Hi there,
The recovery time of the diode affects the max frequency of operation
because the efficiency starts to drop off quickly as the frequency increases.
For high frequencies if you replace the diode with a short circuit (an
approximation to what it looks like at high frequency) you obviously have
a problem with conversion efficiency. The solution is to use a diode
with faster recovery or the so called "zero recovery" type.
An example would be when trying to use a 1N4001 diode for a 50kHz dc to dc
converter where the diode has to rectify the output back into dc. A typical
rectifier diode like 1N4001 which is made for line frequencies (50 to 400Hz)
would provide such poor efficiency we wouldnt want to use it. Going to
a 1N5817 Schottky instead would boost the efficiency quite a bit and make
it worthwhile to use it.
ADDED LATER:
The problem with using a 'slow' diode at high frequency can be visualized
in the following way...
A diode designed for say 100Hz might have a recovery time of 1 percent
of the time it takes for the 100Hz wave to complete one half cycle, but
if that diode is used at 1000Hz (10 times higher) that same recovery
time takes 10 percent of the time to complete one half cycle! Quite
a bit of difference in percent time of the wave.
That diode at 100Hz might not affect efficiency that much, but with
the 10 percent time at 1000Hz it could affect efficiency quite a bit.
Using a faster recover diode, it would work much better at 1000Hz
which would not bother the efficiency of the circuit anymore because
the recovery time when compared to the total wave time would again
be low like 1 percent.