Improve the performance of automotive systems based on tantalum capacitors and niobium oxide capacitors
Various electronic systems are equipped in modern cars, many of which directly affect the reliability of the car and the safety of passengers. Conversely, this also means that the electronic components used in critical electronic systems must be very reliable. Capacitors are widely used in many automotive electronic systems, and their quality, stability and reliability must be strictly checked by car manufacturers.
If we compare tantalum capacitors and niobium oxide capacitors with other capacitor technologies, we can find many significant advantages.
Compared with aluminum electrolytic capacitors
Unlike aluminum electrolytic capacitors, tantalum capacitors and niobium oxide capacitors do not have a drying effect, so they have more stable electrical parameters (capacitance, ESR, leakage current, etc.), which means that their performance and function can be maintained for a long time. Consistent and reliable.
Compared with multilayer ceramic chip capacitors
Tantalum capacitors and niobium oxide capacitors do not have any piezoelectric effect. Piezoelectric effects can cause objectionable additional noise in multilayer ceramic capacitors (MLCC). 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 remain stable within a certain temperature and DC voltage range. Since there is no piezoelectric effect, the capacitance value changes little with temperature, and has nothing to do with voltage. Tantalum or niobium oxide capacitors are more suitable for coupling circuits than MLCC.
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 capacitor and niobium oxide capacitor application guide
In order to properly use tantalum capacitors and niobium oxide capacitors in the design, we must fully consider all important electrical and physical conditions of the target circuit and equipment. The input parameter usually needs to provide the capacitance value, which can be calculated based on the power line filter ratio and the maximum voltage drop. Another important parameter to consider when choosing a capacitor correctly is the DC operating voltage. It is important to recommend this general rule for voltage derating. For all tantalum capacitors, the reduction is 50%, and the niobium oxide capacitor is 20%. This means that the operating voltage of the tantalum capacitor is up to half of the rated voltage Vr. The operating voltage is 80% of its rated voltage. Observing this rule is important because it protects the device from accidental current surges and overvoltages, which is likely to happen in automotive circuits. However, the derating voltage of the tantalum capacitor used in the main output circuit is well isolated from the car battery line, it has a protective effect during overvoltage, and has a slow power-up mode (soft start circuit), such as low-power DC / DC conversion The output of the device. In this case, derating as low as 20% is allowed. The operating temperature range tells us that we mainly consider the maximum temperature value when choosing a capacitor, but we must also realize that when the high temperature exceeds 85 ° C, we must use an additional voltage temperature derating value. The maximum DC voltage allowed by the capacitor at the actual temperature is called the category voltage (the rated voltage is only one of the category voltage values ​​at room temperature 25 ° C).
If the normal operating temperature exceeds 85 ° C, the derating of the work and the derating of the temperature should be considered in combination. For example, tantalum capacitors with a maximum operating temperature of 125 ° C in circuits where surges and voltage spikes may occur: the operating derating is 50%, that is, the maximum voltage is 50% of the rated voltage Vr at 125 ° C (worst case) The temperature derating 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: Block diagram of tire pressure monitoring system.
To avoid power-up or starting current overload of the capacitor, it is important to know the maximum working inrush 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 under test (including the effective series resistance ESR). The maximum working surge current should be less than the maximum allowable surge current of the capacitor Ipmax = (1.1 × Vr) / (0.45 + ESR). When the operating current is too high, a larger derating range can be taken, so the higher the rated voltage selected, the greater the maximum inrush current Ipmax of the capacitor (see the figure above).
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 consumption Pd generated by the current on the capacitor ESR. The larger the capacitor volume, the greater the allowable power consumption, and the power consumption of each volume is constant. The smaller the ESR, the lower the power consumption and the larger the allowed ripple capacitance. See the general formula Pd = ESR × Ir × Ir. For the ripple current with higher requirements, the structure with low ESR, large volume and possibly multiple anodes is the best choice.
The combination of the above application rules can help the designer to select a suitable capacitor with a specific volume, or place the volume on the first element of the standard, and then make appropriate adjustments according to other priority lists during the design process. It may happen that there is only one capacitor that cannot meet the application requirements, so it is necessary to use multiple devices at the same time. At this time, basically only the same capacitor is recommended for integration. 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 will reduce the capacitance (division) and increase ESR (multiplication). For serial connection, it is recommended to use a voltage divider to connect capacitors in parallel. In this case, the resistance value should be calculated based on 10 times the capacitor DC leakage current (category value).
