Choosing capacitors for reliable automotive applications

Choosing the most reliable capacitors for today’s automotive electronics requires the designer to examine a number of different device parameters and performance characteristics. The next step is to consider the automotive environment in which the devices will be used and the specific applications for which they are intended. This article looks at the characteristics of the four major dielectric types of capacitors—tantalum, aluminum electrolytics, poly-films, and ceramic. It explains the concepts of temperature coefficient of capacitance and voltage coefficient of capacitance and how these and other factors affect the choice of capacitor for a given application.



The first thing we notice about various capacitor dielectrics is that each has a typical capacitance and voltage range (Figure 1). But for applications requiring capacitance values from about 0.1µF to 10µF, and voltages less than 50V, there are several overlapping choices. To sort out the performance characteristics of these various capacitor types, we’ll need to cover a few of the capacitors basics.



A single formula can be used to determine the capacitance value of every type of capacitor: C = (K x A)/t. 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.Figure 3 shows the 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.



Figure 4 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 SMD technology.



In the most general of classifications, capacitors fall into two basic categories of construction. These are electrostatics (films and ceramics) and electrolytics (tantalums and aluminums). Electrostatic capacitors typically exhibit very low ESR and impedance and are non-polarized devices, meaning that they can be bulk fed in the assembly process for high-speed insertion on to the printed circuit board. Electrolytics generally offer higher capacitance values but are polarized devices, and must thus be mounted onto the board with the proper orientation. The basic criteria among which these types can be compared are summarized in Table 1.



Each capacitor type, however, has certain unique characteristics and even within a particular type the appropriateness for a given application will depend upon the specific dielectric. For example, tantalum capacitors have no wearout mechanism and are particularly suited for applications requiring long life and stability.



The expected lifetime of aluminum capacitors can be doubled for each 10°C of temperature reduction, but it is important to keep these devices away from cleaning solvents. Ceramic capacitors require no surge current screen, but a highpot screen needs to be performed for devices with high voltage ratings. Film and foil types are especially suited for high-current applications, and metallized types have a self-healing feature that improves reliability.



Specific differences in electrical performance as we cross from one dielectric type to another are too difficult to detail within the scope of this article. However, a few examples are offered in the following Figures which should provide insight into the type of information that may be necessary to fully evaluate a given capacitor application.



Figure 5 shows a comparison of ESR and Z of similar cap values of tantalums and ceramics.



As temperatures change, capacitance values also change. This is termed the temperature coefficient of capacitance (TCC). Figure 6 shows a comparison between tantalum and X7R ceramic materials.



Figure 7 shows a comparison of tantalum and Y5V ceramic. In circuits where a minimum capacitance value is required across the full temperature range, TCC must be considered in the design.



For ceramic capacitors, the voltage applied to the capacitor also affects the capacitance value (the electric field strength across the dielectric changes the effective dielectric constant K of the material). As can be seen from Figure 7, this is not an issue for stable dielectrics such as NP0 or when the percentage of applied voltage is low when compared to the rated voltage. This characteristic is termed the voltage coefficient of capacitance (VCC) and is shown in Figure 8 for several different types of ceramic material.



It is not always easy to determine which dielectric type will best suit a given application. Indeed, choosing a capacitor is a multidimensional problem. The application may require a minimum capacitance value or very low ESR.



The capacitor cost and size must also be considered, as well as its packaging type. End-of-life reliability issues may also be important. Each capacitor type has its own set of characteristics that will make it the most logical choice for a given application.