Variant of a Mosfet's Gate-Source protection

If the TVS truly clamps the "+12V" line to 25 V, then the V_GS 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.

posted 11 months ago

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Olin Lathrop‭

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