If you care about every fraction of a Volt (as you sometimes do), Vf
on Schottly diode tends to be noticeably lower for lower Vmax ratings.
If this matters use the lowest voltage rating that the circuit allows.

eg I selectively use 1N5817 & 1N5819 diodes in 2 locations the same
design where voltages vary and diode drop matters (slightly)  in each
case). eg 0.1V drop driving a 3.3V LED from a smps is 3% less energy
or 3% more "free" battery life. For a say 300 minute battery life
that's 10 more free minutes at full brightness. That's enough energy
to put a long diminishing brightness tail on a portable light. Free is
good. Cost of either diode is usually the same. If you can convince
somebody to sell the lower voltage part at lower cost you get more for
less :-).

For flyback clamping this is usually not overly relevant until you get
to serious dissipation levels.
For very low voltage rectification, such as from low voltage PV
(solar) panels or low voltage inverter outputs, the difference can
make a  few percent difference to the efficiency.

Note also that Schottky may have VERY high reverse leakage currents at
high temperature. So much so that reverse power dissipations may
become serious at high voltage and temperature for high current (say
10A plus) devices.

In some circuits high temperature Schottky leakage may be so high as
to render them unusable. Look at spec sheets, determine the worst case
operating device temperatures 9ambient + dissipation induced) and
check the data sheet. Note that for reverse biased dissipation this
may be an iterative procedure as increased leakage leads to increased
temperature leads to .. . "Over relaxation" allows shortening this
process - or use eg SPICE as long as your model properly models
reverse leakage with \temperature.
Not a concern in most applications.


    Russell McMahon






























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