> > >I've always thought (but might be mistaken) that "fast" diodes just
> > >turned OFF faster than regular diodes, and they don't necessarily
> > >turn ON any faster. For a catch diode accros a relay the most
> > >important is turn ON time, right?, so there is no benefit in using
> > >anything more expensive than a 1N400x?
>
> > You are correct.
>
> Agree. BUT the turn off losses in the 1N400x will be higher. Often
> this is unimportant. But if you repeatedly switch an inductor off at a
> high rate the dissipated energy may exceed the 1N400x's thermal
> ratings. I have a 20 kHZ PWM application which uses a protection diode
> to snub a nominally "non inductive" winding which in fact has a small
> inductance. A 1N400X lasts about 20 seconds there despite being
> nominally rated for the voltage and currents involved. An equivalent
> high speed diode (eg BYV26c) runs cool in the same application.

OK Russell. So for relay snubbing 1N400x is cool. They are cheap,
rugged, and the switching rate of a relay is orders of magnitude
lower than the frequency limits of this diode. No need to complicate
that subject any more.

The higher frequency case is interesting though. I have to admit that
I can't quite visualize what is happening in terms of voltage drop
and current flow during diode turn OFF that causes a high
instantaneous peak power. (Assumption is that because turn-on time of
a "slow" diode is the same as a fast diode, the extra power
dissipation must be happening when the the diode turns off ie, when
the inductive load is turned on gain). Can you explain?

--
Brent Brown, Electronic Design Solutions
16 English Street, Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/txt: 025 334 069
eMail:  brent.brown@clear.net.nz

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