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Factors Affecting the Life of Electrolytic Capacitors (I)
Author：本站Release time：2018-12-17 10:54:56Browsing：
Electrolytic capacitors are widely used in different fields of power electronics, mainly for smoothing, storing energy or filtering after AC voltage rectification, and also for non-precise 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 capacitors will affect the life of electrolytic capacitors. From the design point of view, the design method, material and processing technology of electrolytic capacitors determine the life and stability of capacitors. For the applicants, voltage, ripple current, switching frequency, installation form and heat dissipation mode all affect the life of electrolytic capacitors.
2. Abnormal failure of electrolytic capacitors
Some factors may cause electrolytic capacitor failure, such as extremely low temperature, capacitor temperature rise (welding temperature, ambient temperature, AC ripple), excessive voltage, instantaneous voltage, VHF or reverse bias voltage; among them, temperature rise is the most important factor affecting the working life of electrolytic capacitor (Lop).
The electric conductivity of capacitors is determined by the ionization ability and viscosity of electrolyte. When the temperature decreases, the electrolyte viscosity increases, thus 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, excessive heat will accelerate the evaporation of electrolyte, and when the amount of electrolyte is reduced to a certain limit, the capacitance life will be terminated. When working in cold area (generally below - 25 C), heating is needed to ensure the normal working temperature of electrolytic capacitor. For example, outdoor UPS is equipped with heating plates in Northeast China.
Capacitors are vulnerable to breakdown under overvoltage, but surge voltage and instantaneous high voltage often occur in practical applications. Especially because of the vast territory of our country and the complexity of power grids, AC power grids are very complex, often exceeding 30% of normal voltage, especially single-phase input, phase bias will increase the normal range of AC input. The test results show that the commonly used imported electrolytic capacitor at 450V/470uF105 C for 2000 hours will leak and blow out after 2 hours under the rated voltage of 1.34 times. According to statistics and analysis, the failure of PFC output electrolytic capacitor of communication switching power supply close to the power grid is mainly due to the power grid surge and high voltage damage. The voltage of aluminium electrolytic capacitor is usually reduced to 80% of the rated value by two steps.
Analysis of influencing factors of 3-life
In addition to abnormal failure, the lifetime of electrolytic capacitors is exponentially related to temperature. Because of the use of non-solid electrolyte, the life of electrolytic capacitor also depends on the evaporation rate of electrolyte, which leads to the reduction of electrical performance. These parameters include capacitance, leakage current and equivalent series resistance (ESR).
Refer to RIFA's life expectancy formula:
PLOSS= (IRMS) xESR (1)
Th = Ta + PLOSSxRth (2)
Lop = Ax2Hours (3)
B = Reference Temperature Value (Typical Value 85 C)
A = Capacitance life at reference temperature (varying according to capacitor diameter)
C = The number of degrees of temperature rise required to reduce capacitor life by half
From the above formulas, we can clearly see that there are several direct factors affecting the lifetime of electrolytic capacitors: ripple current (IRMS), equivalent series resistance (ESR), environmental temperature (Ta), and total thermal resistance (Rth) transferred from hot spot to surrounding environment. The highest temperature in the capacitor is called the hot spot temperature (Th). Hot spot temperature is the main factor affecting the working life of capacitors. The following factors determine the external temperature (Ta), the total thermal resistance (Rth) transferred from the hot spot to the surrounding environment, and the energy loss (PLOSS) caused by AC current. The internal temperature rise of the capacitor is linearly related to the energy loss.
When a capacitor is charged or 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 dielectrics. In addition, the energy loss caused by leakage current will lead to the increase of temperature inside the capacitor.
1. The life of 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 electrolytic capacitors determine the life and stability of capacitors. For the applicants, voltage, ripple current, switching frequency, installation form and heat dissipation mode all affect the life of electrolytic capacitors.
2. Abnormal failure of electrolytic capacitors
Some factors may cause electrolytic capacitor failure, such as extremely low temperature, capacitor temperature rise (welding temperature, ambient temperature, AC ripple), excessive voltage, instantaneous voltage, VHF or reverse bias voltage; among them, temperature rise is the most important factor affecting the working life of electrolytic capacitor (Lop).
The electric conductivity of capacitors is determined by the ionization ability and viscosity of electrolyte. When the temperature decreases, the electrolyte viscosity increases, thus 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, excessive heat will accelerate the evaporation of electrolyte, and when the amount of electrolyte is reduced to a certain limit, the capacitance life will be terminated. When working in cold area (generally below - 25 C), heating is needed to ensure the normal working temperature of electrolytic capacitor. For example, outdoor UPS is equipped with heating plates in Northeast China.
Capacitors are vulnerable to breakdown under overvoltage, but surge voltage and instantaneous high voltage often occur in practical applications. Especially because of the vast territory of our country and the complexity of power grids, AC power grids are very complex, often exceeding 30% of normal voltage, especially single-phase input, phase bias will increase the normal range of AC input. The test results show that the commonly used imported electrolytic capacitor at 450V/470uF105 C for 2000 hours will leak and blow out after 2 hours under the rated voltage of 1.34 times. According to statistics and analysis, the failure of PFC output electrolytic capacitor of communication switching power supply close to the power grid is mainly due to the power grid surge and high voltage damage. The voltage of aluminium electrolytic capacitor is usually reduced to 80% of the rated value by two steps.
Analysis of influencing factors of 3-life
In addition to abnormal failure, the lifetime of electrolytic capacitors is exponentially related to temperature. Because of the use of non-solid electrolyte, the life of electrolytic capacitor also depends on the evaporation rate of electrolyte, which leads to the reduction of electrical performance. These parameters include capacitance, leakage current and equivalent series resistance (ESR).
Refer to RIFA's life expectancy formula:
PLOSS= (IRMS) xESR (1)
Th = Ta + PLOSSxRth (2)
Lop = Ax2Hours (3)
B = Reference Temperature Value (Typical Value 85 C)
A = Capacitance life at reference temperature (varying according to capacitor diameter)
C = The number of degrees of temperature rise required to reduce capacitor life by half
From the above formulas, we can clearly see that there are several direct factors affecting the lifetime of electrolytic capacitors: ripple current (IRMS), equivalent series resistance (ESR), environmental temperature (Ta), and total thermal resistance (Rth) transferred from hot spot to surrounding environment. The highest temperature in the capacitor is called the hot spot temperature (Th). Hot spot temperature is the main factor affecting the working life of capacitors. The following factors determine the external temperature (Ta), the total thermal resistance (Rth) transferred from the hot spot to the surrounding environment, and the energy loss (PLOSS) caused by AC current. The internal temperature rise of the capacitor is linearly related to the energy loss.
When a capacitor is charged or 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 dielectrics. In addition, the energy loss caused by leakage current will lead to the increase of temperature inside the capacitor.