Question regarding MOSFET Vgs limits

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tal_tiko

Joined Mar 13, 2012
1
I am planning a simple power switch, which consist of P-CH mosfet and N-CH mosfet
This design should be a high-side switch and logically works
The only problem that i'm afraid of is the fact that MOTOR_PWR can reach 50V and the limitation of the Vgs of the MOSFET Q? is +-20V
When enabling GND on the Gate of Q? i guess that the Vgs that is meassured between Gate and Source is 50V which is greater then 20V
Is this wrong? What can i do to prevent this issue? I thought of using Zener diode but don't know exactly if it should help
I thought of using voltage divider by adding a resistor between the Gate of Q? and the Drain of Q6 so that the voltage on the Gate will not be 0v, and it will be less than (MOTOR_PWR-Vgs[th])
 

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crutschow

Joined Mar 14, 2008
27,693
Yes you need to limit Vgs.

If you are just doing slow speed switching and aren't concerned about the speed of the switch then you can just add a resistor divider as you proposed. If you need fast switching then add a zener from source to gate on Q? and a small resistor in series with the drain of Q6.

You typically want Vgs ON to be 10V or greater for standard MOSFETs (but less than 20V of course).
 

johnreg

Joined Apr 24, 2013
1
Thanks a lot. It's very nice to read the answer from crutschow. I also have a problem with driving a P-channel mosfet with 22V Vgs. Now I got the solution :)
 

Ron H

Joined Apr 14, 2005
7,014
Yes you need to limit Vgs.

If you are just doing slow speed switching and aren't concerned about the speed of the switch then you can just add a resistor divider as you proposed. If you need fast switching then add a zener from source to gate on Q? and a small resistor in series with the drain of Q6.

You typically want Vgs ON to be 10V or greater for standard MOSFETs (but less than 20V of course).
A GS zener with a 10K shunt resistor will still turn off very slowly.
 
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