Why do electrolytic capacitors explode? understand

One, electrolytic capacitor

Electrolytic capacitors are capacitors with an oxide layer formed on electrode as an insulating layer by action of electrolyte, and usually have a large capacitance. The electrolyte is a liquid jelly-like substance rich in ions. Most electrolytic capacitors are polar, that is, during operation, voltage of positive electrode of capacitor must always be higher than voltage of negative electrode.

The high capacitance of electrolytic capacitors is also achieved at expense of many other characteristics, such as high leakage current, large equivalent series inductance and resistance, large capacitance error, and short life.

In addition to polarized electrolytic capacitors, there are also non-polarized electrolytic capacitors. The figure below shows two types of electrolytic capacitors 1000uF, 16V. Among them, largest one is non-polar, smaller one is polarized.

(non-polar and polarized electrolytic capacitors)

The inside of an electrolytic capacitor can be liquid electrolyte or solid polymer, and electrode material is usually aluminum (aluminum) or tantalum (tandal). The figure below shows internal structure of a conventional polarized aluminum electrolytic capacitor. Between two electrode layers, there is a layer of fibrous paper impregnated with electrolyte, and then insulating paper layer is formed into a cylinder and sealed into an aluminum shell. .

(Internal structure of an electrolytic capacitor)

By disassembling an electrolytic capacitor, you can clearly see its basic structure. To prevent evaporation and leakage of electrolyte, capacitor leads are fixed with sealing rubber.

Of course, figure also shows difference in internal volume of polarized and non-polarized electrolytic capacitors. For same capacitance and withstand voltage level, a non-polar electrolytic capacitor is about twice as large as a polar one.

(Internal structure of non-polar and polar electrolytic capacitors)

This difference is mainly due to large difference in area of ​​the internal electrodes of two capacitors. The left side of figure below is a non-polar capacitive electrode, while right side is a polarized electrode. In addition to difference in area, thickness of two electrodes is also different, and electrode thickness of polarized capacitor is thinner.

(Different width of aluminum sheets of electrolytic capacitors)

Second, capacitor explosion

When voltage applied to capacitor exceeds its withstand voltage, or when voltage polarity of polarized electrolytic capacitor is reversed, leakage current of capacitor increases sharply, causing internal heating of capacitor to increase, and electrolyte will generate a large amount of gas.

In order to prevent condenser from exploding, there are three grooves on top of condenser body so that top of condenser can explode first under high pressure and release internal pressure.

(Explosive slot in top of electrolytic capacitor)

However, during manufacturing process of some capacitors, groove at top does not meet requirements, and pressure inside capacitor causes sealing rubber at bottom of capacitor to pop out. At this time, pressure inside condenser will suddenly release condenser and an explosion will occur.

1. Explosion of a non-polar electrolytic capacitor

The figure below shows a non-polarized electrolytic capacitor with a capacity of 1000uF and a withstand voltage of 16V. After applied voltage exceeds 18V, leakage current will increase sharply, and temperature and pressure inside capacitor will increase. Eventually rubber seal at bottom of capacitor exploded, and inner electrodes shattered like popcorn.

(explosion of a non-polar electrolytic capacitor from overvoltage)

By connecting a thermocouple to a capacitor, you can measure how temperature of capacitor changes as applied voltage increases. The figure below shows process in which internal temperature of a non-polar capacitor continues to increase when applied voltage exceeds withstand voltage value during voltage increase process.

(Relationship between voltage and temperature)

The figure below shows change in current flowing through a capacitor during same process. It can be seen that increase in current is main reason for increase in internal temperature. In this case, voltage increases linearly, and with a sharp increase in current, internal group of power supply causes a voltage drop. Finally, when current exceeds 6A, with a loud noise, capacitor explodes.

(Relationship between voltage and current)

Because non-polar electrolytic capacitor has a large internal volume and a lot of electrolyte, pressure generated after overcurrent is huge, resulting in no rupture of pressure relief groove on top of case and sealing rubber. condenser is swollen at bottom.

