# Heat Sink Isolation

What is the best strategy for heat sinking a MOSFET? I have a heat sink all picked out here. I also have a MOSFET picked out here. I know that the tab is electrified with the drain. There are a few choices on how to electrically connect the MOSFET to the heat sink.

1. Set the heat sink to the same potential as the drain and not isolate the tab.
2. Set the heat sink to the same potential as the drain and isolate the tab with a sill pad.
3. Leave the heat sink floating and not isolate the tab.
4. Leave the heat sink floating and isolate the tab with a sill pad.
5. Ground the heat sink and isolate the tab with a sill pad.

What is the best option? Am I missing anything?

The MOSFET will be used along with an MCP73844-8.4 to charge a 2 cell lithium ion at 1A. From my understanding the chip throttle the MOSFET and does not use any high speed switching. The input voltage for charging is going to be around 10V.

There will be no enclosure. The heat sink will be upright in open air.

• Which is most convenient for you? Is having the tab connected to a circuit potential possibly dangerous? or make it more difficult to work on the product? I would usually isolate the heat-sink – Icy Nov 26 '15 at 16:18
• @Icy Would you ground it or leave it floating? – vini_i Nov 26 '15 at 16:21
• @vini_i: if the tab is drain and in your circuit drain is not connected to gnd, grounding it is a nono. Also floating the heatsink is not possible when it is connected to the drain. – PlasmaHH Nov 26 '15 at 16:25
• @vini_i Will the PCB with this heatsink sit inside of an enclosure (perhaps with a fan)? Is the heatsink exposed outside of the enclosure? If this a one-off for your own use, or for distribution? – Nick Alexeev Nov 26 '15 at 18:11
• @NickAlexeev This will be a one off. The heat sink will be upright, in free air. No enclosure. – vini_i Nov 26 '15 at 18:15

Heatsinks almost always float (but are capacitively grounded) or are grounded. I have never seen them connected to MOSFET drain. Under PWM, the heatsink potential would swing up and down, which would draw immense leakage currents through the system. On top of introducing a lot of electrical noise it would also be dangerous.

Use option five- ground the heatsink or option four (but use caps to provide some current path) and use Sil-Pads (http://www.digikey.com/catalog/en/partgroup/sil-pad-k-4-series/1298).

The benefit of floating heatsink is the decreased parasitic capacitance. The MOSFET has some capacitance to the heatsink and the heatsink has some capacitance to ground/DC bus negative/DC bus positive. These two capacitances are in series, which effectively results in lower capacitance and therefore lower leakage currents. It seems that you understand the implications based on your five options.

• In a switching application EMI is basically the [very] nasty side effect of not isolating the heatsink from the FET. But even then the heatsink can pick EM capacitively and re-radiate it as a bigger antenna. Big issue with CPU heatsinks this can be. – Fizz Nov 26 '15 at 16:54

If you have to ground the heat sink, then the best option is to isolate the MOSFET with something that has a low enough thermal resistance that the transistor's $\theta _{JA}$ won't be exceeded. I prefer a mica washer and thermal grease rather than a silicone pad.

If the heat sink is left to float, then you can bolt the TO-220 directly to the heat sink with no isolator, but I'd still use thermal grease.

Unless there's some pressing reason to consider the remaining 3 options, just ignore them and choose whichever of the two above is appropriate.