All,
Here's my take on this, particularly from a VHF contests point of view.
We need good HV SMPS because;
1. Must be lighter, and hopefully smaller and more efficient than
equivalent linear supplies, - lugging heavy PSUs to contest sites is a
(back) pain.
2. It will cope with poor quality power supplies - we currently have to
hire generators for VHF portable contests that are vastly overrated to
cope with the huge swing, poor power factor, and distorted current
waveform from linear PSU's and still give a reasonably constant voltage.
3. In the denser parts of Europe, received signals in a VHF contests on
a good site can be enormous and so signal quality is very important. The
supply therefore needs to be regulated .
a) To cope with smaller poorly regulated generators as above,
b) To allow the most linear operation of the amplifier to reduce
splatter. Tuning a linear amp and throttling it back to give good
linearity needs care even on a decent mains supply. On a mutliband
contest site with poor quality power whose voltage is varying with the
demands of the other stations, it's a nightmare.
4. It needs power factor correction. Again to allow smaller, cheaper,
more efficient (hence lower CO2), generators to be used.
5. It needs low ripple and spurious on its o/p. Any switching ripple on
the high voltage lines will modulate the signal. 100 / 120 Hz ripple on
a linear supply is not too serious as it only needs to be say 40 dB
below peak speech level i.e 1% and its harmonics will get lost in the
intermod / splatter. However beyond say 10 kHz from carrier, outside
the splatter zone, it becomes very serious . A close by station may
well be receiving -20 dBm signals. His noise level in 3 kHz is likely
to be in the -130 to -140 dBm region depending on band thus the ripple
needs to be better than about -110 dB down (say <3 mV). Otherwise local
fixed stations will call you and tell you they can hear you at say 75
kHz intervals up the band. I think this is achievable but it will need
careful design. There is also the need to consider induced ripple into
other nearby equipment, supplies and cables so well screened enclosures
and good mains filtering are important.
6. It must be very quiet on receive. Lower band (6,4 and 2m) VHF
contests and DX are a demanding environment. The best contacts are
often at signal levels at or below the noise floor thus radiated and
induced spurs which may be acceptable on transmit may well not be on
receive. The logging computer is even a problem and I've recently had
to put its plastic brick PSU into a metal box with further filtering.
Modern Fast switching PSU devices produce spurs up to unbelievably high
frequencies which can be radiated out of the box all over, especially
through fan and meter openings. The current answer to this, on 12 V 30
A transceiver SMPS PSUs, of putting a switching frequency tweak control
is not the answer. It may be useful on HF but by 50 MHz the oscillator
harmonics are so wide they are a continuous mush. Thus it needs to
shut down on receive, preferably completely.
7. Design
a. Try and design the circuit so that it is easily understandable or at
least layout the circuit diagram so that its easier to follow, I think
this point should not be overlooked as it seems very difficult, even for
those of us with some circuit design experience, to fathom the operation
of most modern SMPSs. This will help immensely with testing and fault
finding.
b. It needs to use components that are readily obtainable world wide and
ICs which are widely used and unlikely to go obsolete quickly.
c. It should be easily adaptable for different B+ voltage and current
requirements and maybe even provide a screen supply for 4CX users.
So get to it folks, a good proven design will be much appreciated.
73 John G3UUT (G4ZAP/P group)
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