
Uses

Characteristics and Construction

General Definitions

Advantages
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Uses
• Blocking
• Coupling and Bypassing
• Timing Circuits
• Tuning and Oscillations
• Energy Storage
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
Parameters  PET  PP  Units 

Dielectric Constant 1 kHz  3.3  2.2   
Dissipation factor at 1 kHz  50  1  104 
Dissipation factor at 10 kHz  110  2  104 
Dissipation factor at 100 kHz  170  2  104 
Dielectric Strength  400  250  V/µm 
Maximum Temperature  125  105  0C 
Power density at 10 KHz  50  0.6  W/cm3 
Water absorption  50  <0.1  % 
Tensile Strength longitudinal  200  150  N/mm 
Uses
• Blocking
• Coupling and Bypassing
• Timing Circuits
• Tuning and Oscillations
• Energy Storage
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 Qfactor. 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 statetest 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 U^{2} × tan δ
where f = frequency,C = maximum value specified, Urated voltage
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 selfhealing 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