Capacitors: Misconceptions and Selection Guide

A capacitor, also known as “capacitance,” refers to the amount of electric charge stored under a given potential difference, denoted as C, with the international unit being the farad (F).
In general, charges move under the force of an electric field. When a dielectric exists between conductors, it prevents charge movement, causing charges to accumulate on the conductors. The amount of charge stored is referred to as capacitance.

So, what are some common misconceptions when using capacitors? And how should they be selected?

Four Misconceptions About Capacitors

1. The larger the capacitance, the better

Many people prefer to replace capacitors with larger capacitance. While it is true that higher capacitance provides stronger current compensation for ICs, there are drawbacks: larger size, higher cost, and impaired airflow and heat dissipation.

More importantly, capacitors have parasitic inductance. The discharge loop of a capacitor resonates at a certain frequency. At the resonance point, the capacitor’s impedance is low, and its ability to supply current is optimal. However, once the frequency exceeds the resonance point, impedance rises, and the current supply capacity decreases.

The larger the capacitance value, the lower the resonant frequency, which reduces the effective frequency range for current compensation. Therefore, the idea that “bigger is always better” is incorrect. Circuit design usually follows certain reference values.

2. The more small capacitors in parallel, the better

Key capacitor parameters include rated voltage, temperature resistance, capacitance, and ESR (Equivalent Series Resistance). For ESR, lower values are generally better. ESR is influenced by capacitance, frequency, voltage, and temperature. At a fixed voltage, higher capacitance usually means lower ESR.

In PCB design, using multiple small capacitors in parallel is often due to space constraints. Some assume that more parallel capacitors always reduce ESR and improve performance. In theory, this may be true, but in practice, the impedance of solder joints and leads must be considered. Adding many small capacitors in parallel does not always yield significantly better results.

3. The lower the ESR, the better

In power supply design, the requirements vary for input and output capacitors.

• For input capacitors, higher capacitance is often preferred, while the ESR requirement can be slightly relaxed. The main functions are withstanding voltage and absorbing MOSFET switching pulses.

• For output capacitors, voltage and capacitance requirements may be lower, but ESR requirements are stricter to ensure sufficient current flow.

However, ESR is not always better the lower it is. Very low ESR capacitors can cause switching circuit oscillations, requiring damping circuits, which increases complexity and cost. Therefore, in board design, a suitable ESR reference value is used to balance stability and cost.

4. Good capacitors equal high-quality products

The so-called “capacitor-only theory” was once popular, with some manufacturers and media promoting it as a major selling point. In reality, circuit design capability is the key. Just as some companies can achieve greater stability with two-phase power designs compared to others using four-phase, using expensive capacitors alone does not guarantee a better product.

When evaluating a product, it is important to take a holistic approach rather than exaggerating the role of capacitors.

How to Select Capacitors

• Low-frequency applications: A wider range of capacitors can be used, even those with weaker high-frequency characteristics.

• High-frequency circuits: The choice is much stricter. Improper selection can affect the overall circuit performance.

In general:

• Power supplies commonly use electrolytic capacitors and ceramic capacitors.

• For high-frequency circuits, materials like mica capacitors (though more expensive) are required.

• Polyester capacitors and electrolytic capacitors are unsuitable for high-frequency use, as they behave inductively and compromise circuit accuracy.