Had the same thing happen with our RAMPS, the heatbed was drawing too much current and the standard MOSFET got very hot, the fuse got hot and burned.
The fix for us was to change the STP55NF06L MOSFET with a IRLB8743PBF or (IRLB8743). It's the one closest to the fuses.
We also put a small heatsink on the MOSFET and run a 40mm fan over them.
Both MOSFET and fuse can be found on ebay.
The fuse is rated for 11A. Not sure what the bed resistance is for a kossel mini, but our 8"x8" bed is 1.3 Ohms.
So 12v / 1.3 Ohms = 9.2 Amps so it does draw a lot of current.
Steve