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Best Filler for Vacuum potting?

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ConKbot of Doom

Fri May 04 2007, 02:37AM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
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Posts: 329

I'm considering making a boost converter, and to increase the working voltages of the inductor, I'm considering vacuum potting it in something. But what would be best to used? Would just any clear epoxy work? Clear (definitely not the black. carbon content...) RTV silicone? Wax?

We have vacuum pumps and bagging equipment at work so I figure I could put the inductor and potting compound in a paper cup, put that in a mason jar, or other glass jar, tape the vacuum bagging on top of the jar, and put a vacuum port in it and let it suck down for a shift at work.

I can put a vacuum of 25-28 inches of mercury on the part for ~8hours minus whatever setup time I take. If I have to leave it on longer, I can leave it on a 20-25 in Hg vacuum for up to 24 more hours after that.

Any suggestions for compounds?

Steve Ward

Fri May 04 2007, 03:06AM

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

What is the designed output voltage for your boost converter? If it truly is a boost converter (and not a flyback converter) then you must have a very impressive switch that it switches voltages that require vacuum potting of the inductor its switching.

Typically, if you want to boost your DC input by more than 2X, you use a flyback converter. By the sound of your request, me thinks you are building a flyback converter (either that or your boost converter is switched via HV vacuum tube).

ConKbot of Doom

Fri May 04 2007, 11:51AM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
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Hmm, well I guess I beter make sure I'm not totally off base with what I wanna do.

I want to do some research and experimentation with the dielectric breakdown of water. Eventually making a prototype electro thermal gun, if things look promising. I want to be able to charge a capacitor bank to do some tests, just causing a breakdown in water.

I'm not set on my topology yet, I want to build a converter for 12v in to 1000-1100v out, which looks like its is about as far as you can push a boost converter from other posts that Ive read. I guess for now I could buy an isolation transformer and build a CW off of mains, but eventually I would like to make a portable power supply that runs off of a 12v SLA.

Anyway... Ive mainly worked with boost converters, and not flybacks, so I guess I'm just sticking with whats familiar for now, plus finding a suitable transformer, or winding my own vs buying a suitable inductor from digikey or mouser fairly cheaply makes the boost look more desirable. I know 1kv really isnt a ton of voltage, but I hadnt seen a maximum voltage in the datasheets for the inductors ive been looking at online. I figured potting would help make sure it doesn't over volt the inductor, and if I have the resources to vacuum pot it, why not do it right.

Should I just skip on a boost converter for now, and just use an isolated CW off of mains?

Marko

Fri May 04 2007, 12:15PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I'm not set on my topology yet, I want to build a converter for 12v in to 1000-1100v

You do realize how inefficient is that going to be?

If you are going to use a high voltage IGBT with high forward drop and put 20amps trough it you will cook it just from ohmic losses.

Not to say how will the slow device feel about switching such a high current and high output capacitance charged at 1000V!

A small simple circuit like royer with IRF1010 or similar mosfets (wich will run happily on primary side with 12V) would be orders of magnitude more efficient.

If you really need regulation, you'l need a flyback or forward converter; not as efficient, but still good.

Boost converters really aren't much of good for such enormous input/output voltage ratios!

ConKbot of Doom

Fri May 04 2007, 12:54PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
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Any suggestions for sources of transformers to build this? In the future, the voltage I need will probably increase as I want to increase power. Would an automotive ignition coil work for this if I limit the inductive spike on the primary? I dont need regulaton per se.. I just need the charger to not blow the caps up. I'd rather not have to wind my own transformer, or at least the secondary anyway.

Marko

Fri May 04 2007, 01:09PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

If you don't specifically need regulation go for royer 'mazzili' circuit and use a flyback core.

For the secondary coil you'l be fine with a single layer winding and really no any kind of vacuum potting is required.

You can gap the core to limit the current or use ballast inductors, or even a capacitive secondary side ballast since output is sinewave.

ConKbot of Doom

Fri May 04 2007, 01:55PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
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mmm, I found a flyback in my electronics bin, surprised I forgot I had this, tore it out of and old TV I found.

I have a few fairchild mofets and IGBTs laying around, I'm sure I'll find a suitable one. Along with some 1700v fairchild diodes. I think I only need to get some resistors.

How big should the pull down resistors and zener diodes be (wattage wise) for the mazzili circuit?

Also, gapping the core to limit current, I take it that means secondary and primary need to be on opposite pieces of the core then? What kind of spacing are we talking about? thousandths of an inch, millimeters, centimeters?I can get anodized aluminum foil ranging in thickness from .0005" to .003" or so at work, and use a few layers as needed, or would eddy currents be an issue with metallic (though non-ferrous) spacers?

Sparrow338

Fri May 04 2007, 10:02PM

Registered Member #661 Joined: Sat Apr 21 2007, 03:17AM
Location: Ventura, CA
Posts: 41

Gapping means the space in between one core half and the other, normaly the spacers are made of plastic, or cardboard.

ConKbot of Doom

Sun May 06 2007, 08:54AM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
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Posts: 329

ok, put in an order in to mouser to get everything, ended up spending way more then I needed to, but I always like to get spares and lots of extra for my electronics bin.

To check the output of the transformer, to make sure its sinusoidal, and the core isnt saturating, can I wind a few (2-10 windings or so, depending on the volts/turn ratio) and use my sound card as an O-scope for that? With anti-parallel diodes for clamping of course...

Steve Ward

Sun May 06 2007, 04:28PM

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

Yes, winding a "meter winding" on the core should work fine (thats how i do it myself). Just a few turns is plenty to get a pretty good idea of whats going on. Keep in mind that depending on how you drive the thing, the voltage waveform might be kinda messy. In these cases, i watch the waveform as i turn up the input voltage, and watch for any big non-linearities. Saturation is usually pretty "sharp" with ferrites, so you should note a pretty obvious change in waveform if/when it does saturate.

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