Education

Understanding the Aluminum Electrolytic Capacitors

The rapid, ongoing growth in electronics has been aided by technological advances, leading to constant new challenges for manufacturers of these components. Aluminum electrolytic capacitors are one such example. They have been traditionally used for filtering, timing networks, by-pass, coupling and other applications requiring a cost-effective, volumetrically efficient and highly reliable component.

View these technical notes for a more in-depth look at the fundamentals of aluminum electrolytic technology.

 
Introduction

The phenomenal growth in electronics has been aided by technological advances in the areas of miniaturization, cost reduction and improved reliability in electronic components. Both passive and active components have changed dramatically as manufacturers are challenged by new and more stringent requirements.

The aluminum electrolytic capacitor is a passive component which has kept pace with the advancements in technology. This type of capacitor has traditionally been used for filtering, timing networks, by-pass, coupling and other applications requiring a cost effective, volumetrically efficient and highly reliable component.

Nippon Chemi-Con, parent company of United Chemi-Con, is the world's largest manufacturer of aluminum electrolytic capacitors and has pioneered the development of raw material processing techniques and highly automated production systems. The product improvements resulting from these innovations have dramatically improved circuit design flexibility and led to many new applications for aluminum electrolytic capacitors.

United Chemi-Con is presenting these technical notes in order to help explain some fundamentals of aluminum electrolytic technology. We hope this will contribute to the effective and efficient use of aluminum electrolytic capacitors in your design applications.

Technical Developments

Aluminum electrolytic capacitors are used in various applications because they can achieve high capacitance and voltage ratings in small, cost efficient case sizes. In order to understand how this is accomplished, we must examine some of the basic properties of capacitors.

A capacitor is made up of two parallel plates, the electrodes, with a dielectric between them. The amount of capacitance is directly proportional to the surface area of the electrode. If we double the surface area of the electrode, we double the capacitance. The amount of available capacitance is also inversely proportional to the dielectric thickness. If we reduce the thickness by one-half, the capacitance is doubled.

The high volumetric efficiency of an electrolytic capacitor is due to its enhanced plate surface area and a very thin dielectric layer. A very large internal surface can be created on the aluminum electrodes by electrochemical etching. The dielectric is an oxide and has a high dielectric strength which is electrochemically deposited in very thin layers. This combination produces a high capacitance in a small volume.

Miniaturization

Since 1965, the case size of aluminum electrolytic capacitors has been reduced to realize a volumetric reduction of almost five times.

Miniaturization has been achieved by the development of new etching technology, improvements in the oxide layer and advancements in production engineering. All this has been accomplished without the significant increase of ESR (equivalent series resistance) or degradation of frequency and temperature characteristics. An example of the technical advancements in electrolytes and oxide formation is the extended operating temperature range from -25 ~ +70°C to -55 ~ +130°C.

Characteristics of Aluminum Electrolytic Capacitors
Characteristics of Aluminum Electrolytic Capacitors

The aluminum electrolytic capacitor is composed of an anode foil, a cathode foil and separator paper which are wound together and impregnated with an electrolyte. The anode foil has an aluminum oxide layer acting as the dielectric. After the thin aluminum foil (65 to 100 microns) is electrochemically etched to increase the plate's surface area, the dielectric is produced by anodic oxidation on its surface. The cathode foil, in general, utilizes no oxidation process.

Radial Lead (VB Type)Large Can
A Lead Wire G Aluminum Rivet
B Aluminum Tab H Terminal
C Sealing Rubber I Rubber
D Sleeve J Phenolic
E Can K Aluminum Tab
F Element L Element
M Tape
N Can
O Sleeve
P Potting Material
Q Insulating End Disk

The Equivalent Circuit

The equivalent circuit of an aluminum electrolytic capacitor is shown below. Due to the physical design ele-ments and construction, not only does a capacitor have capacitance, but it also has a series resistance and inductance as well as a parallel resistance allowing the flow of current.

RESR = Equivalent Series Resistance
R DCL = Leakage Current
C = Capacitance
L ESL = Equivalent Series Inductance

Capacitance

The capacitance of aluminum electrolytic capacitors as well as other capacitors is expressed by the following equation:

Where:
E = Dielectric constant
S = Surface area of dielectric (cm2)
d = Thickness of the dielectric (cm)

Large capacitance can be obtained when:

  1. the dielectric constant is high
  2. the surface area is large
  3. the dielectric is thin

In aluminum electrolytic capacitors the dielectric constant is only 8 to 10, but the aluminum oxide dielectric layer is extremely thin (about 15 Å per volt). High gain foil produced by the electrochemical etching creates a surface magnification, or gain, as much as 100 times for low voltage foil and 20 to 25 times for high voltage foil.

Therefore, an aluminum electrolytic capacitor can provide a large capacitance compared to other types of capacitors of the same volume.

Equivalent Series Resistance (ESR)

A critical component in the manufacturing of a capacitor that must always be considered is the equivalent series resistance. It is this resistance that leads to heat generation in the capacitor when AC current is applied. The equivalent equation for ESR is shown below.

RESR = R1 + R2 + R3

Where:

  • Foil Length rl
  • Tabbing
  • Lead Wires
  • Ohmic contact resistance

R1 = Resistance due to aluminum oxide thickness
R 2 = Resistance due to electrolyte/spacer combination
R 3 = Resistance due to materials

The amount of heat generated by ripple current depends upon the ESR of the capacitor. In order to have a low ESR, it is necessary to control the characteristics of the electrolyte, the separator paper, the winding alignment of the element, the position of the tabs and the magnification and pit construction of the etched foil. All these things contribute to the ESR of the capacitor.

