WCT1012VLF/WCT1013VLH Consumer MP-A11 (WCT-
15W1CFFPD) V1.0 Wireless Charging Application User’s
Guide, Rev. 1, 05/2019
12
NXP Semiconductors
Power loss FOD extensions
Typically, the RX estimates the power loss inside itself to determine its received power. Similarly, the
TX estimates the power loss inside itself to determine its transmitted power. A systematic bias in these
estimates results in a difference between the transmitted power and the received power, even if there is
no Foreign Object (FO) present on the interface surface.
To increase the effectiveness of the power-loss
method, the TX can remove the bias in the calculated power loss by calibration.
For this purpose, the TX
and the power RX execute the calibration phase before the power transfer phase starts. The TX must
verify that there is no FO present on its interface surface before the calibration phase and FOD based on
the Q factor can work.
Because the bias in the estimates may depend on the power level, the TX and RX determine their
Transmitted Power and Received Power at two load conditions—a “light” load and a “connected” load.
The “light” load is close to the minimum expected output power, and the “connected” load is close to
the maximum expected output power. Based on the two load conditions, the power transmitter can
calibrate its transmitted power using linear interpolation.
Alternatively, the power transmitter can
calibrate the reported received power.
Take the calibrated transmitted power as an example:
𝑃
𝑃𝑇
𝑐𝑎𝑙
= 𝑎 ∗ 𝑃
𝑃𝑇
+ 𝑏
a =
𝑃
𝑃𝑅
(𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑)
− 𝑃
𝑃𝑅
(𝑙𝑖𝑔ℎ𝑡)
𝑃
𝑃𝑇
(𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑)
− 𝑃
𝑃𝑇
(𝑙𝑖𝑔ℎ𝑡)
b =
𝑃
𝑃𝑇
(𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑)
∗ 𝑃
𝑃𝑅
(𝑙𝑖𝑔ℎ𝑡)
− 𝑃
𝑃𝑅
(𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑)
∗ 𝑃
𝑃𝑇
(𝑙𝑖𝑔ℎ𝑡)
𝑃
𝑃𝑇
(𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑)
− 𝑃
𝑃𝑇
(𝑙𝑖𝑔ℎ𝑡)
Therefore, the TX uses the calibrated transmitted power to determine the power loss as follows:
𝑃
𝐿𝑂𝑆𝑆
= 𝑃
𝑃𝑇
𝑐𝑎𝑙
− 𝑃
𝑃𝑅
When the MP-A11 transmitter charges an RX baseline, only the power-loss FOD baseline works. If an
RX extension is placed on the MP-A11 transmitter, the Q factor is measured first to detect if there is an
FO present. If yes, the TX stops charging; otherwise, the TX can proceed to the calibration phase and
the power transfer phase, and the power-loss FOD extension works to detect if an FO is inserted during
the power transfer phase.
For more details about the FOD, see the
WCT1012VLF /WCT1013VLH Consumer MP-A11 Run-Time
Debug User’s Guide
(document WCT101XRTDUG).
4.8. FOD based on Q factor change
A change in the environment of the TX coil typically causes its inductance to decrease or its equivalent
series resistance to increase. Both effects lead to a decrease of the TX coil’s Q factor. The RX sends a
packet including the reference Q factor for the TX to compare and determine if the FO exists, as shown
in
Figure 10
.