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
|
