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Film Capacitor Technical Notes

• 1. Principle and basic theory of capacitor
• 2. Kinds of (fixed) capacitors
• 3. Kinds of film capacitors
• 4. Characteristics and performance
• 5. Manufacturing process
• 6. Applications
• 7. Caution for proper use
• 8. Examples of failure
• 9. Safety and conforming to environmental
• 10. Additional information

1. Principle and basic theory of capacitor

1-1. What is the capacitor?
When voltage is applied between facing conductors, the insulator (or space) sandwiched between them will cause dielectric polarization by electrostatic induction, thus leading to the accumulation of electric charges (charging). The capacitor is a device (part) that performs this charging and discharging of accumulated charges as its function.

1-2. Electrostatic capacity and energy

                    

                                                                                                  Fig.1 Conceptual diagram of polarization

2. Kinds of (fixed) capacitors

2-1. Classification

Fig.2 Capacitor classification

2-2. Comparison of characteristics among various kinds of capacitors (summary)

Table 1 Comparison of characteristics

3. Kinds of film capacitors

3-1. Classification by dielectrics

Table 2 Classification by dielectrics

3-2. Classification by electrodes

Table 3 Classification by electrodes

3-3. Classification by element structures

1) Foil electrode tab structure                                         2)Evaporated electrode extended foil structure

Fig.3 Element structures

                                        ①Dielectric film                    ⑤Undercoating resin
                                        ②Protective film                   ⑥Outer coating resin
                                        ③Aluminum foil                    ⑦Metallized film
                                        ④Lead wire                          ⑧Metallicon

3-4. Classification by armoring

        Resin dipping
        Tape wrapping resin sealing
        Non-metallic case resin sealing
        Metallic case hermetic sealing
        Resin molding
        Simple armoring (chips for surface mounting)

4. Characteristics and performance

4-1. Physical properties of dielectric films

Table 4 Physical properties of dielectric films

4-2. Electrical characteristics

        Fig.4-1 Electrostatic capacity-temperature                      Fig.4-2 Dielectric loss tangent-
                        characteristics                                                                     temperature characteristics

        Fig.4-3 Insulation resistance-temperature                Fig.4-4 Frequency characteristics
                        characteristics

4-3. Features of various kinds of films

Table 5 Features of dielectric films

5. Manufacturing process

(Control items and influencing characteristics by kinds of products and processes)

5-1 Foil electrode tab structure

Table 6 Manufacturing process and control items

5-2. Evaporated electrode extended foil structure (lead wire)

Table 7 Manufacturing process and control items

5-3. Evaporated electrode extended foil structure (power film)

Table 8 Manufacturing process and control items

6. Applications

Table 9 Outlined applications by kinds of dielectrics and electrodes

Fig.5 Example of use

7. Caution for proper use

Please refer here with regard to caution for proper use of film capacitors.

8. Examples of failure

Table 10 Probable causes of failure and failure mode of film capacitors

*) In case of the metallized film capacitors (evaporated metal electrode type), if voltage in excess of the withstand voltage ( or apparently in excess of the withstand voltage due to the lowering of withstand voltage) is applied, self-healing will happen continuously.
Upon such occasion, the film may be melted and carbonized by discharging energy, thus leading not to complete short-circuiting, but to short-circuiting with resistance value.
**) Upon occurrence of short-circuiting with resistance value, if some of the conditions deemed to be its probable causes are combined with each other or critical, it may sometimes lead to fuming and/or ignition. Particularly for the laminated type, it is necessary to pay due attention to overvoltage because the withstand voltage in the portion having been cut upon formation of capacitor elements is low.

·Concrete example
The product of evaporated electrode with extended foil structure became open after 8 years operation in the field.
→ Results of analysis: Judging from the fact that the evaporated metal has been almost lost, it is supposed to have been used in high humidity environment.

