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Ideal FET question

PostPosted: Thu Dec 07, 2006 10:19 am
by Transmitter Man
With many surplus 48v telecomms SMPS's around I'd like to ask the collective brain trust for their suggestions as to what they consider the best switching FET currently on the market.

Using the 100% voltage rating headroom and, to make things interesting let's say we want to operate not just on 160 & 80m but 40m or higher bands.

Up to 500w average/carrier output.

Your thoughts please.

Phil G4ZOW

PostPosted: Fri Dec 08, 2006 7:38 pm
by blackplasma
With pi*Vin*modulation*headroom appearing on the MOSFET, then perhaps you're looking at a 500V FET.

I like the TO-247 IRFP450 MOSFETs, rated at 500V, 14A, 330mΩ Rds
Here's the datasheet:

The input capacitance varies from brand to brand from around 2.2nF - 2.6nF.. I've used them up to 12MHz (25m), they feel very resilient, and they're very cheap from Digikey etc. At lower frequencies I've switched 350W+ with a single FET.

It probably doesn't qualify as "the best FET on the market", but I find them economical and reliable.

PostPosted: Sun Dec 10, 2006 7:52 pm
Phil, there are many power supply FETs that you can use. I prefer the larger devices such as the IRFP32N50K. I recently completed a 1500 watt carrier RTTY amplifier using 10 of these devices yielding 90% efficiency on 40 meters. This amp used transformer drive (see some of my previous spots) and required only 30 watts drive for 1500 out (17db).
It also gave a near perfect 50 ohm match at the input.
Floyd WA2WVL

PostPosted: Mon Dec 11, 2006 2:18 pm
by Transmitter Man
Thanks to both of you for your thoughts.

I wonder, have either of you posted your designs anywhere on the web?

I should like to take a closer look?

I could not find any data on the IRFP32N50K Floyd. I wonder if you can point me to any links?

Many thanks.

Phil G4ZOW

PostPosted: Mon Dec 11, 2006 7:40 pm
Phil, If you go to Google and search for IRFP32N50K you will be able to download a copy of the data sheet. These devices are about $6 in the US but similar devices can be used. 32A 500v 475Watt Diss. Also if you search this forum for all of my spots you will find lots of info including the measured input impedance at 160 80 40 and 20 meters. With transformer drive large input capacity has no effect on driving or efficiency since it is series tuned out"in mass" at the 50 ohm point.
I will be glad to answer any questions you may have.
Floyd WA2WVL

PostPosted: Wed Dec 13, 2006 5:06 pm
by cableman
Interesting Fet Floyd!
It is here in Europe better available than the FQA11N90.

I am interrested in a 100Watt carrier design with that fet for 40 meter.
And i think some more people who want to start.
I am stuck here with 12 watt from the IRF510
100 Watt would be a nice power,and reduces a lot of costs(f.e. powersupply)
50-60V switched powersupply's of 6,7A are very cheap here(appr.$40)
Maybe it is possible to reduse carrier-voltage to 25-30V and use some more fet's to get the 100W

Please can you describe what you would do to create this power.
I do have 20 FT core's Steve is using and a lot ATC-100B caps.
Maybe you can tell some tips for drive,carrier voltage,amount of fet's, amount of cores in output.
TTL square is easily made by a Cypress-clockoscillator i used before.
I can draw instandly a PCB for it and post it here.
I think this power can still be made on a PCB

PostPosted: Wed Dec 13, 2006 8:00 pm
Phil, one way you might approach a new design is this:

1.Current designs on the Forum seem to be for PEP (peak envelope power) equal to 5 times carrier power (500w for 100w carrier)
2.Determine peak modulator voltage available and design for 500w at that voltage. Carrier DC voltage will be about 1/2.4 times the peak modulator voltage.
3. Peak modulator current might be 600/Ep (allowing for 600w DC )
4. If peak modulator voltage was 70v.
DC carrier voltage = 29.1v
5. Design the RF amp to deliver 100 watts at a supply voltage of 29.1v
6. Two pair of IRFP32N50K devices in push/pull will easily do this job.
7. 3/1 input transformers would give 10.2 ohms each or 41 ohms total input impedance for the amp. Zin of each device is 1.13+j3.56 @ 40 meters. 9x1.13=10.2
What do you think? Floyd WA2WVL

PostPosted: Thu Dec 14, 2006 4:07 am
by Transmitter Man
Hi Floyd,

As Cableman says the IRF fet's are more easily available here in Europe and relatively inexpensive so to are plenty of surplus as well as new 50v SMPS's.

Was someone going to produce a PWM board as I seem to remember reading something a while back. Is one in developement?



PostPosted: Thu Dec 14, 2006 9:12 am
Phil, is there any way to modify an SMPS to become a modulator?

PostPosted: Thu Dec 14, 2006 9:23 am
Phil, if I had your Email I would send you a schematic for a 500 watt pep amplifier.

PostPosted: Thu Dec 14, 2006 9:46 am
by Transmitter Man
Hi Floyd,

Use this:

Many thanks.

Phil G4ZOW

PostPosted: Sat Dec 16, 2006 8:19 pm
by Square Wave
@ blackplasma, you have any schematics for the IRFP450? i have a few of those overhere.


PostPosted: Sat Dec 16, 2006 9:04 pm
by blackplasma
hi John
I will draw up a schematic for you this evening, though I warn you it's a little unconventional -- class-E drive goes straight into my primary (coupled to air-cored resonator) in series with the drain circuit; no matching network ^^

edit - schematic as promised:
(**clicky to enlarge**)[/url]

let the criticism begin :D

PostPosted: Sun Dec 17, 2006 11:35 am
by Square Wave
Thanks Black plasma. i hoped on something simple like this
can't you drive the fet directly with the fetdriver? i'm just a beginner heh.

PostPosted: Wed Dec 20, 2006 6:12 pm
by blackplasma
The drivers are transformer-coupled to the FET like that to enable an appropriate impedance transformation to swing it at 13.56MHz (needs lots of current).

There's also a typo on my schematic -- that's supposed to be a 6:1 transformer going to the FET. On the primary you have 30Vp/p (which can be increased if you raise the Vcc on the IXDD414s), and on the secondary you have 5Vp/p. Now you may float the FET's gate at 6V if you dare, and if you're lucky it resonates with a Q of maybe 1.2 - 1.5 (at 13.56MHz).

I've tried to push IFP450s to 16MHz, but couldn't do it, at least not without a DEIC420 or something.