Suggestions for mounting live heatsink in an aluminum enclosure

[AcHmed99]

A

[alm]

How about a bigger heatsink or more FETs so you can deal with the thermal resistance of silpads or mica washers? I imagine the people at UL will not be happy with a live heatsink if it can be reached through holes in the case.

The common way is to mount the heatsink to the PCB, but this one seems way too heavy for that. I don't believe I've seen insulated brackets, although I guess you could mess around with silpads between the heatsink and the bracket. If the heatsink gets hot, the temperature of the aluminum case may be an issue, depending on the type of product. If the voltage on the heatsink is something other than SELV, I would also make sure the case is grounded, since whatever method you settle on probably doesn't count as double isolation.

[IanB]

How hot do you estimate the heat sinks will get? Because anything below 100 C is essentially "not hot" as far as most structural materials are concerned. I would expect your heat sinks to be much cooler than that or they will not remove heat from the FETs effectively. So I would have thought that some nylon bolts and washers would do the job OK?

[amspire]

Another recommendation for insulating the FETs. That is a difficult heatsink design to use anywhere. The fins are too close for efficient use  for external convection cooling, and for internal use, the air has to be forced through the fins - it will go around the heatsink if it can.

So you will probably have to add extra metal around the heatsink to channel the air and then add 3 or 4 small fans to push the air through. Without a well designed air flow through the fins, that heatsink will not be great at all for continuous use. For decent cooling, you need enough air flow to make the air coming out of the box no more then about 20 or 30 degrees above ambient. Less is even better. If it is coming out at 60 degrees above ambient, then you are wasting most of your heatsink.

To insulate the assembly, it would have to be mounted very solidly to the chassis.  You can get plastics blocks from a good plastic supplier that can handle the heat (like delrin or laminated bakelite) and it can be drilled and threaded for bolts quite easily.  You do have to think towards a heavy duty construction.

With 3 parallel FETs and your requirement of no FET insulators, I just wonder if you are trying to get rid of too much heat using a heatsink not purpose designed for fan cooling. Have you calculated the thermal difference between the first and the last FET?

Richard.

[amspire]

OK The data makes it clearer. They are meant to be used with 100mm  47 cu/minute fans bolted on to the fin side blowing the air out. The fin design makes more sense now. I was thinking more like air flow from one side to the other.

You will find that FETS mounted along one edge will probably only get half the cooling compared to evenly spaced FETS along the center line of the heasink.

About the costs of the plastic - blocks or thick sheets of good quality machinable plastic do tend to be a bit pricy unfortunately.

Richard

[amspire]

So you recommend placing the fets in the center of the heatsink vs the edge? I wasn’t sure about that as well.
I havent tapped any holes yet.

Definitely. At high power's you will be very surprised how much heat drop you can get with that thick-looking aluminium slab. The middle location will spread the heat far better. Also when I said evenly spaced, I meant you want to give each FET an equal width of the heatsink for cooling. So if you divide the heatsink into 3, the fets then go into the center of each of the three divisions.

Richard.

[graynomad]

The FETs should definitely be mounted near the centre and spaced as equidistant as possible. I would think a triangle would give you the best spacing.

______
Rob

[IanB]

At high powers you will be very surprised how much heat drop you can get with that thick-looking aluminium slab.

Truly. I have never looked closely at the numbers before, but I really was quite surprised:



That was quite a thick slab too. Many (most) real heat sinks are nowhere near 1 cm thick (although the fin cross-section will compensate for a thinner base).

[T4P]

Not 1cm thick ? 

[vk6zgo]

If you could find some porcelain standoffs in a surplus,(or junk store) they might do the job.
In fact I would suggest having a bit of a nose round in one of these places,as they often get Electrical equipment which they leave out in the yard to deteriorate.
You may be able to find some of the heavy duty plastics or even the ceramic materials they use in stoves,for instance,where the environment is quite hostile.
Teflon could handle the job with ease.but is quite costly,new.

[SeanB]

What about using 3 CPU coolers with integrated clip on fans? One device per heatsink, and using the round Intel  or clone heatsinks gives good airflow, with the centre only being solid, the rest being fin. You can mount 3 in a tube with a 80mm fan blowing through, which will not give ideal airflow or temperature, but which will handle a lot of heat.  Guard on inlet and outlet and you will have a nice unit.