Quote Originally Posted by Roxy View Post
Incidently... I killed my RAMPS board on my Folger Tech i3-2020 printer. I'm not sure how it happened, but I might have shorted something on the Bed Heater circuit. I had a previously killed RAMPS board from another printer in my spare parts pile. I de-soldered two of the MOS-FET's from the other RAMPS board and transplanted them on the freshly killed board. I put one MOS-FET where it normally goes. And then the other MOS-FET got soldered on the bottom of the board to the same pads. So now I have two MOS-FET's switching the bed current.

I know some people are cringing. But I did look at the current / voltage curves for those MOS-FET's. And when the bed is heating up, I can feel each of the doubled up MOS-FET's getting warm. But they are much less warm than they used to get. Just doubling up the MOS-FET's does seem to work and not stress them as hard.
Putting MOSFETs in parallel like this doubles the current/watt handling and also drops the ON resistance in half. The only real drawback is you really need to match the pair for gate turn-on voltage which are notoriously sloppy in FETs of all types. In this case it's probably not that big of a deal unless they are wildly different it should work OK. If you have an IR temperature 'gun' a quick way to get a decent idea if they are handling the current equally which means the gates are fairly well matched is to measure the MOSFET's body temps and the closer they are to being the same the closer your gates match. Unless there is a large difference in temperature for this simple application (as compared to say a high performance, high efficiency motor speed driver) you'll likely have no problem.

Paralleling FETs and bi-polar transistors is an old trick used to make low noise audio and radio preamps because paralleling them also drops your Noise Figure in half which can be significant in the first stage of a microphone or weak signal RF preamp. If you look at the internal schematics of many analog ICs you'll find they use parallel transistors a lot. It's also used a lot in power amps for both audio and RF although in those applications matching is critical since they need to run linear for low distortion, something that doesn't matter at all for turning on a bed heater