【Introduction】Selecting reliable capacitors for today’s automotive electronics applications requires understanding the performance characteristics of each capacitor and the operating conditions in different applications. The operating environment in the application has a greater impact on the actual circuit performance than the specifications in the data sheet, so it is critical to the development of the best price/performance solution.
Capacitance medium capacity and voltage range
Figure 1 shows typical capacitance and voltage ranges for common capacitive media, often with several overlapping options.
Figure 1: Capacitance Diagram
While capacity and voltage are often 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 the four basic capacitors can be seen. The combination of low K value and low dielectric strength, such as polyester film capacitors, results in low volume efficiency. However, these larger devices are still widely accepted due to very low losses, very stable electrical characteristics and low cost.
In operation, the equivalent series resistance (ESR) of a capacitor is the real part of the impedance and represents the losses of the capacitor in the equivalent circuit. These parameter values vary with temperature, frequency and media type. Insulation resistance (IR) determines the amount of direct current (DC) leakage current through the capacitor to the applied voltage, whereas electrostatic (film and ceramic) capacitors typically have much lower leakage currents. The DC leakage current varies with temperature and applied voltage, and the inductive reactance is related to the electrode type.
Important Parameters of Capacitors
Figure 3 shows the important relationships of capacitors: capacitive reactance, dissipation factor, inductive reactance, and impedance. Very high resistance resistors are used to simulate insulation resistance. For convenience, it can be ignored when deriving the total impedance (Z).
Impedance is an important metric for determining the effect of a capacitor on the input signal. During charge/discharge cycles, low ESR is critical to achieve high efficiency, low thermal losses, and reliability. The capacitive reactance (XC) and the inductive reactance (XL) represent the energy storage capacity and the induced magnetic field generated by the capacitor. Note that the device resonant frequency is reached when XC and XL are equal. This is important when selecting decoupling capacitors to eliminate AC component noise in DC signals.
In order to effectively eliminate the AC signal component in the DC link, a capacitor whose resonant frequency is close to the AC noise frequency to be removed should be selected to achieve electrically small impedance and electrically large decoupling grounding.
In-vehicle application types are generally divided into powertrain controls (ECU and transmission) and safety and comfort controls (such as airbags and climate 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. Engine bay applications may be exposed to or submerged in salt spray, water, fuel/oil, operating temperatures of 125 °C or higher, vibration forces up to 15 g, and frequencies up to 200 Hz. These situations 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 the operating temperature limit (85 °C).
Characteristics and Applications of Different Capacitors
In general, electrolytic capacitors (tantalum, aluminum, and EDLC) have high capacitance but are polar, while electrostatic capacitors (polyester and ceramic) are non-polar and typically have very low ESR and impedance.
It is recommended to use tantalum devices with voltage derating. The voltage of solid tantalum capacitors is derated by 50%, and the voltage of polymer and liquid tantalum axial capacitors is derated by 80% to ensure reliability. Capacitors need to be surge tested/screened in order to achieve the extremely low ESR that high capacitance devices often require. Typical failure rates range from 5 FIT (one failure per billion hours of operation) to 15 FIT with voltage derating, and their electrical characteristics are very stable over time and temperature.
High capacitance is a key feature of aluminum electrolytic capacitors; however, temperature has a large impact on device performance, with different product families operating at 85 °C, 105 °C, 125 °C, and 150 °C. The device has a normal wear life of 10,000 hours over the entire rated temperature and ripple current range, so current screening is not required. The service life can be extended by reducing either parameter.
Ceramic capacitors do not have to be voltage derated for reliability, but the voltage factor of capacity must be considered, as capacitors can lose up to 40% of their capacity when operating at or near rated voltage. Typical failure rate is less than 1 FIT, and some ranges can easily operate at 150 °C. The failure mode is short circuit or parameter drift.
Finally, polyester film capacitors are typically rated at 105 °C, although PPS devices can operate up to 125 °C (PET) or even 150 °C (PEN). Voltage derating is not necessary, typical failure rate is about 5 FIT, but surface mount products are limited.
The importance of these features depends on the application and required size, cost and manufacturing process. However, these characteristics do lend themselves to general technology choices when considering actual circuit functionality.
Power supply filtering requires high capacity, low ESR, and high temperature resistance, and is suitable for tantalum, aluminum and some ceramic capacitors. Large-capacity energy storage requires high capacity and low ESR for fast discharge and pulse applications, for which tantalum, aluminum, and some polyester film capacitors are widely used. Tuning and clocking circuits require that the capacity be very stable over temperature and frequency and must be repeatable under thermal cycling.
In this regard, Class I (C0G/NP0 and high-Q) ceramic and polyester film capacitors are usually the best solution. The decoupling/bypass function requires very low ESR and good impedance (Z) performance. Ceramic, Mylar and some specially designed tantalum polymer devices are ideal for this application. Class X/Y safety capacitors for EMI/RFI filtering require high voltage and pulse performance, for which only film and ceramic capacitors can be used.
In conclusion, choosing a capacitor is a multidimensional process. Each capacitor has its own electrical characteristics, performance weaknesses, and mechanical and economic considerations. The importance of each of these depends on the application, environmental conditions, and actual circuit functionality. Since there are so many capacitors to choose from, it is important to refer to each manufacturer’s specifications to select the appropriate capacitor.
About the Author: Andrew Wilson
Andrew Wilson is Senior Manager of Product Marketing for Vishay’s Tantalum Capacitor Division. Previously, he served as Regional Business Development Manager for TTI Corporation, Marketing Manager for Sensata Technologies, and Head Marketing Manager for Osram’s North American OEM components Division. Andrew is a senior mechanical engineer with two patents and extensive experience in electronics packaging integration. He holds a BA from Wentworth Institute of Technology and an MBA from Northeastern University.
About the Author: Florian Weyland
Florian Weyland is currently the Product Marketing Manager for Vishay Ceramic Capacitors. He is a member of the German Ceramic Society, a recipient of the Hans-Walter-Hennicke Award, and a 2017 American Winter Symposium Grant. Mr. Weyland holds a Ph.D. from the Technical University of Darmstadt, Germany.
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