+1 what ChrisP58 said. Also, the ratings they show pertain only to the silicon, and totally ignore the limitations and parasitic properties of the package. The datasheet specs are out to lunch when you try to apply them to real world. For example, IFR1405PBF: datasheet says 169A max @25C continuous, 118A max @ 100C continuous, and max pulsed current 680A. It's in a TO-220AB package. It says right there on the datasheet in the fine print that package limitation is 75A. Really? 75A?. My welder operates around 75A and it needs #2 thick copper cable to carry that. Am I really to believe that the cute little leg of a TO-220AB fet can carry the same amperage? I'm not comfortable with passing more than say 10A through it, and it would probably be pretty darn hot even at that. So I say datasheets are off by several orders of magnitude, and the thumbrule of 50-75% of max is not even a safe bet in some instances. My goal is only to make OP aware of some of the flagrant fouls in datasheets, and to be careful; I know they are not all this bad.
.....