Jim,
## What about these freescale devices ? That company in OH land
manages to get 800w to 1 kw out of just one device. I saw on U
tube, 1 of em being used in a 1250w CCS FM broadcast TX, 88-108 mhz
variety.
That's in saturated service. Dissipation should be no more than perhaps
400 watts. That can be handled.
Also on U tube, was a 2.5 kw CCS eme 144 mhz amp... using 2 of em.
I would think that this one also operated in saturation, at high
efficiency. So it was not linear. Surely it was used in modes that don't
require linearity, like CW or some digital modes that use slowly single
tones. Was it that way?
The cooling used was unique on all 3 of them.
The biggest problem when one designs cooling systems for such devices is
getting the heat away from the small device. There is just simply an
awful lot of heat coming out of their small mounting surface. A copper
spreader is the usual answer, but to get the heat into the bulk of a
heatsink large enough, it might have to take a 10cm long path through
copper, and the device end of this path is narrow and tight. The thermal
resistance in this area is already so much that it severely limits the
maximum safe total dissipation.
I did a design where I got water flow as close as possible to the
device. The trick was getting enough water/copper interface surface,
with short enough in-copper path length.
As soon as the amount of heat is less, or the device mounting surface is
larger, or it's spread out among several devices, or the device can run
hotter, things get easier.
Then the 3 x 150 ohms are all in parallel to make 50 ohms. They are
rated at 800w CCS each. The air cooling was not up to snuff..and one
exploded. Plan B was to lower the fins face down into a tub of ice
water..problem solved.
But when it comes to power transistors instead of resistors, even that
trick is often not good enough. Instead you need copper, not aluminium,
and you need to move the water fast, to have enough turbulence to get a
low thermal resistance between the metal and the water.
And just a little further up the scale you near the point where the
copper between the device and the water is the main limiting factor
(other than the internal resistance of the device, of course). At that
point, it's time to say "enough"!
+ 69dbm was being used at the time.
And may I ask what's the connection between +69dBm and ham radio?
Dick,
M2 markets both a 6 and 2 Meter Monoband Amp, 1250 watts out, uses a
single device. Granted there's a time limit on AM and JT modes, but
there are no limits on SSB.
I just had a look at 2m version's manual. I see that it switches between
class AB and class C, depending on the drive envelope. In CW and such it
runs 950-1000W in class C, while in SSB it runs 1250W in class AB. But
drain matching doesn't change. I didn't check if perhaps they lower the
supply voltage when in class C. That's likely - otherwise I don't see
how they could get the 65-80% efficiency of teh rating!
Anyway, with 1000W output and 60% efficiency in class C, the FET might
be running barely in the green area, as long as the heat sink remains
cool enough.
But with 1250W in class AB, never! It has 1:4 matching, so it has a
swing of +/-88V at 1250W output, which at 50V supply is OK for a push
pull stage - but results in only slightly over 50% efficiency. That
means about 1200W dissipation at the envelope peaks, and an average
dissipation not very much lower, given that efficiency falls at lower
amplitudes. During sustained speech in SSB, this might result in about
900-1000W of dissipation - if the SWR is perfect. The device is rated at
1333W dissipation, if it is kept under 25 degrees Celsius. M2 activates
thermal protection at 90 degrees - I don't know if they measure this at
the heat sink, the spreader, how close to the FET, or the case of teh
FET proper - but certainly the mounting surface of the FET will be
hotter! Even if the spreader had zero thermal resistance, so that the
maximum possible temperature at the FETs mounting surface would be 90
degrees, that would still limit maximum dissipation to 900W - and that
is for 225 degrees junction temperature, which is higher than most
manufacturers would rate their devices to survive!
All this means that unless I'm missing something, this amplifier
stressed the MOSFET to its absolute limit when operating with a very
light hand (no speech processor, short transmissions, lots of RX time,
low ambient temperature). In any other case, the MOSFET gets stressed
beyond its absolute maximum thermal ratings.
I hope I am wrong, but I would expect those amplifiers to fail easily,
due to MOSFET meltdown.
