EDIT : after a while, I had time to finish this project and found this topic about how to connect a MOSFET to a ramps :
https://3dprint.wiki/reprap/electronics/heatbed_mosfet
long story short, if you use a MOSFET for your bed and/or your extruder, you "just" have to connect the signalwire (the control input of the MOSFET) form the MOSFET directly to the bed output from your duet.
Here is the picture from the link.
special thanks to 3Dprintwiki !
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