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1. Film / Foil Structure                                                                                                                                                                              

塑膠薄膜內部結構


Film / Foil stacked structure, mainly used for capacitors with smaller capacitance (100pF to 0.1μF).

The advantage of this structure is that it is easy for the foil electrode to have better contact, so that the capacitor has good pulse strength.

 

Once the dielectric of the film capacitor is broken down, it will inevitably cause a short circuit, thus the capacitor will fail.

In order to avoid the weak point of the dielectric being broken down, the choice of the dielectric is always thicker than the required value of the material with specific breakdown strength in theory.

 

Because of the high percentage of thin spot, any film less than 4μm is not allowed.

Due to the necessity of choosing thicker dielectric, it has adverse effect on the size and materials.

 

In order for the thicker dielectric to reach a certain capacity, the length of the film must be increased by an equal amount, then the core also has formed a more square volume.

The appearance of a thinner point will make the upper and lower surface breakdown, the dielectric must be thick enough to withstand the breakdown strength.

 

Pros: foil electrodes are able to withstand the higher pulse loads due to good contact of terminal surfaces.



2. Metallized structure                                                                                                                                                                              

 

塑膠薄膜內部結構


Metallized type of structure, the characteristic is using a smaller winding volume to obtain large capacity volume (0.01μF~100μF or larger)

Merallized capacitors, coating with aluminum at approximately 0.03µm, so the vacuum evaporation for dielectric as conductive electrode.

 

Metallized capacitors self-healing properties, such as the dielectric breakdown up under the defects in short circuit at some point, and breakdown point of metallized layer can evaporate under the effect of arc melting moment, and form a small metal free zone, the capacitance of the two pole piece to insulation and still can continue to work, greatly improve the reliability of the capacitor.

 

Theoretically, there should be no short-circuit failure mode in the metallized film capacitors, while there are many short-circuit failures in the foil capacitors. This makes it possible to use a thinner dielectric thickness (< 1μm).

 

Pros: this structure is best for volume/volume.



3. Film/Foil and metal film interphase structure                                                                                                                                    

 

塑膠薄膜內部結構


This type of capacitor is a film/foil structure, and the carrier is metallized electrode. This type of capacitor is suitable for high-current load applications.


The capacitor is connected in series, and the current-carrying electrode is composed of two metal foils and a carrier of metallized film as a 'floating electrode', which is connected with lead after the outer edge of the core is sprayed with metal.

 

Only in the floating electrode part, it can be coupled by the capacitive current. In this way, the capacitor has the characteristics of self-healing through the metallized floating electrode, and the metal foil also provides a more reliable way of bonding.

 

The series connection structure has the property of doubling the voltage mass, but the capacity is only half of the theoretical design capacity.

 

Pros: foil electrodes are able to withstand higher pulse loads due to good contact of terminal surfaces.

It is a carrier floating electrode with good self-healing ability, Series connection structure, double voltage.



4. The self-healing process of metallized capacitors                                                                                                                           

Even best plastic film, or ceramic materials, pinholes are unavoidable.

 

However, metallized film capacitors can eliminate this problem by applying higher than the rated voltage. This process is called self-repair, and the possibility of achieving 'zero defects in the medium' is realized. 

 

塑膠薄膜內部結構

Figure 1: self-healing process                                                                   Figure 2: self-healing metal-free areas

 

The local breakdown, after the dielectric into highly dense plasma, the transfer fills the breakdown point, while the dielectric breaks open. (figure 3)

Plasma diffusion, continuous charge at the metal electrode. A high temperature of nearly 6000K is generated, and an insulating zone is formed around the breakdown point. (figure 4)

 

This self-healing process takes a few microseconds, and the plasma is no longer energized until the voltage causes a greater loss, and the instantaneous melting and evaporation at the breakdown point is necessary to avoid further damage.


The pressure between the layers is not so great that the plasma diffuses throughout the damage point, and most of the plasma fills the area with a lower electric field.

The voltage applied for optimal self-healing depends on the thickness of the metallization layer and the composition or ratio.


Here, in addition to the preconditions provided by the manufacturing process, the premise is to obtain the best self-healing.




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