power ON problem with P- MOSFET

[balagbc]

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.

 

[d123]

Hi,

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.

 

[Audioguru]

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.

 

[iop95]

MOSFET is connected wrong (S to Bat and D to load...). Reverse them.

 

[FvM]

MOSFET is connected wrong (S to Bat and D to load...).

Nope, just the right way for PMOS load switch. Otherwise the substrate diode would power the load permanently.

Audioguru hit the point. Use a 2.5V logic level FET, e.g. a tiny Si2323.

 

[iop95]

Correct, my mistake...
Also, maybe Q6 is not fully ON and Vce add to Vth.

 

[balagbc]

Nope, just the right way for PMOS load switch. Otherwise the substrate diode would power the load permanently.

Audioguru hit the point. Use a 2.5V logic level FET, e.g. a tiny Si2323.

will(Si2323) it withstand in low voltage input(3.7v) at minimum 2A load?

 

[treez]

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

 

[Audioguru]

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.

 

[treez]

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

http://www.radio-sensors.se/download/gate-driver2.pdf
thanks but im sure logic level mosfets are well enhanced well under 4.5v=vgs

 

[balagbc]

Nope, just the right way for PMOS load switch. Otherwise the substrate diode would power the load permanently.

Audioguru hit the point. Use a 2.5V logic level FET, e.g. a tiny Si2323.

@FvM your suggestion really helped me and worked perfectly. thanks mate