Re: SSTC Related: Shottky Diodes Unnecessary!!

["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)-

With two diodes in parallel and forward-conducting, a very small
difference in relative forward voltage drop will cause a very large
difference in relative current flow.  I don't see how one can depend upon
the faster diode consistently having a smaller forward drop than the
slower one.  Hence the Schottky.

Ken Herrick

  On Sun, 19 Jan 2003 17:03:15 -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 Dan,
 >
 > I completely agree with you in regard to the absolute necessity of
 > having fast "freewheeling" or "catch" diodes across each MOSFET
 > drain/source. Also, I agree with you that for slow stuff, the
 > MOSFET
 > body diodes can work well.
 >
 > The diodes that I am referring to as being unecessary, are any
 > shottky diodes in *series* with each MOSFET drain. These diodes are
 > present in Richie Burnett's well-known SSTC schematic, as 1N5822's.
 > They are also present in the H-bridge feedback coil schematic on my
 > website...this was before my friend Aron found them, pretty much
 > useless to speak frankly.
 >
 > Although a series shottky diode will *completely and totally*
 > isolate
 > the MOSFET internal body diode from forward conduction, they are
 > just
 > another source of dissipation when the "freewheeling/catch" diodes
 > are plenty fast enough.
 >
 > Also, if a MOSFET in a half or H-bridge fails dead short (typical),
 > the shottkys are in the current path and will see the full,
 > dead-short current for a significant amount of time...usually until
 > the fuse blows or a trace explodes. Fortunately, shottky diodes
 > have
 > huge surge current ratings and they very well can survive.
 >
 > BTW, for low voltage half and H-bridges, you can use shottky's
 > across
 > the drain/sources...this works REALLY good as shottkys are
 > extremely
 > fast.
 >
 > Dan I've been meaning to tell you, nice job on your power
 > controller!!! My bud Aron likes it too. Are you going to leave the
 > enclosure the way it is or will you paint it? I personally like the
 > unfinished metal look.
 >
 > Take care,
 >
 > Justin Hays
 > KC5PNP
 > Email: justin-at-hvguy-dot-com
 > Website: www.hvguy-dot-com
 >
 >
 >
 >
 >