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-precise timing delays. When the MTBF of the switching power supply is estimated, the model analysis results show that the electrolytic capacitor is the main factor affecting the life of the switching power supply, so it is very important to understand and affect the factors affecting the life of the capacitor.

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

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

2. Abnormal failure of electrolytic capacitors

Some factors can cause electrolytic capacitors to fail, such as extremely low temperature, capacitor temperature rise (soldering temperature, ambient temperature, AC ripple), excessive voltage, transient voltage, very high frequency or reverse bias; The operating life (Lop) of electrolytic capacitors is the most influential factor.

The conductivity of the capacitor is determined by the ionization ability and viscosity of the electrolyte. As the temperature decreases, the viscosity of the electrolyte increases, and thus the ionic mobility and electrical conductivity decrease. When the electrolyte is frozen, the ion mobility is very low resulting in very high electrical resistance. On the contrary, excessive heat will accelerate the evaporation of the electrolyte, and when the amount of electrolyte is reduced to a certain limit, the life of the capacitor will be terminated. When working in alpine regions (generally below -25°C), heating is required to ensure the normal working temperature of the electrolytic capacitor. For example, outdoor UPS is equipped with heating plate in northeastern my country.

Capacitors are easily broken down under overvoltage conditions, and in practical applications, surge voltages and transient high voltages often occur. In particular, my country has a vast territory and complex power grids in various places. Therefore, the AC power grid is very complex, and often exceeds 30% of the normal voltage. Especially for single-phase input, the phase deviation will increase the normal range of AC input. The test shows that the commonly used 450V/470uF 105 ℃ imported ordinary 2000-hour electrolytic capacitor will leak liquid and gas after 2 hours at a voltage of 1.34 times the rated voltage, and the top will burst open. According to statistics and analysis, the failure of the PFC output electrolytic capacitor of the communication switching power supply close to the power grid is mainly due to power grid surge and high voltage damage. The voltage selection of aluminum electrolytic capacitors is generally derated in two stages, and it is more reasonable to use it when it is reduced to 80% of the rated value.

3 Analysis of factors affecting life expectancy

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

Refer to RIFA's formula for life expectancy:

PLOSS=(IRMS)²xESR(1)

Th=Ta+PLOSSxRth(2)

Lop=Ax2Hours(3)

B=reference temperature value (typical value is 85℃)

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

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

From the above formula, we can clearly see that there are several direct factors that affect the life of electrolytic capacitors: ripple current (IRMS) and equivalent series resistance (ESR), ambient temperature (Ta), transfer from hot spots to surrounding The total thermal resistance (Rth) of the environment. The point with the highest temperature inside the capacitor is called the hot spot temperature (Th). The hot spot temperature value is the main factor affecting the working life of the capacitor. And the following factors determine the outside temperature (ambient temperature Ta) in practical application of the hot spot temperature value, the total thermal resistance (Rth) transferred from the hot spot to the surrounding environment and the energy loss (PLOSS) caused by the alternating current. The internal temperature rise of the capacitor has a linear relationship with the energy loss.

When the capacitor is charged and discharged, the current will cause energy loss when flowing through the resistor. The voltage change will also cause energy loss when passing through the dielectric. In addition to the energy loss caused by leakage current, the result of all these losses is the internal temperature of the capacitor. rise.