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UCC21750: faulty turn-on behavior

Janine E
Prodigy 10 points
Part Number: UCC21750

Hi TI-Team,

I am observing a problem during double pulse tests with the driver. When I try to characterize a SiC MOSFET at certain operating points, after turn-on for the second pulse, the gate-source voltage is pulled down to turn-off voltage after approx. 100 ns. Sometimes, the driver tries to turn-on again after several hundreds of ns but fails. Since this only occurrs at operating points with high currents/voltages/temperatures I assume it is related to the di/dt and the reverse recovery peak-current. DESAT and AIN are connected to COM and CLMPI to VEE. Also, the FLT  is not showing any error. Do you know what is causing the problem?

Thank you!

  • +1
    TI__Expert 6730 points

    Hi Janie, 

    Welcome to E2E! In order to help with this situation, I have some questions and comments regarding the double pulse testing setup and the system: 

    1. If possible, can you share us the schematic and system condition for the double pulse test? (For example, VCC, VDD, bus voltage, load current, temperature, FET PN, etc.), as well as any waveform you captured. 

    2. How repeatable is this event? After the gate pull-down event, did you observe any damage to the gate driver and/or FET? 

    3. Can you try to monitor the RDY pin during this event, and VCC/VDD? If the power supply is not stable and triggers UVLO, RDY will fall and the output will turn off. If the power supply recovers, the gate driver will attempt to turn on again. 

    4. Can you try to monitor the input waveform during this event? It is possible that noise coupling to the primary side can distort the input waveform, thus influencing the output waveform.

    Please let me know if you are able to capture the additional information, and we would be able to provide a more thorough analysis. 

    Thanks! 

    Vivian

  • 0 in reply to Vivian Qi
    Prodigy 10 points

    Hi Vivian,

    thank you for your reply. Thanks to your advice, I changed the capacitor on the driver input where I use an RC network to filter the input signal. When I changed it from 100pF to 1nF, the problem was solved.

    Many thanks,

    Janine

  • 0 in reply to Janine E
    TI__Expert 6730 points

    Hi Janine, 

    That's great to hear! Please don't hesitate to reach out to us again if you have any questions or comments. 

    Thanks, 

    Vivian