MOFSET Choice Help

I asked this over in the RAMPS forum and received a few conflicting answers. This is probably more appropriate for this forum.

RAMPS requires the use of the STP55NF06L TO-220 N-type MOSFET. The problem is that it is a special order through digikey which is a major electronics supplier here in the US. The STP55NF06 is a stocked product but my real question is what the true requirements for this MOSFET are. I am no electronics whiz but was hoping someone could give the major requirements and then any MOSFET that meets that spec could be used.

Thanks in advance.

If i recall correctly some math i did for the same thing:
- STP55NF06L is 60V and can do like ~8A in still air, and
- IRL3803 is 30V and can do like ~16A in still air and with approx 4 times less disipation on it
The IRL3803 is logic level in all variants and on stock on several packages at digikey. There are probably lots of other choices aswell.

I guess that the fet ideally should:
- be logic level and
- be able to switch the working current in still air / without heatsink - for the best. Which usually translates in a low rds(on) value: the power that the fet needs to disipate would be Iworking^2 * rds(on).

To approximate how much current it can switch in still air / w/o sink, i think it is
= SQRT [ (Tempmax / R-thj-amb ) / (pwrdisipated@25 / Imax ^2) ]
and all of these 4 values can be found in any fet datasheet. Double check the formula though. I think this value should be, with a fair margin, bigger than the current expected to work with.

I hope the above is fairly accurate. Good luck

This should help [forums.reprap.org]

Edited 1 time(s). Last edit at 05/05/2011 08:04PM by Sublime.

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