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Original question:

UCC21750: Reg Fault pin

UCC21750: FLT pin always low

Mert Eren Kandilli
Prodigy 71 points
Part Number: UCC21750

Hello,

I am using UC21750 gate driver for a vienna rectifier application. I tied DESAT pin to COM, but still FLT pin is low.

Note: There are 6 gate driver on the board, two of them work as expected. Therefore;

1- I replaced the gate drivers with new ones

2- I removed the buffer to which the relevant FLT pins were connected.

3- I observed DESAT pin, it is always low (Connected to COM)

4- At the beginning of the program, all drivers' FLT pins are in HIGH state. FLT pins go low after switching starts.

Edit : 10kohm pullups connected to FLT RDY and RST pins.

My question is that what are the conditions could trigger FLT pin other than DESAT? Or could DESAT pin trigger FLT somehow even it is LOW.  

Thank you in advance. 

  • 0
    TI__Genius 13760 points

    Hi,

    Thank you for posting to E2E.

    The FLT pin is designed to go LOW when the voltage on the DESAT pin has exceeded the DESAT threshold voltage for longer than the DESAT de-glitch filter time.

    The DESAT pin shorted to COM disables the gate driver DEAST function (DESAT voltage would never exceed threshold) and FLT pin should not be held low. I have some questions to get a better understating of the issue:

    • When you run this test are you running the system with high voltage applied on the rectifier or is the program only switching the power MOSFETs ON and OFF?
      • If the issue appear with high voltage applied could you try running the test with no high voltage applied?(only switching MOSFETs ON and OFF)
        • Does the FLT pin still trigger LOW with no high voltage applied?
    • If the issue appears with high voltage applied, could you provide more details of the test conditions when the FLT pin triggers?(i.e. what is the bus voltage, what is the current level going through the switches)

    Best regards,

    Andy Robles

  • 0 in reply to Andy Robles
    Prodigy 71 points

    Hi Andy,

    Thank you for your response.

    I do not apply high voltage to the rectifier (about 30V L-N), but still FLT pin is low. When I disabled the gate drivers FLT pin is HIGH.

    Now there is another problem with the system. As soon as it resolved I will try to switch the MOSFETs without applying any voltage and share the results.

  • 0 in reply to Mert Eren Kandilli
    TI__Genius 13760 points

    Hi Mert,

    Thank you for the update. If there's no high voltage being switched it does not make sense to me what could be causing the FLT pin to go low.

    Please do share the results one the other system problem has been resolved.

    Let me know if there's any other gate driver questions in the mean time.

    Best regards,

    Andy Robles

  • 0 in reply to Andy Robles
    Prodigy 71 points

    Hi Andy,

    I replaced R65 with 5K1 pull-up resistor. At first problem seemed to be solved, but still the FLT pin goes LOW as the current level increases. Why might this be happening even though DESAT connected to COM?  

  • 0 in reply to Mert Eren Kandilli
    TI__Genius 13760 points

    Hi Mert,

    Do you mean the FLT is going LOW?

    Were you able to confirm whether the FLT pins goes low when no high voltage is applied?

    Best regards,

    Andy Robles

  • 0 in reply to Andy Robles
    Prodigy 71 points

    Hi Andy,

    Sorry for the confusion, yes I meant FLT goes LOW when no high voltage applied. I think it has to do with current level.

  • 0 in reply to Mert Eren Kandilli
    TI__Genius 13760 points

    Hi Mert,

    I am a bit confused. In the last comment you mention the FLT pin goes low when no high voltage is applied.

    • This doesn't make sense to me. With no high voltage applied there are no power stage transients being applied to the system meaning there is no possibility of conductive or radiated power stage noise being applied to the driver. When no high voltage is applied the gate driver would be working with no stress and FLT should always remain HIGH

    You then mention you think it has to do with the current level.

    • Power stage current level are only present when high voltage is applied.
      • Do you mean power stage output current or which current are you referring to?

    Please correct me if I am wrong in my understanding.

    Best regards,

    Andy Robles