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Calculating self-resonant frequency of HV Transformer?

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Ash Small

Thu Jan 31 2013, 08:46PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve Conner wrote ...

You might want to consider using a copper foil primary to get the height down. A "20 amp" cable might well be under-utilised at 25kHz due to skin effect.

I was considering a Litz primary, to keep the mass down. Would foil have any advantages over Litz? How would foil affect the capacitance?

Patrick

Thu Jan 31 2013, 10:10PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i would use a foil primary, as its a more effcient in terms of volume, than roundish litz.

further more, ive done previuosly (with planar ferrite hv transformers) what your trying to do through calculation. And it didnt end well...

the problem is youll arrive at some specific C value (from Coil to Ground, or coil to coil) then youll see the accuracy, precision and significant digits cuasing your nice little C and/or L value, to be + or - 9,000% which effectivley makes your calculation useless.

your "suck it, and see" comment is basically right.

Newton Brawn

Fri Feb 01 2013, 03:42AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Ash :
What is the Bmax in your ferrite core ?

The max voltage between two layers is 2500V, I believe 5 x 0.1mm mylar or polyester is enough to avoid corona witout oil ...At a safety factor of 1.25...

The isulation between coil and core to be 30 ~35 kV, if the center tap is grounded to core..

Patrick

Fri Feb 01 2013, 06:34AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Newton Brawn wrote ...

The max voltage between two layers is 2500V, I believe 5 x 0.1mm mylar or polyester is enough to avoid corona witout oil ...At a safety factor of 1.25...

insulating the high field-intensity regions does not prevent corona. if air is present, suffciently sharp geometry and high enough voltage, their will be corona. reguardless of how well you insulate the conductors.

Oil is desireable becuase its cheap, easy to get, holds off V, and displaces air in um vacuum drawn production processes.

Ash Small

Fri Feb 01 2013, 12:39PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Newton Brawn wrote ...

Ash :
What is the Bmax in your ferrite core

The thread on maximum volts per turn for these cores is here:

So the two areas to concentrate on are minimizing leakage inductance, and minimizing inter layer capacitance?

What should I be looking for as far as foil primaries are concerned? I'll try the usual suppliers, and see what they have on their websites.

EDIT: Would something like this be suitable for ~20A?

Steve Conner

Fri Feb 01 2013, 03:39PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yeah, good luck though, because most measures to reduce interwinding capacitance also increase leakage inductance.

What thickness of foil is needed? Rod Elliott recommends a current density of 2.5 amps per square millimeter for transformer windings. Copper has a skin depth of 0.4mm at 25kHz. So, the foil needs a cross-section of 8 sq mm, and a thickness no more than about 2 skin depths.

So, the 70mm wide copper tape linked would do fine, if it were at least 0.11mm thick. This is much less than one skin depth, so the current density will be evenly distributed: we don't need to worry about skin effect.

Contrast that with a round wire of 8 sq mm cross-section, which has a diameter of 3.2mm: 8 skin depths.

Did you mention that this transformer would be used under vacuum? There could be serious issues with cooling that would require a lower current density. Under oil, the current density could probably be a good deal higher.

I would also avoid sticky-backed tape under vacuum, who knows what it would outgas.

Ash Small

Fri Feb 01 2013, 04:53PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks for the info re. copper foil primaries, Steve.

The multiplier circuit will be under vacuum, the transformer will be under oil.

Suppose I'm designing for 100kV insulation under oil @25kHz, what minimum clearances should I be looking at? (oil will probably be cheap cooking oil, boiled and vacuum outgassed) I'm currently working to ~10mm oil, and 1mm polystyrene as minimums.

I may start considering other plastics for the bobbins, as the glass transition temperature of polystyrene is only 100C, although dielectric properties are excellent.

Patrick

Fri Feb 01 2013, 06:28PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ash Small wrote ...

I may start considering other plastics for the bobbins, as the glass transition temperature of polystyrene is only 100C, although dielectric properties are excellent.

im getting good at laying out fiberglass, arimid and carbon fiber. for electrical work i think S-2 or E glass is best, with MAX 1618 epoxy which is good for electrical too. you and i could figure out how to make a few bobbins, they would be specific to our purposes, without machining solid plastic.

electrical " pie windings " maybe the solution to your C and L problems, as steve rightly points out, a solution to one, generally screws up the other.

Ash Small

Fri Feb 01 2013, 08:39PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

Ash Small wrote ...

I may start considering other plastics for the bobbins, as the glass transition temperature of polystyrene is only 100C, although dielectric properties are excellent.

The working temp. of most epoxies isn't that much greater than polystyrene. I'm currently looking at PE, PP, PVC and ABS, etc. Interestingly, PP is above it's glass transition temp. already at room temp. I suspect that PE and PVC are as well, but I've not checked.

What are the properties of polyester and epacrin resins?

(I'm not planning on machining the polystyrene, I was planning on using polystyrene cement. I'm forming the elliptical tubes by heating round tube of the correct circumferance to it's glass transition point, and 'forging' it into elliptical tube. For the 'fins' I'm using old CD cases. So far, the only cost has been for polystyrene cement.)

I'm still researching 'pie windings', I seem to be finding differing explanations on different websites. Do you have any good links?

EDIT: Do you mean '2T Pri : 500T Sec : 4T Pri : 500T Sec : 4T Pri : 500T Sec : 4T Pri : 500T Sec : 2T Pri' ?

Steve Conner

Sat Feb 02 2013, 12:07PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

When I hear pie windings, I think the disc-shaped secondary on an old TV flyback. Basically a winding with a pie-shaped aspect ratio. A RF guy might think of something different, like the old basket-weave coils, which can also be made in a pie shape.

I seem to remember that clearances under oil can be one-sixth of what they would be in air.

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