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UCC5871-Q1: OV on Vds of external MOSFET

song.lin
Intellectual 260 points
Part Number: UCC5871-Q1

Hi experts:

For the application of ucc5871-Q1, we have some issue. As the picture showed below, When the short circuit happens, we will open the SIC-MOSFET, there will be a high voltage at Vds of MOSFET because of parasitic loop inductance induced voltage spikes. It is not proper to add absorb circuit to protect the MOSFET because of the high voltage and high current when short happens. So we need to figure out other ways to protect the MOSFET. The Bat voltage is 800V, the parasitic loop inductance is about 30uH, the resistance in loop when short happens is about 200mR.

In the DS of ucc5871-Q1, I checked that it has 2 functions to protect the external MOSFET when fault occurs, STO and 2LTOFF. Which method is better? And what factors need to consider when using these two methods?

And is DESAT function suitable for our application? Is there other methods?

  • 0
    TI__Expert 3735 points

    Hello,

    Yes DESAT function is suitable for Short Circuit events. The DESAT input will monitor the VDSon through an external resistor - diode network. When the voltage of the input charges up to the selectable DESAT threshold, the driver output is pulled into the safe state defined by the driver in the CFG10 register. This turn off is selectable between OUTL, STO or 2STO pulldown.

    the STO and 2LTO protect the MOSFET from OV from this voltage spike. The STO will pull the gate down with a smaller customizable pulldown current and will limit di/dt which limits the loop inductance. The 2LTO also reduces the MOSFET current by reducing the gate to a smaller customizable plateau voltage and then pulling it down (with OUTL or STO) to turn the MOSFET off. The strength of the pulldown current and the timings can be configured in the CFG registers. 

    One factor to consider is the short circuit energy. Reducing the STO current will inherently increase energy loss. You will need to understand what kind of short circuit event the Power Module can handle.

    Let me know if you have any more questions.

    Regards,

    Akshat