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Voltage Multipliers keep destroying drivers

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Andy Kay

Wed Jun 06 2018, 09:32PM

Registered Member #62265 Joined: Sat Apr 28 2018, 11:26AM
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Posts: 24

Newbie to all this, I've been playing with Cockcroft Walton and Marx configurations, and I've been running them from a 12V car battery in order to avoid getting high voltage spikes on my mains electrical system. I've used various kinds of devices for stepping up the 12V to provide a feed for the multiplier, such as CCFL inverters, ionizer circuits, and SS Tesla coil drivers, and every time the system works for a short time before the drive stage for the transformer primary coil burns out. Is there a standard way of dealing with this problem?

ritaismyconscience

Wed Jun 06 2018, 10:13PM

Registered Member #62937 Joined: Wed Jun 06 2018, 08:20PM
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Could it be overcurrent? It could also be inductive spikes that are damaging your transistors. More details would definitely be helpful.

Andy Kay

Wed Jun 06 2018, 11:36PM

Registered Member #62265 Joined: Sat Apr 28 2018, 11:26AM
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Thanks for your interest.

The source is a 12V lead-acid car battery.

This powers a small flyback driver incorporating a JRF2907 MOSFET driving a ferrite core transformer that I reclaimed from a CCFL inverter (an AVT4029 that I destroyed this way on an earlier occasion). The MOSFET failed so I replaced it with what I had, an 1RF1404, and it now works again but I don't want it to go the same way.

The flyback driver feeds a Cockcroft-Walton multiplier consisting of ten stages of 10nF 3kV ceramic disk capacitors and 16kV diodes.

The CW multiplier feeds a Marx Generator consisting of five stages of 1nF 20kV ceramic disk capacitors and 1MOhm 1W metal film resistors. I'm using the same resistor for the ballast resistor.

I think I've covered everything, but if there's anything else you need to know then just say.

Patrick

Thu Jun 07 2018, 12:38AM

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

i need a schematic, you seem to be going from:

battery -> flyback -> CW -> Marx Gen.

if i understand correctly thats a lot to be driving.

Is the flyback driver putting out AC HV ? is this done with a single transistor ?

ritaismyconscience

Thu Jun 07 2018, 03:31AM

Registered Member #62937 Joined: Wed Jun 06 2018, 08:20PM
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I assume it looks something like this (but with a mosfet instead):
6d77d5883e7d911e90036e31e5fbe6fd

My guess is that you need zener diodes to protect the gate.

Andy Kay

Thu Jun 07 2018, 06:42AM

Registered Member #62265 Joined: Sat Apr 28 2018, 11:26AM
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Posts: 24

This is the driver I used most recently:

I will try to attach photos of the setup, which is now dismantled but you can get the idea. There is only one transformer, but I show two in the photos so you can see both sides.

Plasma

Thu Jun 07 2018, 09:51AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
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Hi, what's the capacitor recatance at the frequency you're using, if the cw is a low impedance it will burn most of your energy.
If the capatance is to large it will try to draw to much current, then at high voltage the corna will be a drain for the Marx, but you are using a flyback so I'm assuming 50khz,the capacitor on the cw should be about 100pf

Andy Kay

Thu Jun 07 2018, 01:58PM

Registered Member #62265 Joined: Sat Apr 28 2018, 11:26AM
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Posts: 24

Plasma wrote ...

Hi, what's the capacitor recatance at the frequency you're using, if the cw is a low impedance it will burn most of your energy.
If the capatance is to large it will try to draw to much current, then at high voltage the corna will be a drain for the Marx, but you are using a flyback so I'm assuming 50khz,the capacitor on the cw should be about 100pf

This is interesting. Being new to all this, your comments give me some direction. If my capacitive reactance calculation is correct then you would appear to be saying that the correct reactance for the CW capacitors at 50kHz should be 31.8k. I'm using 10nF ceramic capacitors in the CW, which would give a reactance at 50kHz of 318R -- i.e. 100 times too low. How is the "correct" reactance arrived at?

ritaismyconscience

Thu Jun 07 2018, 04:33PM

Registered Member #62937 Joined: Wed Jun 06 2018, 08:20PM
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I would probably add zener diodes to protect the mosfet. That might help.

Andy Kay

Thu Jun 07 2018, 05:01PM

Registered Member #62265 Joined: Sat Apr 28 2018, 11:26AM
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Posts: 24

ritaismyconscience wrote ...

I would probably add zener diodes to protect the mosfet. That might help.

I was assuming that the company that manufactures this little driver board would have had all the necessary components on the board, but that might be a false assumption. I'll get on it and see if it helps. Thanks.

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