Characteristics ofaluminum electrolytic capacitors and precautions for selection
Aluminum electrolytic capacitor is an important part of most electrical designs, so its correct selection is the key to the success of many designs. Therefore, engineers must understand the working principle of aluminum electrolytic capacitors, and correctly select the appropriate capacitor for the specific design.
Aluminum electrolytic capacitors not only have a high capacitance value per unit volume, but also can be used in almost any electronic system, including for filtering unwanted AC frequencies, and for storing energy in some applications. In addition, because it provides high capacitance and low impedance, it is also often used in DC-DC converters, inverters and power supplies.
For efficiency considerations, people have been paying great attention to the improvement of aluminum electrolytic capacitor technology. Through new manufacturing processes and new materials, their service life has been greatly improved, and at the same time reliability and robustness have been improved. In general, aluminum electrolytic capacitors can achieve a long service life-with a proper design, they can be used for up to 20 years.
All capacitors are composed of two layers of conductive material (or electrodes), which are separated by an insulator made of dielectric material. An electric field is created between these layers, and when there is current charging the capacitor, it can store energy. The characteristic of the aluminum electrolytic capacitor is that its electrodes are made of aluminum foil, and the conductive liquid, that is, the electrolyte, is placed between the two aluminum foil electrodes. Through the electrochemical reaction, the foil will form an oxide layer (Al2O3) on the anode, which acts as a dielectric in the aluminum electrolytic capacitor
The combination of design and materials gives aluminum electrolytic capacitors unique electrical characteristics. These characteristics create obvious advantages, making it ideal for many applications (compared to other capacitors). And in many cases, aluminum electrolytic capacitors are the solution.
First of all, the capacitance value of aluminum electrolytic capacitor is larger (in terms of unit volume and voltage level), so it has a higher volumetric efficiency. Therefore, the capacitance per unit volume of an electrolytic capacitor is higher than that of any other type of capacitor. This is an important feature of aluminum electrolytic capacitors. When properly designed, this feature can provide significant advantages over other technologies in applications.
Second, the rated voltage of most aluminum electrolytic capacitors is higher than that of other types of capacitors. Aluminum electrolytic capacitors with a DC voltage rating of 600V are easily available and can be used in almost a variety of applications.
Third, these capacitors have extremely high energy storage capabilities because they have both high capacitance values and high voltage ratings. The energy stored in the capacitor increases linearly with the capacitance value and exponentially increases with the voltage.
A full understanding of these three characteristics is the key to the correct selection of aluminum electrolytic capacitors for power applications. On this basis, engineers should consider the following four points when evaluating the design of aluminum electrolytic capacitors.
An important consideration for aluminum electrolytic capacitors is the service life requirements of specific applications. It’s important to know how long an application will be able to function as designed for it before it needs to be repaired or replaced.
Due to the many complex factors involved, this seems to be a difficult task, but given the reliability of electronic equipment, it is actually very predictable. Fortunately, an engineer does not need to consult an expert or become an expert to determine the lifespan of a capacitor. Because most reliable capacitor manufacturers provide web-based tools to help engineers calculate the service life of aluminum electrolytic capacitors within the specific design constraints of the application.
The engineer should ensure that the aluminum electrolytic capacitor works within the specified safe voltage working range and under the rated voltage. This can not only extend the service life of the capacitor, but also protect the capacitor from unnecessary damage.
Aluminum electrolytic capacitors are usually used in conjunction with power supplies to provide high capacitance values. However, aluminum electrolytic capacitors are polarized, so they can only be used in DC voltage applications in rectifier circuits.
When evaluating aluminum electrolytic capacitors, engineers should not choose capacitors with a voltage rating much higher than the application requirements. Because electrostatic resistance will also increase drastically.
In addition, engineers should understand the load requirements of the application, including ripple current, ambient temperature, and possible surge voltage. This helps to extend the overall service life of the application.
Although many aluminum electrolytic capacitors can withstand occasional short-term surges and reverse voltages or voltages of opposite polarity, they are not recommended for continuous surges or continuous over-rated reverse voltages. Overvoltage, reverse voltage, and transient surges may damage the capacitor. Engineers should use diodes appropriately to prevent reverse voltage, and the capacitors purchased can always meet the specified application design requirements.
Heat dissipation characteristics
The alternating current flowing through the capacitor will increase its temperature. This current is called ripple current, and self-heating causes power loss. Therefore, the application environment temperature will become a key factor in choosing aluminum electrolytic capacitors.
The cooling measures and surface area of an aluminum electrolytic capacitor determine its thermal resistance or heat dissipation in the application. The heat dissipation is also limited by the ripple current of the contact element.
The ripple current rating of aluminum electrolytic capacitors is usually specified at the upper specification temperature. Not only the application environment temperature is an important factor, but the frequency, thermal resistance, and equivalent series resistance of the AC signal are also important.