Glossary of Technical Terms
27
Glossary of Technical Terms
Glossary
AC-OK SIGNAL. The signal used to indicate the loss of AC input voltage from the
115/230V line.
ALTITUDE TESTING. Generally performed to determine the proper functionality of
equipment in airplanes and other flying objects. MIL-STD-810.
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 power factor into account.
BANDWIDTH. A range of frequencies over which a certain phenomenon is to be considered.
BASEPLATE. All modular products have an aluminum mounting base at which Vicor
specifies operating temperatures and which should be affixed to a thermally conductive
surface for cooling.
BELLCORE SPECIFICATION. A telecommunications industry standard developed
by Bellcore.
BIPOLAR TRANSISTOR. A transistor that 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, to assure discharge of capacitors.
BOBBIN. A device upon which the windings of a transformer or inductor are wound, it
provides a form for the coil and insulates the windings from the core.
BOOSTER CONVERTER. A “slave” module in a driver/booster combination, connected to the
driver module. Several boosters can be connected to a driver module.
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-DC converter topology (configuration) employing two or
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.
BRITISH TELECOM STANDARDS. A telecommunications industry standard developed by
the British PTT authorities.
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.
12 1-800-927-9474
27-1
Applications Manual
Glossary (cont)
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.
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.
C-GRADE. Industry standard where the operating temperature of a device does not drop
below –20 degrees Celsius.
CHASSIS MOUNT CONFIGURATION. A configuration where the modules or AC front ends
are mounted directly to the chassis.
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.
COMPAC. A Vicor DC input power supply that provides EMC filtering and transient
suppression for industrial, military and telecommunications markets.
CONSTANT CURRENT POWER SUPPLY. A power supply designed to regulate output
current for changes in line, load, ambient temperature and drift resulting from time.
CONSTANT VOLTAGE POWER SUPPLY. A power supply designed to regulate 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, that controls the operation of the system to achieve regulation.
CONVERTER. An electrical circuit that 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.
CREST FACTOR. In an AC circuit, 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 that shorts the power supply output to ground
in order to protect the load when an overvoltage fault is detected.
27-2
12 1-800-927-9474
Glossary
Glossary (cont)
CSA. Canadian Standards Association. Defines the standards and safety requirements for
power components.
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.
CURRENT MONITOR. An analog power supply signal that is linearly proportional to output
current flow.
DC-OK SIGNAL. Signal used to monitor the status of the DC output.
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.
DISTRIBUTED POWER ARCHITECTURE. A central power source that is delivered to a local
site, usually in the form of DC.
DRIFT. The change in an output voltage, after a warm-up period, as a function of time when all
other variables such as line, load, and operating temperature are held constant.
DRIVER MODULE. The controlling module in a standalone or driver/booster configuration.
The driver module contains all the control circuitry.
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 largely
line dependent, whereas for most switchers it is largely load dependent, and to a smaller degree
line dependent.
DYNAMIC LOAD REGULATION. The delta in output voltage when the output load is
rapidly changed.
EFFICIENCY. The ratio of total output power to input power expressed as a percentage.
Normally specified at 75% full load and nominal input voltage.
12 1-800-927-9474
27-3
Applications Manual
Glossary (cont)
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.
EMC. Electromagnetic Compatibility, relating to compliance with electromagnetic emissions
and susceptibility standards.
EMI. 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 that
duplicates the performance characteristics of a real capacitor.
FAULT TOLERANT CONFIGURATION. A method of parallel operation, using output
isolating diodes, in which the failure of a single supply (module) will not result in a loss of
power. The total current of the parallel system must not exceed the load requirements to a point
where the failure of a single unit will not result in a system overload.
FET. Field Effect Transistor, a majority carrier-voltage controlled transistor.
FINMOD. A flangeless/finned packaging option available on Vicor’s VI / MI Family
converters and accessory modules.
FLATPAC. A Vicor AC-DC switcher available with one, two or three outputs, with total power
rating from 50 to 600W.
FLOATING OUTPUT. An output of a power supply that is not connected or referenced to any
other output, usually denoting full galvanic isolation. Floating outputs can generally be used as
either positive or negative outputs. Non floating outputs share a common return line and are
hence DC referenced to one another.
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. A switching power supply in which the energy is transferred from
the input to the output during the “on” time of the primary switching device.
