Message-ID: <4F5A8943.0@iup.uni-heidelberg.de>
Date: Fri, 09 Mar 2012 23:50:43 +0100
From: =?ISO-8859-1?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: DK7FC LF PA schematics
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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.
>   =20
I saw your PA schematic some years ago but didn't take it directly to=20
built my design from. Lastly we all learn a bit from each other and copy=20
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=20
consumption without a load. That was uncritical in each situation so=20
far, even while running power. There are some peaks during the switching=20
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.
>   =20
So far it is the same here. The PA runs almost every day (i.e. night)=20
for many hours. In the current design i haven't lost one FET so far.=20
(Problems in December were due to a problem in the IQ converter).

Tnx, 73, Stefan/DK7FC

> Andy
> www.g4jnt.com
>
>
>
> 2012/3/9 Stefan Sch=E4fer<Stefan.Schaefer@iup.uni-heidelberg.de>:
>   =20
>> LF,
>>
>> Now i've finally documented my complete LF PA in 2 documents, for thos=
e 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.pn=
g
>>
>> Questions, hints and constructive advice is welcome.
>>
>> BTW there are further components which complete the TX system here, li=
ke 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 cir=
cuit
>> which allows the PA to easy control the TX power and allows linear mod=
es
>> will be added soon.
>>
>> 73, Stefan/DK7FC
>>
>>
>>
>>     =20
>   =20