Product Information: Electrolytic Capacitor

Aluminum Electrolytic Capacitor Technical Notes

4. Failure Modes

     Please refer here for the representative failure mode and its factors.

5. Life

     Aluminum electrolytic capacitors are greatly affected by the use conditions (environmental conditions, electrical loads, etc.), and come to the end of its usefulness due to decrease in the capacitance and increase in the tangent of the loss angle (tanδ). Degradation of this characteristic is caused by the reduction of the electrolyte in the capacitor element, and generally explained as a diffusion phenomenon, which dissolves electrolyte into sealing rubber material and evaporates to outside.

5-1 Ambient Temperature and Life

     Life of the aluminum electrolytic capacitor is highly dependent on temperature, and the relation between the ambient temperature and the lifetime is expressed by Equation 7 based on the theory (10ºC 2 times law) that the lifetime doubles as the temperature decreases by 10ºC.
Equation 7

     Diffusion of the electrolyte from the sealing rubber material is generally the dominant factor in the life of aluminum electrolytic capacitor, and its speed (diffusion coefficient) is consistent with the Arrhenius law. Figure 20 shows the comparison of the Arrhenius law and the "10ºC 2 times law" commonly used for life calculation of electrolytic capacitors. The Arrhenius law and the "10ºC 2 times law" show good consistency in the range of 70ºC to 90ºC, but there is some deviation of "10ºC 2 times law" in the temperature range < 60ºC or >105ºC.

Fig. 20

Fig. 20 Arrhenius law vs 10ºC 2 times law (105ºC base)

     Therefore, the life calculation formula described in the following section is mainly applied to products with upper category temperature limit of 105ºC or less. For estimating life expectancy of products with a category temperature upper limit of 125ºC or higher, please contact us.

5-2 Ripple Current and Life

     Aluminum electrolytic capacitor generate Joule's heat (self-heating) when ripple current is applied due to higher loss in comparison with other type of capacitors. Due to this self-heating, the internal core temperature of the capacitor (at the element) is higher than the ambient or surface temperature of the capacitor. Since the ESR of the capacitor increases due to electrolyte dry-up, heat generation by ripple current continues to rise. Therefore, it is necessary to consider acceleration which is larger than equation 7 for estimating expected life when ripple current is applied.

(1)Temperature At Top Of Case And At Core Of Capacitor When Ripple Current Is Applied
     Temperature rise of the capacitor when ripple current is applied is expressed by Equation 8.
Equation 8

     Value of β is generally becomes smaller as surface area becomes bigger. β value approximation is expressed as equation 9
Equation 9
     Where β is a factor when heat rise is measured at top of capacitor.

(2)Temperature Slope Between Core And Case Surface Of Capacitor
    Temperature slope between core and case surface of capacitor is expressed as equation 10
Equation 10

Table 1 Temperature Difference Factor (SMD / Radial Lead Capacitors)

Case Dia (mm)Φ4~Φ8Φ10

Table 2 Temperature Difference Factor (Snap-in type)

Case Dia (mm)Φ20Φ22Φ25Φ3035

Note 1 ΔT0 is specified for each series. Please inquire details.
Note 2 Frequency coefficient is specified for each series. By measuring effective value of ripple
             current for each frequency of actual use condition and dividing by frequency coefficient
             described in product catalog, rated ripple current can be converted to effective value at
             defined frequency. (Equation 11)

Equation 11
                 (please refer to multiplier for ripple current of product catalog)

     For industrial equipment and others, forced air cooling by a fan and cooling of the bottom of capacitor by water cooling are carried out. In such case, it is necessary to calculate using more accurate thermal model of the capacitor. Please inquire details.

(3)Heat Rise By Ripple Current And Estimated Life
     We have experimentally calculated the affect of ripple current and its effect on life. It is estimated as shown in Equation 12. In addition, since the influence rate on the life by ripple current depends on the product type (size) as well as dry-up phenomenon, this effect is expressed by coefficient k .
Equation 12

5-3 Applied Voltage and Life

     For products of large size and electrolyte retention, such as snap-in or screw terminal type, not only dry-up of electrolyte but also consumption of electrolyte due to leakage current flowing when voltage is applied also affect lifetime. The life calculation formula incorporating this effect is shown in Equation 13.
Equation 13
     Where Min [A,B ] means taking a smaller value of A and B .
Equation 13-2
     Equation 13 means that when it is used at 80% or less of the rated voltage, lifetime of capacitor is κ times as large as when rated voltage is applied. Please inquire about the value of κ .
     This voltage dependence is applicable only to snap-in and screw terminal type with rated voltage of 160 V or more. It is not applied to small size such as SMD and lead wire type or those with rated voltage of 100 V or less. This is because dry-up effect is larger for small-size products, and voltage dependence is not observed for low-voltage products.

5-4 Life calculation formula for each product type

     Considering the influence of ambient temperature, ripple heat generation, voltage application, our life calculation formulas are as follows.

‹SMD / Lead wire type (maximum category temperature 105 ºC or less)›
①Products specified endurance with applying rated ripple current
Equation 14
②Products specified endurance with applying rated DC voltage
Equation 15

* If applicable below, please contact us.
  ·The case that heat rise (Tj)exceeds 20ºC by applying ripple current.
  ·Product whose maximum category temperature (T max) exceed 105ºC.

‹Snap-in type›
①Product with rated voltage of 100 V or less
Equation 16
②Products with a rated voltage of 160 V or more
Equation 17
*If heat rise (Tj) exceeds 30ºC by applying ripple current, please contact us.

‹Screw terminal type›
①Product with rated voltage of 100 V or less
Equation 18
②Products with a rated voltage of 160 V or more
Equation 19
*Maximum temperature rise (Tj) by ripple current is specified for each ambient temperature to be
  used for the product. Please inquire details.

     Please note that the estimated lifetime is a reference value and not a guaranteed value. Therefore, please select a product that has sufficient margin for the design life of the equipment. If the life calculation result exceeds 15 years, 15 years will be the upper limit. Please contact us if you need further life.

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