Aluminum electrolytic capacitors are used in virtually all types of circuit designs. However, they are commonly used as filtering devices in power supplies.

Reduction of High Frequency Impedance
Improved capacitor performance has extended the use of electrolytic capacitors from filter circuits of linear power supplies to other electronic devices, especially switching power supplies where the impedance char-acteristics at higher switching frequencies is very important. Therefore, the capacitor manufacturers have developed new engineering techniques for decreasing high frequency impedance (see Figure 18). The typical engineering techniques are:

(1) Decrease of ESR
(a) Separator paper, electrolyte and oxide layer
(b) Construction (e.g. the number of tabs)
(c) Swaging of cathode foil

(2) Decrease of ESL
(a) Non-inductive tabbing
(b) Stacked foil type
(c) 4-terminal type

Figure 18. Impedance versus Frequency

Capacitor for Switcing Power Supplies
Switching power supplies have increased in popularity over linear supplies because they are lighter, smaller and more efficient. The useful life of the power supply has become dependent on the design quality of the aluminum electrolytic capacitor because of the reduced power supply size and increased operating temperature. Therefore, capacitors with special characteristics are required for switching power supplies.

Figure 19. Typical Switch Mode Power Supply Circuit

Capacitors for Input Smoothing
Capacitors used at commercial line frequencies require the same amount of energy as series regulators, so the capacitance can be reduced by increasing the input voltage. However, in this case, the ripple current increases in proportion to the capacitor impedance, and the ESR, which contributes to ripple heat generation, is represented by the equation:

The heat generation increase is inversely proportional to the capacitance. Consequently, capacitors used for input smoothing of switching power supplies have to be able to endure high ripple currents.

Switching power supply circuits are shielded to prevent noise generation, and the components are mounted very close together to reduce the overall size causing the operating temperature to increase. Thus, the temperature range selected for the components must be high enough to accommodate this increase in temperature. For aluminum electrolytic capacitors, this is even more important because of the additional internal heat generated by the capacitor itself. Input smoothing capacitors, which are designed for operation under these conditions, have a low ESR. This reduces the power and thus the internal heat generated within the capacitor. The operating temperature range has been extended to 105°C from 85°C by improving the materials used in the construction of these capacitors. Examples of these capacitors are shown in Table 4.

Table 4. Input Capacitors

Series Operating
Temperature
Range
(°C)		 Voltage
(V)		 Capacitance
(µF)		 Remarks
SMH-VN -25~+85 160~450 56~2,700 Snap Mount
KMH-VN -25~+105 160~450 56~2,200 Snap Mount,
High Ripple,
High Temperature
RWE-LG -25~+85 350~550 100~12,000 Large Can,
High Capacitance
RWF-LG -25~+85 350~450 2,700~15,000 Large Can,
High Ripple
KMH-LG -25~+105 160~450 180~27,000 Large Can,
High Temperature
LX-LG -25~+105 160~450 220~12,000 Large Can,
Long Life, High Ripple,
High Temperature

Ripple current at the switching frequency will also flow through the input capacitors if there is not a special filter circuit between the smoothing and switching circuits. As seen from the following equation, this puts additional stress on the capacitor, but it does not create a major problem. In the example shown, Figure 20, this accounts for only 10% of the total heat generated by the ripple current:

PT = Pc + Ps and P = IR2R

Where:
Pc = Power at commercial line frequency Ps = Power at switching frequency

Figure 20. Frequency Characteristics of ESR

Capacitors for Output Smoothing
The necessary condition to determine the rating of capacitors for filtering is:

ZC << ZO

Where ZC is the impedance of the capacitor, and ZO is the load impedance. The relationship between the capacitance and the impedance value of the capacitor at low frequencies (120 Hz) is approximately

Therefore, the rating is determined by the capacitance value. At high frequencies, the relationship would be

Figure 21 shows that the required rating is not determined by the capacitance value alone.

Figure 21. Impedance Characteristics of Aluminum Electrolytic Capacitors

Capacitors designed for output smoothing have been improved in frequency characteristics so that their im-pedance values approach 1/wC at high frequencies. Table 5 lists output capacitors and their ratings.

