Film Capacitor Terminology & Related Info
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.
The applied voltage at which a dielectric no longer behaves as an electrical insulator.
Capacitive Reactance (Xc)
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.
Corona Discharge – (Partial)
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)
Corona Inception Voltage – (CIV)
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.]
DC Leakage Current
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].
DC Voltage Rating
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.
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.
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.
Dielectric Constant – (relative permittivity)
The ratio of capacitance for a given dielectric to the capacitance of the same geometry with vacuum as the dielectric.
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.
Dissipation Factor – (DF or Tangent Delta)
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.
Equivalent Series Resistance – (ESR)
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).
Film/Foil Capacitor (Extended Foil Design)
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.
Film/Foil Capacitor (Hybrid Design*)
SBE’s 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.
*SBE capacitors utilize this design where high AC voltages are required.
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.
Metallized Film Capacitor
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.
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.
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.