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UCC28251PWR

Serg Smirnov
Prodigy 110 points
Other Parts Discussed in Thread: UCC28250

I assembled my converter on UCC28251PWR.

I use the mosfet 7002 switch between the SS pin on GND to turn on / off the output voltage.

When I switch the SS output to GND, I see that the converter continues to work until the output voltage of the capacitors drops to ground.

Could you explain why it continues to work if the SS output should stop the device?

Thank you

Serg

  • 0
    TI__Genius 13660 points
    Hi Serg,

    In theory, pulling SS pin to ground, Controller should be shut down quickly.
    Could you attach SS pin, OUTA pin, OUTB pin and VOUT waveforms? Thanks.

    Regards,
    Teng
  • 0 in reply to Teng Feng
    Prodigy 110 points

    on attached pictures yellow is output DC voltage and magenta are ss pin voltage and output AC voltage (I use bridg rectifier schematic)

    I will try to use additional switch on EN pin to groung both pins...

  • 0 in reply to Teng Feng
    Prodigy 110 points

    Hi Teng
    I have add switch to EN pin and with two switches on EN and SS , generation stopped.
    It looks like to have complete stop and next soft start it is necessary to use both pins.

  • 0 in reply to Serg Smirnov
    TI__Genius 13660 points
    Hi Serg,

    It is better that only driving EN pin to disable or enable device.
    Is the EN pin high level after SS pin shorted to GND in the second picture, did you have output signal waveform after SS pin shorted to GND when EN pin is high, it looks that Output has been disabled.
    I think you can try to decrease R16 and C19 if the charging and discharging time of C19 is long.

    Regards,
    Teng
  • 0 in reply to Teng Feng
    Prodigy 110 points

    From datasheet:
    "If the designer wants to disable UCC28250 with a level signal during an over current condition, the recommended solution is to pull down the SS pin rather than the EN pin."
    I saw start without softstart after stopped IC while overcurrent.
    It was unacceptable for me becouse of high current stress in primary transistors, so I used SS pin instead of EN.
    Using both pins solve problem with poststopped generation
    Thank you for communication and answers
    Serg

  • 0 in reply to Serg Smirnov
    TI__Genius 13660 points
    Thank you for your feedback, I am closing this thread. Thanks.——Teng