Ⅰ. The concept of tantalum capacitors
Ⅱ. The characteristics of tantalum capacitors
Ⅲ. Working principle of tantalum capacitors
Ⅳ. The main performance parameters of tantalum capacitors
Ⅴ. The advantages and disadvantages of tantalum capacitors
Ⅵ. Five functions of tantalum capacitors in the circuit
Ⅶ. Causes of failure of tantalum capacitors
Ⅷ. Precautions for the use of tantalum capacitors
Ⅰ. The concept of tantalum capacitors
Tantalum capacitors are a type of electrolytic capacitors. It uses the metal tantalum as a dielectric, hence the name. Tantalum capacitors were first developed by Bell Laboratories in the United States in 1956. It has excellent performance and is a passive component in a capacitor that is small in size but can achieve a large capacitance. Tantalum capacitors come in a variety of shapes and are manufactured as small and chip components suitable for surface mounting. It is not only used in military communications, aerospace and other fields, but its application scope is also being extended to industrial control, video equipment, communication instruments and other fields.
Ⅱ. The characteristics of tantalum capacitors
1. Tantalum Capacitors Polarity
Tantalum electrolytic capacitors have high polarizability. While polarized aluminum electrolytic capacitors can withstand transient reverse voltages, tantalum capacitors are extremely sensitive to reverse polarization. When a voltage of the opposite polarity is applied, the dielectric oxide decomposes, causing a short circuit. This short circuit can lead to thermal runaway and destruction of the capacitor in the future. In contrast to aluminum electrolytic capacitors, the positive terminal of tantalum capacitors is usually marked on the case.
2. General characteristics
Tantalum capacitors have capacitance values between 1nF and 72mF, which are significantly smaller than aluminum electrolytic capacitors of the same capacitance. Tantalum capacitors are rated from 2V to over 500V. Their equivalent series resistance (ESR) is 10 times lower than aluminum electrolytic capacitors, allowing higher currents to pass through the capacitor while generating less heat. Tantalum capacitors are very stable over time compared to aluminum electrolytic capacitors. Its capacitance does not change significantly over time. When handled properly, they are very reliable, and their shelf life is virtually unlimited.
3. Tantalum capacitor failure mode
According to a paper published by ASM International, the failure modes of tantalum capacitors fall into three broad categories.
(1) Low capacitance/open circuit
This failure is uncommon because the capacitance of tantalum capacitors does not shift under normal operating conditions. A low capacitance value of a tantalum capacitor in any application may indicate a shorted capacitor. Whereas an open circuit fault may be caused by damage to the positive wire and wire connection.
Tantalum capacitors are known to have potentially dangerous failure modes. During voltage spikes, the tantalum anode may come into contact with the manganese dioxide cathode. If the spike energy is sufficient, a chemical reaction may occur. This chemical reaction generates heat and is self-sustaining, while it may also produce smoke and flames. External fault protect circuits, such as current limiters and thermal fuses. They should be used in conjunction with tantalum capacitors to avoid thermal runaway.
(2) High leakage/short circuit
High leakage currents can be caused by applied reverse voltage, which is common during troubleshooting, failure or bench testing. Short-circuit failures occurred in tantalum capacitors containing crystallization due to heating of the cathode due to hot spots formed during the crystallization process.
(3) High equivalent series resistance (ESR)
Mechanical/thermomechanical has a significant impact on the ESR of capacitors when they are exposed to board mount, pick and place, reflow soldering and operational lifetime. External or internal relationships are often compromised by this form of stress, resulting in high ESR.
Ⅲ. Working principle of tantalum capacitors
The working principle of tantalum capacitors is based on the principle of metal capacitors, that is, a fixed electric field is formed between two electrodes. When the capacitor is charged, the positive electrode will attract negative ions, while the negative electrode will naturally flow to positive ions. This results in the formation of a nearby electric double layer region of uniform thickness, the metal capacitor region. In this process, the tantalum metal foil acts as an electrode. The dielectric material can be non-conductive materials such as oxides and nitrates.
Ⅳ. The main performance parameters of tantalum capacitors
The main performance parameters of tantalum capacitors are loss, rated capacity, leakage current, withstand voltage, equivalent series resistance ESR and nominal capacitance value.
1. Loss
A measure of energy loss in a capacitor. It is defined as the ratio of the energy dissipated by a tantalum capacitor to the energy stored. Losses are also frequency and temperature dependent.
2. Rated capacity
In the actual working state, the capacity can determine the response speed and amplitude of the filtered signal. In the pulse charge and discharge circuit, it can determine whether the output current waveform meets the requirements. However, unless the signal to be filtered by a very high operating frequency circuit is weak, high capacity accuracy must be required. For filtering and pulse charging and discharging in general DC-DC circuits, small deviations in capacity will not affect the use effect at all, let alone affect reliability. Therefore, in actual use, the deviation of capacity will not have any impact on reliability.
3. Leakage current
It depends on the applied voltage, the elapsed time after applying the voltage and the temperature of the component. The test condition is to apply the rated voltage, and the leakage current will increase with the increase of temperature and applied voltage.
4. Pressure resistance
It refers to the maximum DC voltage at which a tantalum capacitor can work continuously and stably at 85°C. This is a design value and is the basis for determining other parameters of tantalum capacitors.
