APPLICATION NOTE
AN-886 REVISION 08/26/15 1 ©2015 Integrated Device Technology, Inc.
P9025AC FOD Tuning Guide AN-886
Introduction
The P9025AC wireless power receiver is a WPC-compliant integrated circuit used to receive wireless power inductively. To
achieve WPC compliance, the Foreign Object Detection (FOD) feature must be enabled and tuned on both the transmitter (Tx)
and Receiver (Rx).
In any wireless power transfer system where a magnetic field is induced to transfer energy, the power transfer flux is also present
outside the Tx and Rx coils. Although this field decays proportionally to the inverse square of the distance, it is typical that these
fields intersect with other permeable materials (such as copper, steel or Lithium Ion batteries). These ferrous materials that are
unrelated to the power transfer but necessary to the product, absorb a portion of the transmitted energy and appear as a system
loss in efficiency. Other ferrous objects that are not part of the system may also be present, such as keys, coins, or other foreign
objects - and they too may absorb energy during operation. As a safety precaution, the Tx will disable power transfer whenever
the losses are measured to be higher than a pre-determined threshold.
Most wireless power systems will have some permeable materials present by design on the Tx and Rx side of the system (such
as the final product casing). This is normal and can be accounted for by tuning the FOD parameters of the system. The P9025AC
has internal OTP registers that are used to configure the FOD parameters required to ignore losses from known system design
objects and only report losses associated with actual foreign objects. Additionally, the P9025AC offers an external global FOD
offset adjustment resistor from FOD2 pin to GND. This resistor can add a positive or negative offset to the reported power by the
device based on the pre-programmed FOD curve.
For WPC-compliant systems, the loss allowed due to foreign objects is 350 mW, not including known losses due to the Tx and
Rx systems. For systems where WPC compliance is not a requirement, FOD can be programmed to accept losses greater than
350 mW, or can be disabled completely.
Getting Started
The P9025AC has two external resistors for changing (tuning) the received power value that is reported to the Tx. The resistor
from pin FOD1 to GND is used to select one of several pre-programmed banks of FOD values. The resistor from pin FOD2 to
GND is an additional offset adjustment and shifts the entire reported received power curve up or down. Details on how to adjust
these values is explained later in this guide.
Before starting to tune the system FOD, careful consideration should be taken to determine to what degree FOD is required in
the application. When WPC compliance is not required, increasing the FOD loss threshold or disabling FOD completely can
significantly reduce the effort and costs associated with tuning. The three options are as follows:
Option #1: Disabling FOD – some applications will not need any form of FOD, such as fixed location chargers. This
eliminates the need for tuning completely. Skip to the Disabling FOD section for details on how to disable FOD.
Option #2: “Good Enough” FOD – some applications want FOD to a degree, but do not require a strict FOD detection
threshold. This is typical of systems where the Tx and Rx are paired, and where WPC-compliance is not necessary. This
type of tuning can be accomplished in a short time with just a few simple measurements and two external resistors. This
guide will instruct users on how to achieve this.
Option #3: WPC-compliant FOD – this is for applications that require Qi Certification from one of the WPC certification
centers. This typically requires precision tuning with the help of an FOD expert and special tools. This degree of tuning falls
outside the scope of this guide. IDT will refer customers to a third party support team for tuning and pre-certification testing.
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.