> > > The flyback diode DBUK2 MUST be rated for a peak current capability of
> > > several times the mean output current and MUST be a high speed part
(eg
> NOT
> > > 1N400X).
>
> Olin has suggested that this is NOT the case due to circuit time constants
> etc. I'm not convinced yet but his assertion needs looking at,. The
ability
> to use a low cost general purpose diode here would be an advantage.

It's a bit amgiguous what "this" stands for in your sentence.  I did say
that in MY circuit, the flyback diode didn't need to be high speed.  That
was because I carefully designed the circuit to guarantee that the flyback
current in the inductor would have ceased (and the diode would no longer be
conducting) by the time the next current pulse starts.  However, I make no
such claims for your circuit.

And yes, the flyback diode must be rated at several times the average
current.  The whole point of a buck converter is to feed current to the
output in small but high pulses thru the inductor.  The flyback diode will
be subjected to the same maximum current the inductor is.  My circuit was
designed for a maximum average output current of 1.5A, and the inductor and
the flyback diode are rated at 6A.  Note that you need to take these higher
currents into consideration when calculating the loss in the switching
element.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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