[600MRG] Ferrite core materials for 475 kHz

[Rudy Severns]

You want to be a little careful on your choice of ferrite core materials. 
There are two classes of ferrites use in power applications:

1) manganese-zinc (MnZn)

2) nickel-zinc (NiZn)

The MnZn ferrites work well up to 1 MHz with some usable up to 3 mHz.  the 
NiZn ferrites are intended for RF applications above 1 MHz.  For a given 
flux density the core losses are much higher and the permeabilities are much 
lower in the NiZn materials.  One clue about the higher core losses in NiZn 
is in the data sheets.  Most MnZn data sheets have extensive core loss 
graphs as a function of frequency and flux density.  You rarely see this 
information for NiZn materials.  NiZn must be operated a much lower flux 
densities.  The only reason NiZn ferrites are used is that at HF they are 
better than the MnZn ferrites performance which degrades rapidly above 1-2 
MHz.

At 475 kHz MnZn is by far the most appropriate  material.  In the Fair-Rite 
Products line the type 31 material is MnZn and very appropriate, on the 
other hand the type 43 material is NiZn and not a good choice below 500 kHz, 
at least for a power inductor or transformer.  The Fair-Rite type 77 
material can also be used and may be more available.  Magnetics Inc. 
(Mag-Inc) also has an extensive line of MnZn power ferrites: F, J, K, L, P 
and R materials.  The J material that Pat used has high permeability but may 
not be as available.  The Mag-Inc F and P materials are more common and may 
be available in a greater range of shapes and sizes.  Mag-Inc has always had 
great applications support and a generous hand with samples.

One source of potentially useable cores is salvage from old switching 
regulator transformers.  These will almost certainly be MnZn E-E or E-I 
cores.   You may not get the absolute best material but if you back off on 
the flux density they should work fine.

In a prior life I taught magnetics design to engineers in the power 
conversion field and I still have a ton of old class notes and references, 
so I have some interest in the subject.  What's needed is a short monograph 
on the design of inductors and transformers for 630m with lots of practical 
examples.  This is on my to-do list but I've still got my hands full 
finishing my station and antennas.  I also have to get my HF station and 
antennas, particularly for 160m, going so it will be a while before I can 
pick up that project.

73, Rudy