STDES-WLC38TWS Wireless Power Receiver Quick Start Guide and Test Report

Introduction

The STDES-WLC38TWS reference design, based on the STWLC38, is designed for wireless power receiver applications. It enables users to quickly start a 2.5 W wireless charging project. The integrated circuit requires minimal external components and can operate with an adjustable output voltage, with a default of 5 V. Users can monitor and control the STWLC38 using the STSW-WPSTUDIO GUI via an external USB-to-I²C converter. The STDES-WLC38TWS incorporates safety mechanisms such as overtemperature (OTP), overcurrent (OCP), and overvoltage (OVP) protections, which can trigger an end power transfer (EPT) packet, disable the device output, or short coil inputs.

Figure 1: STDES-WLC38TWS reference design

[Image description: A fully assembled board developed for performance evaluation only, not available for sale. It shows the STDES-WLC38TWS reference design with various components and connections labeled.]

The STDES-WLC38TWS reference design is ready to use, featuring a small size and good thermal performance up to 2.5 W, with stable power transfer when used with standard Qi wireless chargers. To begin, install the I²C drivers and the STSW-WPSTUDIO GUI. Connect the board to your PC using an external USB-to-I²C bridge (connector P2 on the USB-I²C bridge) to communicate, program, and monitor the board's functions. The GUI supports MCP2221 and FT260Q-T USB-I²C converters with standard connections as detailed in the USB-I²C bridge datasheet.

Overview

The STDES-WLC38TWS is optimized for performance and includes the following features:

• High efficiency (98% typical) synchronous rectifier operating up to 800 kHz
• Low drop-out linear regulator with output current limit and input voltage control loop
• Adaptive rectifier configuration (ARC) mode for enhanced spatial freedom
• 4 V to 12 V programmable output voltage
• 32-bit, 64 MHz Arm® Cortex® MO+ core with 32kB RRAM, 16 KB SRAM, 64kB ROM
• 10-bit A/D converter
• Configurable GPIOs
• I²C slave interface
• Multilevel ASK modulator, enhanced FSK demodulator
• Output overvoltage clamping protection
• Accurate voltage/current measurement for foreign object detection (FOD)
• On-chip thermal management and protections
• Flip chip 40 bumps (2.12 mm x 3.32 mm) package
• Small, ready-to-use 20 mm board
• I²C connector, GPIO, and INT connector, SOVP resistor

Figure 2: STDES-WLC38TWS connection overview

[Image description: A diagram showing the STDES-WLC38TWS board connections. Key components and connectors like COIL, SOVP protection, VRECT, VOUT, GPIO, GND, AC1, CTANK, SCL, SDA, and USB-I²C are labeled. It also indicates VRECT and VOUT capacitors and the SOVP resistor.]

The STDES-WLC38TWS is equipped with all necessary components for standalone operation. The TWS coil connects to pads (COIL1 and COIL2), and AC1 serves as the debug output. The rectifier output is labeled VRECT, and the main regulator output is VOUT. The I²C interface is accessible via the SDA, SCL, and GND connectors for connection to a USB-I²C bridge. GPIOs are also available on a header. The board includes capacitors for VRECT and VOUT, CTANK capacitors calculated with the TWS coil, an SOVP resistor, and other essential components.

Test points

The STDES-WLC38TWS features several connectors and test points for easy access to key signals.

Table 1: Connectors and test points

Reference design specifications

The STDES-WLC38TWS target specifications are listed below:

Table 2: Reference design specifications

Default configuration

Basic parameters:

Table 3: Basic parameters

PCB layout

Figure 3: STDES-WLC38TWS top layer

[Image description: Top layer layout of the STDES-WLC38TWS PCB, showing component placement and routing. Key connection points like COIL1, COIL2, GND, VRECT, VOUT, VDD, VAA, SCL, SDA, AC1, and GPIO/INT connectors are visible.]

Figure 4: STDES-WLC38TWS inner1 layer

[Image description: Inner layer 1 layout of the STDES-WLC38TWS PCB, illustrating the internal routing and connections.]

