Electronics > Projects, Designs, and Technical Stuff
Is 30mV still acceptable as the lowest voltage for a linear power supply?
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bloguetronica:
Hi,

This is more a matter of opinion, based on your personal experience. The attached circuit can only achieve an output voltage of 30mV at its lowest (even of the output of the DAC is at absolute 0V). I could solve this by using a different topology (4:1 amplifier and 1:1 error correction, instead of follower and 4:1 correction), or perhaps by driving the output of the first op-amp to negative via a 10K resistor with a charge pump. However, both these solutions will increase noise, with the risk of losing precision as well, and IMHO it is not essential to be able to achieve such low voltage and more precision for the full range is preferable.

What is your opinion?

Kind regards, Samuel Lourenço
ejeffrey:
For most applications 30 mV is more than sufficient.

It is typical to want lab supplies to have floating outputs.  In this type of regulator that would be accomplished by having the 24 V input from a floating supply.  In that case it is easy enough to slightly raise the "ground" level to allow tuning down to zero volts.  However, I notice that you have SPI pins.  If your supply will be externally programmable you would then need to isolate the communications interface somehow.
bloguetronica:
Thanks Jeffrey,

The power supply will be isolated, sure. This is just a test module to see if the concept is feasible. Another issue I'm having is a noise of about 2mVpp at the maximum load.

Kind regards, Samuel Lourenço
David Hess:
One of the amusing things about the design of the old Tektronix PS501 and PS503 power supplies is that they deliberately included an offset in the voltage error amplifier which is slightly greater than the maximum offset voltage of the 301a or 741 operational amplifier.  Since the output has a pull-down to the negative supply, this allows the output to actually go up to 10 millivolts below 0 volts.  Without the deliberate offset, the minimum output voltage could be higher than 0 volts depending on the input offset voltage of the operational amplifier.

I would consider operating down to 300 millivolts acceptable for general use.  But for a source-meter application, I would want it to support operation down to zero volts or even negative.
floobydust:
30mV is a problem if that is the minimum output voltage when in CC mode, if I remember your other design that has CC.
You short the output and CC activates but only takes the output down to say 30mV and lots of unexpected current flows.

That happened on one of my designs, I'd set it for 100mA and all was fine until I shorted the output and I got 3A  :o
Took me a while to figure that out. The problem was CC could not take the output to zero volts and a small offset remained and so (pass transistor) current-limiting took over.
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