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| opamp transistor follower oscillates to buggery when in CC mode |
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| iMo:
The simplest way to estimate the behavior is to simulate its stability (ie. phase margin, etc.) in LTSpice. Just put the essential parts of your PSU into the LTSpice and there is a good tutorial which outputs pretty useful results for example here: https://www.analog.com/en/technical-articles/ltspice-extracting-switch-mode-power-supply-loop-gain-in-simulation-and-why-you-usually-don-t-need.html Except that you may simulate your PSU such you wire a dynamic load to it and watch the currents/voltages at the important nodes. We have done that many times in various threads here already. |
| nemail2:
--- Quote from: David Hess on July 27, 2019, 05:14:08 pm ---Yes, that will work. Like I said, the values are not critical as long as at a minimum, the resistors can sink the worst case Icbo leakage current. I normally use much lower values sized based on the excess available current from the previous stage which helps remove charge from the emitter-base junction more quickly. Ultimately this comes down to how negative feedback is used and the relationship between gain and phase. I do not know of any good practical books on specifically this but I am sure one exists and someone can suggest it. I first picked it up in bits and pieces from various places. --- End quote --- Ok, i bodged them in now, PSU still as stable as before. However I noticed that (doesn't matter whether the base-emitter shunt resistors are in place or not) it starts oscillating if the current consumption of the attached load is JUST exactly as high as the set current limit. if I adjust the load slightly down or the current limit slightly up, the oscillation goes away. also if I adjust the current limit slightly down (or even a lot or doesn't matter how much), no oscillation occurs. it seems that it is only happening in the "transition zone". @literature: feedback and gain I only know in context with opamp feedback resistors and therefore set gain, everything else is chinese to me :) phase shifting and such is chinese as well (or maybe japanese) to me. someone (or some book or video) would really have to start at page 1 with me... --- Quote from: imo on July 27, 2019, 06:35:59 pm ---The simplest way to estimate the behavior is to simulate its stability (ie. phase margin, etc.) in LTSpice. Just put the essential parts of your PSU into the LTSpice and there is a good tutorial which outputs pretty useful results for example here: https://www.analog.com/en/technical-articles/ltspice-extracting-switch-mode-power-supply-loop-gain-in-simulation-and-why-you-usually-don-t-need.html Except that you may simulate your PSU such you wire a dynamic load to it and watch the currents/voltages at the important nodes. We have done that many times in various threads here already. --- End quote --- Thanks for the link, I took a glimpse at it and will read it thorougly tomorrow and try to learn from it. I tried to simulate my PSU (always) but the lack of parts in LTSpice is frustrating. Due to my limited knowledge I never know where I can substitute which parts without significantly affect the simulation results. E.g. LTSpice has no MAX4080. My other (working) circuit uses the LTC6102, which is obviously included in LTSpice, as it is an LT part. But that one is really expensive and I did aim for low cost with this (not very good working) design. OPA2197 isn't included as well, neither is the LM358 but at least for the latter I did find a .lib file to use in LTSpice. Also, with those 3rd party .libs you never know how accurate they are. Anyway, I did not manage to get my current, not working design to oscillate in LTSpice. that makes it quite hard to try to stabilize it :-DD I have attached what I have come up with (rename LM324.ti.txt to LM324.ti.lib). Due to the lack of the MAX4080, I inserted a LTC6102, which is not accurate, I know - but what should I do? |
| nemail2:
i have replaced C13 (original schematics) with 100nF and removed the diode D2. only oscillation that is left ist the one in the attached screenshots, when transitioning from CV to CC mode or vice versa. with C13 being 1nF this oscillation was WILD and irregular. with C13 @ 100nF it got this small and regular. I tried to mess with R11 as well and to put 22pF between IC3B pin 6 and 7 but I didn't manage to get that oscillation away (which looks like ripple to be honest). also touching the circuit with my finger at various points didn't do much - i thought doing this i might find sensitive places which react to capacitance... in SDS00005.png you can see the transition and the oscillation during the transition. in SDS00006.png you can see the continuous oscillation if I manage to tweak the load knob with my tongue at the right angle to just about the set current limit. I'm using this load and I have already tried powering it with batteries, the 50Hz oscillation is still there: https://www.aliexpress.com/item/150W-adjustable-Constant-Current-Electronic-Load-Battery-Tester-12V24V48V-Lead-acid-lithium-Discharge-Capacity-meter-200V/32866240382.html?spm=a2g0s.9042311.0.0.27424c4dTzPJu2 Any ideas? As this is supposed to be a "random kinda crappy, half-decent general purpose" PSU, i might leave it like this but if anyone has suggestions how to get rid of that oscillation, I'd be happy to modify the circuit. Looking at the changes I have made already, I have to respin the PCB anyway. I just don't really want to spend very much time on it anymore so complete redesign won't be the solution for me (maybe another place, another time, another PSU, ...)... Thanks! |
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