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Agency Approval 
UL, CSA and TUV are safety agencies that test specifications such as component spacing, Hi-Pot isolation, leakage currents, circuit board flammability, temperature rating and soon. 

Auto-Ranging 
Automatically senses the input voltage and adapts the powers supply itself to the proper range (115 VAC or 230 VAC). No manual switch is needed. 

CFM (Cubic Feet per Minute) 
A measurement unit of the fan cooling airflow through an average ATX system Typical values range from 25 to 35 CFM. 

Dropout 
A condition that allows the input voltage to the power supply to drop below the minimum specified input voltage. 

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

Electromagnetic Interference (EMI) 
EMI, which is the noise generated by the switching operation of the power supply, can be classified into Conducted EMI and Radiated EMI. Conducted EMI, which is fed back to the power line, is often suppressed by an EMI filter placed at the input to the power supply. Radiated EMI, which is radiated into the free space, is often suppressed by housing the switching power supply with a metal case. The FCC governs Conducted and Radiated EMI emission levels. 

Hold-Up Time 
The time period following a loss of AC input power during which the power supply could maintain output regulation under maximum DC output power. 

Latch Off 
After detecting a fault condition, a power supply usually shuts itself down to protect its electronic load. Even if the fault condition disappears, the power supply does not resume its normal operation unless manual or electronic intervention is invoked. Manual intervention could be done by unplugging and plugging the AC input to the power supply. Electronic intervention could be done through electronic signals in the server system. 

Line Regulation 
The change in the output voltage due to the variation of the input voltage from its minimum to its maximum within specifications expressed as a percent of the nominal output voltage. 

Load Regulation 
The change in the output voltage due to the variation of the output current from its minimum to its maximum within specifications expressed as a percent of the nominal output voltage. 

Mean Time Between Failure (MTBF) 
A measurement of the relative reliability of a power supply based upon measurement or calculation according to MIL-HDBK-217. 

Monotonic Transition 
A waveform transits itself from one level to another in a monotonic fashion without intermediate oscillation. 

Operating Range 
Specify the minimum and maximum allowable input voltage / frequency and ambient temperature for the power supply to safely operate within specifications. 

Overcurrent Protection (OCP) 
A circuit that protects the power supply and the electronic load from excessive current, including Short Circuit Protection (SCP). 

Overvoltage Protection (OVP) 
A circuit that shuts down the power supply whenever the output voltage exceeds a specified limit. 

PFC (Power Factor Correction) 
Power Factor (PF) is the ratio of true ac input power to apparent ac input power. A higher PF signifies a more effective use of electric power. PFC shapes the input current so that the current waveform is in phase with and of the same shape as the voltage waveform of the input voltage. The PF without PFC is around 0.55 to 0.60 while the PF with PFC is around 0.95 to 1. PFC can lower input current and harmonic current, hence reducing power line copper loss. 

Power OK (PWOK) 
A logic output provided by the power supply that signals the computer system that all DC output voltages are within their specified ranges. 

Ripple & Noise 
The peak to peak amplitude of periodic signal and random noise, which are superimposed on the DC output voltage by the switching operation of the power supply, are often measured over a frequency bandwidth of 10 Hz to 20 MHz and specified as 1 percent of the nominal DC output voltage. 

Rise Time 
Rise time is defined as the time required for any output voltage to rise from 10% to 90% of its nominal value. 

Sag 
The condition on which the input voltage drops below the minimum specified input voltage. 

Stand by Voltage 
An output voltage that is present whenever the AC power is inputted to the power supply. 

Surge 
The condition on which the input voltage exceeds the maximum specified input voltage. 

Transient Response 
The time required for the output voltage to return within the regulation envelope following a 50% load change.50