Guidelines For Using Aluminum Electrolytic Capacitors
Aluminum electrolytic capacitors are used in virtually all types of circuit designs. They are commonly used as filtering devices in power supplies. The following blog post includes a number of guidelines to follow when designing with and using electrolytic capacitors. The material has been paraphrased from United Chemi-con‘s technical topics published on their web sit e here.
Aluminum electrolytic capacitors should not be used in AC applications.
In DC applications, always confirm the polarity. If the polarity is reversed, the circuit life will be shortened or the capacitor may be damaged. Generally, an intermittent reverse voltage of 1V DC is allowed. Capacitors used in circuits whose polarity is occasionally reversed or whose polarity is unknown require the use of bi-polar capacitors.
Choose a capacitor whose maximum specified temperature is greater than the operating temperature of the application. This will increase the life of the capacitor. If the temperature rating of the capacitor is less than the temperature of the application, the life of the capacitor will be substantially less than expected or the capacitor could fail catastrophically.
In general, for each 10 degree decrease in operating temperature the capacitor life will double. Conversely, capacitor life will be halved for each 10 degree increase in temperature as determined by the following life expectancy formula.
Lx = Lifetime at actual operating temperature Tx
Lo = Lifetime at maximum rated operating temperature
To = Maximum rated operating temperature (°C)
Tx = Actual operating temperature (°C)
Ripple Current/Load Life
The life expectancy of an aluminum capacitor is not only determined by the ambient temperature, but also by the ripple current. The ambient temperature plus the increase in temperature due to ripple current equals the operating temperature.
Do not apply a ripple current exceeding the rated maximum ripple current allowed for the capacitors as do so will result in shortened capacitor life and may result in the capacitor venting or failing catastrophically.
In many cases capacitor heating due to ripple current is more severe than ambient temperature stress. An acceleration rate of approximately 2x for each 5-10°C temperature increase due to ripple current. The following formula used to determine life expectancy:
Lx = Lifetime under actual ambient temperature and actual ripple current
Lo = Lifetime under maximum rated operating temperature and rated DC voltage with no ripple
To = Maximum rated operating temperature (°C)
Tx = Actual ambient temperature (°C)
T = Inside temperature increase (°C) by actual ripple current
K = Acceleration factor, varied from 5 to 10 by product and conditions
If the applied voltage exceeds the rated voltage of the capacitor, the capacitor may be damaged from an increase in leakage current. When using a capacitor with an AC voltage superimposed on a DC voltage, care must be exercised so that neither the peak value of the AC voltage plus the DC voltage exceeds the rated voltage nor that the minimum AC voltage plus the DC voltage inverts the polarity on the capacitor.
When capacitors are connected in series, the voltage distribution across the series may not be uniform. This is due to the normal DC leakage distribution and should be considered in the design process by using a higher rated voltage capacitor and/or using balancing resistors in parallel with each series capacitor.
General purpose aluminum electrolytic capacitors are covered with a sleeve made of polyvinyl chloride or similar material. In addition to the insulating properties, the sleeving is also used for marking. The aluminum can is not insulated from the cathode, and when the internal element needs to be electrically insulated from the can, capacitors specially designed for these insulation requirements should be used. Also, the dummy terminal is not insulated from the cathode and must not be connected electrically to the anode or cathode.
Incorrect soldering may shrink or break the sleeving of the capacitor. Please read the following information carefully before soldering:
- If the soldering iron comes in contact with the capacitor body during wiring, damage to the polyvinyl sleeve and/or case may result in defective insulation or improper protection of the capacitor element.
- When soldering a printed circuit board, care must be taken so that the soldering temperature is not too high and the wave or soldering time is not too long. Otherwise, there will be adverse effects on the electrical characteristics and the insulating sleeve of aluminum electrolytic capacitors. In the case of miniature aluminum electrolytic capacitors, no harm will occur if the soldering process is performed at less than 260°C for less than 10 seconds.
- During soldering, the sleeve may melt or break if it comes in contact with the circuit board traces. To avoid this problem, do not locate circuit board traces under the capacitor body.
- The sleeving may be melted by solder which migrates up through the terminal holes in the circuit board. To avoid this problem, the same application as stated in item 3 is recommended.
- When soldering adjacent components to the capacitor, preheated lead wires or terminals may tear the capacitor sleeve if these terminals come in contact with the capacitor sleeve. Therefore, mount the capacitors carefully so that the adjacent components’ terminals or lead wires do not come in contact with the sleeve, particularly when mounting on through-hole circuit boards.
For surface mounting capacitors, the reflow soldering conditions are specified in the Surface Mount section of United Chemi-Con’s catalog.