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Technical Paper

Resource

Question

CHARACTERISTICS OF THE PLASTIC FILM DIELECTRICS

Answer

 

Item

PET

PP

PEN

PPS

Dielectric constant

1 kHz/23°C

3.3 (positive as

temperature rise)

2.2 (negative as

temperature rise)

3.0 (positive as

temperature rise)

3.0 (very constant

versus temperature)

Operating temperature

(°C)

-55...+105

-55...+105

-55...+125

-55...+140

Dielectric absorption (%)

0.2...0,25

0.05...0,10

1.0

0.05

ΔC/C versus

temperature (%)

± 5

± 2.5

± 5

± 1.5

ΔC/C versus

voltage (%)

negligible

negligible

negligible

negligible

Dissipation factor (%)

1 kHz

10 kHz

100 kHz

 

0.8

1.5

3.0

 

0.05

0.08

0.25

 

0.8

1.5

3.0

 

0.2

0.25

0.5

ESR

low

very low

low

very low

Ris (MΩ x µF)

25 °C

85 °C

 

10000

1000

 

100000

10000

 

10000

1000

 

10000

1000

Capacitance tolerance

(+/- %)

5/10/20

1/2.5/5/10

5/10/20

2.5/5/10/20

Self-healing

yes

yes

yes

yes

Typical failure mode at

end of life

open

open

open

open

Reliability

high

high

high

high

Piezoelectric effect

no

no

no

no

Polarity

no

no

no

no

 

Question

SELF-HEALING PROCESS IN METALLIZED CAPACITORS

Answer

Even the best plastic films, like ceramic materials, are not free from pin-holes. However, in the case of metallized film capacitors it is possible to eliminate these faults by applying a much higher voltage than the rated voltage. This process is known as self-healing and practically makes a "zero defect dielectric" possible.

 

 

 

 

 

Figure 1: Schematic representation of the

self-healing process

Figure 2: Isolated area after the self-healing

process

 

The self-healing process is started by an electric breakdown, which takes about 10-8 secs. In the breakdown channel, the dielectric is transformed into a highly compressed plasma which is pushed out of the channel and presses the dielectric layers apart (figure 1).

 

In the spreading plasma, discharging continues over the metal electrodes. Temperatures of approximately 6000 K occur and insulated areas are formed around the original failure spot (figure 2). This self-healing process takes a few µsec and the discharging in the plasma has already ceased before a greater loss of voltage takes place. This quick extinction of the plasma is necessary to avoid further damage to the dielectric layer next to the point of failure.

 

The pressure between the layers must not be too great, so that the plasma can spread out from the breakdown channel quickly. Large parts of the plasma get into areas of low field strength.

 

The flawless course of the self-healing process depends on the thickness of the metallization, on the chemical composition and on the rate of the applied voltage; here, apart from the chemical composition, the production conditions have to provide the prerequisites for optimum self-healing.

Question

CALCULATION OF RMS IN VARIOUS WAVEFORMS

Answer

In each waveform, calculate the rms value in the following formula.

 

Question

WARNING NOTICE

Answer

Operation voltage 

The plastic film capacitor varies in the maximum applicable voltage depending on the applied voltage waveform, current waveform, frequency, ambient temperature (capacitor surface temperature), capacitance value, etc. Be sure to use capacitors within the specified values by checking the voltage waveform, current waveform, and frequency applied to them.

 

Operating Current 

The pulse (or AC) current flowing through the capacitor is expressed as: l=C x dV/dt. 

Since dissipation factor of the capacitor will generate the internal heat under the application of high frequency or high pulse current, temperature rise in it will occur and may cause deterioration of withstanding voltage, even lead to break down (smoking or firing). Therefore, the safety use of capacitor must be within the rated voltage (or category voltage) and the permissible current. 

The operating current must be considered by dividing into pulse current (peak current) and continuous current (rms current) depending on the break down mode, and when using, should make sure the both currents are within the permissible values.

 

Charging and discharging

Because the charging and discharging current of capacitor is obtained by the product of voltage rise rate(dv/dt) and capacitance, low voltage charging and discharging may also cause deterioration of capacitor such as shorting and open due to sudden charging and discharging current. When charging and discharging, pass though a resistance of 20Ω/V~1 000Ω/V or more to limit current. 

When connecting multiple film capacitors in parallel in withstand voltage test or life test, connect o resistance of 20Ω/V~1 000Ω/V or more in series to each capacitor.

In additional, capacitors must be discharged with resistor before handling. Because the capacitor hasn't discharge resistor inside, so there is residual but maybe deathful electric energy contained. 

 

Buzzing noise

Any buzzing noise produced by capacitor is caused by the vibration of the film due to the coulomb force that is generated between the electrodes with opposite poles. If the wave-form with a high distortion rate or frequency is applied accorss the capacitor, the buzzing noise will become louder. But the buzzing noise is of no damage to capacitor.

 

Contained temperature rise (△ϴcase)

When continuing current flows through the capacitor, the temperature inside the capacitor will rise, induced by accumulated heat. If the temperature exceeds allowed hot-spot temperature, it might cause a short circuit or fire. The limits described in the catalogue are not exceeded and it's necessary to check the temperature on the capacitor surface when it works. 

 

Flame retardation

Although flame retardation epoxy resin or plastic case is used in the coating or encapsulating of plastic film capacitor, continuous outer high temperature or firing will break the coating layer or plastic case of the capacitor, and may lead to melting and firing of the capacitor element.

 

Storage conditions

Capacitors may not be stored in corrosive atmospheres, particularly not when chlorides, sulfides, acids, lye, salts, organic solvents or similar substances are present. It shouldn't be located in particularly high temperature and high humidity, it must submit to the following conditions (unchanging primal package): 

Temperature: ≤35℃

Humidity: ≤80% RH, no dew allowed on the capacitor. 

Storage time: ≤24 months (from the date marked on the capacitor's body or the label glued to the package)

 

RoHS Compliance 

Aishi infinity caps products in the catalogue are RoHS Compliant.

 

General Remarks

All data, range surveys and application data correspond to the actual state of the art and were elaborated as thoroughly and precisely as possible. They are to be understood as general information, and the right for amendments and construction changes is reserved. Special customized designs which deviate from our catalogue data, irrespective of whether being based on factory standards, specifications or related data, do not release the user from his duty of care with regard to incoming goods inspection and production monitoring. In case of the components being purchased through second or third suppliers we urgently ask to compare the technical details with the data given by the manufacturer. In cases of doubt we recommend use is made of our technical support, since we do not take any responsibility for damages caused by inappropriate use or processing of our capacitors