[ExI] Power factor correction

[hkhenson]

At 08:17 AM 10/15/2007, Jordan Hazen  wrote:
>On Mon, Oct 15, 2007 at 09:25:05AM +0200, Eugen Leitl wrote:
> > On Sun, Oct 14, 2007 at 07:06:29PM -0400, Jordan Hazen wrote:

snip

>On the negative side, typical grid-tie systems have no autonomy, their
>inverters being required to shut down during a utility outage
>("anti-islanding").  Some can revert to standalone mode, with
>batteries for buffering, but at greater cost and lower overall
>efficiency, as safety requirements in most areas limit DC battery bus
>voltage to a nominal 48V.

I have seen it higher, the American Express UPS battery bank was in 
the several hundred volts range.  But that's special industrial, not 
something you would see in a home very often.

> > > reserves at nearby power plants instantly make up any sudden
> > > shortfalls, keeping frequency and voltage within spec.  But, there are
> >
> > What's wrong with DC?
>
>At the low DC voltages commonly used off-grid, I^2R losses over any
>significant distance (even in house wiring) can be high, unless very
>thick conductors are used.  It can work well for low-wattage loads.
>
>For utility-scale transmission and distribution, although DC-DC
>voltage converters are gradually improving, they still tends to be far
>more expensive, less efficient, and more vulnerable to surges &
>spikes, compared to the simple and rugged AC transformer.
>
>High-voltage (700kV - 1M V) DC power transmission is already used in a
>few places, though, and may become more popular over time.
>
>
>For end users, though, there's too much inertia, and legacy equipment
>tied to AC for customer power standards to change anytime soon.
>
>
>(Interestingly, though, most electronics and CFL lighting is perfectly
>happy running on DC, at 1.4 times its rated AC line voltage-- i.e.
>~160V in 120V countries, or 320V in the EU.  Switchmode power supplies
>in this type of equipment tend to immediately rectify the incoming
>line, and are sensitive to peak voltage, not RMS.  Active power-factor
>correction circuitry may complicate things...)

It might, but having recently designed an active power factor 
correction circuit for commercial use, and reviewing the schematics I 
still have on this computer, I think PFC designs could be made to run 
off DC just fine with few if any modifications.  The way they work 
boosting the rectified line voltage to roughly 10% over the AC 
waveform peak, using an inductor and a MOSFET that shorts the 
inductor to ground and a second catch diode that isolates the inductor power.

If you are really serious about understanding what Jordan was talking 
about here is the data sheet for the MC34262 that I used.

http://www.onsemi.com/pub/Collateral/MC34262-D.PDF

>Special power supplies are available for running computers directly
>from DC.  I do this at home, floating most equipment on a 12V bus, and
>avoiding inverter losses.  Just remember the I^2R considerations, and
>try to keep all low-voltage cabling as short as possible.

Jordan knows what he is talking about here.

With respect to up/down conversion loses, transformers give you lower 
loses than boost/buck circuits.  And above about a 6 to one voltage 
ratio, boost/buck circuits are not as efficient as switching circuits 
and transformers.

Keith Henson