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UCC28782EVM-030: Questions about the transition behavior from LPM to ABM

user4243385
Prodigy 60 points
Part Number: UCC28782EVM-030

During the transition from LPM to ABM, the Vds surge of the synchronous rectifying FET jumped up.
I think that the operation is as described in (1) to (4) in the attached file, but is it correct?

Also, what determines the time of ① + ② (ON time of high side FET when shifting from LPM to ABM)?

I would like to shorten the time of ① + ② and suppress the surge of the synchronous rectifier FET (point A in the attached file). Is there any adjustment method, countermeasure circuit, etc.?
Also, for the surge at point B in the attached file, we are considering countermeasures in the direction of using a synchronous rectifier IC with a short minimum ON time, but are there any other possible countermeasures?2148.waveform.pdf

  • 0
    TI__Genius 9790 points

    Hi, Poster

    Your post is under analysis, please expect a response by next Monday.

    Yunsheng

  • 0 in reply to Yunsheng Qu
    TI__Expert 7080 points

    Hello,

    Almost all of your analysis are correct from T1 to T4 .

    The 1st PWMH on time was determined by RDM pin resistor + Vout sense (through VS pin) + Primary peak current (Vcst decided by FB loop). what you can change or revise is the RDM pin resistor . Decrease RDM resistor will short  1st  PWMH on time .

    Once 1st PWMH on time was shorten by decreasing RDM resistor . the clamp capacitor will not been over discharged(voltage drop) . if the secondary reflected voltage >= clamp capacitor voltage in next PWMH on time , there is no reverse current from secondary to primary to charge the Cclamp.

    And if you use a SR controller which has short minimum on time also will help for reducing the reverse current.

    Thanks.

  • 0 in reply to Jaden Ning
    TI__Expert 7080 points

    BTW: I recommend you capture transformer primary winding current instead of only capture current through Clamp caps next time . in this way. both of low side FET and high side FET current will be presented in the one channel . it's more better for analysis . Thanks.

  • 0 in reply to Jaden Ning
    Prodigy 60 points

    Hello.
    Adjusting the RDM resistance was able to improve the symptoms. Thank you.

    Please tell us a little more about the on-time of PWMH.

    What is the formula for the first PWMH on time? Also, please let me know if there is a page in the data sheet that explains the PWMH on-time.

    Thank you.

  • 0 in reply to user4243385
    TI__Expert 7080 points

    Hello,

    It's IC internal control logic , we don't have formula to calculate it and there is no description in datasheet.

    Thanks.