New NPX LDMOS.... 1800w CW
http://www.nxp.com/products/rf/rf-power-transistors/rf-broadcast-and-ism/1-6
00-mhz-broadcast-and-ism/1800-w-cw-over-1.8-470-mhz-65-v-wideband-rf-power-l
dmos-transistor:MRFX1K80H
Ken
N4zed
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Saturday, April 22, 2017 12:38 PM
To: amps@contesting.com
Subject: Re: [Amps] Price per Watt Conversation
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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