Figure 5: STDES-WLC38TWS inner2 layer

[Image description: Inner layer 2 layout of the STDES-WLC38TWS PCB, showing further internal routing.]

Figure 6: STDES-WLC38TWS bottom layer

[Image description: Bottom layer layout of the STDES-WLC38TWS PCB, depicting component placement and routing on the underside.]

Typical performance characteristics

The following table shows the charging performance of the STEVAL-WBC86TX/STDES-WLC38TWS (Tx/Rx) setup at various load currents. Temperature is measured after 5 minutes of continuous operation.

Table 4: Typical performance characteristics

Efficiency and spatial freedom in the XY plane

Efficiency is a key metric for wireless charging performance. Spatial freedom, the area where a receiver can be placed on a transmitter while maintaining sufficient power transfer, is also crucial. The STDES-WLC38TWS efficiency and spatial freedom were measured using the STEVAL-WBC86TX as the receiver. Efficiency was measured from the transmitter's DC input to the receiver's DC output, excluding power losses in the input cable. The test setup included a power supply (HMP4040), a transmitter (STEVAL-WBC86TX), the STDES-WLC38TWS receiver, and an electronic load in CC mode (BK Precision 8500). The maximum efficiency achieved was 67% at 2.5 W (5 V/500 mA) with a 3 mm gap between the Rx and Tx coils.

Figure 7: STDES-WLC38TWS and STEVAL-WBC86TX efficiency test

[Image description: A line graph showing efficiency (%) versus output current (Iout [mA]) for the STDES-WLC38TWS and STEVAL-WBC86TX. Two lines represent 'Center' and 'Center + 5mm (Y)' conditions.]

Efficiency and spatial freedom in the Z-axis

The Z-axis distance, or charging gap, also significantly impacts charging performance. The STDES-WLC38TWS was tested at various charging gap distances. Efficiency curves for Z-axis misalignment are shown below.

Figure 8: STDES-WLC38TWS and STEVAL-WBC86TX efficiency test in the Z-axis

[Image description: A line graph showing efficiency (%) versus output current (Iout [mA]) for the STDES-WLC38TWS and STEVAL-WBC86TX in the Z-axis. Two lines represent 'Center' and '5mm (Z)' conditions.]

A Z-distance of 3 mm is typical for most applications (2 mm on the Tx side + 1 mm on the Rx side). The transmitter can deliver sufficient power even with a 5 mm charging gap, but efficiency decreases rapidly with increasing gap. Minimizing the Z-distance is recommended.

Thermal performance

The following images show the STDES-WLC38TWS thermal performance with a 2.5 W load (5 V/0.5 A) after 10 minutes of continuous operation.

Figure 9: STDES-WLC38TWS thermal performance (1 of 2)

[Image description: A thermal image of the STDES-WLC38TWS board, displaying temperature distribution across the components. A color scale indicates temperatures from 21.9°C to 53.4°C.]

Figure 10: STDES-WLC38TWS thermal performance (2 of 2)

[Image description: A second thermal image of the STDES-WLC38TWS board, providing another view of the temperature distribution. A color scale indicates temperatures from 23.7°C to 54.4°C.]

Note: Temperature measured by thermal-imaging cameras may differ from TRECT values, as TRECT measures internal device temperature.

Startup waveform

A startup waveform for the STDES-WLC38TWS and STEVAL-WBC86TX is shown below. The startup conditions are a centered Rx and Tx coil position, a 3 mm gap between coils, and a 100 mA load on the Rx Vout. The STEVAL-WBC86TX is powered by a 5 V supply.

Figure 11: STDES-WLC38TWS startup waveform

[Image description: An oscilloscope waveform display showing VRECT, VOUT, AC1, and AC2 signals over time. The horizontal axis is 200 ms/div, and vertical scales are 5 V/div for all channels.]

