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[SOLVED] About MOSFET Switching

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Anantha Krishna

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Feb 10, 2011
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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.

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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.

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.


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