> Look at the W6PQL site for info on cooling one of these LDMOS devices.
He solders the MOSFETs to a thick but rather smallish copper heat
spreader, and attaches that to a rather small heat sink.
Soldering is good, far better than using screws and thermal grease. The
spreader is thick, but too small. The edges of the heat sink will be a
several degrees cooler than the middle, wasting capacity.
Want to do some math? Let's do it, very simply and not very accurately,
but enough to get an idea:
The thermal resistance inside the device is 0.15 degrees per watt. This
part is easy, it's in the data sheet.
The thermal resistance of the soldered connection is probably low enough
to ignore.
The copper spreader seems to be about 1cm thick. The hard part is
estimating the thickness of the heath path through the copper. It starts
as small as the MOSFET is, about 3cm^2, and expands to about 50cm^2 at
the spreader's edges. Heat leaves it all along that path. The average
bit of heat might travel about 5cm through the copper - this is a coarse
approximation! Actually we should be using integral calculus here, and
that's beyond my range of most beloved hobbies. So let's take a 5cm long
copper path, that starts with 3cm^2 cross sectional area, and ends with
about 20cm^2, which is what it would have at the 5cm path length periphery.
A cube of copper, 1cm on each side, has a thermal resistance across it
of about 0.27 degrees per watt. The spreader starts with about 3 such
cubes side by side under the MOSFET, so that gives about 0.09 degrees
per watt. As we get farther away from the MOSFET, the path widens, and
resistance per length drops, all the way to about .013 degrees per watt
per cm at the periphery of our mean path length. The average will be
around .03 degrees per watt per cm along the path, it's 5cm long, so we
get about 0.15 degrees per watt total thermal resistance for the
spreader. Remember that this is VERY CRUDE, and it would be good to make
the exact math. If anyone knows how to do that, I would like to learn!
So, we have 0.15 degrees per watt inside the device, and another 0.15 in
the spreader! Now comes the interface to the heatsink, which will add
very little, because it's a large surface. Then comes the thermal
resistance of the heatsink, considering that we have to take a value
slightly worse than its rated one, because the heat isn't spread out
evenly over the full mounting surface!
That heat sink used by W6PWL looks like about 0.2 degrees per watt, if
it gets a good air flow from strong fans. Let's de-rate it to 0.25,
because of the non-even heat distribution. Again this is pure guesswork,
but based on some practical experience.
So we would have a total thermal resistance of 0.15+0.15+0.25=0.55
degrees per watt.
How warm will the air be? Let's suppose, at most 35 degrees Celsius. And
the MOSFET is rated to be able to work at up to 225 degrees. So we have
190 degrees difference. With 0.55 degrees thermal resistance, this
allows continuously dissipating 345 watts. And nothing more!!!
In ICAS one can go higher, because we can store some heat in the
spreader and heatsink and release it later. But we cannot go very much
higher, because some important part of the total thermal resistance is
very close to the MOSFET, even inside it, where the thermal mass is very
small! The heat storage within a few millimeters of the chip is great to
inflate the pulse power capability, but not the dissipation over a one
minute transmission! The spreader and heatsink can store quite some heat
for one minute, though.
So, this arrangement can dissipate about 345W continuously, perhaps 500W
for one minute, and well over a kilowatt for a few milliseconds. This
is enough for 1kW continuous output in high efficiency, well saturated
class C, well over 1kW in class E or class F, but only about 800W or so
PEP in brief, processor-less SSB when running class AB, and even less
when being as long winded on the air as I am in this post! ;-)
Don't forget that I did some gross estimations. Maybe I'm wrong. But
then I would like someone to PROVE me wrong, either by doing the precise
math, or by actually measuring the temperature of the MOSFET's mounting
surface (not the top of its case!!!) during full power operation!
The problem is that this kind of design leads to amplifiers that seem to
work just fine, but then tend to fail very easily, or to fail
consistently after just a few hundred hours of use, for the simple
reason that the MOSFETs are running far too hot.
Phew!
Manfred
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