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Re: OLTC update



Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi Terry,

> The high gate voltage (27 volts = three 9V batteries) is needed since I am
> running about 500 peak amps per IGBT.  2.5X their rating :-))

Yeah works as long as max power dissipation at specific junction
temperature is not exceeded. Something like 10 x Ic_peak_max possible,
AFAIK, but only for a few microseconds.
If your igbts survive longer than a few runs that'd be quite promising! ;)
Those buggers really seem to get a hard life, 2.5x the rating :-)
Well at least it seems they're latch-up free pretty high over ratings, so
one less worry...

> So I need
> "extra" gate voltage to insure the CE gate stays in saturation.  The Ic vs.
> Vge graph shows to get "really high" current you have to have pretty high
> gate voltage.  Of course, I really should have used 15 IGBTs in retrospect
> to keep the gate voltage and current per device down.
> I am not worried about gate failure.  I will test a spare IGBT sometime
> but I bet they are solid to 50 volts.  The gate bonds are big and these
> are copper metalized ICs.  They are built for big currents and fast gate
> switching ;-))

Peak gate current and maximum gate voltage depend on two different things.
Using real metallization instead of poly on the gate "just" reduces gate
series resistance. The maximum gate voltage is mainly restricted by the gate
oxide thickness...
 http://www.irf-dot-com/technical-info/appnotes/an-936.pdf
 http://www.fairchildsemi-dot-com/an/AN/AN-9016.pdf
 (some nice I_c peak calculations there, too...)

I've never heared anyone use more than 20V, really. While it seems that
typical gate oxide punchthrough voltage _might_ be a stunning 70V..80V for
_some_ devices, no datasheets specifically mention that. Probably
an unreliably controllable process parameter.
I.e. if you get all your current igbts to work at 50V for an extended
time, that'd be a small miracle... ;o)

Better stack up more IGBTs. That would solve a lot of problems.
Maybe there's still room on the back side of the heatsink? :)

Anyways, what I get from the is IRG4PC50FD datasheet, as an estimate, is
300A at 12V Vge. Then around 400A at 14V. 500A at 16V? 600A at 18V?
Before c-e saturates. Hard to tell as the curves end "too early" and some
are missing. Voltages look lower for IRG4PC50WD (again, just
an "extend the graph" estimate :-)

Have you measured that they really do saturate?
And it isn't due to unequal current sharing?

> The IGBTs "can" turn on or off an "any" time.  The control circuit turns
> them on and off nicely but the coil is designed to withstand errant
> triggers too.  So I could turn the IGBTs on and turn them off in the middle
> (peak voltage) of the next cycle or something like that.  So to keep the
> thing reliable, even under fault conditions, it has to take any IGBT timing
> signal and survive.  I clamp the gates with high voltage transorbs now but
> they are on the "other side" of the gate resistors.  Thus, a high dV/dT on
> the collector might be able to drive the gates high if the resistors are
> too high of value.  Of course, lowering them increases dV/dT too....  The
> TP250s are no light weights either.

Negative gate bias may be helpful there. Negative bias is a good thing in
any case. :o) Should be easy with the TP250s too, just one 9V battery
more.

> One "very bad" signal occurs if you turn the IGBTs off while the primary
> current is maximum.  That give a BIG positive voltage kick right across the
> IGBTS!!  ""7 Joules!!"" stored on a single turn 404nH coil!!  BIG dV/dT
> there!!!  I have transorbes on that too but the voltage can jump from say
> zero to the clamp voltage terribly fast and perhaps drive the gate voltage
> up with it.

Yeah even fast transorbs are like >5ns clamping response time so with
really huge dV/dt it may not clamp fast enough to prevent Vc-e
overvoltage. And then the capacitive coupling to the gate too (again,
negative bias helps a bit)...

OTOH, you could trade this in with slower igbt turn off (but still use
fast turn on). If interrupted (over)current doesn't happen repetetively
then just _maybe_ the igbts won't explode. More igbts, anyone?  :)

cheers,
 - Jan

--
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 high voltage at http://www.hut.fi/~jwagner/tesla
 Jan OH2GHR