RE: Shottky Diodes in SSTC ---> Not A Necessity

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

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <Marco.Denicolai-at-tellabs-dot-com>

Hi Justin,

Please, find my comments below

 > -----Original Message-----
 > From: tesla-at-pupman-dot-com [mailto:tesla-at-pupman-dot-com]
 > Sent: 15. maaliskuuta 2003 21:37
 > To: tesla-at-pupman-dot-com
 > Subject: Shottky Diodes in SSTC ---> Not A Necessity
 >
<SNIP>
 > 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.

What is "testing"? If testing is watching with a O-scope that MOSFET Id
and Vds are OK, there is no shoot-through, etc. then that's OK to me.
But if testing is only turning the power switch on and observing no
failure...What we are trying to protect from here isn't systematic
faults but stochastic faults. Build hundreds of them, try each one
hundreds of times, will it break? That is the right perspective, IMHO.

 > 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.

You might want to investigate on the cause of these diodes failure.

 > 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.

Current doesn't choose the fastest lane, but divides into paths
proportionally to their impedance. You'll probably have current flowing
in both body and external diodes.

 > 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!!

In my understanding it all depends on your application. If for a reason
or another you want a fast recovery time, you can use FREDFETs or
disable a usual MOSFET body diode with the two-diode trick you
mentioned. You might want the trr to be low, for instance:

- to reduce dissipation in the body diode during free-wheeling
- to operate with higher frequency
- to operate with reduced dead-times (between bridge cycles)
- with highly inductive loads
- ...

Jim and Jan, please correct me if I'm wrong.

I posted to hot-streamer two application notes (APT9803.PDF and
APT9804.PDF) which are almost similar, but reading both helps
understanding. The conclusion is that MOSFET with faster body diodes
break less than slower ones.

http://hot-streamer-dot-com/temp/APT9803.pdf

http://hot-streamer-dot-com/temp/APT9804.pdf

 > 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.

With the same attitute you can very well do without those useless
airbags: car driving is a success also without them, ain't it? :-)
Once again, those things are there to protect you from _occasional_
events.

Nevertheless, I think you should be proud of the results you have
achieved. Well organized site and neat designs!

Best Regards

 > Anyhow...SSTC's are cool!

Yes indeed, even if sometimes I'd like to "educate" them with a
sledgehammer.