I would not recommend doing that.
What are the options and the issues linked with them :
- using a 12V heated bed on a 24V power supply...
I see on the FT website for one of the 12 beds that it is designed for 12V 19A. Just simple application of Ohms law gives a resistance of 0,63 ohms (12/19). If you apply 24V on that the current will be the double : 38Amps...
Power @12V = 228W, same bed at 24V will try to dissipate 912W.
The 24V power supply FT has in their list can deliver 360W or 15A.
No matter what you do : from the moment you switch on the bed it will try to draw that full current.
PWM : pulse width modulation.
A rough example what is meant by this :
If you have a 100W lamp switched on at full power you consume 100% or 100W in one hour (100Wh)
If you want to dim it and consume only 75%, you could switch it on for 45 minutes and then off for 15 minutes. The period used is 1 hour, the duty cycle is 75%. You will be 15 minutes in the dark, but you will indeed consume only 75Wh. During the 45 minutes the lamp will draw full current and use its 100W power.
The same for the bed : the PWM will not limit the actual current through the bed, but the average current.
Risky business. The switching MOSFET on the BED has to cope with 38 Amps. Since the power supply cannot deliver this full current, there will be voltage dips too.
Never tested that (I wouldn't even think off), just reasoning on the physics and the risks...
One way to avoid that is putting the same resistance in series with the bed. Has to be a 0,63 Ohms 228W Resistance but still then the bed will draw 19Amps. The best one you could find for that would be an identical heated bed ...