Capacitors are one of the most common components on a circuit board; you will see them next to just about every microchip and in every power circuit. In systems they are commonly used for stability by decoupling AC signals, cleaning up DC power into a chip, bypassing to reduce the impact of the inductance of conductors, and filtering out unwanted frequencies.
As systems have gotten smaller, capacitors have been a limiting factor since a certain size was needed to get the capacitance required, but that changed with the introduction and usage of tantalum capacitors. In this article we will look at some of the unique characteristics of tantalum capacitors and where their strengths and weaknesses are.
What is a Tantalum Capacitor?
What makes tantalum capacitors unique is the usage of tantalum for the anode of the device. This anode is set up as one chunk of material inside of the case, which is very different from the layering style used in MLCC capacitors where the material is sandwiched on itself and then capped on the ends. Using tantalum results in a very thin dielectric layer, resulting in a higher capacitance per volume and allowing for smaller capacitors that can still meet the needs of modern systems. In addition to the nice gain in capacitance per volume, the tantalum capacitors also have very low ESR or Equivalent Series Resistance reducing system losses. A downside of low ESR is that it may be too low to achieve stability in power supply regulators, which needs to be taken into account.
How to Measure Capacitance:
Why Use Tantalum Capacitors?
The tantalum capacitor also has excellent stability characteristics. Across a wide range of temperatures and frequencies as shown in the KEMET graphic above, the tantalum capacitor will maintain the expected capacitance better. With this stability you get more of the expected behavior of your system, which is especially important for filters; if the capacitance varies too much you may lose frequencies you desire. Beyond the electrical differences the construction of the tantalum capacitor also makes it highly resistant to vibration issues, improving overall system reliability.
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The previous characteristics show how tantalum capacitors can be uniquely suited to help in modern electronics, but they are not without their quirks and there are a couple of major ones to take into account when you want to design these in. Tantalum capacitors are generally polarized devices, meaning that during layout and assembly you need to pay more attention to their orientation. This makes them a little more work than a regular ceramic capacitor, which is just a trait of electrolyte base capacitors. When putting tantalum capacitors into a system, you also need to watch out for their failure modes.
Tantalum Capacitor Failure Modes
Due to their chemistry you also need to derate their voltage capabilities – even small overvoltages will lead to a failure known as Field Crystallization. When tantalum capacitors fail they can do so spectacularly in a fiery pop and potentially fail shorted together, completely removing their benefits from the circuit and potentially presenting a short to ground for power. There are many thoughts on how to approach derating but one of our suppliers, AVX, has an excellent paper covering the reasons for different derating numbers called “Voltage Derating Rules for Solid Tantalum and Niobium Capacitors.” The general rule for voltages is to derate a tantalum capacitor with an MnO2 electrolyte by 50% and a polymer electrolyte by 20%.
When to Use a Tantalum Capacitor?
So when do you want to use a tantalum capacitor? When you need maximum capacitance in a small space like decoupling next to a microchip, excellent stability over a range of temperatures or voltages, and you are aware of their unique characteristics so they can be designed in properly and not risk your system with a fiery failure.