Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!iuvax!rutgers!umn-d-ub!cs.umn.edu!ux.acs!keith
From: keith@ux.acs.umn.edu (Keith MaloneyHuss)
Newsgroups: sci.electronics
Subject: Transistor recommendation
Keywords: transistor
Message-ID: <1345@ux.acs.umn.edu>
Date: 16 Mar 90 16:42:47 GMT
Reply-To: keith@ux.acs.umn.edu (Keith MaloneyHuss)
Distribution: usa
Organization: University of Minnesota, Academic Computing Services
Lines: 19

I have a problem with a power supply that I recently built.  It drives
a laser (5 mW) and is a high voltage (2-10 kV) DC supply.
Basically it uses a 555 that drives a power transistor switching 12 VDC
into the primary of a transformer that steps this up to about 300 VAC
which through a diode/capacitor ladder gived the required high voltage DC.
My problem is that I'm blowing out transistors.  I have a diode across the
transistor to save it from the transformer's backlash (for lack of a better
word).  Is it better to put the diode across the transformer primary, or
across the transistor emitter/collector junction?  Or does it matter?
The problem seems to be overheating.  I am using an NTE248 PNP
transistor that I believe is rated at about 100 watts or so.  It is
in a TO-3 can and is well heat sinked to a chassis with heat transfer goo,
but it still overheats.  If you haven't guessed already, I'm a beginner
at this.  The laser tube draws 6.5 mA at 2000 VDC if that is relevant.
Would a power MOSFET be a better choice for this application?  Or do I
need a more robust Darlington?
I would appreciate any advice.  Thanks.

--keith