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Old 11th November 2006, 09:55 PM   #11
AndrewT is offline AndrewT  Scotland
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Hi,
the layout of the heatsink and mounting devices is covered in detail in a Wakefield paper.

The devices are usually spaced out more at the top and closer together at the bottom.

I seem to recall that for three, the bottom device is only 15% up from the bottom edge, the second is about 40% up from the bottom and the third device is about 60% up from the bottom.

Doing something along these lines will nearly keep the devices at similar temperatures.

Similar temps are critical for temperature derating.

The thickness of the main plate should be no less than 10% of the maximum radius from device centre to the extreme corner of the sink.

The worst location for this thickness to radius ratio is the top device to top corner.
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Old 12th November 2006, 02:18 AM   #12
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Quote:
Originally posted by MattM
Thanks for the replies!

Yes, I am using these.

I plan to mount 3 devices per heatsink (two heatsinks per monoblock). The heatsinks will be mounted with the fins running vertically. In that orientation, they are 12" tall, 5.8" front to back, and the fins are 3" wide. Each heatsink weighs 5-1/2 pounds and has 16 fins. They are Wakefield Engineering part number MD387A, but I can't find them on their web site so they may have been a custom item. They are closest to extrusion profile 8415 - here is a link with some specs.




They will have air space under them, so I am expecting natural convection to create quite a healthy air flow across the 12" height.

Maybe the answer is simply to mount the devices at the same level. I had originally planned to spread them out across the heatsink (as depicted in the "wrong" half of the diagram below). While mounting them close together may or may not make them run a little hotter, at least they will all be at the same temp.

You got a great deal with those babies.

The heat sink will operate most effectively with the devices spread out. Aavid Thermalloy tells you why on their website -- it's that the devices in close proximity act like a point source so the heat takes a bit of time to migrate across the sink.

You can try this experiment to determine the thermal impedance of a heat sink -- really easy -- get a chasis mounting power resistor of (perhaps) 50 Watt rating and mount it on the sink -- use thermal grease -- measure the ambient temperature -- then cook at 25 Watts for one hour and measure the new temperature -- take the delta, divide by the power and you're good to go.

If you have 3 such resistors you can experiment with their position -- and the thermal impedance of the device. Prove it for yourself.

Try mounting the fins horizontally, then compare the result when you mount the fins vertically...

Natural convection is only some tens of feet per minute.
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Old 12th November 2006, 02:45 AM   #13
MattM is offline MattM  United States
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Quote:
Originally posted by AndrewT
I seem to recall that for three, the bottom device is only 15% up from the bottom edge, the second is about 40% up from the bottom and the third device is about 60% up from the bottom.

Doing something along these lines will nearly keep the devices at similar temperatures.

Cool info (no pun intended). Thanks!!!!
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Old 12th November 2006, 07:11 AM   #14
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by AndrewT
The thickness of the main plate should be no less than 10% of the maximum radius from device centre to the extreme corner of the sink.

The worst location for this thickness to radius ratio is the top device to top corner.
remember that guidance?
I see the sink you plan to use is VERY thin. You will run into uneven temperatures across the backlate and that will reduce the effectiveness of the sink to well below the manufacturers specification.

If you were to bolt a thick copper plate between each device and the sink to bring the heat out to nearer the edges then you will get better performance. Still sticking to that 10 to 1 rule a 60mm square plate needs to be at least 3mm thick. Conversely if you can lay your hands on 5mm copper then you could bolt a long strip 100mm wide from near bottom to near top. BTW. chip manufacturers do this to spread the heat out along the mounting plate to improve Rth j-c.

Mounting the plate with the fins horizontal will cook your devices. It really is that bad! Maybe worse for your deep, thin fins.
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Old 12th November 2006, 01:18 PM   #15
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Quote:
Originally posted by AndrewT


Mounting the plate with the fins horizontal will cook your devices. It really is that bad! Maybe worse for your deep, thin fins.

That's why I suggested running the experiment with the fins horizontal and vertical...you'll get two readings of thermal impedance because the volumetric difference.
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Old 14th November 2006, 07:40 AM   #16
MattM is offline MattM  United States
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Quote:
Originally posted by AndrewT
I see the sink you plan to use is VERY thin. You will run into uneven temperatures across the backlate and that will reduce the effectiveness of the sink to well below the manufacturers specification.
Yeah, the base plate is only 4mm thick. I will look into bonding a copper plate to the heatsink as you suggested.

Thanks again!
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Old 14th November 2006, 08:56 AM   #17
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Quote:
Originally posted by MattM


the base plate is only 4mm thick

If heat sinks have the same surface area, I prefer the one having thinner base plate because the thinner base plate make it possible to reach faster thermal balance (or steady-state) with the air temp surrounding the sinks. This means that we can more quickly arrive the final amp's warm operational condition from the cold start. Just my preference . . .
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Old 14th November 2006, 12:19 PM   #18
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post script-- there is nothing linear about this discussion.

rule #301 about heat sinks -- a device doesn't necessarily get rid of its heat by the path that you intended -- a thick slab of metal stores energy -- nice if you like heating hysterisis in your system as it will heat up the junction when the device is dissipating less power as heat.
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Old 14th November 2006, 02:18 PM   #19
sajti is offline sajti  Hungary
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Hi,

I have recommendation from one of the company which makes heatsinks. So this recommendation is how we mount more devices on single heatsink, and keep the temperature equal.

There will 3 pictures:

1,
Attached Images
File Type: gif heatsink1.gif (7.9 KB, 182 views)
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Old 14th November 2006, 02:21 PM   #20
sajti is offline sajti  Hungary
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2,
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File Type: gif heatsink2.gif (76.1 KB, 177 views)
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