Project 'YASSA' - Yet Another Solid State Amplifier
June 2022: This amplifier is an experiment with the MRFX1K80 LDMOS FET to produce a compact 6m amplifier that will pair with my IC-705 ... both as a 'second 6m station' at home, and in a '6m DXpedition package'.
Having heard about 'blown up' LDMOS devices due to an antenna accident or trying to get 'the last mW' (PA running at '101%'), perhaps an analogy about cars is appropriate? They can do 180 km/h, but are safer and more reliable if driven at 100 km/h ... so, while the MRFX1K80 in this project is a 65V 1800W device, it will be run at about 50V and 1000W for safety and reliability.
In addition, the MRFX1K80 "N" variant was selected rather than the more commonly used "H" device due to better thermal properties from its Over-Molded Plastic package (OMP). Here is a comparison ...
... and a $30 lower price tag (the same 'stuff' inside, just better and cheaper). What is not to like?
The compomise is that the OMP format is a little more difficult to solder to the copper heat spreader, but NXP/Freescale's AN1907 'Solder Reflow Attach Method for High Power RF Devices in Over--Molded Plastic Packages' has helpful tips.
The heat sink chosen from my junk box for this PA is only 210mm x 200mm x 50mm (8.5" x 8" x 2") to keep the amplifier's overall size and weight down. To compensate, two high volume fans will assist with the cooling.
As always, the tedious part of an amplifier build is the metal work, so that is being done first. A case to fit the heatsink is under construction using salvaged aluminium and hand tools ...
Copper heat spreader block drilled and polished to an almost mirror-finish ...
The MRFX1K80N device soldered to the copper heat spreader block ...
October 2023: The first test on 6m with a simple 1:9 output transformer was not satisfactory. After adding compensation capacitors to the output transformer the amplifier only produced 660W output at 55% efficiency with a 48V supply. As they say, 'more work to be done' ...
