Файл: wirelesspowerspecificationpart1.pdf

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2 System Overview (Informative)

Operation of devices that comply with this System Description Wireless Power Transfer relies on magnetic induction between planar coils. Two kinds of devices are distinguished, namely devices that provide wireless power—referred to as Base Stations—and devices that consume wireless power— referred to as Mobile Devices. Power transfer always takes place from a Base Station to a Mobile Device. For this purpose, a Base Station contains a subsystem—referred to as a Power Transmitter—that comprises a Primary Coil,1 and a Mobile Device contains a subsystem—referred to as a Power Receiver— comprises a Secondary Coil. In fact, the Primary Coil and Secondary Coil form the two halves of a coreless resonant transformer. Appropriate Shielding at the bottom face of the Primary Coil and the top face of the Secondary Coil, as well as the close spacing of the two coils, ensures that power transfer occurs with an acceptable efficiency. In addition, this Shielding minimizes the exposure of users to the magnetic field.

Typically, a Base Station has a flat surface—referred to as the Interface Surface—on top of which a user can place one or more Mobile Devices. This ensures that the vertical spacing between Primary Coil and Secondary Coil is sufficiently small. In addition, there are two concepts for horizontal alignment of the Primary Coil and Secondary Coil. In the first concept—referred to as Guided Positioning—the user must actively align the Secondary Coil to the Primary Coil, by placing the Mobile Device on the appropriate location of the Interface Surface. For this purpose, the Mobile Device provides an alignment aid that is appropriate to its size, shape and function. The second concept—referred to as Free Positioning—does not require the active participation in alignment of the Primary Coil and Secondary Coil. One implementation of Free Positioning makes use of an array of Primary Coils to generate a magnetic field at the location of the Secondary Coil only. Another implementation of Free Positioning uses mechanical means to move a single Primary Coil underneath the Secondary Coil.

Figure 2-1 illustrates the basic system configuration. As shown, a Power Transmitter comprises two main functional units, namely a Power Conversion Unit and a Communications and Control Unit. The diagram explicitly shows the Primary Coil (array) as the magnetic field generating element of the Power Conversion Unit. The Control and Communications Unit regulates the transferred power to the level that the Power Receiver requests. Also shown in the diagram is that a Base Station may contain multiple Transmitters in order to serve multiple Mobile Devices simultaneously (a Power Transmitter can serve a single Power Receiver at a time only). Finally, the system unit shown in the diagram comprises all other functionality of the Base Station, such as input power provisioning, control of multiple Power Transmitters, and user interfacing.

A Power Receiver comprises a Power Pick-up Unit and a Communications and Control Unit. Similar to the Power Conversion Unit of the Transmitter, Figure 2-1 explicitly shows the Secondary Coil as the magnetic field capturing element of the Power Pick-up Unit. A Power Pick-up Unit typically contains a single Secondary Coil only. Moreover, a Mobile Device typically contains a single Power Receiver. The Communications and Control Unit regulates the transferred power to the level that is appropriate for the subsystems connected to the output of the Power Receiver. These subsystems represent the main functionality of the Mobile Device. An important example subsystem is a battery that requires charging.

The remainder of this document is structured as follows. Section 3 defines the basic Power Transmitter designs, which come in two basic varieties. The first type of design—type A—is based on a single Primary Coil (either fixed position or moveable). The second type of design—type B—is based on an array of Primary Coils. Note that this version 1.1.1 of the System Description Wireless Power Transfer, Volume I, Part 1, offers only limited design freedom with respect to actual Power Transmitter implementations. The reason is that Mobile Devices exhibit a much greater variety of design requirements with respect to the Power Receiver than a Base Station does to Power Transmitters—for example, a smart phone has design requirements that differ substantially from those of a wireless headset. Constraining the Power Transmitter therefore enables interoperability with the largest number of mobile devices.

1Note that the Primary Coil may be a “virtual coil,” in the sense that an appropriate array of planar coils can generate a magnetic field that is similar to the field that a single coil generates.

System Description

Wireless Power Transfer

Figure 2-1: Basic system overview

Section 4 defines the Power Receiver design requirements. In view of the wide variety of Mobile Devices, this set of requirements has been kept to a minimum. In addition to the design requirements, Section 4 is complemented with two example designs in Annex A.

Section 5 defines the system control aspects of the power transfer. The interaction between a Power Transmitter and a Power Receiver comprises four phases, namely selection, ping, identification & configuration, and power transfer. In the selection phase, the Power Transmitter attempts to discover and locate objects that are placed on the Interface Surface. In addition, the Power Transmitter attempts to discriminate between Power Receivers and Foreign Objects and to select a Power Receiver (or object) for power transfer. For this purpose, the Power Transmitter may select an object at random and proceed to the ping phase (and subsequently to the identification & configuration phase) to collect necessary information. Note that if the Power Transmitter does not initiate power transfer to a selected Power Receiver, it should enter a low power stand-by mode of operation.2 In the ping phase, the Power Transmitter attempts to discover if an object contains a Power Receiver. In the identification & configuration phase, the Power Transmitter prepares for power transfer to the Power Receiver. For this purpose, the Power Transmitter retrieves relevant information from the Power Receiver. The Power Transmitter combines this information with information that it stores internally to construct a so-called Power Transfer Contract, which comprises various limits on the power transfer. In the power transfer

2A definition of such a stand-by mode is outside the scope of this version 1.0 System Description Wireless Power Transfer, Volume I, Part 1. However, [Part 2] provides requirements on the maximum power use of a Power Transmitter when it is not actively providing power to a Power Receiver.

phase, the actual power transfer takes place. During this phase, the Power Transmitter and the Power Receiver cooperate to regulate the transferred power to the desired level. For this purpose, the Power Receiver communicates its power needs on a regular basis. In addition, the Power Transmitter continuously monitors the power transfer to ensure that the limits collected in the Power Transfer Contract are not violated. If a violation occurs anyway, the Power Transmitter aborts the power transfer.

