Definition: power supply glossary

The following terms are reproduced with permission from PC Power & Cooling, Inc., Carlsbad, California, a manufacturer of quality power supplies.

agency approval
UL, CSA and TUV are safety agencies that test specifications such as component spacing, hi-pot isolation, leakage currents, circuit board flammability and temperature rating. Hi-pot (high-potential) isolation is the ability to accept voltage surges with safety.

efficiency
Ratio of output power to input power expressed as a percentage.

EMI
(ElectroMagnetic Interference) Noise generated by the switching action of the power supply and other system components. Conducted EMI is radiation reflected back into the power line, which is normally controlled with a line filter. Radiated EMI is that portion that would radiate into free space, but is suppressed by enclosing a power supply's circuitry in a metal case. The FCC governs conducted and radiated emission levels in the U.S.

fan rating
Airflow rated in cubic feet per minute. A 100% increase in airflow will reduce system operating temperatures by 50% relative to ambient temperature. For each 18 degrees (Fahrenheit) of reduction, the life of the system is doubled (Arrhenius equation).

hold-up time
Time period that a power supply's output will remain within specified limits, following power disturbances or a loss of input power. Adequate hold-up time keeps the computer running until a standby UPS takes over within a few milliseconds.

load regulation
Change in output voltage due to a varying load. Expressed as a percent of the normal output voltage, a power supply with tight load regulation delivers optimum voltages regardless of system configuration. This is tested by measuring the difference in output voltage when applying a light load and a heavy load.

line regulation
Change in output voltage due to varying input voltage. Expressed as a percent of the normal output voltage, a power supply with tight line regulation delivers optimum voltages throughout the operating range. This is tested by measuring the difference in output voltages while varying the input voltage from minimum to maximum; for example, from 85 to 135 volts.

MTBF
(Mean Time Between Failure) Measurement of the relative reliability of a power supply based upon actual operating data or calculated according to MIL-HDBK-217.

noise (loudness)
Issues include fan blade pitch and speed, hub size, venturi depth, bearing quality and layout of power supply components. Acoustical noise is measured logarithmically; each 3 db reduction represents 50% less noise.

operating range
Minimum and maximum input voltage limits within which a power supply will operate to specifications. A power supply with a wide input range is recommended when the line voltage is subject to brownouts and surges.

operating temperature
Range of ambient temperatures within which a power supply can be safely operated.

output current
Maximum current that can be continuously drawn from the output of a power supply. PC motherboards and expansion cards draw 5 volt current. Drive motors draw 12 volts.

overcurrent protection
Circuit that shuts down the power supply from excessive current, including short circuits.

overvoltage protection
Circuit that shuts down the power supply if the output voltage exceeds a specified limit.

power good signal
Signal used to prevent the computer from starting until the power has stabilized. The power good line switches from 0 to +5 volts within one tenth to one half second after the power supply reaches normal voltage levels. Whenever low input voltage causes the output voltage to fall below operating levels, the power good signal goes back to zero.

ripple
AC voltage superimposed onto the DC output, expressed as a percent of the normal output voltage or as peak to peak volts. A power supply with clean DC output is essential for computers with high-speed CPUs and memory.

transient response
Time required for the output voltage to return within the regulation envelope following a 50% load change. A power supply with quick transient response will reduce the risk of read/write errors.