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UCC256303: Soft start function

Dae Hyun Kim
Prodigy 125 points
Part Number: UCC256303

Hello Dear.

I'm using UCC256303 for LLC control IC.

(Design - LLC fs/w: about 150Khz, PFC Voltage: 400Vdc, output: 12V/33A)

I want to implement the soft start function.

Please see the attached file.(@90Vac, Load: 33A)

I tried to improve the status to modify the Pin Vcr, Isens and LL/SS.

and added Q19 and other things. You can refer the attached circuit.

please let me know if u can notice some problems and there is any solutions.

Best Regards.

DH.Kim

  • 0
    TI__Guru* 86206 points

    Hello,

    The circuit that you added is to improve burst mode and not startup.

    Go to section 5 of the same application note it talks about adding Zener diode and resistor from RVCC and the FB pin to prevent transformer saturation.

    I believe this is what is causing your soft start issue.  You just need to add 6.1V Zener and a 10 k ohm resistor.  The application note shows the placement.

    I would try that to see if it fixes your problem.

    Regards,

    Mike