• State TI Thinks Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 3 answers
  • Subscribers 63 subscribers
  • Views 316 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC28221: Slope compensation problem

FLAVIO POZZOBON
Prodigy 80 points
Part Number: UCC28221

Hi Team,

Yesterday I opened the thread but I didn,t receive any answer. Maybe I have made some mistakes in doing it...

Anyway this is the subject of mine. I am working on a 28VDC-100A buck interleaved power supply with Vin up to 160VDC.

Rchg and Rdischg are respectively 75 and 22kohm. Rslope at the begginning was 22kohm, but later I put a 100kohm trimmer to adjust easily the value.

The problem is that when Vin approaches 50VDC a subharmonic oscillation at about 3kHz appears on PWM signals. Increasing the input voltage the oscillation increases in amplitude and the board starts to be acoustically noisy.

Reducing Rslope oscillation becomes smaller but  also Vout is reduced due to the reduction of duty-cycle.

Changing the amplitude of the output voltage feedback in order to reach the nominal output voltage value subharmonic oscillation comes back to an increment.

I have tried different value of switching frequency and maximum duty-cycle variyng Rchg and Rdischg but without appreciable improvements.

Do you have any hints?

Kindest Regards

Flavio

  • 0
    TI__Guru 73265 points

    Hi,

    Can you please provide your schematics for a look to see I can have any suggestions?

  • 0 in reply to Hong Huang
    Prodigy 80 points

    Hi Hong,

    Below the main schematic (please save the image and enlarge it).

    R13, R14 and D5 are not present thus unwanted interferences are eliminated.

    RT1 and RT2 are trimming resistors. At the moment RT1 is not mounted and RT2 is a trimmer.

    The current sense transformer have a ratio of 1:100, Ls=7.6mH, Rs=0.75ohm.

    Output chokes have 11.5µH inductance, Output capacitance is 574µF (polymer).

    Regards

    Flavio

  • 0 in reply to FLAVIO POZZOBON
    TI__Guru 73265 points

    Hi,

    Your schematics look not a completed one as I see you feedback loop does not show all parameter values. 

    In your first message, when you reduce Rslope, the sub-harmonic is reduced, while Vout becomes smaller, - it looks your feedback loop is not working as expected, since if Vout is not in regulation, the feedback loop should adjust to increase the duty cycle until Vout becomes regulation.

    I suspect your feedback loop does not work as expected, as your schematics also shows the loop not completed.

    You need to make sure your feedback loop work normally to maintain Vout in regulation after you adjust Rlospe to attenuate the sub-harmonic.

  • 0 in reply to Hong Huang
    Prodigy 80 points

    Hi Hong,

    The Output Voltage Feedback Signal to UCC28221 is provided by U2 output which in turn compares a quote of Vout with a reference voltage (from DZ5). No other parts are missing apart from output overvoltage and temperature derating sections wich refer to R13 and R14 (not mounted).

    The peak current limiter section (left-low corner of schematic) does not influence the CTRL signal because D5 is not mounted.

  • 0 in reply to FLAVIO POZZOBON
    TI__Guru 73265 points

    Hi,

    If your Vout is not in regulation after you change Rslope, your feedback loop should adjust the duty cycle to bring back the regulation. So it looks your feedback loop does not work as expected. You need to check your feedback loop to make sure it works.

  • 0 in reply to Hong Huang
    Prodigy 80 points

    Hi Hong,

    Thank you for the suggestion. I'll double check the feedback loop.

    By