Characteristics and applications of electrolytic capacitors

SMD aluminum Electrolytic capacitor

Tantalum capacitor

Electrolytic capacitors are the most used types in the capacitor family, and there are many places to pay attention to when using them:

1. About the polarity of electrolytic capacitors

The aluminum electrolytic capacitor has two aluminum foil electrodes, and the electrodes are porous materials containing an electrolyte. In order to increase the capacitance, the two electrodes are in a wound form. The prepared electrolytic capacitor is energized with a DC voltage between the positive and negative electrodes, and the electrolyte between the electrodes is electrolyzed to form a thin layer of aluminum oxide on the aluminum foil. It is the insulating medium for electrolytic capacitors. This kind of medium has poor insulation performance, so the insulation resistance of the electrolytic capacitor is sometimes as low as several tens of kiloohms, and the leakage current is quite large. Its loss is also the largest of the capacitors. These characteristics make ordinary electrolytic capacitors have a polar capacitance, and the positive or negative electrode is marked on the outer casing.

In the above case, the electrolytic capacitor can only be used in a DC circuit or a circuit in which an AC component passes, but the voltage across the capacitor can always ensure its polarity, and must not be used in a pure AC circuit.

If the polarity of the electrolytic capacitor is reversed, the internal electrolytic action of the capacitor is reversed after the power is turned on, the normal medium gradually disappears, the leakage current increases rapidly, the capacitor will heat up, eventually causing a burst, and even hurting people!

However, in some cases, large-capacity, non-polar capacitors are required, and manufacturers have introduced bipolar (non-polar) electrolytic capacitors. It is equivalent to two back-to-back connections of electrolytic capacitors, but it must be pointed out that it is not recommended to use a back-to-back connection of two common electrolytic capacitors instead of a stepless electrolytic capacitor.

In AC intermediate power devices that require large capacitance, such as UPS uninterruptible power supplies, intermediate frequency power supplies, and washing machines, non-polar oil-immersed low-capacitors or polystyrene capacitors must be used.

2. If the capacitor in the safety application circuit is fully charged, especially when the voltage is high, it should be discharged with a resistor during measurement or replacement to ensure personal safety.

3. In some applications, the effects of leakage from electrolytic capacitors must be fully considered.

Figure (a) below shows a long delay circuit composed of RC and CMOS inverters. The inverter transfer level Vth ≈ 0.5 VDD in the following figure (b). The output rectangular pulse is delayed by td ≈ 0.69 RC compared to the input signal. However, when the insulation resistance r of R and electrolytic capacitor C can be compared, it can be deduced from the Thevenin's theorem that the maximum charging voltage of the capacitor is only VDD × r / (R + r), and the equivalent internal resistance of the signal source is rR / (r + R). VDD is the amplitude of the input signal, which is the supply voltage. If r is small, there may be a possibility of less than Vth, so that the inverter cannot be flipped and the circuit cannot work normally.

It is one of the solutions to solve the above problems by selecting 钽 and 铌 electrolytic capacitors with small leakage current.

4. Improve the high frequency performance of electrolytic capacitors

The winding structure of electrolytic capacitors has poor high-frequency characteristics. Therefore, when decoupling and other applications are used, a small-capacity Ceramic Capacitor with high-frequency characteristics, such as CC4 monolithic capacitor, is generally connected in parallel. The capacity depends on the operating frequency of the circuit, often between 1000pF and 0.1μF.

5. Method for reducing ESR of electrolytic capacitor

The ESR of the electrolytic capacitor has a great influence on its high-frequency performance, and the effect on the filtering effect of the switching power supply output is very obvious. The choice of ESR small electrolytic capacitors, such as tantalum capacitors is a way. However, paralleling several Aluminum Electrolytic Capacitors is also an effective way.

6. Tantalum electrolytic capacitor

It is divided into solid and liquid. The solid tantalum electrolytic capacitor is constructed by forming a metal tantalum oxide by a molding process, and then chemically oxidizing to obtain a very thin, rough-surfaced tantalum oxide layer as an insulating medium, and then coating thereon. The layer of manganese oxide solid electrolyte is sprayed with a layer of conductive metal foil to solder the leads and package. The liquid tantalum electrolytic capacitor uses a liquid electrolyte as a negative electrode.

Due to the high dielectric constant of yttrium oxide, the capacity/volume ratio is large, the performance is stable, the leakage current is small, the dielectric strength is high, and the service life is long. The disadvantage is that the capacity cannot be made too large (<470UF), and the price is relatively high. expensive.

Tantalum electrolytic capacitors are often used in voltage reference, time reference, measurement amplifiers, etc. where capacitor performance is high.

7. Tantalum electrolytic capacitor

It is made of rare metal antimony and its processing and characteristics are similar to those of tantalum electrolytic capacitors.

8. Pay attention

Ordinary aluminum electrolytic capacitors typically have a lifetime of 2,000 hours.

Military products can only be used in military products that require long life, and the service life can reach 5,000 hours. In some civilian products that also require long life, such as household three meters, it can be solved by using a "redundant backup" method in which several capacitors are connected in parallel.