Re: power supply idea

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.