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"flux cancelling windings" ?

A

amdxjunk

Jan 1, 1970
0
Hi all,
I noted the following (below) on a website.
What is Nc? Number of turns compared to what?
I want to improve a transformer with very low capacitance between the pri
and sec, this might help improve the coupling
while maintaning the low capacitance.
Do you have any more information on this technique?
Thanks
Mike K.

It is also possible to reduce leakage using a pair of "flux cancelling"
windings, each with Nc turns. Wind Nc turns first on bobbin. Then wind
transformer. Then wind Nc turns (so one winding is "inside" the other
"outside"). Connect the windings in parallel. Because the inner and
outer windings have different flux linkages due to leakage, current will
flow thereby creating a flux which counters the imbalance.
 
T

Tony

Jan 1, 1970
0
Nc doesn't matter, as the "Nc" windings are just shorted out anyway.
Could be a single turn of foil for each (as long as the external links
were of similar low resistance.

But bear in mind, this is really a "flux shorting ring"-type of
magnetic shielding, not a flux-canceling technique (it requires an
imbalance to work), and of course it generates losses, and it still
adds capacitance. If you have the space for this method, you might be
better off putting it to more effective use by sandwiching one of your
existing windings between a split version of the other. Maybe some
judicious electrostatic shielding will reduce the effect of the extra
capacitance?

Hi all,
I noted the following (below) on a website.
What is Nc? Number of turns compared to what?
I want to improve a transformer with very low capacitance between the pri
and sec, this might help improve the coupling
while maintaning the low capacitance.
Do you have any more information on this technique?
Thanks
Mike K.


windings, each with Nc turns. Wind Nc turns first on bobbin. Then wind
transformer. Then wind Nc turns (so one winding is "inside" the other
"outside"). Connect the windings in parallel. Because the inner and
outer windings have different flux linkages due to leakage, current will
flow thereby creating a flux which counters the imbalance.

Tony (remove the "_" to reply by email)
 
G

Genome

Jan 1, 1970
0
amdxjunk said:
Hi all,
I noted the following (below) on a website.
What is Nc? Number of turns compared to what?
I want to improve a transformer with very low capacitance between the pri
and sec, this might help improve the coupling
while maintaning the low capacitance.
Do you have any more information on this technique?
Thanks
Mike K.


windings, each with Nc turns. Wind Nc turns first on bobbin. Then wind
transformer. Then wind Nc turns (so one winding is "inside" the other
"outside"). Connect the windings in parallel. Because the inner and
outer windings have different flux linkages due to leakage, current will
flow thereby creating a flux which counters the imbalance.

Errrrrr.... I haven't come across the term "flux cancelling windings" but I
have seen something about 'flux balancing windings' in a Unitrode
Application note about designing transformers with fractional turns. Have a
hunt around the TI website.

It goes something like if you use a fractional turn to get closer to the
right output voltage then leakage inductance goes up because more flux is
diverted to the outer leg of the transformer not associated with the
fractional turn. In order to compensate for it you add a 'short circuited'
balance winding at the end of the transformer which couples the outer legs
together and forces the flux to balance.

DNA
 
T

Terry Given

Jan 1, 1970
0
Genome said:
Errrrrr.... I haven't come across the term "flux cancelling windings" but I
have seen something about 'flux balancing windings' in a Unitrode
Application note about designing transformers with fractional turns. Have a
hunt around the TI website.

see my other post in this thread
It goes something like if you use a fractional turn to get closer to the
right output voltage then leakage inductance goes up because more flux is
diverted to the outer leg of the transformer not associated with the
fractional turn. In order to compensate for it you add a 'short circuited'
balance winding at the end of the transformer which couples the outer legs
together and forces the flux to balance.

DNA


Hi DNA,

Billings has a nice section on fractional turns and flux-balancing.

I once (OK, 15,000) stuck a 7.5 turn winding on a coupled inductor,
without flux balancin. but thats because 14A flowed thru the 3T winding,
25mA thru the 7.5T winding and I had an air gap :)

Cheers
Terry
 
T

Terry Given

Jan 1, 1970
0
ya have now :)
He posts here regularly. Maybe we can get his attention.

probably not. He's notoriously unobservant :)

"Rise Time Reduction in High-Voltage Pulse Transformers Using Auxiliary
Windings, L.M. Redondo, E. Margato, J.F. Silva, IEEE Trans. Power
Electronics Vol. 17 No. 2 March 2002

I have not tried this myself, but the paper is a good read. They used a
transformer with:

Np = 50
Ns = 506
Nc = 25T (they call it Naux)

The xfmr had double P-S insulation and "staircase" secondary
construction to reduce capacitance. They play around with various
resistances connecting the two Nc windings.

Cheers
Terry
 
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