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I had a question this week from a gentleman who made a matching transformer from Fair-Rite 43 material. He noticed a significant temperature rise at 1.8MHz and medium level voltages and wondered if 31 material would be a better choice.
Those 2 materials, 31 and 43, are generally used for inserting loss at higher frequencies for EMI suppression control. If you are going to build a transformer with them then 43 is better and we do use it for inductors up to about 1MHz.
I recommend our 61 (perm =125) or 52 (perm = 250) materials.
Small toroids are readily available from either.
How do I know this? By looking at the complex permeability curves of the materials. At 1.8Mhz I want a material with a low MU”. This factor represents the loss in the material. At 1.8MHz the 43 material has a MU” of about 105. The resistive losses calculate out as R=2*PI*F*L0*MU”. If L0 is just 300nH (a few turns) this is calculates out to 356 ohms of resistive losses. Its easy to understand the temperature rise.
The same calculation with Fair-Rite 61 material results in 1.7 ohms of resistive losses.
Just an example of picking the right magnetic material for the job.
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