ABNORMAL FAILURE. An artificially induced failure of a component, usually as a result of “abnormals” testing for regulatory agency safety compliance.
AMBIENT TEMPERATURE. The temperature of the environment, usually the still air in the immediate proximity of the power supply.
APPARENT POWER. A value of power for AC circuits that is calculated as the product of RMS current times RMS voltage, without taking the power factor into account.
BANDWIDTH. A range of frequencies over which a certain phenomenon is to be considered.
BIPOLAR TRANSISTOR. A transistor which operates by the action of minority carriers across a P/N junction; and is a current controlled device as opposed to a voltage controlled device.
BLEEDER RESISTOR. A resistor added to a circuit for the purpose of providing a small current drain, usually to provide a load for improving output voltage stability, or to assure discharge of capacitors.
BOBBIN. A device upon which the windings of a transformer or inductor are wound, which provides a form for the coil and insulates the windings from the core.
BODE PLOT. A graphic plot of gain versus frequency for a control loop, typically used to verify control loop stability, including phase margin.
BREAKDOWN VOLTAGE. A voltage level at which dielectric insulation fails by excessive leakage current or arcing. In reference to power supplies the breakdown voltage is the maximum AC or DC voltage that can be applied from input to output and/or chassis.
BRIDGE CONVERTER. A DC to DC converter topology (configuration) employing four active switching components in a bridge configuration across a power transformer.
BRIDGE RECTIFIER. A full wave rectifier circuit employing four rectifiers in a bridge configuration.
BROWNOUT. A reduction of the AC mains’ distribution voltage, usually caused deliberately by the utility company to reduce power consumption when demand exceeds generation or distribution capacity.
BROWNOUT PROTECTION. The ability of a power supply to continue operating within specification through the duration of a brownout.
BURN-IN. Operating a newly manufactured power supply, usually at rated load, for a period of time in order to force component infant mortality failures or other latent defects before the unit is delivered to a customer.
CAPACITIVE COUPLING. Coupling of a signal between two circuits, due to discrete or parasitic capacitance between the circuits.
CENTER TAP. An electrical connection made at the center of a transformer or inductor winding, usually so as to result in an equal number of turns on either side of the tap.
CENTERING. The act of setting the output voltage of a power supply under specified load conditions, usually an auxiliary output of a multiple output power supply with all outputs at half load.
COMMON MODE NOISE. Noise present equally on two conductors with respect to some reference point; often used specifically to refer to noise present on both the hot and neutral AC lines with respect to ground.
CONSTANT CURRENT POWER SUPPLY. A power supply designed to regulate the output current for changes in line, load, ambient temperature, and drift resulting from time.
CONSTANT VOLTAGE POWER SUPPLY. A power supply designed to regulate the output voltage for changes in line, load, ambient temperature, and drift resulting from time.
CONTROL CIRCUIT. A circuit in a closed-loop system, typically containing an error amplifier, which controls the operation of the system to achieve regulation.
CONVECTION. The transfer of thermal energy in a gas or liquid by currents resulting from unequal temperatures.
CONVERTER. An electrical circuit which accepts a DC input and generates a DC output of a different voltage, usually achieved by high frequency switching action employing inductive and capacitive filter elements.
COOLING. Removal of heat, which, in a power supply, is generated by transformation, rectification, regulation, and filtering. It can be accomplished using radiation, convection, forced air, or liquid means.
CREST FACTOR. In an AC circuit, Crest Factor is the mathematical ratio of the peak to RMS values of a waveform. Crest factor is sometimes used for describing the current stress in AC mains supply wires, since for a given amount of power transferred, the RMS value, and hence the losses, become greater with increasing peak values. Crest Factor gives essentially the same information as Power Factor, and is being replaced by Power Factor in power supply technology.
CROSS REGULATION. The effect of a load change on one output to the regulation of another output. It usually only applies to non-postregulated (quasi) outputs.
CROWBAR. An overvoltage protection method which shorts the power supply output to ground in order to protect the load when an overvoltage fault is detected.
CURRENT LIMITING. An overload protection circuit that limits the maximum output; current of a power supply in order to protect the load and/or the power supply.
