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.
....

Hello Folks,

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.

I know this is a far stretch but maybe ...

Tom Bruhns wrote:
On Apr 29, 11:31 am, Joerg <notthisjoerg...@removethispacbell.net
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/

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

On Apr 29, 11:31 am, Joerg <notthisjoerg...@removethispacbell.net
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?

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.)

Hello Folks,

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.

I know this is a far stretch but maybe ...

Hello Folks,

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.

I know this is a far stretch but maybe ...

Joerg wrote:
Hello Folks,

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.

I know this is a far stretch but maybe ...

Eric Tart Red "Arbeitsbush fur den HF Techniker"

The very first chapter is about the RF transformers and the simulated
line transformers, their equvalents and the compensation.

But the ratio of 100:1 in one stage doesn't seem reasonable; the
sensible way would be breaking this into the series/parallel connection
of the transformers.

This type of the impeadance matching can be done by a bandpass filter
type of network (also using many stages); I would do it that way.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultanthttp://www.abvolt.com

On Apr 29, 6:43 pm, Joerg <notthisjoerg...@removethispacbell.net
wrote:
Tom Bruhns wrote:
On Apr 29, 11:31 am, Joerg <notthisjoerg...@removethispacbell.net
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/

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

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

... 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.

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.

On Tue, 29 Apr 2008 18:31:08 +0000, Joerg wrote:

Hello Folks,

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.

I know this is a far stretch but maybe ...

A bandwidth of around an octave implies _some_ resonance.

You'll have a huge juggling job between leakage inductance and primary
inductance.

The more loss you can stand the better chance you'll have of making it
work. I'd feel a strong sense of accomplishment if I got 50% of my input
power coming out of my secondary.

I recall reading in some ARRL publication or another (the one on
transmission line transformers, I think) that to achieve extreme ratios
you can often do better using two stages -- in your case perhaps three?
4:1 * 5:1 * 5:1 = 100:1.

Disclaimer: never done it, I wasn't there, it's not my fault, etc.

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.

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:QK0Sj.429$J16.350@newssvr23.news.prodigy.net...
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.

Hmm... how's the efficiency of that approach?

We have a design where there's a four-pin power connector, with two of the
pins being 120-240VAC (nominally) and the other two being jumpered or not to
configure a relay to "configure" a transformer to be either in series or
parallel. Originally the idea was to keep the voltage at the transformer's
secondary the same at 120V vs. 240V as you'd expect since the Vicor module
that the secondary feed didn't have a 2:1 input voltage range. At some point,
though, I found a different power module (thanks to Terry Given) that had very
wide input range and suggested we get rid of the relay and proprietary 4-pin
power connector (just go back to the regular IEC ones), etc., but there was an
objection that running a 240VAC transformer at 120VAC would be "very
inefficient." That didn't seem right to me -- if anything it seems as though
it's probably a skosh more efficient at 120VAC since you're not pushing it
anywhere near saturation -- but I don't have a strong enough background in
power transformer design to rigorously debate the issue.

(...So we stuck with the original design...)

A larger primary inductance reduces core losses, it thus becomes less warm
and under light loads it can be a good thing. At higher currents copper
losses would cause a penalty.

Then main advantage is that the core won't saturate if, say, a unit is
running on generator power and an impatient maintenance guy gooses the
throttle a bit until the old spark plugs that should have been replaced last
year burn themselves cleaner and it stops misfiring.

Hi Joerg,

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:pJ2Sj.12651$GE1.3505@nlpi061.nbdc.sbc.com...
A larger primary inductance reduces core losses, it thus becomes less warm
and under light loads it can be a good thing. At higher currents copper
losses would cause a penalty.

So if you need, e.g., 250W through the transformer (with either 120V or 240V
input), it seems that you'd really want to buy, e.g., a 500W 240V transformer,
yes? ...which of course will be bigger and spendier...

Then main advantage is that the core won't saturate if, say, a unit is
running on generator power and an impatient maintenance guy gooses the
throttle a bit until the old spark plugs that should have been replaced last
year burn themselves cleaner and it stops misfiring.

Ah, good point.

Not quite that bad but yes, you need to provide a bigger one. For high power
you'd be better off buying or designing a good 90VAC-260VAC switcher.

A lot of engineers believe all that can be permanently muffled by some MOV
here and there. However, those work like a bank account and one fine day ...
KABLOUIE.