PSU design question (analog vs. digital control)

[homebrew]

Perhaps a stupid question but why are Dave and other Lab-PSU designers using analog control circuits for voltage and current regulation? In all designs that I am aware of, the microcontroller 'only' sets the desired voltage and current through DACs and the actual regulation part is done in analog hardware.

Why not just do proper PID control in software? The uC could sense current and voltage and drive the series pass transistor through a single DAC (or even PWM).
The big advantage would be that the system could be parameterised completely in software for both accuracy as well as stability. And it might also even be cheaper ...

I don't like to believe that this is a safety thing. If the controller goes bust in the traditional circuit, it could also set an undesired voltage to the output ...

Is there any real reason?

[Dave]

Not fast enough. An analog feedback loop will always be faster than an ADC + data processing + DAC.

[homebrew]

An analog feedback loop will always be faster than an ADC + data processing + DAC.

That's certainly right but what is fast enough? 0.5MSPS + Processing for example seems very doable on nearly any modern uC ...
What would your estimate for a minimum sampling and update rate?

[Dave]

A few microseconds, tops.
That would have to be the time from measuring a change, to applying the appropriate correction to the DAC.

[tszaboo]

Why not just do proper PID control in software? ...
Is there any real reason?

Yes, the reason is that it is way too easy to solve the problem with two resistors. There are countless analog power supplies, which have everything built it, and I can design a new power supply in an hour if I really want. With a microcontroller, you need an external switching device, usually a driver for it, voltage sensing for the feedback, programming port for your micro. Also, you need to take care, so a breakpoint doesnt blow up your carefully designed power supply, or it doesnt blow up the first time you power it.
And then you also need an extra power supply, just for the microcontroller.
Besides, digital control is slower than analog. And you can make a decent PID with 3 resistors, capacitors and an opamp basically.
And after you designed it, you need to characterize your supply, which is already done if you use an off the shelf IC.