Electrical – Mosfet for switching a step-down for a 12V fan

fanmosfetpwmswitches

I'm working on a circuit to control a 12V 4-pin PWM fan from an Arduino (that bit is working), and from a source of up to 42V.

Turning the fan off is what I'm having a problem with. I know some 4-pin PWM fans turn off when there's no PWM signal (type B & type C) but the fans I have must be type A as they run at minimum speed when there's no PWM. Given that I might only be able to get Type A fans, I'd like to have an option to turn the fan off via mosfet or similar.

The best 12V step-down that I can find seems to be the Pololu D24V10F12 but that is only rated up to 36V (and 1A), but it does also have an enable pin which solves the turning-off problem.

Although that will probably be fine, ideally I'd also like to have a footprint on the board for something that will work up to 42V (from solar PV), and it looks like Pololu have/are going to discontinue their older model D24V6F12 which goes up to 42V/600mA.

The Traco TSRN 1-2450 step down looks like a good but expensive option as it's rated to 42V, but it doesn't have an enable pin like the Pololu does, so I presume I need to add a MOSFET too unless there's a better option?

I've done some reading and am happy adding an N-channel MOSFET to the Traco's ground line (e.g. stp55nf06), controlled directly from the 5V Arduino, but I'm not sure that ground line what I should be cutting. I guess my questions are:

Is that Traco and MOSFET combo a good option or is it overkill?
If it is a good option, can I use an N-channel on the ground line or do I need a P-channel on the input, or can I use an N-Channel on the fan's ground line instead? I'm not sure whether that would mean the Traco still consumes some power.

Thanks and apologies for the long post!
Danny

Best Answer

schematic

simulate this circuit – Schematic created using CircuitLab

The fan power is controlled by the NFET, active-high.