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UCC256404: Start-up issue

Marcos Orellana
Prodigy 70 points
Part Number: UCC256404

Tool/software:

Hi!

This is the start-up of my LLC. Before the LLC starts regulating, there are 4 initial attempts to start:

Are these normal? The reason I'm asking is because I did a re-spin of this converter (without changing any components, just imrpoving the layout), and this new version gets stuck in this initial phase, and never starts regulating:

Any idea what could be causing this? Thank you!

  • 0
    TI__Guru* 78506 points

    Hello,

    Your inquiry is under review.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 78506 points

    Hello,

    Your waveforms do not represent normal startup. 

    What your input and output power requirements of the old design?

    What are the input and output power requirements of your new design?

    What did you change in the new design?

    Did you use the excel design tool?

    https://www.ti.com/tool/download/SLUC675

    Regards,

  • +1 in reply to Mike O'
    Prodigy 70 points

    Hi Mike,

    Thank you for your message. A very experienced engineer joined our team today and he figured out what the problem was: my VCC capacitor wasn't large enough, and the bootstrap capacitor was too big. Basically, the inital charge through the HV pin was depleted before the LLC could sustain the VCC by itself.

    The design must have been at the edge: the previous version started after a few attempts, and the new version did not start at all. By adjusting these values, the LLC starts very nicely now: