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Patrick

Mon Mar 22 2010, 11:00PM

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

As per my "About making flybacks as Uzzors2k(Eirik), Steve Ward et al. describe. " thread. I now see the problem that many of you alerted me to, the interwinding and secondary capacitence limiting my upper frequency reach.

EDIT: I will be using a normal ferrite core and a "normal" primary and secondary, i wont be using the flyback with an airgap, i was merely researching uzzors and steves construction methods,which i will likley use as they report great success.

Is it possible to calculate , measure or aproximate this undiserable capacitence? or do i have to build the transformer then measure its resonant freq, and reverse calculate from there? also i have a good LCR meter.

I want the highest frequency possible, because this gives me a high volt per turn, and more power density withless volume/mass of ferrite, which for aircraft and spacecraft (remember im buiding an ionocraft) will be important later. Also several professional sorurces seem to indicate that 100-200khz was used for their 10kw-100kw ferrite power modules.

-patrick

IntraWinding

Tue Mar 23 2010, 12:59AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

I think you might mean 'Intrawinding' there

radiotech

Tue Mar 23 2010, 01:55AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

You can measure this 'self capacitance" with a Q meter.

ref 'condensed manual' Heathkit QM1 Benton Harbor 1959.

QM1 meters are hard to find in mint condition and are sought by experimenters.

This is how it was done with a Heath QM1. You can adapt the technique using a signal generator and a scope to indicate resonance.

Mattski

Tue Mar 23 2010, 02:18AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

You can determine it both by simulation/calculation and by measurement. You want ultimately a simple circuit model based on the physical system which includes an ideal transformer surrounded by various non-idealities like intra- and inter-winding capacitances, resistances, magnetizing and leakage inductances. You can calculate approximate values up front, then with measurements using a signal generator and oscilloscope, you can refine the values in the circuit model.

My first guess would be to take the capacitance from one turn to the next, using a two-wire capacitor model (

). Then you have N turns where each turn is a parallel inductor and capacitor, with inductance per turn and capacitance per turn. Then for N turns you model the terminal characteristics as a single capacitor of C/N in shunt across the terminals. That is the intra-winding capacitance, for the input and output winding.

If you want inter-winding capacitance, if the primary and secondary are not wound on top of each other it will not be smaller. If they are on top of each other then it will be bigger, and you can calculate what it will be roughly.

The first hit gives you more than you ever wanted to know about transformer modeling.

Patrick

Tue Mar 23 2010, 02:37AM

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

yeah intra-winding was what i meant, IntraWinding

McCluskyCustom

Tue Mar 23 2010, 03:23AM

Registered Member #2538 Joined: Sat Dec 12 2009, 06:56AM
Location: San Luis Obispo, CA, USA
Posts: 10

You can calculate it based on the design of the secondary and the wire layering, since most capacitance is from stacking layers. If you can, check out the paper:

Biela, J.; Kolar, J.W.; "Using transformer parasitics for resonant converters
- a review of the calculation of the stray capacitance of transformers,"
Industry Applications Conference, 2005. Fourtieth IAS Annual Meeting.
Conference Record of the 2005 , vol.3, no., pp. 1868- 1875 Vol. 3, 2-6
Oct. 2005

They discuss a number of methods for calculating the distributed capacitance (intrawinding capacitance), but the results are not so encouraging. When comparing calculations to measurements on 4 transformers, the error was over 500% on one, and near 50-150% on the others. As such, calculating this capacitance is probably good for an order-of-magnitude indication, but actual value needs to be measured. Also remember for calculation to take into account the dielectric such as oil or epoxy, which will more than double the capacitance compared to air.

Measuring the SRF is the easiest way to measure the distributed capacitance, since it is easily done with a function generator, resistor, and o-scope. A good paper on it is here. The measurement setup is on pg. 6.

Patrick

Tue Mar 23 2010, 05:12AM

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

yeah mcClusky i feared that, large errors, dam!

Also is the stray capacitence limiting my upper freq, the reason why Steve Ward and others use tapered secondaries. With many turns on the first layers (near ground), and fewer turns on the higher volt outer layers?, since energy stored in capacitence is a square of the voltage across the capacitence?

hey Mattski, what university are you at?

Mattski

Tue Mar 23 2010, 08:02AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

I think it might have been because of insulation breakdown that it was done.

However, I think it would also reduce intra-winding capacitance, because the capacitor size from a wire of one layer, to a wire of another layer is essentially fixed. But by having more layers, there are more of these capacitors in series, so total capacitance is smaller. And you can get away with more layers at the start since the capacitance is mainly where the layers overlap, so each layer has many turns which have little inter-layer capacitance. But you should definitely review the paper McClusky mentioned because it talks a lot about how to actually calculate this stuff.

Patrick, you've actually asked me this already in an old thread ;) but I am at UCSB.

Sulaiman

Tue Mar 23 2010, 02:01PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Don't forget, the intra-winding capacitance (and voltage stress) is much less if at the end of each layer the wire is run back along the length of the winding to start the next layer, that is, each layer starts at the same end.

Steve Conner

Tue Mar 23 2010, 02:12PM

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

It's less still with a stacked pie winding, because the layers are narrow.

And less again with a single-layer winding a la Tesla coil. Steve Ward has built ferrite transformers with single-layer windings, and I always thought this was a clever idea.

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