[0001] This relates to methods and apparatus for wireless transfer of power.

BACKGROUND

[0002] Wireless power transfer may be achieved by coupling energy from an inductive component in a power delivery device (transmitter) to an inductive component in a power receiving device (receiver). Alternatively, capacitive components may be coupled.

[0003] A wireless power transfer system may include multiple receivers. The multiple receivers may be of different designs with different characteristics, or may be of the same design with different characteristics due, for example, to tolerances or aging.

[0004] It would be advantageous to have the capability to effectively transfer power to one or more of a variety of receivers with different characteristics using one transmitter. It would further be advantageous to effectively transfer power to one or more receivers over a range of frequencies without needing to tune components of the transmitter or receivers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 illustrates an example wireless power transfer system.

[0006] FIG. 2 illustrates an example circuit model of coupled power transfer coils.

[0007] FIG. 3 illustrates an example transmitter.

[0008] FIG. 4 illustrates the frequency response of an example transmitter.

[0009] FIG. 5 illustrates an example transmitter coupled to an example receiver.

[0010] FIG. 6 illustrates the frequency response of an example transmitter coupled to an example receiver.

[0011] FIG. 7 illustrates an example wireless power transfer system with multiple receivers.

[0012] FIG. 8 illustrates an example wireless power transfer system with multiple receivers, and including communications links and a power tracker.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0013] For a wireless power transfer system using inductive power transfer components (i.e., power transfer coils) the power is transferred through magnetic coupling between transmitter and receiver power transfer coils. When the transmitter and receiver circuits are resonant circuits and have substantially the same resonance frequency and the transmitter operates at that resonance frequency, the system is capable of maximum power transfer. For example, transmitter resonant circuit output voltage can be significantly higher than the resonant circuit input voltage. [0014] However, such a resonant system may not be desirable, or may be difficult to implement in a system with multiple receivers. For example, power transfer in a resonant system diminishes significantly when multiple receivers are in proximity to the transmitter. The quality factor of a resonant circuit is sensitive to loading; therefore a resonant transmitter circuit with multiple loads in proximity has a much lower quality factor, and hence reduced power transfer. A resonant system must compensate for this lower quality factor, possibly with additional circuitry or software in the transmitter or receiver(s). Further, the resonant system may require costly tuning components such as variable capacitors to maintain resonance through a variety of system conditions. Additionally, a resonant transmitter may require a low voltage direct current (DC) input (e.g., 5V DC) to limit the output voltage on the transmitter power transfer coil and thereby limit the magnetic field intensity because the ratio of output voltage to input voltage can be very high, especially for high quality factor resonant circuits. In many cases the low DC voltage needs to be generated from a high voltage AC source such as an electric outlet or a power grid connected source. This requires an AC/DC conversion from a high voltage (e.g. 110V or 220V) alternating current (AC) to the low voltage DC, possibly requiring several stages of power conversion.

[0015] In the following discussions, a receiver may include a resonant circuit; however, the concepts apply equally to a receiver without resonant circuits. A system may include one or more receivers, and one or more of the receivers may include a resonant circuit.

[0016] A system operated in a non-resonant or quasi-resonant mode overcomes some of the issues that add complexity and cost to a resonant system. For example, a non-resonant system does not exhibit a significant loss in quality factor when multiple receivers are in proximity, so does not require additional circuitry or software to compensate for the quality factor reduction. Further, because resonance is not maintained, high-cost tuning components are not required. Additionally, because the ratio of output voltage to input voltage in a non-resonant system is small, the input voltage of the non-resonant transmitter circuit may be much higher than that of a resonant system. Therefore, the power supply of a non-resonant system does not need as many power conversion stages as is necessary for a resonant system.

[0017] FIG. 1 illustrates an example wireless power transfer system 100 with a transmitter 110 and a receiver 120. Transmitter 110 includes coil 130 having inductance LI . Receiver 120 includes coil 140 having inductance L2. Coils 130 and 140 are coupled with coupling coefficient k, which is a measure of magnetic coupling between the power transfer coils. The coupling coefficient is at least in part a function of the distance between the transmitter and the receiver and the relative orientation of the transmitter to the receiver. When the coupling coefficient between power transfer coils decreases, the power transferred between the coils correspondingly decreases. A mutual inductance M is established between coupled coils 130 and 140 at least in part as a function of coupling coefficient k and inductances LI and L2. Coil 130 may be physically implemented as one or more coils.