Leakage Current (DCL)

The dielectric of a capacitor has a very high resistance which prevents the flow of DC current. However, there are some areas in the dielectric which allow a small amount of current to pass, called leakage current. The areas allowing current flow are due to very small foil impurity sites which are not homogeneous, and the dielectric formed over these impurities does not create a strong bond. When the capacitor is exposed to high DC voltages or high temperatures, these bonds break down and the leakage current increases. Leakage current is also determined by the following factors:

  1. Capacitance value
  2. Applied voltage versus rated voltage
  3. Previous history

The leakage current is proportional to the capacitance and decreases as the applied voltage is reduced. If the capacitor has been at elevated temperatures without voltage applied for an extended time, some degradation of the oxide dielectric may take place which will result in a higher leakage current. Usually this damage will be repaired when voltage is reapplied.

Equivalent Series Inductance (ESL)

The inductance of a capacitor is a constant and is due primarily to the capacitor terminal spacing. The inductance runs anywhere from 4nH for miniature radial capacitors to as high as 34nH for large can capacitors. Generally speaking the inductance does not affect the overall impedance unless the capacitor is operating at extremely high frequencies.

Impedance

Impedance is the resistance which opposes the flow of alternating current at a specific frequency. It is related to capacitance and inductance in terms of capacitive and inductive reactance, and also to resistance. The impedance value is expressed in Ohms.

Factors affecting the impedance value are:

  1. Capacitive Reactance (Capacitance) - Low
    Frequency Range: XC= ½(Pi)fC
  2. ESR (Resistance) - Middle Frequency Range
  3. Inductive Reactance (Inductance) - High
    Frequency Range: XL= 2(Pi)fL

Usually, a lower impedance value in the middle frequency range is preferred. Therefore, the combination of a lower impedance value and a lower ESR value results in better capacitor performance. Due to the physical construction of a capacitor, for the same given voltage, a smaller case size capacitor has a higher ESR value. And for the same given capacitance value, a higher voltage capacitor has a lower ESR value (except for high voltage ranges).

Rated Voltage

The rated (or working) voltage is the sum of the DC voltage and the superimposed AC voltage which can be continuously applied to the capacitor. Derating the ap-plied voltage will reduce the failure rate of the capacitor.

Ripple Current

Ripple Current is the RMS value, expressed in Amps, of the alternating current flowing through a capacitor. To insure the life of aluminum electrolytic capacitors, the maximum permissible ripple current must be computed by using the following equation:

Where:
T MAX = Temperature of Capacitor Center
T a = Ambient Temperature
I RMS = Ripple Current (Amps)
R = ESR (Ohms)
B = Heat Transfer Constant
A = Surface Area of Can
Delta T = Temperature Rise produced by Internal Heating (TMAX - Ta)

Where:
If the ripple current applied is higher than the specified maximum permissible ripple current computed by the equation above, the life of the capacitor becomes shorter (since the applied ripple current causes heat generation) and in extreme cases the capacitor vent will rupture. Also note that the peak voltage must not exceed the rated voltage, regardless of computed ripple current rating.

Capacitor Production Process

Etching

The plates, or electrodes, are made of high purity, thin aluminum foil (0.05 to 0.1 mm thick). To get the maximum capacitance for a given electrode surface area, an electrochemical process called "etching" is used to dissolve metal and increase the surface area of the foil in the form of a dense network of microscopic channels.

The etching process consists of continuously running aluminum foil through a chloride solution with an AC, DC or AC/DC voltage applied between the etch solution and aluminum foil. The increase in surface area is referred to as foil gain and can be increased as much as 100 times for foil being used in low voltage capacitor applications and 20 to 25 times for higher voltage applications. (See examples of electron micrographs.)

Formation

The dielectric of the aluminum electrolytic capacitor is composed of a thin layer of aluminum oxide (Al2O3) which develops or "forms" on the surface of the etched aluminum foil during a process called "formation."

This process of forming the dielectric oxide on the aluminum foil (electrode) requires a continuous application of DC voltage at 140% to 200% of the rated voltage for the capacitor being manufactured. The dielectric thickness of this aluminum oxide film is approximately 15Å/volt. The insulation strength is approximately 107V/cm.

Electron Micrographs of Etched Aluminum Foil


AC Etched Aluminum Foil for Low Voltage


Cross Section of DC Etched High Cubicity Aluminum Foil


Cross Section of AC Etched Aluminum Foil for Low Voltage

Since capacitance is inversely proportional to the dielectric thickness and the dielectric thickness is proportional to the forming voltage, the relationship between capacitance and forming voltage should be:

C (capacitance) X V (forming voltage) = constant

This is true for high voltage foil which has a relatively coarse etch structure. However, for foils that have ex-tremely fine etch structures, the conversion of aluminum to aluminum oxide has a significant smoothing effect on the etch structure resulting in a non-linear relationship.

Slitting

The etched and formed foil is then slit into various widths depending on the specific size of the capacitor.

Winding

A typical capacitor winding (or element) is shown below.


Capacitor Element

Each capacitor contains two foils, the positive foil is called the anode and the negative is called the cathode.

The anode and cathode foils, along with a separator paper called the "separator," are rolled into a cylinder. The separator paper prevents the anode and cathode foils from coming into contact with each other and shorting.

As part of a highly automated winding process, aluminum tabs are attached to the anode and cathode foils. This completed assembly of etched and formed foil, separator paper and attached tabs is called the capacitor "element."

Impregnation


Element Cross-Section

Electrolyte is now added to the assembly by a process called "impregnation." The method of impregnation requires the wound element to be immersed into the electrolyte by either a vacuum/pressure cycle with or without applied heat or simple absorption. The electrolyte contains a solvent such as ethylene glycol and a solute such as ammonium borate.

Should the dielectric film be damaged, the presence of the electrolyte will allow the capacitor to heal itself by forming more oxide. By selecting different electrolytes, we can improve the capacitor characteristics such as operating temperature range, frequency response, shelf life and load life.

Sealing

The impregnated element is then sealed in an aluminum can. The sealing material may be rubber, rubber backed phenolic, molded phenolic resin or polyphylenesulfide (PPS).

Aging

The final process is "aging," during which a voltage greater than the rated voltage of the capacitor is applied at elevated temperatures. The purpose is to reform (or repair) any oxide film which may have been damaged during the slitting, winding and assembly processes.

