Mosfet Id calculation? for fixed Vds & a maximum specified Vgs

[themaccabee]

Hi,

I ve got a circuit in which the Vds of the MOSFET is fixed at 1.9V.
The Vgs (threshold maximum) =2.5V
I can give the Vgs to a maximum value of 5V and not more.
Now i want my Id to be 12A for Vgs =5V
Is it possible?
Can some one tell me how design it properly??
The MOSFET Im considering is FDP8860 from fairchild
(https://www.fairchildsemi.com/ds/FD/FDP8860.pdf)
Thanks

 

[antaryami.mt.er09]

U r sayin Vds is fixed at 1.9 v ,Id ~Vgs shows Vth is nearly 2.5v(@25degC)
To keep ur device in sat u require Vds> Vgs-Vth at least 4.4v is req then apply the sat id eqn and un*Cox is constant, now change the W/L to get 12A
or in linear region it is still possible 2.5<Vgs<4.4(assuming vth=2.5v)

 

[themaccabee]

Im not doing IC level, Im working with a discrete MOSFET circuit.
Im sorry but i couldnt get it clearly.Can you please tell me how the 4.4V is calculated?

 

[antaryami.mt.er09]

Im not doing IC level, Im working with a discrete MOSFET circuit.
Im sorry but i couldnt get it clearly.Can you please tell me how the 4.4V is calculated?

Just see the Id~Vds curve for each Vgs There is a diffirent Vdssat,since Vdsat=Vgs-Vth.To keep in sat Vds >Vdssat for each Vgs
If u r keeping Vds constant at 1.9v,Vds<Vgs-Vth i.e. Vgs <Vds+Vth=1.9+2.5=4.4v.So for ur device in sat 2.5<Vgs <4.4.Now if ur increasing the Vgs >4.4v the device is in linear region(since Vds>Vdssat jst check for Vgs=5v Vdssat is 2.5 which is greater than 1.9v)
Sorry last time i wrote it 2.5<Vgs<4.4 it'll be in linear.
I hope this time it s clear

 

[FvM]

Now i want my Id to be 12A for Vgs =5V

As the datasheet reveals, rdson of the said transistor at Vgs of 5V is only a few mohm. So if you want fix Id and Vgs, Vds would be a few mV only. If Vds is fixed, you have to reduce Vgs.

 

[pinoy]

It depends on your load. For example if you have a resistive load of 100-ohms, Power source of 12v, Vds at 1.9V, then you have an approx Id = (12V-1.9V)/100-Ohms = 101mA (RDS_on is not considered)....

 

[FvM]

It depends on your load.

I don't think so. Vds is the common term for the voltage at transistor terminals, not the supply voltage.