Electronics > Projects, Designs, and Technical Stuff
Is 550uF too big for a power supply that has CC limit?
T3sl4co1l:
The remark about unity gain is not about stability of the amp itself, but about its place in the loop. Namely, even if the amp is severely overcompensated (say by putting 1000uF from out to -in), it still has gain of 1 from +in to out, which means the loop gain has not dropped to an ideal dominant pole as you might've been expecting with such a large cap.
The solution is to apply filtering to +in so it drops at the same rate.
Compare this, which has G = 1 at high frequency,
with this, which has G --> 0 at HF.
For an error amp, R2 and R4 --> infty, so that DC error is as nearly zero as possible.
For a pole-zero compensation of course, a resistor is placed in series with C2 and C4.
The zero itself means HF gain levels off; the point of doing this, versus leaving it alone, is you have control of the gain and frequency where this happens, rather than being stuck with wherever G=1 rolls it off at.
Tim
blackdog:
Hi bloguetronica,
For example, assume that IC7 is set as a 1x amplifier and has 60 degrees of phase space.
Of these 60 degrees little remains, this because of the following components in de current loop: IC6, R7+C12, Gain Q1 and Q4 your power stage, which will eat all phase margin.
45 degrees is the absolute minimum you need for a good en stable power supply, this under all conditions.
That at a certain capacitor value at the output connector, and the power supply does not generate, does not mean that it is stable...
You will have to test this circuit at various output voltages and also at various currents.
Test it dynamically with short pulses, pay special attention to the transition area when your power supply changes from CV to CC.
Kind regards,
Bram
xavier60:
From my experience, designs that have high impedance output stages, also called transconductance or voltage to current transfer function are easy to compensate for good CV and CC stability using what TI calls "Type II Compensator".
This typically applies to so called floating type designs which tend to give very good performance.
With a low impedance or voltage follower type output stage, in CC mode the loop gain would increase steeply with lowering load resistance.
http://www.ti.com/lit/an/slva662/slva662.pdf
xavier60:
Have a close look at a typical LM723 implementation. Although the output stage is voltage follower in CV mode, it becomes transconductance in current limiting mode.
floobydust:
550uF is too much.
Prove it by setting any PSU to say 15V at 10mA and testing some LED's, as a real world example.
Energy is stored in the output capacitor which will be charged at 15V which dumps into the LED load down to 3V. You thought the LED was 12V but it's a 3V part. The CC mode transition takes some time as well.
So the LED might just look like a strobe light (bright) flash happened, or the LED might be dead now due to the impulse.
Some lab power supplies I can't even test LED's, they surge so bad. This is my criterion, a decent PSU is at most ~100uF but if you work with discrete semi's and small parts, 550uF is too big.
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