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UCC21710-Q1: Soft-Turn Off Additional R, C Components between OUTL - VEE as shown by Excel Calculator Worksheet

Kartavya Agarwal
Prodigy 70 points
Part Number: UCC21710-Q1
Other Parts Discussed in Thread: UCC21710

Tool/software:

Hi,

I am finishing a gate driver design using the UCC21710 gate driver IC for SiC MOSFETs.

I have been using the SLUC695 calculator tool to check my design and additional component values. 

In the soft-turn off tab, I noticed that an additional set of R, C components (R_STO, C_STO) are being used to apparently control the pull-down current in the event of a DESAT/OC detection event.

However, the datasheet does not specify this need nor does it explain why this is needed.

As a result, I have 2 questions:

  1. Why is this R_STO and C_STO needed based on the SLUC695 spreadsheet?
  2. Do I need this in my implementation? How can I evaluate whether these components can be eliminated in my design?

Thank you very much for any advice you might be able to share.

  • +1
    TI__Mastermind 21066 points

    Hi Kartavya,

    1. The R_STO and C_STO are not mandatory. The UCC217XX devices have a 'soft turn off' output mode for when an overcurrent or DESAT is detected. The idea is that with a very high overcurrent, you might want to open the switch with a single, very slow edge rate. This will to prevent a large inductive voltage spike that will breakdown the switch you were trying to protect from overcurrent.

    When you use a BJT totem pole buffer on the output, as shown in this tab, the soft-turn off will be amplified into a strong turn-off and won't have any effect. 

    2. If you want to retain the soft-turn off feature and still use a BJT output buffer, you can technically try to use R_STO and C_STO to heavily load the UCC217XX output to just below the BJT gain. That way, the strong turn on/off will still have mostly the same output drive strength, but the soft turn off will be attenuated so much that the BJT buffer will also have a weaker turn-off. 

    I recommend trying to use the UCC21710 without a buffer and without gate resistance. It is indeed strong enough to drive most modules outright, and this will keep the integrated soft-turn off with no extra complexity. If you do want stronger drive strength and a buffer, this is a way to keep the soft-turn off. 

    If you are using relatively inexpensive modules, this feature may be more trouble than its worth. Just don't optimize your normal turn-off for efficiency until you are confident your design is not going to accidentally cause overcurrent.

    Best regards,

    Sean 

  • 0 in reply to Sean Cashin
    Prodigy 70 points

    Hi Sean,

    Thanks a lot for your response.

    So just to clarify, it seems that R_STO and C_STO are only needed if I'm using the output totem-pole buffer to boost the driver's peak current strength?

    By the way, to elaborate, I am not using the buffer to boost the drive strength of UCC21710. So sounds like I shouldn't place R_STO and C_STO in my design?

  • +1 in reply to Kartavya Agarwal
    TI__Expert 7865 points

    Hi Kartavya,

    Yes thats right. If no buffer stage planned, then the R_STO and C_STO is not needed.

    Thanks

    Sasi