P9025AC FOD TUNING GUIDE
2
REVISION 08/26/15
AN-886
FOD Tuning Overview
The WPC Foreign Object Detection (FOD) method involves continuous measurement by the Tx and Rx devices independently.
The Rx reports the measured output power to the Tx through the wireless communication. The Tx then compares the values to
the input power to check if an excessive amount of power is being lost somewhere in-between. If the product is part of a
WPC-compliant system, the acceptable foreign object losses in excess of the known losses is 350 mW or less. For
non-compliant systems, the acceptable losses can be higher, or disabled all together.
During system development, the known Tx power losses need to be determined. Some power is consumed by the transmitter
in the form of digital processing, losses in the DC to AC inverter, passive components, and metallic objects needed for the product
(such as cases, screws, copper foil). These losses are inherent to the specific Tx and are determined by measuring the Tx
against a calibrated Rx. These known losses should be subtracted from the calculation when the system checks for a foreign
object. These losses will be referred to as TX
LOSSES
:
where,
Vin = input voltage to the Tx controller IC (measured at REG_IN pin on P9038)
Iin = input current to the Tx module (measured across RSNS (R2) on P9038)
TX
LOSSES
= Sum of power consumed by Tx system to generate B-field for power transfer and decode communication
packets
Respectively, the Rx device measures the power that it is receiving (i.e. delivering to the load) and reports that value to the Tx
module. Since the Rx circuit and system also consume power during operation, these fixed losses must be added to the reported
received power value so that they are not considered as foreign object losses in the transmitter's calculation. Unfortunately, the
fixed losses are not linear with respect to the load current. Therefore, the P9025AC device has been pre-programmed with
multiple FOD curves that can be selected using the resistor on pin FOD1. These FOD1 settings are intended to include the Rx
operational losses and other end-product specific metallic material losses, such as the receiver's casing. The reported power
calculation is as follows:
where,
OUT = output voltage of the Rx device
Iout = output current from the Rx device
RXLOSSES = Sum power consumed by Rx product to rectify B-field for power delivery determined by the FOD1 bank
selection and the FOD2 resistance to GND.
where,
5V= basis voltage of the Rx device power reporting calculation
Iout = output current from the Rx device
mcf(region) = programmed slope adjustment of FOD tuning by power region
bcf(region) = programmed offset adjustment of FOD tuning by power region
FOD2 Offset = Additional offset set by FOD2 resistor (sometimes referred to as Pa)
The regions vary based on the output current. There are 5 regions, so there are 5 mcf values and 5 bcf values that need to be
programmed in order to report correct RXLOSSES value to the Tx based on the selected FOD1 bank.
