Driver for IGBT? (-ve off and +ve on from CMOS)

Well, in fact, I will place the box with the breadboard, heatsink and everything electronic kind of far away from the reactor and HF circuit. The welder is separated by a wall in the middle, and the circuit will be beyond the wall. It is needed anyway for proper cooling.

That said, would not a capacitor and a resistor protect me from transient spikes? I thought that that's what snubbers are for.

i

Trigger on the gate drive for instance.

Perhaps, stay within its ratings using a voltage divider if necessary.

Very good idea.

Not joking, I have seen large plastic packages explode with the noise of a shotgun blast and similar dispersal of small bits. How much do you value your eyesight?

No, I am talking about pulsing the current without changing the polarity from a low current (background) to a higher current (foreground) at rates from around 10 pulses per sec (for a wide arc) to several hundred pulses per second (for a narrow arc). Changing polarity is useful only for manual TIG of aluminum IMO, and when you do that it is useful to set the reverse polarity time less than the fwd polarity time to get more heat into the work.

Some inverter welders also allow you to switch to constant voltage operation for MIG welding, which is the only reasonable way to do most aluminum welds at reasonable production rates, and to adjust the "stiffness" or droop in either CC or CV modes.

Yes, that was part of my suggestion, but it was a bad suggestion for a beginner as it requires a rather good understanding of switch mode power supply design. Sorry I mentioned it.

Not too likely if you keep your bridge downstream of the reactor, like outside of the welder box. It is a pretty tough machine. And if it does break you have another fun project on your hands :-).

Listen up my ignorant sonny, your enthusiasm is noted, but I was checking out IR's ir2114 and ir21141 (600V), and their ir2214 and ir22141 (1200V) when you were still dribbling in kneepants. The datasheet is now rev C. The oldest version in my computer (out of six files) dates to before there were revs, when it was marked "Advance Data." I even have IR's original press release, which said, "Pricing begins at US $3.50 each in 10,000-unit quantities."

So there!

BTW, with your welder's over-active reactor (flyback city), and your 1200V IGBTs, you should chuck those wimply free ir2114 parts (who wants 'em), and upgrade to the manly ir22141 chips, complete with the all-important sought-after active-bias feature for those awkward, embarrassing desaturation-detector coupled-noise moments.

--
 Thanks,
    - Win

Dreadful indeed, but the iR2214 family at 2/3A isn't much better, when one is considering the huge IGBTs under discussion here.

--
 Thanks,
    - Win

Winfield, sorry, that was a typo. I did mean the IR22141SS chips, in fact. That's what I will, hopefully, receive probably tomorrow. I apologize for the confusion.

i

20J? E = 0.5 LI^2 = 0.5 0.01 400A^2 = 800J. With 50mH, 4000J.

--
 Thanks,
    - Win

Yeah - has anyone noticed little 'i's other thread that he just started, "Demolishing electronic equipment"?

I think he's well on his way to demolishing some IGBTs and possibly a welder and maybe his house. =:-O

Sigh, Rich

The EMI issue is mostly about your low level oscillator and gate drive circuits which must be protected by design and layout, not snubbers. You could in theory protect your IGBTs with snubbers, but when you do the calculation on how big they need to be to handle your reactor energy you will appreciate the approach of never letting both sides of your bridge turn off at the same time in order to avoid the need to snub that much energy. This approach completely screws desaturation protection however because if the desat circuit turns off your IGBTs they will see the full reactor energy. Have you measured your reactor inductance yet? That is the first thing you need to know in order to design a workable snubber, otherwise you may just add an exploding snubber cap to your exploding IGBT!

Mostly snubbers are used to protect from the energy stored in relatively small wiring inductances, not big-ass reactors. If you simply copy someones wiring inductance snubbers you will be needing those safety glasses and face shield the first time your bridge turns off at high current, either due to design error or when EMI kills your control circuit!

["Followup-To:" header set to sci.electronics.design.] On 13 Oct 2005 02:02:05 -0700, Winfield Hill wrote in Msg.

yes, but isn't that some enthusiasm? Maybe I'm a poor judge because I always lean to the pessimistic side, but I've never seen anything as doomed as this TIG welder project. In the end, Ignoramus will be out 50 bucks for his IGBTs and a few dollars for his gate drive circuitry, but that'll be a small price for the knowledge gained.

So... power to him!

robert

I see. The gate driver would isolate them from load electrically, to some extent. Are you concerned about EM waves inducing currents in my circuitry, or about interference arriving through power cables?

If it is the former, I will place this item in a separate compartment of the welder as opposed to where the HF unit is. If it is the latter, I will try to make sure that there is sufficient isolation.

Sure. I mean, I would not mind installing a large capacitor, but that could worsen starting conditions. I will indeed try to select gate resistors so that there is a little bit of cross conduction.

