Improving Automotive System Performance with Tantalum and Niobium Oxide Capacitors

Driven by user demands for greater comfort, safety and functionality, the automotive industry has become a rapidly growing market sector. A wide variety of electronic systems are installed in modern cars, many of which directly affect the reliability of the car and the safety of the passengers. This, in turn, means that the electronic components used in key electronic systems must be extremely reliable. Capacitors are widely used in many automotive electronic systems, and their quality, stability and reliability must be strictly controlled by automobile manufacturers.

If we compare tantalum and niobium oxide capacitors to other capacitor technologies, we can see many significant advantages.

Compared with aluminum electrolytic capacitors

Unlike aluminum electrolytic capacitors, tantalum and niobium oxide capacitors do not experience dry-out and therefore have more stable electrical parameters (capacitance, ESR, leakage current, etc.), which means that their performance and functionality can remain consistent and reliable over a long period of time.

Compared with multilayer ceramic chip capacitors

Tantalum and niobium oxide capacitors do not have any piezoelectric effect, which can generate unwanted additional noise in multilayer ceramic capacitors (MLCCs). Another advantage of tantalum and niobium oxide devices is better parameter stability over a certain temperature and DC bias range. Broadband coupling requires high-end capacitors to be stable over a certain temperature and DC voltage range. Due to the absence of piezoelectric effect, the capacitance value changes less with temperature and is independent of voltage, making tantalum or niobium oxide capacitors more suitable for coupling circuits than MLCCs.

In addition, AVX's tantalum capacitors and niobium oxide (OxiCap) capacitors are produced according to the automotive quality standard TS16949. Each series of capacitors fully meets the AEC-Q200 technical requirements and is ideal for automotive applications.

AVX Tantalum and Niobium Oxide Capacitors Application Guide

In order to properly use tantalum and niobium oxide capacitors in a design, we must fully consider all important electrical and physical conditions of the target circuit and device. The input parameters usually require the capacitance value, which can be calculated based on the power line filtering ratio, maximum voltage drop, etc. Another important parameter to consider for the correct selection of capacitors is the DC working voltage. It is important to use this general rule for the recommended voltage derating, which is 50% for all tantalum capacitors and 20% for niobium oxide capacitors, which means that the working voltage of tantalum capacitors is up to half of the rated voltage Vr and the working voltage of niobium oxide capacitors is 80% of its rated voltage. It is important to follow this rule because it protects the device from unexpected current surges and overvoltages, which are likely to occur in automotive circuits. However, tantalum capacitors used in main output circuits with derating voltage have good isolation from the vehicle battery line, protection in case of overvoltage, and have a slow power-up mode (soft start circuit), such as the output of low-power DC/DC converters. In this case, it is allowed to use a derating as low as 20%. The operating temperature range tells us that the maximum temperature is the main consideration when selecting capacitors, but we must also realize that when the temperature exceeds 85°C, we must use additional voltage temperature derating. The maximum DC voltage allowed for the capacitor at the actual temperature is called the category voltage (the rated voltage is just one of the category voltage values ​​at room temperature 25°C).

If the normal operating temperature exceeds 85°C, the operating derating and temperature derating should be considered together. For example, in a circuit where surges and voltage spikes may occur, the tantalum capacitor with a maximum operating temperature of 125°C: the operating derating is 50%, that is, the maximum voltage is 50% of the rated voltage Vr, and the temperature derating at 125°C (in the worst case) is 33%, that is, the maximum voltage is 66% of Vr. The combination of the two is 0.5×0.66=0.33, which means that the tantalum capacitor can be used at a maximum voltage of 33% of the rated voltage Vr (for the worst operating conditions).

Figure 1: Tire pressure monitoring system module diagram.

To avoid power-on or startup current overload of capacitors, it is important to understand the maximum operating surge current (single peak) through the capacitor. This current can be calculated based on the internal voltage of the power supply and the internal resistance of all devices connected in series with the capacitor to be tested (including the effective series resistance ESR). The maximum operating surge current should be less than the maximum allowable surge current of the capacitor Ipmax=(1.1×Vr)/(0.45+ESR). In the case of too high operating current, a larger derating can be adopted. Therefore, the higher the rated voltage selected, the greater the maximum surge current Ipmax of the capacitor (see the figure above).

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The maximum ripple current of a capacitor is the maximum AC current flowing through the capacitor. It has two main parameters: effective value (rms, ACIrms, Ir) and frequency f. The ripple current is limited by the maximum power dissipation Pd caused by the current on the capacitor ESR. The larger the capacitor volume, the greater the allowable power dissipation, and the power dissipation of each volume is constant. The smaller the ESR, the smaller the power dissipation, and the larger the allowable ripple capacitance. See the general formula Pd=ESR×Ir×Ir. For higher ripple current requirements, low ESR, large volume, and possibly multi-anode structures are the best choice.

The combination of the above application rules can help designers select the right capacitor with a specific volume, or put the volume first in the standard, and then make appropriate adjustments according to other priority lists during the design process. There may be a situation where only one capacitor cannot meet the application requirements, so multiple devices need to be used at the same time. At this time, it is basically recommended to integrate the same capacitors. Parallel connection can increase the capacitance value (multiplication) and reduce ESR (division), while serial connection can increase the total allowable DC voltage value (rated voltage multiplication), but it will reduce the capacitance (division) and increase ESR (multiplication). For serial connection, it is recommended to connect the capacitors in parallel with a voltage divider. At this time, the resistance value should be calculated based on 10 times the DC leakage current of the capacitor (category value).

