How to isolate 2 coil wireless transmitter?

Community
1

Hi guys,

I have two 5V wireless charger (two transmitter) and would like to have 2 charging station.
So the 2 coils will be placed horizontally and at z=0 but close about 10mm.
coil

I’m worrying about interference between the two transmitters.
So thinking to build a magnetic shield around (bottom and sides) to isolate them and concentrate the energy to the top side only. Event this charger have only 20mm max of charging distance, I like to be sure the magnetic field won’t affect any the performance of each other and do not disturb any other equipment that may be close.

How would I do that what kind of material? I hear about copper or aluminum…

Thanks

2

First I would build and try the system to see if there is actually a problem.
The spacing between the phone and the transmitter coil is pretty small so the field may not extend much past the perimeter of the coil.

Although it’s called magnetic shielding it may be useful to note that in principle you cannot shield magnetic fields, only redirect them.

The effectiveness of a material in “shielding” depends on the magnetic properties of the material and the frequency of the source.

Here is some MuMetal film that may work.
https://amzn.to/3Zq8KkP

You could just make a set of rings, glue the MuMetal film on the perimeter and see how that works.

Some similar systems. Don’t know if they work?
shorturl.at/fyzBE

There are some multidevice charging stations that use multiple coils stacked in an overlapping configuration.

2 Likes
3

Thanks donkjr I will then do some experiences to see.

4

You’re right to worry about interference, but copper or aluminum won’t actually solve your problem in fact they usually make wireless charging performance worse. Qi chargers work using an alternating magnetic field (around 110–205 kHz) generated by the wireless charging coil. Conductive metals like copper or aluminum don’t block magnetic fields; instead they create eddy currents that absorb energy, produce heat and reduce efficiency. In many cases the transmitter will start foreign-object detection, throttle power, or keep reconnecting. What you actually need is a magnetic flux guide, not an electrical shield. Commercial chargers handle this by placing a ferrite shielding sheet directly under the wireless charging coil. Ferrite has very high magnetic permeability, so it redirects the magnetic field upward toward the receiver and prevents sideways coupling into nearby transmitters.

For two transmitters spaced about 10 mm apart:

• Put ferrite sheet under each coil (bottom)
• Extend ferrite slightly on the sides facing each other
• Try to keep a small ferrite wall (5–8 mm) between coils
• Avoid copper/aluminum between coils

This will greatly reduce cross-coupling and improve charging stability and efficiency. Without ferrite isolation, each transmitter can detect the other coil as a foreign object and constantly renegotiate power.

1 Like