RE: SSTC Related: Shottky Diodes Unnecessary!!

["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 all,

I agree with Ken. The series Schottky is really needed.

Best Regards

 > -----Original Message-----
 > From: tesla-at-pupman-dot-com [mailto:tesla-at-pupman-dot-com]
 > Sent: 21. tammikuuta 2003 02:57
 > To: tesla-at-pupman-dot-com
 > Subject: Re: SSTC Related: Shottky Diodes Unnecessary!!
 >
 >
 > 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
 >  >
 >  >
 >