The Advantages and Mechanisms of Self-Healing in Film Capacitors
One of the most significant benefits of self-healing film capacitors is their inherent self-healing capability, which has contributed to their rapid growth in the capacitor market. These capacitors exhibit two distinct self-healing mechanisms: discharge self-healing and electrochemical self-healing. The former occurs at higher voltages, also referred to as high-voltage self-healing, while the latter can take place at very low voltages, known as low-voltage self-healing.
Discharge Self-Healing Mechanism
In the case of discharge self-healing, let’s assume there is a defect in the dielectric organic film that separates the metallized electrodes. This defect might be metallic, semiconductor-based, or have poor insulation. If the defect is conductive (metallic or semiconductor), the capacitor may discharge at low voltages, but in the case of poor insulation, the self-healing occurs at higher voltages.
When a voltage VVV is applied to a metallized film capacitor with such a defect, an ohmic current I=V/RI = V/RI=V/R flows through the defect, where RRR is the defect's resistance. The current density J=V/Rπr2J = V/R\pi r^2J=V/Rπr2 flows through the metallized electrode, which results in a higher concentration of current near the defect (as rrr decreases). This causes localized heating due to the Joule effect, where power consumption is proportional to W=(V2/R)rW = (V^2/R)rW=(V2/R)r. As the temperature rises, the defect's resistance drops exponentially, increasing both current III and power WWW.
In regions where the electrode is closest to the defect, the current density J1J_1J1 surges, leading to Joule heating that melts the metallized layer. This forms an arc between the electrodes, which evaporates the metal in the affected area, creating an insulated isolation zone devoid of the metal layer. This arc is then extinguished, completing the self-healing process.
However, this process also subjects the dielectric surrounding the defect to thermal and electrical stresses. As a result, chemical decomposition, gasification, and even carbonization may occur, causing localized mechanical damage to the dielectric material.
Optimizing Discharge Self-Healing
For effective discharge self-healing, it's crucial to optimize the capacitor's design. Key factors include achieving a proper environment around the defect, selecting an appropriate metal layer thickness, maintaining a hermetically sealed environment, and ensuring the core voltage and capacitance are appropriate for the application.
A perfect self-healing process involves a short self-healing time, minimal energy consumption, and precise defect isolation, without damaging the surrounding dielectric. To avoid carbon deposition during self-healing, the organic film molecules should have a low carbon-to-hydrogen ratio and an adequate amount of oxygen. This ensures that the decomposition products include gases like CO2, CO, and CH4, which help extinguish the arc by quickly dissipating the energy as gas.
The energy required for self-healing must be carefully managed—not too large to damage the surrounding media, and not too small to fail in removing the metallized layer around the defect. The amount of energy necessary for self-healing depends on the material, thickness, and environment of the metallization layer. Using low-melting-point metals for metallization helps reduce the energy needed and improves self-healing efficiency.
Additionally, it’s vital that the metallization layer maintains uniform thickness and avoids defects such as scratches, which could lead to incomplete or irregular self-healing. Capacitor manufacturers, like CRE, ensure the quality of their products by using high-quality films and implementing stringent material inspections to prevent defective films from entering the production line.
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