I have not yet, have not had time, but I do want to do that indeed.

I agree 100%. I think that there is a calculation of snubber size based on inductance, in the Fuji manual.

I started drawing schematic of my circuit, and will calculate all needed things in Gnumeric (equivalent of excel). All resistances, capacitances etc.

i

wrong. Typically detect Vce > 10V whilst device is on - called "desat" detection, as IGBTs "de-saturate" and limit current (to about 10x rated, depends on Vg and device) when switched into a short.

there are plenty of ways of creating such a short. Firstly, transient thermal events eventually cause IGBT modules to fail (ditto for T0-247, T0-220 etc), and they fail short-circuit (initially - add in enough energy and they fail "vaporised"). Desat detection can significantly limit damage, greatly reducing MTTR and $TTR.

Its not that hard to arrange a short-circuited load, either - forklifts are particularly useful. Likewise choke insulation can and does fail.....

I reverse-engineered one a few years back. They also play sneaky buggers during fault turn-off, to limit dI/dt. Nice little drivers.

Cheers Terry

its an adiabatic thing. typically, you have about 10us to shut the thing off, and current is limited (by the IGBT) to about 10x its rated value - IOW 2,000A for a 200A IGBT.

yep

Cheers Terry

Note though, that my switching frequency is extremely low. That gives me a little leeway here. The International Rectifier engineer who I asked for help, did not think that driving these particular IGBTs would be difficult, and suggested a particular bootstrap capacitor.

i

carefully. usually with a DSO, so you can catch the damned transient event that snots your IGBTs. I once spent 3hrs in the lab, blanket draped over me & the PM3394 scope in analogue mode, brightness cranked up, to catch a glimpse of something I suspected was there, but couldnt make the digital scope trigger on. I eventually saw one glitch, me & a buddy then reasoned out the cause in about 3 minutes flat, and had a fix running about 1/2 an hour later. metastability causing a race condition causing runt pulse causing very intermittent self-destruct. yay for ISP CPLDs.

then, when it gets destroyed, replace the scope and get a few P5200 diff probes.

1,000J will destroy many plastic packages. A Semikron rep once proudly announced their "rupture-proof" 1/2-bridge modules. So we bolted some to our 100kW prototype, and blew them up. The first one didnt rupture, but the drive leaped about 1' into the air. So we replaced the IGBT, and sat an anvil on top of a sheet of nomex on top of the unit, and the module blew itself to tiny pieces. He was suitably embarassed, but bought us lunch so that was OK. IIRC it was only a kJ or so....

if you mess up the PCB layout, the drivers will fail catastrophically, and take out your IGBTs :)

I first looked at IR 1200V 1/2-bridge driver chips around 1997 or so. IR were trying hard to convince us to buy IR parts (they never succeeded, mostly because they were too damned expensive, and of course only make tiny IGBTs). Our techs annihilated 3 test boards, and about 9 chips, doing nothing nasty.

Apparently they got an idiot to lay out the test PCB, and it was done so badly that they pretty much all failed. oops.

I have also re-worked several designs using these types of chips, that failed for similar reasons.

Cheers Terry

is it?

I worked with a guy who, at Rensslaer IIRC, built a 25kW FET inverter that ran insuide a dewar of liquid nitrogen. Rdson was so low the efficiency was terrific. Almost all switching losses....

Cheers Terry

Excel? "Compatibility"??????

Good Luck! Rich

I'm sorry if I sound harsh or condescending here, but if you can take a DC welder and an H-bridge and get AC to come out of it without dramatically hacking the welder itself, then more power to you, and I'd really like to see it. Don't forget - when you're DCEN, your whole welder and workpiece are positive ground. When you're DCEP, your whole welder and workpiece are negative ground. So you not only have to run AC to the electrode, you have to have the whole welder and workpiece going from negative to positive at your pulse rate.

If _anybody_ could make that work, I'd love to see it.

But that other thread you've started about "Demolishing electronic equipment", where it sounds like you don't even know a resistor from a capacitor, gives me serious qualms about your ability to get this thing accomplished[1].

I don't mean to rain on your parade[2]; as I've already said in one of the other three or four threads about this project, it's your welder, your IGBT's, your money, your eyes, your house, and etc., so, I wish you all the luck in the world.

The thing is, with a project of this magnitude, I have a feeling that you're going to need it. (all of the luck in the world, that is.)

Good Luck! Rich [1] I'd have been a lot less nervous about this if you'd said something like "salvaging". But that's just my not-so-humble opinion. [2] On second thought, I guess that's exactly what I'm trying to do. I'm sorry for that, and like I say, good luck, and try not to win a Darwin award.

Oh, do whatever you want - I was just M$-bashing. :-)

Cheers! Rich