Making square wave AC from DC? (inverting)

I am curious, can;t I simply use IGBT modules instead of MOSFETs? They feature lower saturation voltage and could make a better square wave.

The irony is that I sold four beautiful 150A Fuji IGBTs for $100 a few months ago...

i

Will it be "close enough" to drive a nice arc? That's the real question.

i

excellent points, thanks...

i *fast* to minimize how much the transistors heat. When the transistor is off, it doesn't dissipate any power. When it is fully on, it conducts a lot of current, but has very little voltage across it. For instance, say a transistor is switching 50V at 20A.

you possibly can. Sin wave drive just won't do the trick.

transistors. You put a circuit together, fire it up, and the switch transistor fries. If you have a single-shot scope, or a nice digital scope that is fast enough, you check to see if the scope succeeded in saving the waveforms you were interested in. No? try it again. Yes? stare at the circuit for an hour or so trying to figure out exactly what happened. Fix what you figured out, try it again. Bummer, another transistor blew out. Why this time? This can take a

*very* long time, especially given your level of experience.

I realized that all I need to drive these mosfets or IGBTs is a HP

8011A pulse generator. i

*fast* to minimize how much the transistors heat. When the transistor is off, it doesn't dissipate any power. When it is fully on, it conducts a lot of current, but has very little voltage across it. For instance, say a transistor is switching 50V at 20A.

No argument that quick is better, but it's actually somewhat surprising how well this could work because it only switches at a couple hundred hertz max. Switchers operate 200 KHz and more. You can switch 1000 times slower without dissipating any more avg power.

That said, I'd still use a MOSFET driver. There just isn't any good reason not to.

Oops, I realized that I sold the 8011A, someone will be picking it up this Sat.

I can, instead, use a wavetek 171. I have a working wavetek 171 and it should be very easy to adjust to do whatever I want. It makes up to 8 volts on output. Its output will simply drive mosfets or IGBTs, no extra circuits required. I am a little concerned about "shoot through", with square wave output from wavetek, though.

i

Triacs and SCRs don't shut off on a DC source. They need to see 0V across them in order to shut off. There is a process that can shut off a SCR that is designed for that purpose but it ssn't an easy process. Better to use MOSFETs for this job. Bipolar transistors (npm or pnp) transistors have a large (relatively) on voltage across the ends (E to C) and they need to have a fairly large current into them (B terminal) in order to work and the larger devices also tend to be slwo relative to what is wanted. In addition, bipolar transistors have a positive temp coeeficent (the current goes up with temp) so you have to do emmiter resistors Iincreasing the power losses) in order to keep them from being unruly. MOSFETs have a negative temp coeeficent so they will decrease the current in the device when the temp goes up, making paralleling devices easy. The only downside to using MOSFETs is that they require 10-12V to turn them on fully but that requirement isn't difficult to achieve and the current requirements are minimal as the control terminal is basically a cap that needs to be driven to a particular voltage. The basic H bridge is what you are discussing and there are controller chips for doing that, sorry but I don't have any chip numbers for this, which can make your design easy to implement.

-- Why do penguins walk so far to get to their nesting grounds?

I'll slap her good enough that her kids'll be simple.. no, wait, that's bad.. Aw, dang, this time travel stuff is tough.

John

Can these be simple enough to set one up for, say, each AC motor that needs to be run? Switch the motor through the invertor..

John

You keep focusing on the easy part.

Just getting the triggering square wave signal that provides the input is trivial compared with ensuring that the current drive is sufficient to drive the capacitive load of the MOSFETs fast enough. Then there is the big issue of management of the high current supply and variable load without getting the FETs into destruct mode.

If you can't develop your own adjustable square wave drive for the input, I've got to think that your chances of building a square wave modulator for hundreds of amps into an ugly load are nil.

Lots of people have told you that this is very difficult. I think I know a bit about where you are coming from. When I have started trying new things, I usually jumped in very deep, and usually have had good luck. Many years ago I might have had your optimism on this project. Now, knowing just a little bit about the challenges, I wouldn't attempt it unless I was willing to waste lots of time and money just to learn lessons others have already learned.

Good luck, though. Be sure to wear that eye protection when you start testing with lots of input power.

The suggestion of buying the book AOE seems very good. If you really still want to do this, read all the relevant sections very carefully for a long time (the information is very high density and details are easy to miss until you go back to dig and see if you can figure what went wrong) and hopefully do some less critical experiments first.

Great!!! Now you've done it...or will do it...or something like that. All I know is that you are to blame!

Better to have 100 bucks in your pocket vs 4 burned out parts in your junk box.

chuck

An excellent point.

If I go ahead with it, I will start small.

I will attempt to make a DC square wave inverter for 12V (car battery) and would gently test it with a low voltage load bank (which I have), at low loads like 20A.

If things go bad, well, I will have learned something.

That's a good suggestion. I am reading some application notes now, which are also quite dense.

i

Sorry.. I'm so sorry about that..

No, wait- new information just in- read the post I brought in from misc.survivalism; "OT Neocons in China"- cliffy's a *clown*!

WooHoo! It wasn't me, he's just a clown! No, no.. he's a *she* clown!

Heh-heh-heh-heh. Heh. Hah.. hahahahahaha..

Whew.. I really feel better now.

Johh

She's a clown!! Hahahaha!

I have blown up lots of transistors, capacitors, and even a ferrite toroid. But, from 30 years ago, T0-3 bi polar transistors paralleled with insufficient ballast resistors are the best. When one goes and blows a hole in the steel lid, it causes the next one to go, which.... The machine gun effect sound will make people dive under their benches for cover.