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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