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Wolfram

Sun Dec 18 2011, 11:57AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Case B (second picture) is correct as long as you use the secondary capacitance as C.

Do you have the values figured out? I think R12 represents magnetizing losses in the transformer. L11 represents the magnetizing inductance of the transformer, this is just the measured or calculated inductance of the primary with no load on the secondary. R11 represents the winding resistance, this should also be easy to measure or calculate. L10 represents the leakage inductance, which along with the secondary capacitance and turns ratio is what determines your resonant frequency.

You asked if turn-to-turn capacitance and end-to-end capacitance can be accommodated with a single capacitor. Theoretically, I don't think they can, but using a single capacitor to represent them will probably result in negligible error. That's how I've always done it at least.

The leakage inductance, secondary capacitance and turns ratio are the main factors influencing your resonant frequency. I'm not really sure how you calculate the leakage inductance and secondary capacitance, there are a lot of factors that influence these, so it might be easier to just try building the transformer and measure the resulting resonant frequency.

I guess you are trying to figure out if your transformer design has a chance of working. What is your intended operating frequency? What is the physical size of the transformer?

Steve Conner

Sun Dec 18 2011, 01:17PM

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

I don't know how to calculate leakage inductance, but I've always used the following assumptions:

If you put the primary and the secondary on opposite core limbs, the leakage inductance is about equal to the inductance of either coil if it didn't have the core in it. (so for instance, referred to the primary, it is just the inductance of the primary without core)

That is the worst case. If the primary is put on the same limb as the secondary, it goes down, and then interleaving reduces it further.

I think this tends to be pessimistic, but it is more realistic for HV transformers that have a lot of air space around the secondary for insulation. For little low voltage SMPS transformers it would make them look a lot worse than they are.

Carl Pugh

Sun Dec 18 2011, 05:51PM

Registered Member #1064 Joined: Tue Oct 16 2007, 05:04PM
Location:
Posts: 42

Two excellent books on transformers are
Electronic Transformers and Circuits, Reuben Lee and others.
and
Transformer Engineering, Blume and others.
In particular Electronic Transformers and Circuits explains how to calculate stray capacitance in transformers.

Pinky's Brain

Sun Dec 18 2011, 08:05PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

BTW, aren't diode split secondaries just as relevant to HV bridge converters as they are to HV flyback converters?

Steve Conner

Sun Dec 18 2011, 08:16PM

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

Absolutely, if you want a DC output. The capacitance issues get worse as the square of the number of turns (I think) so splitting the secondary up is a huge win.

You can think of it as a bunch of lower voltage power supplies in series, that just happen to share a transformer core. Except they don't even have to share a core, which might be handy if you have several small cores but can't find a single giant one.

Patrick

Mon Dec 19 2011, 03:20AM

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

Pinky's Brain wrote ...

BTW, aren't diode split secondaries just as relevant to HV bridge converters as they are to HV flyback converters?

Steve Conner wrote ...

Absolutely, if you want a DC output. The capacitance issues get worse as the square of the number of turns (I think) so splitting the secondary up is a huge win.

You can think of it as a bunch of lower voltage power supplies in series, that just happen to share a transformer core. Except they don't even have to share a core, which might be handy if you have several small cores but can't find a single giant one.

I was planning to put 4 or 5 core secondaries in series , with 90-120kV of mylar from core to secondary and another 90-120kV of mylar between Primary and secondary. (the normal max operating V would be about 40 kV at 500 watts.) under oil of course. So, the primaries would be CMC'ed-parralled, clock sync'ed, and the secondaries in series with diodes. Each secondary would contribute about 10kV.

This is what i planned to do, but heres my question, i have the typical full bridge, half bridge, PP-CT, with +2300G B swing followed by -2300G B swing, will the rectified volts-seconds be unequal? I do need HVDC out.

I have no idea if any of this is even possible, so if i can simulate, and identify important factors ill now where to be cautious in strategy and topology. One last thing my cores are small planars, about 0.8 x 1.25 x 0.5 inches. volume for insulation and enough turns is becoming problematic, FEMM analysis is helping though.

My primary and secondary are wound on the central leg, and one feedback winding.

Pinky's Brain

Mon Dec 19 2011, 04:28PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

This might be relevant to your interests :
Optimal Design of a 3.5 kV/11kW DC-DC Converter for Charging
Capacitor Banks of Power Modulators

Patrick

Tue Dec 20 2011, 05:10AM

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

OH Yeah!!!

Becasue the capacitors are in series their final contribution will be small, and im hoping for pF or nF and 10kv for those caps. i need a fast rise time on the HV square wvae.

It does worry me have so much secondary attatched to cap thoughs, even through diodes.

Im trying for a 10% to 90% rise in 0.01 seconds

Patrick

Thu Dec 22 2011, 11:40PM

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

Given this case "B" and the below schematic...

would L6 be divided by the number of isolated secondaries, then refelected over to the primary via turns ratio squared? I guess the C1 would be multiplied by the number of secondaries ?

If so that would give me total control over all significant factors in the design of any transformer circuit for HV, HF, and high power.

i found this source:

1324611812 2431 FT1630 0509understanding Fig03

I think this example is better than my case "B".

jpsmith123

Fri Dec 23 2011, 03:19PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I forget, what voltage and current are you aiming for?

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