Polymer capacitors and their role

What are polymer capacitors?

Polymer capacitors are capacitors that use conductive polymers as electrolytes. They use solid polymer electrolytes instead of liquid or gel electrolytes in ordinary electrolytic capacitors. By using solid electrolytes, electrolyte drying is completely avoided. Electrolyte drying is one of the factors that limit the life of ordinary electrolytic capacitors. There are several types of polymer capacitors, including aluminum 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 that of traditional electrolytic capacitors: usually up to 35 volts, although some polymer capacitors have a maximum operating voltage of up to 100 volts DC.

Polymer capacitors have many qualities that are superior to ordinary electrolytic capacitors: longer life, higher maximum operating temperature, better stability, lower equivalent series resistance (ESR), and safer failure modes. These qualities come at the cost of lower maximum rated voltages and narrower capacitance ranges, as well as higher costs compared to wet electrolyte capacitors. This type of capacitor is not new: production began in the 1980s, and since then they have been used in many applications, including server motherboards and computer graphics accelerator cards.

Polymer Capacitor Definition

Polymer capacitors are capacitors that use a solid polymer as the electrolyte. They have many superior qualities, including safer failure modes, lower losses, and longer life than electrolytic capacitors.

Features

Equivalent Series Resistance

Polymer capacitors have lower equivalent series resistance than ordinary electrolytic capacitors. This allows polymer capacitors to withstand higher ripple currents during normal operation. Ripple current is the AC component that causes the internal resistance of the capacitor to dissipate power and therefore heat the capacitor. The ESR of a polymer capacitor is nearly constant over its operating temperature range, while the ESR of an electrolytic capacitor varies significantly 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 causes the electrolyte to evaporate. As it evaporates, pressure builds up inside the capacitor and it can burst or even explode. Solid polymer capacitors do not have this risk, and their failure mode is much safer - the capacitor either shorts or starts to act like an open circuit. In general, the reliability of polymer capacitors is far superior to that of electrolytic capacitors.

In addition to better reliability and safer operation, solid polymer capacitors have a better service life because the solid electrolyte used in their construction does not dry out. This type of capacitor can operate for a long time at higher temperatures compared to electrolytic capacitors.

Capacitance range, voltage ratings, and polarization

Polymer capacitors have a capacitance between 10μF and 1mF. Typical maximum voltage ratings are up to 35 V, but the maximum operating voltage of polymer capacitors reaches 100 V. Like ordinary electrolytic capacitors, these polymer capacitors are usually polarized.

Structure 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 up to obtain a cylindrical shape. Electrodes are added, and the entire assembly is then encapsulated in an aluminum case. Since solid polymer capacitors are not considered components that may be replaced in the device, they are usually made by SMT (surface mount technology). This allows them to take up less space on a PCB (printed circuit board), but is more difficult to desolder if replacement is needed.

Ordinary electrolytic capacitors have a vented case or housing with a pre-scored surface, which should break in the event of overpressure and vent the gases formed inside the housing in a safe way to prevent explosion in the event of overheating. There are usually no such scoring or venting on polymer capacitors, as there is no significant pressure buildup in the event of failure.

Applications of polymer capacitors

Due to their low ESR, polymer capacitors are used in applications where large ripple currents are permitted. An example of such an application is a switching DC-DC converter. Good examples are buck, boost and buck-boost converters, which keep the voltage across the capacitor relatively constant but generate high ripple currents. In this case, it is preferred to use capacitors with a low ESR to improve power efficiency and increase safety in the event of overload and overheating.

Solid polymer capacitors can be used to smooth the voltage from the power supply to sensitive circuits, thereby reducing power noise. In these applications, they can easily replace standard electrolytic capacitors as long as the operating voltage is low enough.

They can also be used for power bypassing and signal decoupling to reduce signal noise and power noise generated by the device that would otherwise be transmitted to the power supply and potentially affect other devices connected to it.

Polymer capacitors are commonly found on computer motherboards, especially higher quality motherboards such as server motherboards, where they have largely replaced wet electrolytic capacitors.