Re: What kind of UPS do Europeans use? My modified sq wave UPS + PFC'ed load = CRASH !
From: AC/DCdude17 (dude17_at_sacREMOVEbeemail.com)
Date: 07/08/04
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Date: Thu, 08 Jul 2004 17:46:04 GMT
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phil-news-nospam@ipal.net wrote:
> | An example of power factor corrected PSU:
> | http://www.savastore.com/products/product.asp?catalog_name=Savastore&product_id=10262447&pid=44
>
> Do you have any schematics or design theory?
I don't know about that specific power supply, but this is the general
schematic of a passive power factor correction that goes in the input
stage of a switch mode power supply or a ballast:
http://www.irf.com/technical-info/designtp/temp004.pdf
Look at figure 2 on page 2.
This L-C type circuit is tuned to work with sine wave of specific
frequency.
All ballast meant for commercial use have power factor correction of
some sort so that it will get at least PF >0.95 most likely because
there's some regulations mandating it.
Residential application ballast doesn't have any form of front end
circuit to correct PFC and they don't have a problem with my UPS.
Apparently the extremely high harmonic content modified squarewave
doesn't play nicely with L-C circuit tuned to work with sinewave.
The inductors are actually quite large. Despite the weight, it's the
preferred choice for many electronic ballasts, because it's the cheapest
design and works quite well.
>
> | The passive PFC on a power supply like the one linked above is the same
> | type as the one in my ballast. Since computer power supplies in the US
> | typically don't have a PFC, we're fine, but how do Europeans get their
> | PFC equipped computers to get along with their UPS? I searched "PFC
> | UPS" on Google and looks like I'm not the first one to have issues with
> | a UPS interecting badly with a PFC power supply.
>
> I would have thought that a proper power factor corrected load would be
> indistinguishable from a linear resistive load. Maybe what they are
> talking about is just the phase angle, rather than harmonics? Maybe
> this kind of PFC is actually making harmonics worse?
That's the goal, but there's still harmonics. Again the L-C circuit is
designed to limit the current rise so that it doesn't abruptly rise
while the voltage sweeps in sinewave. Since in modified squarewave,
voltage goes from zero to full almost instantaneously, i'm sure the
situation is very different.
This is what modified squarewave looks like:
http://www.fords-mtm.com/electric%20images/waveforma.jpg
even though the sales likes to call that modified sinewave, it's not.
The only reason it's called modified is because there's a slight pause
before polarity is reversed.
This might qualify as modified sinewave though:
http://www.wojack.net/Our%20Solar%20System/MODIFIED_SINE_WAVE.GIF
>
> | My UPS shares the transformer between inverter operation and charging.
> | It's a steel core transformer with center tapped secondary. When
> | operating on battery, the center tap on secondary is tied to one of the
> | battery terminals and the MOSFETs switch the other terminals back and
> | forth between the two taps on the sides. A relatively common design.
>
> And that's not producing any backfeed?
Not sure, I haven't put the effort to reverse engineer the UPS any further.
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