CURRENT MODE. A control method for switch-mode converters where the converter adjusts its regulating pulsewidth in response to measured output current and output voltage, using a dual loop control circuit. Since output current is measured, current mode control allows accurate sharing between power supplies.
CURRENT MONITOR. An analog power supply signal which is linearly proportional to output current flow. Usually only feasible for single output power supplies.
DERATING. A reduction in an operating specification to improve reliability. For power supplies it is usually a specified reduction in output power to facilitate operation at higher temperatures.
DESIGN LIFE. The expected lifetime of a power supply during which it will operate to its published specifications.
DIFFERENTIAL MODE NOISE. Noise that is measured between two lines with respect to a common reference point excluding common-mode noise. The resultant measurement is the difference of the noise components of the two lines. The noise between the DC output and DC return is usually measured in power supplies.
DRIFT. The change in an output voltage, after a warm-up period, as a function of time when all other variables such a line, load, and operating temperature are held constant.
DROPOUT. The lower limit of the AC input voltage where the power supply just begins to experience insufficient input to maintain regulation. The dropout voltage for linears is quite load dependent. For most switchers it is largely design dependent, and to a smaller degree load dependent.
EFFICIENCY. The ratio of total output power to input power expressed as a percentage. Normally specified at full load and nominal input voltage.
ELECTRONIC LOAD. An electronic device designed to provide a load to the outputs of a power supply, usually capable of dynamic loading, and frequently programmable or computer controlled.
EMI. Abbreviation for Electromagnetic Interference, which is the generation of unwanted noise during the operation of a power supply or other electrical or electronic equipment.
ESR. Equivalent Series Resistance. The value of resistance in series with an ideal capacitor which duplicates the performance characteristics of a real capacitor.
FAULT MODE INPUT CURRENT. The input current to a power supply with a short circuit on the output.
FERRORESONANT POWER SUPPLY. Power supply used at higher power levels in fixed applications, since they are very heavy. Can only be used effectively when the line frequency is very stable as they are sensitive to variations of input AC frequencies.
FET. Field Effect Transistor, a majority carrier voltage controlled transistor.
FILTER. A frequency-sensitive network that attenuates unwanted noise and ripple components of a rectified output.
FLOATING OUTPUT. An output of a power supply that is not connected or referenced to any other output, usually denotes full galvanic isolation. They generally can be used as either positive or negative outputs. Non-floating outputs share a common return line, and are hence DC referenced to one another.
FLYBACK CONVERTER. The flyback converter is the simplest type of switcher. In most cases, it uses one switch and only needs one magnetic element – the transformer. Flybacks are limited to outputs of generally lower than 200 Watts.
FOLDBACK CURRENT LIMITING. A type of protection circuit where the output current decreases as the overload increases. The output current reaches a minimum as the load approaches a short-circuit condition.
FORWARD CONVERTER. Similar to flyback converter but the forward converter stores energy in the output inductor instead of the transformer.
FULL BRIDGE FORWARD CONVERTER. The full bridge is more complex than other switcher topologies. It has the capability for very high performance. It can product high power with four switchers and requires only two magnetic elements.
GROUND. An electrical connection to earth or some other conductor that is connected to earth. Sometimes the term “ground” is used in place of “common,” but such usage is not correct unless the connection is also connected to earth.
GROUND LOOP. An unintentionally induced feedback loop caused by two or more circuits sharing a common electrical ground.
HAVERSINE. A waveform that is sinusoidal in nature, but consists of a portion of a sine wave superimposed on another waveform. The input current waveform to a typical off-line power supply has the form of a haversine.
HEADROOM. Used in conjunction with series pass regulators, and is the difference between the input and output voltages.
HEATSINK. Device used to conduct away and disperse the heat generated by electronic components.
HIPOT. Abbreviation for High Potential, and generally refers to the high voltages used to test dielectric withstand capability for regulatory agency electrical safety requirements.
HOLD-UP TIME. The length of time a power supply can operate in regulation after failure of the AC input. Linears have very short hold-up times due to the CV squared energy storage product of their low voltage secondary side output capacitors. Switchers have longer times due to their higher voltage primary side energy storage capacitors.
INDUCED NOISE. Noise generated in a circuit by a varying magnetic field produced by another circuit.
