Wireless Charging Coil Design Guidance Needed for Multi-Coil System?

[Logan_va]

Hello Team, I am currently working on a Qi-based wireless charging system using a multi-coil transmitter design with wireless charging coils and looking to improve charging efficiency and active charging area coverage. I need some technical guidance regarding coil design and system optimization. What are the recommended parameters for coil inductance and Q factor in a multi-coil wireless charging setup for stable and efficient power transfer? Also, how should coil spacing, placement, and alignment of wireless charging coils be designed to ensure consistent performance across all coils? Additionally, I would like to understand the main challenges when scaling from a small setup (3–4 coils) to a larger system (8 or more coils). Are there limitations related to control, EMI, or power distribution when using multiple wireless charging coils? Another question I have is: How does the system manage coil-to-coil interference and ensure only the active wireless charging coil is properly driven during object detection? If possible, please share any reference designs, application notes, or evaluation kits from NXP that support multi-coil wireless charging systems or large-area transmitter designs. Best practices for ferrite shielding and interference reduction with wireless charging coils would also be very helpful for improving overall system performance. Any technical guidance or documentation would be highly appreciated.

[RafaR]

Hello Logan_va

Good Day!

NXP’s multi‑coil reference designs (e.g., WCT‑15WTXAUTO and WCT‑15WAUTO13) are optimized for the Qi operating band around 127–205 kHz.

• Q‑factor:: NXP designs support Q‑factor‑based FOD methods, indicating the system expects high‑Q coils with stable resonant characteristics for efficient detection and power transfer.
• Efficiency: NXP multi‑coil platforms achieve >70–75% efficiency with properly tuned coils and Q‑optimized topologies (e.g., WCT‑15WTXAUTO, WCT‑15WAUTO13)

Design coils with stable, high Q at ~130–150 kHz. Ensure inductance tolerance is tight and consistent across all coils to avoid misalignment of resonant frequencies.

RDWCT-15WTXAUTO

WCT-15WAUTO13

Challenges When Scaling from 3–4 Coils → 8+ Coils

1. Coil Selection Complexity

NXP’s controllers (MWCT1xxx, MWCT2xx3A) include dedicated coil‑selection logic in firmware to manage multi‑coil arrays. As you grow to 8+ coils, the following issues expand:

• More coil scanning cycles
• More candidate coils for object detection
• Higher chance of false positives in FOD and modulation detection
• NXP’s MWCT101xS firmware includes explicit blocks for coil selection, FOD, and error handling to manage this complexity.

2. EMI & Cross‑Coupling

More coils means:

• Greater mutual inductance between neighboring coils
• More susceptibility to EMI and detuning

3. Power Distribution & Thermal Load

Larger coil count means:

• More drivers & switching components
• More power path losses
• NXP highlights efficiency and thermal constraints particularly in automotive environments where 70–75% efficiency and thermal footprint minimization are critical.

4. Control Firmware Scalability

• DSP‑based control loops
• Digital demodulation
• Multi‑coil management with minimal CPU overhead

Coil‑to‑Coil Interference & Preventing Multi‑Coil Mis‑Drive

NXP’s multi‑coil wireless charging ICs implement:

• Digital demodulation (on‑chip) for clearer RX signal separation.
• Firmware‑based coil selection state machine (MWCT101xS).
• Dedicated FOD algorithms using Q‑factor & calibrated power loss to determine the active coil (WCT‑15WTXAUTO, WCT‑15WAUTO13).

Process Overview

• System scans each coil sequentially.
• Performs object detection (modulation, resonant response, Q‑factor).
• Selects the coil with valid RX modulation.
• Enables only that coil’s inverter and disables all others.
• Continuously verifies coil validity during charging.

This prevents simultaneous drive and reduces cross‑coupling artifacts.

You can enter our official website and find many documents that cover different points about your situation, the problem is that many of them are classified as secure files, so it is not possible to share them by this means and to have access to them you must have a valid NDA with us

I hope this information has helped you, please let me know if you need help with anything else.

Have a great day and best of luck.