Capacitor application characteristics

Filter 18.432M

Capacitors can be roughly divided into membrane, paper, ceramic and electrolysis according to different dielectrics. Their equivalent circuits are shown in the figure below.

Where C is the ideal capacitance of the device, R is the leakage resistance, ESR is the equivalent series resistance, and ESL is the equivalent series inductance.

Leakage resistance R is present in any type of capacitor, because all dielectric insulation properties are not ideal, and there is more or less leakage, and the resulting losses not only affect the normal operation of the circuit (for DC and AC) The effect is the same), and for electrolytic capacitors, their lifetime is also reduced due to temperature rise. To reduce this effect, capacitors with large leakage resistance and low loss should be selected in the circuit. Commonly used are tantalum capacitors, polystyrene capacitors, and ceramic capacitors.

ESR is a series resistance caused by electrodes, leads and contact resistance. It also has the same effect on DC and AC. Especially in the application of the power filter must pay attention to the impact of ESR, then the impact of leakage factors on the circuit does not seem important.

The equivalent series inductance ESL is determined by the length of the lead and the size of the capacitance. Any length of wire flowing through a current creates a magnetic field and therefore has a certain inductive effect. For capacitors, the effect of this additional effect is very pronounced (especially in high frequency applications) because the reactance properties produced by the leads are opposite to those of the self reactance.

The size of the ESL, on the other hand, is determined by the capacitance capacity, which is because the relative area of ​​the two poles increases the capacity. The structure of the capacitor is generally made into a sandwich structure or a similar structure as shown in the following figure, and in order to further increase the capacity, most of the capacitors are also wound when the capacitor is fabricated, taking into account the displacement current. This is obviously the structure of an inductor. After considering ESL, a capacitor becomes a series tuned circuit with a low O value. In circuit applications where this must be avoided, capacitors with small capacitance, short leads, or even no leads should be used. The use of chip capacitors meets this requirement. In some occasions where large capacitors must be used, and resonance should be avoided. For example, in a decoupling circuit, a capacitor with a small capacity and a small ESL can be connected in parallel to the large capacitor.