An article to understand the analysis and calculation of electrolytic capacitor life

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An article to understand the analysis and calculation of electrolytic capacitor life [Copy link]

As an important component of electronic products, electrolytic capacitors play an indispensable role in switching power supplies. Their service life and working conditions are closely related to the life of the switching power supply.

In a large number of production practices and theoretical discussions, when the capacitors in the switching power supply are damaged, especially when the electrolytic capacitors are topped out and the electrolyte overflows, the power supply manufacturer suspects that there is a problem with the quality of the capacitors, while the capacitor manufacturer says that the power supply is improperly designed, and the two sides are arguing.

The following is an analysis of the service life and safety of electrolytic capacitors to provide some judgment basis for electronic engineers.

Arrhenius

1.1 Arrhenius equation

The Arrhenius equation is an empirical formula used to describe the relationship between the reaction rate of a chemical substance and the change in temperature. The interior of an electrolytic capacitor is composed of metal aluminum and chemical substances such as electrolyte, so the life of the electrolytic capacitor is closely related to the Arrhenius equation. Arrhenius equation formula: k=Ae－Ea/RT or lnk=lnA—Ea／RT (graphical method) K is the chemical reaction rate R is the molar gas constant. 51)]●T is the thermodynamic temperature

●Ea is the apparent activation energy

●A is the frequency factor

1.2 Arrhenius Conclusion

According to the Arrhenius equation, as the temperature rises, the chemical reaction rate (life consumption) increases. Generally speaking, for every 10°C increase in ambient temperature, the chemical reaction rate (K value) will increase by 2-10 times, that is, for every 10°C increase in the operating temperature of the capacitor, the capacitor life is halved, and for every 10°C decrease in the operating temperature of the capacitor, its life is doubled. Therefore, ambient temperature is an important factor affecting the life of electrolytic capacitors.

Analysis of the service life of electrolytic capacitors

2.1 Formula:

According to the conclusion of the Arrhenius equation, the formula for calculating the service life of electrolytic capacitors is as follows:

●L The service life of electrolytic capacitors when the ambient temperature is T (hour)

●L0 The rated service life of electrolytic capacitors at maximum temperature (hour)

●T0 The rated maximum operating temperature of electrolytic capacitors (deg℃)

●T The ambient temperature (deg℃)

●T0-T temperature rise (deg℃)

2.2 Analysis:

According to formula (1),

when the operating temperature of the electrolytic capacitor is at the highest operating temperature (i.e. T0=T),The minimum service life of the electrolytic capacitor calculated by formula (1) is L=L0×20=L0, which is equal to the rated life, such as 8000 hours, 8000/8760=0.9 years.

When the operating temperature of the electrolytic capacitor is 10℃ lower than the maximum operating temperature, the service life of the electrolytic capacitor calculated by formula (1) is L=L0×2[T0-(T0-10℃）]/10℃=L0×21, which is equal to twice the rated life, that is, 16000 hours, 16000/8760=1.8264 years.

It can be seen that the calculation formula for the service life of electrolytic capacitors conforms to the conclusion of the Arrhenius equation

Calculation of the service life of electrolytic capacitors

In electronic products, the factors that affect the life of electrolytic capacitors include ambient temperature T and ripple current Irms. The load power borne by the capacitor is proportional to the ripple current. The greater the load, the greater the ripple current (the deeper the electrolytic charge and discharge), the greater the heat generated when the internal oxide film decomposes, and the more electrolyte is consumed during repair. See Figure 1. The greater the ripple current, the greater the heat caused. Therefore, the heat caused by the ripple current should be considered in the calculation of the life of the electrolytic capacitor.

3.1 Ripple current calculation

1) Capacitor capacity

[ 51)]

2)Charging time

3)Discharging time

4)Charging Current

51)]5)Discharge current

[attach]6)Ripple current

[color=rgb(51, 51, 51) 51)]

3.2 Power loss calculation

3.3 Electrolytic capacitor heating formula

[51)]When thermal equilibrium is reached, the temperature rise of the center temperature T0 of the container and the ambient temperature T is determined by the heat dissipation method (air heat dissipation, container heat dissipation) and the dissipated power PD, and is described by thermal resistance, thermal resistance (Thermal Resistance) Rq, unit (℃/W):

●△T Self-heating of electrolytic capacitor when ripple current I is added (deg℃)

● I Actual working ripple current (A rms）,

●β Heat dissipation coefficient (W/℃ Cm2)

●S Surface area of electrolytic capacitor (cm2)

●R Equivalent impedance of electrolytic capacitor (ESR Ω)

3.4 Calculation of synthetic ripple current

Because in actual circuits, ripple current includes ripple currents of various frequency waveforms, the calculation of actual circuit ripple current should be obtained by synthetic ripple current Irms:

