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Original question:

UCC28631: DRV signal does not appear

UCC28631: Fault during startup

Soumik Sen
Prodigy 10 points
Part Number: UCC28631
Other Parts Discussed in Thread: UCC28630

I designed a flyback using UCC28631. It does not startup and when I probe the SD pin I obtained this waveform. Please tell me the type of fault indicated by such pulse count. The flyback does not start at all and I believe this is a latching fault.

  • 0
    TI__Mastermind 24245 points

    Hi Soumik,

    Thank you for the query on UCC28631.

    Here are the list of fault codes from the SD pin count. The fault-code pulse train starts with a ~30-us positive pulse (start-pulse - can used for scope trigger), then a train of ~1-us positive pulses, then a low interval of ~10 us to signify end of the pulse train. Count the number of 1-us pulses in between the start/end. The following is fault code map for UCC2863x devices.

    4 - VDD overvoltage
    5 - VDD undervoltage
    12 - overlaod timer time-out - only for UCC28630
    14 - AC (i.e. bulk cap level) under-voltage detection while running
    15 - overtemperature internal (die temp)
    18 - SD pin fault
    20 - output overvoltage
    21 - VSENSE pin fault
    22 - DRV pin fault
    23 - CS pin fault
    24 - X-cap discharge event triggered - only for UCC28630/3
    25 - AC (i.e. bulk cap level) under-voltage detection before startup or restart
    26 - external overtemperature fault (SD pin pulled low)

    Seems to be 21. 

    There are a few possible causes for latched fault mode on the UCC28631:

    - CS pin open (or the series feed resistor is too large > 10k)

    - CS pin shorted to GND accidentally

    - VSENSE pin open (or Thevenin resistance of the divider pair across the aux winding is too large > 20k, or one or other of those resistors is open)

    - VSENSE pin shorted to GND (or Thevenin resistance of the divider pair across the aux winding is too small < 10k, or one or other of those resistors is shorted)

    - DRV pin is externally shorted to GND (or pulled low through too small a resistmace < ~2k)

    So I suspect it could be related to VSENSE pin or too low voltage on VSENSE pin depending upon test conditions.

    Please le us know if you have further questions.

    Regards,

    Harish