op amp oscillation in feedback loop without very large cap at the output - Page 1
Forum section: electronics / projects

author: xReM1x
author_rank: Regular Contributor
timestamp: November 18, 2015, 06:07:07 pm
content: hi, I got this circuit for fun (download below) and it is just supposd to be LM350 CC/CV for testing.as it is now the circuit works, but 47uF 1R_ESR cap at the output to make it stable is probably not a good solution. the op amps probably need compenstion network..what can I do to at least stop the oscilation? also is this circuit will be stable in general even if it would not oscilate?

author: Fungus
author_rank: Super Contributor
timestamp: November 18, 2015, 06:19:50 pm
content: What frequency is the oscillation?Electrolytic caps have ESInductanceas well as ESR. That could be causing problems.You could put some ceramic caps in parallel with the main output cap. They have much lower ESR+ESI and will respond quickly until the electrolytic gets moving. For good measure, add a polyester cap as well, for better mid-range response.In audio terms:ceramic = tweeterpolyester = mid-rangeelectrolytic = woofer

author: xReM1x
author_rank: Regular Contributor
timestamp: November 18, 2015, 06:35:14 pm
content: Quote from: Fungus on November 18, 2015, 06:19:50 pmproblem is that It is in simulation so the cap is pretty much ideal, so adding 100 caps will still not change anything, maybe in real life but if it oscilate in ltspice it will probably oscilate in real life.here is with 1OHM load, CC set to 100mA, and no caps expect at the output of the regulator (10uF, 1R ESR)

author: xReM1x
author_rank: Regular Contributor
timestamp: November 18, 2015, 06:38:57 pm
content: Quote from: xReM1x on November 18, 2015, 06:35:14 pm

author: Kleinstein
author_rank: Super Contributor
timestamp: November 18, 2015, 06:45:05 pm
content: The caps at the output of the OPs may work in the simulation (if the model uses a fixed output resistor model and the resistor has no ESR/ESL), but in real life this is a bad idea. So you need a different method to slow down the OPs.Moving around the ref. Pin of a voltage regulator is usually also not a good idea. If with direct connection to a fixed voltage these regulators are just stable with a load in a resonable range. Having extra OPs and extra feedback through this pin can easily cause instability. Also don't expect the models for the regulators to be that accurate. So even if the sumulation works, real world may not.

author: xReM1x
author_rank: Regular Contributor
timestamp: November 18, 2015, 07:00:37 pm
content: Quote from: Kleinstein on November 18, 2015, 06:45:05 pmye I'm kinda aware of that, thats why I want to breadboard the circuit with some caps on the op amp input pins and at the output and input on a breadboard or perfboard.anyways, just playing around for 1minute and I got it pretty stable with caps from output to the inverting pin. is it any good?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 18, 2015, 07:24:54 pm
content: Put a resistor in series with U4 Vout to -in. Also try a series resistor on the compensation caps, so you get R+C feedback instead. Adjust R and C until the step response for each channel is reasonably clean and fast.Tim