We are developing a wireless charger using Qi, that also uses BLE and NFC using nRF52832.
Right now our NFC antenna (same one as used by Nordic in their DKs) is just on top of the Qi charger coil, connected directly to nRF52 NFC pins, and calibrated with capacitors and VNA.
Everything was working smooth: BLE, NFC tag read by smartphone... but doing some testing while charging the smartphone using the Qi coil, the NFC tag of nRF52832 stopped working.
We have disassembled everything and now we can measure 62 Ohm constant between NFC pins of nRF52 while OFF, and variable resistance of aprox 2 Ohm while ON.
When measuring NFC antenna with VNA, it does not look fine, there is no return loss spike anywhere (there was before).
Now NFC tag is not working, no reader is able to read anything, but BLE, CPU and Qi charger are working fine.
Is our nRF52832 broken? Any way to revert this state / unbrick it or garbage?
Is it broken because of Qi charging next to NFC antenna?
How can we avoid this failure in our design?
We read at devzone (https://devzone.nordicsemi.com/f/nordic-q-a/20621/wireless-charging-circuit/138421) that adding 1nF capacitors in series to the NFC antenna may solve this.
Is there anything else we can do? For example, is it better if we disable NFC in software during Qi charging or it doesn't matter?
Thank you very much for your support,
Yes, QI currents can damage NFC readers/tags, and what you're describing does not seem reversible. Sorry about that. Although we can't guarantee anything, adding the extra capacitor seems to protect the NFC tag sufficiently, so I suggest you give it a go. Additionally, it wouldn't hurt disabling the NFC while you're charging either, but I don't believe it to be crucial.
Due to the summer holidays in Norway, our support team is understaffed this week, and delayed replies must be expected. Sorry for the inconvenience!
Thank you for your answer!