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One well-known fact is that putting a ferromagnetic material in an inductor will increase the magnetic flux and thus the inductance value.

Wireless power transfer (WPT) happens through the magnetic near-field. It seems logical that if the magnetic field strength generated by the transmitter is increased so is the field at the receiver. It therefore seems very desirable.

On the other hand all the WPT solutions I have seen use air cores and only a ferromagnetic shield. The high permeability of the shield makes it equivalent to a much thicker air gap, and thus the field at its other end will not be weakened by any metals.

The situation seems to be similar on the receiver side. There are ferrite rod antennas for far-field reception. The connection between two ferrite core coils at a given air gap seems to be the same as if we would take one ferrite rod, wind the two coils on it, and just cut the ferrite rod in the middle and pull it apart a little. The magnetic field should be concentrated in and around the ferrite cores, shouldn't it? Why aren't ferrite cores used in WPT? What are their drawbacks?


PS: an image search has not resulted with any coil using a ferrite core, but the black pad below the planar coil winding is a ferromagnetic material, which concentrates the magnetic flux density. This way the coils can be placed on conductive surfaces, such as the battery of a mobile phone, without great losses.

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    \$\begingroup\$ Please show the arrangement that you have seen i.e. I have seen use air cores and only a ferromagnetic shield. The devil is in the detail and I have certainly used ferrite cores to optimize power transfer on some jobs. \$\endgroup\$
    – Andy aka
    Jan 24 at 11:28
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    \$\begingroup\$ neosid.de/en/hints-solutions/product-solutions/… \$\endgroup\$ Jan 25 at 4:41
  • \$\begingroup\$ @BruceAbbott that's a nice collection of ferrite-core coils! I wonder what are the trade-offs? Is a ferrite core always the best way to go from the engineering perspective? Are there only economic reasons, why many systems avoid their use? \$\endgroup\$ Jan 25 at 17:40
  • \$\begingroup\$ A ferrite core can be more efficient because the coupling is better. However it requires more accurate alignment and is bulky. for example I have a wireless charging nightlight which usues a 10mm thick coil with ferrite core. This is much too fat for a modern cell phone! It sits in a dedicated cradle shaped to fit it for accurate alignment. \$\endgroup\$ Jan 25 at 18:22

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It is all about the coupling factor between the supply coil and the receiver coil.

A ferrite can funnel the magnetic field through the receiver coil, improving the coupling, but this is not always necessary.

Image a cell phone charger. The phone coil can be very wide - several cm in diameter - and close to the supply coil, maybe only as much as 1 cm away. In this case, even the rapidly dispersing magnetic field of the air coil will essentially pass through the receiving coil with close to 100% efficiency, especially if the supply coil has a similar diameter.

However, if the two coils are further apart the coupling factor rapidly drops for air coils. In this case, field concentration with ferrites can improve the coupling strongly, while the added losses are rather small.

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  • \$\begingroup\$ So, you are saying that the ferrite core is neglected, because though it is good to have, it is not required? It would be close to my understanding: The H starts to drop at a distance of R (R is the radius of our round coil) with the 3rd power of the distance. Closer to it, a ferrite "just" reduces the leakage inductance. dB/dt generates the voltage at the receiver. The normal component of B through any surface stays unchanged (div B=0). It should be true for the ferrite in the RX coil as well, thus a ferrite RX isn't bring much. The same thing boosts H in a ferrite TX core --> nice. \$\endgroup\$ Jan 25 at 21:09
  • \$\begingroup\$ @HorrorVacui In terms of coupling only, it is good to have ferrites, yes. But there are drawbacks, such as mentioned Bruce, i.e. weight, cost and some slight hysteresis losses. So if coupling is geometrically efficient with air coils, the transfer could even worsen with ferrites, due to the losses. \$\endgroup\$
    – tobalt
    Jan 25 at 21:49

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