Dear all,
I have been following this thread, and at some moments shuddered at what
some of you opine, and assented at what others write, and several times
I would almost have replied to something, to set it straight, but always
somebody did it before I got a chance.
Interesting thread, but I can't help the impression that most of the
people who give their opinions have never in their life actually
designed some piece of equipment, let alone a high power solid state
circuit of any sort. And many are also a bit weak in the field of vacuum
state power electronics.
I would like to tell you that I have been working over the last months,
as a background hobby work, on a project of a legal limit HF solid state
power amplifier. At this time I'm getting a pretty clean 1200 watts
output, and saturated 1700 watts, from 160 to 15 meters, with slightly
less power on 10 meters, from a set of MOSFETs costing a total of 75
dollars. No, the number is NOT missing a zero.
I have tried several slightly different but generally similiar MOSFETs
from different manufacturers, so that if one device is pulled from the
market, I have a choice of others to fall back upon. And while I have
been working on this circuit, newer, better and even cheaper devices
have been showing up.
You might also be interested in learning that ALL of the parts for this
amplifier, including the power supply, heat management, protection, low
pass filtering, and automatic bandswitching, will likely end up costing
below 1000 dollars total. That's using all new parts. An antenna tuner
is NOT included in this amplifier, so if you want to use antennas having
an SWR higher than 1.5 or so, you would need an external tuner.
Working on my little pet project, and reading how people write that
solid state amps are not for the ham, or not for the home builder, I
tend to smile, and sometimes I tend to become slightly angry too. People
who say it can't be done shouldn't be discouraging people who are doing it.
Some of the last messages regarding water and vapor cooling have shown
that some people here don't understand the thermal specifications of
semiconductors. So let me explain: The maximum power dissipation, given
in the datasheets, in most cases refers to the maximum power a device
can safely dissipate AS LONG AS _YOU_, the designer/builder, make sure
that the case of the device will never exceed 25 degrees Celsius. In
practice that's almost always imnpossible, and for that reason the
actual power that a device can safely dissipate is always lower than the
rated power. This is not any sort of de-rating for safety or improved
life - it is instead an essential part of designing the circuit, to keep
the device operating AT (not below) its maximum safe internal
temperature, which should result in a reasonable lifetime. If you truly
derate it from this point, so that the silicon runs at significantly
less than 150 degrees Celsius (that's the typical silicon temperature
allowed for plastic-cased devices), additional (speak: essentially
unlimited) lifetime will result.
Vapor cooling isn't practical with solid state devices, because between
water boiling temperature and the device's internal absolutely maximum
temperature you have just 50 degrees Celsius difference, and this
temperature difference must push all the heat from the silicon to the
water. On its path, that heat will find the internal thermal resistance
from the silicon to the case, more resistance from the case to the
copper block, more resistance through the copper into its fins, more
resistance from the fins to the water, and then some, as the water is
stirred around only by the vapor bubbles. If you allow one half of this
50 degree gradient to the whole path from the device's case to the
water, and the other half just to the path from the silicon to the case,
then the maximum allowable power dissipation will be only ONE FIFTH the
rated power! This is because the rated power is based on having 125
degrees from silicon to case, and you are allowing only 25.
It's a bad bargain to use transistors in such an inefficient way, so
forget vapor cooling.
When using water cooling, you have basically the same thermal
resistances, except that the resistance from the fins into the water is
much lower, if you pump the water by fast enough to create a lot of
turbulence. But the greatest benefit of water cooling over vapor is that
your cooling medium might be just 30 degrees Celsius warm, instead of
100 degrees. This gives you much more than twice the temperature
difference from the silicon to the cooling medium, and thus allows
dissipating much more than twice as much power, from the same devices,
mounted the same way on the same copper block!
So there is no point in vapor cooling for solid state devices, as long
as we keep using silicon. When semiconductors come around that can work
safely at higher temperature (maybe silicon carbide?), vapor cooling
would become more attractive.
Vapor cooling comes into its own when your active device can work so hot
that the difference between almost cool water and boiling water is much
smaller than the difference between boiling water and the allowable core
temperature of your device. This IS the case with many big vacuum tubes,
but NOT with solid state devices. When you have a lot of heat to remove,
like hundreds of kilowatts, bringing in a small amount of water, and
moving away a large volume of steam (which is lightweight and moves
easily in a pipe) can be easier than bringing in A LOT of cold water,
and removing that lot of water after it has warmed up.
For my amp I'm using water cooling. Should I say "of course"? Maybe.
Silent operation was just one consideration. Another is that I just
couldn't find a way to move heat from my silicon to air, at the rate
needed, for a very simple reason: The thermal conductivity of copper is
finite! To dissipate a certain amount of heat, without making the air
VERY hot, you need A LOT of air. You can't make the air very hot,
because the heat sink must stay just lukewarm, in order to have enough
thermal gradient between the heat sink and the silicon, to move all that
heat over all those thermal resistances along the way. So, if you need
to evacuate a lot of heat, but not make the air more than lukewarm, you
need A HUGE LOT of air, and for that you need a large heatsink, in
addition to powerful fans. And a large heatsink inevitably has a long
path from the transistors to the average location in the fins! And that
long path has too much thermal resistance, even if it's made from copper.
One solution is using a great many small transistors, each of them
located directly on a fin of the heat sink, not on the baseplate 10cm
away. But that creates a difficult construction, and RF-wise it's not so
easy to have hundreds of paths, each up to 0.02 wavelengths long or even
more. Specially not at very low impedance.
So, it's much easier to use a watercooled copper block. The path length
from silicon to water is just 2cm or so, and the water can be kept
cooler, in average, than the air in a heat sink. The contact surface
between fins and water can be very much smaller than between fins and
air, and this allows using the small cooling block, with the favourable
short thermal path lengths.
Anyway, the kind of class AB amplifiers hams are mostly using
(regardless of whether it's solid state or empty state), and its
associated low efficiency, should be a thing of the past soon. As
LDMOSFETs for UHF and microwaves become more capable, a high efficiency,
low cost switching amplifier for HF is moving closer. The end result
would be a legal limit amp smaller than a shoebox, with its power supply
built in, of course, that can run key-down all day long putting 1500
watts into the antenna, while dissipating only about 100 watts, less
than what a normal HF radio dissipated while running 100 watts into the
antenna. Heat dissipation issues are far less important in such an amp.
Years ago I was experimenting in this line, but didn't get much beyond
40 meters with good efficiency and low distortion, simply because the
FETs I used were too slow. So I decided to develop my low cost,
watercooled conventional class AB MOSFET amplifier first, and perhaps
later on, if my head still works and any ham radio activity is left,
develop a switchmode amplifier.
Without any doubt, even when some day 95% of all hams are using highly
efficient solid state parametric synthesis amplifiers, there will be
some hams left saying it can't be done, and that tubes are best...
That's not to say that tubes don't have their advantages. If a 70 year
old ham has a 55 year old junkbox, well filled with quality parts
covering all history from the development of the first tubes up to 1970
or so, and this 70 year old ham is very comfortable with tube
technology, but has a hard time learning new solid state tricks, I won't
blame him the slightest bit for building a tube type amplifier, even in
2012! But for a young ham who hasn't much of a junk box yet, it would be
rather stupid to try and buy all the antique parts he needs, often at
collector's prices, to build himself an amplifier to an antique design.
He is much better off buying modern, low cost parts, and building a
modern amplifier.
I'm in between, so I think I understand both extremes! ;-)
73,
Manfred
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Visit my hobby homepage!
http://ludens.cl
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