"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...)

---Joel

"Joerg" <notthisjoerg...@removethispacbell.net> wrote in message

news:CV0Sj.981$To6.131@newssvr21.news.prodigy.net...



Tim Wescott wrote:
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.

On extreme ratios you just can't avoid it.

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

Yes, and you can never have half turns like at some restaurants.

Please give some more information about why you can never have half turns.
I remember
overheating a transformer that used a half turn. Never tried half turns
again. But what is the reason the half turn gets hot?
Mike

On May 1, 11:57 am, "amdx" <a...@knology.net> wrote:



"Joerg" <notthisjoerg...@removethispacbell.net> wrote in message

news:CV0Sj.981$To6.131@newssvr21.news.prodigy.net...

Tim Wescott wrote:
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.

On extreme ratios you just can't avoid it.

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

Yes, and you can never have half turns like at some restaurants.

Please give some more information about why you can never have half turns.
I remember
overheating a transformer that used a half turn. Never tried half turns
again. But what is the reason the half turn gets hot?
Mike

With a toroid core, it should be obvious why you can have only
integral numbers of turns. In an E-I core, or a pot core with
openings on both sides, you can have a wire exit a different place
than it entered. The loop then closes around one of the outside
"legs" of the core. Note that this is equivalent to a full number of
turns around the center post, and one turn around the outer post, with
the two connected in series. IF the magnetics are balanced, the field
in the outer leg will be half the field in the center leg. But this
happens only if there is no current in the turn around the outer leg.
Note that the "half turn" is not strongly coupled to the rest of the
turns, and as a result adds a lot of leakage inductance. I don't see
why the "half" turn itself should get hot, but if it diverts the
magnetic field into the other leg in such a way that it significantly
increases the core loss in that leg, it could lead to excess power
loss in the transformer.

Cheers,
Tom

Tim Wescott wrote:
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.


On extreme ratios you just can't avoid it.


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


Yes, and you can never have half turns like at some restaurants.

With a toroid core, it should be obvious why you can have only
integral numbers of turns. In an E-I core, or a pot core with
openings on both sides, you can have a wire exit a different place
than it entered. The loop then closes around one of the outside
"legs" of the core. Note that this is equivalent to a full number of
turns around the center post, and one turn around the outer post, with
the two connected in series. IF the magnetics are balanced, the field
in the outer leg will be half the field in the center leg. But this
happens only if there is no current in the turn around the outer leg.
Note that the "half turn" is not strongly coupled to the rest of the
turns, and as a result adds a lot of leakage inductance. I don't see
why the "half" turn itself should get hot, but if it diverts the
magnetic field into the other leg in such a way that it significantly
increases the core loss in that leg, it could lead to excess power
loss in the transformer.

On May 1, 12:32 pm, Tom Bruhns <k7...@msn.com> wrote:

On May 1, 11:57 am, "amdx" <a...@knology.net> wrote:




"Joerg" <notthisjoerg...@removethispacbell.net> wrote in message

news:CV0Sj.981$To6.131@newssvr21.news.prodigy.net...

Tim Wescott wrote:

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.

On extreme ratios you just can't avoid it.

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

Yes, and you can never have half turns like at some restaurants.

Please give some more information about why you can never have half turns.
I remember
overheating a transformer that used a half turn. Never tried half turns
again. But what is the reason the half turn gets hot?
Mike

With a toroid core, it should be obvious why you can have only
integral numbers of turns. In an E-I core, or a pot core with
openings on both sides, you can have a wire exit a different place
than it entered. The loop then closes around one of the outside
"legs" of the core. Note that this is equivalent to a full number of
turns around the center post, and one turn around the outer post, with
the two connected in series. IF the magnetics are balanced, the field
in the outer leg will be half the field in the center leg. But this
happens only if there is no current in the turn around the outer leg.
Note that the "half turn" is not strongly coupled to the rest of the
turns, and as a result adds a lot of leakage inductance. I don't see
why the "half" turn itself should get hot, but if it diverts the
magnetic field into the other leg in such a way that it significantly
increases the core loss in that leg, it could lead to excess power
loss in the transformer.

Cheers,
Tom


Which got me to thinking: you can keep the magnetics in the two outer
legs balanced (that is, the rate of change of flux per unit time) if
you put a turn around each and put those two turns in parallel.
However, each will see half the flux that's in the center leg, so will
contribute half a turn's voltage...this could be an interesting way to
get a high step-up ratio with fewer secondary turns: only 50 turns
instead of 100, to get a 1:100. That could be an advantage in keeping
the parasitic capacitance on the secondary at bay, though the
effective capacitance is generally a very weak function of the actual
number of turns--and for modest permeability cores, significantly
lowers the pri:sec coupling as compared with having the windings co-
axial.