Looking for some help with SMD Component identification N51K
Couple of options, the first second being most likely of the two.




ZXTN5551FL - 160V, SOT23, NPN High voltage transistor

This has package marking N51, though I can't find any photos of it to find out whether it appears along with the letter K. The manufacturer (Diodes Inc.) do use 1-character shortened date codes on small packages, in which case K would mean October 2015 (when did you buy the card?). However given its a part originally made by Zetex rather than Diodes Inc, I'm not sure if the date codes apply.





SI2305CDS - P-Channel 8 V (D-S) MOSFET

This has a package marking of N5, but from pictures of such devices, the marking appears in front of additional characters which represent date code information. However the package marking on your part doesn't match the lettering style from other photos of this part, and is missing a small overlined character in the middle of the number.

Having said that, there is a similar thread on another forum which is looking for a similar part with number N51Y in a very similar style to your parts, which apparently they were able to confirm from the manufacturer was a SI2305CDS, which leads me to suspect your parts are too.

A P-Ch MOSFET also makes more sense based on your first picture as it looks to be related to switching of a power rail