GATE IN. The Gate In pin of the module may be used to turn the module on or off. When Gate
In is pulled low (<1 Volt @ 4 mA, referenced to –Vin), the module is turned off. When Gate In
is floating (open collector) the module is turned on. The open circuit voltage of the Gate In pin
is less than 10 Volts, referenced to –Vin. A Gate In/Gate Out connection is necessary to run
driver/booster configurations.
GATE OUT. The Gate Out pin is the clock pulse of the converter. It is used to synchronize
booster modules to a driver module for high power arrays.
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.
27-4
12 1-800-927-9474
Glossary
Glossary (cont)
GROUND LOOP. An unintentionally induced feedback loop caused by two or more circuits
sharing a common electrical ground.
HAM (VI-HAM Harmonic Attenuator Module). The VI-HAM is a component level front end
that accommodates universal input voltage (85-264), provides line rectification, filtering,
transient protection, unity power factor, inrush limiting and a DC output compatible with the
300V input families of DC-DC converters.
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, headroom is the difference
between the input and output voltages.
HEATSINK. A medium of high thermal mass that can absorb (sink) heat indefinitely with
negligible change in temperature. Heatsinks are not necessarily needed with Vicor modules, and
their use is highly dependent on the individual application, power and ambient temperature.
HIGH LINE INPUT. The maximum steady-state input voltage on the input pin.
HIPOT. Abbreviation for high potential, and generally refers to the high voltages used to test
dielectric withstand capability for regulatory agency electrical safety requirements.
HOLDUP CAPACITOR. A capacitor whose energy is used to provide output voltage for a
period after the removal of input voltage.
HOLDUP TIME. The length of time a power supply can operate in regulation after failure of
the AC input. Linears have very short holdup times due to the CV2 energy storage product of
their low voltage secondary side output capacitors. Switchers have longer times due to higher
voltage primary side energy storage capacitors.
HOT SWAP. Insertion and extraction of a power supply into a system while power is applied.
IAM (VI-IAM Input Attenuator Module). A Vicor filter module used to provide EMC
conducted noise filtering and transient protection.
I-GRADE. Industry standard where the operation temperature of a device does not drop below
–40 degrees Celsius.
IMPEDANCE. The ratio of voltage to current at a specified frequency.
INDUCED NOISE. Noise generated in a circuit by varying a magnetic field produced by
another circuit.
INPUT LINE FILTER. An internally or externally mounted lowpass or band-reject filter at the
power supply input that reduces the noise fed into the power supply.
12 1-800-927-9474
27-5
Applications Manual
Glossary (cont)
INPUT LINE FILTER. An internally or externally mounted lowpass or band-reject filter at the
power supply input that 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 may be much higher than the steady state input current due to the charging
of the input filter capacitors.
INRUSH CURRENT LIMITING. A circuit that limits the amount of inrush current when a
power supply is turned on.
ISOLATION. Two circuits that are completely electrically separated with respect to DC
potentials, and almost always 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 that may be continuously applied
from input to output and/or chassis of a power supply.
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 Hz current that flows through the EMC filter capacitors
connected between the AC lines and ground (Y caps).
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.
LINE REGULATION. The change in output voltage when the AC input voltage is changed
from minimum to maximum specified.
LINE VOLTAGE (Mains). The sine wave voltage provided to the power supply, usually
expressed in volts rms.
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.
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.
LOW LINE. The minimum steady state voltage that can be applied between the + and - input
pins of a converter and still maintain output regulation.
MAINS. The utility AC power distribution wires.
MARGINING. Adjusting a power supply output voltage up or down from its nominal setting in
order to verify system performance margin with respect to supply voltage. This is usually done
electrically by a system-generated control signal.
27-6
12 1-800-927-9474
Glossary
Glossary (cont)
MEGA MODULES. A chassis mount packaging option that incorporates one, two or
three VI/MI-200 Family converters for single, dual or triple outputs having a combined
power of up to 600W.
M-GRADE. An industry standard where the operating temperature of a device does not drop
below –55 degrees Celsius.
MIL-SPECS. Military standards that a device must meet to be used in military environments.
MINIMOD. A junior size (VI-J00) version of the VI/MI-200 Family of DC-DC converters
offering up to half the power in a 2.28" x 2.4" x 0.5" package.
