One of the most common ways 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 which occurs on the supporting circuitry. Power loss on the power receiver side may occur in the
following locations:
• The receiving coil and series resonant capacitors
• The rectifier
• The post-regulation stage
• In tracks and cables
• Other locations
A high power loss value indicates the probable presence of a foreign object.
STWLC38 uses precise internal AD converters to perform voltage and current measurements and a 32-bit
arithmetic to keep sufficient accuracy. STWLC38 then uses the measured values to create a mathematical model
to estimate the received power, which are then sent to the power transmitter for evaluation.
For more information regarding FOD please refer to the Qi specification.
5.10 WPC Qi wireless power transfer
5.10.1 Wireless power interface
Wireless power interface is the area in which power transfer takes place. It consists of the transmitter (primary)
power interface and the receiver (secondary) power interface. The main component of the interfaces is the
transmitting/receiving coil. The user should avoid placing any objects which are magnetically active and not
required for power transfer on or near the wireless power interface in order to avoid possible damage or injury
(see Section 5.9.7 Foreign object detection (FOD)). Detection of a valid receiver on the primary wireless power
interface is performed by analog and digital pings.
Analog ping is a short burst generated by the power transmitter. The power transmitter then evaluates if the
resonant frequency and/or resonant circuit capacitance changed, as such change might indicate presence of a
potential power receiver. However, the burst does not carry sufficient energy to wake the power receiver.
Digital ping is a more energetic burst of power generated by the transmitter; its parameters are defined by the Qi
specification. The target of this burst is to generate enough power for the receiver to establish communication
between the transmitter and the receiver.
The Qi specification defines a specific way the receiver is expected to answer the digital ping. The reason is that
only a proper answer can inform the transmitter that a valid power receiver was placed on the wireless power
interface.
UM3154
WPC Qi wireless power transfer
UM3154 - Rev 2
page 27/81
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