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UCC28780: Abnormal working waveform of UCC28780

yy lee
Intellectual 310 points
Part Number: UCC28780

UCC28780 + NV6252, 230V, 12V, 1.5A. When working, the VDS waveform is as attached, as if the upper MOS tube is not working.

  • 0
    TI__Mastermind 40350 points
    Hello YY,

    Thank you for your interest in the UCC28780 ACF controller.

    The waveform shows operation in DCM. This is not a normal operating mode for the UCC28780. I suspect that the value of your RDM resistor is too high. The controller is not aware that the high-side MOSFET in the NV6252 is not switching.

    You show the low-side drain waveform, so obviously the controller is delivering PWML pulses to the low-side MOSFET.
    Please place a probe on PWMH to verify that the controller is delivering PWMH pulses.

    I believe that the UCC28780 is delivering both PWML and PWMH pulses, but the high-side of the NV6252 is not responding to the PWMH from the controller. This can happen if the bias voltages for the high-side MOSFET do not meet the specifications for switching. Please check the components around the high-side of the NV6252.

    There is some error in the pdf file that you posted. It will open with Adobe Reader, but it will not show any schematic.
    Without the schematic, I can only guess at the possible problems based on the single waveform shown.
    Please regenerate the schematic file and post it again.

    Also, please provide your design target specifications. The filename says "27W", but the test conditions are 12V x 1.5A = 18W. Does this problem only happen at this condition, or at any other conditions, too?

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Intellectual 310 points

    UCC28780 is delivering both PWML and PWMH pulses, but the high-side of the NV6252 is not responding to the PWMH from the controller.Low input 110V is normal, high voltage 230V is abnormal，High voltage 230V can't take 27W

  • 0 in reply to yy lee
    TI__Mastermind 40350 points
    Hello YY,

    This issue lies with the NV6252 device, then. It is probable that the high-side bias voltage does not reach the correct level under high-input line conditions. At high line, the low-side on-time is shorter than at low-line. In these short low-side on-times, the high-side bias caps may not be able to fully charge up to spec to allow the high-side GaN Fet to operate. At low-line, the low-side Fet is on longer and this can allow the high-side caps to fully charge through the internal bootstrap diode.

    I believe that this diode is limited to ~350mA current, so it may be a "bottle-neck" for the high-side bias charge.
    I can suggest 2 things to try: 1) decrease the VB cap (C18) from 100nF to 22~47nF, and/or 2) add an external bootstrap diode from VCC to VB.

    Also try to decrease the GATEL cap (C46) from 680pF to 330pF, to see if that helps.

    Regards,
    Ulrich