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FET of UCC28251

Takahiro Nishizawa
Genius 5575 points
Other Parts Discussed in Thread: UCC28251

I have  questions about UCC28251.

We try not to use SRA and SRB.

(Not using FET of secondary-side)

Question 1.

Can we use this IC if we do not use FET of secondary-side?

Question 2.

Does the formula of datasheet change if we do not use  FET of secondary-side?

Best regards,

  • 0
    TI__Mastermind 25570 points

    Hello Nishizawa-san

    You can leave the SRA and SRB outputs open circuit and the UCC28251 will work without any problems.

    You can then ignore any of the design issues related to synchronous rectifiers - for example, section 8.3.5 and 8.3.6, page 19

    You will still need to calculate eq 23, 24 and 25 using the maximum duty cycle seen by the output diode. You can follow the calculations on page 37, eq. 28 et seq.

    Please let me know if you need any further information.

    Regards

    Colin

    .

  • 0 in reply to Colin Gillmor
    Genius 5575 points
    Dear Mr. Colin


    Thank you for your answer.

    I understood that formulas of datasheet does not change when we do not use FET of secondary-side.
    Is this understanding right?
    For exampe , the formulas(29~35) do not change if we consider maximum duty cycle seen by the output diode.
    Please tell me all formulas if the formulas change in this condition(not using FET of secondary-side).

    Can does pre-bias operate when we do not use FET of secondary-side?


    Best regards,
  • 0 in reply to Takahiro Nishizawa
    Genius 5575 points
    Dear Mr. Colin


    Thank you for your answer.

    I have questions about UCC28251 when we do not use FET of secondary-side.
    Are the following methods correct?

    1.
    We want to calculate Vramp.
    If we do not use FET of secondary-side, we use eq. 23 and eq. 24 .

    2.
    In eq. 34,
    Vramp is value of eq. 24 and we do not use eq. 29.

    3.
    In eq. 35,
    Vramp is the value of eq. 33.


    Best regards,


    Nishizawa
  • 0 in reply to Takahiro Nishizawa
    TI__Mastermind 25570 points
    Hello Nishizawa-san
    I'm travelling this week so I have little time to answer your question - I will get to it but my answer will be delayed.