[0018] A receiver may be a wirelessly-rechargeable device such as a mobile phone, computer, or GPS device. A receiver may alternatively be a wirelessly-powered device.

[0019] FIG. 2 illustrates an equivalent electrical model 200 for the coils in the system 100 shown in FIG. 1. Inductors 210, 220, and 240 together with ideal transformer 230 model the effect of the coupling of coils 130 and 140 in FIG. 1. Inductors 210, 220, and 240 as modeled have the inductances shown in equations 1, 2, and 3, respectively.

L _kl = {\ - k) - L\ (1)

L _M = k - L\ (2)

L ₇ = (l - k)- Ll

kl > (3)

[0020] Ideal transformer 230 in the equivalent circuit of FIG. 2 is illustrated as having a 1 :n turns ratio, where n is defined in equation 4. (4)

[0021] Coupling coefficient k is defined in equation 5.

M

k

JL\ - LI

(5)

[0022] Although a transmitter is able to deliver maximum power when operated at resonant frequency, the transmitter may also deliver power across a wide frequency band at a reduced level. FIGS. 3 and 4 illustrate this property.

[0023] FIG. 3 illustrates an example transmitter resonant circuit including an alternating current power supply 310 with variable frequency f represented by ω=2πί, a capacitor 320 with capacitance CI, and a transmitter coil 330 with inductance LI . The output voltage across coil 330 is indicated as V coil. The resonant frequency of the circuit is given in equation 6. ° I JETO. (6)

[0024] When a resonant circuit is operated at a substantially different frequency than the resonant frequency, the circuit is operating in a non-resonant mode. The output power on the power transfer coil is weakly frequency-dependent, and therefore the generated magnetic field and the corresponding power transferred is also weakly frequency-dependent.

[0025] When a resonant circuit is operated at a frequency near but not at the resonant frequency, the circuit is operating in a quasi-resonant mode. The output power on the power transfer coil will be frequency-dependent, and therefore the generated magnetic field is frequency-dependent, and the corresponding power transferred may also be frequency-dependent depending on the characteristics of the receiver.

[0026] FIG. 4 represents the uncoupled frequency response of the circuit of FIG. 3 by a plot of the transfer function of the circuit on a semi-log graph. The transfer function plotted is the coil voltage V coil as a function of the input voltage to the resonant circuit, in this case power supply 310 voltage. The upper plot is the magnitude of the transfer function in decibels (dB) and the lower plot is the phase of the transfer function in degrees. The horizontal axis is normalized frequency, f/fO, displayed logarithmically.

[0027] As can be seen from the magnitude plot, the coil 330 voltage spikes to a large value when operating frequency equals the resonant frequency. For the particular circuit of FIG. 3, the quasi-resonant frequency band is approximately f/fO = 0.8 to f/f0=2.0, excluding the resonant frequency. In the quasi-resonant frequency band, the transmitter coil 330 voltage is a function of power supply frequency. Thus, in the quasi-resonant frequency band, the magnetic field intensity at transmitter coil 330, which is related to coil 330 voltage, is a function of transmitter power supply frequency also. A change in power supply frequency will therefore effect a change in magnetic field intensity and correspondingly may alter the maximum rate of power transfer.

[0028] It is further seen from the magnitude plot of FIG. 4 that, at power supply frequencies above the quasi-resonant frequency band, the coil voltage is substantially equal to the power supply voltage and the circuit is operating in a non-resonant mode. In non-resonant mode the circuit is substantially inductive, capacitor 320 appearing as a short circuit to the high

frequencies. For a system designed to operate in non-resonant mode, capacitor 320 may be eliminated. [0029] FIG. 5 illustrates the example transmitter of FIG. 3 coupled to an example receiver. The transmitter includes power supply 310, capacitor 320, and coil 330. The receiver includes power transfer coil 510 with inductance L2, capacitor 520 with capacitance CM, capacitor 530 with capacitance C2, and a load shown as resistor 540 with resistance R load. The voltage across the load is noted as V out. For the particular circuit of FIG. 5, the receiver resonant frequency is approximately f = 1.9 f0.

