The positive electrode of the aluminum electrolytic capacitor, the anode lead electrode and the outer casing are all high-purity aluminum. The medium of the aluminum electrolytic capacitor is a film of aluminum oxide formed on the surface of the positive electrode. The real negative electrode is an electrolyte, which is equivalent to an electrolytic cell during operation, but only The anodized layer on the surface of the positive electrode has been formed, and no electrochemical reaction occurs. The theoretical current is zero, and a slight leakage current is caused by the presence of impurities in the electrode and the electrolyte. From the phenomenon point of view, the common failure phenomena and failure modes of aluminum electrolytic capacitors are: electrolyte dryness, pressure release device action, short circuit, open circuit (no capacitance), and excessive leakage current.
Aluminum electrolytic capacitor application environment
If the aluminum electrolytic capacitor has no problem in quality, the failure problem appears in the application environment. The design and application environment of aluminum electrolytic capacitors mainly include: ambient temperature, heat dissipation method, voltage and current parameters. For capacitor users, short circuits, open circuits are “catastrophic failures†or “fatal failures†that cause them to lose the function of capacitors. Other types of failure modes (ie, failures caused by the second type of factors) are generally classified as "degradation failure" or "depletion failure".
Second, the failure mechanism of aluminum electrolytic capacitor
Depletion failure
Depletion failure (1)
Usually, the end of the life of the electrolytic capacitor is judged by the capacitance falling below 80% of the rated (initial value). Due to the filling of the electrolyte of the early aluminum electrolytic capacitors, the capacitance of the aluminum electrolytic capacitors slowly decreased at the beginning of the work. As the working electrolyte is continuously repaired during the load process, the electrolyte is gradually reduced due to the anodic oxide film damaged by impurities . In the later stage of use, due to the evaporation of the electrolyte, the electrolyte with increased viscosity is difficult to sufficiently contact the oxide film layer on the surface of the roughened aluminum foil which is corroded, thus reducing the effective area of ​​the plate of the aluminum electrolytic capacitor. That is, the capacity of the anode and cathode aluminum foils is reduced, causing a sharp drop in capacitance. Therefore, it can be considered that the capacity reduction of the aluminum electrolytic capacitor is caused by the evaporation of the electrolyte. The most important cause of the evaporation of the electrolyte is the high temperature environment or heat.
Depletion failure (2)
The reason why the aluminum electrolytic capacitor is heated due to the application conditions is that the aluminum electrolytic capacitor flows through the aluminum when it operates in the rectification filter (including the high-frequency rectification and filtering of the switching power supply output) and the power supply bypass of the power electric furnace. Electrolytic capacitors generate losses in the ESR of aluminum electrolytic capacitors and convert them into heat to generate heat.
When the electrolytic solution of the aluminum electrolytic capacitor evaporates more and the solution becomes thicker, the resistivity increases due to the increase of the viscosity, and the equivalent series resistance of the working electrolyte increases, resulting in a significant increase in the loss of the capacitor and an increase in the loss angle. For example, for an electrolytic capacitor with a working temperature of 105 degrees, when the maximum core package temperature is higher than 125 degrees, the viscosity of the electrolyte is suddenly increased, and the ESR of the electrolyte is increased by nearly ten times. The increased equivalent series resistance produces more heat, causing greater evaporation of the electrolyte. In such a cycle, the capacity of the aluminum electrolytic capacitor drops sharply and may even cause an explosion.
Depletion failure (3)
An increase in leakage current often leads to failure of the aluminum electrolytic capacitor.
Excessive application voltage and high temperature will cause leakage current increase
Pressure release device action
Pressure release device action
In order to prevent the electrolyte generated in the aluminum electrolytic capacitor due to the internal high-temperature boiling gas or electrochemical process, the internal high pressure causes the explosion of the aluminum electrolytic capacitor. In order to eliminate the explosion of aluminum electrolytic capacitors, aluminum electrolytic capacitors with a diameter of 8 mm or more are equipped with pressure.
