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Power supply design approach
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Red_Micro:
Hi,

I want to design a power supply with these specs for metering purposes:
Input voltage = 100-600VAC three phase (RMS line to line)
Output voltage = 48V and 3.3V/200mA

I'm looking into two approaches: A cap dropper and a flyback SMPS. Flyback has higher efficiency at the expense of higher cost and complexity. Cap dropper is kind of a cheaper option but capacitors at this high voltage are also expensive.

I'd appreciate any suggestions on trade-offs.
moffy:
Why the three phase input? You only need a single phase.
Kleinstein:
For the low power one can likely use just a single phase. However if available anyway it is only 2 more diodes and fuses to save on the filter cap.

The flyback converter would also need high voltage capacitors of some kind, though possibly a little less. The more tricky part could be the semiconductor: 600 V AC results in up to som 900 V DC worst case. So it would need really high voltage switches (e.g. 1500 V types).
trobbins:
The OP appears to only have 3 phases, no neutral, so the minimum 'single phase' is to use a 'floating' phase-to-phase level that can get to 600Vac.

Is a mains frequency transformer out of the question for some reason?
David Hess:
Wide input range switching regulators exist but a 1:4 ratio is typical.  Some support an even wider input range but finding a suitable one would be a problem.

The usual problem is preventing or handling the transition between discontinuous and continuous conduction modes as the input voltage and output load current vary over their entire range because frequency compensation requirement are variable in discontinuous mode where the duty cycle varies.  This can mean using a much larger inductance to prevent discontinuous mode operation, which compromises performance, or using a topology like constant off-time which does not care.

For safety reasons, it might be acceptable or desirable to use a power transformer for input isolation *before* the switching regulator despite the extra weight and cost.  Some old test equipment did this like the Tektronix 2225 series of oscilloscopes even though off-line switching regulators were available at the time.
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