LF: Re: Toroids

["James Moritz" <james.moritz@btopenworld.com>]

Dear Chris, LF Group,

"Power" ferrites for switch-mode PSU applications like 3C8 / 3C85 / 3C90 
/3C95 / 3F3 / N27 / N67 / N87 / F44 / #77 / #78, etc., are all quite similar 
with mu in the region of 2000 - they are quite well suited to PA 
applications at 136 and 500k. Large toroids don't seem to be very popular 
with manufacturers of SMPSUs, probably because of the difficulty of winding 
more than a few turns, applying insulation between windings, mounting on a 
PCB, etc. with the toroid geometry. So they are quite rare. Pot cores and 
"E" cores with plastic bobbins, such as the ETD format, are much more widely 
available in these materials, and in my view easier to wind where multiple 
windings are needed. E.g. I have used ETD39 core sets for a few hundred 
watts PA output at 136k, and ETD49 up to 1kW. They are available in one-off 
quantities from suppliers like RS components and Farnell.
If you really must have toroids, consider using "EMC supression" type 
cores - these are usually Fair-Rite #43 material with mu around 850, or some 
equivalent. They will require somewhat more turns due to lower permeability 
and flux density, and have a lower maximum operating temperature.
Cheers, Jim Moritz
73 de M0BMU