Design considerations when using a wireless charging battery pack
I want to include a wirelessly charged battery pack into my design. However, I am worried that the wireless charging could damage my main logic board. What considerations, apart from not having loops in main board, have to be taken into account when making design? Are there any specific pitfalls to avoid?
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There are various criteria that govern how well power is taken from a nearby changing magnetic field:
- Distance. The magnetic field diminishes rapidly with distance from the source.
- Shielding.
- Pickup ability. To get current from a changing magnetic field, there has to be a conducting loop with the magnetic field lines piercing the enclosed loop area. Therefore, there are three criteria for pick ability:
- Loop area.
- Loop orientation.
- Number of turns of the conductor around the loop.
- Impedance matching.
All these criteria need to be carefully considered to get meaningful power across a gap with reasonable efficiency.
To avoid unintended coupling, do the opposite. Make sure sensitive circuits are on the far end of the board from the magnetic field, perhaps protected by some shielding (usually not necessary), and keep loops small and oriented with the plane parallel to the magnetic field lines.
If you are only worried about the wireless charger damaging the other electronics, then this is not a difficult problem to design to. Good design of the charger to keep the magnetic field local to only the intended pickup area is a good start. Do the math to find the induced voltages and currents further away, and you'll probably find they are well below damaging levels.
Designing so as to not interfere with sensitive analog signals is another matter. All issues mentioned above should be considered. Good frequency planning can also make a big difference. It helps greatly if the charging frequency is outside the frequency range of interest of the analog signals.
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