Release devices that operate before the pressure inside the aluminum electrolytic capacitor reaches a dangerous pressure that has not caused the aluminum electrolytic capacitor to explode, releasing the gas. With the action of the pressure release device of the aluminum electrolytic capacitor, the aluminum electrolytic capacitor is declared to be invalid.
Aluminum electrolytic capacitor pressure release device (cross in the middle)
Electrochemical process causes the pressure release device to act
The leakage current of an aluminum electrolytic capacitor is an electrochemical process, which has been discussed in detail above and will not be described again. The electrochemical process will generate gas, and the accumulation of these gases will cause the internal pressure of the aluminum electrolytic capacitor to rise, and finally the pressure relief device will be relieved.
Excessive temperature causes the pressure release device to act
If the temperature of the aluminum electrolytic capacitor is too high, the ambient temperature may be too high. For example, if there is a heating element near the aluminum electrolytic capacitor or the entire electronic device is in a high temperature environment;
The second reason why the temperature of the aluminum electrolytic capacitor is too high is that the core package temperature is too high. The root cause of the excessive temperature of the aluminum electrolytic capacitor core package is that the aluminum electrolytic capacitor flows excessively high ripple current. Excessively high ripple current causes excessive loss in the ESR of the aluminum electrolytic capacitor, and excessive heat is generated to cause the electrolyte to boil to generate a large amount of gas to operate the pressure release device when the internal pressure of the aluminum electrolytic capacitor rises sharply.
Instantaneous over temperature
Generally, the core temperature of a core pack of an aluminum electrolytic capacitor is doubled by 10 ° C. This core is located roughly at the center of the capacitor and is the hottest point inside the capacitor. However, when the capacitor heats up to its maximum allowable temperature, for most models of the capacitor at 125 ° C, its electrolyte is driven by the capacitor core package, resulting in a capacitor that increases the ESR by a factor of 10. Under this action, instantaneous over-temperature or over-current can cause the ESR to increase permanently, resulting in capacitor failure. In the application of high temperature and large ripple current, it is especially necessary to be alert to the possibility of instantaneous over-temperature, and pay extra attention to the cooling of aluminum electrolytic capacitors.
Instantaneous overvoltage generation
Power shock
During the power-on process, the filter inductor releases the stored energy into the filter capacitor, resulting in an over-transient overvoltage of the filter capacitor.
Power-on overvoltage indication
Prevention of capacitor overvoltage failure
Capacitors are easily broken down under overvoltage conditions, and transient high voltages in practical applications are common.
Selecting an aluminum electrolytic capacitor with good transient overvoltage performance, some aluminum electrolytic capacitors of RIFA give the instantaneous overvoltage value parameter.
Electrolyte drying is the main reason for the failure of aluminum electrolytic capacitors
The reason why the electrolyte is dry
The electrolyte naturally volatilizes
Electrolyte consumption
The electrolyte naturally volatilizes
The evaporation rate of the electrolyte increases with temperature
The evaporation rate of the electrolyte is related to the sealing quality of the capacitor, and it has good sealing performance at high temperature or low temperature.
Electrolyte consumption
Electrochemical effect caused by leakage current consumes electrolyte
The life of aluminum electrolytic capacitors decreases with increasing leakage current
Leakage current increases with increasing temperature: 25 ℃ leakage flow is only one-tenth of the leak current is less than 85 ℃ when leakage current increases with the applied voltage increases: a withstand voltage of the aluminum electrolytic capacitor rated voltage of 400V The leakage current is about 5 times the leakage current at 90% of the rated voltage.
The time when the electrolyte is dried is the life of the aluminum electrolytic capacitor.
