Hi Andy,
Thanks for the feedback.
Am 09.03.2012 23:13, schrieb Andy Talbot:
Stefan -
Your design has a certain amount in common with my 700 Watt half
bridge design http://www.g4jnt.com/137tx.pdf, in particualr the way
the output FETs are driven in parallel from a common drive
transformer. I took the idea from the Decca transmitters.
I saw your PA schematic some years ago but didn't take it directly to
built my design from. Lastly we all learn a bit from each other and copy
some ideas, probably even the Decca transmitter builders :-)
But driving the FETs in this way gave a tiny but significant period
when both were on. I compensated for this in the same way as the
Decca ones did by adding a small centre tapped choke in series with
the top and bottom devices to absorb the transient.
Did you check for or detect any such transients?
Of course i checked the Drain-Source voltages and observed the power
consumption without a load. That was uncritical in each situation so
far, even while running power. There are some peaks during the switching
periods but they always stay well below 500V.
I found them very
small and probably non damaging, but the small choke did not
otherwise affect performance so was used in the circuit as an extra
precaution. That Tx has been very reliable over the years.
So far it is the same here. The PA runs almost every day (i.e. night)
for many hours. In the current design i haven't lost one FET so far.
(Problems in December were due to a problem in the IQ converter).
Tnx, 73, Stefan/DK7FC
Andy
www.g4jnt.com
2012/3/9 Stefan Schäfer<[email protected]>:
LF,
Now i've finally documented my complete LF PA in 2 documents, for those who
are interested.
The exciter:
http://dl.dropbox.com/u/19882028/LF/LF%20H%20bridge%20PA%20Exciter%2C%20DK7FC.jpg
The power part:
http://dl.dropbox.com/u/19882028/LF/LF%20H%20bridge%20PA%2C%20DK7FC.png
Questions, hints and constructive advice is welcome.
BTW there are further components which complete the TX system here, like a
watchdog savety and PTT sequencer circuit, the IQ transmit converter,
temperature controlled fans, the automatic variometer. These do not
necessarily belong to the PA and aren't shown here. A AM Modulator circuit
which allows the PA to easy control the TX power and allows linear modes
will be added soon.
73, Stefan/DK7FC
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