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UCC256403: VCR voltage fixed 7.2V at burst mode

user5996000
Prodigy 60 points
Part Number: UCC256403

Hi expert, 

I'm using burst mode.
The VCR voltage is fixed at 7.2V just before stopping in burst mode.
please see excel file.

Waveform VCR voltage.xlsx

Why is the VCR voltage fixed at 7.2V?

How can I slove this issue?

thanks

yuki

  • 0
    TI__Mastermind 26215 points

    Hi,

    The device is going into ZCS protection. This can happen at light load if there is noise on the ISNS signal or if the ISNS signal is very small in amplitude at light load.The dead time is being allowed to be longer when ZCS is detected (150us). The VCR ramp current is active during the dead time so this pushes VCR up all the way to 7.2V. 

    Could you share which BW option and what values are being used for LL/SS? UCC256403 ignores the ZCS protection whenever the FB signal is less than the BMTH threshold. We can modify the LL/SS or BW setting to make the device ignore ZCS at light load.

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Prodigy 60 points

    Hi Ben Lough

    Adding a capacitor to ISNS no longer causes the issue.

    I will check if the problem occurs under other conditions.

    THANK YOU!

    yuki

  • 0 in reply to user5996000
    TI__Mastermind 26215 points

    Hi Yuki-san,

    Glad to hear the issue looks to improved. Good luck!

    Best Regards,

    Ben Lough