Hi all,
just a quick comment about a point touched by Joe:
Several of the newer LDMOS designs are using *two* active devices
rated at 1500W each.
I'm not aware of any LDMOSFETs rated at 1500W output in linear
operation. I'm pretty sure that no such device is on the market yet.
Many hams are misinterpreting the datasheets of these devices. Let's
take, for example, the BLF188XR. Its manufacturer calls it a 1400 Watt
LDMOSFET. The application information in the datasheet gives several CW
application examples with output powers ranging from 1200 to 1400W, but
this is _not_ in linear service! Instead these power levels are obtained
by building class AB circuits and driving them deeply into saturation,
resulting in very nonlinear operation. This is fine for typical CW
applications, where CW stands for truly "continuous wave", not for Morse
code. It means applications such as FM broadcast transmitters, or
industrial RF power generators for welding and other such applications.
In this saturated class AB operation, the efficiency is high, like 73 to
85%. In terms of power dissipation, the worst case listed in the
datasheet is 1400W output at 73% efficiency. This results in 518 watts
dissipated as heat, which is about the reasonable limit when using
normal heat spreaders, heatsinks and mounting techniques.
Instead if you want to run a class AB stage in a linear way, you have to
keep the drive low enough to stay out of deep saturation. Under these
conditions, the efficiency tends to be around 50%. In theory a class AB
stage should be able to run at something between 65 and 70% efficiency,
depending on how much quiescent current is used, but MOSFETs (like all
devices) are not perfect, which reduces the efficiency somewhat. And
what's worse, the extremely low drain impedance of these 50V high power
transistors makes it pretty much impossible to properly couple the two
drains together, which is a requirement for correct class AB linear
push-pull operation. The usual, very poor implementations of the output
transformer and feed arrangement result in a further reduction of
efficiency and increase of IMD. That's why practical broadband linear
amplifiers using low impedance transistors often struggle to achieve
even 50% efficiency, at acceptable linearity.
So, you can apply the old rule that in class AB linear service you have
to dissipate about as much power in heat, as goes into the antenna.
Generating 1500W output will generate roughly another 1500W in heat.
And here is the core point: There is no practical way on earth to make
any currently available LDMOSFET dissipate 1500W without overheating!
That's why nobody can make a single device 1500W class AB linear amp yet.
Even using two of these devices, the thermal aspect is a bit marginal. A
very good heatsinking scheme is required, and the devices operate at
very high junction temperature, which counts against their reliability
and life expectance.
The mentioned BLF188XR has a rated internal thermal resistance of 0.1
K/W. For the metrically challenged among you, K/W means kelvin per watt,
and a temperature rise in kelvin is the same as a temperature rise in
old fashioned degrees Celsius. So, the rating means that at a given
power dissipation, the junctions will be one tenth as many degrees
Celsius hotter than the LDMOSFETs underside. The rest is up to the
system designer/builder.
The maximum acceptable short term junction temperature is 225°C. So, if
you wanted to dissipate 1500W from a single BLF188XR, you are
responsible for keeping the external surface of the device below 75°C.
At first sight this might seem possible, but once you do the maths
involved in calculating the thermal resistance of the device-to-spreader
mounting, the internal thermal resistance of the spreader, then the
spreader-to-heatsink thermal resistance, then the thermal resistance
inside the heatsink, then the fins-to-air thermal resistance at a given
amount of air motion, you will realize to what degree it's impossible to
satisfy this requirement!
And that would be just to keep the junctions at the absolute maximum
short-term allowable temeperature! For good reliability they should be
kept much cooler.
When spreading the 1500W dissipation over two devices, you gain in
several ways: First, each device can be allowed to heat up to 150°C at
its underside, before the junctions exceed the absolute limit
temperature. Second, you have twice the surface to extract the heat
from, and the two devices can be physically separated as much as you
want, if you build two pallets and use a splitter/combiner arrangement.
So you end up using two heat sinks, or two quite independent sections of
a single heatsink, each having to extract 750W of heat while having
permission to let the device heat up to 150°C. That's hugely easier than
having to extract 1500W with no more than 75°C device temperature!
How much easier is it? Well, assuming that the air temperature will
never be higher than 25°C, the single-device solution requires a total
device-to-air thermal resistance of 0.033 K/W, and that's impossible to
achieve. Instead the two-device solution requires each of the two
heatsinks to have a thermal resistance of 0.167 K/W. Five times less
stringent a requirement. It's still hard enough to achieve, but with
good engineering it can be done.
The newest upcoming high power LDMOSFET I have heard of is the
MRFX1K80H. It's rated at 1800W output in nonlinear CW operation. Some
people might think that it even has headroom when operating at 1500W!
Well, yes - but not in linear operation!
The internal thermal resistance of this device is 0.09 K/W. Only 10%
better than the BLF188XR. And it's the same size, so whatever heatsink
and spreader you use, it would have the same thermal resistance for
either device. So, if your super heatsink system achieves 0.1 K/W, the
total junction-to-air thermal resistance of the MRFX1K80H is 0.19 K/W,
against 0.20 K/W for the BLF188XR. Just a 5% improvement, nothing more!
And with less marvelous heatsinking schemes, the advantage brought by
the new higher power device is even less than that.
It follows that it doesn't pay to use the highest power rated devices
for dissipation-limited applications like class AB linear amplifiers.
It's much better to use two or more, lower power devices. Instead the
very high power devices come into their own when used in high efficiency
applications such as nonlinear FM amplification.
> They are not nearly as stressed as older
devices run into saturation and should provide significantly better
IMD than marginal designs like the Expert 1.3K.
As far as I know, the Expert 1.3K has a single device with a 50V supply
and a 1:9 output transformer. This allows it to produce roughly 700 to
800W before entering significant saturation. The fully saturated output
power of a typical amplifier is somewhat less than twice its maximum
unsaturated (linear) output. That explains the 1300W rating - it's the
fully saturated, nonlinear, high efficiency output power of this amp,
usable only in modes that don't require linearity. In SSB it should not
be driven beyond 700 to 800W PEP, or severe splatter will result. And at
that power level its efficiency will be roughly 50%.
At 1300W output it's likely that its LDMOSFET is dissipating _less_
power than at 700W output. In any case the dissipation is high enough to
heat the junctions very close to the allowable limit.
And a 1500W linear amplifier using two devices is pushing them just as
hard as the Expert 1.3K does when operating inside its linear range at
750W or so. An 1500W linear amp could be rated at roughly 2600W
saturated output power, if the power supply and output networks are up
to it.
Oh boy, I promised to make just a quick comment! ;-)
Manfred
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