Hi Wolf,
What type of FETs are you using? The standard IRFPs? Many of them have
internal Z diodes, so no addional RC network or Z idode is needed. The
losses are there anyway, of course.
In some older projects i also played with that critical working point,
i.e. where one FET shuts down and the other opens. I've tried simple RC
networks (series R, parallel C) with an 100 Ohm trimmer. That worked
reasonably but usually it is not even necessary. My small MF PA is using
a simple ICL7667 and 2x IRFZ48N (Reichelt...) without problems.
My large PA generates up to 1.2 kW, which is also enough to reach 1 W
ERP, https://dl.dropboxusercontent.com/u/19882028/MF/mod%20MF%20PA.jpg.
It was designed for field experiments (Field day) where MUCH more
inefficient antennas will be tested, e.g. in France, next year, if they
have the allocation then...
73, GL, Stefan/DK7FC
Am 29.12.2013 18:09, schrieb wolf_dl4yhf:
Hi Stefan,
> Hmm, P ~ 300W? :-)
Not really... the design goal was driving the PA with the 13.8 V, "20
ampere" power supply. There actually is some stray inductance (despite
using 17 mm wide copper foil) so the load "seen" by the drains is a
bit higher than 12 V / 20 A = 0.6 Ohm. The max DC input current is now
about 15 A, but that's ok for me. Also, after adding the first lowpass
(which transforms from 12.5 to 50 Ohms, at very 'moderate' RF
currents), the waveform at the drains isn't such a nice square wave
but the HUF75343 can 'absorb' the very narrow spikes (80 ns long, and
over 55 V) when none of the two FETs conduct for a very short time.
According to the datasheet (where the FET are mistreated by driving an
inductive load, as a test) they should survive this, even without an
RC snubber network or an extra Zener diode.
I will find out soon - also if P ~ 150 W is enough to reach 1 W ERP.
From experience with the old PA (in which the FETs were biased into
'almost linear operation', at the expense of efficiency), it should be
possible.
73,
Wolf .
An alternative to the foil could be: 4x
http://www.reichelt.de/index.html?ACTION=3;ARTICLE=57209;SEARCH=CLI
200/120
73, Stefan
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