"I don't think there is an upper limit. It's more a question of what's
practical. There is an increasing number of hams using homebrew amplifiers
that use 16, 24, 32, or even 48 cheap 1-dollar MOSFETs. Those FETs are much
like your IRF510, only for higher voltage. The IRF710 is a typical one used
in those amps."
I have 32 BJTs in my push-pull parallel design. Once a single 220 Watt
module was designed with lots of negative feedback, and its own
self-contained switching power supply, it was easy to splitter/combiner my
way to full power. The advantage of such design is that I simply don't
supply mains voltage to one or more modules, and I have instant lower power.
The disadvantage is that some power is eaten in the output combiner. Without
pre-distortion techniques, I have maximum 3rd order products 37 dB down. I
have a fan on each module, but they are thermally controlled in a PID loop.
At 2 kW into a dummy load, I can just hear them spinning. Another problem
with this design is that the inexpensive output transistors are now obsolete
- good thing I purchased a bunch although none have ever failed.
-Doug Ronald
W6DSR
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Sunday, April 23, 2017 11:04 AM
To: amps@contesting.com
Subject: Re: [Amps] Price per Watt Conversation
Hi Cathy,
> So would it be better to use 4 or or more devices with somewhat lower
> ratings for linear operation?
I do think so. Specifically I would look for devices designed specifically
for linear service. These have a relatively high power dissipation rating,
compared to their output power rating. For example an ancient Motorola
PT9780 transistor, of which I have a bunch in my junk box, is rated for 100W
RF output, and 350W dissipation. That allows it to survive infinite SWR at
full continuous power, by the way. The very well known and much used MRF150
is rated for 150W output and 300W dissipation. That's a bit tighter, so its
survival rating is for near-infinite SWR at 25°C case temperature, which is
impossible to maintain in practice. So it's NOT able to withstand high SWR
for long, without protection circuitry. And among modern LDMOSFETs, one
device that comes to mind is the MRFE6VP5600H, rated for 600W RF output and
1667W dissipation.
Instead the BLF188XR and several other modern LDMOSFETs are NOT designed for
linear use, and you can notice that in the very different ratio between
output and dissipation: The BLF188XR is rated for 1400W RF output but "only"
2000W dissipation. The MRFE6VP61K25H for 1250W output and 1333W dissipation,
and the new MRFX1K80H for 1800W RF output and 2222W dissipation.
Given that all these dissipation ratings are based on the case being kept
below 25°C, which is impossible in practice, they have to be adjusted to
realistic dissipation values by taking into account the case mounting
surface, the heatsink/spreader/fan used, and the mounting technique (greased
or soldered). As a rule of thumb, small devices can be made to dissipate 1/3
to 1/2 of the rated amount of power, while with large devices this gets
worse, like 1/4 of the rating.
So, the 100W output rated PT9780 has a realistic dissipation capability of
around 120 to 170W, which is comfortable for 50% efficient operation, and
has some headroom for overloads caused by high SWR. Instead the 1800W rated
MRFX1K80H can achieve perhaps 600W dissipation in the real world, so in 50%
efficient operation it's good for 600W output - and not for 1800W! But in
nonlinear, saturated operation, at 75% efficiency or better, it can do 1800W
output.
> Is there a cheaper option to use simpler heat sinks that don't have to
> be carefully machine, but push them less hard by spreading the power
> over a larger number of devices?
Yes. By using more devices, cooling becomes much easier. Usually it's good
economy to use more devices, because the higher cost for more devices is
more than offset by savings in the cooling system. Up to a point, of course.
This point must be determined for each case.
> What's the practical upper
> limit of devices that could be used before creating other design
> problems?
I don't think there is an upper limit. It's more a question of what's
practical. There is an increasing number of hams using homebrew amplifiers
that use 16, 24, 32, or even 48 cheap 1-dollar MOSFETs. Those FETs are much
like your IRF510, only for higher voltage. The IRF710 is a typical one used
in those amps.
Manfred
========================
Visit my hobby homepage!
<http://ludens.cl> http://ludens.cl
========================
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