Capacitor application characteristics

Filter 18.432M

Capacitors are generally categorized based on their dielectric materials into membrane, paper, ceramic, and electrolytic types. Their equivalent circuits are illustrated in the diagram below.

In these equivalent circuits, C represents the ideal capacitance of the component, R is the leakage resistance, ESR stands for Equivalent Series Resistance, and ESL refers to Equivalent Series Inductance. Understanding these parameters is crucial for selecting the right capacitor for specific applications.

The leakage resistance R exists in all capacitors due to the non-ideal insulation properties of the dielectric material. This leads to some degree of current leakage, which can cause energy losses and affect circuit performance, whether it's DC or AC. For electrolytic capacitors, this leakage can also lead to increased temperatures, shortening their lifespan. To mitigate this, it's recommended to use capacitors with high leakage resistance and low loss, such as tantalum, polystyrene, or ceramic capacitors.

ESR is caused by the resistance of the capacitor's electrodes, leads, and contact points. It affects both DC and AC circuits similarly. In power filter applications, ESR becomes a critical factor, often overshadowing the impact of leakage currents. Therefore, minimizing ESR is essential in high-performance designs.

ESL, or Equivalent Series Inductance, depends on the length of the leads and the capacitance value. Any wire carrying current generates a magnetic field, introducing inductance. This effect is particularly significant in high-frequency applications because the inductive reactance from the leads opposes the capacitive reactance. As a result, the capacitor behaves more like a series resonant circuit, which can reduce its effectiveness at certain frequencies.

Additionally, the ESL increases with larger capacitance values, as the area between the plates expands. Most capacitors are constructed using a layered or wound structure to maximize capacitance, which inherently introduces inductance. This makes them act like small inductors. To avoid resonance issues, especially in decoupling circuits, it’s advisable to use smaller capacitors with shorter or no leads, such as chip capacitors. When large capacitors are necessary, adding a smaller, low-ESL capacitor in parallel can help prevent unwanted resonance.