Hardware Reference
In-Depth Information
•
Load Regulation (or Voltage Load Regulation)
—When the current drawn from a
particular output increases or decreases, the voltage changes slightly as well—usually
increasing as the current rises. Load regulation is the change in the voltage for a par-
ticular output as it transitions from its minimum load to its maximum load (or vice
versa). Values, expressed in terms of a +/− percentage, typically range from +/−1% to
+/−5% for the +3.3V, +5V, and +12V outputs.
•
Line Regulation
—The change in output voltage as the AC input voltage transitions
fromthelowesttothehighestvalueoftheinputrange.Apowersupplyshouldbecap-
able ofhandling anyAC voltage inits input range with achange inits output of1%or
less.
•
Efficiency
—The ratio of power input to power output, expressed in terms of a per-
centage. Values of 65%-85% are common for power supplies today. The remaining
15%-35% of the power input is converted to heat during the AC/DC conversion pro-
cess. Although greater efficiency means less heat inside the computer (always a good
thing)andlowerelectric bills,itshouldnotbeemphasized attheexpenseofprecision,
stability, and durability, as evidenced in the supply's load regulation and other para-
meters.
•
Ripple (or Ripple and Noise, or AC Ripple, or PARD [Periodic and Random De-
viation])
—Theaverage voltage ofall ACeffects onthepowersupplyoutputs,usually
measured in millivolts peak-to-peak or as a percentage of the nominal output voltage.
The lower this figure, the better. Higher-quality units are typically rated at 1% ripple
(orless),whichifexpressedinvoltswouldbe1%oftheoutput.Consequently,for+5V
that would be 0.05V or 50mV (millivolts). Ripple can be caused by internal switching
transients, rectified line frequency bleed-through, or other random noise.
Power Factor Correction
In order to improve power line efficiency and to reduce harmonic distortion generation,
the power factor of PC power supplies has come under examination. In particular, new
standards are now mandatory in many European Union (EU) countries that require har-
monics to be reduced below a specific amount. The circuitry required to do this is called
power factor correction (PFC)
.
The
power factor
measureshoweffectivelyelectricalpowerisbeingusedandisexpressed
as a number between 0 and 1. A high power factor means that electrical power is being
used effectively,whereas a low power factor indicates poorutilization ofelectrical power.
To understand the power factor, you must understand how power is used.
Generally, two types of loads are placed on AC power lines:
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Resistive
—Power converted into heat, light, motion, or work
