Ambient temperature:
Usually defined as room temperature. Ambient temperature refers to the temperature of the airsurrounding a power supply.
Cross regulation:
When a power supply has more than one output, changes in the load for one output cause changes in voltage for the other outputs. To determine cross regulation, you divide the voltage change by its nominal value.
In order to provide overvoltage protection, a Silicone Controlled Rectifier (SCR) is placed over the output terminals in a power supply. This type of protection is referred to as a crowbar.
Current Limiting Circuit:
There are three types of current limiting circuits: constant, foldback and cycle-by-cycle. These circuits prevent a constant-voltage power supply from overloading.
When an operating parameter is reduced in order to compensate for changes in other parameters, the process is referred to as derating. In the case of a power supply, derating may be achieved by reducing the power level when temperatures become elevated.
Differential-mode Noise:
Separate from common-mode noise, this type of noise is calculated in output return. Differential mode currents flow in opposite directions and are out of phase with each other.
Double Insulation:
Independent insulation applied to basic insulation in order to reduce the risk of electric shock in the event of a failure of the basic insulation.
Dynamic Load:
This is a type of load that can quickly change levels. In order to state this type of load, you must calculate the total change and the rate of change.

Efficiency can be measured in several different conditions, such as full-load or nominal line. If you are using a multiple output switching power supply, you would calculate efficiency based on the total power output and the division of the separate outputs.
EMI (electromagnetic interference):
When switching transistors operate, they can produce a high-frequency energy referred to as EMI. Other causes of EMI include output rectifiers and zener diodes. This can also be called RFI or radio-frequency interference. EMI has the ability to be conducted both in input and output lines, as well as being radiated through space.
ESR (equivalent series resistor):
ESR refers to the amount of resistance in series with an ideal capacitor. If the ESR level is low, the capacitor will operate more effectively. ESR is used to determine the cause of a ripple in switching power supplies.
A filter is a frequency sensitive network that functions by removing unwanted noise and/or ripple components in rectified outputs.
Floating Ground:
A circuit whose electrical common point in not tied to earth ground. The common point potential can be different than that of earth ground.
Flyback Converter:
When you have a power supply that uses a single transistor as well as a flyback diode, this is referred to as a flyback converter.
Foldback Current Limiting Circuit:
A current limiting circuit that functions by reducing output currents during overload conditions. This type of circuit will continue to operate under a direct short circuit until a set minimum current level is attained.

Full-Bridge Converter:
This type of power supply uses four transistors to control high power levels.
Ground Loop:
Some power supplies have difficulties with feedback, usually as a result of two or more circuits on a common electrical line, which is also a common ground line. In order to rectify this situation, single-point grounding is recommended.
Half-Bridge Converters:
This type of power supply uses two transistors. Commonly used for medium-power equipment or applications.
Hi-Pot (High Potential Voltage):
A specification that is necessary for safety. Hi-Pot is the ability of a power supply to effectively handle high voltage potential that comes from either input terminals to ground, output terminals to ground or between the interaction of input and output terminals. This specification can vary on the individual power supply.
Holdup Time:
Holdup time is the total amount of time that output will remain at a regulation band following an input voltage line being turned off. This is measured both at full load and under nominal line conditions.
Input Voltage Range:
The specification for power supplies under different ranges of line voltage.
Inrush Current:
A surge of current which occurs during the turn on phase of a power supply as the bulk capacitors are charged.
To maximize a power supply’s effectiveness, proper input-to-output isolation is essential. This is calculated by discerning the degree of the electrical separation between two points, either by voltage (breakdown) and current (galvanic) or by resistance and/or capacitance.

Leakage Current:
Leakage current occurs when there are flaws in certain electrical components, or in the design of the components themselves. The result is current that flows between the current ground and output buses. It is essential to control leakage current to maintain compliance with UL and VDE safety regulations.
Line Regulation:
Line regulation occurs as the result of output voltage variance caused by input voltage variance. This is defined by the maximum amount of percentage change in output voltage as input voltage varies in a specified range.
Defined as the output current in voltage regulated power supplies.
Load Regulation:
A change of output voltage that occurs as the result of a change in an output’s load (usually from no-load to a full load). This is expressed by a percentage of nominal DC output voltage.
A unit that can control outputs of at least one slave unit. This allows proper load sharing with parallel power supplies.
MTBF (Mean Time Between Failures):
A type of standard used to calculate reliability, using the procedures set down by MIL-HDBK 217.
Multiple Output Supply:
When a power supply has two or more different output voltages, it is referred to as a multiple output supply.
A component, normally random, of deviations in output voltage. Noise is undesirable, and normally will be specified in conjunction with ripples. See also PARD and Ripple.
Nominal Output Voltage:
The model voltage of an output.

