MOSFET acts like a diode to protect against reverse polarity but with lower voltage drop. MOSFET selection: The figures below are for the P-FET design. N-FET design will be similar except with minus signs instead. 1. The absolute maximum Vds should be at least -2Vbat, where Vbat is the voltage of the power supply/battery. This is because in a situation when correct polarity rapidly switches to reverse polarity, you get Vs = Vbat, Vd = -Vbat. 2. The absolute maximum Vgs should be at least +-Vbat. 3. Check the output characteristics to ensure that the FET is thoroughly on when Vgs = -Vbat. Roughly speaking, this corresponds to Vgs(th) being greater than than -Vbat (remembering, e.g., –2 is greater than –9). 4. Check that the FET can handle the current, the power dissipated, and the heat generated. These are three related but very different things that need to be checked independently (the last is calculated using thermal resistance, given in the datasheet).