ST STDES-WBC86WTX Instructions for use

Introduction
The STDES-WBC86WTX reference design, based on STWBC86, is designed for wireless power transmitter applications. It
allows the user to start a 2.5 W wireless charging project quickly.
The integrated circuit requires only a few external components, such as ASK (Qi based communication) demodulation circuit.
The board can work with 5 to 12 V input voltage.
Using an external USB-to-I²C converter, you can monitor and control the STWBC86 using the STSW-WPSTUDIO GUI.
The STDES-WBC86WTX includes several safety mechanisms that provides overtemperature (OTP), overcurrent (OCP), and
overvoltage (OVP) protections.
The STDES-WBC86WTX is a fully assembled reference design developed for performance evaluation only, not available for
sale.
This document summarizes the following fundamental evaluation steps:
• Power-up test
• ASK demodulation test
• Efficiency test
• Thermal performance test
STDES-WBC86WTX Wireless Power Transmitter Quick Start Guide and Test
Report
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Technical note
TN1442 - Rev 2 - August 2023
For further information contact your local STMicroelectronics sales office.
www.st.com

1 Overview
To get started with the STDES-WBC86WTX, you need the following equipment:
•STDES-WBC86WTX
• Additional hardware:
– 1 x USB adapter 5 V/1.5 A or power supply
– 1 x USB cable (can be replaced with a pin cable)
– 1 x USB-to-I²C external board (using MCP2221 or FT260) with pin cable and connection to a PC
– Windows PC
• Software:
–STSW-WPSTUDIO wireless power studio PC GUI installation package
– I²C drivers
Begin by installing the I²C drivers and the STSW-WPSTUDIO GUI.
Connect a 5 V power supply to power the board using the USB or the pin cable. Using a jumper, select your
preferred method of power delivery on the JP4 header.
Using an external USB-to-I²C converter, connect the board to your PC (connector P3 on the board). This allows
you to communicate with the board, program it, and monitor its functions.
Figure 1. STDES-WBC86WTX reference design
Fully assembled board developed for
performance evaluation only,
not available for sale
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Overview
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Figure 2. STWBC86 block diagram
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Overview
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2 Specifications
The STDES-WBC86WTX evaluation board is optimized for performance. The board features:
• STWBC86 wireless power transmitter chip with BPP 1.2.4 compatible FW
• Very few external components, optimized BOM and PCB space
• On-chip high efficiency full bridge inverter
• 32-bit, 64MHz ARM Cortex micro controller with 8KB SRAM
• 9-channel, 10-bit A/D Converter
• On-chip thermal management and protections
• I²C interface for communication with host system (optional)
Figure 3. STDES-WBC86WTX main blocks
In the figure above:
• the grey box indicates the coil connection
• the light blue box indicates the series resonant capacitors (Ctank) and the transmitting coil from a resonant
circuit. The resonant circuit transmits the power signal. So, any components/tracks involved should be
rated accordingly
• the fuchsia box indicates the CBT1 and CBT2, which are bootstrapping capacitors and ensure the proper
functionality of the integrated inverter. This should be taken into consideration during the PCB design as
these nets generate noise. Therefore, they should be routed separately from sensitive circuits
• the red box indicates the ASK demodulation circuit. Apart from transferring power, the power signal is also
used for receiver to transmitter communication. The communication signal is extracted from the power
signal using the ASK demodulation circuit and fed into the VS pin of the STWBC86 for processing
• the yellow box indicates the connection for the external USB/I²C converter. It provides a communication
channel between a PC and the STWBC86. Note that the P3 header connects the external converter I²C
signals to the STWBC86 I²C signals.
• the lilac box indicates the power input (USB connector/pin header). Two separate inputs can be used to
power the board. However, only one must be used at once to prevent damage to the power supplies.
Therefore, it is necessary to select the input using a jumper on the JP4 header.
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Specifications
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2.1 Test points
STDES-WBC86WTX features several connectors and test points to provide easy access to key signals.
Figure 4. STDES-WBC86WTX test points
Table 1. STDES-WBC86WTX test points
Connector/test point Name Description
P1 Coil connection Transmitter coil connection
AC1_COIL AC1_COIL AC1_COIL signal sensing
AC1 AC1 Resonant circuit terminal
AC2 AC2 Resonant circuit terminal
Vrect Vrect Inverter voltage sensing
Vout Vout Input voltage sensing
VS VS VS signal sensing
5V0 5V0 5 V LDO output
V1V8 V1V8 1.8 V LDO output
GND GND Ground
P3 Digital interface I²C and RST signals
JP1, JP2, JP3 Pull up connection Pull-ups connection for I²C
JP4 Vin selection Input selection
J1 Input voltage USB power input
P2 Input voltage Pin cable power input
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Test points
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2.2 Reference design specifications
The STDES-WBC86WTX target specifications are listed in the table below.
