General Technical Information
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Vishay Roederstein
Revision: 13-Jun-12
3
Document Number: 26033
For technical questions, contact: dc-film@vishay.com
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INSULATION RESISTANCE
Insulation resistance as a function of temperature
(typical curve)
Notes
• Dielectrics according to IEC 60062:
KT = Polyethylene terephthalate (PET)
KP = Polypropylene (PP)
KI = Polyphenylene sulfide (PPS)
KN = Polyethylene naphtalate (PEN)
• Polyethylene terephthalate (PETP) and polyethylene naphtalate
(PEN) films are generally used in general purpose capacitors for
applications typically with small bias DC voltages and/or small
AC voltages at low frequencies.
• Polyethylene terephthalate (PETP) has as its most important
property, high capacitance per volume due to its high dielectric
constant and availability in thin gauges.
• Polyethylene naphtalate (PEN) is used when a higher
temperature resistance is required compared to PET.
• Polyphenylene sulfide (KI) film can be used in applications where
high temperature is needed eventually in combination with low
dissipation factor.
• 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.
• Typical properties as functions of temperature or frequency are
illustrated in the following chapters: “Capacitance”, “Dissipation
factor”, and “Insulation resistance”.
DEFINITIONS
The following definitions apply to both film/foil capacitors
and metalized film capacitors.
RATED VOLTAGE (U
R
)
The rated voltage is the voltage for which the capacitor is
designed. It is defined as the maximum DC (U
R
) or AC (U
RAC
)
voltage or the pulse voltage that may continuously be
applied to the terminals of a capacitor up to an operating
temperature of + 85 °C. The rated voltage is dependent
upon the property of the dielectric material, the film
thickness and the operating temperature. Above + 85 °C,
but without exceeding the maximum temperature, the rated
voltage has to be derated in accordance to the dielectric
material used.
TEST VOLTAGE OR DIELECTRIC STRENGTH
The test voltage of a capacitor is higher than the rated DC
voltage and may only be applied for a limited time. The
dielectric strength is measured between the electrodes with
a test voltage of 1.5 x U
NDC
for 10 s, at metalized film
capacitors and of 2 x U
NDC
at film/foil capacitors for typically
2 s. The occurrence of self-healing or clearing-effects during
the application of the test voltage is permitted for metalized
film capacitors.
AC VOLTAGE
The AC voltage ratings refer to clean sinusoidal voltages
without transients. The capacitors must not, therefore, be
operated in mains applications (e.g. across the line). This
applies also to capacitors that are rated with AC voltages
250 V
AC
. Capacitors especially designed for mains
operations (X and Y capacitors) are listed as “RFI
Capacitors”. For operations in the higher frequency range,
the applied AC voltage has to be derated. The derated AC
voltages are provided in the graphs “Permissible AC Voltage
Versus Frequency” on the capacitor datasheet. The
calculations of the graphs are based on the assumption that
the temperature rise measured on the surface of the
capacitor under working conditions does not exceed 10 °C.
P - Dissipation power (W)
- Angular frequency (rads/s)
C - Capacitance (F)
tan - Dissipation factor at frequency (f)
T - Temperature rise (°C)
A - Surface area of the capacitor (cm
2
)
- Heat transfer coeff. [mW/(°C x cm
2
)]
( = 0.96 for plastic boxes with a smooth surface)
G - Component heat conductivity (displayed in datasheet)
MAXIMUM APPLICABLE PEAK TO PEAK RIPPLE
VOLTAGE
When an AC voltage is superimposed to a DC voltage, the
sum of both the DC voltage (U
DC
) and the peak value of the
AC voltage (U
pk
) must not exceed the rated DC voltage (U
R
)
of the capacitor.
PULSE VOLTAGE
The RMS value of a pulse voltage (U
RMS(pulse)
) must not
exceed the rated AC voltage U
RAC
.
The peak value of the pulse voltage (U
pk
) must not exceed
the rated DC voltage.
NOMINAL CAPACITANCE (C
N
)
The nominal capacitance is defined as the capacitive part of
an equivalent series circuit consisting of capacitance and
equivalent series resistance (ESR). C
N
is the capacitance for
which the capacitor is designed. It's value is typically
measured at a frequency of 1 kHz ± 20 %, at voltage of
0.03 x U
RDC
(max. 5 V
AC
) and a temperature of 20 °C.
The capacitance tolerance indicates the acceptable
deviation from the rated capacitance at 20 °C. Since the
dielectric constant of plastic film is frequency dependent,
the capacitance value will decrease with increasing
frequency. High relative humidity may increase the
capacitance value. Capacitance changes due to moisture
are reversible.
Heat coefficient for the capacitor is presented in datasheet
for T calculation.
For critical applications, please forward your voltage and
current waveforms (worst case conditions) for our capacitor
proposal.
T
P x 1000
A x
------------------------
P x 1000
G
------------------------
==
