What are polymer capacitors?
Polymer capacitors are capacitors which use conductive polymers as the electrolyte. They use solid polymer electrolytes instead of liquid or gel electrolytes that are found in ordinary electrolytic capacitors. By using solid electrolyte, the electrolyte drying is completely avoided. Electrolyte drying is one the factors that limit the lifetime of ordinary electrolytic capacitors. There are several types of polymer capacitors, including aluminium polymer capacitors, polymerized organic semiconductors and conductive polymer capacitors.
In most cases, polymer capacitors can be used as direct replacements for electrolytic capacitors, as long as the maximum rated voltage is not exceeded. The maximum rated voltage of solid polymer capacitors is lower than the maximum voltage of classical electrolytic capacitors: usually up to 35 volts, although some polymer capacitors are made with maximum operating voltages of up to 100 volts DC.
Polymer capacitors have a number of qualities superior to ordinary electrolyte capacitors: longer lifetime, higher maximum working temperature, better stability, lower equivalent series resistance (ESR) and a much safer failure mode. These qualities come at a price of lower maximum voltage rating and a narrower capacitance range, as well as a higher cost compared to wet electrolyte capacitors. This type of capacitor is not that new: production started in the 1980s and since then, they have been used in many applications including server motherboards and computer graphic accelerator cards.
Polymer capacitor definition
A polymer capacitor is a capacitor which uses solid polymers as the electrolyte. They have a number of superior qualities including a safer failure mode, lower losses and a longer lifetime than electrolytic capacitors.
Equivalent series resistance
Compared to ordinary electrolytic capacitors, polymer capacitors have a lower equivalent series resistance. This allows polymer capacitors to withstand higher ripple currents during normal operation. A ripple current is the AC component which causes the internal resistance of a capacitor to dissipate power and thus heat up the capacitor. The ESR of polymer capacitors is nearly constant within its operating temperature range, while the ESR of an electrolytic capacitor noticeably changes with temperature.
Reliability and failure modes
The use of a solid electrolyte is a major advantage over electrolytic capacitors. In a wet electrolytic capacitor, overheating can cause the electrolyte to evaporate. As it evaporates, pressure builds up within the capacitor and it may burst or even explode. Solid polymer capacitors don’t have such risk, and their failure mode is much safer – the capacitor either shorts or starts acting like an open circuit. Generally speaking, the reliability of polymer capacitors is much better than the reliability of electrolyte capacitors.
In addition to better reliability and safer operation, solid polymer capacitors have a better lifetime because the solid electrolyte used in their construction cannot dry out. This type of capacitor can operate for extended periods of time at higher temperatures compared to electrolytic capacitors.
Capacitance range, voltage rating and polarization
Polymer capacitors are made with capacitances between 10µF and 1mF. The typical maximum voltage rating is up to 35 V, but there are polymer capacitors with maximum operating voltages reaching 100 V. Just like ordinary electrolytic capacitors, these polymer capacitors are usually polarized.
Construction and properties of polymer capacitors
Polymer capacitors are made by sandwiching a separator sheet containing the electrolyte between two aluminum or tantalum foils. The assembly is then rolled to achieve a cylindrical shape. Electrodes are added and the complete assembly is then encased in an aluminum casing. Since solid polymer capacitors are not regarded as components which are likely to be replaced in a device, they are often made in SMT (Surface-Mount Technology). This allows them to take up less space on the PCB (Printed Circuit Board) at the expense of being harder to unsolder if replacements are necessary.
Ordinary electrolytic capacitors have vented casings or casings with a pre-scored surface which is supposed to break in case of over pressure and vent the gases forming inside the casing in a safe way to prevent explosion in case of overheating. There are usually no such scorings or vents on polymer capacitors as there is no significant pressure buildup in case of failure.
Applications for polymer capacitors
Due to their low ESR, polymer capacitors are used in applications which allow for a large ripple current. An example of such an application is a switched DC-DC converter. Great examples are Buck, Boost and Buck-Boost converters, which hold the voltage at the capacitor relatively constant, but produce a high ripple current. Using a capacitor with low ESR is preferable in such cases, both to improve power efficiency and to increase safety in cases of overload and overheating.
Solid polymer capacitors can be used to smooth out the voltage from a power source to a sensitive circuit, reducing power noise. In such applications, they can easily replace standard electrolytic capacitors, provided that the operating voltage is sufficiently low.
They can also be used for power bypass and signal decoupling, in order to reduce signal noise and power noise generated by the device which would otherwise be transferred to the power source and might affect other devices connected to that power source.
Polymer capacitors can often be found on computer motherboards, especially higher quality ones, such as server motherboards, where they have largely replaced wet electrolytic capacitors.