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Heat Sink

Thread starter #1
I want to mount my Full Bridge(H-Bridge) Switches on a single heat sink so, if any one can guide me to a heat sink with thermal resistance of <2°C/W, that wouldd be great. Also, I have one doubt. as my top switches and bottom switches will be mounted on same heat sink, would there be any problem related to ground. How good will be the quality of the insulation paste? Will it be sufficient to provide electrical insulation for long run. Also, How to avoid the problem with the mounting screws as , they will be in contact with the MOSFET and the heat sink directly.
Thankyou for reading my question.


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It depends on the package your MOSFETs are in. Some have an electrically connected heatsink pad, others do not. You need to check the datasheet. If it is electrically connected, then you need a thermal pad. The cheap crappy ones are thin pieces of plastic or elastomer material. The good ones are actually a thermally conductive, electrically insulative ceramic material like alumimum oxide (or beryllium oxide which is top of the line but is hard to find but very expensive due to materials and because of toxicity if it somehow produces dust).
https://www.digikey.ca/products/en/fans-thermal-management/thermal-pads-sheets/218?k=thermal pad

BTW, you should still use thermal paste with these. At least with the hard thermal pads. I'm not sure if the softer thin plastic ones need it since they might be soft enough to fill in the air gaps.

Do not rely on thermal paste for insulation since by definition the thermal paste is only to fill in the air gaps due to roughness at the thermal interface. Any areas that made direct contact are expected, and should do so since they conduct heat better than the paste.

There should be clearance around the hole in the heatsink pad so the screw does not make contact with the electrically conductive part (some packages anyways, you haven't told us what you're using yet). It should be visible if you look at the pad. Worst comes to worst, you can use nylon screws and bolts.

Heatsinks can also have aluminum oxide (or some other finish) that makes them not electrically conductive. Of course, finishes can always be scratched through.
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Oh, for that, you do not need a thermal pad. Just thermal paste it and bolt it on. The baseplate is not electrical connected (of which the screw holes and heatsink pad are both part of). You can tell because it doesn't label the baseplate as being connected to anything and the note that says "miniBLOC, with Aluminium Nitride Isolation". I actually have a new MOSFET in the same package from the same company down stairs. I had to get the machinist to grind an aluminum block and drill and tape some holes for the package. I don't think you'll be able to find a pre-made heatsink for something like that. At least not somethign reasonably priced. I'd find some aluminum block and a drill press. If you don't have a tap then use long screws and drill all the way through the block and use bolts on the other side.

EDIT: There is this https://www.digikey.com/en/product-highlight/o/ohmite/c40-heat-sink-system

Look at the datasheet since the clips (doesn't use screws) are ordered separately so you need to also order the CLA-S227-21E clips which are for the SOT-227 package. But neither Digikey nor Mouser sell them.

You can also see from the datasheet that airflow (i.e. a fan) is required to get the thermal resistance you want.
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Look at the pinout on that. It clearly says the tab is drain. If it's not isolated, then it will clearly have the connection of the tab labelled somewhere the same as if it were any other pin.
Look at the pinout on that. It clearly says the tab is drain. If it's not isolated, then it will clearly have the connection of the tab labelled somewhere the same as if it were any other pin.
Did You mean in the figure below from datasheet:
So since Back side is marked as 4 which is given as Drain so it is not isolated

Whereas in 247 package in the same datasheet doesnt say somrthing like this, which means tab is electrically isolated.



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The drawing from page one shows that the tab is connected to the drain on both packages, yet the drawing on page two disagrees.

I would defiantly check a real part.

Or better yet, get a confirmation from the factory. If for no other reason than to let them know that their datasheet is ambiguous, and they should fix it.

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