[Amps] New NXP BLF578XR 1200W LDMOS FET is "indestructible"

[jeff millar]

This email describes what I've learned from looking at these modern transistors 
for the last few months.  It seems to me like a fundamental crossover point has 
been reached.  Silicon processing technology has produced rugged VHF/UHF 
transistors, with reasonable impedance matching for a reasonable cost, 
especially considering all the other costs associated with an amplifier.  These 
parts will only get better and cheaper as time goes on.

Brian, WA1ZMS, brought a dual MRFE6VP61 on 2 meters (2 x 1250W) to the last VHF 
contest.  The parts are $271 each at Digikey...

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MRFE6VP61K25HR5CT-ND

His amplifier also used a 1/2 inch thick copper heat spreader but he avoided the 
machining and polishing step by "soldering" the two transistors into the slot 
with a 4 mil film Indium foil, which melts at 156C (hot plate is your friend). 
That fills the voids better than thermal grease and even avoids the need for 
clamping or mounting hardware.

Brian's amp spread the heat through the copper plate into the 1/4 inch aluminum 
plate that formed the bottom of the amplifier and then he attached a water 
cooled plate to the bottom outside the amp.  The water lines circulated in a 5 
gallon bucket for thermal mass.  A separate cooling loop circulated water into 
some fan cooled transmission heat exchangers to dump the heat into the air.

We talked about all that metal for heat spreading and I said that he really 
needs to run a thin copper water pipe directly under the transistor to put as 
little thickness as practical between flowing water and the transistor.  After 
that any extra metal for spreading is not effective or necessary.  That could 
save a lot of size, weight, and cost.

One of the big advantages of transistor vs tubes is all the heat comes out at DC 
and RF ground potential.  Water cooling is a lot easier without HV on it.

Another advantage of modern FETS...A 50V, 56A power supply costs $25 on ebay 
because server farms dumped thousands of them...includes power factor 
correction, no blinking lights in the operating room.

The NXP device at $700 may be fully rugged, but the Freescale device is rated to 
60:1 VSWR for 10 msec pulse.  That provides plenty of time for a controller kill 
the drive or switch off the supply on detecting high VSWR.

The transistors have about 27 dB of gain and only require a few Watts of drive. 
  They can handle drive without any supply voltage without damage.

Operating the amplifier was a very odd experience, since we were on a mountain 
and outside was cool. The amplifier started out at tap water temperature and 
cooled off into the night.  After hours of EME operation, the amp felt COLD to 
the touch.  In retrospect, amplifiers need to operate a bit warm just to prevent 
operating below the dew point and condensing water out of the air.

jeff, wa1hco

On 07/23/2011 10:14 AM, Mike Tubby wrote:
>
> We (Steve G8GSQ) and myself are in the process of putting together a 2m
> amplifier with two BLF578 devices (not 578XR) at the moment.
>
> Big heatsink = 350mm (L) x 250mm (W) x 80mm (H) and two copper spreader
> plates 95mm x 125mm x 9mm machined down from copper bar (4" x 5.5" x
> 3/8") by Simon M0SKC.
>
> The hardest part was getting the copper bar and heatsink flat!  Simon
> had to make several tools for his Bridgeport milling machine to get a
> decent level of flatness and to cut the 1.4mm slot in which the device
> sits, this was then machined down to about 4 thou and then polished with
> toothpaste and jewlers rouge and the devices "lapped in".  That and a
> very fine (thin) layer of Arctic Silver heatsink paste does the job...
> its really just attention to detail...  (reads from Zen and the art of
> motorcycle maintenance)...
>
> Remember that an amp built with these devices should have an efficiency
> of around 71-72% so at 1KW DC input you'll have something like 720W of
> RF output and 280W of heat output, or in our case double that, but a big
> heat sink, good copper spreader and excellent device junction and you're
> okay!
>
>
> Mike