Factors affecting the life of aluminum electrolytic capacitors (temperature 1)
According to the electrolyte of aluminum electrolytic capacitors, the maximum operating temperature of aluminum electrolytic capacitors can be divided into:
General use: 85 ° C
General high temperature use: 105 ° C
Special high temperature use: 125 ° C
Car engine compartment: 140~150°C
Factors affecting the life of aluminum electrolytic capacitors (hours of rated life)
Aluminum electrolytic capacitors by life hours can be divided into:
General use (normal temperature, within 3 years): 1000 hours
General use (normal temperature, hope for a long time): 2000 hours or more
Industrial grade: longer life hours
Factors affecting the life of aluminum electrolytic capacitors (temperature 2)
For every 10°C increase in temperature, the number of hours of life is halved
Factors affecting the life of aluminum electrolytic capacitors (electrolyte)
The electrolyte solution determines the life of aluminum electrolytic capacitors
Factors affecting the life of aluminum electrolytic capacitors (application conditions)
High temperature shortens the life of aluminum electrolytic capacitors
High ripple current shortens the life of aluminum electrolytic capacitors
Operating voltage is too high to shorten the life of aluminum electrolytic capacitors
Parameters and application conditions affecting the life of aluminum electrolytic capacitors
Relationship between operating voltage and leakage current
Relationship between operating voltage and leakage current
Relationship between leakage current and applied voltage of 450V/4700μF/85°C aluminum electrolytic capacitor produced by a company
Temperature and leakage current
Relationship between leakage current and ambient temperature of 450V/4700μF/85°C aluminum electrolytic capacitor produced by a company
The effect of temperature, voltage and ripple current on life
Take an aluminum electrolytic capacitor for an electronic ballast as an example.
Different lifetime of electrolytic capacitors at different voltage and temperature conditions,
Derating life characteristics of aluminum electrolytic capacitors for an electronic ballast
Overvoltage life characteristics of aluminum electrolytic capacitors used in an electronic ballast
Relationship between life of aluminum electrolytic capacitors and temperature and ripple current
Third, aluminum electrolytic capacitor life capacitance calculation method
Simple life calculation
Estimate life based on ESR, thermal resistance, and ripple current
Estimate life based on temperature, ripple current and lifetime
Simple life calculation method 1
Estimation of aluminum electrolytic capacitors that do not contain ripple current operation: The basic basis is "10 °C rule", that is, the ambient temperature is halved for every 10 °C rise in the ambient temperature, and vice versa. This "10 °C rule" is at zero ripple current. Applicable under the conditions. It is not necessarily applicable when an aluminum electrolytic capacitor flows a relatively large ripple current.
Simple life calculation method 2
Recommended method for some domestic brands of aluminum electrolytic capacitors
Japanese brand aluminum electrolytic capacitor recommended method
The basic idea
At rated voltage, the life of an aluminum electrolytic capacitor can be calculated by the following formula.
Among them, L and L0 are respectively: the life at the actual ambient temperature T and the life at the rated maximum temperature T0. It can be seen that the service life of aluminum electrolytic capacitors decreases with temperature every 10 ° C, and the lifetime is doubled, the so-called 10 ° C rule. Therefore, whether it is used or stored, aluminum electrolytic capacitors should be as low as possible at ambient temperatures. For example, an aluminum electrolytic capacitor of 85 ° C / 1000 hours has an actual life of 16,000 hours at 45 ° C, which is equivalent to 1 year and 10 months; 48,000 hours at 29 ° C, which is equivalent to 5 years and 6 months; and 1000 hours is converted into 42 days. It can be seen that reducing the temperature is important for extending the life of aluminum electrolytic capacitors.
Graphical calculation
Problem
The above three methods of estimation are only applicable to the storage state and the operating state of no ripple current (small ripple current) (such as DC blocking capacitors, bypass capacitors for small signal circuits). The above method is not necessarily applicable for working conditions that obviously contain ripple current. In this case, the effect of the ripple current should be considered in the application conditions.
Life estimation method considering ripple current
The heat generated by the aluminum electrolytic capacitor is caused by the internal equivalent series resistance (ESR), and the loss is:
Where I and R (ESR) are: ripple current (A) and equivalent series resistance (Ω). The temperature rise due to heat generation is:
△T, I, A, R(ESR), H are: temperature rise (°C) at the center of the capacitor, ripple current (A), surface area (Cm2) of the capacitor, ESR (Ω), heat dissipation coefficient {1.5~ 2.0) × 10-3W / (Cm2 × ° C)}.
