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Original question:

UCC2897A: Power is shutting down while probing the pins which are controlling the gate signals.

UCC2897A: Which pin of the chip can be used to control the output of the chip in case of any fault (short circuit, over temp etc)

Navin Kumar2
Intellectual 380 points
Part Number: UCC2897A

I am using the UCC2897A chip to generate 12V from 80-120V input. I want to switch off the gate signals in case of over current, short circuit and other faults. Currently, I am trying to use the given circuit(As soon as L or K1 will go high, I want my chip to shut down) at the SS/SD pin of the chip to control its output. But as soon as I am connecting this circuit(given below), my output will fall from 12V to lower voltage as I will connect some load at the output. But if I am not connecting the R17 resister my circuit is working fine at all loads. I am not able to understand why this is happening. Is there any other way in which I can control the chip? Please suggest some ways in which I can overcome this issue. 

Note: - K1 and L are the outputs of the logic gates CD4072BM96 and  CD4001UBM96 respectively. 

  • 0
    TI__Mastermind 34465 points

    Hello 

    The charge current on the SS/SD pin is only 14.5uA
    This means that you will not be able to use bipolar transistor for Q5 and Q6 since the collector leakage current of these parts will be
    significant and cause the soft start capacitor to discharge or partially discharge.
    This will result in  narrow pwm duty cycle and not a full shut down

    You can replace the MMBT3904 bipolars with a MOSFET device such as the 2N7002 low signal MOSFET.
    Alternatively you can connect Q5 and Q6 to the LINEUV pin and this will shut off the gate drive output

    Regards

    John

  • 0 in reply to John Griffin1
    Intellectual 380 points

    Thank you, John!

    What about using the FB pin which the same BJT circuit?

    Also as per the datasheet, both outputs of the UCC2897A controller are off if the synchronization signal stays continuously high. So, is it recommended to use SYNC pin to control the output of the chip? Is there any issue with these pins? 

  • 0 in reply to Navin Kumar2
    TI__Mastermind 34465 points

    Hi Navin,

    You may wish to try out these ideas and let me know how you get on.
    Regards

    John