How We Developed NFC Charger Under Strict Physical and Electrical Limits

Our client, a successful sports-tech startup, had developed their device – a ball equipped with a built-in motion tracking sensor, and brought it to the market.

Thousands of units were already in use when it became clear they needed a charging solution that their own team couldn’t deliver due to workload constraints. Off-the-shelf development kits worked as a temporary fix, but they were expensive, inflexible, and unsuitable for large-scale deployment – so they turned to us.

The challenge was to design a transmitter that fit strict physical and electrical boundaries while ensuring reliable power transfer and full compatibility with unmodifiable hardware already in the field.

The result was a production-ready charging module that achieved reliable operation from the first build, even with significant distance between the transmitter and receiver antennas, combined with additional misalignment, and with no hardware redesign required.

Project Background and Client Requirements

The design had to comply with multiple constraints:

  • A fixed 16 mm receiver antenna inside the ball, which could not be modified
  • A requirement to use specific transmitter antennas (35 mm or preferably 40 mm in diameter) from a pre-approved supplier
  • A targeted distance of at least 8 mm between the transmitting and receiving antennas, with imperfect alignment due to mechanical integration

The system had to be optimized to operate under these conditions, even though the existing design was not best suited for the new requirements.

Our Contribution

Characterization and Simulation

As the final system’s antenna configuration was unique, it was necessary to characterize the coupling effects manually using a Vector Network Analyzer (VNA). These measurements provided the input needed for accurate system modeling, simulation, and optimization.

Key technical steps included:

  • Impedance measurement of both transmitter and receiver antennas
  • Calculation and tuning of matching network components
  • Simulation of multiple configurations using real-world antenna characteristics
  • Assembly and testingof prototypes with component values derived from simulation

Firmware and Control

Custom firmware was implemented to control the charging transmitter and ensure its safe operation:

  • Configuring and managing the NFC charger transmitter
  • Monitoring and safely handling overtemperature or overcurrent conditions
  • Indicating charging state through an LED
  • Logging of internal events, state transitions, and temperature measurements to simplify design verification

The firmware is based on the Zephyr RTOS, selected due to its rich selection of built-in functionality, allowing for rapid development:

  • Toolchain and build system
  • Peripheral drivers
  • Timers
  • Logging framework

The NFC charger IC utilized in the project comes with a vendor-provided SDK for configuration, control, and monitoring.

A custom Zephyr driver was implemented, which:

  • Wraps the vendor-provided SDK and provides a simplified API for easier integration in application firmware
  • Allows configuration of the NFC charger IC through the Zephyr devicetree
  • Allows additional debug features to be enabled through Kconfig

The driver is board-independent, allowing it to be reused in future NFC charging applications.

The firmware is responsible for:

  • Initializing the NFC charger IC and configuring its enabled features and RF frontend
  • Configuring the NFC charger IC to use an on-board current sensor
  • Initializing an on-board temperature sensor for measuring ambient temperature
  • Monitoring the NFC charging state machine, allowing the firmware to react if receivers are detected, removed, or have finished charging
  • Preventing charging in case of overcurrent or overtemperature events
  • Indicating the charging or error state to the user through an LED
  • Logging all relevant events over UART

Testing and Evaluation

To validate system performance, several transmitter antennas were tested: 24 mm, 30 mm, 35 mm, and 40 mm in diameter. Initial testing was conducted under ideal conditions – antennas positioned at zero distance apart and perfectly aligned. This phase established a performance baseline.

Real-world conditions were then emulated by testing:

  • Antenna separation distances ranging from 2 mm to 13 mm
  • Lateral misalignment of the antennas, simulating real positioning offsets inside the device

Measurements included:

  • Transmitter current, for assessing power consumption and thermal behavior
  • Receiver current, as an indicator of power transfer efficiency

All test results were systematically recorded and mapped against antenna distance and offset to identify the most robust configuration.

Results

Despite tight integration constraints, the final design met all key performance criteria:

  • The NFC charger was engineered precisely to meet the client’s specific requirements.
  • Stable operation was maintained at and beyond the required 8 mm antenna gap, with successful charging confirmed at 11 mm and even 13 mm separations, exceeding the original expectations.
  • No hardware revision was necessary: the first iteration of the prototype proved fully functional, indicating precise simulation and accurate modeling.
  • Control firmware and a reusable Zephyr driver enabled stable bring-up, protection (over-current/over-temperature), user indication, and UART logging, making the solution portable for future NFC-charging designs.
  • Efficient energy transfer was maintained across a range of alignments, with significant drops observed only at offsets greater than 6 mm from center.
  • Design for manufacturability was prioritized – component count and cost were minimized without compromising performance.

If you’re facing similar challenges, let’s discuss how we can support your development.