Hi Dimitris,
What i forgot to say: Actually i can not recommend a class E for /p
purposes. A class E is a fine thing and it is fascinating to see that
one can generate 1 kW with a single FET and > 95% efficiency and without
any ferrite transformers. But the efficiency and voltages and currents
are highly dependant on the load resistance and eventual reactive
components. As long as the SWR is < 1:1.5 or so, it is fine but then it
becomes critical.
I lost a FET in the field while using a 100m kite vertical antenna. The
kite dropped and i wasn't watching for a minute and then BAANG! If you
use the class E at home, in a fixed antenna installation with known and
stable SWR, then this is fine. But if you are in the field and don't
know the feed point impedance, i think a voltage- or current-mode class
D using 2 FETs against ground and a ferrite transformer performs better.
Here you also have more flexibility because you can use various taps
(and so power levels and/or impedance matching) on the secondary winding...
For example, my MF class D
https://dl.dropbox.com/u/19882028/MF/100W%20475kHz%20PA.png works at 12V
as well and is much less critical regarding mismatch...
73, Stefan/DK7FC
Am 09.08.2012 03:08, schrieb Dimitrios Tsifakis:
Output impedance is about 1.4 ohm. I have designed it to produce 50 W
at 12 V and of course 200 W at 24V.
Yes, good idea. In /p for moderate power and at home in QRO :-)
That's exactly the idea :-) Also, if all components are selectedfor a
QRO operation, they will probably survive any kind of dodgy condition
you may encounter in the field.
I work in multiples of 12 V as I
normally use big SLA batteries to power my equipment. The FET is a
monster IXYS 500V/55A FET which should be bombproof.
But you could have much lower on-resistances when using a 100V FET. 200W at
24V is no problem though. IXYS is mostly a good choice. But quite expensive.
Once you lost a pair of them you begin to think to go back to the IRFP
types, at least i have done so ;-)
I did not buy that FET, I was given it by a friend who bought it for a
project but ended up not using it. The Rds(on) is 0.09 ohm, which is
not that bad. Higher Rds(on) will result in a bit more heat on the
heat sink and slightly less power for the same supply voltage, but no
other serious side effect. The biggest problem with using a monster
FET is the Ciss which is about 10 nF. This makes it a bit more
difficult to drive than other smaller FETs, even at 137 kHz. But I
found that the TC4420 (6 A peak) FET driver that I had in my junk box,
seems to be happy with that Ciss/frequency combination.
73, Dimitris VK1SV
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