Re: phasing GDTs with MOSFETS

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

Original poster: Jan Wagner <jwagner-at-cc.hut.fi> 


On Mon, 26 Apr 2004, Tesla list wrote:
 > Original poster: "Jim Mitchell" <Electrontube-at-sbcglobal-dot-net>
 > Its very easy to screw this up which can cause a catastrophic faliure of
 > the MOSFETs in the bridge.  My motto is,  check the phasing, and then check
 > it again.

Wouldn't have happened if you followed my instructions posted ;)
In addition, it'd be a very good idea to power up the bridge from a small
PSU first, say 12V 1A, before you plug it to the rectified mains. Unless
you really got a death wish for the transistors ;o)

If your PSU current limiting kicks in or it buzzez loudly, that's a
usually good indicator that something is screwed up. Yes it would be more
obvious if the transistors would explode or there's a dense puff of smoke
from somewhere, but the less dramatic way of troubleshooting is better
for your bank account...

 > That was the problem with my first SSTC, as I had two GDTs and
 > the primaries got reversed, which messed up all the phasing (which I didn't
 > bother to recheck)

As not everybody has floating-channel oscilloscopes for measuring the
mosfet gate signals live in-circuit, the easiest way to work out the
phasing is to measure each GDT at a time, using a signal generator and
oscilloscope.

Stick bits of tape on the wires where you write which wire or pin is
what. Something like

PRI+
----------+""""+------ sec1 +0deg
            """"|
            """"+------ sec1_gnd
            """"
            """"+------ sec2 +180deg
PRI-       """"|
----------+""""+------ sec2_gnd

(where " is supposed to be the xfmr, in very bad ascii art ;-))

That way it's virtually impossible to get it connected wrong to the
bridge later on.

Labeling the wires is probably also a very obvious thing to do, but I
thought I'd mention it anyway.

regards,
  - Jan

--
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  Helsinki University of Technology
  Dept. of Electrical and Communications Engineering
  http://www.hut.fi/~jwagner/ -