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Why pure sine wave in IT?

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thobo:
I just stumbled over this question myself as I was searching for a new UPS for my IT Rack. There is big difference in costs for the modified sine wave and the pure sine wave online UPS's.
What technical reason makes a pure sine wave UPS necessary? Most IT tech uses switching power supplies, in my rack 100%. My assumption is, that switching power supplies does'nt benefit of a pure sine wave UPS, because after their filtering the first thing is a rectifier and ripple cap(s). It shouldn't matter if the input is pure sine wave or modified sine wave, isn't it ?

regards
  Thomas

Jeroen3:
Considering IT racks almost always run on online dual conversion UPS systems, the capacitors inside those switch-mode power supplies will age rather quickly when run on modified or square waves due to the high current surges in the transitions. Remember, those edges are of a very high frequency. And other losses in filter components of course.

Modified sine is a lot less demanding in that regard. And there is less noise emissions, since it's a nice sine wave with maybe some harmonics. And not a repeating series of high frequency impulses.
I think there might also be design limitations in continuous operation of modified sine wave inverters that I'm not aware of.

In theory a DC system would be preferable, but that has other protection and hot plug drawbacks. Maybe we're at a place in time now where this is technically viable if one can make diverting from standards economically viable.

thobo:
I know the modified sine wave inverters have sharp edges on their output, but the switching transistor in a switching power supply generates sharp edges to, so I don't think that this can be the culprit.
The filter might get warmer, but doesn't it smooth the sharp edges from the input?

NiHaoMike:
Most likely a holdover from the days when modems and such were very sensitive to noise. Nowadays, the optimum waveform would be either DC or slew rate controlled square waves at either 340V or 170V.

Pawelr98:
Active PFC circuit may not like the square wave.

DC would be ideal, if suitable DC switches were used.
First connect the power and then a low RDSon transistor would switch on the power to the device.
Physical switches used only for emergency as they wouldn't last long on DC.

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