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Original question:

UCC28064A: Disable burst mode

UCC28064A: BRST threshold

Christian Lenau
Prodigy 20 points
Part Number: UCC28064A

I am using the UCC2864 in a conventional PFC converter. For my application i want to disable the burst mode, to do so i followed the advice given in an other topic to connect the BRST Pin directly to GND.

@230 VAC input the controller still enters burst mode below 50 W.

@ 120 VAC the the controller enters Burdt mode onlx below 20 W.

As referece i used the 400 W evaluation module, which enters BRST below 20 W @230 VAC.

What factors influence the threshold if the BRST pin is grounded.

  • 0
    TI__Genius 10555 points

    I apologize but my original response was not saved. Entering here again:

    Connecting the BRST pin to GND will in effect, disable burst mode.

    But please note that for very light-load conditions, the COMP voltage may fall below 250mV.  If that happens, switching will stop until COMP voltage rises above 250mV again.

    This threshold behavior is not well documented but appears in Section 9.2.2.11 (page 43) of the datasheet

  • 0 in reply to Ray DiCecco
    Prodigy 20 points

    Thanks for the answer,

    i checked thec omp voltage and it falls below 250 mA when the switching stops this happens below 50 W input power. The eval board shows this behavior only below 20 W. The compensator values from my design are very similar to the eval board (1n; 10k; 2.2µ) I already tried to vary them slightly which allwed me to change the ripple frequency on the comp signal as well as the crossover frequency. But the threshold from which i get interruption in the swichching is not changing. Do you have any further recommendations how to lowe this threshold.

  • 0 in reply to Christian Lenau
    TI__Genius 10555 points

    The compensation network won’t have an appreciable effect during a DC event (which I believe is what you're describing), so it makes sense that changing the compensation network doesn't impact this behavior.  The difference is likely related to the boost FET and to a lesser extent, the boost diode you may be using.  You may be using a larger FET for lower conduction losses.  However, larger FETs generally have larger Coss which will raise the power level where COMP drops below 250mV.  I suggest you try a slightly smaller boost FET and compare the impact on the power level where COMP = 250mV as well as on the overall efficiency.

  • 0 in reply to Ray DiCecco
    Prodigy 20 points

    Thank you for the very detailed information, i will try to use different mosfets.