Selecting Reliable Capacitors for Automotive Applications
Selecting reliable capacitors for today's automotive electronics requires careful analysis of a variety of parameters. First, the performance characteristics of various capacitor technologies must be understood. Second, the automotive environment and specific application should be considered to find a reliable solution with an excellent cost-effectiveness ratio. This article will explore the characteristics of four major dielectric capacitors: tantalum electrolytic, aluminum electrolytic, film, and ceramic. In addition, the automotive environment will be explained and the general categories of automotive applications will be listed.
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Figure 4
The equations in Figure 4 reveal the important relationships of capacitors: capacitive reactance, dissipation factor, inductive reactance, and impedance. Note: The insulation resistance (IR) is modeled with a very high value resistor and is often ignored when deriving the total impedance (Z) for simplicity.
Z is important in determining the effect of capacitance on the input signal. Low ESR is key to high efficiency, low heat dissipation, and reliability during the charge/discharge cycle. Capacitive reactance (XC) and inductive reactance (XL) represent the device energy storage capability and inductive field generation. Note that the device resonant frequency is reached when XC is equal to XL. This is important when selecting decoupling capacitors to remove the alternating (AC) component/noise from the direct current (DC) signal. To effectively remove the AC signal component from the DC power rail, a capacitor with a resonant frequency close to the frequency of the AC noise to be removed should be selected to minimize impedance and maximize decoupling to ground.
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Powertrain control systems (electronically controlled engines, transmissions, and emissions control). The current development of electric vehicles has created a large number of new opportunities for power conversion and electronic control components.
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Vehicle controls (anti-lock brakes, active suspension, traction control, power steering and four-wheel steering)
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Safety, comfort and convenience (airbag actuators, collision avoidance, air conditioning, cruise control and anti-theft)
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In-car entertainment
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Driver information display and audio warning system
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Diagnosis and repair
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After introducing the key automotive environments and applications, we will examine the characteristics of four major capacitor technologies and their impact on circuit performance and long-term reliability.
Tantalum capacitors
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Rated voltage 2.5 VDC to 63 VDC Surface Mount Device (SMD) and 125V axial leaded type. Note: For best reliability, the operating voltage should be derated to 50% of the rated voltage for solid tantalum capacitors and 80% of the rated voltage for tantalum polymer and wet tantalum axial capacitors
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Electrical characteristics are very stable over time and temperature
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Surface mount devices up to 2200 μF and axial wet tantalum capacitors up to 10,000 μF
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Larger form factor surface mount devices requiring surge testing/screening (low ESR, high capacitance)
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Typical failure rate at normal voltage derating is 5 FIT - 15 FIT (failures per billion hours)
Aluminum capacitors
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Rated voltages 6.3 VDC to 450 VDC (SMD). Large cylindrical aluminum capacitors with higher operating voltages
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85˚C, 105˚C or 120˚C temperature rating
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Surface mount devices up to 10 mF
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No surge current screening required
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Aluminum capacitors have a natural wear mechanism that limits their service life to 5,000 hours at full rated voltage and maximum temperature. Derating to 80% of the rated voltage can extend the life by a factor of 2.
ceramic capacitor
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Rated voltage is 6.3 VDC to 5,000 VDC (most uses are 100 V or less); voltage derating is not required, but the voltage coefficient of capacitance must be considered. When operating at or near rated voltage, the effective capacitance of multilayer ceramic capacitors (MLCCs) may drop by 40%
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Operating temperatures may exceed 150˚C
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Non-polar devices (high-speed plug-in batch feeding possible)
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Very low ESR and DC leakage current
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Typical failure rate is less than 1 FIT; typical failure modes are short circuit or parameter drift
Film capacitors
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Rated voltages from 16 VDC to 2,000 VDC; no voltage derating required
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Most types operate at 105˚C (125˚C for PPS)
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Ultra-low ESR and DC leakage current
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Typical failure rate is less than 5 FIT; typical failures are open circuit or parameter drift
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Surface Mount Products Limited
The above characteristics help design engineers make choices in capacitor selection. In addition, cost, size and manufacturability are also factors to consider.
Choosing the best capacitor for a particular application is often not easy. Below we provide some general guidelines for the main types of circuits found in automotive and other electronic circuits.
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Power supply filtering : high capacity, low ESR, high temperature resistance - tantalum, aluminum capacitors (some ceramic and film capacitors) |
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Large-capacity energy storage : high capacity, low ESR (for fast discharge and pulse applications) - tantalum, aluminum capacitors (some film capacitors) |
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Tuning and Timing : Content values are stable over temperature and frequency range, repeatable under thermal cycling - ceramic capacitors (NP0 type), film capacitors |
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Decoupling/Bypassing: Very low ESR, good Z characteristics - ceramic capacitors, film capacitors |
Andrew Wilson is currently the Senior Product Marketing Manager for Vishay's Tantalum Capacitor Business Unit. Previously, he served as Regional Business Development Manager for TTI, Marketing Manager for Sensata Technologies, and Chief Marketing Manager for Osram's North American OEM Components Division. Andrew is a mechanical engineer with two patents and experience in electronic packaging integration. He graduated from Wentworth Institute of Technology and received an MBA from Northeastern University.
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