The Fundamentals of a Tantalum Capacitor
The dielectric medium of a tantalum capacitor is an amorphous tantalum pentoxide film. When subjected to high forming voltage, this oxide changes into the more conductive crystalline structure.
This change increases the equivalent series resistance ESR of a capacitor and can lead to failure if used in a circuit with large AC ripple. To avoid this, a proper derating design must be used.
Capacitance
The capacitance of a tantalum capacitor is the amount of electric current it can hold in an electrostatic field. It is measured in farads (F) or picofarads (pF). The capacitance of a capacitor depends on its size, temperature, surge and other factors.
The thin oxide insulating layer of a tantalum capacitor is what allows it to offer much higher levels of capacitance for a given volume than other dielectric types. This makes them ideal for use in high-frequency AC circuits, where they can provide high levels of smoothing and filtering.
However, they are not robust enough to withstand surges of power. In such conditions, they can build up heat very quickly and erode the oxide layer prematurely. This in turn reduces the withstand voltage capabilities of the capacitor, so it is important to select the right type for the job.
In order to avoid this, it is a good idea to design the circuit using a series resistance of R>3V. This will reduce the impact of surge on the tantalum capacitor and limit current flow to a safe level.
It is also a good idea to ensure that the capacitors have a polarity mark and are only used in circuits with correct polarity. Attempting tantalum capacitor to operate a tantalum capacitor with reversed polarity, even briefly, depolarizes the dielectric and causes it to break down. This can lead to catastrophic thermal runaway, in which the capacitor forcefully ejects its burning core.
ESR
The ESR of a tantalum capacitor is the product of its capacitance and its self-inductance. It is the voltage drop across the capacitor at normal operating conditions and is the primary factor determining its ability to withstand AC ripple. It is also a critical factor in the reliability of high-frequency circuits.
The low ESR of tantalum capacitors makes them an attractive choice for many applications that require long-hold duration and precise timing. They are frequently used in sample and hold circuits to achieve high capacitance values at very low rates, and for power supply rail decoupling. They are also used for medical and aerospace equipment that requires extremely high quality and reliability.
While the ESR of a tantalum capacitor varies widely between manufacturers due to differences in raw materials, manufacturing processes, and other factors, the impact on a product’s reliability during use is relatively small. However, it is important to note that the loss size of a tantalum capacitor affects its capacity more than the ESR does.
The leakage current of a tantalum capacitor changes significantly as the operating temperature rises. This is the result of the amorphous structure of the dielectric Ta2O5 being changed into a crystallized structure by field and voltage. This change can cause damage ranging from a few burned areas in the oxide to zigzag burn streaks covering large areas of the pellet. Fortunately, solid tantalum has an excellent “self-healing” capability in which the local short circuit is broken down and replaced by a new one within a few milliseconds.
Leakage Current
The leakage current of a tantalum capacitor depends on the temperature. It also increases slightly with humidity. To reduce this phenomenon, it is important to use a voltage conditioner.
The conductivities of conductive polymers are better than that of manganese dioxide, and they are closer to that of metals. This allows the production of smaller, thinner tantalum capacitors with the same capacitance and ESR values. However, these tantalum hybrid capacitors still have a higher leakage current than solid tantalum capacitors.
Another problem with tantalum capacitors is their ripple current handling capability. When the peak current of a circuit exceeds the maximum AC ripple that a tantalum capacitor can handle, it will experience led light driver a sudden breakdown. This happens because of the serious mismatch between the ESR of the tantalum capacitor and that of the circuit.
As all tantalum capacitors are polar components, their polarity must be connected properly to avoid a catastrophic thermal runaway failure. If a tantalum capacitor is operated with its polarity reversed, it will depolarize and the dielectric oxide layer will break down. This can cause the capacitor to become a short circuit or even explode. During highly accelerated life testing (HALT), the leakage currents of some types of tantalum capacitors decrease with time. The variations in these currents are due to the varying effects of annealing and thermally stimulated depolarization (TSD). The degradation of leakage currents has a small impact on product reliability during usage.
Temperature
The temperature of a tantalum capacitor has a great impact on its performance, especially at high frequencies. It can influence the impedance of its pins and internal electrodes. Moreover, it can also increase its ESR. However, the ESR of tantalum capacitors is lower than that of aluminum electrolytic capacitors.
Due to their low ESR and high ripple current handling capabilities, tantalum capacitors are a popular choice for applications that require stability at high temperatures. They can be used in avionics, defense, and oil drilling equipment. The highest recommended temperature for a solid tantalum capacitor is 125degC, but manufacturers are developing surface mount tantalum capacitors that can operate up to 230degC.
The resistance of a tantalum capacitor increases as its operating temperature rises, and this is known as the leakage current temperature curve. It is important to select a tantalum capacitor with a leakage current curve that stays stable at high temperatures, because products that change their leakage current at high temperatures fail more often than those that do not.
In addition, tantalum capacitors should be kept away from heat sources. The temperature of the air around them can also affect their performance, which is why it is important to store them in a cool, dry environment. Also, they should be stored in a well-ventilated area to prevent condensation, which can cause electrical problems and even melt the solder at the sealing tin.
