[Amps] Combiners for SS amps + cooling issues.

[Leigh Turner]

In answer to your question Jim, my favourite RF power MOSFET today for HF
linear SSB amplifier use is the VRF2933 devices from Microsemi. They have a
VDD drain breakdown rating of 170V which means operation with a 50 V power
supply they have a good margin of safety...a factor circa 3X will ensure
robustness against most mishaps.

Although seemingly compelling at first thought, personally I'm not a fan of
single high power RF devices like the NXP BLF578XRS. Rather than a single
big device like that, I instead like and prefer the idea of multiple smaller
parallel/push-pull 300 Watt RF power devices to share the work load and more
evenly distribute the massive heat flow from the die and mounting flanges
into the copper spreader and heatsink system. RF power devices tend to
prematurely fail when their thermal aspects are not properly attended to,
rather than a failure attributed to any load / VSWR mismatch.

The big ARF1500 devices as use by Tokyo High Power in their HL-2.5Kfx amps
have a 500V VDD breakdown and can be run from a 125 V power supply rail.

I would not consider schemes that directly rectify the mains to provide the
DC supply rail. For my two pallet combined 4 x VRF2933 QRO S.S. amplifier
modules for example, (total of eight VRF2933 FETS) I use a nominal rated 3
kW commercial 50V power-factor corrected switch-mode P.S. made by MeanWell
that delivers 63 Amps. These rugged well made EMC compliant units are
relatively cheap and widely used by the cellphone base-station industry.

Leigh
VK5KLT

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Jim Thomson
Sent: Thursday, 12 September 2013 10:37 PM
To: amps at contesting.com
Subject: [Amps] Combiners for SS amps + cooling issues.
Importance: High

++SNIP++

Leigh, what's the highest vcc you can put on any of these SS devices ??
It sure would be nice to just rectify the 120 vac from the wall plug, with
or without any isolation xfmr. 

I'll let you SS experts sort it all out..but it is exciting stuff.   In the
meantime, I will stick with my 3CX-3000A7 / YU-148 ceramic GG triodes. 

Jim  VE7RF
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