Why Parallel Capacitors with Different Values Are Essential for Bypass?

 

Using different value capacitors in parallel for bypass (or decoupling) serves to filter a wide range of noise frequencies and ensure a stable power supply to sensitive components like ICs. Here’s why:

1. Broad Frequency Coverage

• Large-value capacitors (typically in the microfarad range, like 10 µF or 100 µF) filter out low-frequency noise. These capacitors are effective at stabilizing the power supply by handling larger, slower voltage fluctuations and noise at lower frequencies. However, their internal equivalent series inductance (ESL) and equivalent series resistance (ESR) make them ineffective at high frequencies.
• Small-value capacitors (typically in the picofarad or nanofarad range, like 100 nF or 10 pF) handle high-frequency noise. Due to their lower ESL, these capacitors can filter out higher-frequency transients and noise, but they may not provide sufficient capacitance for low-frequency noise filtering.

By using capacitors of different values in parallel, each one can target noise in its respective frequency range. This allows for more comprehensive filtering across a broad spectrum of frequencies, preventing high-frequency noise from reaching sensitive ICs while also providing stability against slower fluctuations.

2. Decoupling High-Frequency Switching

Modern digital circuits and switching devices (like microcontrollers or switching power supplies) can generate noise at multiple frequencies. Parallel capacitors with different values prevent both high-frequency switching noise (caused by fast switching) and low-frequency ripple from corrupting the power supply.

3. Minimizing Voltage Ripple

Power rails tend to experience both transient and steady-state noise. Large capacitors act like energy reservoirs for slower, larger transient loads, while smaller capacitors respond quickly to sudden, sharp transients, ensuring minimal voltage ripple on the power lines.

Example Configuration

• A 10 µF ceramic capacitor for filtering low frequencies.
• A 100 nF capacitor for intermediate frequencies.
• A 10 pF or 1 nF capacitor for high-frequency noise.

This approach ensures a cleaner, more stable power supply for ICs and other sensitive components. Each capacitor tackles different parts of the noise spectrum, improving overall performance.