Mosfet and Transient voltage protection

Discussion in 'General Electronics Chat' started by akea, Oct 7, 2016.

  1. akea

    Thread Starter New Member

    Oct 7, 2016
    4
    0
    The schematic: 12v solenoid with a flyback diode and a low side N-ch mosfet driven by PWM.

    I'm using a sot23 mosfet 2.5v rated with a 3.3v logic device and its Vgs(max) is at 12v. This is NOT Vgs(th) max, in case there is confusion. As you can see it's close to the max voltage it can block between the gate and source terminals and that's bothering me. While a fast switching diode will deal with the solenoid turn-off, during its active operation there is nothing protecting the mosfet from transient voltage. Also, the use of a unregulated power supply could briefly exceed Vgs(max) from a no-load state.

    So, am I missing something? what's the best approach here?

    Thanks.
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
    3,249
    626
    Welcome to AAC!

    Post a schematic and the part number for the MOSFET.
     
  3. akea

    Thread Starter New Member

    Oct 7, 2016
    4
    0
  4. akea

    Thread Starter New Member

    Oct 7, 2016
    4
    0
    Forget it, I just misinterpreted it.. let's pretend I never asked about Vgs lol.

    Mosfet datasheet parameters explanation from nxp, here .

    But I still would like to add transient protection to the mosfet. How to go about it?
     
    Last edited: Oct 7, 2016
  5. dl324

    Distinguished Member

    Mar 30, 2015
    3,249
    626
    What do you mean by transient protection?

    In the schematic you referenced, the 1k base resistor isn't necessary and a resistor between gate and ground might help with turn-off or keeping the MOSFET off while not being driven.
     
  6. akea

    Thread Starter New Member

    Oct 7, 2016
    4
    0
    It's a 100R resistor to avoid ringing. The schematic isn't mine, sorry. There will be a less than 10k pull down resistor, mostly because because of the uC high impedance on startup or reset, and low enough to quickly discharge the fet and not slow down the pwm signal. No math done yet. It's not a problem in my case though as it's low frequency driven circuit and makes no difference for just a test bench. This is topic is more about the main line power supply side.

    What do you mean by transient protection?
    ex. High frequency power supply/solenoid noise influence on the mosfet. I'm not sure how much this could be a problem.

    And thank you for taking your time for reading and answering it.
     
Loading...