1 of the factors affecting the life of electrolytic capacitors
Electrolytic capacitors are widely used in various fields of power electronics, mainly for smoothing, filtering stored energy or AC voltage is rectified, and also for non-precision timing delays and the like. When switching power supply MTBF forecast model analysis showed that the electrolytic capacitors are the main factors affecting the life of the switching power supply, and therefore understand the factors that affect the life of the capacitor is very important.
Electrolytic capacitor life depends on the internal temperature. Therefore, electrolytic capacitors design and application conditions will affect the life of electrolytic capacitors. From a design perspective, the electrolytic capacitor design methods, materials, processing technology determines the life of the capacitor and stability. And the application were concerned, the use of voltage, ripple current, switching frequency, install the form, and so affect the heat dissipation electrolytic capacitor life.
2 electrolytic capacitor abnormal failure
Several factors can cause failure of the electrolytic capacitor, such as extremely low temperature, capacitance temperature (soldering temperature, ambient temperature, AC ripple), high voltage, instantaneous voltage, VHF or reverse bias; wherein the temperature is electrolytic capacitor working life (Lop) the most important factor.
Capacitance is determined by the conductivity of the electrolyte ionization ability and viscosity. When the temperature decreases, the viscosity of the electrolyte increases, thereby reducing the ion mobility and ability to conduct electricity. When freezing the electrolyte, the ion mobility is very low that very high resistance. On the contrary, excessive heat will accelerate the evaporation of the electrolyte, while reducing the amount of electrolyte to a certain limit, the capacitor life will cease. In alpine region (generally -25 ℃ or less) work, we need to be heated to ensure the normal operating temperature of the electrolytic capacitor. Such as outdoor UPS, in northeast China are equipped with heating plate.
Capacitor easily breakdown overvoltage condition, but the practical application of the surge voltage and high voltage transients often occur. In particular grid vast, complex all over the country, so that the AC grid is very complex, often occur outside the normal voltage of 30%, especially single-phase input, phase deviation will increase the normal range AC input. After testing, import common ordinary electrolytic capacitors 2000 hours 450V / 470uF 105 ℃, and at 1.34 times the rated voltage of the voltage, capacitance occurs two hours after take gas leakage, red open top. According to statistics and analysis, communication with the grid close to the PFC output switching power supply failure electrolytic capacitors, mainly due to the high voltage power grid surges and damage. Electrolytic capacitor voltage selection for secondary general derating ratings dropped 80% use more reasonable.
3 life influencing factors in addition to non-normal failure, life of electrolytic capacitors and temperature relationship exponentially. The use of non-solid electrolyte, the electrolytic capacitor life also depends on the rate of evaporation of the electrolyte, reducing the electrical performance resulting.
These parameters include the capacitor capacitance, leakage current and the equivalent series resistance (ESR). Reference RIFA company expects Formula Life: PLOSS = (IRMS) Xu ESR (1) Th = Ta + PLOSS x Rth (2) Lop = A x 2 Hours (3)
B = reference temperature (typically 85 ℃)
A = capacitor life reference temperature (depending on the diameter of the capacitor changes) C = capacitance leads to life halving the required degree of temperature rise from the above formula, we can clearly see the impact of several electrolytic capacitors life direct factors: the ripple current (IRMS) and equivalent series resistance (ESR), ambient temperature (Ta), transfer from the hot to the total thermal resistance of the ambient (Rth). The highest point of the internal temperature of the capacitor, called the hot spot temperature (Th). Hot spot temperature is the main factor affecting the working life of the capacitor. The following factors and determine the practical application of the hot temperature outside temperature (ambient temperature Ta), transfer from the hot to the total thermal resistance of the ambient (Rth) and energy loss due to the alternating current (PLOSS). Capacitor internal temperature rise and loss of energy into a linear relationship.
Results will cause when capacitor charging and discharging, current flows through the resistor power loss, voltage changes when passing through the dielectric will lead to energy loss, coupled with the energy loss caused by the leakage current, all of these losses are due to an internal temperature of the capacitor increased.
3.1, the design considerations
In the non-solid electrolyte capacitors, the dielectric anodic oxide layer of aluminum foil. As the electrolyte electrical contact with the cathode and anode aluminum foil oxide layer. Electrolyte interfacial layer absorbent paper to become the cathode and the anode aluminum foil between the spacer layer, aluminum foil capacitor connected to the terminal through the connection of the electrode sheet.
By lowering ESR values can be reduced by the internal temperature rise inside the capacitor ripple current. This can be achieved by using a plurality of electrodes measures connection of sheets, laser welding electrodes.
The ESR and ripple current determines the capacitor temperature rise. One of the main measures to promote the capacitor can have the satisfaction of ESR value is: usually connected external electrodes and the core pack with one or more electrodes connection of sheet metal, reducing the impedance between the core package and pin. The connection of multi-electrode on the die package, the lower the ESR value of the capacitor. By means of laser welding technology, connection of the electrodes can add more pieces to the core package, so the capacitor can achieve low ESR values. This also means that the capacitor can withstand higher ripple current and has a lower internal temperature rise, which means a longer working life. It would also help to improve the ability to fight capacitance vibrations, or they may cause an internal short circuit, high leakage current, capacitance loss, ESR values rise and open circuit.
