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UCC28070A: Soft-start into partly charged DC-Buffer

Michael Furxer1
Prodigy 190 points
Part Number: UCC28070A
Other Parts Discussed in Thread: UCC28070

Hello,

I am using the UCC28070A for a 2phase Off-line PFC Regulator with an output voltage of 400V. Normally the PFC gets switched off when there is no load at the output so the DC-Buffer stays charged for a while.

Lets say the DC-Buffer still has 200V when switching the PFC on again. In this situation no Soft-start is used and the PFC tries to charge the DC-Buffer up to 400V with full power which causes it to reach the programmed current Peak-limit of 30A. Ialso causes an overshoot at the output of about 20V.

Is there any way to force the controller to use soft-start after switching on again ?

Thanks for you Help

Michael

  • 0
    TI__Mastermind 25570 points

    Hello Michael

    If the UCC28070 has been switched off by pulling its SS pin below 0.6V then it should go through a soft start process when the SS pin is released.

    Please check this. Also can you tell me how are you 'switching off' the controller and for how long.

    Also, can you look at the EAO pin and see if it has fallen below 0.75V during the 'stopped' interval.

    Please look at the note on P24 of the Data Sheet and make sure that you are observing the required condition that

    'For soft start to be effective and avoid overshoot on VOUT, the SS ramp must be slower
    than the voltage-loop control response. Choose CSS ≥ CVZ to ensure this.'

    Finally, please have a look at the Vout behaviour at initial startup - It would be good to confirm that the overshoot is absent when the system is initially turned on.

    Please let me know how you get on.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 190 points
    Colin,

    thanks for your quick response!

    the PFC has been switched off by switching off the line voltage, the UCC28070A supply stays on.
    The EAO Pin does fall below 0.75V immediately after switching off. The capacitors CSS and CVZ both are 1.5uF.

    At initial startup soft-start is used and no voltage overshoot at the output is observed.

    as i understand it the only way to make the UCC28070A use softstart whne the DC-Buffer is still partly Charged is to short the CSS Capacitor with a small Mosfet when a Line dropout is detected.
  • 0 in reply to Michael Furxer1
    TI__Mastermind 25570 points

    Hi Michael

    That all makes sense now - Removing the Line voltage will not initiate a soft-start so you get an overshoot due to the slow loop bandwidth. This overshoot may or may not be damaging to your system - that depends on component ratings and so on. Usually, the VCC rail to the UCC28070 is derived from the line and it collapses when the line connection is removed - but obviously this will not happen in your case.

    You are correct, pulling the SS pin below 0.6V stops the GDA and GDB outputs and you get a soft start when the SS pin is released - this is what I would recommend. BTW, you don't have to pull SS low before removing the line voltage, just pull it low before reapplying line voltage.

    Regards.

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 190 points

    Thanks Colin, that works for me!