Mosfet protection

Discussion in 'General Electronics Chat' started by edmundopt, Sep 13, 2013.

  1. edmundopt

    Thread Starter Member

    May 4, 2011
    60
    0
    Hello

    I am new to reverse voltage protection with mosfet, I'm still studing the theory, I need some help with the maximum VGS(voltage between gate and source), I have the P-Mosfet FQPF47P06, with a maximum VGS of +- 24 volts.
    There's no graph int the datasheet, so I do not know if this value is linear or not.
    If I am correct, with an aditional 12 volts zener, the circuit will tolerate 36 volts maximum(theory) and if I replace the zener with a 24 volts zener, will it handle a maximum(theory) of 48volts ?

    My question is, is there a rule of thumb, like use only half of the maximum VGS ? or can I use the theory(withou considering transients) ?
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,292
    1,255
    First you should turn the FET around so the source goes to 24 volts so you can turn it on.
    The gate to source voltage is a limit. It may fail above that limit, but no problem up to that limit.
    Most FETs like to have 10 volts gate to source to allow maximum current although some (logic level) are designed for lower voltage. You can see this graph in the data sheets.
    A zener like you have drawn is often used to keep the gate to source voltage safe.
     
  3. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    In my PC monitor repair days, I sometimes found the PSU MOSFET gate protected by a 20V Shottky-barrier diode - they soon break down (hard) if the PIV is exceeded and presumably there's much less Trr if they ever get forward biased.

    In practice, they tended to be quite good at protecting the driver chip when the MOSFET violently punched through due to core saturation - they certainly saved the chip more often than zeners did.
     
  4. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    The N-type,P-type mosfet zener Vgs protection.

    [​IMG]
     
    shortbus and GopherT like this.
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