Open Frame Power Supply:
A power supply that does not have an enclosure. Commonly seen with OEM power supply manufacturing techniques. An open-frame construction may consist of a circuit board that is mounted on a chassis that does not have a cover or a stand-alone printed circuit board.
Operating Temperature:
A specified limit wherein a power supply can perform at optimal levels.
Output Impedance:
The value of an ideal voltage source in conjunction with a fictional resistor that would supply the same amount of AC voltage across a power supply’s terminal that the magnitude and frequency of alternating current would supply.
OVP (overvoltage protection):
A type of mechanism that protects load circuitry by preventing output voltage from exceeding its current, preset level. Whenever output voltage is lessened, input power is recycled to maintain the output of a power supply.
Parallel Operation:
When two or more power supplies are connected, it is referred to as parallel operation. This allows supplies to combine current into a single load.
Peak Transient Output Current:
During transient loading conditions, peak transient output current refers to the utmost amount of peak current that can be delivered to a load.
Phase-Controlled Modulation:
A type of circuit that is commonly employed in switching regulators. It is used when an operating frequency is kept at a constant level, usually 60-Hz. It can control line and load changes simultaneously without causing much dissipation.

Power Factor:
A ratio of actual power to apparent power in a circuit. Also defined as the measure of a fraction of current that is in phase with voltage and contributes to average power.
Power Factor Correction (PFC):
A method of increasing the power factor of a power supply. Typically referring to a European requirement. Needed when input power is above 70 watts - normally 60 watts of output will pass without special circuitry. Elpac power supplies with FWP or MWP in the series name have PFC circuitry included.
Power Fail signal:
A TTL signal which indicates that the input power has failed. This signal gives the user a chance to store information or switch over to backup power before the system goes down.
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.
Pulse-Width Modulation (PWM):
A type of circuit that is used in switching regulated power supplies. This type of circuit holds the frequency constant while the width of power pulse is varied, and controls both line and load changes without major dissipation.
Rated Pulse Power:
When a power supply operates on a pulse basis, this is the maximum amount of power that the power supply can deliver. This amount of rated pulse power is usually averaged out to the maximum continuous output power.
Recovery Time:
The amount of time that is necessary for a transient undershoot or overshoot in a stabilized output quantity to decay, usually within a precise limit.

By connecting more than one power supply, or using parallel power supplies, a redundancy is created. This means that should one power supply fail, the others can continue to provide power to the load. Redundancy is commonly employed when power supply failure is not an option.
A known amount of stable voltage that is used to compare output voltage in order to stabilize the amount of voltage in a power supply.
Remote Sensing:
This type of sensing can compensate for drops in IR in a power distribution bus. It is a way of moving the point of regulation between an output terminal to a load.
Response Time:
The reaction time for an output to react to a dynamic load change. Response time also includes the time that it takes for the load to settle within the tolerance band after a load change.
Reverse Voltage Protection:
A power supply’s capacity to withstand reverse voltage in output terminals when it is hooked up in reverse polarity.

A power supply output voltage AC noise component that is periodic.
Schottky Diode:
This is a type of diode that features a fast recovery time and a low forward voltage drop (0.6V). If a diode is needed for a high current, low voltage supply (5v DC) and when low losses and high speed are important, a schottky diode can be used with great success.
A secondary unit that is controlled by a master, typically used in master-slave paralleling configuration schemes.

A network comprised of a capacitor, resistor and diode that is used in switching power supplies. This network works by trapping high-energy transients as well as for the protection of sensitive components.
Soft Start:
A type of input surge-current limiting that is used in a switching power supply, where the supply drive is gradually ramped on.

Switching Frequency:
When source voltage is switched, either in a switching regulator of by chopping in a DC-to-DC convertor, switching frequency refers to the rate at which this occurs.
Thermal Protection:
A type of device that provides protection through a thermally actuated switch that will stop the operation of a power supply once the temperature inside the supply reaches a specified level.
UL (Underwriters’ Laboratories):
A public safety testing company located in the United States that is an independent, not-for-profit agency. A UL recognition may be mandatory for equipment that is used in certain applications.
UPS (Uninterruptible Power Supply):
A device that can operate either with a DC battery back-up or an AC input line. Commonly used to provide power for equipment during temporary or permanent loss of power.
VDE (Verband Deutscher Elektrotechniker):
A public safety testing company located in Germany. Similar in operation to its United States counterpart, UL.

Warm-Up Drift:
Warm-up drift occurs during a normally 30 minute time period after a cold power supply is turned on. This is calculated at constant load, ambient temperature and AC line and occurs as a result of internal components of a power supply reaching their thermal equilibrium.
An X-capacitor is used in Across-the-Line applications. Typically between the Hot and the Neutral power lines in North American applications and between L-1 and L-2 in European and other power line applications.
An Y-capacitor is used in Line-Bypass applications. Typically between the Hot line and Ground or Neutral and Ground in North American applications and between L-1 or L-2 and Ground in European and other power line applications.