4.9.2 Comparing to reference Q factor
According to the Qi specification, an EPP Rx must provide its reference Q-factor and resonant frequency to the Tx
during the negotiation phase. The firmware of the device compares the received and measured values, and
based on the difference estimates whether a foreign object is present. If the presence of an FO is suspected,
power transfer is stopped and the Event FOD_DURING_NEGOTIATION is indicated in the GUI.
4.9.3 Power balance
One of the most common ways of detecting the presence of FOD is estimating the amount of power lost in the
system. The power receiver indicates the total amount of received power from the power transmitter. This power
consists of power available to the load and power loss on the supporting circuitry.
A high-power loss value could indicate the presence of a foreign object.
STWBC2 calculates the FOD metric, which consists of Rx received power (including Rx loss estimation),
transmitted power and other related parameters.
For more information regarding FOD, refer to the FOD tuning guide.
4.10 EPT reason
Power transfer may be terminated for several different reasons. Determining the exact cause might be difficult, as
more than one fault condition might have been met.
For this reason, the STSW-WBC2STUDIO provides the events log, which indicates the key events of the power
transfer.
An important part of those events is also the end of the power transfer and its reasons. The end of the power
transfer (EPT) may be initiated both by Rx and Tx. If the EPT is initiated by the Rx, the Rx shall indicate the
reason in the EPT packet, as specified by the Qi specification. Upon receiving such a packet, the reason is shown
in the event window of STSW-WBC2STUDIO.
Similarly, if an end of power transfer is initiated by the Tx, the reason is shown in the event window.
4.11
WPC Qi wireless power transfer
4.11.1 Baseline and extended power profiles
The flowchart in the figure below shows the steps required to reach power transfer in the Baseline Power Profile
(BPP) and Extended power profile (EPP) according to Qi 1.3.
Figure 5. Difference between BPP and EPP power transfer startup
UM3286
Device description and operation
UM3286 - Rev 1
page 12/84
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