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MOSFET usues?

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New Member
Dear all,

Can anyone be so kind to describe briefly the use of a MOSFET.
I have come across it recently as I am seraching for an alternative to a transistor for switching purposes.

I understand the difference is that a MOSFET uses a voltage signal instead of a current input in a transistor.

However, how do I calculate what voltage I should put at the gate to get the required current flow at the drain.

Thank you very much for your kind effort and help

Many Thanks
Wil :eek:


Wilbee, :)

I would like to share my little knowledge in MOSFETs. Like bopolar devices, liner FET applications necessitate the application of bias. The method of applying bias will differ accroding to the mode of operation but it will involve the appliction of a gate-source bias voltage. A standing (quiescent) value of drain current will result. Typical values of gate-source bias voltage vary from between -2.5v and + 2.5v, according to mode of opreation.

Power MOSFET's should be de-rated at high operating temperatures (particularly when heat sinking arrangements do not meet the manufacture's recommendations). The normal requirement is to de-rate power dissipation linearly to zero at 100 digrees Centigrade whenever the junction temperature exceeds 40 digrees Centigrade.

Care should be taken when handling and slodering MOSFET devices ats thay can be easily damaged by stray static charges. Also note that a MOSFET's gate terminal has a near-infinite input resistance and if allowed to 'float' can accumulate electrostatic charges that cn destroy the device.

I got some of the above infor. from text books.


New Member

Dear Ravi,

thanks very much for your advice.
However, i have read several texts concerning MOSFETs, and one information bothered me. It says for a power MOSFET, the gate voltage should be 10times the voltage across the drain and source to turn the MOSFET "fully" on.

I am looking to use the MOSFET as a switch with a 0 or 5V gate voltage to turn the MOSFET off and on respectively, I have a +12 and -12 acorss the drain and source and would require a current of 0.1A to drive my load. If this is the case there is no way i can turn the MOSFET fully on. then should i be designing to use the MOSFET within the linear region between gate voltage and current?

Also, am i getting confused is there a difference between power MOSFET and a MOSFET, or are they just the same?

Thank you very much. your help is very much appreicated
Wil :wink:


Wil,My knowledge in MOSFET is very little. But I think you are refering to resistance value between drain and source when the transistor is in the conducting (on) state.right? If that is so, you may found the value as per the example below.

If the maximum current to be controlled is 12A and if the device is rated at 75W,the maximum permissable value of resistance between drain & source would be;

Rds(on) = Pdmax/IxI

Where, Rds(on) - the max. value of resistance between drain & source
Pdmax - the max.drain power dissipation
I - the max.current of the load to be controlled

therefore, 75/12x12 = 75/144=0.52ohms

To ensure reliable operation and to avoid the risk of over-dissipation, we should obtain a device which has an 'on' resistance somewhat less than this value.

Hope this will help you.


I think Ravi's figure of -2.5v to +2.5v is too low for a switching FET. I have used 12v g-d for VN series FETs to switch them.

The safest way is to look up the manufacturer's data sheet that is usually available from a Google search.
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