Dear all
iam facing problem with power supply switched by P-MOSFET. For voltage above 4v the load(GSM modem) works fine but if the voltage fall below4v the modem couldnt get power supply properly. i dont knw wheather problem in mosfet or modem. pls help me with this problem.
Have you measured the voltage from drain to source and added it to the minimum voltage you need for the modem to operate? Maybe under 4V the MOSFET "uses" a portion of the modems supply.
Or - but I think I consider the above a good possibility - perhaps under 4V is just not enough to turn MOSFET on fully.
Nobody makes an NDT2955L, but they make an NDT2955NL. It is spec'd to work well when its gate-source voltage is as low as 4.5V but some of them barely conduct (250uA) when at the threshold voltage of 4V.
Buy a few thousand of those Mosfets and test them all. You might find one or two that work at a little less than 4V.
you need to look for the parameter called "Vgs(th)" in the fet datasheet, and check that it is low enough for your needs..as said above by others, these are called "logic level" mosfets
you need to look for the parameter called "Vgs(th)" in the fet datasheet, and check that it is low enough for your needs..as said above by others, these are called "logic level" mosfets
The Vgs(th) Threshold Voltage for a Mosfet is when it is almost turned off and conducts only 0.25mA. The maximum Vgs(th) for many Mosfets is 4V.
Instead you want the gate-source voltage when it is turned on properly and conducts a lot. A "logic level" Mosfet is guaranteed to turn on properly when its gate-source voltage is 4.5V or more.
Don't look at the graphs because they show a "typical one" that is a gamble, many will be worse.
Bottom left of page 5 of the attached gives the level of the miller plateau, and above this magnitude of voltage, the mosfet is getting well enhanced……Vgs (miller) = Vgs(th) + Id/gfs
Where
gfs = fet transconductance
Id = drain current