> 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.
That doesn't account for thermal resistance (heat transfer between
substrate and water). Although the heat transfer issues from substrate
to spreader/heatsink are the same in both cases, the issue still comes
down to transfer from the substrate/die to the cooling medium.
I suspect (without trying to do any calculations) that circulating
water and an appropriate radiator would be more effective than the
standing water/condenser or vapor phase cooling simply because the
circulation would provide more *effective volume*.
In any case, it would certainly be interesting to experiment with 8 ea.
VRF-2933 (Quadra or PW-1 with MRF-150 replaced by VRF-2933) or a pair
of Freescale/Phillips 1250W modules with water cooling. The only real
issues are cooling system design and control/protection circuits are
well documented.
With 1 dB compression levels in excess of 2 KW, typical IMD of -35 dB
or better and minimum gain of 20 dB (before splitter/combiner and
gain leveling losses), given proper cooling those devices should be
more than satisfactory for "legal limit" amateur service.
73,
... Joe, W4TV
On 5/1/2012 10:59 AM, Paul Christensen wrote:
>> "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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