Cause analysis and Countermeasures of affecting the life of electrolytic capacitor
1.Factors affecting the life of electrolytic capacitor
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 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.
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 its life and stability. For the users, the service voltage, ripple current, switching frequency, installation mode and heat dissipation mode all affect the life of electrolytic capacitor.
2.Abnormal failure of electrolytic capacitor
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, very high frequency or reverse bias voltage; among them, temperature rise is the most important factor affecting the working life (LOP) of electrolytic capacitor.
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, excessive heat will accelerate the evaporation of electrolyte. When the amount of electrolyte is reduced to a certain limit, the capacitor life will be terminated. When working in high cold area (generally 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 over-voltage condition, but surge voltage and instantaneous high voltage often appear in practical application. In particular, China's vast territory, the power grid is complex, AC power grid is very complex, often more than 30% of the normal voltage, especially single-phase input, phase bias will aggravate the normal range of AC input. The test results show that under the voltage of 1.34 times of the rated voltage, the commonly used 450 V / 470uf 105 ℃ imported ordinary electrolytic capacitor for 2000 hours will leak liquid and gas, and the top will burst open. According to statistics and analysis, the failure of output electrolytic capacitor of communication switching power supply PFC close to power grid is mainly due to grid surge and high voltage damage. Generally, the voltage of electrolytic capacitor is reduced to 80% of the rated value.
3. Analysis of life influencing factors
In addition to 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 reduction of electrical performance. These parameters include capacitance, leakage current and equivalent series resistance (ESR).
Refer to RIFA's life expectancy formula:
Ploss = (IRMS) ESR (1)
Th = Ta + PLOSS x Rth （2）
Lop = A x 2 Hours （3）
B = reference temperature (typical 85 ℃)
A = capacitance life at reference temperature (varies with capacitor diameter)
C = the number of degrees of temperature rise required to reduce the life of the capacitor by half
From the above formula, we can clearly see that several direct factors 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 the highest temperature inside the capacitor is called the 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 (TA), the total thermal resistance (RTH) and the energy loss (Ploss) caused by AC current. The internal temperature rise of capacitor is linear with energy loss.
When the 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 passing through the dielectric. In addition, the energy loss caused by leakage current will lead to the increase of the temperature inside the capacitor.
3.1 design considerations
In the capacitance of non solid electrolyte, the dielectric is the anodic aluminum foil oxide layer. The electrolyte acts as the electrical contact between the oxide layer of cathode aluminum foil and anode aluminum foil. The paper dielectric layer absorbing electrolyte becomes the isolation layer between the cathode aluminum foil and the anode aluminum foil, and the aluminum foil is connected to the terminal of the capacitor through the electrode lead piece.
By reducing the ESR value, the internal temperature rise caused by ripple current can be reduced. This can be achieved by using multiple electrode leads and laser welding electrodes.
The temperature rise and capacitance are determined by ESR. One of the main measures to make the capacitor have a satisfactory ESR value is to use one or more metal electrode leads to connect the external electrode and the core package, so as to reduce the impedance between the core package and the pin. The more electrode leads on the core package, the lower the ESR value of capacitance. With the help of laser welding technology, more electrode leads can be added to the core package, so that the capacitance can reach a lower ESR value. This also means that the capacitor can withstand higher ripple current and lower internal temperature rise, that is to say, longer working life. Otherwise, it may lead to internal short circuit, high leakage current, capacitance loss, increase of ESR value and circuit open circuit.
Through the good mechanical contact between the capacitor core package and the bottom of the aluminum shell and through the heat sink in the middle of the core package, the internal heat of the capacitor can be effectively released from the bottom of the aluminum shell to the connected bottom plate.
The design of internal heat conduction is very important for the stability and working life of capacitor. In EVOX RIFA's design, the negative foil is extended to the bottom of the capacitor aluminum shell thickness that can be directly contacted. This bottom becomes the heat sink of the core package, so that the heat of the hot spot can be released. A more comprehensive heat conduction solution with lower thermal resistance (RTH.) can be obtained by installing the capacitor safely on the base plate (usually aluminum plate).
