How to choose the right capacitor for automotive electronic equipment?

Although passive components (such as capacitors) are not as attractive as microprocessors and digital signal processors, they still need to be reliable for automotive applications. Choosing the most reliable capacitors for today's automotive electronics requires design engineers to test a series of different component parameters and their performance characteristics. The next step is to consider the environment used by automotive electronic equipment and the specific applications it serves. Now, let’s take a look at the characteristics of the four major dielectric capacitors-tantalum capacitors, aluminum electrolytic capacitors, multi-film capacitors and ceramic capacitors; explain the concepts of capacitance temperature coefficient and voltage coefficient; see how these factors and other factors are Affect the choice of capacitors for a particular application.

Each different capacitive dielectric has its specific capacitance and voltage range (see below). However, because the application requires the capacitance value to be approximately between 0.1 and 10F, and the voltage is less than 50V, there will be several overlapping options.

In order to find out the performance characteristics of these different types of capacitors, it is necessary to master some basic knowledge about capacitors. We only need to use a formula to determine the capacitance of each capacitor:

Each capacitive dielectric has a fixed dielectric constant (K). Therefore, for a given type of capacitive dielectric, the size of the capacitor is proportional to the surface area (A) of the active plate of the capacitor and inversely proportional to the thickness of the dielectric (t) . The thickness of the dielectric also determines the voltage tolerance (rated voltage) of the capacitor.

The typical dielectric constant and dielectric strength value (withstand voltage) of these four basic types of capacitors are given below. As we have seen, when a low dielectric constant is coupled with a low dielectric breakdown strength (like connected to a multi-film capacitor), a low volumetric efficiency will occur. However, the size of the object is only one of the characteristics of the capacitor. For example, the size of the film capacitor is very large, but it has very efficient and stable electronic characteristics, thus making up for the shortcomings of its large size.

The figure below shows the working process of the capacitor. The equivalent series resistance (ESR) is the real part of the impedance and represents the loss of the capacitor. The ESR value will vary with changes in temperature, frequency and dielectric type. The insulation resistance determines the magnitude of the DC leakage current when a given voltage is applied across the capacitor. The leakage current varies with temperature and the voltage applied to both ends of the capacitor, and the leakage current of thin-film and ceramic (electrostatic) capacitors is much smaller than that of tantalum and lead electrolytic capacitors.

Various types of capacitors are used in automotive applications, but now there is a tendency to use larger and more complex components. Although the automotive industry uses many lead-containing components, old circuit boards are rapidly being replaced by surface mount component (SMD) technology.

In the most common classification, capacitors are divided into two basic structures: electrostatic capacitors (film capacitors and ceramic capacitors) and electrolytic capacitors (tantalum capacitors and aluminum capacitors). Electrostatic capacitors generally exhibit very low ESR and impedance (Z), and are non-polar components, which means that they can be sent to the assembly line in large quantities to be inserted into the printed circuit board at high speed. Generally speaking, electrolytic capacitors can provide higher capacitance, but it is a polar component, so it must be installed on the printed board with the correct polarity. The following table summarizes several basic standards for comparing these types of capacitors.

However, each capacitor and each type has its unique characteristics; even in a particular type of capacitor, whether it is suitable for a given application depends on its specific dielectric. For example, tantalum capacitors have no wear failure mechanism and are particularly suitable for applications that require long life and stability. The life expectancy of aluminum capacitors will double for every 10℃ decrease in temperature, but it is important to keep them away from detergent.

Ceramic capacitors do not need to be shielded from surge currents, but high-voltage shielding is required for components with high rated voltages. Film and metal film capacitors are particularly suitable for high-current applications, and metal-type capacitors have the characteristics of self-repair, which can improve the reliability of the application.

Due to limited space, this article is difficult to describe in detail the specific differences in the electrical properties of various dielectrics. However, the following table gives a few examples to help us understand information about different types of capacitors, which are necessary for us to fully evaluate the application of a given capacitor.

The following compares the equivalent series resistance (ESR) and impedance (Z) of tantalum capacitors and ceramic capacitors with similar capacitance values.

1) Capacitance temperature coefficient

As the temperature changes, the capacitance value also changes. This is called the temperature coefficient of capacitance (TCC). The following is a comparison between a tantalum capacitor and a Y5V ceramic capacitor.

The following curve shows the comparison of tantalum capacitors and X7R ceramic capacitors. In circuit design, when a minimum capacitance value is required for the entire temperature range, TCC must be considered in the design.

2) Capacitor voltage coefficient

For ceramic capacitors, the voltage applied to the capacitor will also affect its capacitance (the electric field strength between the dielectrics will change the effective dielectric constant of the material). As shown in the figure above, when comparing rated voltages, this is not a problem for stable dielectrics such as NPO or when the percentage of rated voltage is low. This feature is called the capacitance voltage coefficient (VCC). The capacitance voltage coefficients of several different ceramic capacitors are shown below.

Conclusion

It is not easy to determine the type of dielectric that is most suitable for a given application. Choosing a capacitor is indeed a design problem in many aspects. The application may require a minimum capacitance value, or a very low ESR value. The cost, size, and packaging of the capacitor must also be considered. The issue of reliability at the end of life is also important. Each type of capacitor has its own set of characteristics that make it the best choice for a given application.