What are the things to pay attention to when choosing automotive grade capacitors?

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In this article, Andrew Wilson, senior product marketing manager of Vishay's tantalum capacitor division, and Florian Weyland, product marketing manager of Vishay's ceramic capacitors, introduce the selection of automotive-grade capacitors.

Choosing reliable capacitors for today's automotive electronics applications requires understanding the performance characteristics of each capacitor and the operating conditions in different applications. Compared with the specifications in the data sheet, the operating environment in the application has a greater impact on the actual circuit performance, so it is crucial to develop the best cost-effective solution.

Figure 1 shows the typical capacitance and voltage ranges for commonly used capacitor dielectrics, often with several overlapping choices.

Figure 1: Capacitor diagram

While capacity and voltage are usually the main parameters for device selection, there are many other parameters that help make the best choice. As shown in Figure 2, typical dielectric constant (K) and dielectric strength values ​​for four basic capacitors can be seen. The combination of low K values ​​and low dielectric strength (such as polyester film capacitors) results in low volumetric efficiency. However, these larger devices are still widely accepted due to very low losses, very stable electrical characteristics and low cost.

In operation, the capacitor equivalent series resistance (ESR) is the real part of the impedance and represents the losses of the capacitor in the equivalent circuit. These parameters vary with temperature, frequency and dielectric type. The insulation resistance (IR) determines the amount of DC leakage current that the capacitor will pass through the capacitor under the applied voltage, and the leakage current of electrostatic (film and ceramic) capacitors is usually much lower. The DC leakage current varies with temperature and the amount of applied voltage, and the inductive reactance is related to the electrode type.

Figure 3 shows the important relationships of a capacitor: capacitive reactance, dissipation factor, inductive reactance, and impedance. A very high value resistor is used to simulate the insulation resistance. For convenience, it can be ignored when deriving the total impedance (Z).

Impedance is an important parameter that determines the effect of a capacitor on the input signal. Low ESR is essential to achieve high efficiency, low heat loss and reliability during the charge/discharge cycle. Capacitive reactance (XC) and inductive reactance (XL) represent the energy storage capacity and the induced magnetic field generated by the capacitor. Note that when XC and XL are equal, the device resonant frequency is reached. This is important when selecting a decoupling capacitor to eliminate the AC component noise in the DC signal. In order to effectively eliminate the AC signal component in the DC link, a capacitor with a resonant frequency close to the frequency of the AC noise to be removed should be selected to achieve electrically small impedance and electrically large decoupling grounding.

The types of in-vehicle applications are usually divided into power control (ECU and transmission) and safety and comfort control (such as airbags and temperature control), and the type of application is important when considering key performance, reliability, and accuracy. Another major difference is the in-vehicle location and the resulting operating conditions. Under-the-hood applications may be exposed to or immersed in salt spray, water, fuel/oil, operating temperatures of 125°C or even higher, and vibration forces can reach 15g with frequencies up to 200Hz. These conditions are very different from the cockpit. In fact, another high-capacity technology (double-layer capacitor EDLC) is limited to cockpit applications such as electronic lock power backup due to its operating temperature limit (85°C).

Generally speaking, electrolytic capacitors (tantalum, aluminum, and EDLC) have high capacitance but are polar, while electrostatic capacitors (polyester film and ceramic) are non-polar and typically have very low ESR and impedance.

Tantalum devices are recommended to be used with voltage derating, with a 50% voltage derating for solid tantalum capacitors and an 80% voltage derating for polymer and liquid tantalum axial capacitors to ensure reliability. In order to achieve the extremely low ESR that high-capacity devices often require, capacitors need to be surge tested/screened. Under voltage derating, the typical failure rate is 5FIT (one failure per billion hours of operation) to 15FIT, and their electrical characteristics are very stable over time and temperature.

High capacity is the main feature of aluminum electrolytic capacitors; however, temperature has a great impact on device performance, and the operating temperatures of different product series are 85C, 105C, 125C and 150°C. Within the entire rated temperature and ripple current range, the normal wear life of the device is 10,000 hours, so current screening is not required. The service life can be extended by reducing any one parameter.

Ceramic capacitors do not have to take voltage derating to ensure reliability, but the voltage coefficient of capacity must be considered because capacitors can lose up to 40% of their capacity when operating at or near rated voltage. Typical failure rates are less than 1FIT, and some ranges can easily operate at 150°C. Failure modes are short circuit or parameter drift.

Finally, polyester film capacitors are typically rated for 105°C, although PPS devices can operate up to 125°C (PET) or even 150°C (PEN). No voltage derating is necessary, and the typical failure rate is about 5 FIT, but the table

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The importance of these characteristics depends on the application and the required size, cost and manufacturing process. However, these characteristics do apply to general technology selection when considering the actual circuit function. Power supply filtering requires high capacitance, low ESR, and high temperature resistance, which is suitable for tantalum, aluminum and some ceramic capacitors. Large-capacity energy storage requires high capacitance and low ESR to meet the needs of fast discharge and pulse applications, for which tantalum, aluminum and some polyester film capacitors are widely used. Tuning and clock circuits require capacitance that is very stable over temperature and frequency, and must be repeatable under thermal cycles. Class I (C0G/NP0 and high Q) ceramic and polyester film capacitors are usually the best solution. Decoupling/bypassing functions require very low ESR and good impedance (Z) performance. Ceramic, polyester film and some specially designed tantalum polymer devices are ideal for this application. X/Y grade safety capacitors for EMI/RFI filtering require high voltage and pulse performance, which can only be used for film and ceramic capacitors.

In summary, capacitor selection is a multi-dimensional process. Each capacitor has its own electrical characteristics, performance weaknesses, as well as mechanical characteristics and economic considerations. The importance of each of these depends on the application, environmental conditions, and actual circuit functions. Since there are many capacitors to choose from, it is important to refer to each manufacturer's technical specifications to select the appropriate capacitor.