5. ESR
It is a combination of capacitive resistive losses, including internal resistance and contact resistance within the component, viscous forces within the capacitive dielectric, and defects caused by bypass current channels. It is mainly the cathode material of the tantalum capacitor that plays a dominant role, and the ESR will decrease with the increase of the frequency. The level of ESR mainly affects the size of the capacitor's anti-surge current capability.
6. Nominal capacity
It refers to the nominal rated capacity of tantalum capacitors. The test conditions are 20°C, 120Hz frequency and 1V applied signal. The capacitance of tantalum capacitors decreases with frequency and increases with temperature.
Ⅴ. The advantages and disadvantages of tantalum capacitors
1. Advantages
(1) High precision, high reliability and good stability
Tantalum capacitors are more reliable than many other types of capacitors. They provide virtually unlimited life as long as they are operated within their rated range. Unlike aluminum electrolytic capacitors, their use is not limited by time.
(2) Wide operating temperature range
Tantalum capacitors have a wide operating temperature range and can be used in a variety of applications. Tantalum electrolytic capacitors have no electrolyte and are suitable for operation at high temperatures, they are usually suitable for the temperature range of -55 to +125 degrees Celsius. This makes them an excellent choice for equipment used in harsh environments.
(3) Small size
Generally, under the same conditions, tantalum capacitors are smaller than aluminum electrolytic capacitors, which is beneficial to system integration.
(4) Good frequency characteristics
Tantalum capacitors use metal tantalum as a medium instead of electrolyte like ordinary electrolytic capacitors. Tantalum capacitors do not need to be wound with aluminum-plated capacitor paper like ordinary electrolytic capacitors. It has almost no inductance, which means that the equivalent ESL of tantalum capacitors is almost very small, so its self-resonant frequency is very large and the frequency characteristics are good.
2. Disadvantages
(1) Tantalum capacitors have a short-circuit failure mode
The open circuit failure mode leads to thermal runaway, fire and micro-explosions. However, this can be avoided by using external fail-safe devices such as current limiters or thermal fuses. Attention must be paid to the open circuit failure mode of tantalum capacitors, and circuits with high safety levels should be "used with caution".
(2) The ability to withstand current is weak
For example, almost no tantalum capacitors are used in the filter capacitors after bridge rectification. Because the voltage fluctuation after bridge rectification is large, and the current has spikes.
(3) The use of rare metal tantalum is expensive.
(4) Not resistant to reverse or overvoltage
Tantalum capacitors are sensitive to reverse voltage and overvoltage. They are even vulnerable to surges. Tantalum capacitors will explode under excessive voltage or reverse voltage, which is quite dangerous.
Ⅵ. Five functions of tantalum capacitors in the circuit
1. Play the role of compensating power factor;
2. It is often used on the ground line, relative to the bypass;
3. It is the most basic energy storage component, which only stores energy without producing and consuming energy;
4. Sometimes it is also used in DC circuits, generally as a function of blocking DC;
5. It is mainly used in pulse circuits and AC circuits, and often plays the role of oscillation, bypass, filtering, coupling, etc.
Ⅶ. Causes of failure of tantalum capacitors
(1) Dielectric breakdown (ignition) caused by reduced dielectric properties or excessively high applied voltage;
(2) Excessive current (surge current) exceeds the instantaneous power dissipation capacity of the local area of the medium, resulting in thermal runaway;
(3) Thermal runaway caused by excessive ripple current;
(4) Overheating due to high-resistance contact between the terminal and the electrode, or the polarity of the applied voltage is reversed.
2. Dielectric breakdown
Dielectric breakdown occurs due to excessive applied voltage or gas ionization inside the capacitor.
3. Open circuit
The causes and mechanism of the open circuit fault are as follows:
(1) The internal connection of the capacitor is separated due to temperature cycling or mechanical vibration;
(2) The internal electrical contacts of the capacitor are melted or vaporized due to strong heat or high current.
Ⅷ. Precautions for the use of tantalum capacitors
1. Do not exceed the rated voltage, otherwise it may cause a short circuit.
2. When designing, a certain margin should be left for the capacity, withstand voltage, impedance, etc. of the capacitor to make it safer and more reliable.
3. Confirm that the temperature range used is within the operating temperature range of the capacitor. The supply current does not exceed the allowable ripple current. When the allowable ripple current is exceeded, heat will increase inside the capacitor, reducing its service lifespan.
4. Limit rapid charging or discharging. It is recommended to add a current limiting resistor in the charging and discharging circuit to make the impact current small.
5. Circuits that cannot be applied: high-impedance voltage holding circuits, coupling circuits, time constant circuits, circuits that affect leakage current, and circuits that increase withstand voltage in series.
6. It is recommended that the voltage applied to the capacitor be 90% of the rated voltage. If the rated voltage is greater than 10V, it is better to take 80% of the rated voltage; if the DC voltage is added to the alternating voltage, the peak voltage cannot exceed the rated voltage; if the total DC voltage is added to the negative peak alternating voltage, it is not allowed to appear negative pressure.
7. Tantalum capacitors are polarized electrolytic capacitors, so the polarity cannot be reversed when used. Reversed polarity will increase leakage or may cause short circuit, or even smoke and explosion.