Production Inspection

After aging, capacitors are 100% tested. All electrical requirements are checked using highly advanced automated test equipment and any rejects are removed. The capacitors are also visually inspected, and only capacitors passing both tests are accepted for packaging.

Final Inspection

The last operation before actual shipment is a final inspection by the quality department using MIL-STD-105D or its equivalent.

Reliability

The Bathtub Curve

With the advancements in aluminum electrolytic capacitor technology, capacitors can be used in equipment that must have very long life or must operate under severe conditions. For example, the capacitors used in telecommunication applications have an expected life of over twenty years. Other aluminum electrolytic capacitors have run continuously at ~130&C for over one year. The careful selection of a capacitor for a particular application and proper installation in the circuit will assure good service life.

All components will eventually fail. Usually this occurs by a slow, steady drift of parameters called wear-out. Sometimes there is a sharp change in capacitor properties which is called a catastrophic failure.

The failure rate of aluminum electrolytic capacitors follows a bathtub curve with time as shown in Figure 1.

Figure 1. Bathtub Curve

(a) Initial Failure Period

This is the period during which failures are caused by deficiencies in design, structure, manufacturing processes or severe misapplications. Such failures occur soon after the components are exposed to circuit conditions. In aluminum electrolytic capacitors, these failures are either corrected through aging or found during the 100% inspection process and thus do not reach the field. The initial failures due to capacitor misapplication such as

inappropriate ambient conditions, over voltage, reverse voltage or excessive ripple current can be avoided with proper circuit design and installation.

(b) Random Failure Period

This is a period during which the failure rate is the lowest. These failures are not related to operating time but to application conditions. Aluminum electrolytic capacitors feature fewer catastrophic failures during this period than semiconductors and solid tantalum capacitors.

(c) Wear-Out Failure Period

This is a period during which the properties of a component gradually deteriorate, and the failure rate increases with time. Aluminum electrolytic capacitors end their useful life during this period. The criteria for judging failures varies with application design factors.

Failure Types

The two types of failures are classified as catastrophic failures or wear-out failures and are defined as follows:

(a) Catastrophic Failures

This is a failure mode which destroys the performance of the capacitor. Short circuit and open circuit are examples of this failure mode.

(b) Wear-Out Failures

This is a failure mode caused by the gradual deterioration of the capacitor's electrical parameters. The criteria for judging failures varies with application and design factors. Table 1 shows the failure criteria specified in JIS C 5141.

Table 1: Failure Judgment Criteria (JIS C 5141)

Item Characteristics
C B E A W
Leakage Current Not to exceed initial specified value
Capacitance Not less than 85% of initial measured value Not less than 80% of initial measured value
Tan d Not to exceed 175% of initial specified value Not to exceed 200% of initial specified value
Appearance Show no remarkable abnormalities

Capacitance decrease and tan d increase are caused by the loss of electrolyte in the wear-out failure period. This is due primarily to loss of electrolyte by diffusion (as vapor) through the sealing material. Gas molecules can diffuse out through the material of the end seal. If the electrolyte vapor pressure within the capacitor is in-creased, by high temperatures for example, the diffusion rate is increased. Swelling of the seal material by the electrolyte vapor pressure may also occur at elevated temperatures. This swelling may further enhance diffusion and mechanically weaken the seal.

Factors that can increase the capacitor temperature, such as ambient temperature and ripple current, can accelerate capacitor wear-out.

Wear-out can also be increased by high internal pressure caused by gas generation from excessive leakage current or attack of the cathode foil by electrolyte. These factors are not present in well-designed capacitors and are generally not a problem.

Failure Modes

Aluminum electrolytic capacitors show various failure modes in different applications. (See Table 2.)

Table 2: Failure Modes and Causes

Failure Mode Table in Adobe PDF format - To view this file, either configure your web browser with a helper application for automatic launching of Abode Acrobat Reader or save to disk and manually open this file with Adobe Acrobat Reader.

Short Circuit

Short circuits are caused by burred or rough foil edges as well as thin regions in the separator paper and deficiencies in the oxide film; both of which lead to dielectric breakdown. Since all products are subjected to aging before shipping, short circuited capacitors are rarely found in the field. However, inappropriate application factors such as excessive operating voltage, excessive ripple current and reverse voltage may result in short
circuits.

Open Circuit

Open circuit or poor contact of terminal connections can be caused by excessive mechanical stress such as high vibration levels while in service. Open circuits caused by the evaporation of electrolyte are normally observed as an end of life phenomenon during the wear-out failure period. When subjected to excessive operating voltages or excessive ripple currents, capacitors will show internal heat rise, and the internal pressure increases will accelerate evaporation of the electrolyte, allowing open circuits to take place. If halide ions come in contact with the capacitor elements, they will corrode foils and ter-minals causing an open circuit condition. The speed of this phenomenon is dependent on temperature and applied voltage. In this failure mode, an open circuit is usually preceded by gradual capacitance drop and tan d increase.

Capacitance Drop/Tan d Rise

In service, the electrolyte gradually evaporates through the seal, resulting in capacitance drop and tan d rise. These changes are accelerated by high ambient temperatures and ripple current. These phenomena are prominent during the wear-out failure period.

Leakage Current Increase

Capacitors in service are free from leakage current in-creases because of the continuous reformation of the dielectric with the applied voltage. However, capacitors stored for long periods of time without voltage applied show small leakage current increases which are allowable for normal applications. For applications in elec-tronic devices such as audio equipment, time constant circuits, etc., the use of specially designed, low leakage capacitors are recommended.

Open Vent

Reverse voltage and AC current are the major causes of an open vent failure. The safety vent is designed to open when abnormal internal pressure increases or heat rises occur due to excessive operating voltage, ripple current or when any other abnormal operating conditions exist.

