50MHz 500W IRF510 based Amplifier.pdf

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50MHz IRF510 Amplifier
http://frenning.dk/OZ1PIF_HOMEPAGE/50MHz_IRF510.htm
50MHz 500W IRF510 based Amplifier
This amplifier project was based on a prototype circuit developed for the HF-bands by OZ3SW, Steen Møller. The HF
project was eventually scrapped because of widely varying gain from 160m to 10m.
PCB now available from
Far Circuits
(see below)
It looked sufficiently interesting, for me to mature the basic circuit design into a singleband 50MHz amplifier.
The basic building block is a 4+4 push-pull configuration, biased at 500mA, delivering 250W out for 10W in at
40V/10A. To make life easy on yourself, use devices from a single batch, as these are most likely to be closely
matched. I've used devices from Intersil (Fairchild). These are no longer being produced, but there should be an
ample supply in the distribution channels. The FQP7N10 device looks to be a very suitable (and somewhat better)
alternative, but I have not been able to make them work as a direct substitution, I suspect that they have a somewhat
higher gain figure, and would need a lower S/D V to work properly. Frankly I don't have the patience or incentive to
experiment further along this line, as my present amplifier works very well and exhibit no problems whatsoever, I
even have a small supply of spare IRF510 from the original batch - should something untoward happen!
The Vishay
IRF510PBF is as the original ones used, with a Drain dissipation of 43Watts, but have not been tested by me.
The individual IRF510s are mounted on the heatsink using Aluminium Oxide heat-transfer isolation bricks and
silicone grease, for sufficiently low coupling between the MOSFETs and the heatsink to insure stability.
The type of Aluminium oxide wafers used to transfer the heat without introducing a large capacitor is a
Fischer Elektronik GmbH (Germany) type AOS220.
I bought them at Arrow Electronics Nordic at .20£ a piece for 100 pieces a couple of years ago.
The input transformer (9:1) uses 4C65 cores, and the output transformer (1:4) uses thin air!
L4 is just a choke, and thus fairly uncritical, a couple of turns of the supply wire on any available piece of ferrite, see
the photos, it's just below the eletrolytic cap. on the module photo.
1z7
14.03.2021, 23:10
50MHz IRF510 Amplifier
http://frenning.dk/OZ1PIF_HOMEPAGE/50MHz_IRF510.htm
T1 is made by threading a piece of wire three times (the primary) through a piece of coax outer conductor (the
secondary) around four 4C65(*See note below on core material) cores glued together using Cyanoacrylate
(Superglue), thus creating a 9:1 impedance transformation and a very reasonable input match. See also fig. 4 & 5 in
AN749
The output transformer is made from two pieces of teflon coax (RG316) 14.5cm in length, the outer conductors joined at the middle and the
ends, for the primary, and the inner conductors in series as two windings (the secondary) giving a 1:4 ratio and a close match to 50 Ohms
(fine tune with C41), the B+ supply must be as near the midpoint of the primary as possible!
*I've used 4 x Ferroxcube core Material TN14/9/5 (4C65); Part No: 4322 020 9718
an equivalent Amidon would be Material Type 61 Part No: FT50-61
A very thorough description of construction of HF-transformers can be found in :
Motorola Application Note AN749
The combiner used, is a simple 1/4-lambda combiner made from 75Ohm Coax, input splitter made the same way.
The end result is then passed through a
low loss Elliptical LP filter
with a 70dB notch at 2nd harmonic and 80dB at
3rd.
Bias is 500mA pr module; IMD3 has not been measured, but reports on the band have been uniformly good.
More than 2000 QSOs have been made so far with this amp. Only mishap was during an F2 opening to NA in 2001,
when i forgot to reduce tranceiver output to 20W, I still made 4 and a �½ QSOs before things went avry (12 of 16
IRF510 blew loudly and smelly) These were quickly replaced (at less than 30c a piece!) and no one ever commented
on the output, despite the vast overdrive.
This event taught me never to rely on manual throttling of drive to an amplifier - things will go wrong, and Murphy
taught us that it'll happen when most inopportune, e.g. when in the middle of a humongous pile-up for the rarest of
DXs! This amplifier now has a power attenuator built-in, to reduce the 100W from my FT-847 to the 20W required
for full output. (As do my other amplifiers - all of them!) An added benefit is the reduction of splatter produced by the
overshoot from the tranceivers output regulator, the more you throttle back the worse it gets - some modern
tranceivers produce spikes of more than 150W on VHF - and the FT-847 is far from the worst of the bunch!
