Electrolytic capacitor 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, materials and processing technology of Electrolytic capacitor determine the life and stability of the capacitor. For the application, the service voltage, ripple current, switching frequency, installation form, heat dissipation mode, etc. all affect the life of Electrolytic capacitor.
2. Abnormal failure of Electrolytic capacitor
Some factors may cause the Electrolytic capacitor to fail, such as extremely low temperature, capacitor temperature rise (welding temperature, ambient temperature, AC ripple), excessive voltage, instantaneous voltage, Very high frequency or reverse bias voltage; Among them, the temperature rise is the biggest factor affecting the working life (Lop) of Electrolytic capacitor.
The conductivity of capacitance is determined by the ionization capacity and viscosity of electrolyte. When the temperature decreases, the viscosity of the electrolyte increases, resulting in a decrease in ion mobility and conductivity. When the electrolyte freezes, the ion mobility is very low, resulting in a very high resistance. On the contrary, excessive heat will accelerate the evaporation of the electrolyte, and when the amount of electrolyte decreases to a certain limit, the lifespan of the capacitor will also end. When working in cold areas (generally below -25 ℃), heating is required to ensure the normal working temperature of Electrolytic capacitor. For example, outdoor UPS is equipped with heating plates in Northeast China.
Capacitors are prone to breakdown under overvoltage conditions, while surge voltage and instantaneous high voltage are common in practical applications. Especially in China, which has a vast territory and complex power grids in various regions, the AC power grid is very complex, often exceeding 30% of the normal voltage, especially for single-phase input. Phase deviation will exacerbate the normal range of AC input. The test shows that the commonly used 450V/470uF105 ℃ imported ordinary 2000 hour Electrolytic capacitor will leak and emit gas after 2 hours under 1.34 times of the rated voltage, and the top will burst. According to statistics and analysis, the failure of the Electrolytic capacitor output from the communication switching power supply PFC close to the grid is mainly due to the grid surge and high-voltage damage. The voltage of aluminum Electrolytic capacitor is generally selected for secondary derating, and it is more reasonable to reduce it to 80% of the rated value.
3. Analysis of Life Impact Factors
In addition to abnormal failure, the life of Electrolytic capacitor has an exponential relationship with temperature. Because of the use of non solid electrolyte, the life of the Electrolytic capacitor also depends on the evaporation rate of the electrolyte, resulting in a reduction in electrical performance. These parameters include capacitance, leakage current and Equivalent series resistance (ESR).
Refer to the formula for the expected life of RIFA company:
PLOSS=(IRMS) ² XESR (1)
Th=Ta PLOSSxRth (2)
B=reference temperature value (typical value is 85 ℃)
A=capacitor life at reference temperature (varies depending on the diameter of the capacitor)
C=Number of degrees of temperature rise required to reduce capacitor life by half
From the above formula, we can clearly see that there are several direct factors affecting the life of Electrolytic capacitor: ripple current (IRMS) and Equivalent series resistance (ESR), ambient temperature (Ta), and total thermal resistance (Rth) transferred from the hot spot to the surrounding environment. The point where the internal temperature of the capacitor is the highest is called the hot spot temperature (Th). The hot spot temperature value is the main factor affecting the working life of capacitors. The following factors determine the external temperature (ambient temperature Ta) of the hot spot temperature value in practical applications, the total thermal resistance (Rth) transmitted from the hot spot to the surrounding environment, and the energy loss caused by AC current (PLOSS). The internal temperature rise of a capacitor is linearly related to energy loss.
During capacitor charging and discharging, the current flowing through the resistor causes energy loss, and the change in voltage also causes energy loss when passing through the dielectric. In addition, the energy loss caused by leakage current leads to an increase in the internal temperature of the capacitor.