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External Gate Drivers

Paul Moore75
Prodigy 225 points
Other Parts Discussed in Thread: DRV8308, DRV8301

Is there a way to disable the VFETOCP functionality so that the U, V, and W, inputs don't need to be connected? Maybe an undocumented register bit might disable this? Maybe tying U/V/W to some special voltage level?? I too, am trying to use my own, higher current, gate drivers and this desaturation sensing is getting in the way.

  • 0
    TI__Guru** 114905 points
    Hi Paul,

    Sorry for the delayed response. Your original post was based on the DRV8308.

    Is that the target device? If so, can you provide more information on the system? What is the DRV8308 VM and what is the motor VM? It may be possible to disable the VFETOCP, but there are risks involved. Inserting additional timing between the DRV8308 and FETs would be the first concern.

    Can you use the DRV8301 instead? This requires an external mcu, but does have the features you are looking for.
  • 0 in reply to Rick Duncan
    Prodigy 225 points
    The DRV8308 controller is the device I am using for sine drive speed loop control. VM is +12V.
    I already have a gate driver so the DRV8301 will not be needed (it is not a speed controller).
    Please also tell me more about the timing concern. Deadtime adjustment does seem to work in an unpredictable way for me right now with the '8308.
  • 0 in reply to Paul Moore75
    TI__Guru** 114905 points
    Hi Paul,

    Adding a driver between the DRV8308 and the FETs introduces delay that the DRV8308 is not aware of.

    An example would be turning off the low side and on the high side of the same phase. The DRV8308 will detect when the low side is off, wait the deadtime, then turn on the high side.

    With the driver inserted in the path, the low side gate is turned off at the DRV8308, but the actual output is delayed by the driver propagation time. If the DRV8308 then turns on the high side gate because the DRV8308 believes the low side is off, both FETs could be on.

    There may ways around this, but it would require extra circuitry. At this point, attempting to do so has not been verified.

    How much gate drive current do need?
  • 0 in reply to Rick Duncan
    Prodigy 225 points
    The gate drive current I need is several Amps as I'm driving paralleled FET's.
    I can probably tolerate a large deadtime to accommodate the added driver delays.
    My big problem is the VFETOCP. Is there a way to disable this?
    Also, where do I measure to see the deadtime that is set in the registers? Is the register value half of the actual deadtime?
  • 0 in reply to Paul Moore75
    TI__Mastermind 20985 points
    Paul, I've measured the actual deadtime (analog path plus the digital adder) to be around 70-110ns longer than the times stated for register DTIME. There's not a way to disable VFETOCP,

    As just an idea, I wonder if you used 200kHz with OCPDEG=11 (for 5us VFETOCP timeout) with input duty cycles <70%. Maybe that could prevent VFETOCP from taking effect?

    Best regards,
    RE