Table 5. Output Capacitors

Series Operating
Temperature
Range
(°C)		 Voltage
(V)		 Capacitance
(µF)		 Remarks
LXE-VB -55~+105 6.3~63 10~10,000 Low Impedance
LXA-VB -55~+105 10~63 0.47~4,7000 Long Life,
Low Impedance
LXF-VB -55~+105 6.3~63 3.3~15,000 Long Life,
Very Low Impedance
EX-VB -55~+125 10~63 0.1~10,000 High Temperature,
Low Impedance
GX-VB -40~+130 10~63 0.47~1,000 Very High Temperature
URZA -55~+105 6.3~250 56~33,000 Large Size,
Very Low Impedance
SMH-VN -40~+85 6.3~450 56~100,000 Snap Mount,
Small Size
KMH-VN -40~+105 6.3~450 56~82,000 Snap Mount,
Small Size,
High Temperature

 

The smaller the capacitor, the less tolerant it is to ripple current. Output capacitors have been designed for low ESR at high frequencies. This reduces the heat generation that is caused by high frequency ripple current. Furthermore, operating temperature ranges have been extended to allow for higher temperatures relieving the stress on the capacitor.

Capacitors for Control Circuits
Since only a small AC current flows through the control circuit, the capacitor requirements are not strict. General application capacitors as well as miniature capacitors with wide temperature ranges and performance characteristics can be used for this circuit design. Table 6 lists some examples.

Table 6. Capacitors for Control Circuits

Series Operating
Temperature
Range
(°C)		 Voltage
(V)		 Capacitance
(µF)		 Remarks
SME-VB -40~+85 6.3~400 0.1~22,000 Small Size
KME-VB -55~+105 6.3~400 0.1~22,000 High Temperature,
Small Size
SMG-VB -40~+85 6.3~450 0.1~22,000 Very Small Size
KMG-VB -55~+105 6.3~450 0.1~22,000 High Temperature,
Very Small Size
KMA-VB -55~+105 6.3~63 0.1~220 Low Profile,
Tantalum Replacement
LLA-VB -40~+85 6.3~50 0.1~15,000 Small Size,
Low Leakage

 

Capacitors for High Frequency Filtering
As previously stated, an aluminum electrolytic capacitor has inductance thus affecting the overall impedance. Today’s switching power supply capacitors are designed to provide an impedance of 20 to 50% of earlier types.

In general, switching regulator capacitors can be used in circuits with frequencies up to approximately 30 kHz without being significantly affected by inductance. Specially designed capacitors can be used in circuits up to 100 kHz. If higher performance is required, as in the case of a spike noise problem, parallel-connected capacitors with smaller capacitance values making up a ladder filter are recommended. A four-terminal capacitor with a ladder-type filter construction inside is available. However, it should be noted that when using this type of capacitor, only a limited amount of direct current can be applied. The overall impedance of the aluminum electrolytic capacitor is governed more by capacitance at relatively low frequencies and by ESR at higher frequencies.

The temperature characteristics of electrolytic capacitors should also be considered when the capacitor is used for high frequency filtering. Figures 22 and 23 show the characteristics of capacitance versus temperature, and ESR versus temperature, respectively. Note that while the capacitance changes very little between -25°C and +20°C, the ESR changes significantly.

Thus, careful attention should be paid to both frequency and temperature characteristics when an electrolytic capacitor is used in a frequency range where ESR governs impedance.

Figure 22. Temperature Characteristics of Capacitance

Figure 23. Temperature Characteristics of ESR

Notes on Series and Parallel-Connected Capacitors
When capacitors are connected in series, they effectively form a voltage divider. It is recommended to equalize the voltage drops across the capacitors by shunting external (balance) resistors across each capacitor. The general practice is to allow ten times the leakage current of the capacitor through the resistors.

Thermal imbalances between capacitors is also important when they are connected in parallel. Thermal runaway may occur if they are unbalanced which could lead to component failure. When capacitors with the same impedance and different ESR values are parallel-connected, heat generation due to ripple current is represented by the equation:

SigmaP = I12R1 + I22R2 + I32R3 + In2Rn (W)

It is obvious that the greater the ESR value, the greater the temperature rise. Likewise, in the case of capacitors having the same ESR values and different impedance values, the smaller the impedance value, the larger the heat generation.

The best method for overcoming any imbalances is to insert inductors in series with the capacitors as shown in Figure 24. The added inductance will limit current flow, and therefore, the stress on the capacitors will be re-duced. Alternatively, the wire length between the capacitors can be increased. (Resistors can be used instead of inductors, but are not recommended because of the increased impedance.) Furthermore, capacitors that are produced specifically for high frequency applications, preferably from the same production lot, should be used.

Figure 24. Capacitor Connection for High Frequency Filtering

Figures 25,26, and 27 show the impedance characteristics when one to five LXF products have been connected in parallel.

Figure 25. Impedance versus Frequency

Figure 26. Impedance versus Frequency

Figure 27. Impedance versus Frequency