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Danielle

Wed Oct 10 2007, 01:13AM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
Location:
Posts: 267

I finished building an inverter tonight I will calibrate the frequency to 60Hz I should post the schematics later and pics but I blew 2 bricks already drawing 5" 300hz arcs from the MOT I pumped it into. for making the design better I was planning to have a PW control instead of running a half bridge pumped and filtered into the MOT I need an over current protection because I dont want to blow more bricks even though I got them for free. (300A 600V slow half bridge). The design is simple, it is a 555 creating a 40-300hz pulse then I have a PW controller (another 555) and a UCC gate driver 21 and 22 then into a GDT then 2 brick drivers (steve's bridge driver) and to the half bridge of IGBTs. The output goes into a MOT and the input is a car battery I get about 200V at 5A I think I can get much more when properly regulated and the bricks arnt even getting warm with no heat sink. I just need to know if I should use a comparator and shutdown connected to a CT for over current on the primary side.

Marko

Wed Oct 10 2007, 12:09PM

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

Man you need to cool down with those bricks. You need a lot more of huge R&D before building something like this...

Is that going to be a DRSSTC bridge? I don't quite get the idea of your attempt to drive MOT's. What kind of GDT do you use to run at 300Hz?

I myself wouldn't dare to build anything like that without enormous amount of experience (which I don't have). Those are darn expensive things to blow up like that.

One of first things you must learn is that IGBT's are very poor switches for low voltages despite their 'big' 300A rating.

By paralleling few TO220 mosfets, like for example six IRFZ44 mosfets (costing $1 each) easily gets you 300A rating and tens of times more efficient operation at currents >100A.
Enormous waste of resources.

I'l gladly discuss this if you are interested further.

Adding some pics and paragraph breaks would also help a lot for readability of your posts.

Pics often tell so much that you can reduce the amount of text a lot.

Not really any kind of rule, but just makes your posts much nicer to read.

Marko

uzzors2k

Wed Oct 10 2007, 12:36PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

At least use punctuation and run a quick spell check. It does wonders for readability.

http://spellcheck.net/

Or even better get Opera and add the integrated spell-checker.

Danielle

Wed Oct 10 2007, 08:08PM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
Location:
Posts: 267

ok thanks I am not planning to waist more of those bricks and no this was going to go into my car as a 12VDC to 120VAC 60Hz inverter.

I will try the mosfet aproch because I just receved 25 N and 25 P 200V 80A mosfets.

I was board so I just decited to do this, another problem I found was that it wasnt running at 50% duty cycle it was running at 98% and that was killing the bricks.

The reason I was trying to use the large bricks was I had 10 now 8 of them and they are too slow for tesla coils unless I can make one at 5Khz.

I used optical isolation instead of a GDT as it wouldnt work at that frequency.

Marko

Wed Oct 10 2007, 09:27PM

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

I would say these bricks are completely useless for 12V high-current application like your idea of using them in a battery inverter.

What bricks do you have?

You know to calculate power dissipation on a component, don't you?

Simply by paralleling 6 IRFZ44 mosfets you'l have 300A 50V switch with 3.6 miliohms total on resistance, and cost $6.

Your brick has saturation voltage of 1-2V, circle to 1.5V.

At say 100 amps of current our $6 mosfet switch will drop 0.36V and dissipate 36 watts,
while IGBT dissipates >150W at that point. And if that isn't enough you can always add several more mosfets in parallel.

You don't need any higher voltage rating than 50V, as switches will never see more than 12 or 24V depending on if you use bridge or push-pull. Don't use 200V mosfets.

What you should do is sell those bricks and save the money before you destroy them.

I can hardly think of an application you could use them in, unless you need a huge 3-phase motor controller or something like that.

IGBT's really take advantage over mosfets only at high voltages because mosfet's ON resistance increases drastically with voltage rating.

IGBT's also take advantage at high peak currents, but that's another story.

For 12V application they are just no use at all, you'l realize that sooner or later.

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