Electrolytic Capacitor - Reliability and Length of Life

Reliability and Length of Life

Aluminum, and to a lesser extent tantalum, electrolytics have worse noise, leakage, drift with temperature and aging, dielectric absorption, and inductance than other types of capacitor. Additionally, low temperature is a problem for most aluminum capacitors: for most types, capacitance falls off rapidly below room temperature while dissipation factor can be ten times higher at −25 °C than at 25 °C. Most limitations can be traced to the electrolyte. At high temperature, the water can be lost to evaporation, and the capacitor (especially the small sizes) may leak outright. At low temperatures, the conductance of the salts declines, raising the ESR, and the increase in the electrolyte's surface tension can cause reduced contact with the dielectric. The conductance of electrolytes generally has a very high temperature coefficient, +2%/°C is typical, depending on size. The electrolyte, particularly if degraded, is implicated in various reliability issues as well.

High-quality aluminum electrolytics (computer-grade) have better performance and life than consumer-grade parts. High temperatures and ripple currents shorten life. Typical basic electrolytics are rated to work at temperatures up to 85 °C, and are rated for a worst-case life of about 2000 hours (a year is about 9000 hours); commonly available higher-temperature units are available for temperatures of 105 °C, and a working temperature of 175 °C is possible. One of the effects of aging is an increase in ESR; some circuits can malfunction due to a capacitor with correct capacitance but elevated ESR, although a capacitance meter will not find any fault (an ESR meter will). Runaway failure is possible if increased ESR increases heat dissipation and temperature.

Since the electrolytes evaporate, design life is most often rated in hours at a set temperature, for example, 2000 hours at 105 °C, which is the highest commonly used working temperature, although parts working up to 175 °C are available. Standard inexpensive consumer-grade electrolytic capacitors are rated for 85 °C maximum working temperature. Life in the operational environment is dictated by the Law of Arrhenius, which dictates that the capacitor life is a function of temperature and DC voltage. As a rule of thumb, the life doubles for each 10 °C lower operating temperature. In our example, it reaches 15 years at 45 °C (for caps rated at 105 °C). The operating temperature however is not just the ambient temperature. Ripple currents can increase it significantly. The actual operating temperature is a complex function of ambient temperature, air speed, ripple current frequency and amplitude, and also affected by material thermal resistance and the surface area of the can case. In general, high amplitude ripple currents shorten the life expectancy, whereas low frequency ripple is more detrimental than high frequency. The EIA IS-749 is a standard for testing electrolytic capacitor life.