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25V for PMOS switch VGS?

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mmitchell

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Hi,

In a load supply using P-MOS, the input to source is 25V. We found unfortunately that the MOSFET we chose and soldered has only 20V maximum gate-source voltage. Although there are voltage dividers consisting of R1 and R2, we are not sure upon the sudden turning on of NMOS, whether VGS could temporarily reach 25V, and whether the P-MOSFET Q1 could stand it.
We haven’t yet found MOSFETs having rated VGS>20V yet.

zxm_pmos.jpg
http://www.diodes.com/datasheets/ZXMP6A17G.pdf

Questions:
1. Could someone recommend PMOS having larger maximum VGS?
2. The application of using PMOS for high voltage switch should be common and VCC=25V is unlikely to be at the high end of typical applications. So how do applications typically handle voltage exceeding maximum VGS?


Matt
 

Gorgon

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You can use a zener diode between source and gate to clamp the max Vgs. A 12V Zener is normal to use. I would think it hard to find Vgs max > +/-20V.
 

mmitchell

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If we are using the PMOS for fast switching applications (~10μs), does the Zener diode work to this speed?
 

schmitt trigger

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You won't be able to switch that fast with only a 30k resistor to discharge the gate capacitance. No way.
You require a NPN active discharge circuit. I'm on a mobile device and can't draw it. But please Google p-channel drive circuits.
 

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