Why pure sine wave in IT?

[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.

[TheMG]

Active PFC circuit may not like the square wave.

BINGO!

That's exactly it. The square wave, modified sinewave, whatever you want to call it, creates problems with the active power factor correction circuitry in some power supplies. With a sine wave output, you avoid the issue altogether.

I don't know if that is still an issue with the more modern PFC controller ICs, but it certainly was a significant factor when active PFC power supplies first started being introduced into the market.

[perieanuo]

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

hi!
well, it's kinda complicated.all the chain powered after the ups works ok with pure sine, which is difficult (costly) to obtain on big powers.with other sine wave, all chopper input stages are exposed to the big chalenges: any harmonic can have a distructive effect on chopper stage.not only as others say, some capacitors suffering.of course they are right, but the risk on the power supply powered by non-pure-sine is multiplied.also the harmonics (high frequency of course) transcend the power supply powered by the ups and get into small final voltages.
So it's more complex then some big fat input capas that dye from non-respected operating conditions.
the answer is worth by a exposeum  like 40 pages with lots of math inside.
regards,pierre

[coromonadalix]

A pure sinewave  will prevent you from having problems with ac/dc adapters and or other stuff

With modified sinewave i had problems with laptop adapters and other appliances, they simply refused to power on and / or start, and some of them need to sense a 50/60hz sinewave in some cases ??

[David Hess]

If a switching power supply has a problem with a modified sine input, then it is defective, whether it has active power factor correction or not.  Still, it does happen.

Some loads, but hopefully not any switching power supply, will perform poorly with a modified sine source.

Pure sine wave inverters like those found in double conversion or online uninterruptible power supplies allow the output to be phase locked to the AC line so switching between the inverter and AC line can occur at any time without any discontinuity.

[coppice]

Active PFC circuit may not like the square wave.

Its not just active PFC. Most server supplies have filter caps directly across the mains input. Put a square wave into those and huge currents can flow, overheating the caps.