Electrolyte Leakage

Capacitors in normal service are free from electrolyte leakage. Electrolyte leakage is caused by mechanical factors such as lead stress or chemical deterioration of the seal. Electrical factors like excessive operating voltage, excessive ripple current, reverse voltage and AC current which cause internal heat rise and pressure in-creases may also cause electrolyte leakage.

Capacitor Life

The life of aluminum electrolytic capacitors is very dependent on environmental and electrical factors. Environmental factors include temperature, humidity, atmospheric pressure and vibration. Electrical factors include operating voltage, ripple current and charge-discharge duty cycle. Among these factors, temperature (ambient temperature and internal heating due to ripple current) is the most critical to the life of aluminum electrolytic capacitors. Whereas, conditions such as vibration, shock and humidity have little affect on the actual life of the capacitor.

Lifetime Acceleration Factors

Aluminum electrolytic capacitors are evaluated by accelerated life tests. The acceleration tests contain three factors (one for temperature, voltage and ripple current) which are shown by the following equation:

LB = LA * AT * AV * AR

Where:
LB = Lifetime under a certain condition "B"
LA = Lifetime under a certain condition "A"
AT = Temperature acceleration factor
AV = Voltage acceleration factor
AR = Ripple current acceleration factor

Effects of Temperature on Life

Because a capacitor is essentially an electrochemical device, increased temperatures accelerate the chem ical reaction rates within the capacitor (usually a 10°C rise in temperature will double the chemical reaction rate). Therefore, higher temperatures cause accelerated changes in decreasing capacitance and increasing tan d due to the gradual evaporation of the electrolyte through the capacitor seal (see Figure 2). The Equivalent Series Resistance change (DESR) can be a measure of electrolyte loss and has been found experimentally to be dependent on temperature as shown in Figure 3. In this figure, the vertical axis is the ratio of DESR at some particular temperature to the normalized ESR at a reference temperature (60° or 85°C for these examples). The DESR at both temperatures is taken over the same test time.

Figure 2. Load Life Dependence on Temperature

Figure 3. Accelerated Temperature Test

The temperature acceleration factor and its relationship with capacitance change, ESR and electrolyte loss are shown below.

(1) The relationship between Delta Cap and ESR versus weight loss of the electrolyte (Delta W) at various temperatures versus time. As might be expected, the degradation of capacitor characteristics occurs more rapidly at higher temperatures as shown in Figure 4. Also note that Delta Cap increases proportionally to the loss of electrolyte while ESR increases in a more rapid fashion.

Figure 4. The Relationship Between Ambient Temperature and Delta Cap, ESR and Delta W
Figure 5. Electrolyte Loss on Life Test versus Temperature

(2) The relationship between the temperature acceleration factor and the weight loss of electrolyte. Figure 5 shows the results of a typical test in which the weight loss of the electrolyte increased 1.9 times with a temperature rise of 10°C. The ESR shows a similar change due to a rise in temperature. Generally the temperature acceleration factor is between 1.7 to 2.3, depending upon the particular samples. From these results we can formulate the equation:

Where:
Delta T = the "acceleration test temperature" minus the "actual operating temperature"
AT = the acceleration factor.

Figure 6 shows the acceleration factor at different operating temperatures for several capacitor ratings, relative to performing at 85°C.

Figure 6. Temperature Acceleration Factor Calculated by ESR Change.
(The factor at 85°C is defined as 1.)

Effects of Voltage on Life

Voltage within the allowed operating range has little effect on the actual life expectancy of a capacitor. However in certain applications or misapplications, the applied voltage can be detrimental to the life of an aluminum electrolytic capacitor.

(a) Operating Voltage

When in service at voltages equal to or below the rated value, the life of electrolytic capacitors is affected less by applied voltage than by operating temperature. Figures 7, 8 and 9 show life test results with various reduced voltages applied. The curves show that the life of the capacitor has not been significantly increased by a reduction in voltage. This is due to the use of proper forming voltages to minimize gas generation and leakage current. From this we can say that when capacitors are used at or below their rated voltage, the acceleration factor AV is equal to 1.

Figure 7. Applied Voltage and Life Expectancy at 85°C

Figure 8. Applied Voltage and Life Expectancy at 105°C

Figure 9. Applied Voltage and Life Expectancy at 85°C

(b) Reverse Voltage

The application of reverse voltage causes internal heating and oxidation of the cathode foil, thus generating gas as shown in the following formula:

2Al + 3H2O - 6e- -> Al2O3 + 6H+
6H+ + 6e- -> 3H2^

This pressure build up may cause the safety vent to open or possibly destroy the capacitor. Deterioration is slow with a reverse voltage of a few volts. (See Figure 10.)

(c) Excessive Voltage

Continuous application of excessive operating voltages will rapidly increase the leakage current. (See Figures 11 and 12.) Internal heating and gas generation caused by increased leakage current may destroy the capacitor.

Figure 10. Reverse Voltage at 85°C

Figure 11. Voltage and IDCL Characteristics

Figure 12. Excessive Operating Voltage at 85°C

(d) Charge-Discharge Duty

When designing heavy duty charge-discharge circuits, customers are recommended to select capacitors specially designed for this application. Graphs showing the characteristics of these capacitors under typical charge-discharge applications are shown in Figures 13-15.

Figure 13. Heavy Charge-Discharge Duty

Figure 14. Heavy Charge-Discharge Duty

General purpose type aluminum electrolytic capacitors will show rapid capacitance drops caused by oxidation of the cathode foil by discharge currents. Furthermore, discharge currents may cause heat rise and pressure increases resulting in venting and the potential destruction of the capacitor.

The rate of deterioration during heavy charge-discharge duty is dependent on temperature and applied voltage. (See Figures 14 and 15.)

Figure 15. Heavy Charge-Discharge Duty

Effects of Ripple Current on Life

Compared with other types of capacitors, aluminum electrolytic capacitors have higher tan d and therefore are subject to greater internal heat generation when ripple currents exist. To assure the capacitor's life, the maximum permissible ripple current of the product is specified.

