As shown at the next page, the impedance value varies with
temperature because the resistance of the electrolyte is strongly
affected by temperature.
Leakage current:
The dielectric of a capacitor has a very high resistance that does not allow DC current to flow. However, due to the characteristics of the aluminum oxide layer that functions as a dielectric in contact with electrolyte, a small amount of current, called leakage current, will flow to reform and repair the oxide layer when a voltage is being applied. As shown below, a high leakage current flows to charge voltage to the capacitor for the first seconds, and then the leakage current will decrease and reach an almost steady- state value with time.
Measuring temperature and voltage influences the leakage current. The leakage current shows higher values as the temperature and voltage increase. In general, the leakage current is measured at 20℃ by applying the rated voltage to capacitor through a resistor of 1000Ω in series, The leakage current is the value several minutes later after the capacitor has reached the rated votage. The catalog prescribes the measuring termperature and time.
Reliability
The bathtub curve:
Aluminum electrolytic capacitors feature failure rates shown by the following bathtub curve.
a) Infant failure period This initial period accounts for the failures caused by deficiencies in design, structure, the manufacturing process or severe misapplications, In other words the initial failures occur as soon as the components are installed in a circuit. In the case of aluminum electrolytic capacitors, these failures do not occur at customers' field because aging process reforms an incomplete oxide layer, or eliminate the defective parts at the aging process and the sorting process. Misapplication of the capacitor such as inappropriate ambient conditions, over-voltage, reverse voltage, or excessive ripple current should be avoided for proper use of the capacitor in a circuit.
b) Useful life period
The random failure period exhibits an extremely low failure rate. These failures are not related to operating time but to application conditions. During this period, non-solid aluminum electrolytic capacitors lose a small amount of electrolyte. The electrolyte loss shows as a slow decrease in capacitance and a slow increase in tanδ and ESR. Non-solid aluminum electrolytic capacitors still exhibit lower catastrophic failures than semiconductors and solid tantalum capacitors. c) Wear-out failure period
This period reflects a deterioration in the component properties of the capacitor; the failure rate increases with time, Non-solid aluminum electrolytic capacitors end their useful life during this period. Failure types:
The two types of failures are classified into catastrophic failures and wear-out failures as following.
1) Catastrophic failures
This is a failure mode that destroys the function of the capacitor like a short circuit or open circuit failure.
2) Wear-out failures
This is a failure mode where gradually deteriorates the electrical parameters of the capacitor. The criteria of judging the failures, vary with application and design factors.
Capacitance decreases and tanδ increases are caused by the loss of electrolyte in the wear-out failure period. This is primary due to loss of electrolyte by diffusion (as vapor) through the sealing material. Gas molecules can diffuse out through the material of the end seal. High temperature increase the electrolyte vapor pressure within the capacitor and the diffusion rate is therefore increased. This increases internal pressure may cause the seal to bulge due to temperature rise. This bulging may accelerate diffusion and mechanically degrade the seal. Factors that can increase the capacitor temperature, such as ambient temperature and ripple current, can accelerate the wear-out phase of capacitor.