Making a 300 Amp square wave AC inverter

I am considering a project to build a 300 amp, about 40V inverter that would invert DC into square wave AC. That's for converting a DC TIG welder into a square wave AC TIG welder. I do not yet have the welder, but may get it in a few days.

I am coming here after some preliminary discussions on rec.crafts.metalworking.

I have some basic knowledge of physics and some knowledge of electricity to start with, but I am not well familiar with solid state electronics. I built a well functioning phase converter, and the only solid state piece that I ever used was a crydom relay with zero crossing.

The objective is to make the inverter cheaply but in an effective manner.

With that introduction in mind, let me describe what is my current plan.

The plan is to set up an H bridge, using 4 arrays of MOSFETs. These mosfets would be controlled by variable frequency AC signal. To generate this signal, I want to use an HP 204C oscillator (which I have), coupled to an appropriate amplifier (which I do not have).

HP 204C produces 2.5 volts and needs to be amplified about 16 times.

Signal from amplifier would control mosfets, although, possibly, I would also use diodes to prevent negative voltage from being sent to the gate of mosfets.

The H bridge would produce inverted, almost square wave AC.

The mosfets that was recommended would be something like this

For gate input, I would need to produce about 40 V AC (if I am not mistaken), that sort of determines what the amplifier should be. Looks like wattage requirements for the amplifier are minimal.

Is that a sensible plan, and if so, what would be your suggestions. I know how to solder, use a multimeter and use a drill press. I am a moderately handy person and am open minded and welcome suggestions.

Thanks!

You are, quite possibly, right, but I would like to know why that is so, what are the technical reasons.

I agree 100%. I will start witth a full powered control circuit, but with a lot less mosfets and a small load.

I already have an old trusty Tek 475, as well as a dual HP DC power supply (handy for a control circuit vs. load). Plus an HP 204C oscillator that I mentioned.

I have to admit that I am a little confused regarding this.

Why is there any asymmetry between top and bottom MOSFETs.

Would it make sense to try to use "comparators" to convert sine wave into square wave with some breaks in between.

Makes perfect sense. I hope that someone could suggest a good formula for computing needed capacitance. I have plenty of caps (60VDC, IIRC).

it does not...

I agree that it isn't worth doing, but you might enjoy trying.

Try building a little one first - e.g. 5 Amps, 40 Volts. When you have that working (with light bulbs as a load), you can think about building the bigger one. This way, you won't blow up quite so many MOSFETs in the learning phase. You will need an oscilloscope, I suggest a Tek 475 or similar, and an isolated adjustable power supply for testing your miniature version.

You need to work out the proper gate voltage you need for each MOSFET. The bottom ones are easy, because the gate either goes to the negatve rail of the H bridge, or 12 Volts above that. It's the top MOSFETs of the H bridge that are hard to drive, because you will have to use all NMOS FETs in the bridge even at the top, and the gate drive of the top FETs has to be controlled relative to their sources, and the source terminal is going up and down too. Really you should probably opto-isolate the upper two gate drive circuits from the clock source, but then the gate driver circuits will need their own isolated powre supplies too.

The HP oscillator isn't really ideal for the signal source since you want a square wave, try using a 555-timer oscillator circuit instead. Look for a circuit that allows you to alter the duty cycle - this could be useful. Then you could feed the 555 output signal into a gate driver IC such as a TC4421 to drive the MOSFET gates. You still need to figure out the opto isolators and the extra power supplies though, so that you can drive the top FETs. You probably also want to alter the logic signals a bit so that the top and bottom FETs cannot momentarily conduct at the same time as this is bad for them.

Also beware the inductance of the welder output. This could cause large voltage spikes if the load gets disconnected, which will blow up your MOSFETs. You will want to put capacitors and varistors (MOV) across the incoming DC.

If your welder has HF starting that will blow up your MOSFETs too.

Chris

On Thu, 08 Sep 2005 23:47:47 GMT, Ignoramus965 wrote:

wrote:

A probably too-short answer: because they are all N-channel (or NMOS as stated above).

FET's have three connections labeled source, drain, and gate. You turn it on by applying approximately 10 volts positive to the gate relative to the drain, and this effectively connects the source to the drain. The bottom FETs in the H-bridge have the drain grounded, so you just "apply 10 volts to the gate" which is understood as a voltage relative to ground. In the upper part of an H bridge, the FET's source is connected to some voltage source, apparently 40 volts in your case. When turned off, the gate and drain are both at zero. To turn on this FET, you must put 50 volts into the gate to make the gate 10 volts above the drain, because the drain rises to 40 volts when the FET turns on. Oops, your power supply is only 40 volts. Well, you can use charge pumps and/or other stuff to get around that, but however it's done, it's messier and needs more "support circuitry" than does switching the low-side FET's in an H bridge. Also keep in mind when you get this working (a 5-amp circuit will only need one power MOSFET in each leg of the H-bridge) that a circuit that drives a single power MOSFET might not drive 10 or 20 or 50 power MOSFET's in parallel, and you'll need somewhere around that many in each leg to switch 300A. See Don's response about gate capacitance. with N devices in parallel, you have N times the gate capacitance, and you need to supply and remove N times as much current through the gate(s) to swith N MOSFETS as fast as the one MOSFET. You have to switch them fast so that the spend as little time as possible in the "linear range" where they turn electric power into heat. If they get too hot, the magic smoke will come out (this can happen in an explosive way, so don't be having your face over something when you first power it up) and they won't work anymore. :(

Yes, you could send the HP's signal through one comparator in a LM339 quad comparator (or use the single-comparator in a package, I forget the part number), but you're using a benchtop device and a 50 cent part smaller than your thumb to do the work that can be done by one or two 50 cent parts smaller than your thumb (plus a few resistors and capacitors at a few cents each). If you want to do something fancy that the HP oscillator lets you do, like vary the frequency or the duty cycle, a couple of potentiometers wired into the right places can allow you to do that, and they're only a dollar or two each.

