I am trying to select a MOSFET that will work as a logical switch on a 24 V power rail, toggling on/off as signaled by a GPIO on an Arduino. The Arduino pin that will connect to the gate on the FET is 5 V, max current is 40 mA. The power rail current is ~1-2 A.
I'd like the On characteristic to be as close to saturation as possible with very low \$R_{DS}\$. Problem is the FETs I'm finding and I see recommended in similar applications either have:
- A \$V_{GS}\$ that is low enough to work with the Arduino, but not a high enough \$V_{GSS}\$.
- High enough \$V_{GSS}\$, but either too high a \$V_{GS}\$, or an \$R_{DS}\$ that is too high given the Arduino output.
As I understand it, although the min or max of the VGS spec may be around 5 V, in order to get the lowest \$R_{DS}\$ spec, you have to run it at much higher than max. One part for example specs min \$V_{GS}\$ as 2.0 V, max at 4.0 V, but the test conditions that result in a 50 mΩ \$R_{DS}\$ are 10 V at 8.4 A, which obviously the Arduino can't do.
Do they just not make FETs with the characteristics I need, or is my approach just wrong?
I am a novice in dealing with transistors, so please correct anything I've written that is incorrect.
Thanks
Best Answer
The main search criteria for your Nch low side switch is 5V Vgs so for low RdsOn est. Vgs>3x Vgs(th) which means Vgs(th) <5V/3 ~ 1.7V max
It is usually called a "logic level" FET drive as opposed to the older styles with Vgs(th) = 2 to 4V threshold and prefer a drive voltage >=9V
Vgss should not be a problem.
If you want a Pch High Side switch then Vgs will end up being 24V and then a different driver is needed such as open collector.