estimates results in a difference between the Transmitted Power and the Received Power, even if there is
no Foreign Object 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 Power RX execute the calibration phase before the power transfer phase starts. The TX needs to verify
that there is no FO present on its interface surface before the calibration phase and FOD based on Q factor
could work.
As the bias in the estimates can be dependent 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 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 a RX baseline is charged by NXP WCT-15WTXAUTO, only the power loss FOD baseline works.
If a RX extension is placed on NXP WCT-15WTXAUTO, the Q factor would be measured at first to
detect if there is a FO presents. If yes, the TX would stop charging; otherwise, the TX can proceed to
calibration phase and power transfer phase, and power loss FOD extension works to detect if a FO is
inserted during power transfer phase.
For details of FOD, see the
WCT1011A /WCT1013A Automotive MP-A9 Run-Time Debug User’s Guide
(WCT101XARTDUG).
4.7 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 TX to compare and determine if FO exists, as shown in
Figure 10
.
The reference Q factor is defined as the Q factor of Test Power Transmitter #MP1’s Primary Coil at an
operating frequency of 100 kHz with RX positioned on the interface surface and no FO nearby. Due to the
differences between its design and that of Test Power Transmitter #MP1, the difference between the
frequency it uses to determine its Q factor and 100 kHz, the TX needs to convert the Q factor it measured
WCT1011A/WCT1013A Automotive MP-A9 Wireless Charging Application User’s Guide, Rev. 0, 10/2017
12
NXP Semiconductors
