4.8.3 Voltage drop
Input voltage drop may indicate an overload of the device. The input voltage is therefore monitored and compared
to a threshold set in the parameters. Either an absolute or a relative threshold can be selected for this protection.
The relative drop is related to the Vmax declared by USB PD for current voltage level or to the Vin voltage at
startup in case of an application without USB PD supply. The default is a relative drop of 1500 mV.
4.8.4 RX Overvoltage protection – static/dynamic OVP, CurrentDrop OVP
STWBC2 features static and dynamic OVP, which protects the receiver from an excessive VRECT voltage.
Static OVP estimates the Rx VRECT based on the operational parameters of the transmitter. When a set
threshold is exceeded, the transmitter ignores all CE packets, handling them as if they were 0. The dynamic OVP
works similarly. However, its purpose is to prevent sudden changes of the VRECT voltage.
The current drop OVP also protects the Rx from excessive voltage, in this case caused by a sudden drop of the
bridge current. When triggered, the Tx terminates the power transfer.
These and other protections are further described in a separate application note.
4.8.5 Overtemperature protection (OVTP)
STWBC2 monitors its internal temperature to prevent reaching dangerous temperatures. There are two
overtemperature thresholds - high temperature and very high temperature. In case of the first threshold being
reached, STWBC2 stops the power transfer. When the second is reached, STWBC2 resets. The thresholds are
by default set to 110 °C and 115 °C, respectively.
4.8.6 NTC
The STWBC2 can monitor voltage from up to two NTC thermistors. These NTCs can monitor critical points of the
board - the H-bridge transistors, DC-DC, main Tx coil, etc.
The STEVAL-WBC2TX70 features two NTCs: one placed near the DC-DC capacitors and the second one close
to the H-bridge. The default threshold is 70 °C. When the threshold is reached, power transfer is terminated.
4.8.7 QFOD
Large metal objects, placed on the power interface, may draw high current during Tx digital pings. To prevent
those, STWBC2 features a QFOD function, which evaluates the Q factor of the coil. If presence of a FO is
suspected, the Tx stops performing the digital ping, until the object is removed.
The QFOD is affected by calibration of the device, which shall be performed after any major change to the board
and/or firmware (the calibration must be performed with no object present on the Tx coil. The calibration can be
executed from the GUI or by appropriate UART command).
If the presence of an FO is suspected, the device does not continue with power transfer and the Event QFOD is
indicated in the GUI.
4.9
Foreign object detection (FOD)
Foreign object is any object placed either on or near the transmitting coil, which is not considered a valid wireless
power receiver and is magnetically active. The presence of the magnetic field generated by the transmitting coil
may cause eddy currents to form in the foreign object (such as coins, keys etc.), which in turn would heat the
object to potentially dangerous temperatures. To avoid possible damage to the device or even injury of the user,
the power transmitter must be able to detect the presence of a foreign object. This detection can be implemented
in several different ways.
STWBC2 implements three methods of foreign object detection. Two are based on Q-factor and resonant
frequency measurements, the third one is based on comparing the difference between Tx transmitted and Rx
received energy.
4.9.1 Presence detection
This method is based on a change of the resonant frequency and Q-factor when an object is placed onto the Tx
coil. A foreign object placed on the power interface may cause damage to the device by both high temperature
and high current, drawn by the device during digital ping.
Another purpose of this function is to lower the power consumption of the device by preventing unnecessary
digital pings when there is no object on the Tx coil.
The presence detection uses the same hardware and a similar principle as Section 4.8.7: QFOD.
UM3286
Device description and operation
UM3286 - Rev 1
page 11/84
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