Marcel Birthelmer wrote:
> On Sun, Nov 15, 2009 at 5:32 PM, M.L. <m@lkeng.net> wrote:
> > On Sun, Nov 15, 2009 at 9:02 AM, Marcel Birthelmer
> > <marcelb.lists@gmail.com> wrote:
> > > > A proven option would be a PIC-controlled SMPS. The PIC16F616 has
> > > > an enhanced PWM module which would work nicely for a buck-boost.
> > > > It has hardware-triggered PWM output shutdown and restart as well.
> > > > It's a pretty inexpensive part.
> > >
> > > Do you have any documents/design guidelines/example projects for this?
> > > I've seen the Microchip App Note for the SMPS buck regulator, but I'd
> > > like to see some more information about how to apply control theory
> > > principles to generate a particular output voltage.
> >
> > You could read a digital control systems book and apply that to what
> > you desire. It's not necessary, but I'm not going to tell you it's a
> > bad idea. I know of a few names on this list who have used PICs in
> > real, working, SMPS designs without doing complex analysis.
> 
> I did read Olin's paper about the PFC (which conceptually isn't really
> different), and I have some background in DSP/Control Theory stuff.
> I'm just wondering about how good (in terms of overshoot, stability,
> etc.) a controller can be made without really having a solid model of
> the converter topology. I mean, sure, you could just have a simple
> feedback system in terms of the current or output voltage and generate
> the switch voltage from that, but there seem to be many gray areas
> that would be unexplained (for example: what sort of parameters are
> appropriate for a PID controller in this case? Are there any unstable
> regions when switching from constant-current to constant-voltage
> output (such as might be the case for a battery charger, for
> instance)?). I'm just trying to get a more theoretical understanding
> of how to come up with a model for such a converter.

You can also ask the opposite question: How good does the regulator need to
be for this application?

Many embedded applications don't have huge dynamic load variations to begin
with.

In a battery charger, moderately large voltage transients aren't really
going to harm the battery, or even introduce significant error into the
charge algorithm. The OP was asking about a solar/wind charger for a 24V
system (presumably lead-acid), which falls into this category.

In a PFC, the feeback loop deliberately ignores fast load changes in order
to maintain a high power factor on the input side.

In other words, you really only have to pay attention to the finer nuances
of the feedback circuit if there's going to be something significant for it
to be doing in the first place.

On the other hand, if you do need to design a regulator for tight voltage
control of dynamic loads, you need to be aware of the limitations of
classical linear control theory -- it often falls apart when dealing with
the nonlinearities of switch-based systems, including your example of
mode-switching. Sometimes it's easier to design a simple system that makes
a few basic guarantees (e.g., minimum/maximum output voltage and/or
current, maximum energy transfer per cycle, etc.), and let it do what it
wants to within those bounds.

-- Dave Tweed
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