P mosfet for Low voltage

[rajijames25]

When I connect a 20 V to the gate of Pmosfet and 1.5V at Source i get only 0.7V at Drain . Rds(on) = 9mohms .Why this happens?

 

[alexan_e]

In a P mosfet you apply a positive voltage in the source pin, to make the mosfet conduct the gate voltage must be lower than the source.

Alex

 

[rajijames25]

sorry that was a typo . i have actually applied 0V

 

[alexan_e]

You need a mosfet that can work with very low Vgs voltage,
you have to look in the datasheet to see the proper Vgs for your specific mosfet,
it probably needs bigger gate-source voltage and it can't give the needed current
to the load you have connected to the drain.

Alex

 

[zahrein]

it seems that the Vtp is higher hence you get 1/2 of VCC which is 0.7V. Need to check what is the Vtp (threshold voltage) and do check the equation that satisfy saturation region below.
VDS < VGS – Vtp or VGS > VDS + Vtp

 

[RCinFLA]

Missing drain to ground load resistor to make any judgement.


Couple of low Vgs Pch MOSFET's are:
FDZ299P
Si8407

These are used in cellphone battery charging path within phone. Similar parts are used for low voltage disconnect in phone's LiIon battery pack. They have to carry the peak current of a GSM transmitter (1.8 amps) with minimal voltage drop.