Cooling System

Project Gamma

Cooling System

Requirements:

Maintain transistor junction temperature below 200° C. at:

1500 watts output
45% efficiency
25° C ambient air temperature
100% duty cycle (optimally, may require an additional external fan)

Do not require a constant supply of cooling liquid

Nice to haves:

As quiet as possible
As small as possible
As light as possible

Design Philosophy:

We use a forced air cooling system due to the cost and maintanence hassles associated with a liquid based system. We use copper heat spreaders to move the heat away from the transistors rapidly, and a large extruded aluminum heat sink to transfer the heat into a continuous flow of air. In order to avoid air being blown into the face of the operator, and in order to facilitate the addition of a second (external to the cabinet) fan for RTTY use, we orient the air-flow sideways in the chassis.

Project Gamma Cooling System

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Components not assembled - just for display

Detailed Design:

The transistors on the RF amplifier modules are mounted using 4-40 x 1/2" screws with 2 ea. split lock washers into holes drilled and tapped in a 6" x 8" x 3/8" copper heat spreader. The heat spreader is sourced from CCI. Arctic Silver 5 thermal compound is used between the transistor bases and the copper.

The heat spreaders are mounted two up (horizontally oriented) using #8 x 1" screws through clear holes to 16" pieces of Wakefield 2199 extruded aluminum heat sink.

The heat sinks are mounted face to face, with a 120 mm square fan pushing from one end. They are screwed onto the bottom of the chassis using #8 screws, and a piece of thin sheet metal in screwed into the tops to form a tunnel.

Quick and Dirty Thermal Engineering Analysis:

Max transistor junction temperature is 200° C
Transistor junction to case thermal resistance is .6° C / watt
Worst case efficiency is 45% (data sheet)
Power output is 94 watts per device (1500 / 16)
94 watts at 45% = 120 watts of heat per device
120 watts at .6° / watt makes ΔT between junction and case 72°
200° - 72° means case temperature must be below 128° C
MRF-150 base is .18 square inches
Arctic Silver 5 has a thermal resistance of .03° C / watt / square inch
.03° x 120 watts / .18 square inches makes ΔT between case and spreader 20°
128° - 20° means spreader surface temperature must be below 108° C
Spreader is .375" thick, 2.75" from edge of heat source to edge of spreader
Thermal resistance of spreader is .055° C per watt per inch
Avoiding the hard math, assume 120 watts in each of four directions
.055° C / W * 120 watts * 2.75" makes ΔT to the edge of the spreader 18° C
So worst case we can assume that the bottom of the spreader needs to be 108° C - 18° C = 90° C
6" x 8" = 48 square inches contact area between the spreader and the heat sink
48 square inches at 480 watts at .03° C / square inch / watt makes ΔT spreader to sink = .3° C - ignore it
So we need a heat sink that will keep its face at 90 ° C at 480 watts
90° C - ambient of 25° C equals 65° C
For ease of calculation, assume that each transistor gets its own piece of heat sink 2" long
65° C / 120 watts means that each 2" piece of sink needs a thermal resistance of less than .542
Aavid #64315 meets the requirement, each 2" piece has a thermal resistance of .5 at about 420 LFM of air flow
In a 6" square tube, 100 CFM = 400 LFM, so 150 CFM (allowing for some back pressure) should be enough
Wakefield#2199 is identical to Aavid #64315, and available from stock at a reasonable price, so we use that

These are the old notes from the olde web site , left here for reference only.

Notes on project Gamma - Cooling

It is pretty clear that there are two alternatives for cooling this beast, forced air or water. Let's look at them separately.

Forced air is the "traditional" approach. Each EB104 board gets mounted on a heat spreader, made of copper, to quickly get the heat off the parts. CCI sells a suitable spreader for $24. I looked into just buying pieces of copper from McMaster Carr, but it appears to be much more expensive to do that. Once you have a heat spreader, then you need a heat sink. CCI sells a 6.5" x 12" x 1.6" heat sink (their #99 ) for $22. This sink is 54 square inches per inch of surface area, and has a thermal dissapation factor of .79 . It weighs 3.6 lbs. I spoke with them on the phone, and they claimed that this sink was sufficient with forced air at an unspecified volume/rate to cool an EB104.

I'm concerned that there isn't much in the way of thermal discussion on the CCI website, and the person that I spoke with on the phone didn't seem too technical. Insufficient heat disappation would be a great way to ruin this project...

Aavid is one the leading manufacturers of heat sinks. They make a sink (#61785, extruded profile) that is 10.78" x 3.77" x whatever length you want. Thermal resistance at 18" length is .31 , and its surface area in square inches per inch is 92.1 . However, it weighs 16 lb/ft which might be an issue :-)

The Aavid #62725 sink, also extruded profile, is 9.75" x 2.28" by length, and weighs 7.1 lbs/ft. Budgetary price from Arrow is $107 for an 8' bar - part number 627251F0000. At 18" length its thermal resistance is .29 . You can see its thermal properties here. From the graph in the lower left of the page, it appears that with a 100 CFM fan in a channel 3" high by 10" wide (assuming that we box in just this one sink) we get 2.438 meters/second of airflow, which would drop the sink temperature to less than 25 degrees C above ambient. If you believe that the ambient air is in the range of 40 degrees C, that gives you a 65 degree C heat sink, which is fine.

On to the question of the mechanicals. I think we need a commercial enclosure so that everyone can easily repeat the mechanical design. One option is the Basic Kit 19" rack mount enclosure from Techmar . They quoted me $165 for an 8.5" usable height by 24" depth by 16.75" usable width rack mount all aluminum chassis. It is available with perforated bottom, top, or sides.

Mouser carries BUD aluminum rack mount chassis. The 8.5" high by 22" deep ones are $83 quantity one without top or bottom. The top/bottom is $33 each, so the total equivilant cost (to the Techmar) is $149. That doesn't include handles. Internal chassis shelves of various widths are available too.

Another enclosure option is Protocase . They make "custom" cases - you pick a style (U shape or rack mount seem to make the most sense for us), specify cutout locations and the number and locations of captive connectors, and they make up the enclosure for you. They quoted me prices of around $240 for a suitable (24" x 17" x 8" U shape with 10 cutouts and 10 captive fasteners) enclosure. The big benefit here is that we get the mounting holes for the display, buttons, connectors, etc. cut to our specs - and thus we get a very professional look.