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From: John Larkin <jjSNIPlarkin@highNONOlandtechnology.com>
Newsgroups: sci.electronics.design
Subject: Re: power supply idea
Date: Mon, 22 Apr 2024 08:53:45 -0700
Organization: Highland Tech
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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, and we have no idea
what crazy stuff the customer might connect to our power supply. So
the less we depend of feedback from the output, the safer things get.

A power supply without feedback is always stable.