I'd like to better understand how to calculate the voltage drop between the Drain and the Source of a FET.
As an example, say we have a MDD1902 N-channel FET. How would I determine the maximum voltage drop that can occur over Vds if the max drain current is 5A, max temperature is 75C, and Vgs is typically 12V?
Best Answer
You estimate it with the charts on page 3 of your datasheet. Specifically figures 3 and 4.
Figure 3 is telling us that the on resistance at 75 C is 1.4 times what it is at 25 C (the y-axis is normalized).
Figure 4 is telling us the resistance between gate and source is going to be around 20 mΩ at 12 V.
$$20 mΩ * 1.4 = 28 mΩ$$
Using Ohm's law: $$5 A * 0.028 Ω = 0.14 V$$
Keep in mind these are rough estimates based on the charts.