Product Information: Aluminum Electrolytic Capacitor Technical Notes/Capacitor, Power Supply Units RUBYCON CORPORATION

Product Information: Electrolytic Capacitor

Aluminum Electrolytic Capacitor Technical Notes

6. Caution for Proper Use

6-1 General Cautions

     For basic precautions on using aluminum electrolytic capacitors, please refer here.

6-2 Charge and Discharge Application

     Performance deterioration of aluminum electrolytic capacitor is accelerated by repeated charge and discharge. Deterioration is accelerated as charge-discharge voltage is higher, discharge resistance is lower, charge-discharge cycle is shorter, and ambient temperature is higher. Safety vent operation and rupture may occur depending on charge-discharge conditions for devices which has frequent regeneration such as servo amplifier and which has large ripple voltage amplitude such as lighting. Therefore, it is required to select proper product considering its operating condition.
     Factors causing characteristic deterioration and failure of capacitor by charge and discharge include heat generation and increase in leakage current due to charge-discharge, deterioration and local destruction of the anodized film, cathodic foil formation due to discharge and gas generation with formation, and so on.

①Heat Rise Caused by Charge and Discharge Current
     For capacitors subjected to frequent charge and discharge cycles through very low discharge resistance (less than a few ohms) such as flash units for cameras and welding machines, heat rise due to high charge-discharge current is the main factor in performance deterioration.
Fig. 21

Fig. 21 Schematic diagram of charging-discharging circuit

     Due to its structure, the aluminum electrolytic capacitor has an internal resistance RE shown in figure 21. The internal resistance is due to the characteristics of the electrolyte, electrode foils and oxide film. Power loss W due to the internal resistance occurring at discharge is indicated as equation 20.
Equation 20
     Heat rise through this power loss causes the internal temperature of the capacitor to increase.
     This temperature increase continues until thermal equilibrium is reached between the heat rise and heat radiation from capacitor surface.
     As internal temperature increases, the oxide film on the anode foil progressively deteriorates, accelerating degradation of the capacitor, which is apparent in an increase of leakage current and internal resistance. Therefore, capacitors must be used that are designed with lower internal resistance to minimize heat rise and promote long life when used with applications that have low discharge resistance and involve frequent charge and discharge. When the charge and discharge current is extremely high, a capacitor must be used that is designed to lower dielectric loss, and with low internal resistance, as dielectric loss of the oxide film on the anode foil is another factor in performance deterioration.

②Effect of Discharge on Cathode Foil
     When the capacitor is subjected to frequent and repeated on-off cycles, such as with power supply for audio amplifiers, formation of an oxide film on the cathode foil is considered a pivotal factor in performance deterioration. This phenomenon relates to the amount of electric charge being discharged and the capacitance value of the cathode foil.

Fig. 22

Fig. 22 Electric charge transfer during discharge

     The behavior of the electric charge from the charging stage until the discharging stage is illustrated in Figure 22. The charge is stored in both the anode foil and the cathode foil as per Figure 22 (a) during the charging stage. When it moves to the discharging stage, each electric charge moves to neutralize polarity. However, when the electric charge stored in the anode foil is greater than that in the cathode foil, extra charges remain after the discharge completes, as per Figure 22 (c). This is the same phenomenon as when the cathode foil is charged with positive polarity. When the voltage exceeds the voltage able to be withstood by the oxide film on the cathode foil, the oxide film starts to grow with the decreasing current flow. Eventually, the capacitance of the cathode foil decreases and the capacitance of the capacitor decreases accordingly, as it is a composition of anode and cathode capacitance. Gas generation caused by this electro-chemical reaction makes the internal pressure of the capacitor increase.
     A detailed explanation is given hereunder of the voltage applied to the cathode foil when discharge is completed.
Fig.23 Fig.24
     When DC voltage is applied to the capacitor, the voltage is distributed to the anode foil and the cathode foil in proportion to the ratio of and Rc , as illustrated in Fig. 23. Where ››Rc , because Rc is insulation resistance of thin oxide film on the cathode in the direction in which electricity easily flows.
     Generally, <Cc , but the relation of the electric charge stored in the anode foil to the cathode foil is indicated in equation 21.
Equation 21
     When the capacitor is discharged at this stage, the amount of the electric charge (Qa-Qc ) remains, and Vc' calculated with equation 22 is applied to cathode foil. (Fig. 24)
Equation 22
     The withstanding voltage of the cathode foil V' must be set higher than the residual voltage Vc' .
Equation 23
     From above equation, stabilization of capacitor performance should be achievable by increasing the voltage of cathode foil and making capacitance ratio cathode foil and making the capacitance ratio of Cc/Cα as large as possible. V' is generally known as being between 1.0 and 1.5 volts. As with standing voltage of oxide film on the cathode foil may be reduced, or its distribution widened, in high ambient temperatures, it is essential to use cathode foil with a stable and delicate oxide film.
There may be occasions when formed foil is used as cathode foil. If this is a concern, please consult us for a specific solution.

6-3 Inrush Current

     Current (inrush and starting current) is a large current temporarily flowing when power is applied to a device using a motor or having a smoothing capacitor with large capacitance. The current is much larger than the steady state current value. Generally, a single-shot / short-time large current load at startup is not a problem for the capacitor, but in the case of a circuit in which a large current load is frequently applied to a capacitor, heat generation of the capacitor may exceed the allowable value or abnormal heat may occur at the connection between the internal electrode and the lead terminal or the connection to the external terminal.

