Need some loop stability advice (LTSpice) - Page 1
Forum section: electronics / projects

author: Boscoe
author_rank: Frequent Contributor
timestamp: April 30, 2022, 10:10:35 pm
content: Hi all,I've never mastered analogue control loops and it bothers me. I'm trying to design this CV/CC lab PSU in LTSpice however not matter how many capacitors I've tried to place in how many configurations, I just cannot get this thing stable in CC. CV is fine.It may well be that it's not viable with this topology and component selection.Could anyone give me some pointers on how to tackle this sort of problem?I understand the loop stability criteria however (in the past) I never seem to be able to improve the gain margin. I try to shift only the gain but the phase will always move with the gain so I just end up lowering loop bandwidth and not improving stability.If anyone has any resources for developing knowledge in this area, that would be awesome too.Many thanks!

author: mawyatt
author_rank: Super Contributor
timestamp: April 30, 2022, 10:32:30 pm
content: Remember that in CV mode the starting point for stability analysis is an Open Circuit load that moves towards a Short Circuit, whereas in CC mode the starting point is a Short Circuit load moving towards and Open Circuit.Best,

author: moffy
author_rank: Super Contributor
timestamp: April 30, 2022, 10:35:26 pm
content: The simulation seems stable when I run it, but U3 provides a lot of gain for the current feedback, I am suprised it is stable.

author: xavier60
author_rank: Super Contributor
timestamp: April 30, 2022, 10:41:07 pm
content: Please post the schematic.

author: Boscoe
author_rank: Frequent Contributor
timestamp: May 01, 2022, 08:19:26 am
content: Hi, it is attached in the OP.

author: Boscoe
author_rank: Frequent Contributor
timestamp: May 01, 2022, 10:21:17 am
content: Quote from: moffy on April 30, 2022, 10:35:26 pmYes this is very strange. I just tested again and it is stable but honestly I've been battling this thing for days!Do you have any recommendations on how to replace U3 or decrease the probability of oscillation? I actually want to make this however I currently have zero confidence it will work in the real world.

author: jonpaul
author_rank: Super Contributor
timestamp: May 01, 2022, 11:19:40 am
content: Suggest that you first study classique 2nd order control system théorieCompensation is easy with a Bode plot.between oscillation underdamped and slow response overdamped, the optimal is often critical damping.I have never used LTspice, or other Simulatorsbuilding the circuit is easierBon courageJon

author: moffy
author_rank: Super Contributor
timestamp: May 01, 2022, 12:01:03 pm
content: Yes, reduce the gain of U3, you might need to reconfigure it a bit to work properly but that is the most obvious issue.

author: Zero999
author_rank: Super Contributor
timestamp: May 01, 2022, 12:06:15 pm
content: It is unstable.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 01, 2022, 01:28:57 pm
content: On a side note:Why use the LT1210?It seems to be a quite expensive part with limited supply voltage rails and current delivery.It does have a quite high bandwidth though (which may add to loop stability problems)I would be more inclined to design around an audio amplifier, maybe a TDA2050 or LM3886.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 01, 2022, 06:56:56 pm
content: Quote from: Boscoe on April 30, 2022, 10:10:35 pmYou attached a schematic set for transient analyses, which makes me think you didn't make any AC simulations for stability. You should break the feedback loop to see if the design met the stability criteria.To open the feedback loop, here's an LTspice example:A nice overview about stability from Apex: Stability for Power Operational Amplifiershttps://www.apexanalog.com/resources/appnotes/an19u.pdfFor more verbose explanations and examples, maybe this series by Tim Green might be of help, called "Operational Amplifier Stability". I've collected the PDFs after they were mostly gone, and now scattered all over the web. Took a lot of searching to find them all (they were published while Burr Brown was not yet bought by Texas Instruments, and now the old links are broken). It took so much time to find all of them that I've burned the day searching where from to download the PDFs, instead of studying them.There are only 11 parts (not 15), 12 to 15 were never published, according to the author. There are some PPT continuations instead, called "Solving Op Amp Stability Issues" 1 to 4.ToC for "Operational Amplifier Stability" by Tim Green01. Loop Stability Basics.pdf02. Op Amp Networks, SPICE Analysis.pdf03. Open and Closed Loop Output Resistance.pdf04. Loop-Stability Key Tricks and Rules-of-Thumb.pdf05. Real World Design of a Single Supply Buffer Circuit.pdf06. Capacitance-Load Stability: RISO, High Gain & CF, Noise Gain.pdf07. When Does RO Become ZO.pdf08. Cap Load Stability: Noise Gain & CF.pdf09. Capacitive Load Stability: Output Pin Compensation.pdf10. Capacitor Loop Stability: Riso with Dual Feedback.pdf11. Modeling Complex Zo for Op Amps.pdfThe last 4 are in fact PPT presentations, easier to find, will not attach them:12. Solving Op Amp Stability Issues Part 1.pdf13. Solving Op Amp Stability Issues Part 2.pdf14. Solving Op Amp Stability Issues Part 3.pdf15. Solving Op Amp Stability Issues Part 4.pdf

author: RoGeorge
author_rank: Super Contributor
timestamp: May 01, 2022, 07:01:54 pm
content: Part 8 to 10 of "Operational Amplifier Stability" by Tim Green

author: RoGeorge
author_rank: Super Contributor
timestamp: May 01, 2022, 07:02:51 pm
content: And the last, part 11 of "Operational Amplifier Stability" by Tim Green

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 02, 2022, 02:49:40 am
content: Hi,Try this:I have attached the model.I will let you figure out the changes I madeJay_Diddy_Bpsu_version_JDB.asc(3.46 kB - downloaded 61 times.)

author: xavier60
author_rank: Super Contributor
timestamp: May 02, 2022, 04:21:28 am
content: Instead of the Zobel network. AC feed back could be coupled from the left side of R2.

author: jonpaul
author_rank: Super Contributor
timestamp: May 02, 2022, 07:35:18 am
content: C4 1 nF very high cap load.Add 5..100 Ohm series Rj

author: moffy
author_rank: Super Contributor
timestamp: May 02, 2022, 09:26:54 am
content: What I did was similar to Jay_Diddy_B, but U4 is also problematic as jonpaul states, and deserving of some compensation. Some opamps don't like capacitive loads, very few do, so a little compensation will help stabilise it.P.S. This is not a final design, but merely indicative of some stabilisation methods.