Variant of a Mosfet's Gate-Source protection
Let's say a 100V voltage spike arrives at +12V and the TVS diode clamps it to 25V. Q2 may or may not be activated. Will this circuit ensure a V_gs lower than 25V for the mosfet Q1 ? Any catch to it ?
Thanks
1 answer
If the TVS truly clamps the "+12V" line to 25 V, then the VGS of Q1 can't exceed 25 V.
However, if you want to limit the gate voltage, limit the gate voltage. A Zener between gate and source guarantees that directly. You may still want the TVS to clamp the incoming power voltage for other reasons.
Your MOSFET probably turns on fully with 12 V on the gate. 25 V is probably its absolute maximum to avoid damage. A 12 V Zener with cathode to the source and anode to the gate would clamp the gate voltage to 12 V. You've got a total impedance of 2.35 kΩ driving the gate. With 25 V power and 12 V across the gate, that leaves 13 V across the resistors. (13 V)/(2.35 kΩ) = 5.5 mA. That times 12 V is 66 mW. It will be no problem to find a Zener that can handle 5.5 mA and dissipate 66 mW.
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