INHIBIT. The ability to electrically turn off the output of a power supply from a remote location.
INPUT LINE FILTER. An internally or externally mounted low-pass or band-reject filter at the power supply input which reduces the noise fed into the power supply.
INRUSH CURRENT. The peak current flowing into a power supply the instant AC power is applied. This peak is usually much higher than the steady state input current due to the charging of the input filter capacitors.
INRUSH CURRENT LIMITING. A circuit which limits the amount of inrush current when a power supply is turned on.
INVERTER. A power supply which produces an AC output, usually from a DC input.
ISOLATION. Two circuits that are completely electrically separated with respect to DC potentials, and almost always also AC potentials. In power supplies, it is defined as the electrical separation of the input and output via the transformer.
ISOLATION VOLTAGE. The maximum AC or DC voltage which maybe continuously applied from input to output and/or chassis of a power supply.
LAYER WINDING. A transformer winding technique where the primary and secondary windings are wound over each other and separated by an insulation layer.
LEAKAGE CURRENT. A term relating to current flowing between the AC supply wires and earth ground. The term does not necessarily denote a fault condition. In power supplies, leakage current usually refers to the 60 Hertz current which flows through the EMI filter capacitors which are connected between the AC lines and ground (Y caps).
LINE REGULATION. The change in output voltage when the AC input voltage is changed from minimum to maximum specified. It is usually a small value, and may be near zero with current mode control.
LINEAR REGULATOR. A regulating technique where a dissipative active device such as a transistor is placed in series with a power supply output to regulate the output voltage.
LOAD REGULATION. The change in output voltage when the load on the output is changed.
LOCAL SENSING. Using the voltage output terminals of the power supply as sense points for voltage regulation.
LOGIC ENABLE. The ability to turn a power supply on and off with a TTL signal. A logic low generally turns the supply off; a logic high turns it on.
LONG TERM STABILITY. Power supply output voltage change due to time with all other factors held constant. This is expressed in percent and is a function of component aging.
MAGNETIC AMPLIFIER. Sometimes abbreviated “Mag Amp,” a saturating inductor which is placed in series with a power supply output for regulation purposes.
MAINS. The utility AC power distribution wires.
MARGINING. Adjusting a power supply output voltage up or down from its minimal setting in order to verify system performance margin with respect to supply voltage. This is usually done electrically by a system-generated control signal.
MINIMUM LOAD. The minimum load current/power that must be drawn from the power supply in order for the supply to meet its performance specifications. Less frequently, a minimum load is required to prevent the power from failing.
MODULAR. A physically descriptive term used to describe a power supply made up of a number of separate subsections, such as an input module, power module, or filter module. Modular construction tends to lower the MTBF.
MTBF. (Mean Time Between Failures) may be calculated or demonstrated. The usual calculation is per Mil-Std 217 rev E. Demonstrated reliability is usually determined by temperature accelerated life testing. Demonstrated MTBF is almost always greater than calculated MTBF.
NOISE. Noise is the aperiodic, random component of undesired deviations in output voltage. Usually specified in combination with ripple.
See: PARD and also: Ripple.
NORMAL VALUE. A usual, average, normal, or expected operating condition. This stated value will probably not be equal to the value actually measured.
OFF LINE. A power supply which receives its input power from the AC line, without using a 50/60 Hz power transformer prior to rectification and filtering, hence the term “off line” power supply.
OPEN FRAME. A power supply where there is no external metal chassis; the power supply is provided to the end user essentially as a printed circuit board which provides mechanical support as well as supporting the components and making electrical connections.
OPTOISOLATOR. An electro-optical device which transmits a signal across a DC isolation boundary.
OUTPUT GOOD. A power supply status signal which indicates that the output voltage is within a certain tolerance. An output which is either too high or too low will deactivate the Output Good signal.
OUTPUT IMPEDANCE. The ratio of change in output voltage to change in load current.
OUTPUT NOISE. The AC component that may be present on the DC output of a power supply. Switch-mode power supply output noise has two components: a lower frequency component at the switching frequency of the converter and a high frequency component due to fast edges of the converter switching transitions. Noise should always be measured directly at the output terminals with a scope probe having an extremely short grounding lead.
