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Re: RE: [TCML] SSTC opinions solicited
Ralph (& all)-
Thanks for the suggestion. That looks like a very good choice; a pair
of them in parallel on the same heat sink would sure beat one of the
STEs. 54A each @1200V?...not too shabby for $4.
It happens that I've just about decided to reconfigure the primary assy
to go back to my prior way of doing it (per my patent). Should have
stuck with that in the first place, I think. I can use the same
(separate) driver assy and l.v. assy so it's not too much of a
re-build. I'll play with that notion for a bit.
In the prior ckt, the reverse-diode of each transistor pair (or 2 pairs
in the case of paralleled TO-247s) serves to clamp the other against the
primary's "inductive kick". I see, for the HGTG18N120BND, that the
reverse recovery of that diode is very good, so that should work well.
I'm using 6, STE40NK90ZD 40A/900V MOSFETs in a ring configuration.
Those are nice transistors, but you do pay a lot for the convenience of
screw terminal connections and an electrically isolated mounting design.
I'd be more inclined to use something like the more common
HGTG18N120BND, which is available for about $2.00. One of these is a
pretty fair substitute for the STE's, except for the power dissipation.
390 vs. 600 watts. You could easily parallel two of the HGTG18N120BND's
and have higher dissipation and less driving power requirements. They
are also have a faster rise and fall time than the STE's.
When I use then in high power circuits, I usually bolt them directly to
the heat sink with no thermal pads. Of course, the heat sinks have to
be electrically insulated, but the transistors sure like it that way.
And yes, I have popped a lot of transistors while designing new gadgets.
I think that's par for the course. If you don't vaporize a few, you
really don't know what the circuit limits are. The latest "Oops!!" for
me was a pair of $58.00 RF power transistors. Which sort of ticks me
off, since they go in a used RF amplifier that only cost me $125 in the
Ralph W5JGV - WD2XSH/7
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