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UCC21750-Q1: UCC21750: Paralleling DESAT

Javier Vara
Expert 1645 points

Part Number: UCC21750-Q1
Other Parts Discussed in Thread: UCC21750

Hello,

 

I have a gate driver design to drive to SiC MOSFETs connected in parallel. I am going to use 2 gate drivers (UCC21750), one per MOSFET. My question is, since they are in parallel, the Vds for both of them will be the same, so no need to have 2 separate DESAT circuits. However, if I only have 1, the second gate driver won't know if a desat event has happened and has to turn off. Could I have just 1 DESAT circuitry (blocking diode, blanking cap, resistor, etc.), and simply joing the 2 DESAT pins of UCC21750 together? In this case I would end up with a current source of 1mA, instead of 500uA, so I would need to adjust the desat resistor accordingly.

 

Thanks

  • 0
    TI__Expert 3267 points

    Hi Javier,

    Yes, you can tie the DESAT pins together and use a single DESAT network, but I would recommend having separate DESAT circuits. 

    Having just one DESAT circuit for your application means when one driver is on and the other is off, the gate driver that's off will pull the node low which would disable DESAT. Using this method means you don't have DESAT unless both drivers are on. 

    Should you decide to use a single DESAT circuit, another thing to take into consideration is doubling the charge current would also halve the blanking time. So, to keep the same blanking time, you would also have to double the blanking capacitor.  

    Best regards,

    Muiz.

  • 0 in reply to Muiz Ishola
    Expert 1645 points

    Hi Muiz,

    Thanks for the quick answer. I'm going to try to fit all the components for the second one but I'm not sure I have the space. Both gate drivers are going to be driven from the same PWM signal so in principle it shouldn't happen that one is ON and the other OFF. In the case that I tied in the DESAT pins, could it be theoretically possible that, due to differences in the tolerancing of the desat voltage threshold on each gate driver, one of them triggered a fault and pulled the desat pin low before the other one has had time to detect the fault?

  • +1 in reply to Javier Vara
    TI__Expert 3267 points

    Hi Javier,

    Yes, when DESAT is activated on one gate driver, it can pull the DESAT node low, potentially preventing the other gate driver from detecting a fault. Is there a specific reason why you want to use two gate drivers? Why not use a single gate driver with a with a current buffer to drive both MOSFETs?

    Best regards,

    Muiz.

  • 0 in reply to Muiz Ishola
    Expert 1645 points

    thanks for the answer. The MOSFETs are SiC and to minimize the gate loop inductance I would prefer to use something for each MOSFET that I can place as close as possible to the gate pins. I could use one gate driver with 2 buffers, or one gate driver with a single buffer, each driving one of the mosfets, but then there would be some difference in the timing of the switching