Can I attach more than one transistor to a common heatsink?

I want to attach four TIP3055 transistors to one piece heat sink. I noticed that some transistors
are sold with a plastic (mica?) isolators, while others such as above mentioned TIP3055 are sold
without it. The TIP3055 have metal backing (which will touch the heat sink) so if I attach them
to a common sink will there be an interference between the transistors when circuit is working?

Also does anybody know whether the heat sink compound is an isolator or is it conductor of electric current?

Les

Heat sink compound is an electrical insulator. Its only use is to remove oxygen between the two surfaces being joined. (Remember to make the layer of compound as thin as possible.)

If all of the transistors being used share a common collector, it doesn't matter if the heat sink electrically connects them.

I wouldn't just rely on the heatsink compound to provide insulation if they need to be isolated use mica and heatsink compound

Just to be clear, if you don’t use mica washers then the collectors of the TIP3055s will all be electrically connected to the heatsink. If that is unacceptable, you need to use the washers. Make sure that you de-burr the holes you drill in the heatsink first, because even a tiny burr can break through the mica and cause a short.

Most NPN power transistors connect their collector lead to the metal mounting tab. If your specific circuit is wiring all the collector terminals together (check carefully) then you don't need to electrically isolate the mounting tabs from the heat sink, however keep in mind that in such a case the heatsink itself will then have the same positive voltage on it, and cannot therefore be mechanical mounted to any metal attachment that is a ground potential.

The more normal situation is to insulate the transistors from the heat sink using either mica or custom rubber insulator pads along with using heatsink compound (just a thin coating) to improve the heat transfer. Never use just heaksink compound alone without the flat insulator. One should use plastic hole insulators that fit into the mounting hole of the transistors tab to prevent any touching of a metal screw to the inside of the tab hole, or use nylon screws.

Lefty

And, of course, you must make sure that the heatsink can dissipate the heat from all four devices.

Pete

I’m curious about the degree to which this is true. Some compounds, such as Artic Silver5 (see the MSDS) have particulate metal as an ingredient. I recall (perhaps incorrectly) at one point about some compound made with copper.

Irrespective of that, if heatsink compound is the only thing between either a metal case, or heatsink tab, and the heatsink, surface irregularities can still result in metal-metal contact as the compound flows under pressure into voids and out from the edges.

justjed: [quote author=James C4S link=topic=87874.msg660066#msg660066 date=1327041211] Heat sink compound is an electrical insulator.

I'm curious about the degree to which this is true. Some compounds, such as Artic Silver5 (see the MSDS) have particulate metal as an ingredient. I recall (perhaps incorrectly) at one point about some compound made with copper.

Irrespective of that, if heatsink compound is the only thing between either a metal case, or heatsink tab, and the heatsink, surface irregularities can still result in metal-metal contact as the compound flows under pressure into voids and out from the edges. [/quote]

I've never seen just compound used to try an insulate a transistor from it's heatsink, ever. Even if it could be made to work what about the mounting screw? Compound only would only be used where there is no need to electrically isolate the device from the heatsink.

Lefty

retrolefty: I've never seen just compound used to try an insulate a transistor from it's heatsink, ever. Even if it could be made to work what about the mounting screw? Compound only would only be used where there is no need to electrically isolate the device from the heatsink.

As you said, use nylon screws and nuts.

As a tangential question, what's the reason for making the case, or heatsink tab, tied to the collector (or anything other then ground -- e.g. in regulator ICs such as LM117 (TO-3) the case is Vout)?

Because it's easier to manufacture them that way [EDIT: and provides the best heat transfer]. The die is bonded directly to the metal case or tab.

justjed:

retrolefty: I've never seen just compound used to try an insulate a transistor from it's heatsink, ever. Even if it could be made to work what about the mounting screw? Compound only would only be used where there is no need to electrically isolate the device from the heatsink.

As you said, use nylon screws and nuts.

As a tangential question, what's the reason for making the case, or heatsink tab, tied to the collector (or anything other then ground -- e.g. in regulator ICs such as LM117 (TO-3) the case is Vout)?

I think it's a result of the normal construction of a transistor, that it's substrate is the collector. Note that there are power transistors avalible that have isolated heatsink tabs, but most likely at higher costs and less heat transfer specifications.

dc42: Because it's easier to manufacture them that way [EDIT: and provides the best heat transfer]. The die is bonded directly to the metal case or tab.

Well, that makes sense. Thanks.