Can MOSFETs handle short power spikes?

Discussion in 'The Projects Forum' started by Jack J. He, Sep 6, 2017.

  1. Jack J. He

    Thread Starter New Member

    Apr 24, 2017
    When measuring my 24V power supply I found that its voltage spikes up to ~29V before stabilizing to 24V when I first plug it in. It lasts a fraction of a second but I feel like it is cutting it very close to my MOSFETs maximum Vds (30V):
    The circuit has been working fine so far but I am worried that there will be one time where it spikes higher and it destroys all of my MOSFETs. Can NMOS withstand that kind of voltage spike? The MOSFETs are all off (no voltage to gate) when this happens if that matters.
  2. Sensacell


    Jun 19, 2012
    You should use a higher rated FET.
  3. Papabravo


    Feb 24, 2006
    It may also depend on how you are doing the measurement and what instrument you are using. It is just barely possible you are seeing things that are not really there.
  4. ebeowulf17

    Distinguished Member

    Aug 12, 2014
    Are there zeners in a suitable voltage range that could be used to clamp the spike somewhere between 24 and 30V? Or maybe a couple zeners in series to achieve the desired voltage if an exact match isn't readily available?

    Just brainstorming - I don't know what's out there, or whether the response time would be fast enough for it to do any good, or whether it's even necessary, etc...
  5. OBW0549

    Distinguished Member

    Mar 2, 2015
    For a 24V circuit, using MOSFETs with a Vds(max) of 30V is asking for trouble; that's nowhere near enough safety margin. My own preference (others may disagree) would be to use a device rated at no less than 50 volts.

    The best practice is to take Absolute Maximum Ratings (or Limiting Values, as they are called in your data sheet) literally, and heed the warnings associated with them:

    To do otherwise is to court disaster.
    shortbus and Kjeldgaard like this.
  6. philba

    Active Member

    Aug 17, 2017
    I'm not one of the "others" (totally agree with your point). I'd say 50V is probably the lowest I would consider. In a part search I'd look up to 100V and pick based on best specs. Since you often get a price break at 10, I'd consider even higher max V so I have a stock of parts that might be usable in other projects as well. (I always buy at least one more than I need anway in case the blue smoke leaks out)

    On absolute max ratings. These are extreme and you should never, ever be testing those limits in your circuits. You may not blow a circuit immediately but that's actually worse as the failure will come later when you least expect it and can cause additional damage to other components.
    Kjeldgaard and OBW0549 like this.
  7. OBW0549

    Distinguished Member

    Mar 2, 2015
    One thing to watch out for when doing that, though: once you get above a hundred volts or so of Vds(max), Rds(on) often takes a hit: whereas low-voltage MOSFETs can have ON resistances in the low milliohms, high-voltage units often are in the hundreds of milliohms or even higher.
    Kjeldgaard and philba like this.
  8. philba

    Active Member

    Aug 17, 2017
    yeah, that's why I said "and pick based on best specs". Actually the key spec with Rdson is with respect to Vgs when you are interfacing to logic (which is about all I do). The data sheets often promote a low Rdson when, in fact, that is with a Vgs of 10V. For a Vgs of 4.5V it's often kind of sucky.

    But on the other hand, if you aren't pushing a lot of current, Rds isn't that important.
    OBW0549 likes this.