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Date: Wed, 16 Dec 2009 16:11:00 +0100
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Thread-Topic: "Gain" between qrss3 and qrss10 / PA design
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From: =?iso-8859-1?Q?Stefan_Sch=E4fer?= <schaefer@hst.tu-darmstadt.de>
To: <rsgb_lf_group@blacksheep.org>
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Subject: LF: AW: "Gain" between qrss3 and qrss10 / PA design
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Dear Jim, LF,=20
Thank you for these explanatory notes. And i have nothing to disagree ;-)
Yes, with a B2 recifier, the "minus-pole"/ground have a potential to earth=
 and so an isolating transformer is needed. And to make output power adjus=
table in steps, there is a need for an output transformer since the DC vol=
tage is constant and the load also.=20
One way to economise the output transformer would be to use a M3 rectifier=
 (3 phase system with one diode for each phase) but that's really not the=
 best choice since the voltage gets even 1,73x higher and hence the output=
 power 3x.=20
What about coupling capacitors, e.g. 1=B5F, to decouple the mains from the=
 coax screen (that should be earthed then without ground fault circuit int=
errupter)? Then, the RF could be permitted to the antenna without making=
 a short circuit of the mains. Well, of course, this is just a theoretical=
 consideration, but a possibility when output power doesn't need to be adj=
usted. And, if one really wants to built a 10kW PA I am not sure if a TV=
 line transformer would be suited. If we stay on the ground (I mean, norma=
l power ranges), surely a class D or E is very suited for our amps. From=
 my sight, Class D is more complex in the topology and class E relatively=
 difficult to adjust since Ls and Cs have to be dimensionated very well.=
 So both sides have their pros and cons.
Jim, which kind is your PA? I have the described 1kW Class E PA (80V DC)=
 and a 40W/120W class E PA for 12V DC. Since I have built a switching powe=
r supply with adjustable output (12V input range(for /p operation and/or=
 solar supply at home)) I can adjust to any power within the limit...
And are you still regular qrv in qrss/dfcw3? Are you qrv in CW? I would be=
 glad to do a longer qso again :-)
73s es gl de Stefan/DK7FC
PS: Still searching for CW QRQ partners ;-)



-----Urspr=FCngliche Nachricht-----
Von: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacks=
heep.org] Im Auftrag von James Moritz
Gesendet: Mittwoch, 16. Dezember 2009 14:31
An: rsgb_lf_group@blacksheep.org
Betreff: LF: "Gain" between qrss3 and qrss10 / PA design

Dear Stefan, LF Group,

DK7FC wrote:
...> Yes, on 137kHz a half bridge is also a good idea since the losses are=
=20
quite small. But, for a half or full bridge like in these decca=20
transmitters, one needs an output ferrite core that can handle the whole=
=20
output power. I think these are expensive (?), not easy to get and could=
 be=20
driven into saturation when dimensioned well....>

I have used ETD49-size SMPSU cores successfully for transformers at 136k=
 at=20
the 1kW level. These are readily available from big component distributors=
=20
in this country such as RS Components and Farnell - the set of core halves=
,=20
bobbin and clips costs about 5 - 10 GB Pounds. I suppose you should add so=
me=20
more for wire and insulation, but this is a lot cheaper than a suitably=20
rated 50Hz transformer! In my experience, temperature rise due to hysteres=
is=20
and other losses limits the power capability at these relatively high=20
frequencies, core saturation is not the limiting factor.

In the overall cost of a QRO TX, the most expensive part is usually the DC=
=20
power supply. So, if you are trying to achieve a low-cost TX design, and=
 you=20
already have a high power DC supply, it is sensible to design the TX to ma=
ke=20
optimum use that supply. If you don't already have a PSU, economy is the=
 big=20
advantage of a 330V DC off-line rectified supply. The TX output power is=
 a=20
function of the DC supply V, the load resistance, and the circuit topology=
=20
used. The DC supply is fixed by the mains voltage if we use a direct=20
rectified supply. If, for further economy, we try to design an output stag=
e=20
with no output transformer, or other impedance matching, then the load=20
impedance is also fixed. So the power output is then fixed for a particula=
r=20
circuit configuration. For a push-pull or full-bridge design we really nee=
d=20
an output transformer to drive a single-ended load, but this leaves the=20
Class D half-bridge and Class E configurations, which are single-ended.

Using  the text-book formulae for Pout of an ideal circuit with 330V DC=20
input and 50R load gives about 1.2kW for an "optimum" Class E, and 440W fo=
r=20
the half-bridge Class D. The class E circuit is perhaps simpler, at least=
 on=20
paper, and has attractive output power, but further calculations show that=
 a=20
switching device handling peak voltage 1200V and peak current about 10.5A=
 is=20
required, which would be a problem. We could re-design the class D stage=
 to=20
produce 1.2kW by reducing the load impedance to 18.4ohm, which could be=20
achieved using an LC matching network, if transformers must be avoided. Th=
en=20
the peak current and voltage in the two switching devices would be 330V ,=
=20
11.4A, which is a relatively easy requirement. So for this kind of power=
=20
level, and off-line DC supply, the class D design is probably more practic=
al=20
. With current switching transistor technology, the 1200V peak voltage is=
=20
the problem for any class E off-line design; but in the future it might no=
t=20
be, since there seems to be a lot of interest in developing high speed, hi=
gh=20
voltage switches based on GaN, SiC semiconductors at the moment.

But, in the off-line TX, another important reason to have an output=20
transformer is to isolate the output RF ground and antenna system from the=
=20
mains supply, partly for safety - but with a full-wave rectifier, the "0V"=
=20
DC terminal, and so the RF ground of the output stage, is not at ground=20
potential anyway. The same also applies to the gate drive input side. So,=
 if=20
you don't have a suitable DC PSU, an off-line directly rectified supply is=
=20
attractive, but it will probably need RF transformers! In any case, an=20
output transformer does give a lot more flexibility in the design.

Cheers, Jim Moritz
73 de M0BMU