The active part of the electrolytic capacitor, a so-cal […]
The active part of the electrolytic capacitor, a so-called wound battery, consists of aluminum (anode and cathode foil), paper and electrolyte, but the film capacitor is made of a metal-coated plastic film constituting its electrodes. The electrolytic capacitor is characterized by its "liquid cathode": the total surface of the highly rough aluminum anode foil coated with alumina as a dielectric can be completely contacted by a conductive electrolyte to achieve a high specific capacitance of the technique. The film capacitor is made of a dry material: the capacitor plate is composed of metal vapor, and the metal vapor is deposited on a plastic film used as a dielectric. Typically, the dielectric is polypropylene, consisting of a polymer chain, preferably oriented in the machine direction and in the transverse direction (also referred to as BOPP for biaxially oriented polypropylene).
The different electrical properties of these two technologies stem from the different materials used therein. The actual aluminum electrolytic capacitor has an energy density ten times higher than that of a polypropylene film capacitor. Since the ions flowing through the electrolyte promote the flow of current in the aluminum electrolytic capacitor, the viscosity of the electrolyte has a significant influence on the temperature dependence of the ESR value: at a low temperature, the electrolyte becomes more viscous and inhibits the free movement of the electrolyte. Ions, resulting in higher ESR values. At temperatures above 60 ° C, there is almost no change in ESR. In addition, the capacitance of the aluminum electrolytic capacitor decreases with a two-digit percentage of temperature drop. However, the ESR and capacitance of the film capacitors are largely unaffected by temperature fluctuations: the capacitance over the entire temperature range varies only by about 3 to 5%, and the ESR value remains almost constant.
These parameters show similar performance to frequency: for electrolytic capacitors, both capacitance and ESR exhibit strong frequency dependence, while film capacitors exhibit nearly constant capacitance and ESR values in a technically interesting frequency range, ranging from 100 Hz~200 kHz. Film capacitors offer higher voltage ratings than e-caps: single components can withstand voltages up to 1500 V, while e-cap voltage ratings are limited to 650 V. The voltage (and ripple current) limits of each electrolytic capacitor require multiple capacitors in series. Connected in parallel to build a "capacitor bank." When the electrolytic capacitors are connected in series, active or passive balancing facilitates ensuring a uniform distribution of the DC link circuit voltage across the capacitors. This extra effort can be very useful because the relatively new "3-level inverter" topology has lower losses, smaller intermediate circuit loads, and lower inverters with higher output power and switching frequency. The specific cost is impressively proven.
Electrolytic capacitors and film capacitors are referred to as "self-healing": defects in the dielectric layer of the electrolytic capacitor are repaired by anodization, consuming oxygen from the electrolyte. However, defects in the film capacitor are burned and thus electrically isolated, but each burn-in defect causes a small loss of the dielectric film, that is, a small decrease in capacitance. Both technologies show "elegant" end-of-life behavior in view of operating conditions within the limits of the specification, the main feature of which is parameters rather than catastrophic failure. Operating parameters temperature, voltage and ripple current determine the life of the electrolytic capacitor. For film capacitors, temperature, voltage and humidity limit the service life. The effect of ripple current on lifetime does not go into the equation because the self-heating caused by the particularly low ESR value in the film capacitor is negligible. The typical end-of-life change limit for ESR is two or three times the initial ESR value for both technologies. The common capacitor loss at the end of the film life is 3%, and the aluminum electrolytic capacitor is 30%.
Cost is an important criterion for selecting technology: the specific cost of storing a given amount of energy using an aluminum electrolytic capacitor is significantly lower than that of a film capacitor (approximately three times). On the other hand, the excellent current carrying capacity of a film capacitor is about twice as high as that of an electrolytic capacitor per amp. These significant differences indicate that both technologies will appear on the market in the future.