p-mosfet was not turned off

[ilikebbs]

my p-mosfet was not turned off even if the Vgs=15v..
from the datasheet, it says that the maximum voltage for Vgs is 12v. I do not know if this is the root cause.
now it takes me several days to ask for new parts samples.

so i will appreciate that if anyone can confirm that when Vgs is greater than the maximu value, the mosfet will not under control.

 

[CrabMan]

no that is not the problem, there is something else that is wrong. are you using the mosfet on the high side or the low side and how are you driving the gate.....these things effect the turn on and off of the device

 

[ahsad_syed]

then u have two cases,
case1: for a pmos to turn on, Vds≤(Vgs-Vt)
Vd≤(Vg-Vt)
Vd< 12 - 0.4
Vd< 11.6

so for a 0.18µ tech...Vd< 11.6 volts ,

other wise the probable cause is the noise margin, u have to make the sizing such that it is minimum with respect to NM.
If u see a NM curve it mite tend towads the on stte even when your Vg is high, so work out with the sizing of the particular transistor and also otherparasitic connections attached to it...lemme know if that works...

if not we can check some other ways

 

[M!k]

Make sure not to mix up source and drain. Inside the transistor there's a body diode across source and drain. So connected the wrong way it will never turn off.

 

[ilikebbs]

now i have changed the mosfet whose Vgs is greater than 15v.
but the mosfet is still on. I do not find the reason.

g pin of P-MOSFET is connected to ground.
s pin of P-MOSFET is connected to +15v.
d pin of P-MOSFET is connected to a connector.
so why?

Added after 2 hours 7 minutes:

I remeber that when Vgs = high, p-mosfet will be turned off. Vgs=low, p-mosfet will be turned on.
It seems that I am wrong.

 

[jkahn]

To turn the gate of the P Mosfet on you have to apply a neg voltage, which you have done. If you want to turn off your mosfet, you should lower Vgs to 0V to completely turn it off, also placing a resistor from g to s should pull the voltage to gnd faster when vgs from -15V to 0V to create faster switching times.