Tantalum Capacitors in Automotive Applications

Standard reliability capacitors, such as AVX's TAJ/TPS (1%/1000 hours), are well suited for general automotive systems. TAJ devices offer standard ESR performance: designers who require lower ESR values ​​should select TPS devices. These capacitors are available in capacities up to 2200μF, with a maximum voltage rating of 50V and an operating temperature range of -55°C to 125°C. The low-ESR TPS series capacitors are well suited for applications that require energy pulses so that the capacitors support the power at the load. Figure A shows a bridge circuit for a stepper motor used to set the seat position. The capacitors are placed next to the motor to help provide current pulses to the motor.

Figure 2: Automobile gasoline engine unit.

Switching mode power supply (SMPS) circuits also require low ESR capacitors at the main output capacitor position. Almost all electronic components used in automobiles adapt to DC/DC power supplies at the power input, and most of them are SMPS, thanks to their high efficiency values. TPS or TPM (multi-anode structure) capacitors used in SMPS help improve efficiency and reduce output ripple voltage (at underload) because of their low ESR.

The TRJ professional tantalum capacitor series developed by AVX is suitable for use in harsh environments that require high performance under electrical and mechanical stress. After a series of technical modifications, the capacitor structure has been further enhanced, making this capacitor have more robust performance in these environments.

There are several key differences between the standard reliability TAJ/TPS and TRJ products that enhance reliability (0.5%/1000 hours): The use of well-tested tantalum powder ensures long-term reliability of electrical performance. Conservative design rules are followed during design and manufacturing. Very strict quality control and additional testing measures are taken. 100% strict surge current screening and the use of extended electrical testing and accelerated burn-in processes enable these capacitors to achieve and verify high robustness. Leakage current is reduced to 75% of the standard tantalum specification, which is a great advantage in battery applications such as tire pressure monitoring systems.

The enhanced performance of the TRJ series capacitors is extremely valuable for high-reliability circuits in automotive electronics, such as ABS and ESP systems, airbag control systems or communication buses (Figure C). Today's cars use more and more electronic devices to solve space constraints and eliminate the weight caused by interconnecting cables. Using a communication bus with fewer cables and reliable under all operating conditions is an obvious solution. Figure C shows the use of TRJ capacitors in bus drivers. TRJ capacitors can speed up digital response due to their good support for the sending amplifier.

Figure 3: Injection and fuel efficiency control unit.

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Standard tantalum capacitor technology typically operates over a -55°C to +125°C temperature range, limiting its use to in-car entertainment environments and other lower temperature applications. Some manufacturers have expanded the use of tantalum capacitors to underhood systems with automotive-specific product lines (Figures 2 and 3), which require continuous operating temperatures up to 150°C. However, the automotive industry requires components to operate up to 175°C. AVX's THJ series capacitors meet this requirement, with an operating range of -55°C to +175°C. The category voltage, which is the maximum operating voltage after accounting for the actual operating temperature, is half the rated voltage at 175°C. THJ series tantalum capacitors also offer greater reliability (failure rate of 0.5%/1000 hours) and a higher category voltage at 125°C (78% of rated voltage) than standard devices (typically only 66% of rated voltage Vr). THJ capacitors are available with a maximum capacitance of 150μF and a maximum rated voltage of 50V.

OxiCap Niobium Oxide Capacitors in Automotive Applications

NOJ and NOS low ESR niobium oxide capacitors use niobium oxide powder as the main material of the anode. Compared with pure metal materials such as tantalum or niobium powder, niobium oxide has a much higher ignition energy (200 times) and a much lower burning rate. This characteristic means that niobium oxide capacitors will not go up to the category voltage. If the circuit is overloaded by voltage spikes or large current surges, another typical failure mode is high resistance (usually 20 to 200 kΩ). This overload will cause increased leakage current and reduced capacitance. Niobium oxide capacitors can continue to provide full capacity and function even when subjected to surges, at the expense of increased power consumption.

NOJ niobium oxide series capacitors have very high reliability (failure rate of 0.5%/1000 hours), while low ESR NOS devices have a lower failure rate (0.2%/1000 hours), which is even more reliable than tantalum capacitors. These capacitors are suitable for applications with rail voltages up to 8V, such as in-car entertainment systems, seat adjustment modules, airbag controls, etc. In addition, niobium oxide devices can provide noise-free performance and have very good temperature stability, which is the preferred feature for high-quality sound and is also a major advantage for car audio equipment.

We can use niobium oxide capacitors in two key places in the signal path of an audio circuit. The first is the input coupling capacitor, which is usually connected to the low impedance output of one audio stage and the high impedance input of another audio stage. Capacitors as low as 1 to 10 μF are commonly used here. The second place is the output coupling capacitor, which is usually connected to the low impedance output of the amplifier through a DC blocking capacitor to the output electro-acoustic transducer, such as headphones or speakers. This application requires the use of much larger capacitors: hundreds to thousands of μF.

In summary, tantalum and niobium oxide capacitors are well suited for use in all modern automotive electronic systems, whether these systems require higher reliability, wider temperature range or lower leakage current.