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UCC28180: Ccomp MAX/MIN recommendation

Farzaneh Faridi
Intellectual 605 points
Part Number: UCC28180

Tool/software:

Dear Members,

We have utilized this chip in our Power Factor Correction (PFC) design. It operates with an input voltage range of 85VAC to 265VAC, and the maximum PFC voltage should be 440VAC with current of 3A and peak of  6A. To achieve the minimum and maximum input voltages, we reduced the Ccomp value to 100pF. However, this resulted in an unstable system. Increasing the Ccomp to 1nF significantly improved stability.

My question is: Is there any recommendation for the minimum and maximum values of this capacitor? The only relevant information I found was on page 32 of the datasheet, which states:

"Analysis on the completed converter may be needed to determine the ideal compensation pole for the current averaging circuit. A capacitor that is too large on ICOMP will add phase lag and increase iTHD, whereas a capacitor that is too small will result in insufficient averaging and an unstable current averaging loop."

I have reviewed almost all the forum posts on this topic but could not find any relevant discussions.

I would greatly appreciate any assistance or insights.

Regards,
Farzaneh

  • 0
    TI__Mastermind 36940 points

    Hello Farzaneh, 

    Reviewing the UCC28180 Excel Calculator tool equations for Cicomp, it shows that the maximum and minimum recommended values for Cicomp are based on setting an averaging pole frequency between 1/40th of fSW and 1/20th of fSW.   I don't know the reasoning behind these criteria.  I can guess that 5kHz in the example calculation of equation (101) on page 32 is 1/20th of 100kHz, which is close to the chosen fSW of 118kHz (on page 25).

    I am confused by your statement: "...input voltage range of 85VAC to 265VAC, and the maximum PFC voltage should be 440VAC with current of 3A and peak of  6A. "
    Are you saying the maximum PFC input voltage is 440Vac and the total input range is from 85Vac to 440Vac?  
    Or, did you mean the maximum PFC output voltage is 440Vdc ?

    The UCC28180 is designed for CCM operation over the normal AC input range of 85Vac to 265Vac, with ~385Vdc < Vout < ~405Vdc.  Operation outside of this range may be possible, but performance becomes degraded the further away from the normal ranges the inputs and outputs are. 
    In particular, inductor current may change from CCM to DCM at very high input voltage, and filtering (averaging) of the inductor ripple current may be inadequate with low values of Cicomp. 

    There may not be a single value of Cicomp that is ideal for all conditions, and a compromise value must be chosen by empirical testing and evaluation.

    Regards,
    Ulrich   

  • 0 in reply to Ulrich Goerke
    Intellectual 605 points

    Hello Ulrich,

    It was my mistake.Let me clarify a few points regarding our design parameters and recent observations:

    1. Input and Output Specifications:

      • Input: 85VAC to 265VAC, with a transitory voltage of 440VAC (lasting 10 seconds) at 3A and 6A.
      • Output: 376Vdc to 395Vdc, with a maximum of 415Vdc at 0.6A and 1.2A (lasting 10 seconds).

    2. Voltage Specifications: The PFC Vdcmax is 415Vdc, and the nominal is 395Vdc. You mentioned that "~385Vdc < Vout < ~405Vdc", but I couldn't locate this range in the datasheet. Could the 415Vdc maximum pose any potential issues?

    3. Switching Frequency and Compensation: We've considered a switching frequency (fSW) of 120KHz and a pole frequency of 5Hz. Initially, we used a 1nF capacitor for Ccomp, but the system failed to start at 85VAC. Reducing Ccomp to 100pF allowed the system to start at 85VAC; however, we are experiencing significant noise on the input signal, both with and without load.

    Please let me know if you need further details or additional information.

    Thanks for your help
    Farzaneh

  • 0 in reply to Farzaneh Faridi
    TI__Mastermind 36940 points

    Hello Farzaneh, 

    I am sorry, I still do not fully understand your inputs and outputs for this PFC.
    85Vac to 265V input is a standard range, and 376V to 395V are typical output levels, although 376Vdc will not work for 265Vac input (too close to the line peak).  My output range of "~385Vdc < Vout < ~405Vdc" is simply my own observation.  It is not listed in the datasheet. 
    415Vdc output will not pose any significant issues.  
    Maximum output power appears to be 415V x 1.2A = 498W for 10s duration.  

    I do not understand the transitory [input] voltage of 440VAC (lasting 10 seconds) at 3A and 6A. This implies two things: 

    1. output voltage must be higher than 440Vac x 1.4142 = 623Vdc (for normal boost-PFC), and 
    2. input power can be be as high as 440Vac x 6A = 2640W (assuming PF is close to 1).  

    For ICOMP compensation, I suggest to stay with the 1nF value, or somewhere around that value. 
    Start-up at 85Vac should not be dependent on the Cicomp value.  

    What happens when the system fails to start with 85Vac input?  What is the load condition at that time?
    Is it possible that SOC or PCL is being triggered? 
    What minimum input voltage is needed to successfully start up (with 1nF)?  
    If you have waveforms of the issue, that would be helpful. 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 605 points

    Dear Ulrich,

    Dear Ulrich,

    I appreciate the assistance you've provided thus far. At this time, due to other project commitments, I need to focus on a different aspect of my work. If I'm unable to resolve the issue, I will reach out again. Thank you for your understanding, and I apologize for having to step away from this thread for now.

    Best regards,
    Farzaneh

  • 0 in reply to Farzaneh Faridi
    TI__Mastermind 36940 points

    Hello Farzaneh,  

    Good luck with your other project issues and with resolving this one. 
    I will close this thread for now.  
    If you need help in the future, you can reopen this thread, or start a new one. 

    Regards,
    Ulrich