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 supply from failing.
MODULE EVALUATION BOARD. A test fixture used to evaluate Vicor DC modules. AC and
DC input versions are available.
MTBF (Mean Time Between Failure). MTBF is the point at which 63% of a given population
no longer meet specification. It can either be calculated or demonstrated. The usual calculation
is per MIL-STD-217 Rev. E. Demonstrated reliability is usually determined by temperature
accelerated life testing and is usually greater than calculated MTBF.
NOMINAL INPUT. The center value for the input voltage range.
NOMINAL 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 that 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 that provides mechanical support
as well as supporting the components and making electrical connections.
OPERATING TEMPERATURE. The range of temperatures in which a unit can operate within
specifications.
OPTOISOLATOR. An electro-optical device that transmits a signal across a DC
isolation boundary.
OR’ING DIODES. Diodes used to isolate supplies from one another under a fault condition.
OUTPUT FILTERING. Filter used to reduce switching power supply noise and ripple.
12 1-800-927-9474
27-7
Applications Manual
Glossary (cont)
OUTPUT GOOD. A power supply status signal that indicates the output voltage is within a
certain tolerance. An output that 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 usually 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.
OUTPUT POWER RATING. The maximum power in watts that the power supply can provide
and still maintain safety agency approvals.
OUTPUT VOLTAGE ACCURACY. See Setpoint Accuracy.
OVERLOAD PROTECTION. A power supply protection circuit that limits the output current
under overload conditions.
OVERSHOOT. A transient output voltage change exceeding the high limit of the voltage
accuracy specification caused by turning the power supply on or off, or abruptly changing line
or load conditions.
OVERTEMP WARNING. A TTL compatible signal that indicates an overtemperature
condition exists in the power supply.
OVERVOLTAGE PROTECTION (OVP). A circuit that either shuts down the power supply or
crowbars the output in the event of an output overvoltage condition.
PARALLEL BOOST. VI/MI-200 Family booster modules may be added to a driver to create
multi-kilowatt arrays. Boosters do not contain any feedback or control circuitry.
PARALLEL OPERATION. Connecting the outputs of two or more power supplies together 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 output power
capability and the resulting average power should not exceed rated specifications.
PI FILTER. A commonly used filter at the input of a switching supply or DC-DC converter
to reduce reflected ripple current. The filter usually consists of two shunt capacitors with
inductance between them.
27-8
12 1-800-927-9474
Glossary
Glossary (cont)
POST REGULATOR. A secondary regulating circuit on an auxiliary output of a power supply
that provides regulation on that output.
POWER FAIL. A power supply interface signal that gives a warning that the input voltage will
no longer sustain full power regulated 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 AC input current to
the power supply.
PRELOAD. A small amount of current drawn from a power supply to stabilize its operation.
PRIMARY. The input section of an isolated power supply, it is connected to the AC mains and
hence has dangerous voltage levels present.
PRODUCT GRADE. The environmental and acceptance tests performed on Vicor products.
PULSE WIDTH MODULATION (PWM). A switching power conversion technique where the
on-time (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 that utilizes a centertapped 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 are significantly 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 that 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, temperature and time.
REGULATORY AGENCIES. CSA: Canadian Standards Association; FCC: Federal
Communications Commission; FTZ: Fernmelde Technisches Zentralamt; TÜV: Technischer
Überwachungs Verein; U.L.: Underwriters Laboratory; VDE: Verband Deutscher
Electrotechniker.
12 1-800-927-9474
27-9
Applications Manual
Glossary (cont)
REMOTE INHIBIT. A power supply interface signal, usually TTL compatible, that commands
the power supply to shut down one or all outputs.
REMOTE ON/OFF. Enables power supply to be remotely turned on or off. Turn-on is typically
performed by open circuit or TTL logic “1”, and turn-off by switch closure or TTL logic “0”.
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, 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 EMC.
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 at the frequency of the AC mains. For a switching power supply, it is usually at 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, it is isolated from the AC
mains and specially designed for safety of personnel who might be working with power on
the system.
SELV. An acronym for Safety Extra Low Voltage, a term generally defined by the regulatory
agencies as the highest voltage that can be contacted by a person and not cause injury. It is
often specifically defined as 30Vac or 42.4Vdc.
SETPOINT ACCURACY. Ratio of actual to specified output voltage.
