Saturday, December 18, 2010
Weak current College】 【impact Electrolytic Capacitor life causes and countermeasures】.
<br> <br> Impact factors affecting the life of electrolytic capacitor <br> <br> Electrolytic Capacitor is widely used in different areas of power electronics, mainly used for smoothing, store energy or AC voltage rectifier after filtering, also for non- .precision timing delay, etc. In switching power supply's estimated MTBF, model analysis results show that the electrolytic capacitor is switching power supply life of main factors, so understanding, impact factors affecting the life of the capacitance is important. <br> Electrolytic Capacitor's life .depends on its internal temperature. Therefore, Electrolytic Capacitor design and application conditions can affect to a Electrolytic Capacitor's life. From a design perspective, Electrolytic Capacitor design methods, materials, processing technology determines the capacitance of the life and stability. While the application is concerned ., on the use of voltage, ripple current, switching frequency, installation forms, thermal methods affect the longevity of the electrolytic capacitors. <br> 2 Electrolytic Capacitor failure of non-normal <br> A number of factors can cause Electrolytic Capacitor failure, .such as extremely low temperature, capacitor temperature rise (welding temperature, ambient temperature, AC ripple), high voltage, instantaneous voltage, high frequency or reverse bias; the temperature rise is the working life of electrolytic capacitors (Lop) factors having the greatest effect .. <br> Capacitance of conductive capacities from electrolyte ionization and viscosity. When the temperature decreases, the increase of viscosity electrolyte, ion mobility and conductive capacity reduction. When frozen electrolyte, ion mobility is very low due to a very high resistance. On the .contrary, excessive heat will accelerate the electrolyte evaporation, when the amount of the electrolyte is reduced to a certain limit, the capacitor life also terminates. In the Alpine region (General-25 ℃), the need for heating, guarantee the electrolytic capacitor's normal .operating temperature. If outdoor UPS, in the Northeast are equipped with heating plates. <br> Capacitor in overstress condition susceptible to breakdown, and the practical application of surge voltage and instantaneous high voltage is frequent. In particular, our vast parts of the power .complex, AC power system is complex and often exceeds 30% of the normal voltage, particularly single phase input phase offset would increase the AC input of the normal range. The tests showed that the usual 450V/470uF105 ¡ .æ import common electrolytic capacitors, 2000 hours at rated voltage of 1.34 voltages, 2 hours after leakage capacitance appears, at the risk of air force at the top. According to statistics and analysis, and communication power close to switching power supply with PFC output Electrolytic .Capacitor failure, mainly due to a power surge and high pressure damage. Electrolytic Capacitor voltage select General secondary Derating, dropped to 80 per cent of the nominal value is more reasonable to use. <br> 3 life expectancy factors <br> In addition to .the non-normal failure, electrolytic capacitor of life and temperature have exponential relationships. As a result of the use of non-solid electrolyte, electrolytic capacitor of life also depends on the speed of evaporation of the electrolyte, the resulting electrical performance. These parameters .include capacitance tolerance values, leakage current and the equivalent series resistance (ESR). <br> Reference RIFA company is expected life of the formula: <br> PLOSS = (IRMS) ² xESR (1) <br> Th = .Ta + PLOSSxRth <br> (2) <br> Lop = Ax2 Hours (3) <br> B = reference temperature (typical of 85 ° c) <br> A = reference temperature capacitor life (based on the capacitor varies in diameter .) <br> C = cause capacitors to reduce by half the life of the required temperature (in degrees) <br> From the above formula, we can clearly see that the impact of electrolytic capacitor life of several factors: the ripple current directly ( .IRMS) and equivalent series resistance (ESR), ambient temperature (Ta), passing from hotspots to the surrounding environment of total thermal resistance (Rth). Capacitance internal temperature of the highest point, called the hot spot temperature (Th). Hot temperature .value is affecting capacitance working life. The following factors also decides the hot temperature value in the practical application of the outside temperatures (ambient temperature Ta), passing from hotspots to the surrounding environment of total thermal resistance (Rth) and caused by AC current energy .consumption (PLOSS). The capacitance of the internal temperature and energy loss into a linear relationship. <br> Capacitor charging and discharging, the current through resistor in energy consumption when the voltage changes in through the dielectric is also cause energy loss, leakage current .of energy loss, the loss of all of these result is the internal temperature rise capacitance. <br> 3.1, design considerations <br> In non-solid electrolyte capacitors, dielectric oxide layer for anode foil. Electrolyte as cathode foil and anodized aluminum oxide .layer between the electrical contact. Absorption of electrolytes for aluminum foil paper layers become cathode and anode foil isolated layer between the foil by electrode wire movie connected to a capacitor terminals. <br> <br> · By reducing the ESR values, which can reduce .the ripple current in the capacitor and the internal temperature rise. This can be through the use of multiple electrode wire, laser welding electrodes, and the measures implemented. <br> ESR values and ripple current determines the capacitance of the temperature rise. Prompted the .capacitor can be satisfied with ESR values of one of the main measures are: usually use one or more pieces of metal electrode wire electrode and the connection of external cladding, reduce cladding and PIN between the impedance. Cladding of electrode wire on the Tablet, the .capacitance of the lower ESR values. With the help of laser welding and cladding technology, you can