How to determine FET switch specifications

Question

I am working on a DIY drone, and I (preferably) need to be able to disconnect my ESCs from the battery via the transmitter (a Raspberry Pi). After looking for solutions for a while, I concluded FET switches are my best bet to automate this. However, I am not certain how to determine the requirements for the switches (I have no engineering knowledge, I am learning on the fly). Considering that the maximum current draw for the motors is 12A, the battery is 11.5V, and the RPI's pins for control are 3.3V and can take a maximum draw of 16mA, are my assumptions correct:

1. Switches need to have Vds>11.5V, and Vgs<2.5V
2. The draw on the gate can be neglected because it is ~0

Also, what considerations should be made about the S->D current draw? And am I missing other factors?

Answer 1

Generally, it is a good idea to over-specify in such a situation, especially as FETs don't quickly get unreasonably bulky or expensive at these power levels.

Specifically you will need to consider that most ESCs have capacitors at their battery inputs which can draw quite a lot of current when they are connected (that's why you can sometimes see sparks when connecting an ESC).

You are right about the gate current, as long as you don't continuously toggle the FET, the gate current is negligible. Also, as you stated, the transistor needs to be in saturation/ cutoff at the GPIO high and low voltages, respectively.

However, you've completely overlooked a very important specification of FETs: RdsON. This is the drain-source resistance when the FET is in saturated mode. For an application like yours where switching is only occasionally performed this is the main cause for power dissipation and thus heating, which can effectively destroy your switch if it cannot dissipate the heat quickly enough.