[Luke11]

When your PCB real estate is extremely tight, leaving no room even for a conventional USB port. Beyond the main MCU solution, you also need to account for the PCB footprint occupied by the charging management chip. How do we deliver power for compact wearable devices under these conditions?

Many hardware teams hit this bottleneck during wearable development and consider using traditional pogo-pin connectors. However, from my experience with wearable products adopting this solution, exposed charging contacts tend to corrode over time due to sweat, cosmetics, or oil contamination.

Is there a solution that can handle both data transmit-receive functions and charging management to save PCB space? The NFC-WLC solution uses a tiny PCB coil to harvest energy from a 13.56 MHz magnetic field while supporting bidirectional data communication. It eliminates the PCB space required for pogo-pins or Type-C ports. It also simplifies mechanical housing design and reduces risks of mechanical wear and liquid-induced corrosion.

 

[Jason Hogan]

Here is a constructive, embedded-hardware perspective to post in reply to Luke11:

Spot on about the pogo-pin corrosion issue, Luke. Sweat and salt ingress on exposed metal contacts remain a major cause of RMA returns for fitness bands and smart rings.

NFC Wireless Charging (NFC-WLC) standardized under the NFC Forum is definitely an elegant way to tackle this. By using the 13.56 MHz carrier frequency instead of standard Qi (which operates around 110–205 kHz), you can shrink the physical antenna/coil footprint dramatically down to a small flex PCB trace.


That said, while NFC-WLC solves the mechanical sealing (IP68) and corrosion problem, hardware teams need to keep a few engineering trade-offs in mind when evaluating it against traditional pogo-pins or Qi charging:

Key Engineering Considerations for NFC-WLC​

• Power Transfer Limits: NFC-WLC specs typically top out around 250 mW to 1 W (depending on power class). For small Li-Po / LiFePO4 batteries in smart rings, earbuds, or medical patches (~30 mAh to 150 mAh), this is plenty. But for larger devices needing fast charging, the lower efficiency means longer thermal build-up during charging.
• Coil Placement vs. Metallic Enclosures: You need non-metallic windows (polycarbonate, ceramic, glass, or bio-compatible plastics) for the 13.56 MHz field to penetrate. If your housing is full aluminum or stainless steel, eddy currents will attenuate the field and generate heat unless you carefully position ferrite shielding behind the PCB antenna.
• NFC Listener IC Footprint: While you eliminate the bulky connector and charging port ESD protection diodes, you still need an integrated NFC Receiver IC (like ST's ST25DV / Panthronics or NXP series) that combines the energy harvester, rectifier, LDO, and I²C/SPI interface to the main MCU.
• Bidirectional Data Throughput: Data transmission over the NFC field is great for firmware updates (DFU), diagnostic logs, and provisioning without needing to establish a BLE bond first. However, data transfer rates are capped by the standard (106 kbps to 424 kbps), so it is best suited for small payload transfers.

If the product power budget sits under 1W and requires complete hermetic sealing, NFC-WLC is hard to beat for PCB density.

What controller/listener chip series are you looking at for your current design?

 

[Luke11]

Here is a constructive, embedded-hardware perspective to post in reply to Luke11:

Spot on about the pogo-pin corrosion issue, Luke. Sweat and salt ingress on exposed metal contacts remain a major cause of RMA returns for fitness bands and smart rings.

NFC Wireless Charging (NFC-WLC) standardized under the NFC Forum is definitely an elegant way to tackle this. By using the 13.56 MHz carrier frequency instead of standard Qi (which operates around 110–205 kHz), you can shrink the physical antenna/coil footprint dramatically down to a small flex PCB trace.


That said, while NFC-WLC solves the mechanical sealing (IP68) and corrosion problem, hardware teams need to keep a few engineering trade-offs in mind when evaluating it against traditional pogo-pins or Qi charging:

Key Engineering Considerations for NFC-WLC​

• Power Transfer Limits: NFC-WLC specs typically top out around 250 mW to 1 W (depending on power class). For small Li-Po / LiFePO4 batteries in smart rings, earbuds, or medical patches (~30 mAh to 150 mAh), this is plenty. But for larger devices needing fast charging, the lower efficiency means longer thermal build-up during charging.
• Coil Placement vs. Metallic Enclosures: You need non-metallic windows (polycarbonate, ceramic, glass, or bio-compatible plastics) for the 13.56 MHz field to penetrate. If your housing is full aluminum or stainless steel, eddy currents will attenuate the field and generate heat unless you carefully position ferrite shielding behind the PCB antenna.
• NFC Listener IC Footprint: While you eliminate the bulky connector and charging port ESD protection diodes, you still need an integrated NFC Receiver IC (like ST's ST25DV / Panthronics or NXP series) that combines the energy harvester, rectifier, LDO, and I²C/SPI interface to the main MCU.
• Bidirectional Data Throughput: Data transmission over the NFC field is great for firmware updates (DFU), diagnostic logs, and provisioning without needing to establish a BLE bond first. However, data transfer rates are capped by the standard (106 kbps to 424 kbps), so it is best suited for small payload transfers.

