Bed doesn't heat over 90°C, 24V to RAMPS ok?

Hi, I've got a Prusa i3 from charlies3dtechnologies with RAMPS 1.4 and a MK2a Heatbed.

As per specification on 12V it only heats up to barely 90°C.

but can I simply put the 24V output of the PSU to the RAMPS 1.4 11A power input?
I'm having real issues stepping up the voltage that significantly without any info...

Some simple answers:

1) The Ramps board will handle 24 volts and up to 11 amps.

2) The rest of your electronics likely will not like the 24 volts very much at all.

3) If you have a normal 1.2 ohm PCB heater board, it will pull 24 amps at 24 volts. The Ramps will not be happy at 24 amps.

Best advice - don't do this. Work on insulating your heater board instead. It should be pulling the max current already.

Here's a little more complete answer:

You can keep +12 on the 5A input and take the 11A input to +24 volts. That will only impact the two outputs on the 11A line.
When you go over 11A the first thing that fries is the green screw connector that the heater bed attaches to. You would need to pull it and replace it with something heavier.
The MOSFET is rated at 30 amps, but that's with a real heat sink on it. You would need to rig a real heat sink (not one of the little stick on gizmos). There's not a lot of room in there for that.

Once you did all that, you would find out if the pc board is good to more than 11 amps. That's going to depend a lot on who made your pc board.

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An alternative - leave the 12 volts as is. Run the 12 V output into another (well heatsunk) MOSFET plus inverter and use that to drive the 24 (or 48) volt powered hot bed. Less chance of frying the Ramps board that way. Of course you might then find out what it takes to fry the hot bed board it's self.