DC bench supply - regulation speed, reverse current

[hlavac]

Hi,

I'm trying to design a bench DC supply with several voltage outputs and I have two questions:

1) How fast current/voltage regulation is fast enough? I have seen some designs with a microcontroller in the loop, looks a bit slow to me - how fast should regulation respond to changes in load to be considered good enough? Maybe some theory on destroying stuff would help (how much energy is needed to actually damage something?). Currently i consider a hybrid microcontroller / op-amp solution (micro to set the limits/monitor, op-amps to enforce the limits/regulation)

2) Having several voltage outputs, and maybe with some inductive loads, it is possible that current will want to flow in reverse direction then intended (for example, from +12V to +5V). I understand that usual power supply designs will have problem with this. Any suggestions on how to do a regulator circuit that regulates both ways? I mean, classic designs seem to basically pull up the voltage until its high enough - what if something causes the output to go even higher? How can i make it so it pulls it back down to where it should be?

[scrat]

1) I don't have a good spec by hand, but you could find the ones of a mid-range power supply around and have an idea.
I won't certainly use an MCU for the current/voltage control.

2) Isn't it possible to prevent reverse current by using a series diode for each output? With feedback compensating for the drop, of course.

[Kiriakos-GR]

1) How fast current/voltage regulation is fast enough?

I would worry more about how clean the regulation are from hum and noises.
Especially if you plan to work with preamplifier s and sound circuits.

[Zero999]

1) I have seen some designs with a microcontroller in the loop, looks a bit slow to me

Are you sure?

What normally happens is the DAC output is used as a reference for the power supply and the ADC input is just used to monitor the voltage and display it.

[johnmx]

1) I have seen some designs with a microcontroller in the loop, looks a bit slow to me

Are you sure?

What normally happens is the DAC output is used as a reference for the power supply and the ADC input is just used to monitor the voltage and display it.

Did you read the complete paragraph? What you say is what he calls as a hybrid solution.
That sentence refers to a control circuit using a microcontroller in the feedback path.

[alm]

2) Having several voltage outputs, and maybe with some inductive loads, it is possible that current will want to flow in reverse direction then intended (for example, from +12V to +5V). I understand that usual power supply designs will have problem with this. Any suggestions on how to do a regulator circuit that regulates both ways? I mean, classic designs seem to basically pull up the voltage until its high enough - what if something causes the output to go even higher? How can i make it so it pulls it back down to where it should be?

A diode anti-parallel over the output is the easy solution, you'll just short any reverse current. Some supplies have down programming to deal with this (mainly to discharge caps on the output that may have been charged with a higher voltage). Even more advanced is a four quadrant power supply, which can 'supply' positive voltages, negative voltages, positive currents and negative currents.

[Zero999]

Did you read the complete paragraph? What you say is what he calls as a hybrid solution.
That sentence refers to a control circuit using a microcontroller in the feedback path.

Yes I did. I've read it again and it still doesn't make much sense - it's poorly worded. The only time an MCU would be used in the feedback loop is in an SMPS.

[hlavac]

I guess I was referring to this: http://www.tuxgraphics.com/electronics/200707/bench-power-supply-unit.shtml

They use ATmega8 which has maximum ADC sampling frequency of around 15kHz for full 10-bit resolution (at 200kHz ADC clock, single-ended free running mode, 13 ADC clocks/converison). And they do two ADC converions per loop (current and voltage).
That would put the maximum control loop frequency at around 7.5kHz, or 133us per loop, at best.

Now why this seems slow to me is that in L7805 datasheet from ST there is a graph of transient response that looks like it takes somewhere around 5us for the whole regulation to happen after a step load change...

Maybe they run the ADC over the spec (you can do it but lose LSBs...)? Minimum conversion time is stated as 13us in the datasheet, that is at maximum 1MHz ADC clock, and that is at like 7 bit accuracy at best according to datasheet... 26us per loop. Still not close to 5us reaction time of a linear regulator (and that is just how long it takes for the regulator to notice it has to do something, it does not take into account any actual stabilization AFTER the DAC has been updated to compensate...)

EDIT:

I have checked the actual project sources, it runs on 4MHz cpu clock with 1/32 divider = 125kHz ADC clock = 9615 Hz sample rate, resulting in 208us loop. Even slower than i thought.

[hlavac]

A diode anti-parallel over the output is the easy solution, you'll just short any reverse current.

Oh, i like that one, will do it 

I have also seen a diode across a regulator back to voltage source (fig. 28 here), used to protect the power transistor in the regulator from reverse current (aka input short circuit protection)...

[johnmx]

I guess I was referring to this: http://www.tuxgraphics.com/electronics/200707/bench-power-supply-unit.shtml

That’s not a good power supply. Keep all control circuitry in the analog domain.
You can easily find better circuits in the web or in any electronics magazine.

Regarding the time constant of a power supply, you can find that info in any manual of a decent brand power supply. Search the forum for manuals.

[Zero999]

No it's not how it should be done. That power supply is horrible. They've also used a zener and transistor reference which will give poor drift and regulation - not good at all.

The best way is to build a standard analogue regulator and use the DAC as the reference and the the ADC as a monitor so you can display the voltage/current.