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UCC28019A: Why does the 30khz frequency occur?

Matthew85
Prodigy 90 points
Part Number: UCC28019A


IC is fixed at 65kHz frequency.

There is a section in which the switching frequency operates at 30~35kHz.

Is this a problem caused by soft start? Can I increase the Vsense fillter value?

  • 0
    TI__Genius 9140 points

    Hi Matthew85,

    can you descript a little more on the condition that the controller outputs 30-35k? is it at start up or at input voltage peak line period?

    the condition that the allows the controller to output a frequency less than 65kHz I am guessing it is related to the loop calculation. during the 30k-35k period, maybe it is just that the controller loop has a zero duty at this period.

    if the guess is correct, then there is no need to do any change.

  • 0 in reply to David Huang33
    Prodigy 90 points

    In the emi test, values in multiples of 30 kHz are measured.

    It is necessary to check why the low frequency value is displayed.

    65kHz comes out at more than 20% load.
    Low frequencies come from a load below 20%.
    It does not continue to be measured. The frequency is changed.

    If it has to do with loop calculation, how is it possible to change or eliminate the frequency?

  • 0 in reply to Matthew85
    TI__Genius 9140 points

    can you please test the Vgs/vds and the PFC inductor current waveform for further analysis, and also attached the schematic in this post.

    at light load condition, especailly at no load condition, there is a chance that the loop calculate the pulse width to be zero, then you see the frequency lower than 65k.

  • 0 in reply to David Huang33
    Prodigy 90 points

    Nomal state opration. Output current 10A

     

    Nomal state opration. Output current 10A

     

     Low current opration Outpur current 5A

    Low current opration Outpur current 5A

     

    When I measured the waveform, the same 30 kHz waveform was found in both the steady state and the low load state.
    However, in the emi test, the frequency in the 30kHz band is measured only in a low load condition.

    The frequency is not correct for the result of discontinuous mode.

    Please answer. Thank you.

  • 0 in reply to Matthew85
    TI__Genius 9140 points

    Hi Matthew85,

    the last waveform you add(low current operatin output current 5A) shows that the driver pulse still is 65kHz.(blue probe)

    So all of the waveforms show UCC28019A is working normally.

    I guess you consider the current and VDS and thought it is 30k. Please keep in mind that during DCM, VDS is self oscillation with PFC inductor+ MOSFET Drain pint capacitance, so it look like the frequency is decreasing.

    the only way that can improve the phenomenon is to increase the PFC inductor value to allow the controller works at CCM for more load range. Please also keep in mind, at no load condition, it is impossible to avoid controller works at CCM, because the PFC inductor value can not be increased stintlessly.