Electronics > Beginners
Problems with stability of power supply circuit
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mtimmermans:
Hi,

After watching the power supply design videos of Dave, I got inspired to build my own. For my power supply however, I would like to replace the LT3080 with an op amp plus series pass transistor. The circuit worked fine when I used a resistive load, and I was able to change the output voltage by changing V3. But as Dave warned for in his video, I run into stability issues when adding a capacitive load. Of course I could just use the LT3080 to fix my problem, but for educational purposes I would like to fix this problem without one. I've added my circuit as an attachment to this post.

I am taught the basics of control systems / barkhousen stability criteria at university. So I did the analysis, derived the transfer function and plotted the transfer function in Matlab. This showed a phase margin of 36 degrees, which I guess is not enough and causes the oscillations. What they did not teach in our classes however, is how to actually solve this problem. I could of course decrease the output capacitance, but if I ever use a load with a capacitance greater than 20 uF, the circuit starts oscillating. Does anyone have a suggestion what I could do to solve this problem?
exe:
The simplest way is to add a capacitor (try 1n..10n) between inverting input and output. That would be the simplest type of so-called frequency compensation. Basically, find minimum capacitance that makes it stable and double it (this type of compensation is dominant pole compensation). Be sure to test it well with step load and test under several operating points to be sure it really is stable. The topic is quite complex, if someone can suggest a good practical guide would be great.

Anyway, the best starting point I know is https://www.allaboutcircuits.com/technical-articles/negative-feedback-part-4-introduction-to-stability/ . I suggest read the whole series of articles.
David Hess:
A real output capacitor has ESR which adds phase lead to the frequency compensation and this is why many regulators are unstable when a large low ESR ceramic or film capacitor is used.  So the first step is to include the ESR of the output capacitor in your simulation if it is an electrolytic capacitor

After that and depending on the circumstances, Exe's suggestion of external frequency compensation may be needed.  Usually this takes the form of a capacitive feedback network from the output of the operational amplifier to its inverting input.  But the LM358 is so slow that this should not be necessary.  Note that the LM358 is not too slow to make a good error amplifier; it will work fine.

Another common problem is driving the large input capacitance of a power MOSFET directly from an operational amplifier.  Doing so reduces phase margin and may result in outright oscillation.  A separate driver for the MOSFET gate would help.

A more advanced technique is to add some resistance in series with the source/emitter of the output transistor and take some AC feedback from before this point.  This allows using a low (or no) ESR output capacitor for zero AC output impedance while maintaining stability.  Integrated regulators may do this with special output transistor structures.
T3sl4co1l:
The pictured waveform seems more like simulation noise than ordinary oscillation.  Try building a more physically representative circuit: add series resistance to the gate, capacitor, inductance to the drain, etc.  Adjust simulation parameters (the various TOLs; try GEAR 2 integration; etc.).

Tim
exe:
I suggest add a compensation capacitor even if it seems to be stable, unless you can do a bode plot to verify that phase and gain margins are good-enough not to bother with compensation.
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