You can, of cause, combine an arbitrary number of the 250W modules to obtain your desired output level, my reason
for deciding on 500W was the availability of a suitable powersupply.
Schematic here (in PDF format)
2z7
14.03.2021, 23:10
50MHz IRF510 Amplifier
http://frenning.dk/OZ1PIF_HOMEPAGE/50MHz_IRF510.htm
PCB lay-out (click to obtain full resolution image), Size of PCB: 118mm x 85mm split along the horizontal line.
Etched on double sided material. All drill holes for component mounting to be counter-sunk on the back-side, the rest
are being used for "through plating" to connect the ground areas on both sides of the PCB.
Far Circuits (http://www.farcircuits.net/ ) have now (Feb. 2012) manufactured a set of PCBs, complete
with eyelet inserts for connecting front and back, selling for US 18$ for the set.
The finished module (since the PCB design, I have added the bias circuit in the upper right corner - dead bug fashion)
Click to see hi-res version. The small circuit added to the heat-sink, next to the bias circuit, is a fan-speed controller
(see
elsewhere on my homepage)
The completed amplifier (as usual, click to get hi-res version)
The large finned object is a surplus commercial 800W Switch Mode PS. On top of that the Elliptical LP filter. The
small PCB over the PS, contains 12- and 24V regulators and changeover relays (not seen, but cheap open frame 20A
Washing machine type, but still better than -20dB return loss at this frequency!) The grey RS-232 cable connects
contol LEDs on the front to the PS.
The coils of coax cable are the splitters and combiners
3z7
14.03.2021, 23:10
50MHz IRF510 Amplifier
http://frenning.dk/OZ1PIF_HOMEPAGE/50MHz_IRF510.htm
Last, but not least, the whole thing - ready to plug in - weighs just under 12.5Kg (< 30 pounds, if you don't
understand metric), ideal for /P use or DXpedition travel - and that's with the present heavy duty steel cabinet, I could
probably shave off another 3-4 Kg with an Aluminium enclosure.
ADDENDUM July 2006:
I've had a small, but sometime very annoying, problem right from the start: In Rx mode I had a small but just audible
broadband noise on a very quiet band, no problem when working a contest, ES-opening or F2, but bad enough to be a
problem when working very weak signals in JT6M or JT65 modes. Just recently i was hunting a very rare Norwegian
square, and had to turn off the amplifier during recieve periods in order to be able to decode him at all! That's when I
decided to finally do something about it!
It quickly turned out to be RF-Hash coming from the SMPS and the Fan, but various attempts to get rid of it it proved
fruitless, and, I realized that, instead of curing the decease, I had to do something about the symptoms. The noise
entered my signal path at several points internally to the amplifier (demonstrated by taking the two coax-cables
connected by a female-female adapter, routing this through the amp, with no detectable noise when the amp was
turned on. The main culprits were determined to be the open frame RF-relays and coax joints done on the cheap at
various points.
I thus decided to completely redo internal RF-cabling, using high quality coax relays, cable (RG400) and crimped
N-connectors throughout. I also decided to redo the 25V and 12V power adaptation, and added a transistorized PTT-
circuit while I was at it.
I'm happy to report that the rebuild has been a complete success, I can no longer measure or hear anything coming
from the amp in Rx mode (at least below -100dBm).
4z7
14.03.2021, 23:10
50MHz IRF510 Amplifier
http://frenning.dk/OZ1PIF_HOMEPAGE/50MHz_IRF510.htm
The modified internal layout!
Addendum Jan. 2010:
It seems that OH7LMQ / Timo has solved the oscillation problem with other/newer IRF510s. With Timo's permission
I quote:
Hello
I contacted you about month ago, for the 50Mhz amplifier oscillating issues. The reason of this
message is to share information and experiences, if some builder asks for help.
Looks like i have found a remedy for the parasitic oscillation problems, which may occur with
vishay brand irf510:s.
I did install 4r7 SMD resistors in series to gate lead, right to fet:s gates, and that was right for the
money ! Now the amp looks like it is full stable at any bias voltage, what i have tested few hours.
5z7
14.03.2021, 23:10

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