help with mosfet biasing to use it as switch

[elimenohpee182]

help with mosfet biasing

I'm confused on how to calculate the Vds voltage when trying to use a mosfet as a switch. In my attachment, the source is grounded and the load is connected at the drain. If for example I'm using this mosfet:
**broken link removed**
how would I be able to calculate the voltage at the drain? I ask because the current Id seems seems it is a function of the gate voltage and the drain-source voltage. The whole purpose is to switch a led that requires 1amp and has a voltage drop of 3.5V. Also, how would you go about picking the proper resistor value R?

If you can explain why and how you chose your values of voltage and resistance that would be so appreciated!

 

[patricky]

Re: help with mosfet biasing

If you don't need an exact answer, this will do. If you crank up the input voltage until there's no tomorrow (5V is pretty high), the on-resistance will be almost zero. Thus, there will be almost no voltage drop across the transistor. One way to see this is that when the transistor is in triode/linear region, R_ds ~ 1/(Vgs-Vt). At such a high overdrive voltage, R_ds will be pretty low. In the datasheet, we also see that the on-resistance is low ~ 200mOhms.

You get the approximate answer that Vds ~ 0.

Thus, you can treat the switch as a short, and find the right R. Since there will be a 3.5V voltage drop across the diode, there is ~1.5V drop across R. By ohm's law,
R = 1.5/I. Where I is the current you want flowing through the diode.

Finding the exact answer, I believe, would require knowing the exact I-Vds characteristic for the MOSFET at Vgs = 5V, then doing load line analysis with a variable range of R's. I don't believe there is an analytical solution to the problem (at least not a closed form one).