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UCC21750: pwm input is clamped to 1.3V leading to no waveform output

user4268103
Prodigy 80 points
Part Number: UCC21750

Hello,

I am sorry to disturb you. I have used the ucc21750 to design a driver board and it works well for a long time. However, it doesn’t output the square signal yesterday when I turn on the 5V power supply, and I smelled something. My power stage is a T-Type inverter so I need twelve ucc21750 for my topology. Unfortunately, four of them were troubled and the phenomena were the same. The output of the controller is 4V pwm signal but after a RC the input of the ucc21756 is 1.3V pwm signal as shown in the figure. I think the ucc21750 is troubled but the reason is confusing. 

  • 0
    TI__Genius 13760 points

    Hi,

    Welcome to E2E!

    This does seem rather unusual. Would you be able to answer the following questions for initial diagnostic:

    1. Before this failure happened, did you run any specific stress tests on the UCC21750?

    2. Would you be able to SWAP the UCC21750 for another unit?

    This will enable to narrow down on the root cause. The issue could follow the unit you're testing right now, or if this happens on all the units then the problem would be somewhere on the board.

    3. If the problem remains with another unit could you try replacing the RC for new components and re-test?

    Best regards,

    Andy Robles

  • 0 in reply to Andy Robles
    Prodigy 80 points

    Thanks for your reply,

    1、I never did the specific stress tests on the ucc21750, and does the stress tests  mean that I rise the Vcc voltage.

    2、Does it means I need to test the driver board on another power board?

    3、How should I select the rc?

  • 0 in reply to user4268103
    TI__Genius 13445 points

    Hello,

    While Andy is out of office, another team member will address your question promptly.

    Thanks for patience.

    Regards,

    -Mamadou

  • 0 in reply to user4268103
    TI__Genius 13760 points

    Hi,

    1. By stress test you have the right understanding. Have you ever applied a high voltage or has this pin seen any transient that could cause a voltage spike beyond the voltage rating of the pin?

    • DC max = VCC
    • Transient max = VCC+5V for transient <100ns

    2. No. By this point I mean to de-solder the UCC21750 that you have on the driver board for a new UCC21750 and test to make sure the unusual behavior follows the suspected gate driver.

    3. By this point I mean to replace the resistor and capacitor with different components with the same value. I want to make sure there's nothing wrong with these components or with their solder connection.

    Points 1 and 2 above are to confirm this is an issue with the gate driver and not the board.

    Let me know if there's any questions.

    Best regards,

    Andy Robles