Electronics > Projects, Designs, and Technical Stuff
Half-Bridge 0-30V, 0-20A Feedback loop problem
xavier60:
Removing R1 made no difference?
paladyn:
xavier60
I tested different feedback configurations and sometimes I could see high-frequency sinusoidal waveforms, but this is only when testing on the values of elements that deviate a lot from those calculated in accordance with the book. On the other hand, for the calculated values theoretically good, I have not observed such things. At higher power (600W and more) a sawtooth or trapezoidal waveform appeared, I do not remember too well now - I would have to check it out.
Maybe I can do some discharges from the oscilloscope, I will post it.
Snubbery actually lose some power but 2W resistors can handle it.
On the side of the second snubbery, you only use the diddy or transformer. I used it only at the transformer and this change has improved a lot with respect to the noise at the output.
I have not yet tried to change the R1 resistor, but today I will try to do something with it.
Have you never had problems with squeezing the transformer at low power?
Multiple pulse suppression occurs only with the shutdown function (PIN 10 SG3525)? Is it also at the fault amplifier?
Using the pin 10, it is possible to implement a smooth control of the output current?
Edit:
Do you have a proven material, article or book from which I could calculate the value of the compensator for transconductance mode?
xavier60:
--- Quote from: paladyn on June 11, 2019, 08:53:17 am ---xavier60
I tested different feedback configurations and sometimes I could see high-frequency sinusoidal waveforms, but this is only when testing on the values of elements that deviate a lot from those calculated in accordance with the book. On the other hand, for the calculated values theoretically good, I have not observed such things. At higher power (600W and more) a sawtooth or trapezoidal waveform appeared, I do not remember too well now - I would have to check it out.
Maybe I can do some discharges from the oscilloscope, I will post it.
Snubbery actually lose some power but 2W resistors can handle it.
On the side of the second snubbery, you only use the diddy or transformer. I used it only at the transformer and this change has improved a lot with respect to the noise at the output.
I have not yet tried to change the R1 resistor, but today I will try to do something with it.
Have you never had problems with squeezing the transformer at low power?
Multiple pulse suppression occurs only with the shutdown function (PIN 10 SG3525)? Is it also at the fault amplifier?
Using the pin 10, it is possible to implement a smooth control of the output current?
--- End quote ---
I used pin 8, the Soft Start pin for fast current limiting. It gives smooth current regulation but it's not accurate. It is important for the CT to have lots of inductance so that its waveform has a rising slope. If the slope is falling, the fast current limiting wont be smooth.
A slow and more accurate CC loop can then take over. It can be made to reduce the CV loop's reference voltage.
"LATCHING PWM TO PREVENT MULTIPLE PULSES" prevents the steering flip-flop toggling multiple times if switching noise gets into the error amplifier, and ensures no more than one drive pulse per phase per half cycle. https://www.st.com/en/power-management/sg3525.html
If too much switching noise gets into the error amp, it still can upset the loop causing rattling or squealing sounds.
Mine makes a small noise that can be heard when the cover is removed when the load is very small at low voltage. This is only because the SG3525 cant go below a certain duty cycle causing the drive pulses to become intermittent.
I did use a DIY transformer with bifilar center tapped secondary. I use a 39Ω and 2.2nF snubber across the outer legs to reduce 60V peak ringing down to a 20V spike.
Edit: maybe the Shutdown pin can be used, it's able to discharge the soft start capacitor also. But it might need a good logic level pulse to work properly.
paladyn:
Did you measure how much noise you have on the output?
From what is known, current control requires much less elements than voltage control.
Please write in which ranges you can regulate the output voltage?
Do you have current regulations, but more accurate?
xavier60:
--- Quote from: paladyn on June 11, 2019, 09:42:21 am ---Did you measure how much noise you have on the output?
From what is known, current control requires much less elements than voltage control.
Please write in which ranges you can regulate the output voltage?
Do you have current regulations, but more accurate?
--- End quote ---
It regulates from 3V to 33V
I dont actually have a slow regulation loop for the SMPS because the linear post regulator mainly does the current limiting.
I cant remember what the output noise is, It wasn't much. There are two types of noise. The 80KHz ripple which is easy to filter.
And the high frequency hash, bursts of damped oscillations caused by the fast switching of the MOSFETs and output diodes. This hash gets into everything and can be difficult to deal with. Good PCB layout helps a lot.
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