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From: John Larkin <jjSNIPlarkin@highNONOlandtechnology.com>
Newsgroups: sci.electronics.design
Subject: Re: power supply idea
Date: Mon, 22 Apr 2024 12:16:07 -0700
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On Mon, 22 Apr 2024 17:30:40 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

>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.

Capacitor ESR, native or added, helps.

I also want to kill the Q as seen from the load side, so it doesn't
ring much if they switch an inductive load.

The load-from-hell is of course some box with a switching regulator
power supply, that looks like a negative resistance load.