1. Design considerations
In a capacitor with a non-solid electrolyte, the dielectric is an anodic aluminum foil oxide layer. The electrolyte acts as an 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 the electrode lead piece.
By lowering the ESR value, the internal temperature rise in the capacitor caused by the ripple current is reduced. This can be achieved by using a plurality of electrode tabs, laser welding electrodes, etc.
The ESR value and the ripple current determine the temperature rise of the capacitor. One of the main measures to promote a satisfactory ESR value of the capacitor is to reduce the impedance between the core package and the pin, usually by connecting the external electrode and the core package with one or more metal electrode leads. The more electrode leads on the core pack, the lower the ESR value of the capacitor. With the help of laser welding technology, more electrode tabs can be added to the core package, so that the capacitor can reach a lower ESR value. This also means that the capacitor can withstand higher ripple currents and has a lower internal temperature rise, I .e. a longer operating life. This also helps to improve the ability of the capacitor to resist vibration, otherwise it may lead to internal short circuit, high leakage current, capacitance loss, ESR value rise and circuit open.
Through good mechanical contact between the capacitor core pack and the bottom of the aluminum shell and through a heat sink in the middle of the core pack, the internal heat of the capacitor can be efficiently released from the bottom of the aluminum shell to the base plate to which it is coupled.
The internal heat conduction design is extremely important for the stability and working life of the capacitor. In EvoxRifa company's design, the negative aluminum foil is extended to directly contact the bottom of the capacitor aluminum shell thickness. This bottom becomes the heat sink of the core pack so that the heat of the hot spot can be released. If the installation method with bolts is selected, the capacitor can be safely installed on the bottom plate (usually aluminum plate), and a more comprehensive heat conduction solution with lower thermal resistance (Rth.) can be obtained.
The loss of electrolyte can be greatly reduced by the use of a phenolic plastic cover with an integral electrode and a double special seal that is tightly engaged with the aluminum shell.
Evaporation of the electrolyte through the gasket determines the long life of the electrolytic capacitor operating time. When the capacitor's electrolyte evaporates to a certain extent, the capacitor will eventually fail (this result will be accelerated by the internal temperature rise). The double-layer sealing system designed by EvoxRifa company can slow down the evaporation rate of the electrolyte, so that the capacitor can reach its longest working life.
These characteristics ensure that the capacitor has a long operating life in the required field.
3.2. Application factors affecting life
According to the life formula, it can be concluded that the application factors affecting the life 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 size of 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 aluminum electrolytic capacitor is also an important factor. In the application, the environmental heat dissipation method, the heat dissipation intensity, the distance between the electrolytic capacitor and the heat source, the installation method of the electrolytic capacitor, etc. can be considered.
The heat inside the capacitor is always conducted from the "hot spot" with the highest temperature to the part with relatively low ambient temperature. There are several ways of heat transfer: one is through aluminum foil and electrolyte conduction. If the capacitor is mounted on a heat sink, a portion of the heat will also be transferred to the environment through the heat sink. Different installation methods and spacing and heat dissipation methods will affect the thermal resistance of the capacitor to the environment. The total thermal resistance to transfer from the "hot spot" to the surroundings is represented by Rth. Using clip mounting, the capacitor is installed on a heat sink with a thermal resistance of 2 ℃/W, and the obtained capacitor thermal resistance Rth = 3.6 ℃/W; When the capacitor is installed on the heat sink with a thermal resistance of 2 ℃/W and the forced air cooling rate is 2 m/s, the obtained capacitor thermal resistance Rth = 2.1 ℃/W. (Take the PEH200OO427AM type capacitor as an example, the ambient temperature is 85 ℃).