Equation (5.17) shows that the temperature rise of the capacitor is proportional to the square of the ripple current and the equivalent series resistance ESR, which is inversely proportional to the surface area of ​​the capacitor. Therefore, the magnitude of the ripple current determines the amount of heat generated and affects its use. The life, the type of capacitor and the conditions of use affect the magnitude of the ΔT value. In general, ΔT < 5 °C. Figure 5.26 shows the measurement of the temperature rise caused by the ripple current.
Temperature test of aluminum electrolytic capacitor
Use test results and consider the life formula for ambient temperature and ripple current:
Among them, L, L0, K, T0, T, â–³T are: actual service life, service life under DC working voltage, ripple current coefficient (the ratio of the effective value of the actual ripple current to the effective value of the rated ripple current, When K=2, the ripple current is within the allowable range, when K=4, it exceeds the ripple current range), the maximum use temperature, the operating temperature, and the center temperature rise.
When the capacitor operates at the rated ripple current and the upper limit temperature, the life of the capacitor can be obtained by conversion (5.18), as follows:
(5.19)
Among them, L0 and â–³T0 are: the life (hours) at the rated ripple current and the maximum operating temperature, and the allowable temperature rise of the capacitor at the highest operating temperature.
In fact, the equations (5.18) and (5.19) are expressed after the temperature of the capacitor case is tested, and the magnitude of the ripple current is not known. If the magnitude of the ripple current is known, the equation (5.19) can be changed. for:
Among them, I0 and I are respectively: rated ripple current (A) at the highest operating temperature: actual ripple current (A).
problem
The problem with equation (5.17) is the determination of the heat dissipation coefficient. The manufacturer cannot accurately calculate without giving the user, and if the rough estimate will produce a deviation of at least 30%. Since it is difficult to directly measure the internal temperature rise of the capacitor, the conversion relationship between the surface temperature and the core package temperature can be listed using the following table.
Relationship electrolytic capacitors of the core pack and the housing temperature
problem
The above formula for calculating the life is applicable in principle to the ambient temperature of +40 ° C to the maximum operating temperature range, but due to factors such as aging of the sealing material, the actual estimated life time is generally up to 15 years.
The expressions and calculations of Equations 5.17 through 5.20 are very cumbersome, and the calculation results are inaccurate due to the test and the difference in thermal conductivity of the individual capacitors, and can only be used as approximate estimates.
problem
The best way to get a more accurate estimate is to use the temperature life curve given by the capacitor manufacturer. More responsible foreign aluminum electrolytic capacitor manufacturers can give the relationship between aluminum electrolytic capacitor life and ambient temperature and ripple current.
Estimate life based on ESR, thermal resistance, and ripple current
Relationship between life and temperature and ripple current of EPCOS B43550 aluminum electrolytic capacitor
RIFA life estimation method
Estimated life based on power loss, thermal resistance, and actual ESR
In order to calculate the working life (LOP) of an aluminum electrolytic capacitor, it is necessary to know:
Operating voltage (Uapplied),
The effective value (IRMS) of the ripple current flowing through the aluminum electrolytic capacitor,
Ambient temperature (Ta)
Thermal resistance (Rth).
Related formula
Calculation method
First, in the ESR matrix, the "ESR" value corresponding to the different frequency and core package temperature (Th) is found, and then the "power loss (PLOSS)" generated when the ripple current IRMS flows through the aluminum electrolytic capacitor is calculated. If the IRMS consists of multiple harmonics, the power loss produced by each harmonic is calculated and added sequentially. The "thermal resistance value" of the capacitor winding core package to the ambient temperature can be found in the "thermal resistance matrix", so that the actual "core package temperature (Th)" can be calculated, if the actual value is compared with the previous selection of the ESR value. If the value Th does not match, the hypothesis needs to be corrected, the ESR value is re-detected, and the iterative calculation is repeated until the results agree.