Tantalum capacitors in automotive applications
Standard reliability capacitors, such as AVX's TAJ / TPS (1% / 1000 hours), are very suitable for general automotive systems. TAJ devices provide standard ESR performance: designers who need lower ESR values ​​should use TPS devices. The capacitance of these capacitors can be up to 2200μF, the maximum rated voltage is 50V, and the operating temperature range is -55 ℃ to 125 ℃. The low ESR TPS series capacitors are ideal for applications requiring energy pulses so that the capacitors support the power supply at the load. Figure A shows the bridge circuit of the stepper motor used to set the seat position. The capacitor is placed next to the motor to help provide current pulses to the motor.
Figure 2: Automotive gasoline engine unit.
Switch mode power supply (SMPS) circuits also require the use of low ESR capacitors at the location of the main output capacitor. Almost all electronic components used in automobiles are adapted to the DC / DC power supply at the power input, and most of them are SMPS, thanks to their high energy efficiency value. The TPS or TPM (multi-anode structure) capacitors used in SMPS help improve efficiency and reduce the output ripple voltage (under load) because they have a lower ESR.
The TRJ professional tantalum capacitor series products developed by AVX are suitable for use in harsh environments that require high performance under electrical and mechanical pressures. After a series of technical transformations, the capacitor structure has been further enhanced to make this capacitor have more Strong robust performance.
There are several major differences between standard reliability TAJ / TPS and TRJ products. It is these differences that enhance reliability (0.5% / 1000 hours): the use of tantalum powder that has been tested and proven to be effective can ensure Long-term reliability of electrical performance. Follow conservative design rules in the design and manufacturing process. Very strict quality control and additional testing measures have been taken. Strict 100% inrush current screening and the use of extended electrical testing and accelerated burn-in processes make these capacitors achieve and verify high robustness. The leakage current is reduced to 75% of the standard tantalum specifications, which has great advantages in battery applications such as tire pressure monitoring systems.
The enhanced performance of the TRJ series capacitors is extremely valuable for the high reliability circuits of automotive electronic equipment. These circuits include ABS and ESP systems, airbag control systems, or communication buses (Figure C). Today's automobiles use more and more electronic devices in order to solve the problem of space limitation and eliminate the weight caused by interconnecting cables. Using a communication bus that has fewer cables and is reliable under any operating conditions is an obvious solution. Figure C shows the use of TRJ capacitors in the bus driver. Since it can well support the transmission amplifier, TRJ capacitors can speed up the digital response speed.
Figure 3: Injection and fuel efficiency control unit.
The operating temperature range of standard tantalum capacitor technology is usually -55 ℃ to +125 ℃, thus limiting these capacitors can only be used in automotive entertainment environments and other lower temperature applications. Some manufacturers specialize in the launch of automotive series products, and expand the application of tantalum capacitors to the engine compartment system (Figure 2, Figure 3), this environment requires a continuous operating temperature of up to 150 ℃. However, the automotive industry requires components with a maximum operating temperature of 175 ° C. AVX's THJ series capacitors can meet this requirement. The working range of this capacitor is -55 ℃ to + 175 ℃. The category voltage, ie the maximum operating voltage after considering the actual operating temperature, is half of the rated voltage at 175 ° C. THJ series tantalum capacitors can also provide stronger reliability (failure rate is 0.5% / 1000 hours), and the category voltage (78% of rated voltage) at 125 ° C is also higher than that of standard devices (generally only rated voltage Vr 66%) High. The maximum capacity of the THJ capacitor is 150μF, and the maximum rated voltage is 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 feature means that niobium oxide capacitors will not be as high as the category voltage. If the circuit is in an overload state such as voltage spikes or large current surges, then another typical failure mode is high resistance (usually 20 to 200 kΩ). This overload will cause an increase in leakage current and a decrease in capacitance. The niobium oxide capacitor can continue to provide full capacity and function even if it is impacted by the surge, at the cost of increased power consumption.
NOJ niobium oxide series capacitors have very high reliability (failure rate is 0.5% / 1000 hours), while NOS devices with low ESR values ​​have lower failure rates (0.2% / 1000 hours), and are 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 control, etc. In addition, niobium oxide devices can provide noise-free performance and have very good temperature stability, which is the preferred feature of high-quality sound and is also a major advantage for car audio equipment.
We can use niobium oxide capacitors at two key locations in the signal path of the audio circuit. The first is the input coupling capacitor, where the low-impedance output of one audio level is usually connected to the high-impedance input of another audio level. Generally, capacitors as low as 1 to 10 μF are used here. The second place is the output coupling capacitor, where the low-impedance output of the amplifier is usually connected to the output electroacoustic converter through a DC blocking capacitor, such as headphones or speakers. This application requires the use of much larger capacitors: hundreds to thousands of μF.
In short, tantalum capacitors and niobium oxide capacitors are very suitable for use in all modern automotive electronic systems, regardless of whether these systems require higher reliability, a wider temperature range or lower leakage current.
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