Dual Mode Coil Design Consideration
8
Dual Mode Coil Design Consideration
8.1
Shielding
Implementation of a shielding mechanism is recommended as part of receiver device. Shielding provides
protection from possible bi-directional interference between wireless charging system and consumer
electronic device. The interference causes may include coupling interference causing development of heat
due to eddy currents, impact on wireless charging data transfer, and so forth. It is recommended to apply
the shield on all the magnetically active area (between the secondary coil and the electronic device).
8.2
Receiver Detection: Attraction and Alignment
When a PMA Transmitter is in standby phase and a receiver is placed on the charging surface, the
transmitter detects the presence of the receiver by using either passive method with a “Hall Effect Sensor”
or by active method with digital pinging.
In the passive method, the transmitter uses a Hall Effect Sensor to detect the presence of a receiver coil.
It measures the voltage difference between no coil on TX and a full aligned coil on TX. If the difference in
the hall sensor measurements between the two cases is above 200 mV, the power transfer phase will
start as required by PMA. In the active method, it uses a digital pinging method instead. This detection
method uses a periodic short pulse or short burst of pulses applied to the primary coil. By measuring the
resultant interference on the primary coil, the presence of a receiver can be detected.
For both methods, a PMA-compliant receiver coil design shall include materials that can trigger the Hall
Effect Sensor and create enough interference on the digital pinging on the PMA transmitters. The
implementation is vendor specific and may rely on the magnetic material used for alignment aid, the
shielding, or any other material designers select.
The alignment aid ferrite should have no more than 2 mm distance from charged device outer surface.
The recommended shielding ferrite placed on the receiver antenna and the alignment aid fit into the hole
at the center of the shielding ferrite as indicated in
.
Figure 10. Dual Mode Coil Example
8.3
Inductor Value
A PMA-compliant receiver operating frequency is between 232 kHz and 278 kHz. For best efficient PMA
only systems, the recommended coil self-inductance is around 4-µH range and coil self-resistance is
around 300 m
Ω
. The operating frequency for PMA is higher than WPC. Thus, for dual mode solution the
inductance has to be increased to meet WPC requirements. For dual mode applications, a recommended
coil self-inductance is about 7.5 µH and coil self-resistance is around 300 m
Ω
.
In this EVM, the 760308102210 coil from Wurth electronics is used as an example for dual mode solution.
Other coils also can be used such as KNCWZ08C409 from Panasonic and ASC-353583M08-S0 V1.0 from
Amotech. The resonant capacitors are not required for PMA mode only. To support WPC and PMA, tuning
resonant caps according to WPC requirement and with the final configuration of the board is a must. Note
that the coil inductance and the required shielding may vary from application to application depending on
the final configuration of the board.
17
SLUUAX6B – February 2014 – Revised June 2014
Dual Mode (WPC and PMA) Integrated Wireless Receiver Power Supply
Copyright © 2014, Texas Instruments Incorporated
