I am designing a PCB, and I want to add cooling to the project. To accomplish this I have added a connector for small (40mm x 40mm) brushless cooling fan that will draw off an existing 12V supply from a DC/DC SMPS.
Do I need to add any back EMF protection to the fan connector? Would a Schottky diode be sufficient, or do I need something like a MOV?
Can't hurt to put it in. Even if the fan you chose today does not need it, another in the future might. Either for future boards or when that fan wears out.
You don't need a Schottky diode, just about any ordinary diode should do (1N4148, 1N4005, M5 etc). The difference is that when the transistor turns off it will see a few hundred mV more with a regular diode, and less leakage added to the fan current when it is on (likely negligible under sensible conditions and a Schottky diode such as 1N5819).
The diode is likely not necessary if you're connecting it directly across the 12V supply if there is always significant capacitance to ground. But if the 12V rail is not bypassed and the power supply is 'hot' unplugged there could conceivably be an issue with other things connected to the rail.
Edit: It's always good to see empirical measurements. This gentleman measured a 100V+ transient on a 12V fan.
The fan may already have something like that built in, seeing as how it's brushless and has a controller in it. With removal of power, the controller may also shut off the switching so the coils are not connected to the output.
If you aren't switching the fan on and off then it's probably not necessary. If you really want to find out, test the fan and see if it has an over-voltage and how high it goes when the supply turns off (if anything).
I would suggest a simple 1N4000 series (1N4001 .. 1N4007).
Given a brushless fan, I've seen a circuit attempt to PWM drive one from the 5V rail... that had approximately 33uF bypass capacitance built in. The resulting -1.1V spikes on the 5V supply rail at 40kHz explained intermittent failures elsewhere... Though the recovery response of the 5V regulator impressed me. Don't try to PWM drive one. Do allow for the capacitor charging current spike.
Your 12V supply bus should already have a reverse protection diode connected reverse biased from 12V to GND. This diode will handle reverse EMF from hot disconnects, where the board and wiring are inductors. It should cover fans and such as well. It’s not normally necessary to have this diode next to the fan connector. It depends somewhat on how the distributed capacitance is laid out between the 12V and 0V planes. But in most cases you need the reverse protection parallel diode for the supply, and small loads that aren’t pure inductors will not need separate diodes.
