Glossary of Power Supply Terms

Ambient temperature

the temperature of the still air surrounding a power supply.

Brownout

the condition created during peak utility company grid usage, intentionally reducing their line voltage by approximately 10 - 15% to counter excessive demand.

Common-mode noise

the component of noise in a power supply that is common to both the dc output and return lines with respect to the source power return point (input neutral).

Cross-regulation

in a multiple output power supply, the voltage change at one output caused by the load change on another, expressed as a percentage of the nominal voltage.

Derating

The reduction in output power at elevated temperatures. (see temperature curves by supply)

Differential-mode noise

the component of noise in a power supply (excluding common-mode noise) that is measured between the dc output and output return.

Efficiency

the ratio of total output power to input power, expressed as a percentage. Efficiency must be specified at a specific combination of load and input voltage.

Electromagnetic interference (EMI)

unwanted high-frequency energy that is conducted through the input/output lines or radiated into space by switching power supplies. Also known as radio-frequency interference (RFI).

Forward converter

power supply switching circuit that transfers energy to the transformer secondary when the switching transistor is on. Forward converter circuits store minimal energy in the transformer.

Ground loop

a condition that causes unwanted feedback when two or more circuits share a common electrical return or ground lines.

Holdup time

the time during which a power supply's output voltage remains within specified limits following the loss or removal of input power. Holdup time is normally measured at full load and nominal line conditions.

Inrush current

the peak instantaneous input current drawn by a power supply when it is initially turned on.

Isolation

the electrical separation between the input and output of a power supply due primarily by the power transformer. The isolation is a function of materials and spacings throughout the supply.

Isolation voltage

the maximum ac or dc voltage that may be continuously applied from input to output.

Line regulation

the maximum change in output voltage, expressed as a percentage, that occurs as the input voltage varies over its specified limits, with load and temperature constant.

Load regulation

the change in output voltage, expressed as a percentage of nominal voltage that occurs as the load changes from minimum to maximum, at constant line and constant temperature. Load change may be specified for other than no load to full load as, for example, 50% load to full load.

Mean time between failure (MTBF)

the failure rate of a power supply, expressed in hours, either predicted as prescribed by Military Standard MIL-HDBK-217 or measured as prescribed by Military Standard MIL-STD-781C.

Operating temperature range

the range of ambient or case temperatures through which a power supply may operate safely and perform within specified limits.

Output current protection (OCP)

a protective feature that keeps the output current of a power supply within predetermined limits during overload to prevent damage to the supply or the load. The supply automatically returns to normal operation following the removal of the overload. The term “hiccup” is commonly used as the supply tries to restart during constant intervals.

Overload protection

a protective feature that limits the output current of a power supply under overload conditions so that it will not be damaged.

Overvoltage protection (OVP)

a protective feature that shuts down a power supply (reduces the output voltage to a minimal level) to prevent damage to the load when the output voltage exceeds a predetermined limit.

Parallel operation

the connection of the outputs of two or more power supplies of the same output voltage to obtain a higher output current than either supply can provide alone. Parallel operation requires power supplies that are specifically designed to share the load.

Periodic and random deviation (PARD)

the unwanted periodic (ripple) or periodic (noise) deviation of the power supply output voltage from its nominal value. Ripple is a function of the input line and switching components. PARD is expressed in millivolts peak-to-peak or rms, at a specified bandwidth.

Pulse-width modulation (PWM)

a method of regulating the output voltage of a switching power supply by varying the width, but not the height, of a train of pulses that drives a power switch.

Rated output current

the maximum load current that a power supply is designed to provide at a specified ambient temperature.

Regulator

the power supply circuit that controls or stabilizes the output voltage at a preset value.

Remote sensing

a technique for regulating the output voltage of a power supply at the load by connecting the regulator error-sensing leads directly to the load. Remote sensing compensates for voltage drops in the load leads.

Reverse voltage protection

a feature that protects a power supply from damage caused by a voltage of reverse polarity applied at the input or output terminals.

Short-circuit protection

a protective feature that limits the output current of a power supply to prevent damage to the supply caused by short circuits.

Soft start

a feature that ensures the smooth, controlled rise of the output voltage. This feature protects the switching transistors from transients when the power supply is turned on.

Storage temperature

the range of ambient temperatures through which an inoperative power supply can remain in storage without degrading is subsequent operation.

Switching frequency

the rate at which the dc voltage is switched in a dc-dc converter or switching power supply.

Switching regulator

a high-efficiency switching circuit that operates in a closed loop system to regulate the voltage across a load, generally by means of a pulse-width modulator.

Temperature coefficient

the average change in output voltage per change in degree of a baseplate temperature, expressed as a percentage of nomimal output voltage, over a specified temperature range.

Thermal protection

a protective feature that shuts down a power supply if its internal temperature exceeds a predetermined limit.

Tracking

a characteristic of a multiple-output power supply that describes the changes in the voltage of one output with respect to changes in the voltage or load of another.

Transient recovery time

the time required for the output voltage of a power supply to settle within specified output accuracy limits following a step change in output load current or input voltage.

Warm-up time

the time required after a power supply is initially turned on before it operates according to specified performance limits