Which UPS?

[James Knott]

Scott Allen wrote:
> It's not a similar DC voltage. The AC input supply rectifies 120VAC to
> 170VDC. The DC input supply (that you're questioning my reasoning
> about) requires 48VDC.
> We're talking about 120VAC to 48VDC. It's not just rectification. We
> have to get the voltage down to 48VDC. I Assumed that this would be
> done by a switching supply in the UPS, since the voltage would have to
> be regulated to charge the battery, hence my 85% efficiency estimate.
>
> If the PC supply accepted a fairly large voltage swing, you could get
> away with just a 120VAC to 48VAC transformer then rectify and filter
> it (and then just regulate the battery charger circuit). I don't know
> how efficient the AC transformer would be but we're now less efficient
> than just strait rectification to 170VDC. Also, a transformer that
> could provide the 500 or more watts required for a typical UPS would
> be big, heavy and possibly expensive.

As I mentioned, large data centers use high voltage DC for this, whereas 
telecom runs on -48V.  In the high voltage DC supplies, there'd be 
little difference in the converter/regulator section, compared with AC 
supplies.  With 48V, it would be tailored for the higher current, lower 
voltage, but otherwise similar.  As for transformers, AC supplies 
rectify the AC to high voltage DC and use that to power the 
converter/regulator,.  You'd have a high frequency transformer 
regardless of the input supply, though -48v supply would require a 
transformer with heavier wire, but fewer turns in the primary.  
Eitherway, efficiency would be similar.  Where there is a significant 
difference is in the DC distribution within the site, where you'd need 
larger gauge wire to avoid resistance losses at 48v.  Modern rectifiers 
are much more efficient than they used to be, with switching supplies 
commonly used, as with many other power supplies.  Rectifiers intended 
for this service will also have multiple AC inputs and permit multiple 
rectifiers on the same battery bank for redundancy.  They'll also often 
run on 3 phase power, which makes filtering easier.

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