RE: Dell Inspiron 3847 PSU upgrade
24 pins is EPS12V. 20 pins is ATX.
Power supplies that guarantee 150W combined on the 3.3v 5v rail and 15 to 20W on the +5vSB Rail with 450W or more on the 12v rails would work.
This is why a Corsair CS750M is a recommended upgrade.
Systems that are 650W and lower that only have 130W on the 3.3v/5v rail tend to not work well if at all.
Systems with less than 15W on the +5vsb rail tend to not work well if at all.