous control schemes that employ the
`circuit depicted in the schematic diagram of FIG. 1, the volt
`age control scheme uses an extra DC-DC power converter
`stage (labeled DC/DC Conversion) to control the DC link
`voltage for the power inverter as shown in FIG. 6. The corre
`sponding timing diagram and inverter output Voltage wave
`form are shown in FIG. 7a. The gating signals of the diagonal
`pair of switches are identical and at a full duty cycle of about
`0.5 (except for a small dead time between them in practice to
`avoid shoot-through). The Switching frequency of the inverter
`4 remains constant. The inverter 4 basically controls the fre
`quency of its output voltage V. The magnitude of the
`inverter output voltage is controlled by the front-end power
`converter that varies the DC link voltage for the inverter. FIG.
`7b and FIG. 7c show the simulated waveforms of this scheme
`with high and low DC link voltages respectively. It can be
`seen that the primary Voltage and current can be controlled by
`controlling the DC link Voltage in the front power stage.
`0074 The voltage control scheme has the following
`advantages. It is simple in concept and the power control is
`linear and simple to implement. Individual power converter/
`inverter modules can be designed independently and put
`together. The current in the primary winding can remain
`sinusoidal and thus minimizing harmonic interference and
`signal jamming problem with the antenna (i.e. good RF per
`formance). However, there are disadvantages for the Voltage
`control scheme, as follows:
`0075 (i) The two power conversion stages (i.e. the
`requirement of one extra power converter for controlling the
`DC link voltage for the inverter) will reduce the energy effi
`ciency of the entire wireless energy transfer system.
`0076 (ii) More components and higher costs result from
`one more power converter.
`0077. After analyzing the four types of control schemes
`and considering the energy efficiency and the RF perfor
`mance together, their advantages and disadvantages are sum
`marized in Table 1 below. Surprisingly and unexpectedly, it
`can be seen that the two phase-shift control schemes stand out
`to be the best schemes among all the schemes under consid
`eration. While phase-shift control may require relatively
`expensive customized integrated control circuits, it can be
`implemented with digital control (Such as a microprocessor
`unit, which is good for complex control implementation).
`Due to the use of one power stage, the cost is low, energy
`efficiency is high, bidirectional communication is feasible
`and the RF performance is good. Thus, the phase-shift control
`scheme is the optimal scheme for wireless energy transfer
`system when the RF aspects of the load or loads are consid
`ered.
`
`TABLE 1
`
`Duty-Cycle
`Control
`
`Summary of disadvantages and advantages of control schemes.
`Disadvantages
`Advantages
`1. Simple to design.
`2. Without front-stage power
`converter.
`
`1. Serious current harmonics.
`2. Harmonics cause conduction loss
`and reduce energy efficiency.
`3. Harmonics cause signal
`mixing jamming effect in antenna.
`
`Ex.1005
`APPLE INC. / Page 21 of 24
`
`

`

`US 2011/O 199045 A1
`
`Aug. 18, 2011
`
`TABLE 1-continued
`
`Summary of disadvantages and advantages of control schemes.
`Disadvantages
`Advantages
`
`4. Potential problem in stability due to
`voltage floating.
`5. Potential problem in amplitude
`modulation demodulation
`(communication).
`6. Poor RF performance.
`1. Non-linearity
`2. More expensive components due to
`high frequency for low power.
`3. Interfered by secondary resonance.
`4. Possible interference to different
`frequency bands (poor EMC EMF).
`1. Relatively complex control scheme.
`2. Can only be applied to full-bridge
`power inverter.
`
`1. Poor efficiency due to front-stage
`power converter.
`2. More components (add
`additional circuits), high cost.
`
`Frequency
`Control
`
`Phase-Shift
`Control
`
`Voltage
`Control
`
`1. Applicable for higher power.
`2. Simple to design.
`3. Without front-stage power
`converter.
`
`1. Without front-stage power
`converter (thus, higher efficiency
`and lower cost).
`2. Less RF interference.
`3. No voltage floating problem.
`1. Simple and off-the-shelf
`design.
`2. Good waveform on coil (less
`interference).
`3. No voltage floating problem.
`
`0078. The present invention incorporates phase-shift con-
`trol, together with the Surprising and unexpected results and
`advantages this type of control offers in the context of wire
`less power transfer applications, such as those the present
`invention contemplates. These advantages include the favour
`able RF aspects as well as higher energy efficiency and lower
`COStS.
`0079. As described previously, in order to further enhance
`the signal reception an