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Oscillations with LM358 electronic load

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salami738:
Hi,

i build an electronic load based on the schematics from: https://www.eevblog.com/forum/beginners/lm358-in-consant-current-circuit-problem/
My changes are:
- digital controlled by MCP4725 12 Bit DAC / Arduino
- The power stage is multiplied 3 times in parallel
- mosfet: STP80NF55L-06 (datasheet: https://www.st.com/resource/en/datasheet/stp80nf55l-06.pdf)

Schematic


The problem is, that the circuit becomes unstable above approximately 1600mA current (~530mA per mosfet).

I checked the load current fluctuation with my oscilloscope and got this results:

500mA:
1000mA:
1600mA:
2100mA:
3000mA:

What is the problem with this schematic? How is this fixable? Thanks for your help.

exe:
It can be due to power supply you are loading: two control loops counteract each other. Try increasing compensation cap, or use, e.g., a battery to check if the problem is in load, not in power supply.

MarkF:
Try changing C3 to 10nF instead of 1nF.
I wasn't looking for switching speed in my eLoad so I used 10nF (My opamp and MOSFET are also different).
See if that stops the oscillation.  Then, back it off from there.  I expect you can go as low as 4nF.

Also see:

Kleinstein:
It may take an RC series element (e.g some 10 Ohms and 1 µF)  for damping the output.

The output impedance of the constant current circuit tends to look like a capacitor. The output impedance if the usual simple voltage regulator looks a lot like an inductor. So the two together tend to form a resonant circuit and if one of the two has a little more than the ideal 90 degree phase shift they can oscillate. It is sometimes hard to tell which of the two is at fault.

The LM358 has an output stage (class B) with quite some cross over distortion this can be a problem for stability, is in the trun over point it adds delay and thus destabilizing phase shift. One could try a different OP, like TLC272. Also a resistor from the OPs output to ground to add some load to the OP may help. 

salami738:

--- Quote from: exe on July 13, 2019, 02:13:45 pm ---It can be due to power supply you are loading: two control loops counteract each other. Try increasing compensation cap, or use, e.g., a battery to check if the problem is in load, not in power supply.

--- End quote ---

Hi, i checked the electronic load with a 18650 cell as power supply and got similar results. So it is not a control loop problem.

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