​Electrolytic capacitors are widely used in different fields of power electronics, mainly for smoothing, energy storage or filtering after AC voltage rectification, and also for non precision timing delay. In the MTBF prediction of switching power supply, the model analysis results show that electrolytic capacitor is the main factor affecting the life of switching power supply, so it is very important to understand and influence the factors of capacitor life.

1. The life of electrolytic capacitor depends on its internal temperature.

Therefore, the design and application conditions of electrolytic capacitor will affect the life of electrolytic capacitor. From the design point of view, the design method, material and processing technology of electrolytic capacitor determine the life and stability of capacitor. For the application, the service voltage, ripple current, switching frequency, installation form and heat dissipation mode all affect the life of electrolytic capacitor.

2. Abnormal failure of electrolytic capacitor

Some factors can cause the failure of electrolytic capacitors, such as extremely low temperature, capacitor temperature rise (welding temperature, ambient temperature, AC ripple), high voltage, instantaneous voltage, VHF or reverse bias; Temperature rise is one of the factors that affect the working life (LOP) of electrolytic capacitors.

The conductivity of the capacitor is determined by the ionization ability and viscosity of the electrolyte. When the temperature decreases, the viscosity of electrolyte increases, so the ionic mobility and conductivity decrease. When the electrolyte is frozen, the ion mobility is so low that the resistance is very high. On the contrary, too much heat will accelerate the evaporation of electrolyte. When the amount of electrolyte is reduced to a certain limit, the life of capacitor will be terminated. When working in high cold area (below - 25 ℃), heating is needed to ensure the normal working temperature of electrolytic capacitor. For example, outdoor UPS is equipped with heating plate in Northeast China. Capacitor is easy to be broken down under overvoltage condition, but surge voltage and instantaneous high voltage often appear in practical application. In particular, China has a vast territory and complex power grids. Therefore, the AC power grid is very complex, which often exceeds 30% of the normal voltage. Especially for single-phase input, phase deviation will aggravate the normal range of AC input. The test results show that the commonly used 450V / 470uf105 ℃ imported ordinary 2000h electrolytic capacitor, under the voltage of 1.34 times of the rated voltage, after 2 hours, the capacitor will leak liquid and gas, and the top will burst open. According to statistics and analysis, the failure of PFC output electrolytic capacitor of communication switching power supply close to power grid is mainly due to power grid surge and high voltage damage. The voltage selection of aluminum electrolytic capacitor is generally two-stage derating, and it is more reasonable to reduce to 80% of the rated value.

Analysis of the influencing factors of the service life

In addition to the abnormal failure, the life of electrolytic capacitor has an exponential relationship with temperature. Due to the use of non solid electrolyte, the life of electrolytic capacitor also depends on the evaporation rate of electrolyte, resulting in the electrical performance degradation. These parameters include capacitance, leakage current and equivalent series resistance (ESR).

Refer to RIFA's life expectancy formula:

PLOSS=(IRMS)2xESR（1）

Th=Ta+PLOSSxRth（2）

Lop=Ax2Hours（3）

B = reference temperature (typical 85 ℃)

A = capacitor life at reference temperature (varies with capacitor diameter)

C = the temperature rise required to reduce the life of the capacitor by half

From the above formula, we can clearly see several direct factors that affect the life of electrolytic capacitor: ripple current (IRMS) and equivalent series resistance (ESR), ambient temperature (TA), and total thermal resistance (RTH) transferred from hot spot to surrounding environment. The point with high temperature inside the capacitor is called hot spot temperature (th). The hot spot temperature is the main factor affecting the working life of capacitor. The following factors determine the external temperature (ambient temperature Ta), the total thermal resistance (RTH) from the hot spot to the surrounding environment and the energy loss caused by AC current (Ploss). The internal temperature rise of the capacitor is linear with the energy loss.

When the capacitor is charged and discharged, the current will cause energy loss when it flows through the resistance, and the change of voltage will also cause energy loss when it passes through the dielectric. In addition, the energy loss caused by the leakage current, all these losses lead to the increase of the internal temperature of the capacitor.