Help with fixing oscillating opamp in fb with dc-dc converter - Page 1
Forum section: electronics / projects

author: tmadness
author_rank: Regular Contributor
timestamp: July 13, 2020, 10:30:12 pm
content: I ended up creating a high power oscillator. My original intention was to use a DC-DC converter in CC mode (yes it is designed for that) with a DAC Current setting. The issue seems to be that the system has significant enough phase shift that it oscillates at 0.7- 1.5 KHz. I've attached a schematic (used DaveCAD). My understanding of the theory of compensation tells me that adding a RC pole after the summing amp and a miller cap in the feedback loop of the summing amp should help. I would like to call upon the grey beards of the forum to help we develop a practical understanding and solution to the issue.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 13, 2020, 10:59:38 pm
content: If the DUT is inductive, that can make for quite a phase shift. To offset that, add a resistor between FB and OP2 (try 1k for a start) and a series RC network between Vout and FB. You'll have to play with the values to get the right balance between stability and fast response.

author: tmadness
author_rank: Regular Contributor
timestamp: July 13, 2020, 11:29:51 pm
content: The DUT is purely restive actually. BUT, What you said about inductive phase shift rang a few bells in my head. The DC-DC conv's inductor probably is the main reason for the phase shift. I will try out the series RC

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 14, 2020, 03:00:42 am
content: Why go to all this trouble? Strip it back to just the shunt resistor. FB straight to it.Now put a series resistor to FB. Now put a resistor from DAC to FB. Voila, you have a voltage divider from DAC to FB to shunt. By biasing up the DAC, you fake a current signal and the converter throttles down.A little algebra will solve what voltage ranges and resistor ratios are necessary.Tim

author: tmadness
author_rank: Regular Contributor
timestamp: July 14, 2020, 04:53:33 am
content: I inherited the idea, I was told that the dynamic range was not possible with a resistor current adder. BUT, I did do some back of the envelope calculation and this does not seem true! it is possible to achieve what I want with simple resistors. I will get back to you if it works out

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 14, 2020, 05:34:35 am
content: Dynamic range will be limited by the converter's performance, anyway. It might go into some pulse skipping mode, for example. At that point, turn it off completely and use another method, say just a linear source since you probably aren't worried about efficiency down in the mA?The resistor divider and DAC work as well as the components do; dynamic range means getting arbitrarily close to zero, which is just a calibration error for the DAC, and the resistors can similarly be calibrated out as a gain error.Perhaps the converter's reference is drifty, so that calibration isn't feasible to meet some stability requirement? But then, if it can't hold zero, it won't hold any other value any better!Note that op-amps cannot help here as their output goes to the same thing, it all comes back to the converter's error amp and ref.Tim

author: tmadness
author_rank: Regular Contributor
timestamp: July 15, 2020, 03:26:34 am
content: Update: I used through-hole run of the mill resistors and there were still oscillation. However, i took NiHaoMike's advice and put a RC filter from fb to Vout, this slowed my response down, but, I can live with it. So thank you T3sl4co1l and NiHaoMike.

author: David Hess
author_rank: Super Contributor
timestamp: July 15, 2020, 04:43:55 pm
content: Quote from: tmadness on July 13, 2020, 11:29:51 pmThat is exactly the case; the LC filter formed by the output inductor and capacitor introduces 180 degrees of phase shift. Current mode operation is popular because it removes the phase lag of the output inductor. Alternatively the ESR of the output capacitor may be relied upon to add 90 degrees of phase lead which is why having too low of an ESR may cause oscillation.

author: tmadness
author_rank: Regular Contributor
timestamp: July 15, 2020, 09:32:57 pm
content: Interestingly, the manufacturer of the DC-DC converter recommends only using X7R Caps. I think this may be due to efficiency reasons.