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UCC28910: UCC28910 question about Bode plot and output accuracy.

Bob Lee62837
Mastermind 7230 points
Part Number: UCC28910
Other Parts Discussed in Thread: UCC28700, UCC28730, UCC28911

Dear support member,

My customer used UCC28910.

Specification condition
Vin：74V～264V
Vout：12.7V
Iout：0.5A

Primary Vcc：19V
Vcc Current：0mA～51mA

I have two question.


Q1.
When the load of 19 V system changes with 0 mA - 51 mA,
I think that the Vcc voltage also changes a little.

By setting resistance as Pre-Load,
Is it possible to limit the 12 V output accuracy on the secondary side to within ± 7%?


Q2.
Where should the measurement position the Bode plot measurement?
Is it measured with the resistance of the VS pin?

Best regard.
Bob Lee.

  • 0
    TI__Expert 7395 points
    Bob,
    Thanks for the interest in TI here.
    For Q1, are you asking if it is possible to achieve +/-7% Vout accuracy from a regulation standpoint? If so, yes it's possible with proper design. +-5% is possible with these primary side regulated devices.
    For Q2, you can't break the loop here to measure traditional Bode Plot. These devices are internally compensated and as long as you meet the Cout requirement in the datasheet for stability the converter should be stable. To view the loop response, you can view output load transient response as an indicator of rough stability. You can view an example of how to do that here: www.ti.com/.../slva381b.pdf

    Let me know if this helps with your questions.

    Regards,
    John
  • 0 in reply to John Stevens
    Mastermind 7230 points

    Dear John Stevens,


    Thank you very much for reply.
    May I ask add about Q2?

    I understood that stability and phase margin using load step response.

    But my customer definitely must Bode plot measuerment.

    Because,it is in the evaluation item for my customer DIN.

    I attached file about question.
    Could you check attached file?

    Best regard.
    Bob Lee.

    UCC28910_Question_180119.pdf

  • 0 in reply to Bob Lee62837
    TI__Expert 7395 points

    Bob,

    Unfortunately, no that won't work the way you hope. Because of the integration of the device and the internal compensation you do not have access to all the feedback loops to break them an inject an AC disturbance. A load transient will show relative stability and as long as you meet the effective capacitance required in the datasheet the loop will be stable.

    Regards,

    John

  • 0 in reply to John Stevens
    Mastermind 7230 points

    Dear John Stevens,
    Thank you very much for reply.

    A flyback device of PSR (a device controlled by VS pin such as UCC28700) is
    Can not measure the loop characteristics of Bode diagram?

    Or Can not measure this UCC28910 alone?

    I attached file about question.
    Could you check attached file?


    Best regard.
    Bob Lee.

    UCC28910_Question_180122.pdf

  • 0 in reply to Bob Lee62837
    TI__Expert 7395 points

    Bob,

    All of TI's PSR controllers like ucc28700,ucc28910, ucc28730, and others have a similar control loop with internal compensation. Having or not having an internal power MOSFET like ucc28910 or ucc28911 doesn't change it compared to ucc28700.

    If you would like we can review your design and Cout selection to offer some confidence for you on stability of you prefer.

    Regards

    John

  • 0 in reply to John Stevens
    Mastermind 7230 points

    Dear John Stevens,


    Thank you very much for reply.

    I send my customer your answer.

    I will feedback you.


    Best regard.
    Bob Lee.