I'll propose that considerable effort be put toward ensuring long-term
reliability. I haven't met many switching supplies that would hold up to
20 or 30 years of intermittent use without failing.
Low-frequency big iron and oil-filled caps may be heavy, relatively
inefficient and yesterdays news, but they do seem to work for very long
periods of time without inconvenient and expensive failures.
My .02,
73 - Jim, N7CXI
John Wilson wrote:
> 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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>
>
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