Electronics > Projects, Designs, and Technical Stuff
Half-Bridge 0-30V, 0-20A Feedback loop problem
paladyn:
I've done more tests. I switched off the voltage measurement point for the error amplifier on the common mode choke + 4 capacitors 1000uF caused very high loop instabilities. The loop itself was able to arouse. I did not try on the choke itself.
However, I left the voltage measurement point as in the diagrams, but additionally soldered 200nF between the comp and gnd plus plus the choke common mode choke and 4 capacitors 1000uF. The feedback loop was calculated as if the output capacity was 7000uF and the ESR was 3.57mohm.
Now the whole thing works much better. The transformer at small loads is slightly playing but not terribly loud. Certainly much lower than before.
He also adds graphs of output voltage, evaluate how it looks.
xavier60:
Trying to filter with capacitors can trade one problem for another if the roll off frequency is too low causing phase lag and that ~1.4Khz oscillation in the loop.
It is difficult to figure out the effect the 220nF has at the output of internal error amp. Its output would tend to follow the input signal then start slew limiting when the 100µA limit is reached.
You could try a low pass filter at the input.
It looks promising though.
I would like to see the implementation of the common mode choke.
That 100Hz ripple at full current is likely to be normal.
paladyn:
Direct connection of the output voltage by the common mode choke can be problematic. Mainly because the voltage measurement is single and should be rather differential. When measuring Vout with one common ground wire, I will not have a second signal to pass through the common mode choke.
Unless I understand it wrong?
As for the low-pass filter, I will try to check this option, just please point me at what frequency should this filter be? Unfortunately, the filter itself will strongly influence the calculated feedback loop coefficients.
I met with such a solution that the output voltage was read differently and additionally an offset of about 100mV was added. Have you met with such solutions?
I will also write that when I connected the oscilloscope to the COMP pin which still did not contain 200nF capacity on the oscilloscope, there was a sharp chaff, no periodic run.
Maybe in fact some disturbances penetrated the measuring circuit and the whole was excited.
I will try to check what I get on the output when I remove the common mode choke together with 4 capacitors 1000uF
xavier60:
The CMC is a temporary solution that should get the cleanest voltage signal from the output capacitors into the voltage sense input of the control board.
Experiment with an RC low pass filter at the voltage sense input with a roll-off about 10KHz.
"I met with such a solution that the output voltage was read differently and additionally an offset of about 100mV was added. Have you met with such solutions?"
Not yet.
paladyn:
I checked the inverter's reactions to the ones used
RC filter on the voltage measurement input. 6.8 kohm resistor + 2.2nF capacitor. The transformer is loud and the output waveforms were much worse than at the 200nF capacity between COMP and GND. I expected this reaction, because according to what I read, this filter will enter into the calculation of the compensator coefficients.
I have also tested the influence of the common mode choke, I have not noticed anything beneficial here either. There are still loud squeaks of the transformer at different frequencies. You can see that there is no stability.
I wonder how your inverter works since virtually everything is connected on wires in the air. Life suprises...
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