Re: Slope Compensation and Vodka

From: Tim Wescott (tim_at_wescottnospamdesign.com)
Date: 08/11/04 

Date: Tue, 10 Aug 2004 20:49:36 -0700

Genome wrote:
> "terry" <the_domes@xtra.co.nz> wrote in message
-- well, someone will jump in and castigate us all for
    extraneous text if we don't take it out with a big SNIP --

>
> That's a bum. I can't say why it takes place but I can believe why it
> happens.
>
> I don't believe in an analysis that talks about 'stability'..... or dare
> I say sampled data behaviour.
>
Oh crap. That leaves me out.

Actually it doesn't, because most of the chips that I know of _don't_
sample it -- they have a comparator that's run directly off of the
sawtooth (or triangle) generator and the output voltage.

And I won't talk about 'stability' -- I'll leave off the quote marks.

So now that we've established that (and bearing in mind that I don't
know crap about SMPS, but I _do_ know about time-varying feedback loops):

I think the subharmonic oscillation has to be due to the time-varying
circuit. While this is _not sampled_, if you have any hope of finding a
linear model of it it will be with a z transform -- and if you have a z
transform the most likely place for a pole to escape the stability (see,
no quote marks) region will be at z = -1, which is your 1/2 harmonic
oscillation point.

I'm not going to go beyond this until I'm once again between contracts,
but it's there if you want to chew on it.

> I do believe in multiple solutions without slope compensation that get
> reduced to a unique solution with slope compensation. Its just the
> method required to prove it.
>
> But I wouldn't tout that one around.
>
> Smidley's in it for the bucks, has to roll his shirt sleeve up to take a
> *** and someone else washes his trousers for him.
>
> DNA
>
> 

-- 
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com