The very tiny cores on MiniCircuits transformers will start to
saturate at hundreds of mA. The effect is that the magnetizing inductance
drops, which matters more at low frequencies than high frequencies. I try
to avoid feeding DC to an amplifier through a transformer winding.
Instead I use a separate RF choke for that. However, it would
probably work OK for, say, up to 25 mADC for a small signal transistor,
but
why take a chance.
If you are using a DC feed through a transformer winding, be careful
not to accidently short circuit it, causing the full available current
from the power supply to flow through the transformer. This can
actually magnetize the core and permanently damage it. Saturation
via DC is much more deleterious than saturation via AC.
It is easy to calculate the flux density using Ampere's law, which
is one of the four Maxwell's equations. H = I/(2piR). Since R
(radius) is in the denominator, cores saturate from the inside
first before the whole core is saturated. Multiply H by mu,
(as any time nut knows) to get B. If R is 1 mm, and I is 628 mA,
then H = 10 ampere turns per meter. If mu-relative is 1000, then
B = 4piX10^-7 X 1000 X 10 = 125 mT. That is a hefty 1250 Gauss.
Some materials may be affected at 1/10 this flux density.
Now a days, a lot of RF is differential, in which case you are
free to feed DC through the output transformer without worrying
about this issue.
I worked for several companies where those 6 hole cylindrical chokes
were ubiquitous. I specifically characterized those and established
a maximum current rating of only 100 mA. Of course, many production
designs exceeded this limit and "worked" anyway. I actually observed
someone try to put 20A through one of these. The tantalum capacitors
on the "cold" side of the bead actually exploded due to RF current.
Rick Karlquist N6RK
Thanks for the insight, Rick. You confirmed many of my own assumptions
about RF transformers and cores. I hadn't thought about permanently
magnetizing a core with excessive DC current, but it makes sense. Same
theory applies to line frequency power transformers and inductors; if the
core saturates, inductance takes a nose-dive and current goes wild.
A couple weeks ago, I sent an email to the Minicircuits technical support
folks in hopes of getting this, or similar, info about a couple of their
transformer models (specifically, T1-1 and T4-1-KK81), but so far, I'm still
waiting. Guess I should give them a call.. got great technical advice from
them when I called for help some time ago.
Cheers,
Dave M
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