Figure 12: STDES-WLC38TWS startup waveform with ARC mode and Z gap 14mm

[Image description: An oscilloscope waveform display showing VRECT, VOUT, AC1, and AC2 signals under ARC mode and a 14mm Z gap. The horizontal axis is 200 ms/div, and vertical scales are 5 V/div.]

Schematic diagrams

Figure 13: STDES-WLC38TWS circuit schematic

[Image description: A circuit schematic diagram for the STDES-WLC38TWS. It shows the STWLC38 IC, associated passive components (capacitors, resistors), connectors (P1, P2, P3, P4), and power/signal lines.]

Bill of materials

Table 5: STDES-WLC38TWS bill of materials

Conclusions

The test results confirm that the STDES-WLC38TWS reference design can automatically detect TX and receive power from the STEVAL-WBC86TX transmitter board. The peak efficiency for both the STEVAL-WBC86TX and STDES-WLC38TWS is greater than 67% at 2.5 W. The STDES-WLC38TWS reference design meets its expected performance targets when used with the STEVAL-WBC86TX transmitter board.

Appendix A: Reference design warnings, restrictions and disclaimer

Important Notice – Read Carefully

The reference design is not a complete product and is intended solely for evaluation in laboratory/development environments by technically qualified electronics experts. Users must be familiar with the dangers and application risks associated with handling electrical/mechanical components, systems, and subsystems.

Danger: Exceeding specified reference design ratings (including input/output voltage, current, power, and environmental ranges) can cause property damage, personal injury, or death. Contact an STMicroelectronics field representative for clarification on ratings before connecting interface electronics, input power, or intended loads. Applying loads outside the specified output range may lead to unintended operation or permanent damage. Some circuit components may reach high temperatures during normal operation, including linear regulators, switching transistors, pass transistors, and current sense resistors.

STMicroelectronics reference designs are provided to assist designers in developing systems incorporating STMicroelectronics semiconductor products. Buyers are solely responsible for independent analysis, evaluation, and judgment in designing their own systems. STMicroelectronics has conducted only the measurements and tests described in the published documentation. STMicroelectronics may enhance or improve its reference designs in the future.

Reference designs are provided "as is." STMicroelectronics does not warrant that reference designs are accurate or error-free and disclaims all warranties, express or implied, regarding their accuracy or completeness. STMicroelectronics also disclaims any warranty of title, merchantability, fitness for a particular purpose, or non-infringement of intellectual property rights.

STMicroelectronics shall not be liable for any damages arising from the use of its reference designs. Buyers agree not to use reference designs in any legal or administrative proceedings.

Revision history

Table 6: Document revision history

List of tables

• Table 1: Connectors and test points (Page 3)
• Table 2: Reference design specifications (Page 4)
• Table 3: Basic parameters (Page 5)
• Table 4: Typical performance characteristics (Page 8)
• Table 5: STDES-WLC38TWS bill of materials (Page 13)
• Table 6: Document revision history (Page 17)

List of figures

• Figure 1: STDES-WLC38TWS reference design (Page 1)
• Figure 2: STDES-WLC38TWS connection overview (Page 2)
• Figure 3: STDES-WLC38TWS top layer (Page 6)
• Figure 4: STDES-WLC38TWS inner1 layer (Page 6)
• Figure 5: STDES-WLC38TWS inner2 layer (Page 7)
• Figure 6: STDES-WLC38TWS bottom layer (Page 7)
• Figure 7: STDES-WLC38TWS and STEVAL-WBC86TX efficiency test (Page 9)
• Figure 8: STDES-WLC38TWS and STEVAL-WBC86TX efficiency test in the Z-axis (Page 9)
• Figure 9: STDES-WLC38TWS thermal performance (1 of 2) (Page 10)
• Figure 10: STDES-WLC38TWS thermal performance (2 of 2) (Page 10)
• Figure 11: STDES-WLC38TWS startup waveform (Page 11)
• Figure 12: STDES-WLC38TWS startup waveform with ARC mode and Z gap 14mm (Page 11)
• Figure 13: STDES-WLC38TWS circuit schematic (Page 12)

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