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UCC28070 peak current issue during AC_dropout

Other Parts Discussed in Thread: UCC28070

Hello TI community,

I'm russel. we have current 2400W PSU project using UCC28070 controller.

Right know we are experiencing peak current > 30A during 10ms AC_dropout.

If you experienced this issue before kindly share with us the solution on how to minimize peak current issue.

here our finding.

During dropout, and eventual recovery, the PFC doesn’t soft start. At recovery, the PFC tries to immediately return the bulk voltage (now at 350V) to 400V AND at the same time continue support the output load (2.4kW). This increases the input current a few cycle after recovery.

see attached waveform

Thank you very much.

  • Russel,
    Can you explain what the green waveform (acfail) is and how it is generated?
    Also what does the signal at VSENSE look like during this dropout?
    It is very strange that the controller is responding almost immediately to the recovery in the input voltage.
    For soft start to be effective and avoid output voltage overshoot, the SS ramp should be slower than the voltage loop response.
    This means CSS>CVZ
    Did you use the calculator tool to calculate the circuit component values?
  • Hello John,

     Thanks for your feedback.

     AC_fail signal:
    Within 5ms after falling outside the input voltage requirements, this signal must be driven HIGH. The signal is driven HIGH by the PSU as long as the input voltage is not within required limits.

    I will provide VSENSE waveform to you tomorrow.

    I will check with my RD if they use calculator tool to calculate the components values.

    By the way did you experience this kind of issue before?

    Thank you very much.

    Regards,

    Russel 

  • For the PFC response, the multiplier gain (IMO) was reduced to limit the current during this transient condition. R_imo was reduced from 24k to 12.1k. Highlighted in the Red circle shows the reduced PFC current upon AC recover. However, the yellow circle still shows thePLD issue April 20.docx spike current.