When ripple current flows through the capacitor, heat is generated by the power dissipated in the capacitor accompanied by a temperature increase. Internal heating produced by ripple currents can be represented by:

W = IR2 * RESR + V * IL........(1)

Where:
W = Internal power loss
IR = Ripple current
RESR = Internal resistance (Equivalent Series Resistance)
V = Applied voltage
IL = Leakage current

Leakage currents at the maximum allowable operating temperature may be 5 to 10 times higher than the values measured at 20°C, but compared to IR, lL is very small and negligible.

Thus, Equation (1) has an approximation:

W ~= IR2 * RESR.........(2)

Determining the condition at which internal heating is brought into steady state with heat dissipation results in the following equation:

R2 * RESR = b * A * DeltaT........(3)

Where:
A = (Pi/4)D(D + 4L) Where: D = Can diameter, L = Can length
b = Heat transfer constant (the value of b varies by can size and is between 0.0007 and 0.0013)
A = Surface area of container
DeltaT = Core temperature rise produced by internal heating

From Equation (3) internal temperature rise produced by ripple current is given by:

DeltaT = ...(4)
for (f=120 Hz)
where tan d and C are measured at 120Hz and w represents 2Pif

From Equation (4) the temperature rise is proportional to the internal resistance (RESR) and is inversely proportional to the heat transfer constant (b) and the surface area of the can. To increase the allowable ripple current, capacitors should have a lower internal resistance (RESR), greater surface area (A) and a higher heat transfer constant (b).

Finally, the relationship between temperature and life approximately meets the following equation:

Where:
L1 = Significant life at temperature T1
L2 = Significant life at temperature T2
T1 = Maximum rated operating temperature
T2 = Ambient temperature
A = Acceleration coefficient due to ambient temperature
DeltaT1 = Allowed change in core temperature due to ripple current at rated temperature
DeltaT2 = Actual change in core temperature due to ripple current at operating conditions
K = Acceleration coefficient due to applied ripple current

When T2 is less than T1, it is commonly observed that:

A ~= 2 (This acceleration coefficient varies slightly with product series.)

The acceleration coefficient K varies with the change in temperature due to ripple current and the product series.

K ~= 5 ~ 10

For example, with A = 2, K = 5 (typical), T1 = 105°C, T2 = 65°C, DT1 = 5°C and DT2 = 20°C in Equation (1) causes the life to increase by a factor of 2. The lower the operating temperature, the longer the expected life. To achieve still longer life, products having higher maximum allowable operating temperature are recommended.

The ripple current which flows through the capacitor consists of the charging current and the discharging current. When the discharging current flows through the capacitor, the current flows to the anode electrode from the cathode electrode so that, in principle, an oxide layer could form on the cathode electrode. However, in practice this is prevented by suitable capacitor design and selection of cathode foil. Figure 16 shows some examples of how ripple current affects the increase of ESR during a life test.

Figure 16.ESR Change on Life Test with Ripple Current at 85°C

Maximum allowable ripple currents (RMS) are normally specified at 120 Hz (sine wave). Since internal resistance (RESR) is frequency dependent, maximum allowable ripple current varies with frequency. (See Table 3.)

Table 3. Ripple Current Multiplier for Frequency
(Typical Values for WV <= 100)

Nominal Capacitance (µF)Frequency (Hz)
50/60100/1203001K10K50K
4.7 or below 0.65 1.00 1.35 1.75 2.30 2.50
10 to 47 0.75 1.00 1.25 1.50 1.75 1.80
100 to 1,000 0.80 1.00 1.15 1.30 1.40 1.50

Excessive ripple currents will reduce the life of capac-itors. When designing high ripple current circuits, the customer should select capacitors specially designed for higher ripple current duty. (See Figure 17.)

When applying AC + DC voltage, the peak value of the DC plus superimposed AC voltage should not exceed the rated voltage, and the bottom value should not fall below -1V.

Figure 17. Ripple Current Duty at 85°C

Capacitor Applications

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

SeriesOperating
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

SeriesOperating
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

SeriesOperating
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

Guidelines for Using Aluminum Electrolytic Capacitors

Polarity

In DC applications, 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 1 volt DC is allowed. Capacitors used in circuits whose polarity is occasionally reversed or whose polarity is unknown requires the use of a bi-polar capacitor. Also note that an aluminum electrolytic capacitor cannot be used for AC applications.

Insulating Sleeving

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.

Insulation From The Aluminum Can

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.

Operating Temperature

Choose a capacitor whose maximum specified temperature is greater than the operating temperature of the application. This will increase the life of the capacitor. However, if the temperature rating of the capacitor is less than the temperature of the application, the life of the capacitor will be severely decreased or the capacitor could fail catastrophically.

In general, for each 10 degree decrease in operating temperature the capacitor life will double and conversely it will be halved for each 10 degree increase in temperature as determined by the following life expectancy formula.

Where:
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, and 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 this 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 the ambient temperature stress, and an acceleration rate of approximately 2 for each 5-10°C temperature increase is realized. Following is the formula used to determine life expectancy.

Where:
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

Rated Voltage

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 the peak value of the AC voltage plus the DC voltage does not exceed the rated voltage.

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.

Surge Voltage

The surge voltage rating is the maximum over-voltage including DC, peak AC and transients to which the capacitor may be subjected for short periods of time (not exceeding 30 seconds every 5 minutes). According to JIS C5141, the test shall be conducted for 1000 cycles at room temperature under test condition W of JIS C5141or at the maximum operating temperature under test condition B and C of JIS C5141. Under test, the capacitor shall have voltage applied through a current limiting resistor of 1000 ohms without discharge. After test, the electrical characteristics of the capacitor are specified in JIS C5141. Unless otherwise specified, the rated surge voltages are as follows:

Rated Voltage (V) 6.3 10 16 25 35 50 63 80 100 160
Rated Surge Voltage (V) 8 13 20 32 44 63 79 100 125 200
Rated Voltage (V) 200 250 315 350 400 450 500
Rated Surge Voltage (V) 250 300 365 400 450 500 550

Heavy Duty Charge/Discharge Applications

The standard aluminum electrolytic capacitor is not suitable for circuits in which there is a frequent charge and discharge cycle. If a standard capacitor is used in circuits in which the charge and discharge cycles are frequently repeated, the capacitance value may drop and the capacitor may be damaged. Please consult our engineering department for assistance in these applications.