I see. So, an appropriate chip would solve these issues by combining voltages, supplying adequate power quickly, etc etc. Is that correct?

That makes perfect sense. I kind of like the idea of using a signal generator to control frequency, but I could give it up. I now am trying to find something to understand how to drive these chips and what it is that they do to convert input into output.

Rich *is* right. Out of courtesy to your request, I'll mention a few technical issues below. First, let me say I think experimentation is great and I encourage it. But what you are talking about should be undertaken only by those who would not need to post as you did. This is not a knock on you. Screwing around with 300 amps is not a "give me the schematic and a few pointers and I'll cobble it together" project.

Some technical issues:

2) Assuming you do have the DC source, how do you know it is capable of driving the inverter? How do you intend to protect the source?

Ed

It does tend to be rather spectacular! :-(

Note, also, that welding is a rather nasty load.

Further, you said you didn't intend to have a high frequency start. I assume you are interested in this because you wish to weld aluminum. Is this for TIG? I've not seen or heard of an AC welder without HF start.

Ted

Well, I am aware that it is 12 kW, but, quite possibly, not aware of all implications of that.

Not yet, but if I acquire a CC DC welder on Monday, I will have a DC source. If I do not acquire such a welder, I will not go ahead with this project.

I suppose that a DC welder has internal protection.

Well, I would attempt to do that.

Well, check out "arc stabilizers" on ebay, they are add on devices.

You've got no hope. At least not cheaply, you might get there after a few dozen iterations, with blown output transistors in ma of those. And in many cases you won't understand what went wrong, as you don't have adequate measurement equipment (at minimum probably 8 channels of digital scope).

You're trying to design a competitive rally car, and at the moment you're saying "Ok, so I need a gearbox between the engine and the wheels. I'm now trying to understand what sort of duct tape I should use to secure it to the frame".

You are looking at several weeks of study, before you even know the questions to ask. A not-bad first step would be to get "The art of electronics", and read it cover-cover several times, omitting the chapters on microprocessors and low power stuff.

The only chances that you'd save money would be to essentially learn everything in AoE, probably something going into FET pathology a bit more, and learn how to accurately work a simulator.

I'd say the best part of a year, and even then I'd say that it's almost certain that it'll die either immediately, or a few hours of welding later.

Yes. You could use PMOS devices at the top if you really wanted to but if the main supply voltage is much more than 20 Volts, it's pretty much just as complicated, and you need at least twice as many FETs for a given current if they're PMOS therefore the cheapest and best design will use all NMOS.

You have source and drain mixed up. The gate voltage is relative to the SOURCE, for both NMOS and PMOS.

Again, sourcedrain swapped!

Again....

Again...

And things connected to welders tend to get blown up, better use the 50c chip. I would not use the comparators with a sine wave input, I would use a monostable chip of some sort, or maybe just some R-C delay circuits and

You want to be a bit careful here, the HF start on the TIG I used to use was basically a Tesla coil with a thick secondary winding in series with the electrode wire. It would tend to blow your MOSFETs and probably your scope too while you're building it. Avoiding this happening might involve expensive trial and error.

Thanks. I just ordered this book. If anything, it will help me make more money on ebay (I sell electronic test equipment as a hobby) and be able to tell which equipment can be profitably taken apart and which parts to salvage.

I just said that they are available, I do realize that adding HF to an inverter may present problems...

Ok. When you can advise someone how to correctly handle the issues raised in those questions, you're ready to start the project. Bring deep pockets (for replacement parts), an in depth understanding of mosfets and gate drive circuits and excellent construction techniques to bear. You'll need a dedicated 60 amp 240 volt AC mains circuit to power the DC welder. A flameproof working environment might restrict the failures to the equipment if you stand far enough away.

Ed

I will read a little, I bought the art of electronics book. It will be useful for me regardless of whether I proceed with this project.

I installed a 60A subpanel in my garage, upgradeable to 75A.

The welder is actually 3 phase.

I built a rotary phase converter relatively recently, which has served me well. Right now it is 10 HP and, therefore, cannot support the full power of this welder, but I do not need full 300A power anyway. I will start playing with thie welder at much lower amperages.

If I find need for higher power than I can currently support, I will simply add an extra idler motor to this converter.

I would like to use some sort of a metal enclosure.

Damn, Ian, don't be so snotty. You remind me of some no real experience professors i have had. Moreover, and in particular, SPICE cannot handle the problems inherent in modeling an arc welder at all.

I am not terribly worried. I will try to implement my project the right way, with gate drivers, snubber circuit etc. I already have some parts working. Before putting it to the toughest test, I will get it through a series of nicer, milder tests.

If, in the end, it all blows up, I will lose perhaps $200-300 and will gain valuable experience. The experience would help me make more $$ on ebay and will be generally enlightening.