I have purchased a large 300×400 mm silicone heater for one of my customer’s CNC to 3d printer conversion project. The heated bed is rated as 1200watt. In order to control it with gnexlab controller boards I use a SSR (Solid State Relay). SSR have the magic to drive very large loads with 3-30v inputs. You can directly connect the input side of the SSR to one of your power mosfet output. The other side is the heater side. Note that to select proper SSR according to your heater specs. You can ask me if you have any trouble.
Converting a CNC Machine to a 3D Printer 
I am very interested in a large heated bed for a larger 3d printer. Could you please share with me any design info and things I need to buy.
Thansk Steve
Hi Steve,
I will soon start to sell a power booster which will help us to drive large heated beds. We will use it instead of SSR
Other needed Items: one of the Gnexlab controller Board, Slicon Heated bed, Power supply