Functions and Usage Considerations of Capacitors

As one of the fundamental passive components, capacitors primarily serve the following purposes:

I. Applications in Power Circuits

Capacitors in power circuits perform bypassing, decoupling, filtering, and energy storage. Below is a detailed breakdown:

1) Filtering

Filtering is one of the most critical functions of capacitors, widely used in nearly all power circuits.

• Theory: Larger capacitors exhibit lower impedance, allowing higher frequencies to pass.

• Reality: Electrolytic capacitors (>1μF) have significant parasitic inductance, causing impedance to rise at high frequencies.

• Solution: Parallel a large capacitor (e.g., 1000μF for low frequencies) with a small one (e.g., 20pF for high frequencies).

• Analogy: A capacitor acts like a "reservoir," smoothing voltage fluctuations by converting them into current changes.

2) Bypassing

Bypass capacitors provide localized energy storage, stabilizing voltage for ICs by:

• Charging/discharging like a small rechargeable battery.

• Placement: Must be placed close to the power and ground pins of the load to minimize impedance and prevent ground bounce.

3) Decoupling

Decoupling capacitors mitigate noise caused by sudden current demands from load circuits:

• High-frequency bypass: Small capacitors (0.1μF, 0.01μF) divert switching noise.

• Low-frequency decoupling: Larger capacitors (≥10μF) buffer current surges.

• Key Difference: Bypassing filters input noise; decoupling prevents output noise from affecting the power supply.

4) Energy Storage

Energy-storage capacitors (e.g., 40–450V, 220–150,000μF aluminum electrolytics) collect charge from rectifiers and deliver it to the output. High-power systems (>10kW) often use large can-type capacitors.

II. Signal Circuit Applications

1. Coupling: Blocks DC while allowing AC signals to pass (e.g., in transistor amplifiers).

2. Oscillation/Synchronization: Used in RC/LC oscillators and crystal load circuits.

3. Time Constant: Forms integrator circuits (e.g., *i* = (V/R)e^(–t/CR)).

III. Usage Guidelines

A. Capacitor Selection Myths

1. "Bigger capacitance is better":

   • Larger caps have lower resonant frequencies, reducing high-frequency performance.

   • Follow design reference values instead.

2. "More parallel small caps improve performance":

   • While lowering ESR, PCB trace impedance can negate benefits.

3. "Lower ESR is always better":

   • Excessively low ESR may cause switching circuit oscillation. Balance ESR with cost.

4. "Premium capacitors guarantee quality":

   • Circuit design matters more than capacitor brand. Avoid marketing gimmicks.

B. Capacitor Failure ("Popcorn Effect")

1. Causes:

   • Overvoltage, reverse polarity, excessive ripple current, or frequent charge/discharge cycles.

   • Primary culprit: High temperature. Every 10°C rise halves capacitor lifespan.

2. Failure Modes:

   • Input capacitors (C1): Fail due to poor power quality (voltage spikes).

   • Output capacitors (C2): Fail less often but are sensitive to ambient heat.

3. Lifespan:

   • Rated for ~20,000 hours. High-quality caps endure 100°C for thousands of hours.

   • "End of life" means capacitance drifts >10%, not immediate failure.