add more of electrode wire, therefore the capacitor can achieve lower ESR values. This also means that the capacitor can withstand higher ripple current and low internal temperature rise ., which means a longer working life. This will also help to improve the capacity of the capacitor against vibration, or they may cause internal short circuit, high leakage current, capacitance value losses, ESR values rise and circuit. <br> · Through the .bottom of the capacitor and cladding and aluminium mechanical contact between the good and through the middle of the heat sink and cladding, capacitance internal heat effectively from aluminium and released to the bottom of the backplane. <br> <br> Internal thermal design for capacitance .stability and workersFor life is extremely important. The design of the company in EvoxRifa, anode foil was extended to may come into direct contact with the bottom of the capacitor of thick aluminum housing. This will be at the bottom of the heatsink, and cladding to .the hotspot of heat energy release. If you choose to install with bolts, safely capacitor installed backplane (usually aluminum), you can get more comprehensive with low thermal resistance (Rth.) of heat transfer solutions. <br> · Through the use of .a whole note around the electrode of phenolic plastic covers and double-crafted letter pad and aluminum case closely, you can significantly reduce electrolyte loss. <br> <br> Electrolyte through the gasket of evaporation have decided the long life of electrolytic capacitors work time .. When the capacitor of electrolyte evaporation to a certain extent, the capacitor will ultimately fail (this results from the internal temperature rise and acceleration). EvoxRifa company designed double sealing systems can reduce electrolyte capacitor evaporation speed reaches its maximum length of working life. < .br> These characteristics ensure a capacitor in the required fields have a long working life. <br> <br> 3.2, influence the life of the application of the factors <br> According to the life of the formula, it can be concluded that .the factors affecting the application of life: the ripple current (IRMS), ambient temperature (Ta), passing from hotspots to the surrounding environment of total thermal resistance (Rth). <br> 1. the ripple current <br> Ripple current of .size, has a direct impact on the electrolytic capacitor internal temperature of the hot spots. Query Electrolytic Capacitor manual, may obtain the ripple current of the allowed range. If out of range, you can use parallel approach. <br> 2. the ambient .temperature (Ta) and thermal resistance (Rth) <br> According to the hot temperature formula, electrolytic capacitor of ambient temperature are also important factors. In the application, you can consider environmental heat, thermal intensity, electrolytic capacitors and heat source distance, .Electrolytic Capacitor installation mode, etc. <br> Capacitor of heat, the temperature is always from the top "hot spots" to the ambient temperature is relatively low conduction of parts. Heat transfer in several ways: first, the conduction through the aluminum foil .and electrolyte. If the capacitor is installed in the heatsink on the part of the heat will also pass through the heat sink into the environment. Different installation methods and spacing and cooling modes is affects the capacitance of the thermal resistance of the environment. From the ."hot spot" is passed to the surrounding environment for total thermal resistance of Rth to represent. A clamp installed, install the capacitance in thermal resistance 2 ℃ / W of heat sink, the thermal resistance value Rth = capacitance 3.6 ¡ .æ / W; the use of bolts installation mode, the capacitor is installed in the thermal resistance is 2 ° / W heatsinks, forced air cooling rate of 2 m / s, the thermal resistance value Rth = capacitance 2.1 ¡ .æ / W. (In PEH200OO427AM capacitor as an example, the environment surrounding temperature of 85 ° c). <br> In addition to extension of cathode foil direct contact with the capacitor shell, is a good way to reduce the thermal resistance. At .the same time it should be noted that shell will therefore negatively charged, cannot be negative. <br> Capacitor must be correctly installed in order to achieve its design work life. For example: RIFA PEH200 series PEH169 series and should be installed in vertical or .horizontal mounting up. At the same time ensuring safety valve, such hot electrolyte and steam to the capacitor failure from the smooth discharge of safety valve. <br> When the capacitor arrangement is compact when adjacent capacitance between at least 5 mm should be left out .in order to ensure the appropriate amount of interval. When installing using the bolt nut torque control is very important. If the screw is too loose, the capacitor and the heatsink room cannot close contact; if tightened too hard, they may make the thread damage .. At the same time it should be noted that the capacitor should not be installed upside down, otherwise it may cause the broken bolts. <br> Capacitor installation should be as far away as possible from heating element, otherwise too high a temperature will shorten .the life of the capacitor, allowing capacitors as whole circuit life minimum parts. In the ambient temperature is high, try to use forced air cooling, the capacitor is installed in the inlet area. <br> 3. frequency of <br> If the .current from the base frequency and number of harmonic composition shall be calculated each harmonic generation power loss value and calculation results to find the value of total loss. <br> In the high-frequency applications, capacitance at both ends of the wire should be as .short to reduce the equivalent inductance. <br> <br> Capacitor resonance frequency (fR), due to the capacitors type. For lug and bolted type aluminum electrolytic capacitors, resonance frequency in 1.5kHz-150kHz. If the capacitor is higher than the .resonant frequency is used, the external characteristics are emotional. <br> 4 closing <br> To sum up, avoid irregular fail, select the correct application of the conditions and environment, the life of the electrolytic capacitors can be guaranteed. <br> .<br> <br>.