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UCC25660: Questions Regarding Soft Start Operation of UCC25660 Series

Conor
Guru 12540 points
Part Number: UCC25660


Tool/software:

Hi,

We would appreciate your guidance on the soft start operation of the UCC25660 series, particularly regarding the following points described on page 34 of the datasheet:

  1. How is the soft start time of a soft start lamp determined?

  2. How are the switching frequency and dead time during soft start determined? If there are any factors involved in soft start or calculation methods, I would appreciate it if you could teach me.

Conor

  • 0
    TI__Intellectual 2050 points

    Hi Conor,

    Looking into your questions, will respond shortly. Meanwhile, please refer to the other sections which include soft start.

    Thanks for your patience.

    Yamini.

  • 0 in reply to SIRIPURAM YAMINI
    TI__Intellectual 2050 points

    Conor, 

    1.The soft start ramp time is fixed (25ms).

    2. The switching frequency (fsw) is dynamic and a function of output capacitance and load conditions. During soft start the fsw would go very high to limit the switching currents. Frequency may be in the range of 1.1MHz. The dead time is self-adaptive, and the controller can extend or terminate the gate drive to ensure that there is enough dead time to ensure ZVS.

    Yamini.

  • 0 in reply to SIRIPURAM YAMINI
    Guru 12540 points

    Hi Yamini,

    SIRIPURAM YAMINI said:
    The switching frequency (fsw) is dynamic and a function of output capacitance and load conditions. During soft start the fsw would go very high to limit the switching currents. Frequency may be in the range of 1.1MHz. 

    I would appreciate it if you could tell me what the switching frequency would be under the conditions of an output capacitor of 300uF, no load, and soft start. Also, if there are any other conditions or factors that determine the switching frequency, please let me know.

    My customer performed an experiment on a custom board, but for some reason the switching frequency did not increase. If you know the reason for this, please let me know.

    Thanks,

    Conor

  • 0 in reply to Conor
    TI__Intellectual 2050 points

    Hi Conor,

    The switching frequency at startup cannot be determined straight forwardly with the capacitance value, the actual LLC design also affects the fsw at the startup.

    and a correction to my statement ' fsw can go beyond 1.1 MHz '(no need to be always >1MHz), but Fsw at startup will be more than the resonant frequency.

    sorry for the inconvenience with the statement.

    Yamini.