A polymer capacitor, or more accurately a polymer electrolytic capacitor, is an electrolytic capacitor (e-cap) with a solid conductive polymer electrolyte. There are four different types:
Polymer tantalum electrolytic capacitor (Polymer Ta-e-cap) Polymer aluminium electrolytic capacitor (Polymer Al-e-cap) Hybrid polymer capacitor (Hybrid polymer Al-e-cap) Polymer niobium electrolytic capacitors Polymer Ta-e-caps are available in rectangular surface-mounted device (SMD) chip style. Polymer Al-e-caps and hybrid polymer Al-e-caps are available in rectangular surface-mounted device (SMD) chip style, in cylindrical SMDs (V-chips) style or as radial leaded versions (single-ended). Polymer electrolytic capacitors are characterized by particularly low internal equivalent series resistances (ESR) and high ripple current ratings. Their electrical parameters have similar temperature dependence, reliability and service life compared to solid tantalum capacitors, but have a much better temperature dependence and a considerably longer service life than aluminium electrolytic capacitors with non-solid electrolytes. In general polymer e-caps have a higher leakage current rating than the other solid or non-solid electrolytic capacitors. Polymer electrolytic capacitors are also available in a hybrid construction. The hybrid polymer aluminium electrolytic capacitors combine a solid polymer electrolyte with a liquid electrolyte. These types are characterized by low ESR values but have low leakage currents and are insensitive to transients, however they have a temperature-dependent service life similar to non-solid e-caps. Polymer electrolytic capacitors are mainly used in power supplies of integrated electronic circuits as buffer, bypass and decoupling capacitors, especially in devices with flat or compact design. Thus they compete with MLCC capacitors, but offer higher capacitance values than MLCC, and they display no microphonic effect (such as class 2 and 3 ceramic capacitors).
History Aluminium electrolytic capacitors (Al-e-caps) with liquid electrolytes were invented in 1896 by Charles Pollak. Tantalum electrolytic capacitors with solid manganese dioxide (MnO2) electrolytes were invented by Bell Laboratories in the early 1950s, as a miniaturized and more reliable low-voltage support capacitor to complement the newly invented transistor, see Tantalum capacitor. The first Ta-e-caps with MnO2 electrolytes had 10 times better conductivity and a higher ripple current load than earlier types Al-e-caps with liquid electrolyte. Additionally, unlike standard Al-e-caps, the equivalent series resistance (ESR) of Ta-caps is stable in varying temperatures.
During the 1970s, the increasing digitization of electronic circuits came with decreasing operating voltages, and increasing switching frequencies and ripple current loads. This had consequences for power supplies and their electrolytic capacitors. Capacitors with lower ESR and lower equivalent series inductance (ESL) for bypass and decoupling capacitors used in power supply lines were needed. see Role of ESR, ESL and capacitance. A breakthrough came in 1973, with the discovery by A. Heeger and F. Wudl of an organic conductor, the charge-transfer salt TCNQ. TCNQ (7,7,8,8-tetracyanoquinodimethane or N-n-butyl isoquinolinium in combination with TTF (Tetrathiafulvalene)) is a chain molecule of almost perfect one-dimensional structure that has a 10-fold better conductivity along the chains than does MnO2, and has a 100-fold better conductivity than non-solid electrolytes.
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