Re: OLTC update

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

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

Hi Terry,

> My top priority now is to gear it up for full 240VAC operation.  It has
> only run in the 1/4 power mode (120VAC) so far and Bill's lab only has 120
> VAC right now due to an "odd" wiring system.  The things I need to change
are:
<snip>
>
> 3.  Increase gate drive to 27 Volts from 18.

That sounds rather bad, as the absolute maximum rating for the gate
voltage is 20V.

It seems like the gate oxide can "usually" survive, sometimes, short peaks
of 30V to 40V. But I wouldn't really recommend going beyond the specs of
the datasheet! :-)

If the Miller capacitance (Cres, ~50pF?) really is a problem, IMHO you
should rather try to get a beefier igbt driver, or place an additional
bipolar buffer stage after the TLP250 opto to get more output sink/source
current and thus a more rigid gate signal.

> 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.

This charges and discharges the gate-emitter capacitance a bit. I'm not so
sure whether the change in gate voltage is significant, though...

The current is caused by the primary voltage ringing, regardless of the
gate resistance. So a lower gate resistance should AFAIK help here to
clean up the gate signal a bit, by sourcing and sinking those 15mA.

Probably a spice or aplac simulation would be nice at this point... ;o)

cheers,
 - Jan

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