What should be noted when choosing capacitors?

What parameters should be considered when choosing electrolytic capacitors? The basic parameters of electrolytic capacitors include the following:

1) Capacitance value and allowable deviation

The capacity of electrolytic capacitors depends on the impedance under AC voltage. Therefore, with changes in operating frequency, voltage, and measurement methods, the capacitance value, i.e. the AC capacitance value.

According to the standard JISC5102, when the frequency is 120Hz, the maximum AC voltage is 0.5Vrms, and the DC bias voltage is 1.5~2.0V, it can be concluded that the capacity of aluminum electrolytic capacitors decreases with increasing frequency.

2) Rated voltage

The series value of the highest DC operating voltage that can be used for capacitors within the specified ambient temperature range is called the rated voltage. Although the rated voltage is not a measurement parameter for capacitors, it is generally stipulated that the operating voltage value should be lower than the rated voltage value due to its close relationship with capacitors. Reducing the application voltage plays an important role in improving the service life of products.

3) Loss tangent value

In communication circuits, actual capacitors must consume a small amount of useful signal power. The ratio of active power to reactive power consumed under a sinusoidal voltage of a specified frequency is called the loss angle cut, which is approximately 0.01 to 0.20, but is closely related to the type, capacity, rated voltage, temperature, and frequency of the electrolytic capacitor. Sometimes it is also expressed as a percentage.

4) Impedance

The resistance that obstructs alternating current at a specific frequency is referred to as impedance (Z). It is closely related to the capacitance and inductance values in the equivalent circuit of capacitors, as well as to ESR.

As the frequency increases, the capacitance reactance (XC) of the capacitor gradually decreases in the low-frequency range. When the frequency continues to increase to the intermediate frequency range, the reactance (XL) decreases to the ESR value. When the frequency reaches the high frequency range, the induced impedance (XL) becomes dominant, so the impedance increases with the increase of frequency.

5) Leakage current

The dielectric of capacitors has a significant hindering effect on direct current. However, due to the electrolyte being immersed in the aluminum oxide film medium, a very small current called leakage current is generated when applying voltage, re forming and repairing the oxide film. Usually, leakage current increases with temperature and voltage.

6) Ripple current and ripple voltage.

In some data, they are referred to as ripple current and ripple voltage, which means that capacitors can withstand ripple current/voltage values. When the ripple current increases, even if the ESR remains constant, the ripple voltage will double.

In other words, as the ripple voltage increases, the ripple current also increases, which is why capacitors need to have a lower ESR value. After being added to the ripple current, the equivalent series resistance (ESR) in the capacitor generates heat, which affects the service life of the capacitor. Generally speaking, ripple current is proportional to frequency, so ripple current is relatively low at low frequencies. In a circuit, the actual voltage of a capacitor must not exceed its withstand voltage value.