This calculation is the most accurate!
Required conditions:
Thermal resistance parameter of capacitor
ESR parameters
The thermal resistance of aluminum electrolytic capacitors from different manufacturers is different!
Fourth, the need to pay attention to the purchase of aluminum electrolytic capacitors
Do not apply aluminum electrolytic capacitors with unknown sources (1)
Why not apply an aluminum electrolytic capacitor with unknown sources? The most fundamental reason is that China's electronic components market has experienced the situation of disassembled parts, parallel imports and counterfeit goods. With the development of the times, this phenomenon is becoming less and less.
However, it should be noted that the semiconductor device can still be used for disassembling parts, and aluminum electrolytic capacitors must not be used for disassembling parts! The reason is that the life of aluminum electrolytic capacitors is the shortest in various electronic components. Among the disassembled parts of electronic products scrapped abroad, the remaining life of aluminum electrolytic capacitors is the least. If you just experiment, you can barely try it. If you try to make a product, the result is imaginable.
Do not apply aluminum electrolytic capacitors with unknown sources (2)
The film is a common method for the "refurbishment" of counterfeit aluminum electrolytic capacitors. Usually, the aluminum electrolytic capacitor sleeve that is bought is raised by two new heat-shrinkable tubes with rated voltages, and the printing level can be faked.
"Electric stealing voltage" of aluminum electrolytic capacitors
Since the surge voltage of the aluminum electrolytic capacitor is 1.15 times of the rated voltage, the surge voltage of the 350V withstand voltage aluminum electrolytic capacitor is 402V, which is higher than 220V+20% corresponding to the rectified peak voltage of 370V; and for the 300V rated voltage of aluminum The surge voltage of the electrolytic capacitor is 345V.
Meet the 243V AC rms input voltage. If the anode foil with a rated voltage of 300V is used instead of the anode foil with a rated voltage of 400V, the cost of the aluminum electrolytic capacitor will be greatly reduced. Some aluminum electrolytic capacitor manufacturers use this "stealing voltage" method to reduce production costs.
The consequences of stealing "voltage"
In general applications, in order to avoid excessive leakage current and prolong the life of aluminum electrolytic capacitors, it is common to reduce the use voltage of aluminum electrolytic capacitors, generally less than 0.9.
If the aluminum electrolytic capacitor that steals the "voltage" is selected, the voltage richness that has been left out is lost, and even the working voltage exceeds the rated voltage, which greatly increases the leakage current, thereby greatly reducing the aluminum electrolytic capacitor. life.
Disassemble parts
The disassembled parts are usually the components on the electronic equipment that are scrapped abroad. If the disassembled parts are carefully selected, most of the parts are screened and can be used in general performance. Only aluminum electrolytic capacitors are absolutely unusable.
The reason is that the life of aluminum electrolytic capacitors is relatively short, only 1000~4000 hours at rated temperature. Although most aluminum electrolytic capacitor manufacturers claim to be able to use 10 or even decades at 40 °C, the application environment is Unpredictable, such as the internal temperature of the display is much more than 45 ° C! Therefore, in a few years, the effective life of aluminum electrolytic capacitors is generally low and cannot be used in electronic products.
Refurbished aluminum electrolytic capacitor
Refurbished aluminum electrolytic capacitors are also relatively harsh. They are usually collected from overseas aluminum electrolytic capacitors and re-dipped with electrolyte. Although the detection of foreign famous aluminum electrolytic capacitors is stricter than domestic ones, some off-line aluminum electrolytic capacitors are still close to the domestic true-rate electrolytic capacitors. However, its performance is still not as good as domestic aluminum electrolytic capacitors.
Downline capacitor (1)
In the domestic electronics market, you can see the lower-line aluminum electrolytic capacitors, usually some irresponsible sellers in order to gain market competitiveness, to the aluminum electrolytic capacitor production plant to buy off-line aluminum electrolytic capacitors, eliminating the lack of capacitance and breakdown. After getting the market to sell.