The various Power Transmitter designs employ different methods to adjust the transferred power to the requested level. Three commonly used methods include frequency control—the Primary Coil current, and thus the transferred power, is frequency dependent due to the resonant nature of the transformer—duty cycle control—the amplitude of the Primary Coil current scales with the duty cycle of the inverter that is used to drive it—and voltage control—the Primary Coil current scales with the driving voltage. Whereas the details of these control methods are defined in Section 3, Section 5 defines the overall error based control strategy. This means that the Power Receiver communicates the difference between a desired set point and the actual set point to the Power Transmitter, which adjusts the Primary Coil current so as to reduce the error towards zero. There are no constraints on how the Power Receiver derives its set point from parameters such as power, voltage, current, and temperature. This leaves the option to the Power Receiver to apply any desired control strategy.

This version 1.1.1 of the System Description Wireless Power Transfer, Volume I, Part 1, defines communications from the Power Receiver to the Power Transmitter only. Section 6 defines the communications interface. On a physical level, communications from the Power Receiver to the Power Transmitter proceed using load modulation. This means that the Power Receiver switches the amount of power that it draws from the Power Transmitter between two discrete levels (note that these levels are not fixed, but depend on the amount of power that is being transferred). The actual load modulation method is left as a design choice to the Power Receiver. Resistive, capacitive, and inductive schemes are all possible. On a logical level, the communications protocol uses a sequence of short messages that contain the relevant data. These messages are contained in Packets, which are transmitted in a simple UART like format.

Annex A provides two example Power Receiver designs. The design shown in the first example directly provides the rectified voltage from the Secondary Coil to a single-cell lithium-ion battery for charging at constant current or voltage. The design shown in the second example uses a post-regulation stage to create a voltage source at the output of the Power Receiver.

This version 1.1.1 of the System Description Wireless Power Transfer, Volume I, Part 1, does not define how a Power Transmitter should detect an object that is placed on the Interface Surface. Annex B discusses several example methods that a Power Transmitter can use. Some of these methods enable Power Transmitter implementations that use very low stand-by power—if there are no Power Receivers present on the Interface Surface, or if there are Power Receivers present that are not engaged in power transfer.

Annex C discusses a few use cases that deal with locating Power Receivers on the Interface Surface of a type B Power Transmitter. In particular, these use cases describe how to find the optimum location for the Active Area—through which the Power Transmitter provides power to the Power Receiver—and how to distinguish between multiple closely spaced Power Receivers.

Finally, Annex D discusses how a Power Transmitter should detect heating of Foreign Objects on its Interface Surface, using the power loss method. Typical examples of such Foreign Objects are parasitic metals such as coins, keys, paperclips, etc. If a parasitic metal is close to the Active Area it could heat up during power transfer due to eddy currents that result from the oscillating magnetic field. In order to prevent the temperature of such parasitic metal from rising to unacceptable levels, the Power Transmitter should timely abort the power transfer.

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The Power Conversion Unit on the right-hand side of Figure 3-1 comprises the analog parts of the design. The inverter converts the DC input to an AC waveform that drives a resonant circuit, which consists of the Primary Coil plus a series capacitor. Finally, the current sense monitors the Primary Coil current.

The Communications and Control Unit on the left-hand side of Figure 3-1 comprises the digital logic part of the design. This unit receives and decodes messages from the Power Receiver, executes the relevant power control algorithms and protocols, and drives the frequency of the AC waveform to control the power transfer. The Communications and Control Unit also interfaces with other subsystems of the Base Station, e.g. for user interface purposes.

3.2.1.1Mechanical details

Power Transmitter design A1 includes a single Primary Coil as defined in Section 3.2.1.1.1, Shielding as defined in Section 3.2.1.1.2, an Interface Surface as defined in Section 3.2.1.1.3, and an alignment aid as defined in Section 3.2.1.1.4.

3.2.1.1.1Primary Coil

The Primary Coil is of the wire-wound type, and consists of no. 20 AWG (0.81 mm diameter) type 2 litz wire having 105 strands of no. 40 AWG (0.08 mm diameter), or equivalent. As shown in Figure 3-2, the Primary Coil has a circular shape and consists of multiple layers. All layers are stacked with the same polarity. Table 3-1 lists the dimensions of the Primary Coil.

do

di

dc

Figure 3-2: Primary Coil of Power Transmitter design A1

Table 3-1: Primary Coil parameters of Power Transmitter design A1

3.2.1.1.2Shielding

As shown in Figure 3-3, soft-magnetic material protects the Base Station from the magnetic field that is generated in the Primary Coil. The Shielding extends to at least 2 mm beyond the outer diameter of the Primary Coil, has a thickness of at least 0.5 mm, and is placed below the Primary Coil at a distance of at most mm. This version 1.1.1 of the System Description Wireless Power Transfer, Volume I, Part 1, limits the composition of the Shielding to a choice from the following list of materials:

Material 44 — Fair Rite Corporation.

Material 28 — Steward, Inc.

CMG22G — Ceramic Magnetics, Inc.