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

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

Original poster: "K. C. Herrick by way of Terry Fritz <teslalist-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>

Justin (& all)-

In 3, I don't quite follow your parenthetical comment, but as to the
respective Vf's--I wonder if either a) you have been fortunate or b)
"ultrafast" diodes in general tend to have Vf's smaller than intrinsic
MOSFET diodes and thus tend to conduct the majority of the current.

Right now, as you may have noted, I'm in the process of changing to the
use of IGBTs.  Absent (continuing) misfortune, perhaps I can chuck all
those dozens of MOSFETs & Schottkys.  If I'd only kept all those I've
burned out & put them in a bag, it would have made a dandy doorstop.

How do you handle crossover in your MOSFET bridges?

Ken Herrick

On Sat, 15 Mar 2003 12:36:32 -0700 "Tesla list" <tesla-at-pupman-dot-com>
writes:
 > 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
 >
 >
 >
 >