Vent

The safety vent needs adequate clearance to work properly. It is advisable to leave a minimum clearance above the vent of 2mm for the can diameters of 16mm and smaller, 3mm for the can diameters of 18-35mm, and 5mm for the can diameters of 40mm and larger.

Adhesive & Coating Materials

When an adhesive is used on the rubber seal of the capacitor to anchor it to the printed wiring board, the adhesive must not contain any halogenated hydrocarbon nor any chemical which could damage the rubber seal or PVC sleeve.

Also, after solvent cleaning and before using an adhesive or coating material on the capacitor, evaporate the solvent residue from the rubber seal of the capacitor for at least 10 minutes at 50-85°C by forced air.

Mechanical Stress on Lead Wires & Terminals

If excessive force is applied to the lead wires and terminals, they may be broken or their connections with the internal element may be affected. (For strength of terminals, refer to JIS C5102, C5141 and C5142.) The distance between the terminal holes on the circuit board should be the same as the spacing between the lead wires or terminals on the capacitor.

1. Axial and Radial Lead Types

Improper insertion of the lead wires into circuit boards may cause electrolyte leakage, lead wire breakage or impair the lead wire connections with the internal element. When the distance between the two terminal holes on the circuit board cannot be made the same as the distance between the lead wires, formed capacitor leads are recommended.

2. Snap-In Type

Improper insertion of the terminals into the circuit boards may break the terminals or impair their electrical connections with the internal elements. The blank terminal of a multi-terminal capacitor should be considered to be at the same potential as the electrolyte, or cathode, and should therefore be isolated from the circuit.

3. Screw Terminal Type

Too much torque applied in tightening the screws into the terminal will result in stripping the threads and possibly increasing the contact resistance. On the other hand, if the screws are not tightened enough, the high contact resistance will cause localized heating at the terminals resulting in early failure.

Soldering

Incorrect soldering may shrink or break the sleeving of the capacitor. Please read the following information carefully before soldering.

1. 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.

2. 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, nothing abnormal will occur if the soldering process is performed at less than 260°C for less than 10 seconds.

3. 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.

4. 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 paragraph 3 is recommended.

5. 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 H7 catalog.

Cleaning

Aluminum can be aggressively attacked by halide ions, particularly by chloride ions. Even small amounts of chloride ions inside the capacitor will cause corrosion which contributes to rapid capacitance drop and vent-ing. Therefore, the prevention of chloride contamination is the most important check point for quality control in production.

Solvent-proof capacitors are required when chlorinated hydrocarbons are used for cleaning. If aluminum electrolytic capacitors without the solvent-proof construction are present on the circuit board, alcohol based solvents are recommended for cleaning.

The mechanism of corrosion in aluminum electrolytic capacitors by chloride ions can be explained as follows:

Chlorinated solvents are absorbed and diffuse through the polymer seal entering the capacitor. Various chemical reactions may occur depending upon the particular solvent and electrolyte, but the final result is the release of chloride ions.

Chloride ions can penetrate through imperfections and micro-cracks in the aluminum oxide dielectric layer reaching the underlying aluminum metal. At these points, the aluminum metal is attacked by soluble chloride as shown in the following anodic half-cell reaction:

Al + 3Cl- -> AlCl3 + 3e........(8)

There is always at least 1 to 2% water in the electrolyte and this is sufficient enough to hydrolyze the AlCl3:

AlCl3 + 3H2O -> Al (OH)3 + 3H+ + 3Cl-....(9)

This reaction releases the chloride ions to further attack the aluminum. The hydrogen ion increases the local acidity which causes the oxide dielectric to dissolve. Thus, localized corrosion occurs at an accelerated rate with the attack of both the metal and the dielectric.

Recommended cleaning solvents therefore are those free of halogens. When halogenated solvents must be used, solvent-proof capacitors whose seal constructions are specially designed for this application are recommended. A terpene or petroleum base solvent swells and damages the rubber seal of the capacitor. An alkaline saponification detergent may damage the aluminum metal and marking. The cleaning solvents compatible with our products are as follows:

Non-Halogenated Solvent Cleaning

Solvents Higher Alcohol base:
Pine Aplah ST-100S
Clean Through 750H, 750K, 750L & 710M
Technocard FRW-14 to 17
Cleaning Conditions 60°C max. within 10 minutes
Immersion (with or without ultrasonic)
Remarks
  1. The wash, rinse and drying process should be so arranged that other components and PC boards do not rub off the marking of the capacitor. Shower cleaning may affect the marking.
  2. For water rinse, control the conditions to avoid sleeve shrinkage.
  3. For alkaline solvents like Clean Through 750H, etc., do not leave residual alkaline on the capacitor after the cleaning process.

HCFC Solvent Cleaning

The following HCFC solvent cleaning is compatible with the solvent-proof type capacitors.

Solvents HCFC (AK225AES)
Cleaning Conditions 5 minutes max. (3 minutes max. for SREC and KRF; 2 minutes max. for KRE) with one immersion, ultrasonic or vapor cleaning. Immersion (with or without ultrasonic).
Applicable Series (Only solvent-proof products) Surface Mount MF, MFK, MV, MVK, MF-BP, MFK-BP, MV-BP, MVK-BP
Low Profile KRE, SRAC, KMA, SRG, KRG
Radial Lead SMEG(~250V), KMG, SME(~250V), KME(~250V), SME-BP, KME-BP, SXE, LXF, KMF(~100V), EX, LX, LXA, GXC, LLA, KRF, TXG
Axial Lead SME, KME(250V & less with diameter 18mm & smaller)
Remarks Where the capacitor seats on the PC board without any gaps between its end seal and the PC board, the solvent might remain there. The residual solvent should be sufficiently evaporated by forced air drying at least 10 minutes at 50-85°C immediately after the solvent cleaning.

