Selecting the most reliable capacitors for today's automotive electronics requires the design engineer to examine a number of different device parameters and performance characteristics. The next step is to consider the automotive environment in which the electronics will be used and the specific applications for which they are intended.
Here is a look at the characteristics of the four major dielectric types of capacitorstantalum, aluminum electrolytics, poly-films, and ceramic; an explanation of the concepts of temperature and voltage coefficients of capacitance; and how these and other factors affect the choice of capacitor for a given application.
The various capacitor dielectrics each have a typical capacitance and voltage range (see below). But for applications requiring capacitance values from about 0.1 to 10 F, and voltages less than 50V, there are several overlapping choices.
To sort out the performance characteristics of these various capacitor types consider a few capacitor basics. A single formula can be used to determine the capacitance value of every type of capacitor:
The dielectric constant (K) is fixed for each capacitor dielectric type. So for a given capacitor dielectric type, the amount of capacitance is directly related to the surface area (A) of the active plates of the capacitor and inversely proportional to the dielectric thickness (t). The dielectric thickness also determines the voltage withstanding capability (voltage rating) of the capacitor.
Given below are typical dielectric constant and dielectric strength (withstanding voltage) values for the four basic types of capacitors. As we see, when a low K is coupled with a low dielectric breakdown strength (as is the case with poly-film capacitors), the result is low volumetric efficiency. But physical size is just one characteristic of a given capacitor type. For example, film capacitors are rather large in size, but compensate for their relative bulk with extremely high efficiency and stable electrical characteristics.
The figure below is a schematic representation of how a capacitor works. The equivalent series resistance (ESR) is the real part of the impedance and represents losses in the capacitor. The value of ESR varies with temperature, frequency, and dielectric type. The insulation resistance (IR) determines the amount of DC leakage current that the capacitor passes for a given applied voltage. Leakage current varies with temperature and the magnitude of applied voltage and is typically much lower for film and ceramic (electrostatic) capacitors than for tantalum and aluminum (electrolytic) types.
All types and styles of capacitors are used in automotive applications, but the trend is toward devices with increased capabilities and greater complexity. Although many leaded devices are used in the automotive industry, older circuit boards are rapidly being replaced by surface-mount device (SMD) technology.