Tantalum Capacitors

What are Tantalum Capacitors?

Tantalum Capacitors Tantalum capacitors are a sub type of electrolytic capacitors. They are made of tantalum metal, which acts as the anode, covered by an oxide layer that acts as a dielectric, surrounded by a conductive cathode. The use of tantalum allows for very thin dielectric layers. This results in higher capacitance values per volume, excellent frequency characteristics, and excellent stability over time compared to many other types of capacitors. Tantalum capacitors are usually polarized, which means that they can only be connected to a DC power supply observing the correct terminal polarity. The disadvantage of using tantalum capacitors is that their unfavorable failure mode can lead to thermal runaway, fire, and small explosions, but this can be prevented by using external fault protection devices such as current limiters or thermal fuses. Technological advances have allowed tantalum capacitors to be used in a variety of circuits, commonly in laptops, the automotive industry, mobile phones, etc., usually in the form of surface mount devices (SMDs). These surface mount tantalum capacitors take up much less space on printed circuit boards and allow for greater packaging density.

Tantalum Capacitor Definition

Tantalum capacitors are electrolytic capacitors that use tantalum metal as the anode. They are polarized capacitors with excellent frequency and stability characteristics.

Features

General Features

Tantalum capacitors have capacitance values ranging from 1nF all the way up to 72mF, and they are much smaller in size than aluminum electrolytic capacitors of the same capacitance. Tantalum capacitors have voltage ratings ranging from 2V to over 500V. Their equivalent series resistance (ESR) is ten times smaller than the ESR of aluminum electrolytic capacitors, which allows greater current to pass through the capacitor with less heat generated. Tantalum capacitors are very stable over time, and their capacitance does not change significantly over time, especially when compared to aluminum electrolytic capacitors. If handled properly, they are very reliable, and their shelf life is virtually unlimited.

Polarity

Tantalum electrolytic capacitors are abnormally polarized devices. While polarized aluminum electrolytic capacitors can survive a briefly applied reverse voltage, tantalum capacitors are very sensitive to reverse polarization. If a reverse polarity voltage is applied, the dielectric oxide breaks down, sometimes forming a short circuit. This short circuit can later lead to thermal runaway and destruction of the capacitor.

It should be noted that tantalum capacitors are usually marked with their positive terminal, in contrast to aluminum electrolytic capacitors, which have the negative terminal marked on the case.

Failure Modes

Tantalum capacitors have a potentially dangerous failure mode. In the event of a voltage spike, the tantalum anode can come into contact with the manganese dioxide cathode, and if the energy of the spike is sufficient, it can start a chemical reaction. This chemical reaction generates heat and is self-sustaining, and can produce smoke and flames. To prevent this thermal runaway from occurring, external fault protection circuits such as current limiters and thermal fuses should be used with tantalum capacitors.

Construction and Performance of Tantalum Capacitors

Like other electrolytic capacitors, tantalum electrolytic capacitors consist of an anode, some electrolyte, and a cathode. The anode is isolated from the cathode so that only a very small leakage DC current can flow through the capacitor. The anode is made of pure tantalum metal. The metal is ground into a fine powder and sintered at high temperatures into particles. This forms a very porous anode with a high surface area. The high surface area translates directly to increased capacitance values.

The anode is then covered with a layer of insulating oxide, which acts as a dielectric. This process is called anodization. This step must be precisely controlled to reduce tolerances and ensure the correct capacitance value, because the extent of oxide growth determines the dielectric thickness.

In the case of solid tantalum capacitors, the electrolyte is added to the anode by pyrolysis. The solid tantalum capacitor is then immersed in a special solution and baked in an oven to produce a manganese dioxide coating. The process is repeated until a thick coating is present on all inner and outer surfaces of the particle. Finally, the particles used in solid tantalum capacitors are immersed in graphite and silver to provide a good cathode connection. In contrast to solid tantalum capacitors, wet tantalum capacitors use a liquid electrolyte. After the anode is sintered and the dielectric layer is grown, it is immersed in a liquid electrolyte inside a casing. The casing and electrolyte together serve as the cathode in wet tantalum capacitors.

Applications of Tantalum Capacitors

Applications using tantalum capacitors take advantage of their low leakage current, high capacity, and long-term stability and reliability. For example, they are used in sample and hold circuits, which rely on low leakage current for long hold durations. They are also commonly used for power filtering on computer motherboards and cell phones because of their small size and long-term stability, usually in surface mount form. Tantalum capacitors are also available in military specification versions (MIL-SPEC) that offer tighter tolerances and a wider operating temperature range. They are a popular alternative to aluminum electrolytes in military applications because they do not dry out and change capacitance over time. Medical electronics also rely on tantalum for its high stability.