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TO-220 MOSFET PCB Placement

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alvarop:
I'm planning on using the STP16NF06 NMOS device with TO-220 footprint on my next PCB layout.
I have two doubts. First, the datasheet says the drain is connected to "TAB". I know the drain is going to pin #2, but does TAB mean that it is also connected to the pad on the heatsink?
For the PCB layout itself, I'm thinking of using some copper under the body to use the board as a heatsink. Assuming this is what I need to do, should those pads be floating or connected to something?

Thanks for the help!

Joshua:
Hello,

This may mean nothing to you, but once I was playing around with a regulator or something, and just assumed that the tab was connected to ground. It ended up being connected to the output instead... I shorted it out and burned my self.

Joshua

mkissin:
Yeah, as you suspected, the entire metal tab on the back and top of the TO-220 is electrically connected to the centre pin of the package. In this case, that's the MOSFET drain pin.

I personally wouldn't bother with copper under the tab on the PCB. You won't be able to get enough copper there to make any thermal resistance with the current layout you have. Unless, of course, you have a ground plane there, in which case you'll have to isolate the three tabs from the copper (and each other). You'd normally do that with a silicone or mica pad, which goes between the tab and the metal surface it's mounted to. The bonus of using the silcone pad is that it's slightly squishy, and you don't need to use any thermal compound.

Also, you'll need plastic bushings which isolate the screw that you'll need to use to hold the whole thing together. Depending on where you get the pads, they may come in a pack with the bushings.

As a side note...if you get the TO-220FP package, rather than the vanilla TO-220, the tab is encased in plastic and you don't need to worry about isolating it. The drawback is crappier thermal transfer.

Jon Chandler:

--- Quote from: PianoKid1994 on April 15, 2011, 03:29:30 am ---Hello,

This may mean nothing to you, but once I was playing around with a regulator or something, and just assumed that the tab was connected to ground. It ended up being connected to the output instead... I shorted it out and burned my self.

Joshua

--- End quote ---

I think a lot of us may have a TO-220 tab-shaped scar from a burn on a thumb!  I think my case was thinking the pinout of a negative regulator is the same as a positive regulator.

alvarop:
Thanks for the help! I'm glad I asked before burning myself.
I moved the ground plane to the front and will get some thermal pads to insulate them.

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