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Common glossaries for power supply units

(1) Power / Voltage / Current / Power / Efficiency




Power Supply Unit (PSU)

A power supply unit converts mains AC to low-voltage regulated DC power.

Alternating Current (AC)

Alternating current flow of electric charge that periodically reverses.



The number of AC power cycles in a given time period and measured in Hertz.

Direct Current (DC)

The unidirectional flow of an electric charge.



Ripple is the residual periodic variation of the DC voltage which has been derived from an alternating current (AC) source.

Aapparent Power (S)


Apparent power is a measure of alternating current (AC) power.  Apparent power (S)² = Active Power (P)² + Reactive Power (Q)²

Active Power (P)


Also called real power, or true power. Active power actual consumed power. Electricity bill is metered in real power consumption of a facility.

Reactive Power (Q)


Also called phantom power. The inductive or capacitive loads which bring phantom power increase power stations’ burden to generate electricity in vain.

Power Factor (PF)


Power Factor is defined as the ratio of reactive power to apparent power. PF = Active Power (P) / Aapparent Power (S).

Power Factor Correction (PFC)

The power factor correction is designed to bring PF to near 1. There are two types of PFC, one is passive PFC, the other one is active PFC.

Rated Power


The maximum power output available from PSU. Watts (W) = Voltage (V) * Current  (A).

Output Voltage


The voltage output available from PSU. Voltage (V) = Watts (W) / Current (A).

Output  Current


The current output available from PSU. Current (A) = Watts (W) / Voltage (V).

Efficiency (EFF)


Efficiency (EFF) is defined as the ratio of input power to output power. Maximum EFF is up to 100%.

(2) Functions




Derating Curve

The derating curve is drawn from output power of PSU according to environmental temperature changing.

Hold Up Time 


The time which PSUs can maintain power output in the event of main power failure.

Constant Voltage (CV)


The PSUs can provide constant voltage regardless of change in load. Suitable for normal load.

Constant Current (CC)


The PSUs can provide constant current regardless of change in load.  Suitable for chargers, LEDs , or serial load.

No-load Power Consumption


No-load power consumption is the electric power only consumed by PSU.

Peak Power


The PSUs can provide high peak power out of rated power in short period.

Power Density


Power density is the amount of power output of PSU per unit volume.

Redundancy Design

The redundancy PSU can work with other redundancy PSUs to improve power output stability.

Common Redundant Power Supply (CRPS)

CRPS is the Intel’s standard for redundant power supply.

Digital Power Supply

The digital PSUs are built-in MCU and DSP.


This standard is for digital PSU to communicate the system.

(3) Safety




Surge Suppression


Surge Suppression is designed to protect electrical devices from voltage spikes.

Inrush Current


Inrush current is the instantaneous high input current drawn by a power supply at turn-on.

Over Current Limit


The PSUs limit maximum current to protect the system.

Over Power Protection (OPP)

The PSUs shut down power to protect the system when load is over limit.

Over Current Protection (OCP)

The PSUs shut down power to protect the system when current is over limit.

Over Voltage Protection (OVP)

The PSUs shut down power to protect the system when voltage from main power is over limit.

Short Circuit Protection (SCP)

The PSUs shut down power to protect the system in case of short circuit.

Over Temperature Protection (OTP)

The PSUs shut down power to protect the system when environmental temperature exceed limit.

Mean Time Between Failure (MTBF)


Mean time between failure (MTBF) refers to the average amount of operation time before PSUs fail.

(4) Certification




Operating Temperature


The PSUs can operate normally in the ambient temperature range.

Storage Temperature


The PSUs can store in the ambient temperature range.

Operating Humidity


The PSUs can operate normally in the ambient humidity range.

Storage Humidity


The PSUs can store in the ambient humidity range.

Operating Altitude


The PSUs can operate normally in the ambient altitude range.

Electromagnetic Interference (EMI)

Electronic devices generate EM fields which interfere with other electronic devices.

Electromagnetic Susceptibility (EMS)

The ability of a device resists EM fields which is generated by other devices.

Electromagnetic Compatibility (EMC)

Electromagnetic Compatibility (EMC) refers to Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC).


80Plus is an efficiency certification for PSUs. There are six levels namely Base, Bronze, Silver, Gold, Platinum, and Titanium.

Energy Star

Energy Star is an American efficiency certification for appliances, electronics, etc.

Ingress Protection (IPXX)

Ingress Protection ratings are defined levels of sealing effectiveness of electrical enclosures against intrusion from foreign dirt and water.

IEC 62368

IEC 62368 is a new safety standard for ICT and AV equipment compiled by the IEC technical committee.

EN 62368

EN 62368 is a new safety standard compiled by Technical Inspection Association (TUV) in Germany, available for Europe.

UL 62368

UL 62368 is a new safety standard compiled by Underwriters Laboratories Inc. (UL) in USA, available for North America.

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