The loss of electrolyte can be greatly reduced by using the phenolic plastic cover with the electrode and the double special sealing pad tightly occluding with the aluminum shell.
The evaporation of electrolyte through the gasket determines the working time of electrolytic capacitor with long life. When the electrolyte evaporates to a certain extent, the capacitor will eventually fail (this result will be accelerated by the internal temperature rise). The double sealing system designed by EVOX RIFA company can slow down the evaporation rate of electrolyte and make the capacitor reach its longest working life.
These characteristics ensure that the capacitor has a long working life in the required field.
3.2 application factors affecting service life
According to the life formula, it can be concluded that the application factors affecting the lifetime are: ripple current (IRMS), ambient temperature (TA), and the total thermal resistance (RTH) transferred from the hot spot to the surrounding environment.
1. Ripple current
The ripple current directly affects the hot spot temperature inside the electrolytic capacitor. The allowable range of ripple current can be obtained by consulting the manual of electrolytic capacitor. If it is out of range, it can be solved in parallel.
2. Ambient temperature (TA) and thermal resistance (RTH)
According to the formula of hot spot temperature, the application environment temperature of electrolytic capacitor is also an important factor. In application, we can consider the environmental heat dissipation mode, heat dissipation intensity, the distance between electrolytic capacitor and heat source, and the installation mode of electrolytic capacitor.
The heat inside the capacitor is always transferred from the "hot spot" with the highest temperature to the relatively lower part of the surrounding temperature. There are several ways of heat transfer: one is through aluminum foil and electrolyte conduction. If the capacitor is installed on the heat sink, part of the heat will also be transferred to the environment through the heat sink. Different installation methods, spacing and cooling methods will affect the thermal resistance of the capacitor to the environment. The total thermal resistance from the "hot spot" to the surrounding environment is expressed in terms of RTH. When the capacitor is installed on the heat sink with a thermal resistance of 2 ℃ / W, the thermal resistance value of the capacitor is 3.6 ℃ / W; when the capacitor is installed on the heat sink with the thermal resistance of 2 ℃ / W and the forced air cooling rate is 2m / s, the thermal resistance of the capacitor is rth = 2.1 ℃ / W. (taking peh200oo427am capacitor as an example, the ambient temperature is 85 ℃).
In addition, direct contact between the extended cathode aluminum foil and the capacitor aluminum shell is also a good way to reduce the thermal resistance. At the same time, it should be noted that the aluminum shell will be negatively charged, so it is not allowed to make negative connection.
The capacitor must be installed correctly to reach its design working life. For example: RIFA peh169 series and peh200 series should be installed vertically or horizontally. At the same time, make sure that the safety valve is upward, so that the hot electrolyte and steam can be discharged from the safety valve smoothly in case of capacitor failure.
When the capacitor arrangement is very compact, at least 5mm interval should be left between adjacent capacitors to ensure proper air flow. The control of nut torque is very important when using bolts for installation. If it is too loose, the capacitor and the heat sink can not be in close contact; if it is screwed too tightly, the thread may be damaged. At the same time, it should be noted that the capacitor should not be installed upside down, otherwise the bolt may be broken.
When the capacitor is installed, it should be far away from the heating element as far as possible, otherwise the service life of the capacitor will be shortened due to too high temperature, which makes the capacitor become the shortest life component in the whole circuit. In the case of high ambient temperature, forced air cooling should be adopted as far as possible, and the capacitor should be installed at the air inlet.
4. Influence of frequency
If the current consists of fundamental frequency and multiple harmonics, the power loss value generated by each harmonic must be calculated, and the total loss value can be obtained by adding the calculated results.
In high frequency applications, the leads at both ends of the capacitor should be as short as possible to reduce the equivalent inductance.
The resonant frequency (FR) of the capacitor varies with the type of capacitor. For soldered and bolted aluminum electrolytic capacitors, the resonant frequency is between 1.5khz and 150kHz. If the capacitor is used above the resonant frequency, the external characteristics are inductive.
To sum up, under the condition of avoiding abnormal failure and choosing the right application conditions and environment, the life of electrolytic capacitor can be guaranteed.