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Is 550uF too big for a power supply that has CC limit?

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bloguetronica:
Hi,

I'm designing a linear power supply that has both voltage and current control. Through both simulation and experimentation, I've concluded that 270uF is marginal in order to stabilize the power supply. So, I had to double the capacitance just to make sure.

However, by seeing the most recent video on EEVBlog, I've concluded that I might have impaired the CC mode of my supply. Should I be worried?

The attached schematic is just the output stage. There is also a pre-regulator stage, and you can see the discussion here:
https://www.eevblog.com/forum/projects/is-there-any-variable-dc-dc-converter-suitable-for-pre-regulation/

By the way, this project had a few corrections, in order to filter some instability caused by the interaction of the pre-regulator with this final stage. Also, the CC mode was unstable, so I had to add a capacitor (C12) near Q1. The new boards are not made yet, but they are now under way. Lets hope this works, because I've fried the whole control section of my "old" output stage board (a mystery).

Kind regards, Samuel Lourenço

bson:
The IC4B based sense amplifier lacks external compensation.

Kleinstein:
How much capacitance is acceptable at the output depends on the current range and application. 500 µF is not good, but about normal for many 3 A supplies.  The capacitor C12 is odd - it slows down the CC mode a lot and has good chances to make it oscillate with an inductive load.  Both the current and voltage sensing are using differential amplifiers in a way that is not good for accuracy and depending on tolerances can cause stability problems. So the whole concept is not that good.

Normally a supply with an emitter-follower  output stage can work with a much smaller ouput capacitor. So with compensation at IC4B it should work with a much smaller (e.g. 10 µF) cap.

Zero999:
It's all but impossible to design a PSU with constant current and constant voltage and have good load regulation for both. A capacitor will improve the transient response of the CV mode, at the expense of CC. No capacitor an a large inductor will improve the CC mode, at the expense of CV mode.

Normally we optimise for CV mode and accept that current surges beyond the current limit can occur. I've toyed with the idea of building a CV/CC power supply with a three position switch which adjusts the filtering to be optimum for CC, CV or the best of both, because no PSU I've seen offers this, although I've never got round to it.

T3sl4co1l:
Delete C12, add a resistor in series with C11 and tweak their values; do the same with IC4B, an R+C from out to -in.

Don't expect it to go terribly fast with that 2.7k (R5) supplying a darlington.  I don't know offhand what C and ESR would be necessary to compensate this combination when tuned for best performance, but 100s of uF is probably not unreasonable.

Why two different DACs?  Seems a waste of a BOM line.  Similarly, the different op-amps and comparator used could be simplified.  But these don't affect the correctness of the circuit, in any case.

Tim

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