The characteristic of the lower-line aluminum electrolytic capacitor is that it is normal to test the capacitance with a multimeter. However, if the leakage current is tested, the leakage current and loss factor of the aluminum electrolytic capacitors under the line will be higher than that of the genuine aluminum electrolytic capacitors. The leakage current and loss factor are generally not tested by the user.
Downline capacitor (2)
This off-line aluminum electrolytic capacitor acts as a coupling capacitor, which causes the bias voltage of the rear pole to shift, but there is no reason. If it is used for delay applications, the delay time will be significantly less than the set value or the delay time. Stable phenomenon, which is the reason why the leakage current of the aluminum electrolytic capacitor is effective after applying a DC voltage.
Due to the large leakage current of the offline aluminum electrolytic capacitor, the actual life is significantly shorter than the expected life, for example, the life expectancy is 5 years, and actually only 2 to 3 years. This is not seen at all in the early days of the use of electronic products. From here, we will also see the importance of the 9000 series quality certification.
If the off-line aluminum electrolytic capacitor is used in applications with large ripple current or high temperature, it is extremely dangerous. The reason is that excessive leakage current will cause the aluminum electrolytic capacitor to have higher temperature in large ripple current or high temperature application. The life is shortened, and in severe cases, the aluminum electrolytic capacitor will "explode" and fail.
Film capacitor
The film is a common method for the "refurbishment" of counterfeit aluminum electrolytic capacitors. Usually, the aluminum electrolytic capacitor sleeve that is bought is raised by two new heat-shrinkable tubes with rated voltages, and the printing level can be faked. For example, an aluminum electrolytic capacitor with a rated voltage of 250V is replaced with an aluminum electrolytic capacitor with a rated voltage of 400V, and the "value" of such an aluminum electrolytic capacitor will be doubled or even higher. The manufacture of laminated aluminum electrolytic capacitors is the habit of voltage derating when drilling applications. After voltage derating, it may be the surge voltage value of the laminated aluminum electrolytic capacitor. At this voltage, the laminated aluminum electrolytic capacitor Still can be used for a while.
Foreign electrolytic capacitor manufacturers
Some foreign aluminum electrolytic capacitor manufacturers are high-quality electrolytic capacitors when they supply to developed countries.
The capacity of electricity supplied to China is always negative, and its quality is not as good as that of developed countries.
The reason is not only to discriminate against me abroad, but the most important thing is that in China, their aluminum electrolytic capacitors can't buy the price, and can only compensate for the stealing capacity.
In order to reduce the price in the Chinese market, foreign aluminum electrolytic capacitor manufacturers often introduce "economical" aluminum electrolytic capacitors with relatively short life. For example, CDE will be converted to China's 2000-hour aluminum electrolytic capacitors, which are in the European and American markets. Can not be seen, therefore, do not think that foreign aluminum electrolytic capacitors have a long life.
A simple method to test the withstand voltage of aluminum electrolytic capacitors
Possibility: The dielectric film of an aluminum electrolytic capacitor is obtained by anodizing, so that a short-time small current "breakdown" does not damage the aluminum electrolytic capacitor.
Basic method: connect aluminum electrolytic capacitors in series with a 10k resistor; slowly increase the applied voltage, keep the charging current below 1mA (the voltage on the corresponding resistor is lower than 10V), until the applied voltage and current are not reduced even if the applied voltage is not continuously increased The corresponding voltage is the breakdown voltage of the electrolytic capacitor; 90% of this voltage is the rated voltage of the electrolytic capacitor. This method is only applicable to aluminum electrolytic capacitors.
Test circuit
Guidelines for the purchase of aluminum electrolytic capacitors
Purchased from reputable aluminum electrolytic capacitor manufacturers or agents of reputable aluminum electrolytic capacitor manufacturers.
Try not to buy at the electronics market or under-represented agencies.
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