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Film
Capacitor Terminology and Related Information |
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AC
Voltage/Current Ratings
Traditionally
expressed as line frequency sine wave RMS values with allowable
peak to peak voltage 2.828 times the AC voltage rating. Although
related, our AC sine wave current ratings are not the same as
the "Ripple Current" ratings used for electrolytic
capacitors because the typical ripple current waveform is not
a sine wave. For arbitrary AC waveforms the high frequency content
must also be considered. The allowed AC voltage drops off with
increasing frequency because of self heating. Because capacitor
losses [especially polyester] are frequency dependant, and capacitor
voltages must be kept well below corona inception, a measured
RMS current cannot by itself verify a safe application.
For polypropylene
film capacitor applications, refer to the voltage performance
curves. [Current ratings can be derived from these curves for
sine or near sine applications.] These curves are based on natural
convection at +85°C ambient temperature. The voltage limit
[sine wave RMS] specified by the horizontal line in each chart
is determined by either dielectric strength or corona inception
voltage. Operation within the AC rating insures that the dielectric
is not overstressed, there is no internal corona discharge, and
temperature rise between the internal hot spot and ambient is
less than 20°C. When film/foil polypropylene capacitor applications
approach the high frequency current rating limits the capacitors
are often heated more from lead wire losses than from losses
within the capacitor itself. (see also Skin Effect). In these
cases circuit board traces should be large enough to avoid unexpected
temperature rise. Special cooling techniques or operation below
maximum rated temperatures may allow higher AC voltage operation
at high frequencies. Because polyester capacitor dielectric loss
is 10-50 times higher than polypropylene [and a function of both
temperature and frequency] polyester capacitors are less suitable
for higher frequency applications.
We encourage
you to contact us with your specific application requirements. |
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| Breakdown
Voltage - |
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The
applied voltage at which a dielectric no longer behaves as an electrical
insulator. |
| Capacitive
Reactance (Xc) - |
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Reactance
is the loss free opposition to the flow for an alternating current,
expressed in ohms. Current flow through this reactance does not
dissipate heat as it would through a resistance of equal value. |
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| Corona
Discharge - (Partial) |
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A
small but locally intense electrical discharge that injects charge
into the insulating film adjacent to edges of foil/metallization
or a location where air is trapped between foil/metallization and
the film. The discharge is caused by a voltage gradient large enough
to ionize molecules in either the film or small air pockets. Each
discharge does some small but cumulative damage to the film. Corona
is an important consideration for AC and/or pulse applications
where the cumulative damage can rapidly accrue and cause dielectric
failure. For film/foil parts this will result in a short circuit.
For capacitors employing metallized film the "clearing" around
the dielectric failure sites results in progressive capacitance
loss. (see also dV/dt) |
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| Corona
Inception Voltage - (CIV) |
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The
peak to peak voltage at which corona discharge begins, also known
as the Corona Start Voltage [CSV]. It is traditionally expressed
as the RMS value of a sine wave with the above peak to peak value.
[Occasionally seen as Corona Offset Voltage, where it is expressed
as a peak to peak value.] |
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| DC
Leakage Current - |
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Undesired
current flow through a capacitor resulting from an applied constant
voltage. Film/foil capacitor leakage current is extremely low [Insulation
resistance is typically greater than 1E6 Megohms for polypropylene
film at room temperature]. |
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| DC
Voltage Rating - |
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Also
known as DC working voltage [DCWV]. The maximum continuous voltage
that the capacitor can withstand without expectation of failure
during the life of an application. This voltage is reduced at the
upper end of the temperature range for each dielectric type. |
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| dV/dt
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The
maximum guaranteed repetitive rate of voltage change [slew rate]
a capacitor can withstand without damage during the lifetime of
an application, expressed in Volts/µsec or KV/µsec. dV/dt
also expresses current pulse capability without requiring a nearly
impossible pulse current measurement. Pulse current is only an
issue for metallized capacitors where pulse currents above the
rated value can destroy the connection to the metallization. With
leads welded directly to extended foil, film/foil capacitor dV/dt
is only limited by corona inception [AC voltage rating] and application
circuit inductance |
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| Dielectric
Absorption - |
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Quantifies
the percent charge stored in a capacitor dielectric [rather than
on the foil surfaces] which cannot be removed quickly. If the voltage
across a charged capacitor is brought to zero for a short time,
the capacitor will appear to "self recharge" slightly
after the discharge circuit is opened. Dielectric absorption can
be approximated by the ratio of the equilibrium value "self
recharge" voltage to the voltage before discharge. This is
an important parameter for "sample and hold" applications.
