Mica isolation layer TO247 package

[GBolamp]

Hi all,
I was surfing the forum searching for an answer to my question but even though I have found many threads on this topic I wasn't able to find the answer to my question, so there it is:

I am building a dual channel power supply, this supply has 4 TIP142 (TO247 package) for each channel, these NPN have to be mounted on a common large heat sink, these transistors have a back which is shorted to the Collector (not isolated back) they have to be isolated electrically from the heat sink, together with the transistors I have received some mica layers that have to be inserted between the transistor and the heat sink so they have an isolation, my question is:

Where does thermal paste have to be used, Do I have to put thermal paste in between the transistor and the mica layer then also on the back of the mica layer that goes on the heat sink or just in between the transistor and the mica layer is enough?

If there is already an answer to my question would you please link it below.

Thanks in advance to all.

[wraper]

On both sides of mica. Thermal paste is used to avoid air gaps. And air gap on either side of mica is just as bad for heat transfer.

[GBolamp]

Thank you,
that is what I though also but on the instructions they specified to put the paste just between the transistor and the mica, which sounded a bit strange to me because I knew the story as you have written, but then I had this doubt in my head, I had to hear another opinion on the topic.

Anyway, thank you very much for the fast answer!