Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

[SOLVED] About MOSFET Switching

Status
Not open for further replies.

Anantha Krishna

Member level 1
Joined
Feb 10, 2011
Messages
41
Helped
1
Reputation
2
Reaction score
1
Trophy points
1,288
Activity points
1,642
Hi all,

I am using MOSFET NDS356AP[P-type] in my circuit for switching. I am using 3.6 V supply to uc and the same voltage to switch the MOSFET.

The problem I am facing here is that when the Vcc, that I want to switch, is more than 4.5V, the switching is not happening properly. When the Vcc voltage crosses 4.5 V, the MOSFET is not switching off. But when I increase the control voltage accordingly, the MOSFET switches off. i.e. when the input Gate voltage increases, the MOSFET switches off.

As I am using the uc supply as 3.6 V, I cannot increase this Gate supply. Please help to solve this issue.



P_MOSFET.JPG
 
Last edited:

xaccto

Full Member level 5
Joined
Oct 30, 2005
Messages
253
Helped
41
Reputation
82
Reaction score
35
Trophy points
1,308
Activity points
3,300
the mosfet gate turnon voltage is always in respect to its source terminal. So when the source voltage is at 4.5v and the gate is still at 3.6 it could already start turning on (logic level mosfet too ? ), so you drop the gate voltage to turn it on hard, sure it might not turn off then, no mystery there :smile:
You must level shift the gate voltage to pull it down with respect to its source.
Use another transistor, npn, collector -resistor (say 1k) - mosfet gate + mosfet gate - resistor (say 100k) - mosfet source.
 

alexan_e

Administrator
Joined
Mar 16, 2008
Messages
11,895
Helped
2,021
Reputation
4,158
Reaction score
2,031
Trophy points
1,393
Location
Greece
Activity points
64,377
The mosfet turns on when the (mosfet)source voltage is higher compared to the gate (or the gate lower compared to the source), the amount of voltage difference is specified with the gate threshold voltage which is 0.8v min, 1.6v typical for your mosfet.
When you apply 3.6v to the gate with a voltage of 4.5v to the source then you have a difference of 0.9v so the mosfet is conducting.
A simple solution would be to use a NPN transistor to drive the mosfet but the drive signal will be inverted.
When the transistor is off then no current flows through it and R3 works as a pull up providing at the gate the same voltage as the source so the mosfet turns off.

Pmosfet_drive.gif

Alex
 
Last edited:

    V

    points: 2
    Helpful Answer Positive Rating
Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top