. Through the good mechanical contact between the capacitor and the bottom of the core package and aluminum core package by the middle of the heat sink, the heat can be efficiently released internal capacitance from the bottom to the aluminum base plate coupled thereto. Internal heat transfer design is extremely important for the stability of a capacitor and working life. In Evox Rifa's design, the negative electrode foil is extended to be in direct contact with the capacitor thick aluminum bottom. This is the bottom of the bag has become the core of the heat sink so that the heat can release hot. As with the choice of mounting bolts securely installed in the end the capacitor plate (usually aluminum), to obtain a more comprehensive with a lower thermal resistance (Rth.) Heat transfer solutions.
By using an electrode of a whole note about Bakelite lid and double special glands close engagement with the aluminum can greatly reduce the loss of electrolyte.
Electrolyte determines the long-life electrolytic capacitors working hours by evaporation gasket. When the capacitor electrolyte evaporation to a certain extent, the capacitor will eventually fail (due to the result of internal temperature and acceleration). Evox Rifa double sealing system can be designed to slow the rate of evaporation of electrolyte, the capacitor has reached its maximum operating life.
These properties ensure that the capacitor has a long working life in the field of the requirements of.
3.2, the application of the factors that affect the life
According to life equation can be drawn from the application of life factors as: the ripple current (IRMS), ambient temperature (Ta), transfer from the hot to the total thermal resistance of the ambient (Rth).
1. ripple current
The size of the ripple current, electrolytic capacitor directly affects the internal temperature of the hot spot. Discover electrolytic capacitors use manual, you can get the ripple current allowable range. If out of range, you can use a parallel manner.
2. The ambient temperature (Ta) and the thermal resistance (Rth)
According to the formula the hot spot temperature, ambient temperature electrolytic capacitor applications are also important factors. In the application, you can consider the environmental cooling method, the cooling intensity, distance from the heat source and electrolytic capacitors, electrolytic capacitor installation methods.
Capacitor internal heat, the highest temperature always conducted from the "hot spots" to the relatively low ambient temperature part. There are several ways to heat transfer: One is through the aluminum foil and the electrolyte conductivity. If the capacitor is mounted on the heat sink, part of the heat to the environment also passes through the heat sink. Types of installation and spacing and heat dissipation will affect capacitance to ambient thermal resistance. Passed from the "hot spots" to the surroundings of the total thermal resistance
Represented by Rth. Use clip installation, the capacitor is mounted on the thermal resistance of 2 ℃ / W heat sink, the thermal resistance of the resulting capacitor Rth = 3.6 ℃ / W; bolt installation, the capacitor mounted on the thermal resistance of 2 ℃ / W heat sink, forced air cooling rate of 2m / s, the resulting capacitor thermal resistance Rth = 2.1 ℃ / W. (In PEH200OO427AM type capacitor, for example, ambient temperature around 85 ℃).
In addition to prolonged direct contact with the cathode foil capacitor aluminum, it is also a good way to reduce the thermal resistance. Also note that aluminum will therefore negatively charged, can not be connected to the negative.
Capacitors must be properly installed in order to achieve its design working life. For example: RIFA PEH169 series and PEH200 series should be mounted vertically upwards or horizontally. While ensuring the safety valve upwards, so that the heat and steam to the electrolyte in the case of capacitor failure, the smooth discharge from the safety valve.
When the adjacent capacitor arrangement is compact should set aside between capacitance of at least 5mm intervals to ensure that the right amount of air flow. When using the installation bolt, nut torque control is very important. If you twist too loose, the capacitance between the heat sink will not be close contact; if screwed too tightly, and may cause damage to the thread. Also note that the capacitors should not be mounted upside down, or it may cause broken bolt.
Capacitance should be installed as far away from the heating element, otherwise the high temperature will shorten the life of the capacitor, so that the entire circuit capacitor life became the shortest member. At higher ambient temperatures, try to use forced air cooling, the capacitor mounted on the air inlet.
3. Impact of Frequency
If the current is many times the fundamental frequency and harmonic structure, it shall be calculated for each power loss value generated harmonics and the results summed to obtain the total loss value.
In high frequency applications, across the capacitor leads should be as short as possible to reduce the equivalent inductance.
Capacitor resonant frequency (fR), due to the type of capacitor differ. For lug type and bolted between aluminum electrolytic capacitors, the resonant frequency at 1.5kHz to 150kHz. If the capacitor is used when the resonance frequency is higher than the external characteristic is inductive.
4 Conclusion
From the foregoing, in the case to avoid abnormal failure, select the correct application and environmental conditions, electrolytic capacitor life is guaranteed.