Non-clean flux: Both ionic halogen and nonionic halogen damage the capacitor when they penetrate into the inside of the capacitor through the rubber seal. Some of the flux called non-halogenated flux contains less ionic halogen activator with a large amount of nonionic halogen added.

Some adhesives, dampproofing agents and dustproofing agents also contain halides and should be used with caution.

Electrolyte and Separator Paper

An aluminum electrolytic capacitor uses a flammable separator paper in the internal element which is impregnated with a flammable, electrically conductive electrolyte. If the electrolyte should leak out onto the PC board, it may short or erode away the copper traces and might catch on fire with a voltage applied. Be careful with the location of the vent, copper land and copper trace when the PC board is designed. Where a through-hole PC board is used, any copper traces and land should not be located under the capacitor. If it is, leave a space above the trace of at least 1-2 mm.

Storage

The electrical characteristics of aluminum electrolytic capacitors are dependent on temperature; the higher the ambient temperature, the faster the deterioration of the electrical characteristics (i.e., leakage current increase, tan d increase, capacitance drop, etc.). If an aluminum electrolytic capacitor is exposed to high temperatures such as direct sunlight, heating elements, etc., the life of the capacitor may be adversely affected. When capacitors are stored under humid conditions for long periods of time, the humidity will cause the lead wires/terminals to oxidize and thus impairing solderability. Therefore aluminum electrolytic capacitors should be stored at room temperature, in a dry place and out of direct sunlight.

A voltage treatment/reformation process to electrolytic capacitors may have to be applied after a capacitor has been stored for more than 2 or 3 years. If aluminum electrolytic capacitors are stored above room temperature for long periods of time, the anode foil may react with the electrolyte increasing the leakage current. After storage, the application of even normal voltages to these capacitors may result in higher than normal leakage currents.
In most cases the leakage current will return to normal
levels in a short period of time. However in extreme cases, the amount of gas generated may cause the safety vent to open.

Capacitors that are stored for long periods of time should be subjected to a voltage treatment/reforming process (Note 1) which will reform the dielectric and return the leakage current to the initial level. Leakage current increase during storage will vary with the working voltage of the capacitors, normally in this order:

Low voltage capacitors < Middle voltage capacitors < High voltage capacitors.

Note 1: In the reformation process the applied voltage is gradually increased up to the rated voltage without exceeding the initial specified leakage current of the capacitor. After reaching the rated voltage, continue applying this voltage for 30 to 60 minutes.

Figure 28. Effect of Storage Time and Temperature

Performance Confirmation Tests

Polarity

In DC applications, 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 1 volt DC is allowed. Capacitors used in circuits whose polarity is occasionally reversed or whose polarity is unknown requires the use of a bi-polar capacitor. Also note that an aluminum electrolytic capacitor cannot be used for AC applications.

Insulating Sleeving

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.

Insulation From The Aluminum Can

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.

Operating Temperature

Choose a capacitor whose maximum specified temperature is greater than the operating temperature of the application. This will increase the life of the capacitor. However, if the temperature rating of the capacitor is less than the temperature of the application, the life of the capacitor will be severely decreased or the capacitor could fail catastrophically.

In general, for each 10 degree decrease in operating temperature the capacitor life will double and conversely it will be halved for each 10 degree increase in temperature as determined by the following life expectancy formula.

Where:
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, and 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 this 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 the ambient temperature stress, and an acceleration rate of approximately 2 for each 5-10°C temperature increase is realized. Following is the formula used to determine life expectancy.

Where:
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

Rated Voltage

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 the peak value of the AC voltage plus the DC voltage does not exceed the rated voltage.

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.

Surge Voltage

The surge voltage rating is the maximum over-voltage including DC, peak AC and transients to which the capacitor may be subjected for short periods of time (not exceeding 30 seconds every 5 minutes). According to JIS C5141, the test shall be conducted for 1000 cycles at room temperature under test condition W of JIS C5141or at the maximum operating temperature under test condition B and C of JIS C5141. Under test, the capacitor shall have voltage applied through a current limiting resistor of 1000 ohms without discharge. After test, the electrical characteristics of the capacitor are specified in JIS C5141. Unless otherwise specified, the rated surge voltages are as follows:

Rated Voltage (V) 6.3 10 16 25 35 50 63 80 100 160
Rated Surge Voltage (V) 8 13 20 32 44 63 79 100 125 200
Rated Voltage (V) 200 250 315 350 400 450 500
Rated Surge Voltage (V) 250 300 365 400 450 500 550

Heavy Duty Charge/Discharge Applications

The standard aluminum electrolytic capacitor is not suitable for circuits in which there is a frequent charge and discharge cycle. If a standard capacitor is used in circuits in which the charge and discharge cycles are frequently repeated, the capacitance value may drop and the capacitor may be damaged. Please consult our engineering department for assistance in these applications.

Vent

The safety vent needs adequate clearance to work properly. It is advisable to leave a minimum clearance above the vent of 2mm for the can diameters of 16mm and smaller, 3mm for the can diameters of 18-35mm, and 5mm for the can diameters of 40mm and larger.

Adhesive & Coating Materials

When an adhesive is used on the rubber seal of the capacitor to anchor it to the printed wiring board, the adhesive must not contain any halogenated hydrocarbon nor any chemical which could damage the rubber seal or PVC sleeve.

Also, after solvent cleaning and before using an adhesive or coating material on the capacitor, evaporate the solvent residue from the rubber seal of the capacitor for at least 10 minutes at 50-85°C by forced air.

Mechanical Stress on Lead Wires & Terminals

If excessive force is applied to the lead wires and terminals, they may be broken or their connections with the internal element may be affected. (For strength of terminals, refer to JIS C5102, C5141 and C5142.) The distance between the terminal holes on the circuit board should be the same as the spacing between the lead wires or terminals on the capacitor.

