Electronics > Projects, Designs, and Technical Stuff
Help with fixing oscillating opamp in fb with dc-dc converter
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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