Application Note
5 of 36
002-34970 Rev. **
2022-05-03
Foreign object detection tuning guide for wireless power transmitters
Applicable for WLC ICs
Getting started
2
Getting started
The WLC power transmitter provides Qi-compliant FOD features. Refer to the FOD chapter in the Qi Standard
for further details (see
and
). The following sections discuss the WLC power transmitter implementation
of the FOD functionality.
2.1
FOD mechanism
The Qi Standard defines two mechanisms for FOD:
1.
FOD before power transfer or pre-power transfer method
2.
FOD during power transfer or in-power transfer method
2.1.1
FOD before power transfer
The pre-power transfer FOD method has the following mechanisms to detect a FO before power transfer
between Rx and Tx starts:
1.
Q factor FOD (FOD/qf)
2.
Resonance frequency FOD (FOD/rf)
The receiver sends FOD/rf and/or FOD/qf packets during the negotiation phase. The Tx responds to these
packets with an ACK packet if no FO is found and with a NAK packet if a FO is found. The Rx measures the
Q factor and resonance frequency values with a standard MP-A1 coil. The transmitter may measure Q factor
and resonance frequency values different from the receiver-reported values, as it may have a different coil. A
scaling factor is used to convert the receiver-reported Q factor and resonance frequency values to those
equivalent to the transmitter coil.
The scaling factor is a function of the shielding material and friendly materials present in the Rx. This is derived
by looking at the relation between the Q factor and resonance frequency devised by the MP-A1 coil and the
experimentally measured Q factor and resonance frequency values. The scaling factors for Q factor and
resonance frequency measurements can be unique. The FOD/qf packets are sent by all extended power profile
(EPP) receivers compliant with Qi 1.2 or above. The FOD/rf packets are sent by all EPP receivers compliant with
Qi 1.3 or above.