2. Explosion of polarized electrolytic capacitors

For polarized electrolytic capacitors, apply voltage. When voltage exceeds withstand voltage of capacitor, leakage current also rises sharply, causing capacitor to overheat and explode.

The figure below shows a limited electrolytic capacitor with a capacity of 1000uF and a withstand voltage of 16V. After overpressure, process of internal air pressure is released through upper pressure relief groove, thus avoiding explosion process of capacitor.

The figure below shows how temperature of a capacitor changes as applied voltage increases. As voltage gradually approaches capacitor's withstand voltage, capacitor's current increases and internal temperature continues to rise.

(Relationship between voltage and temperature)

The figure below shows change in capacitor leakage current. The electrolytic capacitor is designed for a withstand voltage of 16V. During testing, when voltage exceeds 15V, leakage of capacitor starts to increase sharply.

(Relationship between voltage and current)

Through experimental process of previous two electrolytic capacitors, we can also see withstand voltage limit for such conventional 1000uF electrolytic capacitors. In order to avoid high voltage failure of capacitor, when using electrolytic capacitors, it is necessary to leave a sufficient margin in accordance with actual voltage fluctuations.

Third, electrolytic capacitors connected in series

Under appropriate circumstances, parallel and series connection can be used to obtain larger capacitance and higher capacitance withstand voltage, respectively.

(Popcorn on an electrolytic capacitor after an overvoltage explosion)

In some applications, voltage applied to capacitor is AC voltage, such as speaker coupling capacitor, AC phase compensation, motor phase shifting capacitor, etc., which require use of non-polar electrolytic capacitors.

The manuals provided by some capacitor manufacturers also give use of traditional polarized capacitors in an anti-parallel connection, that is, to connect two capacitors in series, but with reversed polarity to obtain non-polarized capacitors. , Effect.

(Electrolytic capacitor after blowing overvoltage)

The following is a comparison of change in leakage current with increasing applied voltage under three conditions of supply of forward voltage, reverse voltage and two electrolytic capacitors in series. to form a non-polar capacitor with a polar capacitor.

1. Forward voltage and leakage current

Measure current flowing through capacitor by connecting a resistor in series within withstand voltage range of electrolytic capacitor (1000uF, 16V), gradually increase applied voltage from 0V, and measure relationship between corresponding leakage current and voltage.

(Positive Series Capacitor)

The figure below shows relationship between leakage current and voltage of a polarized aluminum electrolytic capacitor, which is a non-linear relationship, and leakage current is less than 0.5mA.

(Relationship between voltage, voltage and current after direct series connection)

2. Reverse voltage and leakage current

Using same current to measure relationship between applied directional voltage and leakage current of an electrolytic capacitor, figure below shows that when applied reverse voltage exceeds 4V, leakage current begins to increase rapidly. According to slope of following curve, the reverse electrolytic capacitor is equivalent to a 1 ohm resistor.

(relationship between reverse voltage voltage and current)

3. Capacitors in Series

Two identical electrolytic capacitors (1000uF, 16V) connected in series to form a non-polar equivalent electrolytic capacitor, and then measure ratio between their voltage and leakage current.

(Series Capacitor of Straight and Reverse Polarity)

The figure below shows relationship between capacitor voltage and leakage current. It can be seen that after applied voltage exceeds 4V, leakage current will increase and current amplitude will be less than 1.5mA.

And this measurement result is indeed a little surprising, you will see that leakage current of two capacitors in series is actually greater than leakage current of a single capacitor when forward voltage is applied.

(Relationship between voltage, voltage and current after direct and reverse series connection)

However, due to time reasons, this phenomenon has not been retested. It is possible that one of capacitors has just used a reverse voltage test capacitor, and internal part has been damaged, so above test curve has been obtained. . . .