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UCC28600: Voltage fluctuation

Christeena Joseph
Prodigy 50 points
Part Number: UCC28600
Other Parts Discussed in Thread: TL431

We are designing an SMPS (36V/8A) using UCC28600D. We used Webench design tool for the reference circuit. The input given is 220V AC. The problem we are facing is that the output voltage is fluctuating between 9V - 36V. The input voltage given to the IC UCC28600D is also varying from 9V-12V and hence the output pin of UCC28600D is giving a burst of 128Khz pulse. I am attaching the schematic and the waveform of the voltage at VCC pin of UCC28600. What would be the reason for the voltage fluctuation? 

  • 0
    TI__Guru 66135 points
    Hello Christena,

    When I look at the schematic it looks likes the TL431 opto feedback may not be setup correctly.
    The TL431 needs 1 mA of bias current to operate. It looks like your output is set for 30V. 1mA (R15+R14) = 40V. I would drop R15 down from 20K to 500K. Then make resistor R16 a 499 ohm resistor.

    I noticed that the resistor for the current sense R13 is 31.1 Mohms? I think that is in error.
    I would double check what the Webench tool gave you for a design. This does not seem correct.

    There is an excel design tool and MathCAD file that you might find useful for evaluating the design.

    www.ti.com/.../slua399b.pdf
    www.ti.com/.../slvc104

    It might even be worthwhile to order one of the UCC28600 EVMs to evaluate.
    www.ti.com/.../sluu256

    Regards,

    Mike