Summary: Parasitics often play a greater role in determining the suitability of a capacitor for a given application than the actual capacitance value of the part itself. In this series of blogs, we’ll take a look at some of these parasitics and discuss how they affect capacitor selection. This month, we’ll examine series resistance.
Last month we looked at various aspects of the parallel resistance between the electrodes of a non-ideal capacitor. This month, we’ll expand on that topic by examining the series resistance that a non-ideal capacitor puts in the signal path. Let’s first consider DC behavior and, returning to our bucket analogy from Part 1A, envision the capacitor as a place to deposit and retrieve energy.
If we’re using the capacitor as a reservoir and have a spigot on the side, how long does it take to fill a container from it (assuming the liquid hasn’t already leaked out of the hole in the bottom)? The answer depends on both the viscosity of the fluid (i.e., the amount of charge the capacitor is carrying and the properties of its dielectric material) and the size of the spigot. The wider the spigot, the less resistance to flow and the faster the fill.
In a capacitor, the charge is stored in the dielectric, which has its own characteristic viscosity that affects the rate at which it can release stored energy. Additionally, the speed with which the energy can be transferred to the next part of the system is dependent upon the electrical resistance it encounters on the way out. Together, these elements are referred to as the equivalent series resistance (ESR) of the capacitor.
In the case of a multilayer ceramic capacitor, when voltage is applied, charge is stored, as the dipoles within the dielectric material align with the electric field across the electrode plates. When a load is connected across the plates, the dipoles relax and release the charge at a rate dependent upon the crystal structure of the material (which also defines the different classes of ceramic dielectric materials). The released charge has to pass through the conductive materials connecting the electrode plates to the external termination, then to the circuit board, and then to the load.
As voltage is applied to the capacitor plates, a charge will build up, which establishes an electric field across them. If there is a dielectric material in this space, then the electric dipoles of the molecules within the material will align with the field, increasing its strength and capacity for holding charge. If the polarity of the battery is changed rapidly back and forth, the dipoles will also change alignment rapidly, but the extent to which they can keep pace with polarity reversal will depend on the type and structure of the dielectric material.