Analysis of the causes of capacitor failure

The concept of capacitive breakdown

　　Capacitor dielectric withstand the electric field strength is a certain limit, when the bound charge from the atomic or molecular bondage and participate in the conductivity, it destroyed the insulation properties, this phenomenon is called dielectric breakdown.

Capacitor breakdown conditions

　　The conditions for a capacitor to be breakdown reach the breakdown voltage.

　　Breakdown voltage is the limit voltage of the capacitor, more than this voltage, the capacitor dielectric will be broken down. The rated voltage is the voltage that the capacitor can withstand when working for a long time, it is lower than the breakdown voltage. It is safe and reliable for capacitors to work at voltages not higher than the breakdown voltage, so don’t make the mistake of thinking that capacitors are only normal to work at rated voltage.

　　Define the voltage corresponding to the critical breakdown of PN junction as the breakdown voltage BV of PN junction, BV is an important parameter to measure the reliability and use range of PN junction, and the higher the value of BV, the better if other performance parameters of PN junction remain unchanged.

Is a general capacitor breakdown an open circuit or a short circuit?

　　General capacitor breakdown is equivalent to short circuit, because when the capacitor is connected to DC it is seen as open circuit, when it is connected to AC it is seen as short circuit, the capacitor has a nature of through-crossing isolation, the word breakdown is understood as short circuit in electrician, the breakdown is formed mainly because of the permanent damage caused by the external voltage exceeding its nominal voltage, called breakdown.

　　When a destructive discharge occurs in a solid dielectric, it is called breakdown. Breakdown, leaving traces in the solid dielectric, so that the solid dielectric permanently lose insulation properties. For example, when an insulating cardboard breaks down, it leaves a hole in the cardboard. It can be seen that the term breakdown is only used in solid dielectrics.

　Causes of capacitive breakdown

　　The fundamental cause of capacitance breakdown is the destruction of the dielectric insulation and the generation of polarization. The causes of dielectric insulation breakdown are.

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　　● The operating voltage exceeds the maximum withstand voltage of the capacitor;

　　● Bad quality of capacitor, high leakage current, gradual increase of temperature and decrease of insulation strength.

　Methods to avoid dielectric breakdown.

　　● Use materials with high insulation strength;

　　● The insulation material has a certain thickness and does not contain impurities, such as air bubbles or moisture;

　　● Try to make the electric field distributed as required to avoid the power lines from being too dense in some places.

　　● The polarity of the polarized capacitor is reversed or connected to the AC power supply.

　Can the capacitor recover after breakdown.

　　● If the dielectric is gas or liquid, it is a self-recovering insulating medium and the breakdown is reversible;

　　The dielectric is solid, the breakdown is not reversible, it is the only insulation medium that cannot be recovered after breakdown.

　Failure problems of safety gauge capacitors:.

　　Here the failure problem of safety gauge capacitor is carried out separately, mainly because there are some differences between safety gauge capacitor and conventional capacitor. Briefly introduce the safety capacitor, which mainly includes X capacitor and Y capacitor.

X capacitor is divided into X1, X2 and X3, the main differences are

　　● X1 capacitor voltage withstand value is greater than 2.5kV, less than or equal to 4kV.

　　● X2 capacitor with voltage withstand value less than or equal to 2.5kV.

　　● X3 capacitor voltage withstand value is less than or equal to 1.2kV.

Y capacitor is divided into Y1, Y2, Y3 and Y4 capacitors, the main differences are

　　● Y1 capacitor with a voltage withstand value greater than 8kV.

　　● Y2 capacitor has a voltage withstand value greater than 5kV.

　　● Y3 capacitor has no special restriction on the withstand voltage value.

　　● Y4 capacitor has a voltage withstand value greater than 2.5kV.

　　X capacitor is mainly used between L and N of AC power line, after using X capacitor, when the capacitor fails, the capacitor is in open circuit state, not to produce short circuit between lines. the test condition of X capacitor is: working at 1.5 times of AC voltage rms for 100 hours, plus at least 1kV of pulse high voltage test.

　　Y capacitance mainly acts between L, N and ground of AC power line, or between common ground and shell of other circuits. The capacitor across these locations can lead to electric shock hazard (especially to the case part) if there is a failure short circuit, when Y capacitor must be forced to use (the failure mode of Y capacitor is open circuit).The test condition of Y capacitor is: 100 hours of operation at 1.7 times the AC voltage rms, plus at least 2kV pulse high voltage test.

To sum up: the failure of conventional capacitor is generally short-circuit, and the failure of safety capacitor is generally shown as open circuit, so remember! You can’t use the conventional capacitor to replace the safety capacitor in the occasion of using large AC voltage to prevent the capacitor failure to cause electric shock accident to people.

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