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Help with fixing oscillating opamp in fb with dc-dc converter

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tmadness:
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.

NiHaoMike:
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.

tmadness:
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

T3sl4co1l:
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

tmadness:
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

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