IVP wrote:
> Both are ultrafast, with the pdf suggesting use in SMPS, but
> no mention of linear supplies
>
> Would either be suitable in the attached schematic ?

The speed issue of a diode is how fast it stops conducting after a reverse
voltage is applied.  This is more accuratelly referred to as "reverse
recovery time".

This is important in switching power supplies since they can be operating a=
t
a high frequency.  For example, consider a basic boost regulator run in
continuous mode.  The diode is forward biased and therefore conducting when
the switch turns on.  Before it turns off, the input supply is basically
shorted thru the diode and the switch.  This not only wastes power, but can
really beat up the switch.  I've had a switching FET fail in this case when
the wrong diode got installed.

Let's say the supply is switching at 100kHz, which is 10uS period.  The
switch on time duration must therefore be less.  A slow diode that might
conduct for 1us or even a few 100 ns would be doing that for a significant
fraction of the switch on time.

This is one reason Schottkey diodes are used in these applications when the
voltages allow it.  Ordinary diodes are a P-N semiconductor junction.
Remember that conduction is all about availability of minority carries in
the depletion region.  The reverse recovery time is the time during which
the remaining minority carriers are swept out of the depletion region.
Schottkey diodes are a metal-semiconductor junction, so have half the
depletion region to start with, and can also flush out the minority carrier=
s
in the depletion region that is there faster.  For most purposes Schottkey
diodes have infinitely fast recovery.

Back to your situation.  You are switching the diodes at the 50Hz line
frequency.  Even slow diodes will be no problem since the voltage is
changing slowly at the crossover point.  The diodes being faster does no
harm since even slow ones are plenty fast enough.  However, specifically
fast diodes may have a larger forward drop at the same current due to
tradeoffs made in the design to get the faster speed.  However, this looks
like it will be a minor issue, if at all, at your voltage.

This application is just asking for the common 1N400x diode series.  These
are meant for line power rectification.

another thing to watch out for is that labels like "fast" and "ultrafast"
are pretty meaningless.  If speed matters, check the datasheet for the real
numbers.  I have seen "ultrafast" diodes that are significantly slower than
"fast" diodes.  Beware of marketing math in use.


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