Russell McMahon wrote...

>It is traditional in step down converters to "feed forward" the=20
>effect of the input voltage to reduce the on time as input=20
>voltage rises.

In the context of switching regulators, feedforward is a technique
used (when it is needed and appropriate) for a very specific and
narrow purpose: to improve a regulator's line transient response. =20

The essence of this technique is to provide a means whereby sudden
voltage changes at the input are quickly, but only approximately,
cancelled out by the feedforward mechanism, while allowing the
feedBACK mechanism (notice the emphasis) to take care of the
longer-term input variations and maintain the output voltage against
changes in load. =20

Appropriate use of feedforward techniques can make the design of a
feedBACK (emphasis, again) system easier by partially relieving it of
the burden of having to make large adjustments very rapidly in
response to sudden process changes. =20

In loose, intuitive terms, the feedforward mechanism allows the
feedback mechanism to concentrate more on "fine-tuning" the output and
less on rapidly compensating for sudden input changes.=20

Other than for this particular purpose, I have never seen feedforward
used in a switching regulator. =20

And if I had, I would have considered it a Band-Aid applied by the
designer to cover up some shortcoming in his feedback system.  As
Richard pointed out, these appliques tend to be a "select on test"
sort of thing.

>Presumably the split resistor and zener in the original were placed =
there as
>a result of practical experience by the original designers.

In Richard's application, as in yours, the circuit has to work with an
extremely wide range of input voltages- 4:1 or even more.

In my own work I seldom have to deal with more than a +/- 30% input
variation, so I eliminated those components in my quest to see just
how simple I could make Richard's circuit. =20

Obviously, these components perform an important function (that is,
stabilizing the hysteresis feedback voltage) when huge input
variations are expected and should, when needed, be included.

Dave

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