How to choose reliable capacitors for automotive applications
Article By : Andrew Wilson
Automotive engineers can review characteristics of the four major capacitor dielectric types: tantalum, aluminum, poly-films, and ceramics.
Choosing a capacitor for reliable performance in today’s automotive electronics requires an examination of several parameters. The performance characteristics of the various capacitor technologies must be understood first. Following this, the automotive environment and specific application must be considered in order to determine the most cost-effective and reliable solution.
This article will look at the characteristics of the four major capacitor dielectric types: tantalum electrolytics, aluminum electrolytics, poly-films, and ceramics. In addition, the automotive environment will be described, and the general categories for automotive applications will be listed.
Figure 1 shows the typical capacitance and voltage ranges for some of the more popular types of capacitor dielectrics. It’s interesting to note that for applications requiring capacitance values from about 0.1 μF to 100 μF, and voltages of less than 50 V, there are several overlapping choices. To further understand the performance characteristics of these various capacitor types, we will need to cover a few of the capacitor basics.
Figure 1 Capacitor technology comparisons highlight overlapping choices with voltage and capacitance value ranges. Source: Vishay
Figure 2 shows the typical dielectric constant (K) and dielectric strength values for the four basic capacitor types. A combination of low K and low dielectric breakdown strength—such as the case with poly-film capacitors—results in low volumetric efficiency. However, physical size is only one characteristic of a given capacitor type. For example, although film capacitors are rather large in size, they offer extremely high efficiency and stable electrical characteristics.
Figure 2 Capacitor dielectric characteristics are critical in making the right design choice. Source: Vishay
The equivalent circuit for any capacitor is shown in Figure 3. The equivalent series resistance (ESR) is the real part of the impedance and represents losses in the capacitor. The ESR value 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. The leakage current is typically much lower for film and ceramic (electrostatic) capacitors than for tantalum and aluminum (electrolytic) types. DC leakage varies with temperature and the magnitude of applied voltage.
Figure 3 The capacitor equivalent circuit highlights the roles of ESR and IR. Source: Vishay
The formulas provided in Figure 4 mark important capacitor relationships: capacitive reactance, dissipation factor, inductive reactance, and impedance. It’s important to note that the resistor used to model IR is a very high value resistor, so it’s neglected for simplicity in the derivation of overall impedance (Z).
Figure 4 Formulas shown above embody important capacitor relationships. Source: Vishay
Z is important in determining how the capacitor affects incoming signals. During charge/discharge cycles, low ESR is critical for achieving high efficiency, low heating loss, and reliability. Capacitive reactance (XC ) and inductive reactance (XL) tell us something about the energy storage capacity and inductive field generation of the device. Note that when XC and XL are equal, the resonant frequency of the device is achieved. It’s important when choosing a decoupling capacitor to remove AC components/noise from a DC signal. To efficiently remove AC signal components from a DC power rail, select a capacitor with a resonant frequency near the frequency of the unwanted AC noise for minimum impedance and maximum decoupling to ground.
Automotive applications for electronic components can be categorized into six general areas:
- Powertrain control: electronic engine, transmission, and emission control. Today’s push toward electric vehicles adds many new opportunities for power conversion and control electronics.
- Vehicle control: antilock brakes, active suspension, traction control, power and 4WD steering
- Safety, comfort, and convenience: air bag actuators, collision avoidance, climate control, cruise control, and anti-theft
- In-car entertainment
- Driver information displays and audio warning systems
- Diagnostics and repair
Some automotive environmental conditions are more demanding than others. Figure 5 characterizes under-the-hood and passenger compartment conditions.
Figure 5 A sneak peek of the automotive environment highlights under-the-hood conditions. Source: Vishay
Having described the primary automotive environments and applications, we will now look at the four major capacitor technologies and describe the characteristics that will affect circuit performance and long-term reliability.
In the most general of classifications, most capacitors fall into one of two basic categories of construction: electrostatics (poly-films and ceramics) and electrolytics (tantalums and aluminums). Electrostatic capacitors are non-polarized devices that typically exhibit very low ESR and impedance. Electrolytics generally offer higher capacitance values, but are polarized.
Tantalum electrolytics
- Rated voltage from 2.5 VDC to 63 VDC SMD and 125 V axial leaded. Note: for best reliability, derate the application voltage to 50% of the rated voltage for solid tantalum, and 80% for tantalum polymer and wet slug axial tantalum.
- Very stable electrical characteristics over time and temperature
- Capacitance values up to 2,200 μF for SMD and 10,000 μF for axial wet tantalum
- Surge test/screen and larger SMD case sizes (low ESR and high capacitance)
- Typical failure rates from 5 FIT to 15 FIT (failures per billion hours) with normal voltage derating
Aluminum electrolytics
- Rated voltage from 6.3 VDC to 450 VDC for SMD devices and higher voltages for large can styles
- Available in 85°C, 105°C or 120°C temperature ratings
- SMD capacitance up to 10 mF
- No need to surge current screen
- Aluminum electrolytics have a natural wear-out mechanism that could limit their useful life to 5,000 hours under full-rated voltage and maximum temperature conditions. For 2x longer life, derate to 80% of rated voltage.
Ceramic electrostatics
- Rated voltage from 6.3 VDC to 5,000 VDC (most usage is 100 V or less); no voltage derating is necessary, but the voltage coefficient of capacitance must be considered. MLCCs may lose up to 40% of their effective capacitance value when operated at or near rated voltage.
- Operating temperatures may exceed 150°C
- Non-polar (may be bulk fed for high-speed insertion)
- Very low ESR and DC leakage
- Typical failure rates under 1 FIT; typical failure mode is short or parametric shift
Poly-film electrostatics
- Rated voltage from 16 VDC to 2,000 VDC; no voltage derating necessary.
- Operating temperatures up to 105°C for most types (125°C for PPS)
- Ultra-low ESR and DC leakage
- Typical failure rates under 5 FIT; typically fail open or parametric shift.
- Surface-mount offerings limited
General selection guidelines
The characteristics listed above will help design engineers make general decisions regarding the choice of a capacitor. Cost, size, and manufacturability are also important factors.
It’s not always easy to determine which capacitor type will best suit a given application. A few general guidelines are offered below for the primary types of circuit applications found in automotive and other electronic circuits.
- Power filtering: High capacitance, low ESR, and high-temperature capability—tantalum, aluminum and some ceramics and poly-films
- Bulk energy storage: High capacitance, low ESR for quick discharge and pulse applications—tantalum, aluminum and some poly-film
- Tuning and timing: Stable capacitance values across temperature and frequency and repeatable under thermal cycle—ceramics (NP0 type) and poly-films
- Decoupling and bypass: Very low ESR and good Z characteristics—ceramics and poly-films
Choosing a capacitor is a multidimensional problem; each capacitor type has its own set of characteristics that may make it the most logical choice for a given application. A capacitor’s cost, size, packaging type, and end of life reliability issues are important considerations. With many choices available, it is essential to reference each manufacturer’s specifications for the right capacitor.
This article was originally published on EDN.
Andrew Wilson is senior manager of product marketing at Vishay’s Tantalum Capacitor Division.