Shottky Diodes in SSTC ---> Not A Necessity

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Original poster: "Justin Hays by way of Terry Fritz <teslalist-at-qwest-dot-net>" <pyrotrons2000-at-yahoo-dot-com>

Hi All,

Many of you may remember me posting a message with a similar subject
line, several weeks ago. To be honest, I am still not happy with the
general consensus that these shottky diodes are even remotely
necessary. So, I'm back to bug ya ; )

The shottky diodes I am referring to, are ones placed between the
MOSFET drain pin, and the ultrafast "antiparallel" diode cathode
pin/lead in typical SSTC schematics including ones shown on my
website. These schematics will be revised soon, just haven't had time
lately.

Four Very Good Reasons to exclude series shottky diodes in your next
SSTC project:

1. Huge amount of testing by a friend (Aron) and I. I have personally
tested the no-shottky SSTC design in several completely different
SSTC's that include a). +2kW IRFP460 H-bridge running with 16 inch
streamers. b). IRF640 half-bridge, 120V input with only 30V headroom.
c). IRF840 H-bridge, 10" arcs -at- 340VAC input.

Aron has also tested several different setups, which include a
IRFP460 half-bridge, making 12" arcs with 340VDC input and a IRF840
full bridge -at- 11" arcs.

2. Have you ever noticed that you have dead shottky diodes in your
SSTC circuit, only to realize that everything still works fine? I
have, many times. If you are using 1N5822's in a powerful circuit,
chances are, one or more of these diodes are dead and things are
still working just fine.

3. MOSFET body diodes are EXTREMELY SLOW. An IRFP460 MOSFET body
diode has a rated reverse recovery time of: TYP=580nS and MAX=1200nS.
This is like hours in an SSTC circuit. Most ultrafast diodes blow
body diodes away in Trr (rev. rec. time), hence no need for a series
shottky to isolate the two diodes. I do agree with Jan Wagner, and
Ken Herrick, that this diode should have a forward voltage drop
significantly less than the MOSFET body diode (we are talking forward
recovery time/response time here, which is similar for both diodes,
so Vf would be important *breath*). I didn't agree with this before,
but I do now.

4. I don't see any commercial electronic designs using series
shottkys. Not in motor control OR in power conversion, the two big
ones. If someone can find a design using the shottkys, please email
it to me!!

----

(Refer to #1 above). For all testing on small SSTC's less than 500W,
Aron and I used "antiparallel" dioes with 50 and 75nS reverse
recovery times, in DO-201 packaging (same package as 1N5822). Max
avg. forward current rating was only 3 amps for the smaller SSTC's.
For our larger ones (0.5 - 2KVA), we used 16A TO-220 packaged ones.

You're not going to believe this, but I can *almost* say that
back-to-back zeners from gate to source are unecessary too. This is
pushing it, I know, but I can't ignore my own findings. I have gotten
16" arcs without any zeners in a IRFP460 full bridge. The gate
signals were still very clean with no Vspikes past + and - 20V. I
did, however, use a very powerful driver I designed that has an
output impedance of less than 100 milliohms.

Anyhow...SSTC's are cool!

Take care,

Justin Hays
KC5PNP
Email: justin-at-hvguy-dot-com
Website: www.hvguy-dot-com