9. Safety and conforming to environmental

9-1. Safety
  Countermeasures against fuming and ignition of evaporated electrode type
    ·Improvement in screening accuracy by charging/discharging test and tanδ measurement at higher frequency.
    ·Improvement of safety by attaching security mechanism to vapor deposition pattern.(for electrical equipment and anti-noise)

Fig.6 Structure and destruction mode of security mechanism

9-2. Conforming to environmental

Please refer here with regard to conforming to environmental of film capacitors.

10. Additional information

10-1. Service life of capacitors

(1)Applied voltage and service life
For the relation between the applied voltage and service life of film capacitors, the Formula 4 is taken up generally.

(2)Operating ambient temperature and service life
The temperature-dependence of film capacitor' s service life follows the Arrhenius Law, so-called the 10 ºC Law. Namely, if the temperature rises by 10 ºC , the service life will be reduced down to 1/2. Therefore, the relation between the ambient temperature and service life is given by the Formula 5.

(3)Failure rate calculation
Since electrolyte is not used in film capacitors, there is no lifetime due to dry-up like aluminum electrolytic capacitors, and therefore it is considered reasonable to express by failure rate in general. Failure rate has a dependence on temperature and voltage, and according to the literature, it is calculated following relational expression (formula 6), but the numerical value calculated by these is an estimated value, and it does not guarantee this.

(4)Derating of the rated voltage depending on the operating temperature
If a capacitor is used at high temperatures, its service life will be shortened due to thermal deterioration. In case when a capacitor is to be used at high temperatures, please derate the operating voltage in accordance with the graphs as given below.

                        Fig 7-1                                                       Fig 7-2
                        Application series P2S                                 Application series
                                                                                         MPK,PCK,MMB,MMG,MMK,F2D

(5)Ripple current and service life
In case when ripple current is applied on a capacitor, Joule heat will be generated. The capacitor temperature rise on this occasion is given by the Formula 7.

Since significant self heating of capacitor may lead to the deterioration and/or damage to the capacitor, self temperature rise of capacitor should be limited to lower than 15ºC for polyester film capacitors and to lower than 10ºC for polypropylene film capacitors.
In case of use at high temperature/high frequency, the voltage derating factor is different from that of DC voltage. This is partly because heating by ripple current is severer in conditions than the Arrhenius 10ºC Law and partly because in the polyester film capacitors, etc. tanδ may change with the temperature, thus leading to the change in self temperature rise accordingly.

(6)Allowable voltage in various voltage waveforms
Allowable voltage in various voltage waveforms varies with the kinds of waveforms and frequency. Allowable voltage in various voltage waveforms shall be the value obtained by multiplying sine wave allowable voltage at each frequency by any of the coefficient as given below. Moreover, in case of the voltage containing DC bias component, the AC voltage obtained by subtracting the bias voltage from the DC rated voltage shall be the allowable value. Then, this voltage should be derated for use depending on the respective frequencies and waveforms.
(In case when the voltage values on the voltage derating curves for various kinds of products are given by effective values, the value obtained by increasing the allowable voltage value by 2 √ 2 shall be the Vp-p value. Then, the value obtained by multiplying it by any of the coefficients as given below shall be the allowable voltage value Vp-p at the frequency of the relevant waveform.)

Table 11 Allowable voltage coefficient

10-2. Self-healing (Clearing)
Since an electrode is evaporated as a very thin metal film (about 150 to 400 Å) on the dielectric, even if dielectric breakdown is caused on the weakest portion in the dielectric, only the weakest portion and its peripheral portion will be dispersed instantaneously by large current energy and the functions as a capacitor will not be still lost. This phenomenon is called the self-healing.

Higher evaporation resistance and thinner evaporated film bring better self-healing property. However, this may lead to the poor connection with metallicon and increase in ESR in the high frequency zone. Therefore, full investigations should be made on the applications upon its design.

Fig.8 Evaporation resistance and AC break-down voltage

Photo 1 Self-healing point

*Self-healing means the recovery of capacitor insulation. It does not mean that the disappeared evaporated metal film regenerates and recovers (Insulation is recovered by local disappearance of the deposited metal film around the insulation defect portion.).

FILM CAPACITORS CHARACTERISTICS
Typical characteristics are shown below.(Capacitances are 0.1µF)    PDF