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.
73
Roger (K8RI)
On 8/28/2016 Sunday 2:02 PM, Manfred Mornhinweg wrote:
John,
I wonder if a high power transistor could mount on a chill block sold
for puck type very high power devices. For instance,
http://catalog.chtechnology.com/viewitems/extruded-bonded-fin-heat-sinks/liquid-chill-block?&plpver=10&forward=1
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.
Steve,
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.
Manfred
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