Table 2. STDES-WBC86WTX design specifications
Parameter Description
Tx application PCB area 16 mm x 11 mm
Tx coil specifications Inductance 6.8 µH, DCR 110 mOhm
Dimensions 20.5 mm diameter x 2.8 mm
Input voltage (Vin) 5 V
Input current (Iin) 1 A
Host MCU STM32 used as a reference, the reference I²C driver can be ported to any other
MCU family
Efficiency 58.64% (2.5 W operation) with STDES-WLC38WA
60.23% (peak efficiency) with STDES-WLC38WA at 2 W
Applicable charging gap between Tx
and Rx coils (z-distance)
4.4 mm (2.5 W output) with STDES-WLC38WA receiver, maximum 7 mm – stable
communication without output enabled
Operational modes Transmitter only
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Reference design specifications
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3 Test setup
3.1 Test conditions and equipment
The test setup consists of:
• a power supply (HMP4040 by Rohde & Schwarz)
• a pin power cable
• the STDES-WBC86WTX as a transmitter
• the STDES-WLC38WA as a receiver
• an electronic load in CC mode (BK Precision 8500)
Figure 5. STDES-WBC86WTX and STDES-STWLC38WA test setup
3.2 Test procedure
To achieve the best performance, follow the procedure below.
Step 1. Test the power supply.
The board is powered by voltage supplied by either a USB or a pin cable. This voltage can be
monitored via the VOUT test point. The input voltage also supplies power to the internal 5 and 1.8 V
LDOs. The regulator outputs can be monitored using the appropriate test points. The STDES-
WBC86WTX derives its own power supply directly from the input voltage.
The aim of the test is to check whether the device is powered correctly. The table below shows the
reference range.
Table 3. Internal regulators: output voltage ranges
Item Minimum [V] Typical [V] Maximum [V]
V5V0 3.5 5.0 5.5
V1V8 1.62 1.8 1.98
Step 2. Download the firmware.
The firmware offers all standard functionalities. The device automatically starts in TX mode and starts
pinging after the power-up.
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Test setup
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Step 3. Test the ping functionality.
During pinging, the input current switches between two distinct values periodically. The AC1 and AC2
voltage oscillates with a 5 V amplitude.
Step 4. Test the power transfer start.
Place the receiver onto the transmitter and wait for the status LED indicating an ongoing power transfer
on the receiver to light up. The output should be enabled shortly. A switching waveform can be seen on
the AC1 and AC2 pins.
Step 5. Test the ASK demodulation.
The demodulated signal can be seen on the VS test point. The signal should be clear and carry Qi
encoded packets. This signal is fed to the STWBC86 for communication decoding.
Step 6. Test the efficiency.
Establish the power transfer between the STDES-WBC86WTX and the STDES-WLC38WA. Measure
the input power and output power of the system and calculate the efficiency.
Step 7. Test the thermal performance.
Establish the power transfer and let the system achieve a steady operating temperature. Read the
temperature using either the device internal thermal sensor or a thermal camera.
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Test procedure
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4 Measurements, waveforms, and test data
4.1 Power-up waveform
The figure below shows a startup waveform, from the transmitter perspective.
Figure 6. Start-up sequence of the STWBC86WTX and STWLC38WA
The pinging start can be identified from the AC1 and AC2 waveforms. After registering the power signal, the
receiver initializes communication. When the initial communication is resolved, the receiver enables its output.
The power receiver maintains communication with the transmitter over the course of the power transfer, as it must
provide feedback to ensure a proper regulation.
4.2 ASK packet example
In the figure below, an ASK packet communication example is shown. The VS signal is demodulated from the
power signal.
Figure 7. ASK packet example
The message shown in the figure is a Qi-based CE packet, providing regulation feedback to the transmitter.
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Measurements, waveforms, and test data
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4.3 Typical performance characteristics
The following table shows the charging performance of an STDES-WBC86WTX/STDES-WLC38WA (TX/RX)
setup at various load currents, with the temperature being measured after 5 minutes of continuous operation.
Table 4. STDES-WBC86WTX typical performance
Vin [V] (TX) Iin [mA] (TX) Vout [V] (RX) Iout [mA] (RX)
5,09 263 5,035 100
5,042 382 5,032 200
5,026 502 5,032 300
4,986 670 5,03 400
4,985 860 5,028 500
4.4 Efficiency and spatial freedom
Efficiency is one of the most important metrics of wireless charging performance evaluation. Spatial freedom, the
size of the area for the XY plane in which a power receiver can be placed on the power transmitter, which still
allows sufficient power to be transmitted, is another important metric.
The z-axis distance between the coils, also known as the charging gap, is an additional parameter, which
significantly affects charging performance. Therefore, the STDES-WBC86TX was also tested at various charging
gap distances.
Efficiency and spatial freedom of the STDES-WBC86TX were measured with the STDES-WLC38WA as the
receiver. The efficiency was measured from the transmitter DC input to the receiver DC output. The measurement
does not include any power losses in the wall adapter or the USB cable.
A typical setup is with the fully aligned coils with a 3 mm charging gap (2 mm Tx spacer and 1 mm Rx spacer).
The maximum efficiency achieved with this setup was 60.23%.
Figure 8. Efficiency measurements of the STDES-STWLC38WA and STDES-WBC86TX
The transmitter can deliver 2.5 W even with a 4 mm misalignment in the XY plane or a 4.4 mm charging gap. It is
also able to start communication, with disabled Vout, with up to 7 mm misalignment in the XY plane or distance in
the Z axis.
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Typical performance characteristics
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