3.5 Rated operating temperature

The industry regulations for electrolytic capacitors stipulate that at the rated temperature T0, the maximum heat generated by the allowable rated ripple current I is △t≤5 deg℃

Therefore, when the actual ripple current is Ir, the heat generated by the capacitor itself is

●△t is the maximum allowable temperature rise of the capacitor when the rated ripple current is added to the rated temperature (deg℃)

●Ir is the rated ripple current of the capacitor (Arms)

●I is the (calculated) actual working ripple current (Arms)

3.6 Calculation of electrolytic capacitor life From the above analysis, it can be seen that the life calculation formula of the electrolytic capacitor after considering the ripple current is: 416167 T0 is the rated temperature (for example 105℃) Δt Is the maximum allowable temperature rise of 5℃ at rated temperature

●T is the ambient operating temperature (for example 55℃)

●ΔT is the heating value generated by ripple current at temperature T (for example 20℃)

Example

A capacitor ED33uF/200V/105℃, rated life L0=8000 hours, the allowable ripple current I=195mA/120Hz, applied in a 110V/60Hz circuit at an environment of 55℃.

4.1 Triangle wave

4.2 Sine wave

4.3 Synthesis

4.4 Fever

4.5 Lifespan

1) Service life without considering ripple current

2) Actual service life considering ripple current

From the above example calculation, we can see that the ripple current has a great influence on the life of electrolytic capacitors. When designing and using electrolytic capacitors, circuit engineers must not only consider the ambient temperature of the capacitor, but also the influence of circuit ripple current on the life of electrolytic capacitors, so as to extend the service life of electrolytic capacitors as much as possible.

The circuit is capacitive or highly inductive, which will affect the safe switching of transistors, increase transistor loss, increase heat generation, and superimpose a high single-peak ripple current on the electrolytic capacitor, making the charge and discharge ripple current narrower and higher, and finally causing the electrolytic capacitor to heat up seriously until it is damaged, manifested as top venting, steam emission, leakage or explosion.

Try to choose electrolytic capacitors of good quality and good sealing performance. Do not disassemble electrolytic capacitors whose service life is halved. Providing electrolytic capacitors with a safe working environment and reasonable design is the solution to solve the problems of electrolytic capacitors bubbling, steaming, and leaking to extend their life.

Source: Network compilation. If copyright is involved, please contact us to delete.

51)]1) Service life without considering ripple current

2) Actual service life considering ripple current

Conclusion

[color=rgb(51, 51, 51) 51)]From the calculation in the above example, it can be seen that the influence of ripple current on the life of electrolytic capacitors is very large. When designing and using electrolytic capacitors, circuit engineers should not only consider the ambient temperature of the capacitor, but also the influence of circuit ripple current on the life of electrolytic capacitors, and extend the service life of electrolytic capacitors as much as possible.

The circuit is capacitive or highly inductive, which will affect the safe switching of transistors, increase the loss of transistors, increase the heat, and superimpose a high single-peak ripple current on the electrolytic capacitor, so that the charge and discharge ripple current becomes narrower and higher, and finally the electrolytic capacitor is seriously heated until it is damaged, which is manifested as top bulging, steam emission, leakage or explosion.

Try to choose electrolytic capacitors with good quality and good sealing performance. Do not disassemble electrolytic capacitors whose service life is halved. Providing a safe working environment for electrolytic capacitors and reasonable design is the solution to solve the problems of electrolytic capacitors bubbling, steaming and leaking to extend their service life.

Source: Network compilation. If copyright is involved, please contact us to delete.

51)]1) Service life without considering ripple current

2) Actual service life considering ripple current

Conclusion

[color=rgb(51, 51, 51) 51)]From the calculation in the above example, it can be seen that the influence of ripple current on the life of electrolytic capacitors is very large. When designing and using electrolytic capacitors, circuit engineers should not only consider the ambient temperature of the capacitor, but also the influence of circuit ripple current on the life of electrolytic capacitors, and extend the service life of electrolytic capacitors as much as possible.

The circuit is capacitive or highly inductive, which will affect the safe switching of transistors, increase the loss of transistors, increase the heat, and superimpose a high single-peak ripple current on the electrolytic capacitor, so that the charge and discharge ripple current becomes narrower and higher, and finally the electrolytic capacitor is seriously heated until it is damaged, which is manifested as top bulging, steam emission, leakage or explosion.

Try to choose electrolytic capacitors with good quality and good sealing performance. Do not disassemble electrolytic capacitors whose service life is halved. Providing a safe working environment for electrolytic capacitors and reasonable design is the solution to solve the problems of electrolytic capacitors bubbling, steaming and leaking to extend their service life.

Source: Network compilation. If copyright is involved, please contact us to delete.

小明哥

Great, thanks for sharing!