Electrolytic Capacitor - Types

Types

Unlike capacitors that use a bulk dielectric made from an intrinsically insulating material, the dielectric in electrolytic capacitors depends on the formation and maintenance of a microscopic metal oxide layer. Compared to bulk dielectric capacitors, this very thin dielectric allows for much more capacitance in the same unit volume, but maintaining the integrity of the dielectric usually requires the steady application of the correct polarity of voltage or the oxide layer will break down and rupture, causing the capacitor to lose its ability to withstand applied voltage (although it can often be "reformed"). In addition, electrolytic capacitors generally use an internal wet chemistry and they will eventually fail if the water within the capacitor evaporates.

Electrolytic capacitance values are not as tightly-specified as with bulk dielectric capacitors. Especially with aluminum electrolytics, it is quite common to see an electrolytic capacitor specified as having a "guaranteed minimum value" and no upper bound on its value. For most purposes (such as power supply filtering and signal coupling), this type of specification is acceptable.

As with bulk dielectric capacitors, electrolytic capacitors come in several varieties:

• Aluminum electrolytic capacitor: compact but lossy, these are available in the range of <1 µF to 1 F with working voltages up to several hundred volts DC. The dielectric is a thin layer of aluminum oxide. They contain corrosive liquid and can burst if the device is connected backwards. The oxide insulating layer will tend to deteriorate in the absence of a sufficient rejuvenating voltage, and eventually the capacitor will lose its ability to withstand voltage if voltage is not applied. A capacitor to which this has happened can often be "reformed" by connecting it to a voltage source through a resistor and allowing the resulting current to slowly restore the oxide layer. Bipolar electrolytics (also called Non-Polarised or NP capacitors) contain two anodized films, behaving like two capacitors connected in series opposition. These are used when one electrode can be either positive or negative relative to the other at different instants, on alternating current circuits. Bad frequency and temperature characteristics make them unsuited for high-frequency applications. Typical ESL values are a few nanohenries.

• Tantalum: compact, low-voltage devices up to several hundred µF, these have a lower energy density and are produced to tighter tolerances than aluminum electrolytics. Tantalum capacitors are also polarized because of their dissimilar electrodes. The anode electrode is formed of sintered tantalum grains, with the dielectric electrochemically formed as a thin layer of oxide. The thin layer of oxide and high surface area of the porous sintered material gives this type a very high capacitance per unit volume. The cathode electrode is formed either of a liquid electrolyte connecting the outer can or of a chemically deposited semi-conductive layer of manganese dioxide, which is then connected to an external wire lead. A development of this type replaces the manganese dioxide with a conductive plastic polymer (polypyrrole) that reduces internal resistance and eliminates a self-ignition failure.
  Compared to aluminum electrolytics, tantalum capacitors have very stable capacitance, little DC leakage, and very low impedance at high frequencies. However, unlike aluminum electrolytics, they are intolerant of positive or negative voltage spikes and are destroyed (often exploding violently) if connected in the circuit backwards or exposed to spikes above their voltage rating.
  Tantalum capacitors are more expensive than aluminum-based (with liquid electrolyte) capacitors and generally only available in low-voltage versions, but because of their smaller size for a given capacitance and lower impedance at high frequencies they are popular in miniature applications such as cellular telephones.

• Solid Aluminum electrolytic capacitor with organic semiconductor electrolyte or OS-CON (which stands for OrganicSemi-CONductive) : a new generation capacitor in which the aluminum foil layers are not immersed in a liquid electrolyte solution but in a solid semiconductive material derived from isoquinoline. The single crystal N-n-butyl-isoquinoline is thermoformed to final shape enhancing its conductivity substantially, thus protecting the capacitor from excess heat surges, and finally the OS-CON cans are sealed with epoxy. These capacitors are stable in use between -55°C to practically 125°C in theory. The main advantages of using this particular semiconductor are fairly low ESR, wider frequency range and greater stability in use compared to liquid electrolyte aluminum and tantalum solid polymer capacitors. OS-CON capacitors are often found as SMD.