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

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Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 08:08 17/03/03 -0700, you wrote:

> > 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.

I would say the crux of the matter is the IV characteristics of the diodes 
in question. I think this might have been mentioned before, but what the 
heck. Purpose-made fast diodes tend to have a lower forward voltage than 
the body-drain diode. The difference might not be very great, but the 
dynamic impedance of both diodes is very low, so the external diode will 
take the vast majority of the current. You could help to promote this by 
choosing your add-on diode to have a low forward voltage and a high current 
rating. However, you can't guarantee that it'll work unless you use the 
series Schottky.

P.S. If you do use series Schottkys then they need to be rated and 
heatsinked to take the full MOSFET drain current. A 2kW SSTC like Justin 
and Aron's would need TO-220 diodes on small heatsinks. I'm not surprised 
those 1N5822s are blowing out.

Steve C.