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
On 23/07/2011 00:58, Baruch Zilbershatz wrote:
> Igot 4 devices 2 were 578XR and two had no printing at all on.
>
> Got them as samples so I have no idea about price sorry.
>
> As far as I can say this device needs cooling.
>
> But all that scary talk about super cooling is a little out stretched ( in
> Hebrew it sounds good)
>
> A good sized copper slab polished to mirror flatness and CLAMPING(!)
>
> The device with another pre shaped copper clamp is my method, works fine
> also for MRF6VP11KH,MRF6vp121KH that are similar devices by Freescale but
> not as rugged.
>
> All these had been tested and amps were built already.
>
> The issue is not the cooling but the matching of the devices at the output.
>
> At first a heat sink with copper inserts on a regular shape 4 sleeve trafo
> was used.
>
> The temp rise of the cores I had was a dead end road.the temp gradient was
> too high and I was lucky not to loose the Fet.
>
> I moved to TLT approach with a ferrite loaded coax at a 1;9 configuration
> with a balanced to un balanced 1:1output coax balun with 43 type ferrites
> loading it.
>
> Coax lengyh of all sections is 26.4 cm to stay away from the 90MHz upper
> limit that was my design goal.
>
> Ferrites should have enough cross sectional area "meat" in order to take
> the temp rise on that coa x
>
> I used CK type caps ,many in parallel in the RF pathes since I had no ATC's
> with the needed values.
>
>
>
>
>
> That is just the tip of that iceberg story of my affair with the 578
>
> Thanks
>
> 4z4rb Baruch
>
> Shalom from Israel
>
>
>
>
>
>
>
> 4z4rb
>
> Baruch Zilbershatz
>
> Nitzanay-Oz 118
>
> Nitzanay -Oz
>
> 42836
>
> ISRAEL
>
> phone: (+)972-052 8753838
>
>
>
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