On 7/22/2011 4:16 PM, Paul Decker wrote:
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> Over the years I've heard many people say that adequately cooling a
> transistor is difficult because it is so small. It's really difficult for me
> to wrap my brain around that statement though after playing with the
> 3cx400A7/3cx800A7 series tubes. If one were to take the anode cooler off the
> tube, they would end up with a piece of ceramic about 1.4" in diameter and
> about .75" high. This is on par with the size of these transistors.
Transistors are limited to a much lower temp than any part of a power
tube, so even if you could get the same kind of connection, you would be
unlikely to be able to keep the transistor cool enough.
Also there is a delta T within the transistor material and silicon has a
poor thermal resistance compared to copper, brass, or Aluminum.
Transistors, including FETs are basically constructed on a Silicon
substrate. Insulating layers which are likely oxides, but may be
nitrides or other compounds are *difused* into layer(s) of Silicon.
Capacitors may also be included in the power transistor's design.
Connections to the Emitter, base, gate, source, drain...what ever they
are called for that particular transistor are deposited onto the surface
of the exposed portions of the transistor. External connections to
these "pads" may be soldered, or just pressure points.
The problem is that in general the poorest portion of thermal transfer
from the Silicon die to the outside world is the transistor itself.
Exotic heatsink transfer compounds can lessen the delta T between the
device while heat spreaders can speed the spread of heat through the
heatsink. Thermal pipes/heatpipes actually work quite well, but they
still have a limited capacity. Refrigerating the heatsink or heatsink
and spreader also has distinct advantages and one major disadvantage
which is condensation. You can not cool the heatsink much prior to
applying power to the device or you will get condensation, but the
transistor has very little mass or thermal mass. This means the internal
workings get hot fast with power, but the greater the temp difference
between the transistor and the heatsink the faster it will dissipate
heat and the cooler it will keep the transistor. Unfortunately there is
a limit to how cold you can make the heat sink as the transistor has
temperature limitations on the low side as well.
One problem with transistors has been the migration of the typing
materials that create the gates and junctions so their performance
deteriorates with time due to heat.
A tube will barely notice a heat spike that would destroy a power
transistor under the best of conditions.
Great strides have been made in developing rugged transistors. The NXP
BLF578XR appears to be an example.
So far high power transistor amps have required a lot of protective
circuitry for the fragile and expensive transistors.
Theoretically a couple of transistors that could run the legal limit,
key down, 24 X 7 could be manufactured for a fraction of the cost of an
equivalent tube.
With bipolar transistors gain is inversely related to frequency so
transistors rated for VHF and UHF have far too much gain to be stable on
the lower HF range. With these FETs I "would assume" (because I don't
know for sure) that the 10 MHz lower limit is *probably* due to the gate
voltage required at lower frequencies. It's likely that some one on here
has a much better background with FETs than I and can give a good answer
to that question.
73
Roger (K8RI)
> It seems to me the dissipated power per area is roughly the same when
> looking at the devices minus their coolers. What would be interesting is if
> a transistor manufacturer took a page from the tube world and integrated
> similar cooling.
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