Return to KLUBNL.PL main page

rsgb_lf_group
[Top] [All Lists]

LF: AW: "Gain" between qrss3 and qrss10 / PA design

To: <[email protected]>
Subject: LF: AW: "Gain" between qrss3 and qrss10 / PA design
From: Stefan Schäfer <[email protected]>
Date: Wed, 16 Dec 2009 16:11:00 +0100
In-reply-to: <88FB4290CF234F66933047AD790B3719@JimPC>
Reply-to: [email protected]
Sender: [email protected]
Thread-index: Acp+VTLTYyAIoSUITii030LHU+fnswAB8jwg
Thread-topic: "Gain" between qrss3 and qrss10 / PA design
Dear Jim, LF, 
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 adjustable in 
steps, there is a need for an output transformer since the DC voltage is 
constant and the load also. 
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. 
What about coupling capacitors, e.g. 1µF, to decouple the mains from the coax 
screen (that should be earthed then without ground fault circuit interrupter)? 
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 adjusted. 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, normal 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 power 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 ;-)



-----Ursprüngliche Nachricht-----
Von: [email protected] 
[mailto:[email protected]] Im Auftrag von James Moritz
Gesendet: Mittwoch, 16. Dezember 2009 14:31
An: [email protected]
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 
quite small. But, for a half or full bridge like in these decca 
transmitters, one needs an output ferrite core that can handle the whole 
output power. I think these are expensive (?), not easy to get and could be 
driven into saturation when dimensioned well....>

I have used ETD49-size SMPSU cores successfully for transformers at 136k at 
the 1kW level. These are readily available from big component distributors 
in this country such as RS Components and Farnell - the set of core halves, 
bobbin and clips costs about 5 - 10 GB Pounds. I suppose you should add some 
more for wire and insulation, but this is a lot cheaper than a suitably 
rated 50Hz transformer! In my experience, temperature rise due to hysteresis 
and other losses limits the power capability at these relatively high 
frequencies, core saturation is not the limiting factor.

In the overall cost of a QRO TX, the most expensive part is usually the DC 
power supply. So, if you are trying to achieve a low-cost TX design, and you 
already have a high power DC supply, it is sensible to design the TX to make 
optimum use that supply. If you don't already have a PSU, economy is the big 
advantage of a 330V DC off-line rectified supply. The TX output power is a 
function of the DC supply V, the load resistance, and the circuit topology 
used. The DC supply is fixed by the mains voltage if we use a direct 
rectified supply. If, for further economy, we try to design an output stage 
with no output transformer, or other impedance matching, then the load 
impedance is also fixed. So the power output is then fixed for a particular 
circuit configuration. For a push-pull or full-bridge design we really need 
an output transformer to drive a single-ended load, but this leaves the 
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 
input and 50R load gives about 1.2kW for an "optimum" Class E, and 440W for 
the half-bridge Class D. The class E circuit is perhaps simpler, at least on 
paper, and has attractive output power, but further calculations show that a 
switching device handling peak voltage 1200V and peak current about 10.5A is 
required, which would be a problem. We could re-design the class D stage to 
produce 1.2kW by reducing the load impedance to 18.4ohm, which could be 
achieved using an LC matching network, if transformers must be avoided. Then 
the peak current and voltage in the two switching devices would be 330V , 
11.4A, which is a relatively easy requirement. So for this kind of power 
level, and off-line DC supply, the class D design is probably more practical 
. With current switching transistor technology, the 1200V peak voltage is 
the problem for any class E off-line design; but in the future it might not 
be, since there seems to be a lot of interest in developing high speed, high 
voltage switches based on GaN, SiC semiconductors at the moment.

But, in the off-line TX, another important reason to have an output 
transformer is to isolate the output RF ground and antenna system from the 
mains supply, partly for safety - but with a full-wave rectifier, the "0V" 
DC terminal, and so the RF ground of the output stage, is not at ground 
potential anyway. The same also applies to the gate drive input side. So, if 
you don't have a suitable DC PSU, an off-line directly rectified supply is 
attractive, but it will probably need RF transformers! In any case, an 
output transformer does give a lot more flexibility in the design.

Cheers, Jim Moritz
73 de M0BMU




<Prev in Thread] Current Thread [Next in Thread>