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Original question:

DRV8353RH-EVM: Dead time problem

DRV8353: Vgs threshold for deadtime insertion

Tobias Schwering1
Prodigy 70 points
Part Number: DRV8353

Hi all,

we are currently designing a motor driver using the DRV8353. While doing the math to calculate the dead time compensation and loss numbers I always came across the point that the amount of automatically inserted dead time is hard to grasp with the information I found in the datasheet and the smart gate drive allication sheet. Could you perhaps explain at which point of the gate discharge process the DRV8353 regards the gate sufficiently discharged and inserts the additional fixed dead time? That would really help a lot with our design process.

Kind regards,

Tobias

  • 0
    TI__Intellectual 840 points

    Hi Tobias,

    The DRV8353 will insert deadtime while VGS slews off. When the VGS threshold is reached, this automatically inserted deadtime will end and the fixed deadtime will then start. This fixed deadtime can be set on the SPI part, and is set to 100ns for the HW part. I have reached out to design to get the VGS threshold and will update you as soon as I get a response. Hope that clears things up a bit.

    Best regards,
    Omar

  • 0 in reply to Omar Naamani
    Prodigy 70 points

    Hi Omar,

    thanks for the explanation, but I could get most of this information from the datasheet. Have you alreade received an answer from design about the thresholds? So far it seems to be really low, leading do dead times that are way above what we are aiming for. It really would be a pity if we had to switch the DRV8535 to independant mode and insert the dead times via Microcontroller as the adaptive dead time insertion really sounded promising.

    Best regards,
    Tobias

  • 0 in reply to Tobias Schwering1
    TI__Intellectual 840 points

    Hi Tobias,

    My apologies for the delay, I have reached out to design for the threshold value you are looking for. I requested an update on this information and hope to hear back soon. Ideally, I would like to have this info posted here within the first half of this week. Again, sorry it has taken so long to get back to you on this.

    Best regards,
    Omar

  • 0 in reply to Omar Naamani
    TI__Intellectual 840 points

    Hi Tobias,

    Based on simulations, the typical values I would expect are as follows:

    • Turning off:
      • HS = ~1V
      • LS = ~1.5V
    • Turning on:
      • HS = ~1.1V
      • LS = ~1.5V

    Please let me know if this is not consistent with your observations. Thanks for your patience.

    Best regards,
    Omar

  • 0 in reply to Omar Naamani
    Prodigy 70 points

    Hi Omar,

    thanks a lot for these simulations! I will try to check them against our observations as soon as I can gat one of our boards for he testbench.

    But could ou elaborate perhaps a bit more on how to interpret these numbers? Mainly the differentiation between turning off and on confuses me because I thought that these deadtimes would be inserted when switching from HS to LS FET conducting and vice versa, so I thought there thould be only 2 separate cases.

    Best regards,

    Tobias

  • 0 in reply to Tobias Schwering1
    TI__Intellectual 840 points

    Hi Tobias,

    Sorry for the confusion, I posted the results of the simulation but you only need the turn off thresholds for deadtime insertion. The turn on thresholds are where the device's comparator determines that the FET is on, they are not used for initiating fixed deadtime insertion. Hope that clears things up!

    Best regards,

    Omar