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UCC2897A: Feedback Loop Compensation

Robert Ruchar
Prodigy 70 points
Part Number: UCC2897A

Dear Gents,

Looking at reference documents SLUA535 (section 6, Compensating the feedback loop) and SLUA702, I spotted notable differences in modelling of the active clamp circuit:

In SLUA535, it is stated that double pole at frequency (1-D)*fres (where fres is Lm with Cclamp resonance) occurs. On the contrary, in SLUA702 comprehensive analysis is done proving that there is a double zero at the aforementioned frequency and a double pole at slightly lower frequency. Can you please advise which one is true or why are the results different ?

In addition to that, I'd like to ask whether the same frequency response characteristics (poles, zeroes implied by the active clamp circuit) apply to voltage mode control (e.g. LM2025A) or is there any difference ?

Best regards,

Robert

  • 0
    TI__Expert 6080 points
    Hello Robert,

    Voltage mode control loop has a different frequency response and a transfer function to a peak current mode control loop. May I please refer you to the paper below it is a good starting point and talks in brief about the loop responses of the two.
    pdfs.semanticscholar.org/.../e6525eafc7c54faba90898e9dd7ddbcf4c49.pdf

    Regards,
    Sonal
  • 0 in reply to Sonal Singh
    Prodigy 70 points
    Dear Sonal,

    Thank you for your reply and the useful link. I'm quite aware of differences between voltage mode control and peak current mode control in standard isolated and non-isolated topologies. However, my skills are not up to the task of incorporating the effect of active clamp circuit on the frequency response (forward converter topology). As I wrote before, information in the appnotes is confusing. My questions still are:
    1. which information related to the loop compensation of the forward converter with active clamp described in appnotes SLUA535 (section 6, pages 38+39) and SLUA702 (page 6) - (both related to the same control IC UCC2897A, peak current mode) is valid ?
    2. is the effect of the active clamp circuit on the small signal loop response (introduction of additional zeroes, poles) the same for peak current mode control and voltage mode control ?

    Best regards,
    Robert
  • 0 in reply to Robert Ruchar
    TI__Expert 6080 points
    Hello Robert,

    Please refer to the SULA535 app note, it also has a reference section which quotes very useful IEE papers that will bring you up to speed in this regard. Meanwhile, I will try to identify the correct way of modifying the other app note, thanks for pointing that out.
    An active clamp forward converter operating in voltage mode has two poles due to the output LC filter and one zero due to the output capacitor's ESR. In addition, it has two poles introduced by the active clamp network.

    I hope this answers your question.

    Regards,
    Sonal
  • 0 in reply to Sonal Singh
    Prodigy 70 points
    Hi Sonal,

    Thank you for your response. Unfortunately, I haven't been able to find the answer in the reference documents (some of them I had read before).
    In SLUA535, two poles introduced by the active clamp network are described; on the other hand, in SLUA702, two poles and two zeroes are presented as a result of small-signal modelling. I can't see any justification for this essential difference (considering the same PWM IC UCC2897A - current mode control). Have you got an idea ? Is one of the results correct and the other incorrect or is there more than meets the eye ?

    In other resources (e.g. Chris Basso), modelling results in a notch introduced by the active clamp network (voltage mode control). The topic looks quite foggy .. Unfortunately, my modelling skills are not good enough so that I could arrive at the answer on my own.

    Thank you for your attention.

    Best regards,
    Robert
  • 0 in reply to Robert Ruchar
    TI__Expert 6080 points
    Hello Robert,

    Please refer to SLUA535 for your design, I was unable to validate SLUA702 thank you very much for pointing out the difference. We shall have the documents updated soon.

    Regards,
    Sonal
  • 0 in reply to Sonal Singh
    Prodigy 70 points
    Dear Sonal,

    Thank you for your support. Meanwhile, I've finally arrived at the answer. In voltage mode control, control-to-output frequency response of the forward converter with active clamp exhibits a notch at the aforementioned frequency (1-D) *fresclamp. The notch is a product of a double pole and double zero at the same frequency. The double zero polynomial has higher Q than the double pole, hence the brief gain dip (notch) and certain phase turbulence. For mode details, see the last section of

    cbasso.pagesperso-orange.fr/.../Chris Basso APEC seminar 2014.pdf

    Current mode control response is likely to be different, possibly as described in one of the two TI's appnotes.

    Best regards,
    Robert