[0030] FIG. 6 illustrates the coupled frequency response of the transmitter/receiver of FIG. 5 by a plot of the transfer function of the circuit on a semi-log graph. The transfer function plotted is the output voltage V out as a function of the power supply 310 voltage. The upper plot is the magnitude of the transfer function in decibels (dB) and the lower plot is the phase of the transfer function in degrees. The horizontal axis is normalized frequency f/fO, displayed logarithmically.

[0031] In the magnitude plot, receiver load voltage V out spikes to a large value when the transmitter power supply 310 frequency is equal to the transmitter resonant frequency, with a smaller spike when the transmitter power supply 310 frequency is equal to the receiver resonant frequency of f = 1.9 f0. Also in the magnitude plot, receiver load voltage V out is a function of transmitter power supply 310 frequency over the frequency range shown.

[0032] From FIG. 6 it can be seen that a receiver may still receive power from the transmitter even though the transmitter and receiver resonant frequencies are different. It can also be seen from FIG. 6 that the transmitter may operate away from resonance and away from the resonant frequency of the receiver yet still deliver power to the receiver load. Therefore, power transfer occurs over a wide frequency band.

[0033] There are multiple benefits from operating in non-resonant or quasi-resonant mode. For example, operation in non-resonant or quasi-resonant mode may reduce power supply implementation costs as follows. When operating at resonant frequency, the voltage on the transmitter coil is amplified and may be significantly larger than the input voltage to the resonant circuit, and therefore the input voltage must be limited to prevent the coil voltage from becoming excessively high. In non-resonant or quasi-resonant mode, coil voltage is lower or not much higher than the power supply voltage, therefore the power supply voltage does not need to be limited. For a grid connected power supply, for example, 110V/60 Hz, 220/60 Hz or 220V/50 Hz AC power is typically converted to high- voltage DC power (e.g., 100-400V), then stepped down to low-voltage DC power (e.g., 5V), then converted to low voltage AC power for input to the resonant transmitter. In contrast, it may be possible to eliminate the step-down converter for a non-resonant or quasi-resonant transmitter, thereby saving cost.

[0034] A transmitter power supply may include switches for converting DC power to AC power. For example, a transmitter may include one or more H-bridges. For zero-voltage switching, capacitors may be included electrically in parallel to the switches.

[0035] Another benefit of operation in non-resonant or quasi-resonant mode is cost savings from not implementing variable components, or from using lower-precision variable components than needed for resonant systems. Maximum power transfer may be achieved when operating at the resonance frequency and additionally by using a resonant circuit with a high Q-factor.

Changing system conditions may require adjustments to a resonant circuit to maintain operation at a resonant frequency or to maintain a high Q-factor for the resonant circuit. Such adjustments are made, for example, using variable components, which tend to be expensive. In a non- resonant or quasi-resonant mode, the transmitter circuit does not need to be adjusted. For low cost systems it may be preferable to use a lower quality factor coil and operate in quasi-resonant or non-resonant mode. In addition to the lower coil cost, frequency tuning may not be necessary. However, lower Q-factor generally results in higher power loss, and system design must take this into account.

[0036] Another benefit of operation in non-resonant or quasi-resonant mode is the ability to perform power transfer from one transmitter to multiple receivers substantially simultaneously. In a resonant wireless power transfer system, when both transmitter and receiver operate at resonant frequency, if another receiver is also coupled to the transmitter there will be a degradation of the voltage on the transmitter coil and a corresponding degradation of the magnetic field due to a reduced quality factor of the system. The field may degrade to the point where the power transferred to a receiver is below the minimum power required by the receiver. This phenomenon is a major limitation to the use of a resonant transmitter for the case of multiple receivers. In contrast, in non- or quasi-resonant mode, the transmitter is largely unaffected by additional receivers being coupled to the transmitter because the transmitter circuit does not rely on amplification provided by the high quality factor of a resonant circuit. Further, the non- or quasi-resonant transmitter may adjust frequency or voltage as necessary for various operating conditions for single or multiple receivers.

[0037] FIG. 7 illustrates an example wireless power transfer system with one transmitter 610 and a number 'n' of receivers 620, shown as receivers 620 1 through 620_n. Transmitter 610 includes power transfer coil 630. Each receiver 620 includes a power transfer coil 640, shown as coils 640 1, 640 2, and 640_n corresponding to receivers 620 1, 620 2, and 620_n,

respectively. The receiver coils 640 are coupled to transmitter coil 630 with a corresponding coupling coefficient and mutual inductance. For example, receiver coil 640 1 is coupled to transmitter coil 630 with coupling coefficient kl and mutual inductance Ml .

