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voltage developed across the series capacitance can reach levels exceeding 200 V pk-pk.

Power Transmitter design A1 uses the Operating Frequency and duty cycle of the Power Signal in order to control the amount of power that is transferred. For this purpose, the Operating Frequency range of the half-bridge inverter is kHz with a duty cycle of 50%; and its duty cycle range is 10…50%

at an Operating Frequency of 205 kHz. A higher Operating Frequency or lower duty cycle result in the transfer of a lower amount of power. In order to achieve a sufficiently accurate adjustment of the amount of power that is transferred, a type A1 Power Transmitter shall control the Operating Frequency with a resolution of

or better. In addition, a type A1 Power Transmitter shall control the duty cycle of the Power Signal with a resolution of 0.1% or better.

When a type A1 Power Transmitter first applies a Power Signal (Digital Ping; see Section 5.2.1), it shall use an initial Operating Frequency of 175 kHz (and a duty cycle of 50%).

Control of the power transfer shall proceed using the PID algorithm, which is defined in Section 5.2.3.1. The controlled variable ( ) introduced in the definition of that algorithm represents the Operating Frequency. In order to guarantee sufficiently accurate power control, a type A1 Power Transmitter shall determine the amplitude of the Primary Cell current—which is equal to the Primary Coil current—with a resolution of 7 mA or better. Finally, Table 3-2, Table 3-3, and Table 3-4 provide the values of several parameters, which are used in the PID algorithm.

LP

Figure 3-4: Electrical diagram (outline) of Power Transmitter design A1

System Description

Wireless Power Transfer

3.2.2Power Transmitter design A2

Power Transmitter design A2 enables Free Positioning. Figure 3-5 illustrates the functional block diagram of this design, which consists of three major functional units, namely a Power Conversion Unit, a Detection Unit, and a Communications and Control Unit.

Figure 3-5: Functional block diagram of Power Transmitter design A2

The Power Conversion Unit on the right-hand side of Figure 3-5 and the Detection Unit of the bottom of Figure 3-5 comprise the analog parts of the design. The Power Conversion Unit is similar to the Power Conversion Unit of Power Transmitter design A1. 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. The Primary Coil is mounted on a positioning stage to enable accurate alignment of the Primary Coil to the Active Area of the Mobile Device. Finally, the voltage sense monitors the Primary Coil voltage.

The Communications and Control Unit on the left-hand side of Figure 3-5 comprises the digital logic part of the design. This unit is similar to the Communications and Control Unit of Power Transmitter design A1. The Commnuications and Control Unit receives and decodes messages from the Power Receiver, executes the relevant power control algorithms and protocols, and drives the input voltage of the AC waveform to control the power transfer. In addition, the Communications and Control Unit drives the positioning stage and operates the Detection Unit. The Communications and Control Unit also interfaces with other subsystems of the Base Station, e.g. for user interface purposes.

The Detection Unit determines the approximate location of objects and/or Power Receivers on the Interface Surface. This version 1.1.1 of the System Description Wireless Power Transfer, Volume I, Part 1, does not specify a particular detection method. However, it is recommended that the Detection Unit exploits the resonance in the Power Receiver at the detection frequency (see Section 4.2.2.1). The

16 © Wireless Power Consortium, July 2012

reason is that this approach minimizes movements of the Primary Coil, because the Power Transmitter does not need to attempt to identify objects that do not respond at this resonant frequency. Annex C.3 provides an example resonant detection method.

3.2.2.1Mechanical details

Power Transmitter design A2 includes a single Primary Coil as defined in Section 3.2.2.1.1, Shielding as defined in Section 3.2.2.1.2, an Interface Surface as defined in Section 3.2.2.1.3, and a positioning stage as defined in Section 3.2.2.1.4.

3.2.2.1.1Primary Coil

The Primary Coil is of the wire-wound type, and consists of litz wire having 30 strands of 0.1 mm diameter, or equivalent. As shown in Figure 3-6, the Primary Coil has a circular shape and consists of multiple layers. All layers are stacked with the same polarity. Table 3-5 lists the dimensions of the Primary Coil.

do

di

dc

Figure 3-6: Primary Coil of Power Transmitter design A2

Table 3-5: Primary Coil parameters of Power Transmitter design A2