SEQUENCING. The technique of establishing a desired order of activating the outputs of a
multiple output power supply.
SOFT START. A technique for gradually activating a power supply circuit when the power
supply is first turned on. This technique is generally used to provide a gradual rise in output
voltages and inrush current limiting.
27-10
12 1-800-927-9474
Glossary
Glossary (cont)
SOFT LINE. A condition where there is substantial impedance present in the AC mains feeding
input power to a power supply. The input voltage to the power supply drops significantly with
increasing load.
SPLIT BOBBIN WINDING. A transformer winding technique where the primary and
secondary are wound side-by-side on a bobbin with an insulation barrier between them.
STANDBY CURRENT. The input current drawn by a power supply when shut down by a
control input (remote inhibit) or under no load.
STIFF LINE. A condition where there is no significant impedance present in the AC mains
feeding input power to a power supply. The input voltage to the power supply does not change
appreciably with load.
SWITCHING FREQUENCY. The rate at which the DC voltage is switched on and off in a
switching power supply.
TEMPERATURE COEFFICIENT. The average output voltage change expressed as a percent
per degree Celsius of ambient temperature change. This is usually specified for a predetermined
temperature range.
TEMPERATURE DERATING. Reducing the output power of a power supply with increasing
temperature to maintain reliable operation.
THERMAL PAD. A graphite laminate used as a thermal interface between the converter and a
heatsink or chassis.
THERMAL PROTECTION. A power supply protection circuit that shuts the power supply
down in the event of unacceptably high internal temperatures.
TOPOLOGY. The design type of a converter, indicative of the configuration of switching
transistors, utilization of the transformer, and type of filtering. Examples of topologies are the
Flyback, Forward, Half Bridge, Full Bridge, Resonant and Zero-Current-Switching.
TRACKING. A characteristic in a multiple output power supply where any changes in the
output voltage of one output caused by line, load, and/or temperature are proportional to similar
changes in accompanying outputs.
TRANSIENT RECOVERY TIME. The time required for an output voltage to be within
specified accuracy limits after a step change in line or load conditions.
TRUE POWER. In an AC circuit, true power is the actual power consumed. It is distinguished
from apparent power by eliminating the reactive power component that may be present.
UNDERSHOOT. A transient output voltage change which does not meet the low limit of the
voltage accuracy specification and is caused by turning the power supply on or off, or abruptly
changing line or load conditions.
12 1-800-927-9474
27-11
Applications Manual
Glossary (cont)
UNIVERSAL INPUT. An AC input capable of operating from major AC lines worldwide,
without straps or switches.
VOLTAGE BALANCE. The difference in magnitudes, in percent, of two output voltages that
have equal nominal voltage magnitudes but opposite polarities.
VOLTAGE MODE. A method of closed loop control of a switching converter to correct for
changes in the output voltage.
WARM-UP DRIFT. The initial change in the output voltage of a power supply in the time
period between turn-on and when the power supply reaches thermal equilibrium at 25 degrees
Celsius, full load and nominal line.
WARM-UP TIME. The time required after initial turn-on for a power supply to achieve
compliance to its performance specifications.
X-CAPACITOR. A capacitor connected across the supply lines to suppress normal mode
interference.
Y-CAPACITOR. Power conversion modules generally require bypass capacitors from line to
chassis (earth ground) to shunt common mode noise currents and keep them local to the converter.
In cases where the converters are operating from rectified AC line voltage, the failure of a bypass
capacitor could result in excessive leakage current to the equipment chassis thus creating a ground
fault and shock hazard. For this reason, a special classification of capacitor, referred to as a
Y-capacitor, is recommended. These capacitors contain a dielectric with unique “self-healing”
properties to help prevent against excessive leakage.
To meet general EMC requirements (see chapter 10 of the Vicor Applications Manual), Vicor
recommends the use of Y-capacitors with all power conversion modules. Y-capacitors meet
IEC384-14, EN132400, an UL 1283 standards.
ZERO-CURRENT-SWITCHING. The turn-on and turn-off of a switch at zero current, resulting
in essentially lossless switching. The zero-current-switching topology allows Vicor converters
to operate at frequencies in excess of 1 MHz, with efficiencies greater than 80% and power
densities ten or more times greater than conventional topologies.
27-12
12 1-800-927-9474
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement