There's a variety of snap-on wireless charging solutions on the market. For example, this one includes a charging mat and a plastic case that is put onto iPhone.
I suppose the winding is in the middle of the case back and occupies a rather huge area. For comparison, MIFARE Ultralight smart cards are size of a credit card and wires can be seen if the card is lit through with a bright light source – they run right along the card perimeter. This question contains a scan of a stripped card.
Now when the phone is placed onto the mat the charging system air core transformer completes and I suppose the magnetic field is rather strong there. How does that field not induce currents in the phone circuits and not interfere with the phone functioning?
I mean, if the phone was shipped with inductive charging the manufacturer would have to design the phone appropriately from the beginning. Yet the described system is shipped by a third party and clearly it was designed separately.
How do such third party snap-on systems not interfere with the device functioning?
Best Answer
For maximum sensitivity, an antenna would be sized to the wavelength of the incoming signal. The sizes of most wires (traces) in a phone's circuit board are too small by maybe 2-3 orders of magnitude to be a good antenna for the wireless power signal. The size of wires inside the chips would be orders of magnitude smaller than that. The wireless power receiver wire loop is sized to resonate with the transmitter, and has circuitry to actively guide resonance. The frequencies involved in wireless power are very low compared to most of what's going on in a phone - kHz compared to GHz.
Any digital processing would be immune to noise so long as it remains well below the threshold of creating bit errors. The more sensitive parts of the phone to wireless power would be the analog areas, such as RF receiver and audio. Audio would be especially susceptible as the signal frequencies are closer together, both in the kHz range, although possibly still one or two orders of magnitude apart (4kHz for voice vs 100?kHz for power).
Most people don't use their phone while it's on the charger, so whatever problems are caused wouldn't usually be noticed, so long as no permanent damage is caused. Worst case scenario might be some missed calls, or bad-sounding audio if charged while connected to the stereo.