Electronics > Beginners
Why am I having so Much Trouble Choosing a Mosfet?
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neppo1345:
DOH!  :palm:, okay yes, that makes perfect sense!

For some reason I was failing to make the connection that the voltage differential is only 5V when the current is 0A and when the mosfet is 'on' the Vds will be a product of the Rds for the given gate voltage.
KL27x:
After sourcing FETs a few times, you develop a frame of reference.

Just looking at max Id (27A) and max Vds (20V), your FET sucks for what you want (which is to drive a very modest current of 250mA at a very modest 5V). To directly drive a FET from a 3.3V micro pin, you need a sensitive FET. Max Id is going to be around 300mA to 3A and max Vds is going to be 12V.

The thing you really want to look at is the Rds over Vgs graph. But that's not a searchable parameter in a database. You can search for things like "logic level" and whatnot, but that is an arbitrary and limiting term, and it might exclude some FETs which are better/cheaper but which weren't labelled as such in search terms.

Depending on how fast you want to switch it, gate charge/capacitance can also come into play.

The bigger the max Ids doesn't mean you have more margin of capacity/safety. To use a "more powerful" FET, you generally need more gate voltage and charge.  It would be nice if we could pay a bit more to get a FET that does "everything," but that's not how it works. Most FETs are good FETs (at something). There are very few for sale that are simply inferior at anything.

Depending on your needs and micro, you could potentially use PFET directly if your micro has an open drain output pin on it.  Technically, it's not the fact that it is open drain, but that open drain output specifically will not have an ESD diode to Vdd, because they are specifically made to interface with higher voltage. (If not for ESD diode, you could drive PFET directly with any I/O pin by switching between digital low/ground and high Z).
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