IGBT driver update: I called International Rectifier

If I were to develop a unit with those chips, I would get an entire

*TUBE*. And I would probably use them, too. Until your gatedrive works properly, it doesnt work properly. And gatedrives that dont work properly cause explosions. Even when you are experienced and careful.

But there is a *LOT* to be learned from such an exercise.

Cheers Terry

solid groundplane underneath the circuit. And keep IGBT gatedrive connection inductance low - 50nH is really too much, 10nH is better.

if you use a DIP proto-board, it will go *BANG* the first time you try to use it on the welder.

Cheers Terry

sneaky. bet that wasnt easy to figure out.

Cheers Terry

:~p

Good point, so that'd be um, 0.15 ohm emitter resistors or something in the gate drive circuit then? (If not for: )

Ok. But why? I get good results (into MOSFETs, granted they don't handle as much current) using an emitter follower and transformer for high and low side gate drive. (Three state voltage output: +12V for high side, -12V for low side, 0V inbetween states for both off.)

I can't do that here since I want a square wave that goes from +10 or +15V to -5 or -10V, and I need some sort of active circuitry because just passing that waveform through a transformer is going to poop on the DC offset, especially as duty cycle is varied (from perhaps 25 to 48%; ultimately it will be set to the upper limit, but for testing I want control). A requirement of 15V on-state gate drive doesn't work when duty cycle is less than 50% and your negative gate voltage limit is -20V!

You lost me, where does it handle 320V differential?

a pullup to (say) +15V.

Eh, that MIGHT work... :p

So what of the high side Vsat? Do I need to float a 393 by the +rail?

Ah, true. Although, I don't really see just how high it can be if I'm switching it over say 0.5 to 1uS. That's 50-100A/uS, but still... hm, but wait, diode you say? Ah yes, that will want to switch at around 1A/nS, good point! So a small resistor and 1N4148 on the comp?

Tim

-- Deep Fryer: a very philosophical monk. Website:

driver

no, not really. the problem is the inevitable large physical loops that exist. these pick up H fields, and convert them to troublesome voltages. that in a gate drive always lead to a loud bang.

honestly, it would be far cheaper, at US$50 a pop, to buy a couple of semikron drivers. the advantage here is that (provided you connect it well to the IGBTs, minimising inductance) even if your logic goes screwy, the gatedriver will prevent you from being able to blow the IGBTs up.

or you could dead-bug your hardware, on top of a Cu-clad PCB as a ground plane. I've built some pretty tricky stuff this way, and have seen pictures on the web of some truly amazing gear - IIRC google "manhattan style"

keep circuit loop inductances small. current flows in loops. even logic signals.

Cheers Terry

That's true. Not always thinking clearly, as you've noticed. Yeah I'd go for say beta = 5 or 10.. typical saturation figure for power transistors. Obviously that depends on what type I pick, which I haven't picked yet...

Well okay, then swap the zener for something else...

I was told by a friend it would work, but he deals more with AF amps than HF class D, too...

Well, what the heck do I need an extra fourteen pins for when I can use a couple transistors to do the same thing? (Okay, so in the circuit I put down four transistors which makes 12 pins which would probably cover a wider footprint, but,,, that's beside the point!)

What, you mean use the gate drive for supply *and* switching? I don't think the SG3524 would like that, although it does have 300mA output which would go reasonably far at 12V (=4W). Haven't estimated gate power yet...

So what if there's a ground fault condition? I can hear a shotgun shell going off yet the bottom transistor desat is aloof.

If I had some 400+ Vceo PNP's this could be a lot easier... (Yeah, so buy some... I'm buying plenty of other things after all)

Hm.. I'm not seeing it work with a grounded op-amp that doesn't have 300V common mode input range..

Um, abuse of units? Unless you typo'd uF instead of uH.

Yeah... >>So a small resistor and 1N4148 on the comp?

If you mean not so much keeping things working as getting erroneous output bunnies, I was going to integrate it a little since the gate voltage is at least 200nS ahead of the collector.

Izzat fine for IGBT + diode packages then? Well, not really... hmm in fact it would be better to have an external diode since I could connect very near the diode junction and keep it to a few nH.

Or I could put an integrator on to snub the 100nS-ian rectifier switching hubub. Desat detector won't mind another half uS lag.........

Tim

-- Deep Fryer: a very philosophical monk. Website:

?!

if its a complementary emitter-follower output stage, driving Cg thru Rg, then you can happily use a base resistor Rb = Beta*Rg without really buggering up your output. pick a seriously pessimistic beta though, about 25% rated....

I'm referring to the ratshit desat circuit here. that threshold will be truly awful.

[snipped bit was here]

then you have to do something trickier. the basic idea of the icky gatedrive would work, but dont be afraid to hiff an LM339 or suchlike in there. If you are careful, you can also suck your supply rails out of the same xfmr - eg voltage doubler for +ve and -ve rails, and a couple of regulators (the -ve regulator can be pretty crude).

Or run the gatedrive xfmr at 10MHz, MSK and toss in a demodulator. that way you get nice supply rails for a diode and a cap....

I have seen and used both approaches.

maybe. depends on how good a gate drive you want. I'd not bother, and just sense desats on the lower igbts, its not very high power.

its probably not that hard to have a ground-referred high-side desat detector, either. you would be looking for Vc of the lower transistor failing to get within about 10V of Vdc when the upper transistor is on.

100A/us * 1uF = 100V negative spike. watch something break. 100A/us * 100nH = 10V negative spike. 100A/us * 10nH = 1V negative spike. thats enough to make an LM339 output go bonkers if its running from a unipolar supply. And I'd like to see you keep the inductance between the diode cathode and your Vc takeoff point below 10nH for *any* package.

and what do you use as a detector? R + comparator = beta-dependent threshold....

all depends on what you are trying to achieve. If you want a floating desat detector, a dual or quad comparator is probably the way to go. a quad allows simple UVLO too. I have designed gatedrives without desat detectors, but *never* without UVLO. Low Vg is one of the best ways to destroy FETs/IGBTs, and it usually happens at power-up (or down).

Pg is tiny cf 4W.

very true. so add desat to the upper gatedrives. or feed both DC bus wires (in opposite directions) thru a DCCT, and trip on overcurrent. or think up some other scheme.

not hard to hang a low-power supply just below +Vdc - eg a zener in series with your cap "balancing" (OK, discharge) resistors. Then a simple opto (low dV/dt) can give you a logic sigbnal for Vceupper > (say) 10V, which you can combine with gatedrive signals and filter to develop a floating desat circuit. Hell, you probably dont even need the supply....

oops. I suffer from dailysex

usually best to filter prior to the comparator, and use Vg_off to discharge the filter cap. but either one would work. can do desat delay digitally too.

then the inductance between the diode and igbt will cause problems. IGBTs have piss all reverse-blocking capacity. If you look inside a half-bridge module, you will find that the top diode is closest to the bottom IGBT and vice-versa, to minimise the current commutation loops....

Firstly, optimise the power path. Then deal with the effects that has on the signal paths.

you have 5-10us to get around to desatting, depending on IGBT. for little ones its less.

Cheers Terry