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Re: SSTC Related: Shottky Diodes Unnecessary!!



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