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Re: [Amps] 'heat spreader' sizing ??

Subject: Re: [Amps] 'heat spreader' sizing ??
From: "Roger (Sub1)" <>
Date: Wed, 02 Feb 2011 17:38:28 -0500
List-post: <">>
On 2/1/2011 6:32 PM, Dr. David Kirkby wrote:
> On 02/ 1/11 02:42 AM, Dan Sawyer wrote:
>> The project is to mount a 1KW, 2 transistor, board on a heat sink. The
>> transistor centers are about 2 inches. The question is: What should the
>> dimensions of the heat spreader be? I have seen dimensions of 1/4 x 2 x
>> 4 up to 1/2 x 4 x 7; that represents quite a range. The questions are:
>> 1. What thickness is required. Is 1/4 sufficient?
>> 2. What is the minimum effective distance from the edge of the devices?
>> 1/2 inch? 1 inch?
>> Thanks,
>> Dan
I don't know if this will help or not, but in generalities:

I'd look at the size of the heat sinks on commercial 1KW and legal limit 
SS amps and use those as a minimum. It'd certainly save a lot of 
computing time, unless you want to go through the entire design 
exercise.  It's been so long since I worked with something like this I'd 
cheat and take the easy way out. empirical testing works quite well.

The most critical part of the installation is getting the heat from the 
device to the spreader and heat sink. on high power devices the delta T 
often becomes the ultimate limitation for the device in use.  Heat sink 
compounds like Artic Silver (expensive but efficient) are far, far 
better than the old silicone heat sink compounds.  The spreader can be 
made large. Me? I'd make it as large as the heat sink and a 1/4 to 3/8" 

In the early days of SS amps I had a 600 watt 2-meter amp. The heat sink 
on that thing was a good 12 X 16 inches and no fans.

Actually the calculations can get a bit complicated.  You need to 
radiate so many watts (calories) and keep the device below a given temp. 
Sometimes it appears there is just no way to get the device to run full 
power and stay within the temp ratings.   At any rate you have a device 
that has a specific contact area available.  You need a heat sink 
compound that will transfer that many calories to the spreader.   The 
spreader has a given heat resistance per cm which is how fast it can 
transfer heat.  To radiate that many calories per minute you need a heat 
sink of a given size for a particular design.  The spreader needs to be 
able to efficiently transfer the heat to the heat sink

It's easiest to think of each step as a resistor just like in an 
electrical circuit but in this case were are transferring heat.  The 
heat sink area and design will determine how many watts or calories it 
can dissipate at a given ambient air temp and the data should be 
available. So the two data points become the area for getting rid of the 
heat on the device (pretty small) to the spreader and a large enough 
heat sink to get rid of the can always add fans.


Roger (K8RI)

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