Common Misconceptions About Capacitors – And How to Choose the Right One

A capacitor, also called capacitance, refers to the ability of a component to store electrical charge under a given potential difference. It is denoted by C, with the international unit being the farad (F).

In simple terms, when an electric field is applied, charges move under force. If a dielectric material is placed between conductors, it blocks charge movement, causing charges to accumulate on the conductor surfaces. The amount of stored charge is what we call capacitance.

But when it comes to practical use, there are many misconceptions about capacitors. So, what are the common mistakes, and how should you select the right capacitor?

Four Misconceptions About Capacitors

1. “The larger the capacitance, the better.”

It’s common for people to replace capacitors with ones of higher capacitance, assuming bigger is better. While a larger capacitor can provide stronger current compensation for ICs, there are trade-offs: larger size, higher cost, and potential impact on airflow and heat dissipation.

More importantly, capacitors have parasitic inductance. In a discharge loop, resonance occurs at a certain frequency, where the impedance is at its minimum and energy compensation is most effective. However, beyond that resonance point, impedance increases and the capacitor’s ability to supply current decreases.

A higher capacitance lowers the resonance frequency, narrowing the effective frequency range. From the perspective of high-frequency current compensation, “bigger is better” is a misconception. Circuit design usually has well-defined reference values.

2. “Using more small capacitors in parallel is always better.”

Key parameters of a capacitor include voltage rating, temperature rating, capacitance, and ESR (Equivalent Series Resistance). Generally, lower ESR is better. ESR depends on capacitance, frequency, voltage, and temperature. With voltage fixed, larger capacitance usually means lower ESR.

In PCB design, multiple small capacitors are often paralleled due to layout and space limitations. Some assume that more parallel capacitors automatically lower ESR and improve performance. While this may be true in theory, in practice the solder joints and interconnect impedance of multiple capacitors can offset the benefits.

3. “The lower the ESR, the better.”

In power supply design, input and output capacitors serve different roles:

• Input capacitors require higher capacitance and sufficient voltage rating. ESR requirements are less strict because their main function is to absorb switching pulses from MOSFETs.
• Output capacitors can have lower capacitance and voltage rating but require stricter ESR performance, since they must handle higher ripple current.

However, ultra-low ESR is not always desirable. Very low ESR may cause oscillations in switching circuits, requiring additional damping circuitry, which increases complexity and cost. For this reason, capacitor selection usually follows reference guidelines to balance performance and stability.

4. “High-end capacitors always mean high-quality products.”

In the past, some manufacturers promoted products heavily by highlighting the use of “premium capacitors,” creating the illusion that capacitors alone determine quality.

In reality, circuit design is the true foundation of performance. For example, a well-designed two-phase power supply may outperform a poorly designed four-phase one, regardless of capacitor quality. Overemphasizing capacitors, while ignoring overall system design, is misleading. A product must be evaluated from multiple perspectives, not just by the components used.

How to Choose the Right Capacitor

• Low-frequency circuits are more forgiving, and capacitors with weaker high-frequency characteristics can still perform adequately.
• High-frequency circuits, however, are much more demanding. The wrong capacitor choice can disrupt the entire system’s operation.

In general:

• Power supplies typically use electrolytic and ceramic capacitors.
• High-frequency applications require capacitors with excellent high-frequency performance, such as mica capacitors.
• Polyester film and electrolytic capacitors are unsuitable in high-frequency circuits due to their inductive behavior, which affects precision and stability.

✅ In summary: Bigger capacitance or lower ESR doesn’t always mean better. The correct capacitor depends on the application, frequency, and design considerations. Optimal performance comes not just from component quality, but from thoughtful, balanced circuit design.