PSU Terminology


The 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.



The ratio of output power to input power expressed as a percentage.



Electromagnetic interference is the noise generated by the switching action of the power supply. Conducted EMI, that portion reflected back into the power line, is normally controlled with a line filter. Radiated EMI, that portion which is radiated into free space, is suppressed by enclosing the circuitry in a metal case. The FCC governs conducted and radiated emission levels.



Power factor is the ratio of true power (watts) divided by apparent power (volts x amps or VA). A standard power supply has a power factor of 0.70-0.75, while a power supply with active power factor correction (PFC) has a power factor of 0.95-0.99. A power supply with power factor correction is better able to convert the current into power. This results in lower peak current and lower harmonic current, putting less stress on wiring, circuit breakers, and transformers.



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



The change in output voltage due to the output load varying from minimum to maximum with all other factors held constant. It is expressed as a percent of the nominal output voltage. A power supply with tight load regulation delivers optimum voltages regardless of system configuration.



The change in output voltage due to variation of the input voltage with all other factors held constant. It is expressed as a percent of the nominal output voltage. A power supply with tight line regulation delivers optimum voltages throughout the operating range.



The 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 during access.



The magnitude of AC voltage appearing superimposed on the DC output, specified in peak to peak volts or expressed as a percent of the nominal output voltage. A power supply with clean DC output is essential for computers with high-speed processors and memory chips.



The time period, following a loss of input power, that a power supply’s output will remain within specified limits. Adequate hold time keeps the computer running during the transfer time required by a UPS unit.



A delay circuit used to initialize the computer and provide a logic signal upon low line voltage.



A circuit that shuts down the power supply if the output voltage exceeds a specified limit.



A circuit that protects the power supply and computer from excessive current, including short-circuit current.



UL, CSA, and TUV are safety agencies that test specifications such as component spacing, HI-pot insolation, leakage currents, circuit board flammability, and temperature rating.



The range of ambient temperatures within which a power supply can be safely operated.



Airflow in cubic feet per minute. A 100% increase in airflow will reduce system operating temperatures by 50% relative to ambient. For each 10°C (18°F) reduction, the life of the system is doubled. (Arrhenius equation)



Acoustical noise in dB(A) at 1 meter. Logarithmic scale. Each 3dB reduction represents 50% less noise. Issues include the pitch and speed of the fan blades, the hub size, the venturi depth, the bearing quality, and the layout of the power supply components.



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