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From: "Roger (K8RI)" <>
Date: Wed, 31 Aug 2016 04:19:03 -0400
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There should be hundreds of those heat transfer blocks (solid Copper) available at computer repair shops. Every time they install a new CPU that came with a cooler, be it water or vapor phase, there are pipe(s) into and out of the block. Ready made to hook up the water. Use chilled water close to freezing, just be careful of condensation.


Roger (K8RI)

On 8/28/2016 Sunday 2:02 PM, Manfred Mornhinweg wrote:

I wonder if a high power transistor could mount on a chill block sold for puck type very high power devices. For instance,

Let's take the first of those as an example. It's 2*2 inches, with a 0.008°C/W rating (but they don't say at what water flow rate and pressure). If you solder a typical high power LDMOSFET to it, you will be using roughly one eight of the mounting surface, which will make the thermal resistance increase nearly, but not fully, 8 times. It might leave you with 0.05 to 0.06°C/W of thermal resistance at the unspecified flow rate. And that's still jolly good! It means that when dissipating 600W, which is a typical ballpark value for one of these LDMOSFET devices working in ham service, the transistor's mounting surface will heat roughly 30-35°C above the water temperature. No practical air-cooled heatsink and spreader can be that good!

So this confirms again that water cooling is a good way to go, when using devices that give off a lot of heat from a small surface.

Of course, getting the RF into and out off the device with this cooler in the way is tricky.

I don't think this is a really big obstacle. I would solder the FET and two copper strips to the cooler, in one operation. Being brazed, it should have no trouble surviving soldering temperature. The two copper strips would run under the gate and drain terminals, and be full width. And then I would solder the FET and those straps to my printed circuit board.

Given the numbers above, it might actually be possible to use a single BLF188XR at full legal limit in linear service! That's not a possibility with air cooling, at least not with a good reliability.

Water cooling does have its complexities, but I think it's worth using it, when using these high power devices.

Is there a place where one can buy single quantities of such coolers, off the shelf? I mean, without having to contact a sales department, and trying to convince them to sell a single one to an experimenter.


With water cooling, why not sit the transistor over a pocket and
run the coolant directly against the flange? That's a method I saw
in production test at the Philips (as it was) factory.

The thermal resistance of a metal-water interface depends strongly on the flow velocity, and thus on turbulence. If you want to remove a kilowatt or so of heat from a surface as small as that of an LDMOSFET's flange, you would probably need a very high water velocity. That means a high pressure pump, noise, and risk of cavitation, erosion, and so on.

I haven't done the maths for this specific case, but just from feeling I would say that it's not practical. It's better to solder the LDMOSFET to a suitably machined chunk of copper, that provides much more water contact surface, along with the shortest and widest possible heat path through the copper. So you can use far lower water velocity, using just a simple, cheap, low power, quiet pump.


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