[0038] A power transfer system may be designed such that transmitter 610 and receivers 620 have different resonant frequencies. Moreover, receivers 620 may be designed independently from the overall system and thus have resonant frequencies different than transmitter 610. Thus, transmitter 610 may be designed to be receiver agnostic and to provide power across a specified frequency band, such that any receiver 620 in which the resonant frequency is within the specified frequency band may be charged or powered by transmitter 610.

[0039] For the case of a resonant system with receivers at different resonant frequencies the transmitter will need to tune to the different receiver resonant frequencies to obtain the maximum power transfer. The non- or quasi-resonant system does not require a tunable transmitter, hence system cost can be reduced.

[0040] As an example, a transmitter may be designed to operate at a fixed quasi-resonant frequency. Multiple receivers may be designed to receive power from the transmitter. One of the multiple receivers may be designed with low-power components to meet cost savings, receiver size, or other constraints. To keep from electrically overstressing the low-power components, it may be desirable for the receiver coil voltage to be kept relatively low. The low-power receiver may be designed with a resonant frequency within the transmitter quasi-resonant frequency band, such that the receiver voltage remains below the design limits for that receiver. Another receiver in the system may be designed for quick-charge, and thus the quick-charge receiver resonant frequency is designed to be substantially equal to the transmitter resonant frequency. The quick- charge receiver will then receive maximum power transfer.

[0041] A transmitter may be designed to communicate with receivers and adapt operation to optimize power transfer to multiple receivers based on the communication.

[0042] FIG. 8 illustrates an example wireless power transfer system similar to the system of FIG. 7, with one transmitter 810 and a number 'n' of receivers 820, shown as receivers 820 1 through 820_n. The system of FIG. 8 further includes a communication link Com l between receiver 820 1 and transmitter 810, a communication link Com n between receiver 820_n and transmitter 810. As shown in FIG. 8, receiver 820 2 does not have a communication link to transmitter 810, illustrating that communication links are optional.

[0043] Communication links may be wired or wireless links, and may use standard or proprietary protocols for exchanging information. Although illustrated as bidirectional, a communication link may be unidirectional. A communication link may be used, for example, to identify the type of receiver 820, its resonant frequency, or its maximum power limit.

Transmitter 810 may determine its optimal operating frequency that provides, as one example, the fastest power transfer for the combination of proximal receivers 820 while keeping power transfer under the maximum limits for each receiver 820.

[0044] A transmitter may change from one operating mode to another to adapt to changing system conditions. For example, if the transmitter is receiving power from a battery, and the battery voltage is decreasing, the transmitter may switch into resonant mode from quasi-resonant or non-resonant mode to provide amplification of the input voltage in the resonant circuit. For another example, if the transmitter is receiving power from a low- voltage battery and then is switched over to a power grid source, the transmitter may switch from a resonant mode to a non- resonant or quasi-resonant mode. In these latter two examples, switching to and from resonant mode may be accomplished simply by adjusting the transmitter power supply frequency.

[0045] A transmitter used for wireless power transfer may be operated at a resonant frequency, a quasi-resonant frequency, or a non-resonant frequency to accommodate the needs of the receiver or receivers coupled to the transmitter.

[0046] Effective power transfer in a wireless power transfer system may be performed over a range of frequencies without tuning the transmitter or receiver resonant circuits, by operating in a non-resonant or quasi-resonant mode.

[0047] Operation in a non- or quasi-resonant mode allows for adjustment of power supply frequency to effect a change in coil voltage and thereby a change in magnetic field intensity.

[0048] In non-resonant or quasi-resonant mode, transmitter frequency may be adjusted to adjust the amount of power transferred to a given receiver based on the power needs of the receiver. Also in non- or quasi-resonant mode, the transmitter frequency may be adjusted to increase power transferred when multiple receivers are present.

[0049] Further, a system may be designed in which the resonant frequency of a receiver is purposely different from the resonant frequency of the transmitter. The greater the difference in frequency, the less power delivered, and the system may therefore be designed to accommodate receivers with different power needs.

[0050] Those skilled in the art to which the invention relates will appreciate that many other modifications may be made other than those described, and also that many other additional embodiments are possible within the scope of the claimed invention.