System Description
Wireless Power Transfer
Version 1.0 System Overview (Informative)
© Wireless Power Consortium, July 2010 7
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 1.0 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.
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Note 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.
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