If the product power budget sits under 1W and requires complete hermetic sealing, NFC-WLC is hard to beat for PCB density.

What controller/listener chip series are you looking at for your current design?

Yes, we are using the ST25R3916B for our NFC‑WLC charging dock. Currently we are looking for a small‑form‑factor NFC IC to build a demo board for the receiver side of compact devices.

 

[Jason Hogan]

Nice choice with the ST25R3916B for the charger/poller side — it's an absolute tank when it comes to active power output and dynamic power control (DPC).

For the small-form-factor receiver/listener demo board, here are three solid chip options from ST and NuCurrent depending on your target footprint and how much integration you want:

1. ST25DV-I2C Series (STMicroelectronics)

• Package: UFDFPN8 ([imath]2 \times 3\text{ mm}[/imath]) or WLCSP ([imath]1.7 \times 1.4\text{ mm}[/imath])
• Why it fits: It's ST's flagship dynamic NFC tag with energy harvesting. It exposes an [imath]I^2C[/imath] bus directly to your main MCU and outputs harvested energy via an [imath]V_{EH}[/imath] pin.
• Demo Board: You can grab the STEVAL-25DV64K expansion board to quickly evaluate power harvesting and [imath]I^2C[/imath] data pass-through.

2. ST25RU3993 / Specialized ST WLC Receiver ICs

• If you want a fully integrated solution that handles both NFC listener protocol AND dedicated lithium-ion battery charging without needing an extra standalone charger IC, check out ST's dedicated NFC Wireless Charger listener ICs (like the ST25WLC series). They reduce external passives to almost nothing on tiny flex PCBs.

3. NuCurrent NuPRNP Series

• NuCurrent specializes in ultra-compact NFC-WLC modules and listener ICs specifically designed to pair with ST poller chipsets. Their evaluation kits come with pre-tuned miniature PCB coil antennas, which saves a ton of time on vector network analyzer (VNA) matching tuning.

A Quick Tip for the Receiver Demo Board:

When layout out your miniature antenna coil on the flex PCB, keep the resonant circuit tuning capacitors ([imath]C_{1}[/imath], [imath]C_{2}[/imath]) placed as close to the listener IC pins as humanly possible. At [imath]13.56\text{ MHz}[/imath], trace parasitic inductance on tiny flex boards can shift your resonant frequency right off target, drastically reducing harvested power.

Keep us posted on how the prototype layout goes!

 

[Luke11]

Nice choice with the ST25R3916B for the charger/poller side — it's an absolute tank when it comes to active power output and dynamic power control (DPC).

For the small-form-factor receiver/listener demo board, here are three solid chip options from ST and NuCurrent depending on your target footprint and how much integration you want:

1. ST25DV-I2C Series (STMicroelectronics)

• Package: UFDFPN8 ([imath]2 \times 3\text{ mm}[/imath]) or WLCSP ([imath]1.7 \times 1.4\text{ mm}[/imath])
• Why it fits: It's ST's flagship dynamic NFC tag with energy harvesting. It exposes an [imath]I^2C[/imath] bus directly to your main MCU and outputs harvested energy via an [imath]V_{EH}[/imath] pin.
• Demo Board: You can grab the STEVAL-25DV64K expansion board to quickly evaluate power harvesting and [imath]I^2C[/imath] data pass-through.

2. ST25RU3993 / Specialized ST WLC Receiver ICs

• If you want a fully integrated solution that handles both NFC listener protocol AND dedicated lithium-ion battery charging without needing an extra standalone charger IC, check out ST's dedicated NFC Wireless Charger listener ICs (like the ST25WLC series). They reduce external passives to almost nothing on tiny flex PCBs.

3. NuCurrent NuPRNP Series

• NuCurrent specializes in ultra-compact NFC-WLC modules and listener ICs specifically designed to pair with ST poller chipsets. Their evaluation kits come with pre-tuned miniature PCB coil antennas, which saves a ton of time on vector network analyzer (VNA) matching tuning.

A Quick Tip for the Receiver Demo Board:

When layout out your miniature antenna coil on the flex PCB, keep the resonant circuit tuning capacitors ([imath]C_{1}[/imath], [imath]C_{2}[/imath]) placed as close to the listener IC pins as humanly possible. At [imath]13.56\text{ MHz}[/imath], trace parasitic inductance on tiny flex boards can shift your resonant frequency right off target, drastically reducing harvested power.

Keep us posted on how the prototype layout goes!

Thanks for your suggestions. Applying these into our subsequent projects will take some time.