Re: Models for RF transformers with extreme ratios?

Tom Bruhns wrote:
On Apr 29, 6:43 pm, Joerg <notthisjoerg...@xxxxxxxxxxxxxxxxxxxxx>
wrote:
Tom Bruhns wrote:
On Apr 29, 11:31 am, Joerg <notthisjoerg...@xxxxxxxxxxxxxxxxxxxxx>
wrote:
...
Is there any model, research results etc. for RF transformers that
feature extreme turns ratios such 100:1 and more? I am mainly interested
in leakage inductance, bandwidth and such. Bandwidth doesn't have to be
more than an octave, single digit MHz range. It just can't be resonant,
at least not a lot.
...
What impedances?
About what you assumed, around an ohm at roughly rectified AC level.
That one ohm will be a whole 'nother story but I'll get it there.
Somehow :-)



Try searching for current-sense applications (where the primary is a
single turn).
A reasonable starting point for a broadband transformer is a primary
inductance whose reactance is the same as the primary side impedance.
So if the bottom end is 2MHz and the primary is designed for 1 ohm,
you'd want about 80nH or a bit more. An FT114-61 should give you
about that with a single turn. But 100 turns gives you 800uH which
will resonate with only 2pF at 4MHz. Properly loaded (10k ohms
resistive) it should be reasonably damped. A Spice model will tell
you pretty nicely what the response will be for any reasonable assumed
coefficient of coupling; for example, assuming 80nH:800uH with k=0.9
(which seems like it ought to be pretty easy), and 2pF||10k secondary
load, driven from a 1 ohm source, you get a 3dB bandwidth from about
1MHz to 12MHz, with no apparent resonance effects. k=0.8 and
Cload=4pF only cuts the top end to about 6MHz. (The bottom is
determined mainly by the reactance. Get to low enough coupling and
the effective turns ratio drops, but at k=0.8, the mid-band is only a
fraction of a dB below the "theoretical" 40dB voltage stepup.)
Thanks, Tom. I am currently at around 300-400nH primary but any
capacitance on the stepped up side is killing things. The secondary in
those cases is always largish because it needs to withstand a lot of
breakdown voltage. The only feasible method is to wind the packet on top
of the primary or use compartmentalized bobbins and make sure the other
layer maintains enough clearance. The latter not so much for HV
breakdown but to avoid stray capacitance to ground.

Probably the best avenue is to get a suitable large core and make one. I
was hoping there was some example data from core/bobbin manufacturers
and such but the usual suspects didn't have anything.

--
Regards, Joerg

http://www.analogconsultants.com/

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Oh, yeah, and I forgot to ask: how much power? ;-) ...


Not at liberty to tell ;-)


... My comments came
from a low-power context, though they tranlate. The
reactance:impedance thing should stay the same, assuming you avoid
core nonlinearity. It actually came as a bit of a surprise to me how
low a winding reactance is when you get to the low frequency cutoff.


Yes, it's a compromise to push the upper end a bit. On line transformers it's to save cost on the copper. So when I need really low standby power I often use a 230V transformer at 120V.


The relatively simple model I suggested has worked well for me: get
the coupling coefficient up to extend the high end. The model matches
several RF transformers I've measured. Up till resonances and other
capacitive effects get to you, you can generally extend the response
of a transformer by driving it with a lower source impedance and
loading it with a higher impedance. I have a 1:1 audio transformer
that, when driven with a low impedance and loaded with about 2k ohms,
is flat within +/-0.1dB from 0.6Hz to 109kHz, but quite a bit worse if
driven from 600/loaded with 600, and it shows resonant peaking at the
high end if loaded too lightly.

Suggest you go for a core with modest permeability, probably around
100 (depending on path area and length), so you can drop the
inductance down some from where you are. Harry D. seems to know a lot
about this sort of thing; maybe he'll have some ideas.


Yes, a really low drive impedance is key. I'll just whip up a few and measure them on the bench. The data I can find at manufacturers is mostly based on usage at the impedance they are marketed for.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
.