6-4 Overvoltage Application

     When voltage exceeding the rated voltage of the capacitor is applied, current flows and formation of oxide film progresses until withstand voltage of the anode matches the applied voltage, and it will cause decrease in the capacitance and increase in tan δ (ESR). Since this reaction is associated with heat generation and gas generation, it may result in safety vent operation of the capacitor due to rise in internal pressure or internal short-circuit failure.

Fig. 25

Fig. 25 Capacitance change when overvoltage is applied

6-5 Reverse Voltage Application

     Aluminum electrolytic capacitors have a polarity. When reverse voltage is applied, current flows and formation of oxide film progresses until withstand voltage of the cathode matches the applied voltage, resulting in decrease in capacitance, increase in tan δ (ESR), and gas generation. When high reverse voltage is applied, safety vent of the capacitor may be activated due to internal pressure rise caused by gas generation.

Fig. 26

Fig. 26 Capacitance change when reverse voltage is applied

6-6 Series / Parallel Connection

①Series Capacitor Connection
Fig. 27
        Fig. 27 Series capacitor connection

     When two capacitors are connected in series, voltage at terminals of each capacitor on charging is applied in reverse proportion to the capacitance of each capacitor as shown below.
Equation 24-26
     This means that voltage applied to either capacitor may be over the rated voltage to cause safety vent operation if capacitance values of them are much different. After the completion of charging, terminal voltage on each capacitor varies with the level of leakage current. Then over voltage may be applied to the terminals on either capacitor if another capacitor has high leakage current, which possibly causes safety vent operation.
     To prevent difference in terminal voltage values, it is useful to put Voltage Distribution Resistors as shown in Fig. 28 and to select two capacitors with minimal difference in capacitance. We recommend to use the capacitors in same production lot. Follow the formula 27 to use Voltage Distribution Resistors.

Fig. 28

        Fig. 28 Series capacitor connection with balance resistance

Equation 27
Note: In a circuit with a large charge / discharge load, there is a case resulting in failure.
          Failure causes because leakage current of the capacitor increases over time, voltage balance
          may be lost, and a voltage exceeding the rated voltage may be applied to one of the capacitors,
          even if a balancing resistor is attached.

②Parallel Capacitor Connection
     When connecting capacitors in parallel, as shown in Fig. 29 (a), since wiring resistance of individual capacitors will be different, current flows preferentially to the capacitor with small wiring resistance and its heat generation increases. In such a case, deterioration of the characteristics (capacitance reduction, ESR increase, etc.) of the capacitor located at a specific position (place where the wiring resistance is low) is accelerated leading to breakdown, and there is a possibility that the expected life of the device may not be satisfied. Therefore, in the case of parallel connection, please design the circuit so that it becomes equal-length wiring as shown in Fig. 29 (b).

Fig. 29

Fig. 29 Wiring for parallel connection of capacitors

6-7 Restriking Voltage

     When charged aluminum electrolytic capacitor is discharged by shorting the terminals and left open for a while, the voltage between terminals of the capacitor rises again. This increased voltage is called “regeneration voltage”. The mechanism of this phenomenon is explained as follows.
     In general, the structure of a capacitor is as shown in Figure 30, with a dielectric substance between two electrodes. Dielectric of an aluminum electrolytic capacitor is an oxide film formed on surface of aluminum foil by forming process. When voltage is applied to the dielectric, polarization occurs due to dielectric effect. The polarization does not immediately respond to the electrical field and may delay by the elastic viscosity of the molecules. There are various types of polarization, including space charge polarization, atomic polarization, and electronic polarization.

Fig. 30

Fig. 30 Dielectric polarization during capacitor charging

     When voltage is applied to a dielectric, atomic polarization and electronic polarization are completed in a short period of time, but other types of polarization, such as space charge polarization, are thought to require longer time to complete. When the voltage between the terminals is allowed to discharge to zero and the circuit between the terminals is left open thereafter, the polarization that requires more time appears between the terminals, creating recovery voltage.
     Recovery voltage peaks between one to three weeks after the terminals are disconnected, and then gradually decreases. Recovery voltage tends to be higher in larger capacitors such as capacitors with screw terminals and self-supporting terminals.
     If recovery voltage is present, shorting the terminals will create a spark. This could frighten a person working with the capacitor, and there is also the risk of damaging low-voltage devices in the circuit such as CPUs and memory. To prevent this happening, it is recommended to discharge the capacitor with a resistor of about 1 kΩ before use. We have also dealt with the countermeasure packaging against for restriking voltage, so please consult us.

6-8 Use at High Altitude

     When aluminum electrolytic capacitor is used for equipment used in high altitude such as mountains and aircraft, although it is assumed that the pressure inside capacitor will be relatively higher due to decrease in the outside air pressure, there is no problem on the sealing performance of the capacitor for use in the atmosphere up to about 10,000m. Also, there is no problem in terms of sealing performance for use under vacuum. However, since temperature decreases as altitude increases, please check the operation of the equipment taking into consideration that aluminum electrolytic capacitor has property of decreasing capacitance and tanδ (ESR) at low temperature. For reference, Table 3 shows the relationship between altitude and temperature / atmospheric pressure.

Table 3 Altitude and temperature / atmospheric pressure

Altitude[m]Temperature[ºC]Pressure[hPa]
015.01013.3
2,0002.0794.9
4,000-11.0616.3
6,000-24.0471.7
8,000-37.0355.9
10,000-50.0264.3

     Also, the heat dissipation from the capacitor to the outside air decreases (the thermal resistance increases) at high altitude or under reduced pressure and vacuum condition, so it is necessary to apply a certain derating to rated ripple current value of the catalog. For details, please contact us.

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