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IGBT driver update: I called International Rectifier

Discussion in 'Electronic Basics' started by Ignoramus1797, Oct 11, 2005.

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  1. Jamie

    Jamie Guest

    i know that one, had little hot balls falling out of 690+ series
    today! bye bye IGBT's
     
  2. Yes, worst case, I will learn something. I do not have the time to go
    through an entire tube of stuff, blowing them up. If it does not work
    on the first and second time, I will just give up.

    i
     
  3. That's what I was, in fact, planning on doing. :-(

    Is there some proto board type thing that is acceptable?

    i
     
  4. Terry Given

    Terry Given Guest

    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
     
  5. Thanks. I am reading about it now. If I have to do it, I have to do it.

    i
     
  6. Tim Williams

    Tim Williams Guest

    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!
    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
     
  7. Terry Given

    Terry Given Guest

    ?!

    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.
     
  8. Tim Williams

    Tim Williams Guest

    That's true. Not always thinking clearly, as you've noticed. <g>
    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!)
    <snip interesting modulator>

    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
     
  9. Terry Given

    Terry Given Guest

    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
     
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