I am working on adjustable voltage controlled current source (2 to 3 A) for my project.
I tried different NMOS for the projects, but they all burned.
In my application, the resistance of my load is very low, like 0.5 ohm to 1 ohm.
I want to control the current on the load from 2 to 3 A (maximum 5 A also fine) using the DAC function of a microcontroller. In my opinion, MOSFET is burns due to limitations in Pds.
In my opinion, my mosfet is burnt due to limitation in Pds.
You are probably right.
With a 0.5 ohm load and 3 A output, the output FET is consuming $$3.5\ {\rm V}\times 3\ {\rm A}=10.5\ {\rm W}$$
Even if your FET is designed for this kind of power consumption it will only work with an adequate heat sink. In the case of SMT FET's, that heatsink will be provided by the copper of the circuit board it is mounted to.
Your options include:
Design your board to provide adequate heat sinking
Choose a FET in a package like TO-220 that can fit an air-fin heat sink.
Redesign your circuit entirely to use a switching design that doesn't require using a transistor that consumes much more power than the actual load.
You are operating a part designed to be used as a switch, as a linear pass element. The surface-mount package can dissipate very little heat. The datasheet is very clear about the thermal limitations of this component.
With R3 = 0.5 ohm and 3 A, the FET package is trying to dissipate 10.5 W. At that power level, even a TO-220 part would burn up without a heatsink. Note that the FET would be preforming as a 1.167 ohm resistor, nowhere near its rated Rdson. In this type of application, the Rdson spec rarely applies.
Choose a FET designed for this power level and add an appropriate heatsink.
You need an active cooler sink & fan, the size of a CPU heatsink (est.) from an old PC tower.
Active Load Design Specs
Notes
If dummy load R is a 25W 0.5 Ohm resistor with sink mount flange then both FET and fixed R load can share heatsink with a rating of 1’C/W which is like a standard modern tower CPU heatsink & fan 5V with silver oxide heat grease & FET drain TO-220 insulator.
This way the desired 25W max. power is shared by 2 parts to increase surface area to the sink.
To compute final thermal resistance, and temp rise, the equivalent R network adds all series and parallel parts to the shared heating Rca (case to ambient).
Added
Also if you want 5A always choose a 50 mV max drop or less.
Even if you use a 2W resistor in this case for R3 that is excessive.
R3 temp rise
I^2R = 450 mW for 3A and 1.25W for 5A. To estimate R temp rise using Pd/Max *125’C. So for a 2W part dumping only 1.25W heat rises 80’C which will sizzle water.
