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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: power supply idea
Date: Mon, 22 Apr 2024 17:30:40 +0100
Organization: Poppy Records
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John Larkin <jjSNIPlarkin@highNONOlandtechnology.com> wrote:

> On Mon, 22 Apr 2024 11:10:41 +0100, liz@poppyrecords.invalid.invalid
> (Liz Tuddenham) wrote:
> 
> >John Larkin <jjSNIPlarkin@highNONOlandtechnology.com> wrote:
> >
> >> If one had, say, a 48 volt power bus, you could hang a half-bridge
> >> switcher to ground, and a lowpass filter out. If the drive has duty
> >> cycle n, the output voltage is 48*n. So we have a programmable power
> >> supply with no feedback, which will be stable into any load.
> >> 
> >> The load regulation will be mediocre, but we could almost sell it
> >> as-is.
> >> 
> >> So now, sense the output voltage and compute the error against the
> >> target, run through a slowish integrator, and tweak the PWM to get
> >> zero output voltage error. Gross transient response is basically the
> >> response of the output filter, with some modest drool from the
> >> integrator.
> >
> >In thory, pulse-width contol of the output could give excellent
> >stability under load -- but the filter is going to cause droop.  Unless
> >you are very careful about the design of the filter, the phase shifts it
> >creates will make the feedback loop unstable.  An integrator in the loop
> >will stabilise this at the expense of a much slower response time.
> >
> >Somewhere in the loop you need a dominant pole so that (to use audio
> >amplifier terminology) your roll-off is 6dB per octave until the loop
> >gain has dropped far enough for stability when all the other phase
> >shifts kick in and the slope increases to 12dB per octave or more.
> >Rather than integrating the feedback, transferring the dominant pole to
> >the filter will result in less output noise and a faster response to a
> >step increase in the load.
> 
> An LC filter is at least 2-pole, usually more,

If you made it three poles, with one of them significantly lower
frequency than the other two, stability would be much easier to obtain.


-- 
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk