FC Wireless Charging for Smart Rings: Sealed TX/RX Coil and PCBA Design

The smart ring is the smallest wearable to reach commercial volume, and its hardest design constraint is not the sensor stack — it is the shell. A ring has to stay sealed against water, sweat and dust while sitting on a hand that moves all day. At a few millimetres of wall thickness there is no room for a connector, and none for the pogo pins a watch can hide in a lug — yet the ring must be charged every few days. YQM ELECTRONICS supplies an NFC wireless charging solution that moves the energy across that sealed wall: the ring is set on the dock, the transmit and receive coils couple at 13.56 MHz, and the cell charges with no opening, no contacts and no plug.

1. Application Scenario: The Charging Port a Ring Cannot Have

Smart rings are worn as jewellery, which is exactly why the category grew so quickly — and exactly why charging became the bottleneck that held it back. The design pressures all point the same way: the shell must stay closed for ingress protection, the internal volume is almost entirely consumed by the sensor stack, battery and antenna, and every additional opening is one more failure point in a device that lives on a wet, moving hand. Three requirements follow directly:

• Sealing must survive the charging method: water, sweat and dust resistance is a core selling point, so a charging method that opens the shell works against the product itself.
• There is nowhere to put a connector: a ring wall a few millimetres thick cannot host a USB-C socket, a pogo-pin array or a contact plate, and the internal volume is already spoken for.
• Charging has to be a one-second gesture: a ring comes off several times a day; a fiddly contact array or a plug turns every charge into a chore the user will postpone.

2. Solution Features

• No physical plug or contacts: charging begins when the ring is set on the dock — there is no connector to wear out and no orientation to get right.
• Industrial-grade, compact and sealed: the module is built for a wearable environment, so the ring keeps its water- and contamination-resistant construction intact.
• The charging link also carries data: because power moves over an NFC link, the dock can identify the ring it is charging and exchange status or pairing information without a second radio.
• Thin FPC receive coil matched to the ring: the receive coil is produced on flexible substrate so it follows the inner curvature of the band, and the TX/RX pair is tuned together rather than independently.

3. Solution Architecture: A Dock-Side Transmitter and a Ring-Side Receiver

The system splits cleanly into two halves. The dock carries the NFC controller, the transmit coil, the driver stage and the power supply; the ring carries the receive coil, the rectifier and regulator, the charging IC and the cell, all inside the sealed shell. Bringing the ring into proximity completes the coupling: the transmit coil establishes an alternating field, the receive coil picks it up, and the rectified output charges the cell. Because the same link is an NFC link, the dock can also identify the ring and exchange status while it transfers energy.

Figure 1: A wide-format FPC transmit coil for the dock side. The large aperture is what gives the dock enough coupling to charge a receiver coil only millimetres across.

Figure 2: A compact two-pad FPC receive coil (model R0608-98B). The flex tail terminates in pads that solder directly to the ring's charging PCB.

The coil pair is where the engineering effort concentrates. A ring offers almost no aperture, its wall is curved in two axes, and both the shell material and the wearer's finger sit close enough to shift the resonant frequency. The TX and RX coils are therefore designed as a matched pair: the geometry is paired first, then tuned together under the actual housing conditions instead of on a bare board.

4. Core Technical Principles: 13.56 MHz Near-Field Coupling Through a Sealed Shell

The link operates at 13.56 MHz and follows the NFC Forum wireless charging specification, which defines how a transmitter and a small receiver exchange both power and data over the same near-field magnetic coupling. The specification's power class reaches up to 1 W — ample for a ring cell measured in tens of milliampere-hours. The transmit coil is driven to establish an alternating magnetic field; the receive coil, tuned to the same frequency, couples to it, and the induced AC voltage is rectified and regulated into the charging current. Because the two coils form a coupled resonant system, transfer efficiency depends on how accurately they are tuned and how well they are aligned.

Three things make the ring application harder than a dock for a larger device, and all three land in the antenna design. First, aperture: a coil that fits inside a ring couples weakly, so turns, trace width and substrate have to recover as much coupling as the geometry allows. Second, detuning: the metal shell and the human body both load the coil and pull its resonant point, so the design is tuned in the presence of the actual housing rather than on a bench. Third, alignment tolerance: the user simply sets the ring down, so the dock geometry and the field shape have to tolerate the ring arriving slightly off-centre.

5. Services & Support

• Application scenario planning: YQM identifies the industry pain point, draws out the requirement and matches it against the technology.
• Solution delivery: a concrete solution is produced against the customer's requirement and actual operating environment.
• Project assessment: theoretical and simulation-based evaluation against the deployment conditions and specification targets.
• Technical team support: R&D and FAE engineers provide guidance, on-site analysis and project follow-through.
• Product design support: substitution with mature designs, custom development and high-efficiency RF design.
• Reporting: site environment analysis reports, improvement recommendation reports and commissioning test reports.

6. Cooperation Model & Expandable Products

YQM supports both ends of the link, so a ring brand can take the complete charging pair or only the side it does not build in-house.

The same transmit-and-receive pair extends to the other sealed wearables that share the ring's problem: TWS earbuds and their cases, hearing instruments, smart glasses, styluses, and any body-worn device where an opening for a connector would cost more than the charging hardware itself.

7. Product Showcase

7.1 Coil and Antenna Samples

The samples below show the range of footprints produced across the transmit and receive families, from wide-format coils for larger apertures to compact two-pad coils. Each model is produced on flexible substrate so it can be shaped to the enclosure it serves.

Figure 3: A wide-format FPC coil with a single flex tail — a second footprint in the same coil family.

Figure 4: Model T0810-98B, a compact two-pad FPC coil from the transmitter series.

Figure 5: Model R0609-98B, a compact two-pad FPC coil from the receiver series, with the same pad pitch as the ring-side receiver.

Figure 6: Model T0811-98B, a compact two-pad FPC coil from the transmitter series — the fourth compact footprint in the family.

7.2 Smart Ring Reference

Smart rings of the class this charging solution is designed for. The sealed shell, the component stack behind the band and the absence of any charging opening are the constraints the coil design has to work within.

Figure 7: A reference smart ring with the flexible circuit and coil visible through the band — the component stack the receive coil has to fit around. (Terminal product image source: network; contact for removal if copyright concern.)

Figure 8: Reference smart-ring designs with fully sealed metal shells. The charging method must work without any opening in the housing. (Terminal product image source: network; contact for removal if copyright concern.)

Conclusion: NFC Wireless Charging — Powering a Smart Ring Through a Sealed Shell

For a smart ring, the charging method is not an accessory decision — it determines whether the product can be sealed at all. NFC wireless charging removes the opening, removes the contacts and removes the ritual of plugging something in: the ring is set down, the coils couple at 13.56 MHz, and the cell charges through the shell. The engineering difficulty sits entirely in the coil pair, which has to recover usable coupling from a tiny curved aperture while tolerating the detuning caused by the housing and the wearer, and while staying thin enough to live inside the ring. YQM ELECTRONICS designs and produces both sides of that pair — the coil, the tuning, the PCBA and the rigid-flex board — so a ring maker can adopt contactless charging without building an RF team for it.

Explore YQM ELECTRONICS' complete custom module solutions to add wireless charging to your wearable platform. Our engineering team covers coil design, RF tuning, PCBA and volume manufacturing.

Further reading: Explore the complete custom module solutions or reach out to our engineering team to discuss your specific NFC wireless charging requirements.