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TPS2492: Turn-on into a short with dV/dt slew rate control

Jessica Riley90
Prodigy 180 points
Part Number: TPS2492
Other Parts Discussed in Thread: LM5069

Hello,

I am working on a design using the TPS2492 and I am wondering what happens when the device is turned on into a short circuit output when using slew rate control to limit the in-rush current.  It seems like with the current is limited to a level below Plim / Vcc, it should never trip the current or power limits and will continue to conduct into the short circuit.  Is that correct?  Is there a turn-on timeout where it will give up after a set time?

Thanks and best regards,

Jessica

  • 0
    TI__Guru** 105611 points
    Hi Jessica,

    Interesting!. We never tested that way. There is no time out on the GATE voltage and I believe the device will continue to conduct. Did you happen to test something? Can you please share your findings with me.

    What are your system specifications and end equipment?

    Best Regards
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 180 points

    I have not tested it (I am still in the design phase), I'm just trying to do my due diligence to make sure the FETs can survive direct turn on into a short circuit. 

    I ran a TINA spice simulation for the LM5069 (on another forum question it was suggested I switch to that part), and it seems like the output current will eventually ramp up until the power limit is reached at which point the switch turns off.  But the time it takes to reach that limit is really slow (as a result of the dv/dt limited current) and unless the power is equally distributed between the FETs (something I don't think can be assumed during turn-on), the FETs will overheat before they are disabled.  

    Since the design calculator suggests that turn-on into a short is still survivable even with soft-start control I was hoping there was something I was missing.

    My design inputs are:

    • 60V input
    • 110A current limit (Rsns = 0.5mOhm)
    • 612.8W power limit (Rpwr = 38.3kOhm)
    • 1nF timer capacitor
    • 4 x FDMT80080DC mosfets in parallel

  • 0 in reply to Jessica Riley90
    TI__Guru** 105611 points
    Hi Jessica,

    With slow dV/dt control, Power on into short is risky. Also, the FET stress/failure depends on the impedance of the output short. Low impedance short are safe. High impedance shorts at the output takes longer time to reach the limit and to trip the circuit.
    How much soft start time you are targeting and the reason?

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 180 points

    I am looking to turn on as fast as possible which still remaining within the limits of the FETs.  The speed is needed more for recovery behavior than initial turn on, but is an important component.

    I know it is hard to design for the full range of short circuits (or even larger than planned output capacitance), but at the very least I want the switches to survive turn on into a very strong short circuit.  It doesn't seem like that will be possible with the dV/dt limit required to charge up such a large capacitance at the output without tripping the power limit.  I think my next best option is to add an external pre-charge path to check for output shorts before turn-on.

  • 0 in reply to Jessica Riley90
    TI__Guru** 105611 points
    Hi Jessica,

    The power limit function help to limit inrush current. Please make use of models to verify the behavior as per your system conditions. www.ti.com/.../toolssoftware

    Best Regards,
    Rakesh