Re: OLTC update

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Jan,

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 :-))  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 ;-))

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.

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.  I seem to have plenty of room to fiddle With all this so it's
not a really big deal but I do want to do it right the first time :-))  I
should (according to MicroSim) put small caps across that coil, but then I
get HF ringing...  This is what I was warned about early on and finally
figure out ;-))

Cheers,

	Terry


At 03:41 PM 9/30/2002 +0300, you wrote:
>> > 4.  Decrease gate drive resistance to 10 ohms.
>>
>> Ah, ok, you were using 82 Ohms?
>>  http://hot-streamer-dot-com/temp/OLTC-IGBTdrive.gif
>>
>> Miller causes
>>   I_cg = 2*pi*100kHz * 50pF * 480Vpp = 15mA
>> just as a guesstimate, for each igbt.
>
>Oops, my apologies! I just forgot the parallel diode in the IGBT, and
>then the collector-emitter voltage of course never will be 480Vpp during
>igbt ON state. So please ignore. ;)
>
>But less gate resistance should nevertheless help to get a better signal
>and less noise on the gate. With all the HV RF around...
>
>cheers,
> - Jan
>
>--
>*************************************************
> high voltage at http://www.hut.fi/~jwagner/tesla
> Jan OH2GHR
>
>