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

[Gary K9GS]

Hello Jeff,

Any pictures available of the copper heat spreader, aluminum plate, etc??

On 7/23/2011 10:34 AM, jeff millar wrote:
> 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
>
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-- 


73,

Gary K9GS

Check out K9NS on the web:  http://www.k9ns.com
Greater Milwaukee DX Association: http://www.gmdxa.org
Society of Midwest Contesters: http://www.w9smc.com

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