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Original question:

UCC21750: UCC21750 external RC for soft turn off (STO) with external BJT current buffer

UCC21755-Q1: Is PWM Input disabled when Short Circuit is detected (Desat)?

Philipp Heider
Prodigy 40 points
Part Number: UCC21755-Q1

Hello,

unfortunately I can't find this information in the datasheet.

GateDriver has slow turn off feature to limit the overvoltage in fault case. 

My question is: What happens when a fault event happens close to the end of the PWM Cycle so that the IN+ PWM Signal turns low during the fault event. (Controlled by MCU)

Is the Input In+/In- ignored as soon as a fault is detected?

At which point of time is the Input disabled?

What happens if there is a hard turn off and during the normal turn off event a fault is detected? Is the turn off current the reduced form 10A to 400mA during the turn off process?

Thanks a lot

Kind regards 

Philipp

  • 0
    TI__Expert 6730 points

    Hi Philipp, 

    We have to consider two timing parameters in this case: the PWM propagation delay and the DESAT deglitch time. The propagation delay is 60ns minimum and 130ns maximum per the datasheet. The DESAT deglitch time is 50ns minimum and 230ns maximum per the datasheet. 

    The PWM output follows the PWM input after a propagation delay, and DESAT is enabled/disabled based on the PWM output. Say that the input transitions low and is kept low. After one propagation delay, if the DESAT deglitch has not expired, then output will experience a regular turn-off (with 10A of peak sink current). If the DESAT deglitch time expires before the output transitions low, then output will experience a soft turn-off (400mA typical for UCC21755-Q1). 

    During regular/hard turn-off, as output transitions low, DESAT will be disabled so soft turn-off will not initiate once regular turn-off has started. 

    Hope this helps! 

    Thanks, 

    Vivian

  • 0 in reply to Vivian Qi
    Prodigy 40 points

    Hi Vivian, thank you for your detailed reply.

    One more question:

    What happens when soft turn off is triggered and the PWM low is applied? Is the PWM Input disabled when the Desat was triggered?

    Thank you

    Best regards Philipp

  • 0 in reply to Philipp Heider
    TI__Expert 6730 points

    Hi Philipp, 

    If soft turn-off is triggered before PWM goes low, then the output will continue STO until the Miller Clamp threshold is reached (if using the Miller Clamp). After Miller Clamp activates, output would be pulled low by the CLMPI pin with 4A of max current. 

    Thanks! 

    Vivian