..modern RF transistors are enclosed in a so called cavity package: the
upper side of the die and the top cover are separated by an air gap. The
vast majority of the heat is transferred through the mounting flange, not
the top of the case.
Besides the temperature mentioned in the data sheet is the junction
temperature, not the case temperature. The latter is usually a few tens of
degrees centigrade lower that the maximum junction temperature.
Vapor phase cooling was in vogue for tubes mainly due to the danger of
humidity and high voltage in combination. Cooling water contaminates very
quickly in an amplifier due to ion decomposition. The situation is slightly
better with vapor cooling but still, it's an invitation to arcing problems.
For amateurs there's nothing better than air cooling and a hefty heatsink.
At most, what we can reasonably use is a mounting plate drilled for water
circulation. This is exactly what Philips recommend for their BLFXXXX series
of power devices. It depend mainly on water circulation speed and the air
flow over the external heat exchanger.
Guys, remember, the fluid cooling is intended for situations where size is
critical: it's complicated, needs a lot of protection devices and circuits,
take up energy for blowers and pumps and so on....
All of these exotic systems were invented in order to solve specific
problems of size and/or heat transfer to a point more amenable to cooling.
It came to a point where an airborne system had the heat exchanger mounted
under the airplane's wing, while the equipment itself was inside the plane.
Do we really have this kind of situation?
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Paul Christensen
Sent: Tuesday, May 01, 2012 5:59 PM
To: amps@contesting.com
Subject: Re: [Amps] Tubes vs. Solid State (was) Expert
Amps2K-FA:AnyOpinions?
> "I doubt that ... the solid state devices would not appreciate the die
temperatures with vapor phase cooling. However, a small radiator and closed
water system would work very well. With a suitable pump the
radiator/reservoir/fan could be located outside or in another room for
essentially zero noise."
Joe, good point. It's an interesting thought exercise...
I looked up the maximum operating temperature for the Motorola MRF150 and
Microsemi ARF1500. From the datasheets, maximum operating temperature is
between 175-200 degrees C which is > 75 degrees C above the H2O boiling
point. As I recall, water can never exceed the boiling point temperature
under normal atmospheric pressure. Additional heat does not raise water
temperature but causes steam and steam too never increases beyond the
boiling temperature at normal atmospheric pressures. But under pressure,
it's a completely different ball game. Added pressure raises the boiling
point and the temperature of steam can get extremely high.
Assuming a set of transistors mounted onto a dense copper block (copper, to
avoid water contamination) and immersed in distilled water, it seems to me
that these transistors could operate well under their maximum ratings with a
lot of room to spare. Worst case is the temperature of the transistor
substrate could never exceed 100 degrees C, at least not very long. That
is, until water is boiled out of the reservoir. This cooling system needs
a means to monitor water level (as does the Alpha 70V) since it's so
critical to stay under maximum temp ratings.
A normal vapor-phase cooled system is vented to atmospheric pressure so
there's no danger of rising temperatures due to rising boiler temperature.
But as long as the transistor is immersed, the worst effect is generated
steam that simply moves from the boiler to the condenser where its cooled
and re-circulated in the vented closed loop. That's the normal cooling
function of a vapor-phase system. So, if I've got my facts straight, a
solid-state amp using vapor-phase cooling would utilize 1/17th the volume
when compared to a water-cooled system, thereby keeping the system contained
within a desktop size enclosure.
Paul, W9AC
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