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
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