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UCC28070: PWM bounce during no load

shuang qiu
Prodigy 190 points
Part Number: UCC28070

Hi,

we are invesigating the UCC28070. our applicaiton is 85Vac input, 300Vdc output. It is found PWM bounced like burst mode. Attached waveform channel1 is output voltage, channel4 is PWM. Could you please advise if something need be checked? thanks.

  • 0
    TI__Expert 6080 points
    Hello ,

    For better understanding, I would like to look at the COMP pin during the burst periods and also the schematic of your layout.

    Regards,
    Sonal
  • 0 in reply to Sonal Singh
    Prodigy 190 points

    Hi,

    I tested VAO, it dropped below 0.75V that cause PWM shutdown.  seems that is output loading related, if increasing loading, the PWM will work normal. How can improving that during no load and light load?

    Another question, the spec shows VINAC and VSENSE divider attenuation shall be the same, please clarify what's the problem if it's not the same. Thanks.

  • 0 in reply to shuang qiu
    TI__Mastermind 34465 points

    Hello shuang qiu,

    Any pfc controller will go into a burst type of operation if the load is extremely low. So what you see here is normal.
    You will see a repetitive switching operation at higher loads.

    The resistive divider ratios on VSENSE  and VINAC is recommended to be made equal.
    This is in order to improve the accuracy of the  calculated value for the  RSYNTH resistor and also it affects the accuracy of the voltage levels detailed in Table 1 for the quantized multiplier. The system will work if VINAC and VSENSE have slightly different divider ratios but the overall harmonic distortion will be worse.

    Regards

    John