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UCC28780EVM-021: UCC28780 question, about at 264Vac the rise of Q4（Lowside） Gate and the timing of ILr changing?

Bob Lee62837
Mastermind 7230 points
Part Number: UCC28780EVM-021
Other Parts Discussed in Thread: UCC28780

Dear support member,

I used UCC28780EVM-021.
I measured about waveform SW, Highside Gate, Lowside Gate, ILr.

I have a question.

（Wave form)
At 90Vac, ILr rose from 0A when Q4（Lowside） Gate rose.

At 264Vac, the rise of Q4（Lowside） Gate and the timing of ILr was changed.
ILr did not rose from 0A.


（Question)
Why is the timing change of 264Vac?

Best regard.
Bob Lee.

  • 0
    TI__Guru 67065 points

    Hello,

    The average capacitance of the  FET changes with voltage applied across the FET during switching.  The UCC28780 adjust the clamp switch turnoff to store enough energy in the magnetizing inductance to achieve ZVS. It will also delay the turn on of the main FET to achieve ZVS.

    From the waveforms that you sent it looks like the average capacitance increases with bulk voltage. To achieve ZVS at highline the controller allowed for more negative peak current to energize the transformer magnetizing inductance to supply enough energy to achieve ZVS at highline.

    Please note that the rate of energizing the magnetizing inductance will change with bulk voltage.

    V=L*di/dt , E = (1/2)L*(di)^2

    Regards,

  • 0 in reply to Mike O'
    Mastermind 7230 points

    Dear Mike O',

    Thank you very much for your answer.

    I received an answer from you.
    Please let me check.

    （Question）
    Q1
    Is this EVM behavior no problem?
    Or there is problem behavior?


    Q2
    There was your answer about the operation,
    I want to know about the movement linked to the schematic（magnetizing inductance, controller, Gate waveform).

    Is there any other material on the behavior besides the data sheet?

    Best regard.
    Bob Lee.

  • 0 in reply to Bob Lee62837
    TI__Guru 67065 points

    Hello Bob,

    The behavior does not look to be an issue.  It appears to be normal.

    There are waveforms in the UCC28780EVM-021 User's guide that show what the ACF switching waveforms look like.

    Regards,