Detrimental impact of magnetic field on PCB and battery?

[tchains]

Hi there, we are developing a small consumer electronics product consisting of a small 4-layer PCB along with a few PDM microphones and BLE chip/ and antenna, as well as a LiPo battery. We are locating a small but pretty strong magnet next to these components in the enclosure. It is an N52 (Surface Field: 3032 Gauss, Brmax: 14,800 Gauss, BHmax: 52 MGOe). Are there any long term effects of the magnet which may be detrimental to the battery or PCB electronics?

 

[Rive]

Static magnetic field might effect some components which has magnetic parts, for example speakers, hall sensors, some microphones, some signal isolation devices and such. You should check if there is any such device in your design based on the component datasheets.
Also, the combination of magnets and mechanical shocks/oscillations/sounds can induce voltages/currents in the PCB tracks, turning the design into a microphone. You should either handle this somehow in the desing or at least ensure that the effect is small enough.

 

[berkeman]

BLE chip/ and antenna, as well as a LiPo battery. We are locating a small but pretty strong magnet next to these components

I doubt the radio circuit will work well if at all with the strong magnetic field present. Small magnetic components (inductors, baluns, etc) are typically used in such circuits, and saturating them will alter their values and transfer functions.

Can you shield the magnetic field at the source? What is the purpose of the magnet? Hopefully you can choose a magnet geometry that will result in very little field near the radio circuit...

https://5.imimg.com/data5/MO/YO/MY-1200312/industrial-strength-magnets-500x500.jpg

 

[ravida marcos]

LiPo Battery - The battery will be fine. A good way to verify this is to see if your magnet is attracted to the battery at all. There can be some weird variations on LiPo chemistry, but as far as I know, there aren't any ferromagnetic materials in most, if not all, LiPo cells. High strength (even N52 grade) magnets are frequently used to attach LiPo batteries in hobbyist RC products, and many other consumer products.

Magnetic Cores - I know you didn't mention this, but this is really the only thing you need to worry about. Static magnetic fields generally won't impact how the circuit itself behaves (ignoring magnetic cores, of course), but static magnetic fields will impact any bulk magnetic material used in the circuit. This includes any inductors that don't use an air core and ferrite beads.

All magnetic core materials can saturate. With inductors, this is usually specified as saturation current, which is the current through the inductor that, if you exceed, will cause a rapid drop in inductance. Measuring it as current is just for our convenience, and what is really occurring during saturation is that the magnetic field in the core has reached saturation.

Any nearby static magnetic fields will cause some degree of magnetization in the core material, and any current or ripple current through the inductor will be another field superimposed on the core material. Depending on the exact properties and geometry of the core, this can potentially lower the saturation of the core well below spec, or increase losses in the core.

So this is very much worth considering, especially if you are using any power inductors (like in a buck or boost converter) that you're running near saturation.

That said, the saturation field for power ferrites is usually in the range of 300mT (conservatively). So that is already equal to the surface field of your magnet. Considering that magnetic fields fall off at distance cubed (as opposed to squared), so you can be extremely close to a magnet and experience almost negligable field strength from it.

Case in point: assuming your magnet is an N52 square 10mm x 10mm x 2.5mm (which would yield a surface field of about 3000 gauss, give or take. Unless the dimensions are different by a huge amount, numbers will be similar), the static magnetic field just 2mm away from the center, in the direction of the poles, will be ~2140 gauss or 214mT. 5mm, and it is down to 993 gauss. This is low enough that I would not be concerned.

So the short answer to if it will affect your circuit is: maybe. But probably not, especially if you put even the tiniest amount of distance between the magnet and inductors that might present on board or inside the bluetooth module.

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ravida marcos