My circuit's voltage input can range from 6V to 12.6V. I want to use a p-ch mosfet for reverse bias protection. However, the FET I want to use has a maximum VGS of +/- 8V, so I have to protect it in case the input voltage is on the high side. Am I correct that I should choose a zener diode voltage lower than 6V but higher than gate threshold? Any gotchas with the zener to be sure it works with the full range of voltage input I'm working with? I'm not too familiar with the ins and outs of zeners. Specifically, is there any harm in using TOO LOW voltage zener here (so long as the gate threshold is below the zener voltage). Reason is economies of scale as I have 3V zener used on the board elsewhere.

Also: Would a simple potential divider do the job? Or is a zener better?

Thank you.

  • \$\begingroup\$ A voltage divider with an NPN transistor (like in audio amp bias circuits) can be more flexible in term of the voltage drop. Zeners come only with specific voltages; with a BJT transistor you can regulate the voltage. \$\endgroup\$ – ilkhd Jun 22 '15 at 9:26

If the specified gate threshold voltage is (say) 2 volts this doesn't mean the FET is turned on like a relay contact when you apply that gate-source voltage. There are graphs in the data sheet that will show you (typically) what sort of gate drive you need for a certain gain current with a certain volt drop across drain to source. Vgs(threshold) is just a single parameter and certainly does not guarantee reasonable on-resistance.

You should look at the graphs, estimate the gate-source voltage needed then get a zener that is slightly higher than this voltage but, below the 8V limit. Provide a data sheet link and I'll show you the relevant graph.

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