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Designing a high voltage battery powered capacitor charger

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TheMerovingian

Wed Mar 14 2007, 11:12PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

For my coilgun project i'm going to build a self-resonant push pull driver ( the royer flyback driver but for 330V). I found a nice transformer on a power supply (very compact, fits my needs). I desolder it and found that it is actually an inductor. Immediately I dismantled it to see if it has gaps (as I expected from an inductor). I found a large gap, making it unsuitable for push pull (too much leakage inductance, the mosfets would overheat ). It has cool shaped and was very small, ideal for compact designs. It would work as a boost or flyback converter, but it throuws too much noise on the circuit

. How much filtering do I need to prevent spurious signals on my device? Any suggestions?

...

Wed Mar 14 2007, 11:39PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Have you considered the mazzilli driver? It will run with a pretty decent gap, and there are no sharp transients

Dr. Shark

Thu Mar 15 2007, 10:51AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Wait, the "self-resonant push pull driver ( the royer flyback driver but for 330V)" and the " marzzilli driver" are the same thing, right?
I would also imagine this driver to work rather well with an air-gap, since it resonates the leakage inductance with a resonant capacitor. More leakage inductance means you can get away with a smaller capacitor, and you might have to adjust some other circuit components. Other than that, definitely try it out!

Dr. Dark Current

Thu Mar 15 2007, 03:17PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

if you mean the "mazzilli" ("zvs") flyback driver, then it can run with no core at all, so a core with a big airgap is no problem..

Sulaiman

Thu Mar 15 2007, 05:23PM

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

Royer/Marzilli/zvs/cfpr are essentially the same.

An excellent topology for constant output voltage;
not really suitable for capacitor-charging.

A non-blocking flyback/boost convertor is probably the best
for which an inductor with a (relatively) large airgap is ideal.

The higher the primary supply voltage, the more efficient will be the boost convertor
e.g. Two small 12V batteries in series is better than the same batteries in parallel.
The efficiency will be determined largely by the ON voltage of the switching mosfet or igbt.

A forward-convertor is possibly the most efficient I think.

Steve Conner

Thu Mar 15 2007, 06:11PM

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

I got a Mazzilli oscillator built around an ATX power supply transformer to function fine as a cap charger. The secret is to use a small value capacitor in series with the high voltage winding, this acts as a ballast to limit the current. Capacitors don't normally make good ballasts, but in this case the Mazzilli's sine wave output lets it work fine.

With an ATX transformer, you really need a voltage doubler rectifier on the HV winding. By making the caps in the doubler small, it also functions as the ballast. I had a prototype of this charging a 1000uF cap to 350V in a few seconds, while drawing about 10A at 12V. I posted a schematic and explanation years ago, it'll be in the "Older" archives afaik.

TheMerovingian

Thu Mar 15 2007, 07:21PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

Exactly I meant that. The Mazzilli driver. I used it in my coilgun rifle switching off the power with a relay and it works very good (5 sec 250J). I was worried about the spikes due to the leakage inductance and energy stored in the gap. The idea of the ballast is cool, i'll try it out (i understood why smaller diodes kept blowing, too much current). Since the regulation is impossible and the relay isn't the best option for a compact design i will try to bring down the mosfet gates to turn it off with using two small mosfets. The boost converter is compact but throws tons of noise on sensitive circuits (i'm using microcontrollers, so high risk). I don't know how to design a flyback converter for best efficiency. All my flyback converter test resulted in lots of heating and small power.

Steve Ward

Thu Mar 15 2007, 09:28PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Mero,

For the royer oscillator there *must* be an air gap, precisely because the primary must store a good bit of energy (and ferrite cores suck at storing energy). So the air gap stores the energy.

Sounds like you need some reading about power circuits? Flyback and boost converters are practically the same thing (use flyback when you need extra high voltages, thus turns ratio helps you greatly). Both will be noisy. A half-bridge is probably your best option from a noise point of view, and are typically more efficient, but at the cost of added components (size). A forward converter might be ok too, but i havent worked with them much, and they might be just as noisy as a boost/flyback. Forward converter must not have an air gap, though.

TheMerovingian

Thu Mar 15 2007, 10:16PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

Why people remove the core spacers from flyback transformer?

Steve Ward

Fri Mar 16 2007, 04:22AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Removing the spacers can be a good thing depending on the driver topology. If using a half-bridge, then you would want to get rid of any air gap, as energy storage in the core is not required. But for true flyback operation, an air gap is essential. It can work without the gap, but this isnt ideal.

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