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Causes affecting the life of Electrolytic capacitor (II)


1. Design considerations

In non solid electrolyte capacitors, the dielectric is an anodic aluminum foil oxide layer. The electrolyte serves as the electrical contact between the cathode aluminum foil and the anode aluminum foil oxide layer. The paper dielectric layer that absorbs the 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 an electrode connector.

By reducing the ESR value, the internal temperature rise caused by ripple current in the capacitor can be reduced. This can be achieved by using multiple electrode lugs, laser welding electrodes and other measures.

The ESR value and ripple current determine the temperature rise of the capacitor. One of the main measures to ensure a satisfactory ESR value for capacitors is to use one or more metal electrode connectors to connect the external electrodes and the core package, reducing the impedance between the core package and the pins. The more electrode connectors on the core package, the lower the ESR value of the capacitor. 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 capacitors can withstand higher ripple currents and have a lower internal temperature rise, which means a longer working life. This is also beneficial for improving the capacitance's ability to withstand vibrations, otherwise it may lead to internal short circuits, high leakage currents, loss of capacitance, an increase in ESR value, and an open circuit in the circuit.

By maintaining good mechanical contact between the capacitor core package and the bottom of the aluminum shell, as well as by passing 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 bottom plate connected to it.

The internal heat conduction design is extremely important for the stability and working life of capacitors. In EvoxRifa's design, the negative aluminum 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, allowing the heat from the hot spot to be released. If a bolted installation method is used to safely install the capacitor onto the substrate (usually aluminum), a more comprehensive thermal conductivity solution with lower thermal resistance (Rth.) can be obtained.

The loss of electrolyte can be greatly reduced by using a phenolic plastic cover with electrodes wrapped around it as a whole and a double specially designed sealing pad that tightly engages with the aluminum shell.

The evaporation of electrolyte through the sealing gasket determines the working time of the long-life Electrolytic capacitor. When the electrolyte of the capacitor evaporates to a certain extent, the capacitor will eventually fail (this result will accelerate due to internal temperature rise). The double-layer sealing system designed by EvoxRifa company can slow down the evaporation rate of the electrolyte, allowing the capacitor to reach its longest working life.

The above characteristics ensure that capacitors have a long working life in the required field.

3.2 Application factors affecting service life

According to the lifespan formula, the application factors that affect lifespan are ripple current (IRMS), ambient temperature (Ta), and the total thermal resistance (Rth) transmitted 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 operation manual of Electrolytic capacitor. If it exceeds the range, parallel connection can be used to solve it.

2. Environmental temperature (Ta) and thermal resistance (Rth)

According to the formula of hot spot temperature, the application environment temperature of aluminum Electrolytic capacitor is also an important factor. In application, environmental heat dissipation mode, heat dissipation intensity, distance between Electrolytic capacitor and heat source, installation mode of Electrolytic capacitor, etc. can be considered.

The heat inside a capacitor always conducts from the hottest "hot spot" to the relatively cooler parts around it. There are several ways of heat transfer: one is through aluminum foil and electrolyte conduction. If the capacitor is installed on a heat sink, a portion of the heat will also be transferred to the environment through the heat sink. Different installation methods, spacing, and heat dissipation methods will affect the thermal resistance of capacitors to the environment. The total thermal resistance transmitted from the "hotspot" to the surrounding environment is represented by Rth. Using a clamp installation method, the capacitor is installed on a heat sink with a thermal resistance of 2 ℃/W, resulting in a thermal resistance value of Rth=3.6 ℃/W; By using a bolt installation method, the capacitance is installed on a heat sink with a thermal resistance of 2 ℃/W and a forced air cooling rate of 2m/s, resulting in a capacitance thermal resistance value of Rth=2.1 ℃/W. (Taking PEH200OO427AM capacitor as an example, the ambient temperature is 85 ℃).

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