Tantalum capacitors, their benefits and applications

Tantalum capacitor is an electrolytic capacitor, where porous tantalum metal is the anode, and its Titanium oxide layer acts as dielectric, with a conductive electrolyte cathode (either liquid or solid) surrounding it. They offer high capacitance density by volume, have low ESR, excellent long term stability over its life, and superior frequency performance. They are much smaller than aluminium electrolytic capacitors, and have almost unlimited shelf life. Solid tantalum capacitors are replacing wet aluminium electrolytic capacitors.

The dielectric layer, tantalum oxide Ta₂O₅, has a very high dielectric constant of 25-30. Thickness of this layer is about 1.7 nm/V, with a breakdown voltage of 625 V/μ. This gives a very high capacitance density. Formation voltage is usually mentioned as multiple of rated voltage, known as formation ratio. For example, a 10 V rated capacitor, formed at 40 V, will have a formation ratio of 4.

They are rated for low voltages, which not of concern since working voltages are low in most electronic circuits. These are extensively used in electronic circuits, laptops, mobile phones, automobiles as also military equipment due to their small size as also reliability. Tantalum capacitors commonly have voltage ratings below 35 V.  There is no known wear-out mechanism due to Solid nature of tantalum capacitors.

DC leakage (DCL) of titanium capacitors is a function of dielectric thickness, and is specified as

DCL (μA) = 0.01 x Capacitance (μF) x Voltage

For example, a capacitor of 20μF 10 V rating will have a DCL of 2.0 μA. In most applications, the capacitors are easily recharged to replenish the charge lost to leakage, and is of no concern.

Types of Tantalum capacitors

1. Wet tantalum capacitors:  These can work at high voltages, from 100V to 630 V, with low ESR and lowest leakage current among electrolytic capacitors. They have self-healing properties, allowing thinner dielectric oxide layer, and high capacitance per unit volume.
2. Solid leaded tantalum capacitors: They have higher capacitance density than wet aluminium electrolytic capacitors or solid tantalum type. Higher electron conductivity makes them sensitive to voltage spikes or surge currents.
3. Solid SMD tantalum capacitors: These capacitors use solid electrolyte, and are sensitive to voltage spikes or current surges. Characterized by high capacitance density within a specific temperature range. They are used in several electronic applications, but characteristics are little different from polymer electrolyte capacitors. These are available in standard values from 10μF to 2200 μF.
4. Polymer SMD tantalum capacitors: These have polymer electrolytes, which give them low ESR. These can handle higher ripple currents, making them particularly useful for power supplies in integrated circuits. Polymer SMD capacitors are available from 0.15μF to 1500 μF.

SMD tantalum capacitors are much cheaper than aluminium electrolytic counterpart, and they can also withstand soldering process better. This makes their usage more widespread. The thin titanium oxide does not give high working voltage. Their voltage is mostly limited below 35V. Tantalum capacitors are made in one of three styles;

1. SMD capacitors, forming 80% of total tantalum capacitors
2. Axial-lead capacitors, with solid / non-solid electrolyte.
3. Resin dipped, single ended for RCB mounting tantalum ‘pearls’.

Tantalum capacitor construction

Construction of tantalum capacitor is based on spongy and highly porous structure produced from powder. Tantalum powder is first compressed around a tantalum wire, and then sintered at high temperature to create the anode to create a highly porous structure. Size and shape of powder, and sintering process have a bearing on voltage and capacitance density.

The tantalum is then subject to anodizing process by passing a controlled current through it. This forms a layer of Ti₂O₅ dielectric on the surface throughout the volume. The oxide layer, formed as a result of oxidation by current, acts as a semiconductor, and is stable only in one direction. Reversing the current can destroy the layer. Hence titanium electrolytic capacitor is polar in nature.

Cathode is made of manganese dioxide deposited over the dielectric, followed by layers of other materials like carbon and silver. This layer of MnO₂ helps correct any defects developed in capacitor in service (self-healing). A process of ‘burning out’ (self-healing) is carried out during manufacture. Defective components can be removed at this stage.

Wet tantalum electrolytic capacitors also use a sintered tantalum pellet embedded in wire. they use a tantalum outer can, and gelled sulfuric acid electrolyte. They have low ESR since Tantalum oxide is inert and stable in strong acids.

SMD Tantalum Capacitors

Tantalum capacitors in SMD form, being quite small with high capacitance density, are quite popular in electronic, power filtering, mobile phone, computers etc. They have contact surfaces on both sides for surface mounting on PCB. These capacitors are a good alternative to ceramic capacitors in many applications. Ninety percent of tantalum capacitors manufactured worldwide today are SMD capacitors

Benefits of tantalum capacitors

• High volumetric capacitance density: Thin dielectric layer, with high dielectric constant gives a smaller volume of capacitor than aluminium electrolytic capacitors. This makes them preferred choice for mobile phones, tablets, laptops and several PCB mount circuits where space is a constraint. This facilitates miniaturization of devices.
• Low ESR: This means less heat generation compared with aluminium electrolytic capacitors.
• Low Noise generation: They do not resonate during voltage fluctuations, and there is no undesired noise generation.
• Good frequency characteristics than most other electrolytic capacitors: This makes them suitable where goof frequency response is desired.
• Higher service temperature ratings: Ambient temperature ratings cab be from -55℃ to +125℃, or even higher. Some designs may stand up to 175-200℃ (hermetically sealed solid capacitors). In fact, voltage withstand capacity (BDV) goes up a bit at low temperatures.

Limitations of tantalum capacitors

• High cost: They are costlier than aluminium electrolytic or ceramic capacitors.
• Sensitive to higher voltages: Rated voltages must not exceed, else they can get damaged.
• Smaller range of capacitance values: Range of values for aluminium electrolytic capacitors is much larger than tantalum.
• Lower ripple current ratings: Low ripple current capability comes from small size. This factor limits their use for high current levels.