Zeners on Mosfet Gates

Discussion in 'Electronics Resources' started by bytraper, May 29, 2012.

  1. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    Hi everyone,

    I'm just wondering the best way to go about fitting a zener to mosfet gates.
    Do i need to put a zener after each resistor (on the gate side) to ground?, or can I cheat and just put a zener at the output trigger?

    I've seen it done both ways, but I'm not sure why some designers favour one way over another?

    See the image

  2. crutschow


    Mar 14, 2008
    There's no need for individual zeners. But you do need a resistor is series with the zener, otherwise the zener can draw high current from Q2/Q3. To do that, place a resistor in series with the output of Q2/Q3 going to the gate resistors and then add the zener to GND.
  3. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    A resistor? That's something new!

    What size would you use for current drop?
  4. panic mode

    Senior Member

    Oct 10, 2011
    size eh?

    if you need both hands to lift it up, it is probably too large... ;p
    but seriously, where are your numbers?

    current through zener is always limited. sizing depends on circuit, supply voltage, zener voltage and zener power.

    for example if your zener is 10V, 1W and supply voltage is 24V then
    max zener current is I=P/V=1W/10V=0.1A
    voltage across resistor in worst case is 24V-10V=14V
    resistor then need to be larger than 14V/0.1A=140 Ohm.
    resistor power rating need to be greater than 14V*0.1A=1.4W.
    but all of the shown calculations are on the edge of ratings and good engineering practice is to oversize (within reason).

    for example we could use 150 or 220 Ohm, but also 100kOhm. large value for resistor would mean low current and therefore low power but it will have negative effect on transistors. large R combined with input capacitance of mosfets creates time delay and signal edges are not as sharp (vertical). as a result mosfets would turn on and off slowly and this means they are not operating like a switch (sudden transitions). this means higher losses and heating if the transistors.
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011

    He, I was drinking hot chocolate while reading this, I almost spilled it all over my table!
  6. panic mode

    Senior Member

    Oct 10, 2011
    darn, so i was this close... :D;)
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Why do you think you need the zener?