What are the common failure modes of aluminum electrolytic capacitors?
Issuing time:2020-03-10 17:05
Simply put, an aluminum electrolytic capacitor is an aluminum cylinder equipped with a liquid electrolyte, and then a special surface-treated aluminum foil strip is inserted as the positive electrode, and it is made after sealing. Aluminum electrolytic capacitors are characterized by large capacity, positive and negative electrodes, and are suitable for power supply filtering or low-frequency circuits. Common failure modes of aluminum electrolytic capacitors include leakage, breakdown, open circuit, deterioration of electrical parameters, burnout, explosion, etc.
Liquid leakage is one of the reasons for the failure of capacitors, and aluminum electrolytic capacitors are no exception. The working electrolyte of aluminum electrolytic capacitor is acidic. If it overflows, it will seriously pollute and corrode other components and printed circuit boards around the capacitor. At the same time, inside the electrolytic capacitor, the working electrolyte gradually dries up due to liquid leakage, and the ability to repair the anodic oxide film medium is lost, resulting in capacitor breakdown or deterioration of electrical parameters and failure. There are many reasons for liquid leakage, such as: aluminum electrolytic capacitors are not well sealed; rubber plugs are used to seal aluminum electrolytic capacitors, and they may be used for too long, resulting in rubber aging and cracking and causing liquid leakage; mechanical sealing technology exists Problem: installation problem, the general manufacturer will take into account the problem of liquid leakage, they will clearly stipulate in the corporate standard to install vertically, while some companies use horizontal installation and so on.
The breakdown of aluminum electrolytic capacitors is caused by the rupture of the anodic aluminum oxide dielectric film, which causes the electrolyte to directly contact the anode. The aluminum oxide film may be locally damaged due to various materials, processes, or environmental conditions. Under the action of an external electric field, the oxygen ions provided by the working electrolyte can re-form an oxide film on the damaged part, so that the anodic oxide film can be filled and repaired. However, if there are impurity ions or other defects in the damaged part, the filling and repairing work cannot be completed, then micropores will be left in the anodic oxide film, which may even become penetrating holes, causing the aluminum electrolytic capacitor to break down. In addition, with the increase of use and storage time, the solvent in the electrolyte is gradually consumed and volatilized, which causes the acid value of the solution to rise, and corrodes the oxide film layer during storage. At the same time, due to the aging and drying of the electrolyte, oxygen ions can no longer be provided to repair the oxide film under the action of the electric field, and the self-healing effect is lost. Once the oxide film is damaged, the capacitor will break down. Process defects are also a major cause of breakdown of aluminum electrolytic capacitors. For example, when the riveting process is not good, the burrs on the lead-out chaff will severely damage the oxide film, and the leakage current at the puncture site will be large, and local overheating will cause thermal breakdown of the capacitor. In use, over-temperature, over-ripple current or over-mechanical stress may cause capacitor breakdown.
The electrical parameters deteriorate, the capacitance decreases and the loss increases. The capacitance of the aluminum electrolytic capacitor decreases slowly in the early working period. This is due to the continuous repair of the working electrolyte and the thickening of the anodic oxide film during the load process. In the later stage of use of aluminum electrolytic capacitors, due to more electrolyte loss and thickening of the solution, the resistivity rises due to the increase in viscosity, which increases the equivalent series resistance of the working electrolyte, resulting in a significant increase in capacitor loss. At the same time, the electrolyte with increased viscosity is difficult to fully contact the oxide film layer on the surface of the corroded uneven aluminum foil, which reduces the effective area of the aluminum electrolytic capacitor and causes a sharp drop in capacitance. This is also the performance of the capacitor near the end of its life. In addition, if the viscosity of the working electrolyte increases too much at low temperatures, it will also cause the consequences of increased loss and a sharp drop in capacitance. In use, over-temperature and over-ripple current may decrease the capacitance and increase the loss.
An increase in leakage current often leads to failure of aluminum electrolytic capacitors. Low process level, serious damage and contamination of oxide film, poor working electrolyte formulation, low raw material purity, long-term stability of electrolyte chemical and electrochemical properties, low purity of aluminum foil, high impurity content, etc. It may cause the leakage current to exceed the tolerance. Chloride ions in aluminum electrolytic capacitors are severely contaminated, and the leakage current causes the oxide film at the contaminated part to decompose, causing perforations and further increasing the current. In short, the presence of metal impurities in the aluminum foil will increase the leakage current of the aluminum electrolytic capacitor, thereby shortening the life of the capacitor. Overvoltage in use may increase the leakage current of the capacitor.
Aluminum electrolytic capacitors are burnt out, generally caused by the following reasons: positive and negative poles are connected inversely. Aluminum electrolytic capacitors are capacitors with positive and negative poles. If the positive and negative poles are connected incorrectly when the aluminum electrolytic capacitor is installed, the capacitor will burn out; Not enough. When the voltage exceeds the withstand voltage of the aluminum electrolytic capacitor, the capacitor will burn out; the quality of some manufacturers' aluminum electrolytic capacitors is unqualified, which may also cause the capacitor to burn out. Aluminum electrolytic capacitors may experience open circuit failure during long-term operation in a high temperature or hot flash environment. The reason is that the anode lead foil suffers from electrochemical corrosion and breaks. For high-voltage large-capacity capacitors, there are many such failure modes. In addition, after the anode lead foil and the anode foil are riveted, if they are not sufficiently flat, poor contact will cause intermittent open circuit of the capacitor. In use, excessive mechanical stress may open the capacitor.
The AC component of the aluminum electrolytic capacitor is too large in the working voltage, or the oxide film medium has many defects, or there are harmful anions such as chloride and sulfate, so that when the leakage current is large, the rate of gas generated by electrolysis is faster, and the work The longer the time, the greater the leakage current, the more gas in the shell, and the higher the temperature. The pressure difference between the inside and outside of the capacitor metal case will increase with the increase of working voltage and working time. If the sealing is good and there is no explosion-proof measures, the increase of the air pressure to a certain extent will cause the capacitor to explode. At present, the explosion-proof shell structure has been widely adopted. A fold is added to the upper part of the metal shell. When the air pressure is high, the fold is opened to increase the volume of the shell, thereby reducing the air pressure and reducing the risk of explosion. In use, such as applying an overload voltage, rapidly charging and discharging the capacitor, or applying a reverse voltage, the capacitor may explode.