> > There is one major problem with the FET used as shown. This is an
> N-Channel
> > device
>
> No it's not.  I've got the spec sheet from IR right in front of me.  I
don't
> know where you got this from.

Aaaagh! - you're no doubt correct - my mistake.
I thought I'd downloaded the datasheet but when I went back and looked just
now I find it's for the IRF3205 - I must have written down the wrong number
and also I suppose the price refers to that too.  Nice FET though :-). If
yours is a P Channel then all is well.


> > Buck converters are usually best run in continuous current mode ...

> I see nothing inherent to buck converters to support that statement.

Continuous and discontinuous modes are quite different in the relationship
between input and output voltages. The continuous mode can be considered as
a PWM waveform filtered by the output LC filter. The discontinuous mode adds
extra terms for frequency of switching and inductance in the Vin/Vout
relationship. Swapping between modes in one design is generally
ill advised as design criteria for both are hard to meet simultaneously.
Arguably the continuous mode is most desirable. That said, any buck
converter will be forced to run in discontinuous mode under a light enough
load and this may well be acceptable for a normally continuous mode design .

Texas Instruments report SLVA057, "Understanding Buck Power Stages in
Switchmode Power Supplies" March 1999 gives a good overview of the two
modes and their differences.
They note -

        "It should be noted that the buck power stage is rarely
        operated in discontinuous mode in normal situations,
        but discontinuous conduction mode will occur anytime
        the load current is below the critical level."

This of course doesn't mean you shouldn't design a discontinuous mode
version if there are major advantages in doing so.




regards


            Russell McMahon

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