Specific procedures exist to more precisely quantify dielectric
absorption. We encourage you to contact us with your specific application
requirements. |
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| Dielectric
Constant - (relative permittivity) |
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The
ratio of capacitance for a given dielectric to the capacitance
of the same geometry with vacuum as the dielectric |
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| Dielectric
Strength - |
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The
ratio of the breakdown voltage to the dielectric thickness. Refers
to an average value. Usually expressed at room temperature. Dielectric
strength falls at the upper temperature range for each dielectric,
requiring a corresponding reduction in DC voltage rating. |
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| Dissipation
Factor - (DF or Tangent Delta) |
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The
ratio of the sum of all loss phenomena (dielectric and resistive)
to capacitive reactance, usually expressed as a percent. It is
also the ratio of the current in phase with the applied voltage
to the reactive current. DF must be given at a specific frequency
to be meaningful. DF is an industry standard for comparing capacitor
quality. Lower DF indicates less power dissipated under otherwise
identical conditions |
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| Equivalent
Series Resistance - (ESR) |
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A
mathematical construct [expressed in ohms] that allows ALL capacitor
losses at a single specific frequency to be expressed as a single
series resistance. It allows capacitor heating to be easily calculated
if DF and the sine wave RMS current is known. ESR can be used to
compare capacitor quality if frequency and capacitance are the
same. ESR CANNOT be used to determine losses for a non-sine arbitrary
waveform current [even if the true RMS current is known] because
arbitrary waveforms contain 2 or more harmonic frequencies. ESR=DF*(capacitive
reactance) |
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| Film/Foil
Capacitor - (Extended Foil Design) |
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Film
and discrete foils are utilized rather than metallized film. This
allows the leads to be welded directly to the extended foil. This
method of lead attachment creates a highly reliable connection
resulting in lowest possible DF/ESR. It also allows highly reliable
operation at extremely high pulse currents. |
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| Film/Foil
Capacitor - (Hybrid Design) *SBE capacitors utilize this
design where high AC voltages are required |
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SB
Electronics’ hybrid design incorporates a floating metallized
common between discrete foils (refer to capacitor construction
diagrams). Offers the advantages of both the extended foil and
metallized capacitor design. It allows the high current, high frequency
capabilities of extended foil design to co-exist with the self-healing
characteristics of metallized film |
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| Metallized
Film - |
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An
extremely thin layer of metal vacuum deposited directly on dielectric
film. This layer is conductive but takes almost no volume, aiding
in capacitor size reduction. |
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| Metallized
Film Capacitor - |
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Metallized
film is used rather than film and separate metal foils. Electrical
connection to the metallized film is made with a layer of molten
metal droplets sprayed on each end of the capacitor, with lead
wires welded to this "end spray". The connection of the
metallized film to the end spray is not continuous; small metal
particles contact the metallized layer at discrete locations. There
is more resistance from this connection than for the foil/wire
weld in a film/foil capacitor, resulting in higher DF and ESR.
The current density at each contact point between the end spray
and metallized film is high, and this connection is subject to
deterioration and failure if pulse currents are excessive [results
in lower dV/dt ratings]. Unlike a film/foil capacitor, dielectric
breakdown does not result in capacitor failure (see "self
healing") so metallized capacitors can be made with thinner
film than required for film/foil. This, along with removal of the
foil, allows smaller size for given capacitance and voltage ratings. |
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| Self-healing
- |
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For
capacitors made with metallized film, self-healing or "clearing" removes
a fault or short circuit in the dielectric film by vaporizing [from
high current density] the metallization near the defect. The metallization
is so thin that negligible film damage is done during the clearing
process. The vaporized metal oxidizes over time, aiding in the
isolation of a fault area. |
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| Skin
Effect - |
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The
tendency for AC current to flow on the outside of conductors at
high frequency. This is caused by the conductor resisting the rapid
internal magnetic field changes created by the current. This phenomenon
causes higher conduction losses than one would otherwise expect
based on material and cross section area, and is a major contributor
to the rise in capacitor DF at higher frequencies seen in even
the best film/foil polypropylene dielectric capacitors. |
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