UPS sine wave vs square wave output?

yep. careful selection of the "dead" time gives about the right RMS value, and reduces harmonic content. More steps = better but circuit complexity increases. Lots of work has been done on so-called multi-level inverters for high power drives - partly to minimise harmonics, partly because of the lack of availability of high voltage IGBTs - although 6kV IGBTs are readily available, they have very high Vcesat and switch incredibly slowly. Trade that off versus gazillions of bits. 3-level inverters seem to be a good compromise in that regard.

Cheers Terry

In article , John Tserkezis wrote: [...]

I think you have this statement backwards. The peak of a perfect squarewave is equal to the RMS. The peak of a perfect sine wave is

1.414... times the RMS value.

Most "square wave" converters don't really make a square wave. They make a signal like this:

ASCII Art:

....******............******....... ................................... ...................................

***......***......***......***.... 0V ................................... ................................... .............******............****

The peaks are about 20% higher than the RMS rating. This make the output of a capacitively filtered rectifier end up about 20% lower than expected.

Fortunately most electronics can handle this. Unfortunately not all electronics can and electric motors also can have trouble with it.

This waveform has high frequency components in it. Many transformers and motors and the like are made from iron that becomes quite lossy at higher frequencies.

Induction motors are double trouble in this regard. They always appear fairly lossy at higher frequencies when they are running. The exact explaination is long but the simple version is the that the motor is trying to run at both its normal speed and at 5 times that speed. As far as 5 times the speed is concerned, the motor's rotor is nearly blocked (not turning). this leads to very high losses.

The word "somewhat" is becoming less true. Newer supplies rectify the input and run the pulsating DC with nearly no filtering into a DC-DC converter that is designed so that over the period of a few cycles its input current is proportional to the applied voltage. This regulator does not regulate very well and is followed by a large filter capacitor and a second DC-DC converter. This is how modern power supplies make themselves appear to be resistive loads to the power grid.

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kensmith@rahul.net   forging knowledge

Something just occured to me as being the sort of thing I should already know; during the 0V periods, is the output high impedence as one would expect with minimum FETs, or is there an extra set of FETs that clamp it to 0V? If it is high impedence, I would expect some interesting waveforms as the inductance of the load drives the line. If it is clamped to 0V, it would be interesing to see hopw they derive that 0V in single phase hot/neutral systems where the bottom trace is zero volts relative to earth/ground.

Huh? The crest factor of a sq-wave is 1 (0 dB). The crest factor of a sine-wave is 1.414 (3 dB). Why do you believe what you wrote?

I read in sci.electronics.design that Ken Smith wrote (in ) about 'UPS sine wave vs square wave output?', on Fri, 25 Feb 2005:

What are these power supplies used for? What is the incentive to make the load resistive (outside Europe)?

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The only one I've looked closely at used 4 MOSFETs in a full bridge and did a plus-sproing-0V-sproing-neg-sprong-0V pattern as the MOSFETs we all off breifly at each change in voltage.

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If you get a largish supply (1KW) for running DC equipment in the lab, there is some chance that you will find it is one of these. In Europe there is some sort of silly set of rules called CE that it is claimed specify the harmonic content of the input current. This is just a rumor though.

In the past, efforts had to be made to keep the peak current in the recifiers within reason. This often involved largish (mechanically) resistors and inductors. The spiky current shape made for more line drop than the simple math would lead you to believe. Today a "power factor correction" chip and the related stuff doesn't cost an arm and a leg. These factors have gotten together to make the designers start to use the PFC chips. Generally, the PFC section is just a simple booster. It is non-isolated.

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Because your statement only holds true for a square wave operating at 100% duty. That certainly is not the case for most of the time and probably not the case for ANY of the time in many cases.

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I read in sci.electronics.design that Ken Smith wrote (in ) about 'UPS sine wave vs square wave output?', on Fri, 25 Feb 2005:

Yeah, right!

Indeed, the EMC requirements in this case gel with sensible design decisions. However, there are no harmonics emission requirements for 1 kW lab power supplies. YET.

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Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I think you mean 50% duty cycle. BTW, 50% duty cycle is the definition of "square." Other duty cycles are called "rectangular," since "on sides" and "off sides" are not necessarily equal, but which square implies. Square is a special case of rectangular. I did assume a zero DC square wave, since we're talking the output of a UPS. Also, I should note we are defining a "power" crest factor since we're using RMS instead of a full-wave rectified average.

In article , John Woodgate wrote: [...]

I was using a power supply in a rack mounted system. I discovered that the fool thing made about 1V spikes from corner to corner on its chassis when running. ie: with the lid on, if you grounded the scope to the back of the metal cover and probed the front of the very same chunk of aluminum, you'd see the spike.

The frequency content of this noise would drift around until it landed in the bandwidth of interest.

Even without a government action, there is a limit to the emissions. It is when the thing you are trying to run won't operate if the supply is on.

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I read in sci.electronics.design that Ken Smith wrote (in ) about 'UPS sine wave vs square wave output?', on Sat, 26 Feb 2005:

Presumably that spike wasn't associated with mains harmonics, but your statement is correct. 'Auto-EMC' can be quite a problem with compact designs, especially with mixed technologies as well.

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Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

The switcher was about a 100KHz "constant frequency" design. At light loads it had all sorts of subharmonic oscillations and chaos. At heavier loads it was true PWM. It had an "EMI filtering power entry module" at the back and another chassis ground connection near the front so a lot of the switcher's return current flowed through the sheet metal.

The design in question could only be compact in the most generous use of the term. It was a 2 foot tall 19 inch rack which had a lot of air space inside. The rack helped to conduct the noise to where it was least welcome as racks always will.

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little APC UPS' use a push-pull LV primary. During the 0V time, both FETs are switched on.

Cheers Terry