2 Capacitor Technologies

Multilayer ceramic capacitors (MLCCs) are immensely popular in many applications because of their volumetric efficiency and relatively low price. The advantage of this capacitor technology lies primarily in its use of special dielectric materials. To understand why, recall that the capacitance of a simple parallel-plate capacitor follows Equation 1.

where k is the relative permittivity of the dielectric material placed between the plates (otherwise known as the dielectric constant), ε₀ is the permittivity of free space, A is the area of the capacitor plates and d is the distance between the plates.

Equation 1 shows that materials with higher dielectric constants enable smaller capacitor volumes for a given capacitance value. This accounts for the large variations in the size of a 10-µF capacitor with a particular voltage rating, since it all depends on the capacitor dielectric.

MLCC capacitors are organized into different classes depending primarily on their thermal range and stability over that range. Class II ceramics are often referred to as “high k” because their relative permittivities range from 3,000 (X7R) up to 18,000 (Z5U). By contrast, Class I C0G/NP0 capacitors tend to have relative permittivities in the range of 6 to 200. They are “high-performance” ceramic capacitors because their capacitance is more stable than most other dielectrics.

Plastic film capacitors that use materials like polyethylene or polypropylene tend to have even lower relative permittivities, typically less than 3, and also offer very good stability. Table 1 shows the relative permittivity of some common dielectric materials used in capacitors.

In portable electronics like smart speakers,^[1] it is tempting to use high-k MLCCs due to their small size and low cost. It is important to remember, though, that while their relative permittivity is very high, their capacitance changes significantly over applied voltage and temperature, which can degrade signal-chain performance. This variation in capacitance is primarily due to the use of heavy concentrations of barium titanate in the dielectric. Barium titanate is ferroelectric in nature, which means that increasing the electric field intensity inside the material decreases its relative permittivity. According to Equation 1, this will also lead to a decrease in capacitance. Thus, applying a time-varying voltage to the capacitor results in a time-varying capacitance, distorting the current flowing through the capacitor. The change in capacitance with applied voltage is known as the capacitor’s voltage coefficient, and it can be the dominant source of distortion in the low-frequency spectrum where capacitor impedance is relatively high. Furthermore, as the signal amplitude increases, greater distortion occurs. At higher frequencies, the distortion is less noticeable due to lower capacitor impedance, leading to a negligible voltage drop across the capacitor.