In power supply design, bypass and decoupling capacitors are essential for filtering out voltage noise and ensuring stable operation. But with so many values and package types available, many engineers ask:

“Should I choose one large capacitor or combine several?”
“What value of capacitor will effectively suppress high-frequency noise?”
“What is the self-resonant frequency (Fsr), and why does it matter?”

Let’s break it down simply — and practically.

Capacitor Impedance and Fsr: The Real-World Behavior

Ideally, a capacitor's impedance decreases as frequency increases, following the formula:

$$Z = \frac{1}{j\omega C}$$

But in the real world, every capacitor includes parasitic inductance (ESL) and resistance (ESR), forming an LC series resonant circuit.

• At its self-resonant frequency (Fsr), the capacitor exhibits lowest impedance.

• Below Fsr, it behaves like a capacitor.

• Above Fsr, it becomes inductive and loses its filtering ability.

This means a capacitor is only effective up to its Fsr.

Why Combine Capacitors of Different Values?

Large capacitors like 10μF are great at filtering low-frequency ripple, but they typically have low Fsr (a few MHz). For high-frequency transients, they’re practically useless.

On the other hand, small capacitors — like 100nF or 100pF in 0402 or 0603 packages — have higher Fsr (100–500 MHz), making them excellent for high-frequency noise suppression.

Combining capacitors with different values — say 10μF + 0.1μF + 100pF — helps cover a wide frequency range. The general rule of thumb: use values at least two decades apart.

How to Know a Capacitor’s Fsr?

You can find Fsr values in the capacitor’s datasheet. For example:

• 100pF 0402 ceramic: Fsr ≈ 500 MHz

• 0.1μF 0603 ceramic: Fsr ≈ 100 MHz

• 10μF 0805 ceramic: Fsr ≈ 5–10 MHz

• Electrolytic 10μF (DIP): Fsr may be < 1 MHz

Smaller packages = higher Fsr due to lower parasitic inductance.

Layout Tips: Keep Capacitors Close to Ground

Bypass capacitors should always be placed as close as possible to the power and ground pins of ICs. This reduces trace inductance and ensures effective high-frequency suppression. Remember, long traces = unwanted inductors!

Quick Design Checklist

“In practice, one capacitor is rarely enough. A smart combination — backed by understanding of Fsr — can make or break your power design.”

Conclusion

Choosing the right filter capacitors isn’t just about value — it’s about behavior across frequency. By understanding Fsr, ESL, and combining capacitors of different sizes, you can ensure stable, noise-free power delivery across your entire design.

Need help with component selection or PCB layout advice? Contact our engineering team — we're happy to assist.