High permeability core material at high frequency

Question

I am trying to design a transformer with 1 turn at the primary and 10 turns at the secondary. For application purposes, I need a long wire at the primary and a large conductors at the secondary, which makes it difficult to consider a higher number of turn at the secondary. I do not care about losses as the transformer is not use during a significant time. It works at 500 kHz.

So I have a large leakage inductance at the primary and to not loose a lot of voltage due to the voltage divider between the magnetizing inductance and the leakage inductance, I need a large magnetizing inductance, so a high permeability core. It will also reduce the peak current at the primary if I have a high magnetizing inductor value and then it will reduce the effect of the leakage inductance. I need also to have a high density flux saturation Bsat

I am looking for a graph which shows the permeability of different magnetic materials in function of the frequency. Does anyone know a material that have a good permeability at those frequency and a high Bsat? Better than ferrite?

It is said that ferrites is good material for high frequency, i.e. that the permeability is still high for high frequency. But I know that the Bsat of ferrites is very low and the permeability is not very high compared to supermendur.

Have a nice day!

Answer 1

There's a good reason for the many different materials in transformer cores. Core loss is one of the many parameters involved in the trade-offs required, and at high frequencies (>500Khz) ferrite would be chosen over the other materials in your chart because core losses would tend to dominate in the other materials you have in your graph.

For a tape-wound material like most of the ones in your graph, the tape is kept as thin as possible to reduce eddy currents in the core. The construction allows for high flux density, but at high frequencies even 1/2-mil tape would have unacceptably high core losses. And as @Neil_UK pointed out, there are significant magnetizing losses due to the large hysteresis - remember that this type of loss occurs each time the core magnetization is reversed, so higher frequency means more magnetizing loss.

Source: Magnetics Inc. Tape Wound Core Catalog

Ferrite has acceptable core losses as seen below, but at a price: the flux density at which you can operate will be lower. Here is a curve of Magnetics "L" Ferrite:

Source: Magnetics Ferrite Cores Catalog

So you will be able to operate at high frequency. But this is only one of the many considerations you must make when choosing an approach. It is unusual to choose the frequency first in a power transformer design, but if you have to run at the high end, you are on the right track with ferrite.

Good luck!