RE: A s.s. head-scratcher

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Aron Koscho" <kc5uto@xxxxxx>

Ken,

Are your gate waveforms too perfect? You can tell if the gate has 
opened with a current probe (TEK P6021 or the like) or check across 
the gate resistor. It is highly unlikely the part would fail open 
with out you noticing (BANG). Is it possible something is setup wrong 
regarding the measurement? I am inclined to think this is one of 
those things that you spend two days trouble shooting only to find 
there was a bad solder joint or broken wire. Or maybe you just got a 
resistor is backwards...

Aron




-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Wednesday, March 14, 2007 9:19 PM
To: tesla@xxxxxxxxxx
Subject: A s.s. head-scratcher

Original poster: Ken or Doris Herrick <kchdlh@xxxxxxxxx>

Steve Ward et al-

This one stumps me but perhaps not someone else:  I'm implementing my
push-pull s.s. primary circuit, using two Powerex CM300DY-24H
IGBTs.  Previously it appeared to function but not satisfactorily so
I've redesigned and rebuilt the drivers.  Waveforms now, right at the
IGBT gates, appear perfect.  However...  The IGBTs will conduct no
current; nada, zip, neither of them.  Nothing within them is shorted,
per my ohmmeter, and the gate:emitter nodes are "open" per the ohmmeter.

In order to accommodate my revised driver circuit, which has
back:back zener protection, I had carefully removed the 10K resistors
that were protecting the gates.  Was I not careful enough, do you
suppose?  Could the gates have actually opened up within the
packages?  >Both< of them??  Not hardly, I should think: they'd be
shorted, if anything.  Does anyone have a clue, before I have to
un-mount them & jury-rig a test-setup?

Ken Herrick