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Characteristics and Construction

COMMON FILM DIELECTRICS USED IN FILM CAPACITORS PRODUCTS & THEIR CHARACTERISICS KT = polyethylene terephthalate (PETP)
Polyethylene terephthalate (PETP films are generally used in general purpose capacitors for applications typically with small bias DC voltages and/or small AC voltages at low frequencies. The most important propertyof the film is high capacitance per volume due to its high dielectric constant and availability in thin gauges. KP = polypropylene (PP)
Polypropylene (KP) films are used in high frequency or high voltage applications due to their very low dissipation factor and high dielectric strength. These films are used in AC and pulse capacitors and interference suppression capacitors for mains applications. A comparison chart of the important parameters is given below

ParametersPETPPUnits
Dielectric Constant 1 kHz3.32.2-
Dissipation factor at 1 kHz50110-4
Dissipation factor at 10 kHz110210-4
Dissipation factor at 100 kHz170210-4
Dielectric Strength400250V/µm
Maximum Temperature1251050C
Power density at 10 KHz500.6W/cm3
Water absorption50<0.1%
Tensile Strength longitudinal200150N/mm
Film capacitors are build up by two electrodes (the capacitor plates) with plastic dielectric material in between.The electrodes of metallised film capacitors consist of a thin metal layer (0.01 to 0.05 micron thickness, approx., of Al. or Al/Zn Alloy) which is vacuum deposited on the dielectric film. Due to low metal thicknes, their current carrying capacity is limited. In some constructions double side metalized plastic film is used as electrode .The plastic material has only the function of carrier: the self healing properties are maintained and the current carrying capability is increased a lot in comparison with single metalized types. Depending on the AC voltage in the application, single or series constructions are used. In a series construction two or more sections are placed internally in series in one capacitor. Single section capacitors are normally used for products with an AC rating up to 300 Vac. Series construtions are used for higher voltages. The connection of metallised capacitors is accomplished by means of a metal spraying process and by welding the leads on to the sprayed ends. Finally the capacitor cells can be protected for severe environmental conditions or to withstand passive flammability. Encapsulation with epoxy materials in plastic boxes is common used for fixed outline dimensions. Epoxy dipped capacitors have a more rounded and easy to handle shape. All these encapsulations are flame retardant materials fulfilling the UL 94 classification system. Axial types are typically of the wrapped end construction. An extra wrapped film and epoxy at the end connections protects the cell.

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Uses

• Blocking
• Coupling and Bypassing
• Timing Circuits
• Tuning and Oscillations
• Energy Storage

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GENERAL DEFINITIONS


Voltage

While a DC capacitor is designed essentially for application with direct voltage, an AC capacitor is designed primarily for application with alternating voltage. Rated voltage DC:-The maximum DC voltage (in V) which may be continuously applied to a capacitor at any operating ambient temperature below the rated temperature. Rated AC voltage :-The maximum RMS voltage (in V) at specified frequency (mostly 50 Hz), that may be continuously applied to a capacitor at any operating ambient temperature below the rated temperature.

Capacitance

The capacitance of a capacitor is the capacitive part of the equivalent circuit composed of capacitance,series resistance and inductance Rated capacitance:-The rated capacitance, normally marked on the product, is the value for which the capacitor has been designed. Capacitance tolerance:-The percentage of the allowed deviation of the capacitance from the rated capacitance. This is measured at a free air ambient temperature of 23 ± 1 °C and RH of 50 ± 2 %.

Dissipation factor


The dissipation factor or tangent of loss angle (tan δ) is the power loss of the capacitor divided by the reactive power of the capacitor at a sinusoidal voltage of specified frequency. The tan δ reflects the polarization losses of the dielectric filman d the losses caused by the internal contact resistances (terminal, metal spray, electrodes) of the capacitor. The tan δ is temperature and frequency dependant. The reciprocal value of tan δ is also known as Q-factor. Q = 1/tan δ

Insulation resistance and time constant

The insulation resistance (Rins) is defined by the applied DC voltage divided by the leakage current after a well defined minimum time. The time constant is the product (in s) of then ominal capacitance and the insulation resistance between the leads.

Equivalent series resistance

The equivalent series resistance (ESR) is the resistive part of the equivalent circuit composed of capacitance, serie resistance and inductance. Parallel losses cam in general be neglected at frequencies higherth an 1 kHz, due to the high insulation resistance.

Rated voltage pulse slope (dU/dt)

The maximum voltage pulse slope that the capacitor can withstand with a pulse voltage equal to the rated voltage.For pulse voltages other than the rated voltage, the maximum voltage pulse slope may be multiplied by rated voltage and divided by the applied voltage or: Vsignal x (dU/dt)signal < Vrated x (dU/dt)R

Climatic category

The climatic category code (e.g. 50/100/56) indicates to which climatic category a film capacitor type belongs. The category is indicated by a series of three sets of digits separated by oblique strokes corresponding to the minimum ambient temperature of operation, the maximum temperature of operation, and the number of days of exposure to damp heat (steady state-test Ca) respectively that they will withstand

Self healing

The process by which the electrical properties of the capacitor, after a local breakdown of the dielectric, are rapidly restored to those before the breakdown.

Power dissipation

The power dissipated by a capacitor is a function of the voltage across or the current (I) through the equivalent series resistance ESR.
P = 2 π f C U2 × tan δ where f = frequency,C = maximum value specified, U-rated voltage

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ADVANTAGES

The main advantages of metallised capacitors are:
1. relatively small dimensions, a result of vacuum deposited electrodes, and,
2. self healing property. Metalized capacitors have a self-healing behavior as an.intrinsic characteristic. Self healing is the ability to recover after a dielectric breakdown. Owing to the self healing property, relatively thinner films can be used for metallised capacitors