Inductors and ferri/ferromagnetic materials

Hi dear friends,

I have some questions about ferri/ferromagnetic materials for boosting Inductors Q factor. This sort of things is out of my knowledge so I don't know how to properly choose the right magnetic material for an application. As far as I understand things, for any magnetic material permeability varies with frequency, and the higher is the permeability, the higher I will increase my Inductance value L and Q factor. It looks like there is a kind of trade off between high permeability and high frequency for any magnetic material (similarly to the trade off between the bandwidth and amplification for an amplifier).

First question, what data do I need to look for in a datasheet?

Second question, suppose the permeability of the ferrite is constant, if I combine the inductor and the ferrite, will it move the resonant frequency of the inductor?

Third question, the resonance of the ferrite sheet should be before/after/same as the resonant frequency of the inductor? What will happen if it's different?

Fourth question, how to choose the thickness of the ferrite material?

My final objective would be to use a magnetic material (ferrite sheet or other if thickness is needed) in order to isolate and boost the quality factor of a 5cm inductor working in the range of 1 to 10MHz for Wireless power transfert

If some have interesting lecture concerning this, do not hesitate to give the reference.

Thank you

Boosting Q isn't of much use in wireless power transfer because Q will be primarily limited by the real load impedance. Similarly increasing the permeability above a certain amount (e.g. 50 or 100) has little effect on the effective µr of an open magnetic path, respectively doesn't further decrease the magnetic reluctance.

To give some brief answers sorted bottom up:

4. Thickness of ferrite sheet/plate must be sufficient to avoid saturation. Depends on intended field strength.

3. There's no thing like a ferrite resonance frequency.

2. As you said, ferrite does increase coil inductance, resonance frequency will be reduced, can be compensated by changing the resonance capacitor.

1. No simple answer. You'll preferably use "medium frequency" ferrites with µr of 50 to 100 with low loss in the 10 MHz range. Most WPT applications use lower frequency, e.g. around 100 kHz and power ferrites with higher permeability and higher saturation flux.

FvM said:

Boosting Q isn't of much use in wireless power transfer because Q will be primarily limited by the real load impedance.

Click to expand...

Boosting Q is of importance in WPT systems because it allows to transfer the power to higher distances i.e. with lower coupling between the coils.
Of course the influence of the load impedance is different in each resonant topology used.

1) Another parameter which you would like to keep low is the magnetic loss tangent of the ferrite shield for low energy loss in it.