Can I use a stock SIGNAL GENERATOR to drive an H bridge?

My objective is to make a square wave inverter for high amperage DC for TIG welding, from a 300A CC DC welding power supply.

I would actually be content with 200A current. AC would be used for aluminum only.

I own a Wavetek 171 signal generator:

Can I simply use it to drive either MOSFETs or IGBT's that make up the H bridge?

thanks

Not directly.

You'll need some level translation.

If you don't know what that means then you shouldn't be attempting to make a welder.

Unless you fancy welding *yourself* to something ? Terminally maybe ?

Btw - electronics hobbyists are advised to try out *low current* stuff before progressing to the 'arcs and sparks' scenario !

Graham

do you refer to signal voltage level?

I would love to hear you share your knowledge.

There's quite a bit of gate capacitance in larger MOSFETS that has to be quickly filled or emptied. Drivers are by no means trivial.

In fact, the design of the driver stage is usually much more difficult than the output stage.

Don, what are your thoughts on using IGBT's for switching?

Google IR2011.

That'll explain driving H bridges a bit. Be especially aware of the term 'high side drive'.

Graham

Graham replies that they're very good these days.

Mosfets are best at the higher frequencies though.

IGBTs have their own little 'quirks'.

trouble is.... you really have to have been doing this stuff for ages to pick up the relevant issues. For a beginner, jumping in at the deep end involves a cliff-like learning curve.

Graham

side drive'.

Thanks. I just read their spec sheet. Is there some other document that you have in mind?

Oh, boy! Now, you're REALLY getting in deep water!

The FETs are a lot more forgiving of shorts and linear operation, and at the voltages you are looking at, they work a lot better, too.

But, still, you need perhaps 2-3 A of gate current for EACH FET in your circuit! If you put 5 in parallel, you need 10-15 A of gate current to charge the gates. Now, that is only for 50 nS or so, then the current in the gate drops to zero. But, if you don't provide this kind of current, the transistor will burn up in the linear region before it ever gets fully turned on.

With IGBTs, if they are ever allowed to operate in the linear region, even for a hundred nS, they are destroyed! The FETs have what is called negative temperature coefficient, when they get hot, the current they will conduct drops. This allows them to distribute current evenly across the transistor, and across multiple transistors, even when in the linear region. IGBTs flatten out in the saturation region, but have a strong POSITIVE temp coeff. in the linear region. Current will "hog" to the hottest part of the transistor die in the linear region, and the transistor will self-destruct.

So, you really want to avoid the IGBT.

And, you will not be able to use your signal generator because it can't provide several amps. You could do a low-power test, maybe lighting an LED with it, to see the rise-times, etc. on your scope.

For an H bridge, you need 4 different gate drives. Two of them can be ground-referenced, but two of them (for the "high-side" transistors) will have to be floating. You don't want to use complementary transistors, ie. P channel, as they have MUCH lower performance than N channel. So, using all N channel, the high side transistors have their source connected to the floating output terminal. So, the gate driver must supply gate voltage referenced to the floating output voltage.

The IR 2113 and related chips can handle much of this stuff for you.

One final problem is the DC welder will have a huge inductor at the output. Thus, the output will probably have large voltage excursions when the electrode touches the workpiece and the arc starts. So, you can't depend on getting a steady 40 V DC or whatever. When the arc breaks or sputters, you may have hundreds of Volts on the electrode cable. I think the standard practice is to move the inductor to the electrode cable AFTER the switching circuit. But, the design of the welder may make this hard to do. Certainly in AC "buzz box" welders, the output inductance is just built into the transformer, it is not a separately-connected coil. I guess they can't do that in a DC welder, so you may be able to change the connection there.

Jon

'high side drive'.

I can think of loads ! You can't imagine how many data sources I used before venturing into this arena.

You need to examine the application notes really.

The ANs normally contain 'nuggets of wisdom' worth as much or more than gold.

Graham

Yeah, like hanging on the cliff while the enemy is strafing you! Actually, that is being showered by incandescent particles of your expensive IGBTs, right after the cannon-like bang when they explode! I know, I've been there. The last inverter design I worked on, I had two glowing IGBTs shoot past my head on either side. After that, I took to powering the thing up while crouching under the table with ear protectors on. Once it survived power-up, I got up and looked at the picture on the scope with a lot less fear.

I told all this stuff about keeping out of the linear region, etc. to a guy working on a hybrid electric car drive, but he had to do it his way! He ended up with a Ford Taurus wagon filled with batteries almost doing a wheelie inside a garage full of people when the IGBT popped!

Jon

My frequencies will be low, think 50-300 Hz or so.

I am hoping that I can get some help from learned and esteemed experts here... Doubtless, I could not undertake this project alone. I hope that there will be people who may take enough interest to stop me if I do something stupid. I am not really afraid of, say, losing $80 that I would spend on components that blow up, although I would prefer not to endanger anyone's safety.

I already did many interesting things by "standing on the shoulders of giants", such as repair of a diesel generator or building of a phase converter etc. I am saying this not to brag -- these were relatively simple things compared to electronic design issues that some of you are experts in -- but rather to underscore that projects can be brought to successful completion with a bit of care, open mind and good advice.

side drive'.

Better read the application notes really carefully too. What you don't know will fry expensive semiconductors, typically a few at a time.

Best regards, Spehro Pefhany

'high side drive'.

venturing into this

Thanks. I will read them closely tomorrow, when I print that document out.

That said, can I get some intro on how these chips are used? Are they connected to DC power (for sending signals to MOSFETs), as well as to a signal generator that "drives" the chip? How do I tell the chip what to do?

Is that big enough for a 12 kW power transmission device, or is it just an example?

I apologize if this question is answered in application notes.

'high side drive'.

Trouble is....

Even when you do know it, you're likely to fry several by accident for good measure anyway !

Fzzztttzzzzzzttt ! poPPPP ! BANG !!

Graham ;-)

Got it.

That's not good...

Okay.

Very nice. I will try to read the PDF file about it to see if I can make sense out of it and understand how it should be wired.

got it.

Thanks.

I once tried to start a diesel generator (Onan DJE,

A surprise for me was that when it started, it jumped off the pad.

Fortunately, I forgot to disconnect chain hoist from it and it saved the generator as it kept it in an upright position.

'high side drive'.

venturing into this

So many issues !

When you've read a few ANs about gate drivers pop back in and ask again.

Ohh - and you do know about how to control circulating currents in pcbs ?

Graham

'high side drive'.

before venturing into this

Thanks. I will spend quite a while reading that stuff. That said, all I find are spec sheets. Is there some secret location for application notes, say for the IR 2113 driver?