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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 11:10:41 +0100
Organization: Poppy Records
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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.


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