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System Description

Wireless Power Transfer

4 Power Receiver Design Requirements

4.1Introduction

Figure 4-1 illustrates an example functional block diagram of a Power Receiver.

Figure 4-1: Example functional block diagram of a Power Receiver

In this example, the Power Receiver consists of a Power Pick-up Unit and a Communications and Control Unit. The Power Pick-up Unit on the left-hand side of Figure 4-1 comprises the analog components of the Power Receiver:

A dual resonant circuit consisting of a Secondary Coil plus series and parallel capacitances to enhance the power transfer efficiency and enable a resonant detection method (see Section 4.2.2.1).

A rectification circuit that provides full-wave rectification of the AC waveform, using e.g. four diodes in a full-bridge configuration, or a suitable configuration of active components (see Section 4.2.2.2). The rectification circuit may perform output smoothing as well. In this example, the rectification circuit provides power to both the Communications and Control Unit of the Power Receiver and the output of the Power Receiver

A communications modulator (see Section 4.2.2.4). On the DC side of the Power Receiver, the communications modulator typically consists of a resistor in series with a switch. On the AC side of the Power Receiver, the communications modulator typically consists of a capacitor in series with a switch (not shown in Figure 4-1).

An output disconnect switch, which prevents current from flowing to the output when the Power Receiver does not provide power at its output. In addition, the output disconnect switch prevents current back flow into the Power Receiver when the Power Receiver does not provide power at its output. Moreover, the output disconnect switch minimizes the power that the Power Receiver draws from the Power Transmitter when a Power Signal is first applied to the Secondary Coil.

A rectified voltage sense.

The Communications and Control Unit on the right-hand side of Figure 4-1 comprises the digital logic part of the Power Receiver. This unit executes the relevant power control algorithms and protocols; drives the communications modulator; controls the output disconnect switch; and monitors several sensing circuits, in both the Power Pick-up Unit and the load—a good example of a sensing circuit in the load is a circuit that measures the temperature of, e.g., a rechargeable battery.

Note that this version 1.1.1 of the System Description Wireless Power Transfer, Volume I, Part 1, minimizes the set of Power Receiver design requirements (see Section 4.2). Accordingly, compliant Power Receiver designs that differ from the example functional block diagram shown in Figure 4-1 are possible. For example, an alternative design includes post-regulation of the output of the rectification circuit (e.g., using a buck converter, battery charging circuit, power management unit, etc.). In yet another design, the Communications and Control Unit interfaces with other subsystems of the Mobile Device, e.g. for user interface purposes.

4.2Power Receiver design requirements

The design of a Power Receiver shall comply with the mechanical requirements listed in Section 4.2.1 and the electrical requirements listed in Section 4.2.2. In addition, a Power Receiver shall implement the relevant parts of the protocols defined in Section 5, as well as the communications interface defined in Section 6.

4.2.1Mechanical requirements

A Power Receiver design shall include a Secondary Coil, and an Interface Surface as defined in Section 4.2.1.1. In addition, a Power Receiver design shall include an alignment aid as defined in Section 4.2.1.2.

4.2.1.1Interface Surface

The distance from the Secondary Coil to the Interface Surface of the Mobile Device shall not exceed mm, across the bottom face of the Secondary Coil. See Figure 4-2.

Mobile

Device

dz

4.2.1.2Alignment aid

The design of a Mobile Device shall include means that helps a user to properly align the Secondary Coil of its Power Receiver to the Primary Coil of a Power Transmitter that enables Guided Positioning. This means shall provide the user with directional guidance—i.e. where to the user should move the Mobile Device—as well as alignment indication—i.e. feedback that the user has reached a properly aligned position.3

(Informative) An example of such means is a piece of hard or soft magnetic material, which is attracted to the magnet that is provided in Power Transmitter design A1. The attractive force should provide the user with tactile feedback, when placing the Mobile Device on the Interface Surface. Note that the Mobile Device cannot rely on the presence of any alignment support from the Base Station, other than the alignment aids specified in Section 3.

4.2.1.3Shielding

An important consideration for a Power Receiver designer is the impact of the Power Transmitter’s magnetic field on the Mobile Device. Stray magnetic fields could interact with the Mobile Device and potentially cause its intended functionality to deteriorate, or cause its temperature to increase due to the power dissipation of generated eddy currents.

It is recommended to limit the impact of magnetic fields by means of Shielding on the top face of the Secondary Coil. See also Figure 4-2. This Shielding should consist of material that has parameters similar to the materials listed in Sections 3.2.1.1.2 and 3.3.1.1.2. The Shielding should cover the Secondary Coil completely. Additional Shielding beyond the outer diameter of the Secondary Coil might be necessary depending upon the impact of stray magnetic fields.

The example Power Receiver designs discussed in Annex A.1 and Annex A.2 both include Shielding.

4.2.2Electrical requirements

A Receiver design shall include a dual resonant circuit as defined in Section 4.2.2.1, a rectification circuit as defined in Section 4.2.2.2, sensing circuits as defined in Section 4.2.2.3, a communications modulator as defined in Section 4.2.2.4, and an output disconnect switch as defined in Section 4.2.2.5.

4.2.2.1Dual resonant circuit

The dual resonant circuit of the Power Receiver comprises the Secondary Coil and two resonant capacitances. The purpose of the first resonant capacitance is to enhance the power transfer efficiency. The purpose of the second resonant capacitance is to enable a resonant detection method. Figure 4-3 illustrates the dual resonant circuit. The switch in the dual resonant circuit is optional. If the switch is not present, the capacitance shall have a fixed connection to the Secondary Coil . If the switch is present, it shall remain closed4 until the Power Receiver transmits its first Packet (see Section 5.3.1).

CS

Cd

LS

Figure 4-3: Dual resonant circuit of a Power Receiver

3The design requirements of the Mobile Device to determine the range of lateral displacements that constitute proper alignment.

4The switch shall remain closed even if no power is available from the Secondary Coil.

The dual resonant circuit shall have the following resonant frequencies:

In these equations, is the self inductance of the Secondary Coil when placed on the Interface Surface of a Power Transmitter and—if necessary—aligned to the Primary Cell; and is the self inductance of the Secondary Coil without magnetically active material that is not part of the Power Receiver design close to the Secondary Coil (e.g. away from the Interface Surface of a Power Transmitter). Moreover, the

other Power Receivers. The quality factor Q of the loop consisting of the Secondary Coil, switch (if

Figure 4-4 shows the environment that is used to determine the self-inductance of the Secondary Coil. The primary Shielding shown in Figure 4-4 consists of material PC44 from TDK Corp. The primary Shielding has a square shape with a side of 50 mm and a thickness of 1 mm. The center of the Secondary Coil and the center of the primary Shielding shall be aligned. The distance from the Receiver Interface Surface to the primary Shielding is mm. Shielding on top of the Secondary Coil is present only if the Receiver design includes such Shielding. Other Mobile Device components that influence the inductance of the Secondary Coil shall be present as well when determining the resonant frequencies—the magnetic attractor shown in Figure 4-4 is example of such a component. The excitation signal that is used to determine and shall have an amplitude of 1 V RMS and a frequency of 100 kHz.

Primary Shielding

Figure 4-4: Characterization of resonant frequencies

4.2.2.2Rectification circuit

The rectification circuit shall use full-wave rectification to convert the AC waveform to a DC power level.

4.2.2.4Communications modulator

The Power Receiver shall have the means to modulate the Primary Cell current and Primary Cell voltage as defined in Section 6.2.1.5 This version 1.1.1 of the System Description Wireless Power Transfer, Volume I, Part 1, leaves the specific loading method as a design choice to the Power Receiver. Typical example methods include modulation of a resistive load on the DC side of the Power Receiver, and modulation of a capacitive load on the AC side of the Power Receiver.

4.2.2.5Output disconnect

The Power Receiver shall have the means to disconnect its output from the subsystems connected thereto. If the Power Receiver has disconnected its output, it shall ensure that it still draws a sufficient amount of power from the Power Transmitter, such that Power Receiver to Power Transmitter communications remain possible (see also Section 6.2.1).

The Power Receiver shall keep its output disconnected until it reaches the power transfer phase for the first time after a Digital Ping (see also Section 5). Subsequently, the Power Receiver may operate the output disconnect switch any time while the Power Transmitter applies a Power Signal. This also means that the Power Receiver may keep its output connected if it reverts from the power transfer phase to the identification & configuration phase.

(Informative) Note that the Power Receiver may experience a voltage peak when operating the output disconnect switch (and changing between maximum and near-zero power dissipation).

4.3Power Receiver design guidelines (informative)

4.3.1Large-signal resonance check

In the course of designing a Power Receiver, it should be verified that the resonance frequency of the dual resonant circuit remains within the tolerance range defined in Section 4.2.2.1, under large-signal conditions. The test defined in this Section 4.3.1 serves this purpose.

Step 1. Connect an RF power source to the assembly of Secondary Coil, Shielding and other components that influence the inductance of the Secondary Coil—e.g. a magnetic attractor, see Figure 4-4—and series resonant capacitance ; see Figure 4-5. The presence of the parallel capacitance is optional.

Figure 4-5: Large signal secondary resonance test

Step 2. Position the assembly and an appropriate spacer on primary Shielding material, as shown in Figure 4-4.

Step 3. Measure the input voltage as a function of the frequency of the RF power source in the range of

5(Informative) Note that the dual resonant circuit as depicted in Figure 4-3 does not prohibit implementation of the communications modulator directly at the Secondary Coil.