I have a TO-220 package of irfz44n mosfet that have the middle pin (drain) connected to the mosfet sink tap , why is that?. And do I have to insulate the heatsink from the body of the mosfet ?

I've searched for similar stuff about the subject and I found many topics that talk about insulating the mosfet but I don't know if that connection has a purpose as it will reduce the efficiency of cooling (increasing the themal resistance between the mosfet and the heatsink).

  • \$\begingroup\$ Do you really mean TO-22 and not TO-220? How about linking a datasheet of the IRFZ44N? A Picture of your setup would help a lot... \$\endgroup\$ – Curd Apr 21 '17 at 14:18
  • \$\begingroup\$ @Curd Sorry, I've edited it. \$\endgroup\$ – iMohaned Apr 21 '17 at 14:23

This has to do with how a Power MOSFET is build up:

enter image description here

The Drain is at the bottom. This is convenient as the drain is also the location where the power is dissipated in such a device.

So this Drain can get hot and to cool it the heat must be conducted away from the device. This works best if the Drain has a large metal plate like the back of the MOSFET you mention.

At the same time it is a convenient way to fabricate the MOSFET this way. This has to do with doping levels and processing steps. Read the wikipedia page to learn more.

To cool a transistor you should mount it on a heatsink. If that heatsink can be electrically connected to the Drain of the MOSFET you have no problem.

If electrically connecting the Drain of the MOSFET to the heatsink is a problem then you need some form of electrical isolation. To maintain cooling capabilities you should use a material which does conduct the heat but does not conduct electricity.

Here's a practical example of that: enter image description here

Note that the screws will connect to the heatsink so plastic rings might be needed for proper isolation.

  • \$\begingroup\$ So choosing to insulate it or not is completely back to me or there are any other factors that I've to put in my mind? \$\endgroup\$ – iMohaned Apr 21 '17 at 14:25
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    \$\begingroup\$ @iMohaned if you are only using one transistor, or several that have their drains connected to the same schematic net, and there's no hazardous voltage, then you don't need to insulate. Otherwise you do. \$\endgroup\$ – pericynthion Apr 21 '17 at 14:31
  • \$\begingroup\$ @FakeMoustache is that your photo or taken from the internt? I'm asking because I have the same heatsink taken from a power supply but I don't know its thermal resistance ( or even a range for how much it could be) so if you have any information about it, it would be great :d . \$\endgroup\$ – iMohaned Apr 21 '17 at 14:48
  • \$\begingroup\$ @iMohaned It is not my photo. Many heatsinks look like that so it will very likely not even be the same. My guess is that such a heatsink is OK for dissipating 5 Watt total (all transistors together). If there is a fan blowing nearby that could be 10 W total. \$\endgroup\$ – Bimpelrekkie Apr 21 '17 at 15:48
  • \$\begingroup\$ @FakeMoustache Thank you, you really saved a lot of time for me. \$\endgroup\$ – iMohaned Apr 21 '17 at 18:12

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