OVERLOAD PROTECTION. A power supply protection circuit that limits the output current under overload conditions.
OVERSHOOT. A transient output voltage change which exceeds the high limit of the voltage accuracy specification and is caused by turning the power supply on or off, or abruptly changing line or load conditions.
OVERTEMP WARNING. A TTL compatible signal which indicates that an overtemperature condition exists in the power supply. Most commercial power supplies are designed to shut down if an overtemperature condition exists.
OVERVOLTAGE PROTECTION. A circuit which either shuts down the power supply or crowbars the output in the event of an overvoltage condition.
PARALLEL OPERATION. Connecting the outputs of two or more power supplies with the same output voltage for the purpose of obtaining a higher output current. This requires power supplies specially designed for load sharing.
PARD. Periodic and random deviation, referring to the sum of all ripple and noise components on the DC output of a power supply, regardless of nature or source.
PEAK POWER. The absolute maximum output power that a power supply can produce without immediate damage. Peak power capability is typically well beyond the continuous reliable output power capability and should only be used infrequently.
POST REGULATOR. A secondary regulating circuit on an auxiliary output of a power supply to provide full regulation on that output.
POWER FACTOR. The ratio of true power to apparent power in an AC circuit. In power conversion technology, power factor is used in conjunction with describing the AC input current to the power supply.
POWER FAIL. A power supply interface signal interface signal which gives a warning that the input voltage will no longer sustain full power regulated output.
PRELOAD. A small amount of current drawn from a power supply to stabilize its operation. Preloads are usually provided by a bleeder resistor.
PRIMARY. The input section of an isolated power supply which is connected to the AC mains and hence has dangerous voltage levels present.
PULSE WIDTH MODULATION (PWM). A switching power conversion technique where the on-line (or width) of a duty cycle is modulated to control power transfer for regulating power supply outputs.
PUSH-PULL CONVERTER. A switch mode power supply topology which utilizes a center-tapped transformer and two power switches. The two switches are alternately driven on and off.
QUASI REGULATED OUTPUT. The regulation of an auxiliary output that is accomplished by regulation of the main output. A transformer turns ratio, commensurate with the desired auxiliary output voltage, is used in conjunction with the output around which the main control loop is closed. Quasi regulated outputs can be reasonably well regulated, but are signifiCan’tly affected by second order effects in the converter.
RATED OUTPUT CURRENT. The maximum load current that a power supply can provide at a specified ambient temperature.
REFLECTED RIPPLE CURRENT. The RMS or peak-to-peak AC current present at the input of the power supply which is a result of the switching frequency of the converter.
REGULATION. The ability of a power supply to maintain an output voltage within a specified tolerance as referenced to changing conditions of input voltage and/or load.
REGULATION BAND. The total error band allowable for an output voltage. This includes the effects of all of the types of regulation: line, load, and cross.
REMOTE INHIBIT. A power supply interface signal, usually TTL compatible, which commands the power supply to shut down one or all outputs.
REMOTE SENSE. Wires connected in parallel with power supply output cables such that the power supply can sense the actual voltage at the load to compensate for voltage drops in the output cables and/or isolation devices.
RETURN. The designation of the common terminal for the power supply outputs. It carries the return current for the outputs.
REVERSE VOLTAGE PROTECTION. A protection circuit that prevents the power supply from being damaged in the event that a reverse voltage is applied at the input or output terminals.
RFI. An abbreviation for Radio Frequency Interference, which is undesirable noise produced by a power supply or other electrical or electronic device during its operation. In power supply technology, RFI is usually taken to mean the same thing as EMI.
RIPPLE AND NOISE. The amplitude of the AC component on the DC output of a power supply usually expressed in millivolts peak-to-peak or RMS. For a linear power supply it is usually the frequency of the AC mains. For a switching power supply, it is usually the switching frequency of the converter stage.
SAFETY GROUND. A conductive path to earth that is designed to protect persons from electrical shock by shunting away any dangerous currents that might occur due to malfunction or accident.
SECONDARY. The output section of an isolated power supply which is isolated from the AC mains and specially designed for safety of personnel who might be working with power on the system.
