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Electronics => Beginners => Topic started by: alvarop on April 15, 2011, 03:22:58 am

Title: TO-220 MOSFET PCB Placement
Post by: alvarop on April 15, 2011, 03:22:58 am
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 (http://www.st.com/stonline/books/pdf/docs/7980.pdf) 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!

(http://www.alvarop.com/files/rbgledcontroller.png)
Title: Re: TO-220 MOSFET PCB Placement
Post by: Joshua 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
Title: Re: TO-220 MOSFET PCB Placement
Post by: mkissin on April 15, 2011, 04:56:09 am
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.
Title: Re: TO-220 MOSFET PCB Placement
Post by: Jon Chandler on April 15, 2011, 05:23:29 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

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.
Title: Re: TO-220 MOSFET PCB Placement
Post by: alvarop on April 15, 2011, 08:06:16 am
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.
(http://www.alvarop.com/files/rbgledcontrollerv2.png)
Title: Re: TO-220 MOSFET PCB Placement
Post by: Rufus on April 15, 2011, 02:16:51 pm
Because the copper is so thin the effectiveness of 'PCB' heatsinks drops off rapidly as they become larger and insulation is a pain. I would put 3 patches of copper on the bottom layer with a bunch of vias and the mounting screw thermally coupling them to the devices.  That also gives a flat area on the bottom you could clamp (an insulated) heat sink to if you run into trouble.

You should also put resistors in series with the gates to reduce the possibility of high frequency oscillation and consider adding pull down resistors on the gates to prevent them floating should whatever drives them be disconnected.

Title: Re: TO-220 MOSFET PCB Placement
Post by: alvarop on April 15, 2011, 06:12:20 pm
... consider adding pull down resistors on the gates to prevent them floating should whatever drives them be disconnected.

I didn't even think about that! Here's the updated version.
I just ordered some of the RGB LED strips from Adafruit and figured I'd make my own driver for them.
Thanks again for the help.

(http://www.alvarop.com/files/rbgledcontrollerv3.png)
Title: Re: TO-220 MOSFET PCB Placement
Post by: Rufus on April 15, 2011, 06:37:05 pm
Pull downs would probably be better on the input side of the gate resistors to avoid tapping down the gate drive.

Don't know what you intend to drive the gates with but they are not logic level MOSFETs so I/O from a 5v processor for example is already a bit marginal.

Title: Re: TO-220 MOSFET PCB Placement
Post by: alvarop on April 18, 2011, 02:18:44 am
Don't know what you intend to drive the gates with but they are not logic level MOSFETs so I/O from a 5v processor for example is already a bit marginal.

You're right. I tried driving it with 3.3v logic and it didn't quite do it. I added a BJT at the gate and it seems to do the trick!

Thanks!
Title: Re: TO-220 MOSFET PCB Placement
Post by: DavidDLC on April 18, 2011, 04:23:21 am
Have you tried this circuit before you build the PCB ?

That will save you some time and money if for some reason you miss something.

Title: Re: TO-220 MOSFET PCB Placement
Post by: alvarop on April 18, 2011, 11:37:39 am
I just tried it yesterday and it worked. I only built did one out of the three, but it shouldn't be a problem.
Title: Re: TO-220 MOSFET PCB Placement
Post by: Zad on April 18, 2011, 03:27:30 pm
I may just be reading this wrongly but, in the first picture, isn't the red track leading to the 3-pin component (connector?) at the bottom right going to be drilled through by the top pin?
Title: Re: TO-220 MOSFET PCB Placement
Post by: alvarop on April 18, 2011, 03:41:25 pm
I think it would have, but the top and bottom-right pins are both connected internally. I got rid of that trace in the later version.
Title: Re: TO-220 MOSFET PCB Placement
Post by: alvarop on April 21, 2011, 03:08:23 am
Thanks for all the help!
I ordered the PCB and here's a quick video of the prototype circuit:
RGB LED Strip Test (https://www.youtube.com/watch?v=W6iP8ZVTvc4#ws)