The biggest benefit of organometallic film capacitors is that they are self-healing, which makes these capacitors one of the fastest growing capacitors today.

There are two different mechanisms for self-healing of metalized film capacitors: one is discharge self-healing; the other is electrochemical self-healing. The former occurs at higher voltage, so it is also referred to as high-voltage self-healing; because the latter also occurs at very low voltage, it is often referred to as low-voltage self-healing.

Discharge Self-Healing

To illustrate the mechanism of discharge self-healing, assume that there is a defect in the organic film between two metallized electrodes with a resistance of R. Depending on the nature of the defect, it may be a metallic defect, a semiconductor or a poorly insulated defect. Obviously, when the defect is one of the former, the capacitor will have discharged itself at low voltage. It is only in the latter case that the so-called high voltage discharge heals itself.

The process of discharge self-healing is that immediately after applying a voltage V to a metalized film capacitor, an ohmic current I=V/R passes through the defect. Therefore, the current density J=V/Rπr2 flows through the metallized electrode, i.e., the closer the area to the defect(the smaller r is) and the higher its current density is within the metallized electrode. Due to the Joule heat caused by the defect power consumption W=(V2/R)r, the resistance R of a semiconductor or insulating defect decreases exponentially. Therefore, the current I and power consumption W increases rapidly, as a result, the current density J1= J=V/πr12 rises sharply in the region where the metallized electrode is very close to the defect, and its Joule heat can melt the metallized layer in the region, causing the arc between the electrodes to fly here. The arc quickly evaporates and throws away the molten metal, forming an insulated isolation zone without metal layer. The arc is extinguished and self-healing is achieved.

Due to the Joule heat and arc generated in the discharge self-healing process, the dielectric around the defect and the insulation isolation area of the dielectric surface is inevitably damaged by thermal and electrical damage, and thus chemical decomposition, gasification and carbonization, and even mechanical damage occurs.

From the above, in order to achieve a perfect discharge self-healing, it is necessary to ensure a suitable local environment around the defect, so the design of the metalized organic film capacitor needs to be optimized in order to achieve a reasonable medium around the defect, a suitable thickness of the metalized layer, a hermetic environment, and a suitable core voltage and capacity. The so-called perfect discharge self-healing is: self-healing time is very short, self-healing energy is small, excellent isolation of defects, no damage to the surrounding dielectric. In order to achieve good self-healing, the molecules of the organic film should contain a low ratio of carbon to hydrogen atoms and a moderate amount of oxygen, so that when the decomposition of the film molecules occurs in the self-healing discharge, no carbon is produced and no carbon deposition occurs to avoid the formation of new conductive paths, but rather CO2, CO, CH4, C2H2 and other gases are produced to extinguish the arc with a sharp rise in gas.
In order to ensure that the media around the defect is not damaged when self-healing, the self-healing energy should not be too large, but also not too small, in order to remove the metallization layer around the defect, the formation of insulation (high resistance) zone, the defect will be isolated, to achieve self-healing. Obviously, the required self-healing energy is closely related to the metal of the metallization layer, thickness, and environment. Therefore, in order to reduce the self-healing energy and achieve good self-healing, metallization of organic films with low melting point metals is performed.In addition, the metallization layer should not be unevenly thick and thin, especially to avoid scratches, otherwise, the insulation isolation area will become branch-like and fail to achieve good self-healing. CRE capacitors all use regular films, and at the same time strict incoming material inspection management, blocking defective films at the door, so that the quality of capacitor films is fully guaranteed.

In addition to discharge self-healing, there is another one, which is electrochemical self-healing. Let’s discuss this mechanism in the next article.