ALEPH 3 HEATSINK - thermal coupling between single transistors

[lokuspokus]

Hi,

I plan to build an ALEPH 30. I am therefore in progress collecting information and design ideas especially concerning the heatsinks. I must admit that I have no experiences with circuits dissipating that large amount of heat.

I have seen several designs using for example four smallsized heatsinks with say 1 K/W rated and connected with a large bar made of aluminum. That is exactly what I plan to do because I can get them cheap. (There is no question that the faces where the bar is mounted to the heatsinks have to be near ideal flat and not too small.)

By using certain shape of heatsinks the bar might get quite long. So my question is:
How does the spacing between the output transistors effect the heat distribution ? Should the transistors in any case be as close as possible to each other (could result in local overheat to my opinion) or rather uniformly distributed along the bar (no thermal coupling between neighbour transistors) ?

Thanks for your advice.

Regards

Lukas

[SeanPool]

Lukas,

A local heatsink shop said that he wouldn't recommend connecting transistors to heatsinks using bars, as they would give you more thermal resistance, especially if the bar was not attached to the sink properly or too small. In those cases, you'd end up with a hot bar with the heatsink just warm. So I think you just have to make sure that you put greases well on the bar and use a big and thick bar.

I don't know about the answer to your other question (which is a very good one) as I'm not sure if thermal coupling output mosfets would do any good. Anybody?

Since I cannot find big heatsinks here, my plan is to mount each mosfet directly to each of the smaller heatsinks and connect the sinks together with bars (of course, with greases) to allow for an even heat distribution among the mosfets.

-Sean

[lokuspokus]

Hi,

I already thought nobody will try to answer my question ;-)

Some posts on this website already described the problem of a hot mounting bar and quite cool heatsink. In one of the tutorials available around the internet I read, that smoothing of the transistor's back as well as the mounting plane can improve thermal coupling a lot (using additional thermal compound of course). That should be done with finest sandpaper and very carefully with having an ideal flat working plane beneath.

Yes, mounting each transistor directly to one of the individual heatsinks, that is what I was thinking about. But as mentioned before I got unsure about that because thermal coupling between transistors will be low - also with connecting just the heatsinks. I am not sure if good thermal coupling is required for stabilizing Q-point by keeping all of them at equal temperature.

If somebody has tried out different arrangements, do not hesitate an let me know.

Regards.

Lukas