I am working on a project which requires me to use a battery to run a computer. I planned to work with a 24V battery connected to a DC input (24V,1000W) ATX power supply and connect it to the PC. But now that the ATX power supply is very bulky and more concerning is its efficiency (around just 75%). Since buck has very good efficiency i can do some work around and get the voltages 12V, 5V & 3.3V. But i am not sure how to design on the power good and power ON signals(For instance i know both signals operate at 5V but not more than that). As for the protection circuit, we can have a fuse installed or i can install a circuit breaker in the buck converter.
You're on the right track. Take the next step and consider using a 48V source, following emerging practice in data center technology.
48V ATX power units are available from various sources in the same form factors as for standard IT gear.
Going even smaller, this one in particular is a module that plugs directly onto the motherboard ATX header, accepts 6-32V: https://www.mini-box.com/DC-DC