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A question concerning my Cockroft Walton multiplier

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si2030

Mon Feb 13 2012, 07:19AM

Registered Member #1571 Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44

Hi there,

I need some help on understanding this system and where I have gone wrong...

OK I have a large half wave Cockcroft Walton multiplier. Each capacitor is 500p and rated for 120Kv. There are 15 stages.

I have an air coil with a primary of 200uH and a secondary of 2H. (just under 300mm wide).

I needed the secondary to be resonant with the Cockcroft Walton multiplier as a result of reading this web page. http://blazelabs.com/e-exp17.asp

I have a full bridge driver with IGBTs (irgp50b60pd1 rated at 75Amps continuous)

I have inserted a capacitor in series with the primary so the primary and this capacitor work at resonance - the same frequency as the secondary and CW - so double tank.

Surely once the Cockcroft Walton Multiplier is full and there is no load at the top the system and as a whole should not draw an exorbitant amount of power..... its open circuit at the top so should just be a very high potential once the caps have filled up.

However, after running it I have found that because of such a low primary inductance the bridge is effectively seeing a short!

I have run spice simulations on LT spice that indicate this should work and should anyone like to see it working I have attached the files to this post.

It indicates that this should work but as I have said its producing a short circuit across the bridge...

Can I ask, given these above constraints:

1. Why its effectively shorting (low primary inductance but required in order to create 50kv on secondary)... important.

2. What can I do to fix this... (what can I change to make this work) really important.

Really hoping someone can explain this to me and more importantly point me in the right direction. Can I fix this?

Cheers

Simon
]full_bridge_v1.zip[/file]

Inducktion

Mon Feb 13 2012, 03:25PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

You can't use an air core... You need some sort of coupling. Otherwise the primary coil will effectively BE a short, and not a transformer.

si2030

Mon Feb 13 2012, 03:27PM

Registered Member #1571 Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44

I know this is a stupid question but isnt an air core transformer still a transformer?

Steve Conner

Mon Feb 13 2012, 04:23PM

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

Yes, even when it's a Tesla coil

Air core transformers behave differently to ones with magnetic cores. The magnetising inductance is the same order of magnitude as the leakage inductance, and that leads to some counter-intuitive behaviour. Tesla coil theory can be more useful than regular transformer theory in this case.

jpsmith123

Tue Feb 14 2012, 01:57AM

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

In your circuit I see a primary inductance of 16uH in series with a 2uF cap, so that's a series resonant frequency of about 28.1 kHz and for the secondary I see 1.3H and 39pF + 5pF, which gives about 21.0 kHz.

Is that 39pF capacitance a value derived from a measurement of the resonant frequency of your secondary?

teravolt

Tue Feb 14 2012, 03:13AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Hi jpsmith123, I dont know if this a stupid question or not shouldn't your secondary output be connected to D2 cathode and C4 and C5 be grounded

si2030

Tue Feb 14 2012, 03:17AM

Registered Member #1571 Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44

The primary is about 200uH and the secondary is around 2.2H. The CW's equivalent capacitance is calculated as just under 60pF so the resonant frequency for the secondary/CW is 14770Hz. I added a capacitor that would also resonant the primary at that frequency. ie double tank.

jpsmith123

Tue Feb 14 2012, 03:51AM

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

So the values from the LTSpice circuit schematic were incorrect, or did I misread it?

Anyway, if I understand correctly, you didn't make any actual measurements of the resonant frequency of the coil (either by itself or connected to the multiplier) thus the 14.7 kHz is a calculated number?

Ash Small

Tue Feb 14 2012, 12:59PM

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

This was suggested in another thread back in December, What are you planning to use this for, Simon?

Ash Small wrote ...

I'll have a look at the figures over the weekend, Steve. I had a quick look, but it'll need some studying.

I have another thread here somewhere about a multiplier using VMI(?) diodes from RS, and mounting the multiplier inside the vacuum chamber, with capacitor plates, to get up to ~650 kV (~15 stages with 50kV input). I'll try and find the link for you.

Steve Conner wrote ...

If you have your heart set on 50kV input, some sort of Tesla coil-like air cored transformer might make more sense than a ferrite one. It is really hard to design insulation to stand 50kV of high frequency AC, and also hard to get the stray capacitance of the 50kV winding low enough for efficient operation (or even to make it work at all)

You could put it inside the vacuum chamber too and have a vacuum cored transformer!

Wolfram

Tue Feb 14 2012, 02:37PM

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

teravolt wrote ...

Hi jpsmith123, I dont know if this a stupid question or not shouldn't your secondary output be connected to D2 cathode and C4 and C5 be grounded

It's correct as drawn. The way you describe would not work.

Simon, for the simulation it can be an advantage to simplify the bridge by using ideal switches with some series resistance. That way simulation will be much faster. It won't be as accurate as using the IGBT models you use now, but it will be a lot faster to simulate so it will be a much better way of finding out if the transformer part will work at all. Waiting hours for the simulation to finish seems a bit meaningless when a simplified model would work just as well to show if you're on the right track with the transformer.

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