1. Axial and Radial Lead Types

Improper insertion of the lead wires into circuit boards may cause electrolyte leakage, lead wire breakage or impair the lead wire connections with the internal element. When the distance between the two terminal holes on the circuit board cannot be made the same as the distance between the lead wires, formed capacitor leads are recommended.

2. Snap-In Type

Improper insertion of the terminals into the circuit boards may break the terminals or impair their electrical connections with the internal elements. The blank terminal of a multi-terminal capacitor should be considered to be at the same potential as the electrolyte, or cathode, and should therefore be isolated from the circuit.

3. Screw Terminal Type

Too much torque applied in tightening the screws into the terminal will result in stripping the threads and possibly increasing the contact resistance. On the other hand, if the screws are not tightened enough, the high contact resistance will cause localized heating at the terminals resulting in early failure.

Soldering

Incorrect soldering may shrink or break the sleeving of the capacitor. Please read the following information carefully before soldering.

1. 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.

2. 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, nothing abnormal will occur if the soldering process is performed at less than 260°C for less than 10 seconds.

3. 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.

4. 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 paragraph 3 is recommended.

5. 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 H7 catalog.

Cleaning

Aluminum can be aggressively attacked by halide ions, particularly by chloride ions. Even small amounts of chloride ions inside the capacitor will cause corrosion which contributes to rapid capacitance drop and vent-ing. Therefore, the prevention of chloride contamination is the most important check point for quality control in production.

Solvent-proof capacitors are required when chlorinated hydrocarbons are used for cleaning. If aluminum electrolytic capacitors without the solvent-proof construction are present on the circuit board, alcohol based solvents are recommended for cleaning.

The mechanism of corrosion in aluminum electrolytic capacitors by chloride ions can be explained as follows:

Chlorinated solvents are absorbed and diffuse through the polymer seal entering the capacitor. Various chemical reactions may occur depending upon the particular solvent and electrolyte, but the final result is the release of chloride ions.

Chloride ions can penetrate through imperfections and micro-cracks in the aluminum oxide dielectric layer reaching the underlying aluminum metal. At these points, the aluminum metal is attacked by soluble chloride as shown in the following anodic half-cell reaction:

Al + 3Cl- -> AlCl3 + 3e........(8)

There is always at least 1 to 2% water in the electrolyte and this is sufficient enough to hydrolyze the AlCl3:

AlCl3 + 3H2O -> Al (OH)3 + 3H+ + 3Cl-....(9)

This reaction releases the chloride ions to further attack the aluminum. The hydrogen ion increases the local acidity which causes the oxide dielectric to dissolve. Thus, localized corrosion occurs at an accelerated rate with the attack of both the metal and the dielectric.

Recommended cleaning solvents therefore are those free of halogens. When halogenated solvents must be used, solvent-proof capacitors whose seal constructions are specially designed for this application are recommended. A terpene or petroleum base solvent swells and damages the rubber seal of the capacitor. An alkaline saponification detergent may damage the aluminum metal and marking. The cleaning solvents compatible with our products are as follows:

Non-Halogenated Solvent Cleaning

Solvents Higher Alcohol base:
Pine Aplah ST-100S
Clean Through 750H, 750K, 750L & 710M
Technocard FRW-14 to 17
Cleaning Conditions 60°C max. within 10 minutes
Immersion (with or without ultrasonic)
Remarks
  1. The wash, rinse and drying process should be so arranged that other components and PC boards do not rub off the marking of the capacitor. Shower cleaning may affect the marking.
  2. For water rinse, control the conditions to avoid sleeve shrinkage.
  3. For alkaline solvents like Clean Through 750H, etc., do not leave residual alkaline on the capacitor after the cleaning process.

HCFC Solvent Cleaning

The following HCFC solvent cleaning is compatible with the solvent-proof type capacitors.

Solvents HCFC (AK225AES)
Cleaning Conditions 5 minutes max. (3 minutes max. for SREC and KRF; 2 minutes max. for KRE) with one immersion, ultrasonic or vapor cleaning. Immersion (with or without ultrasonic).
Applicable Series (Only solvent-proof products) Surface Mount MF, MFK, MV, MVK, MF-BP, MFK-BP, MV-BP, MVK-BP
Low Profile KRE, SRAC, KMA, SRG, KRG
Radial Lead SMEG(~250V), KMG, SME(~250V), KME(~250V), SME-BP, KME-BP, SXE, LXF, KMF(~100V), EX, LX, LXA, GXC, LLA, KRF, TXG
Axial Lead SME, KME(250V & less with diameter 18mm & smaller)
Remarks Where the capacitor seats on the PC board without any gaps between its end seal and the PC board, the solvent might remain there. The residual solvent should be sufficiently evaporated by forced air drying at least 10 minutes at 50-85°C immediately after the solvent cleaning.

Non-clean flux: Both ionic halogen and nonionic halogen damage the capacitor when they penetrate into the inside of the capacitor through the rubber seal. Some of the flux called non-halogenated flux contains less ionic halogen activator with a large amount of nonionic halogen added.

Some adhesives, dampproofing agents and dustproofing agents also contain halides and should be used with caution.

Electrolyte and Separator Paper

An aluminum electrolytic capacitor uses a flammable separator paper in the internal element which is impregnated with a flammable, electrically conductive electrolyte. If the electrolyte should leak out onto the PC board, it may short or erode away the copper traces and might catch on fire with a voltage applied. Be careful with the location of the vent, copper land and copper trace when the PC board is designed. Where a through-hole PC board is used, any copper traces and land should not be located under the capacitor. If it is, leave a space above the trace of at least 1-2 mm.

Storage

The electrical characteristics of aluminum electrolytic capacitors are dependent on temperature; the higher the ambient temperature, the faster the deterioration of the electrical characteristics (i.e., leakage current increase, tan d increase, capacitance drop, etc.). If an aluminum electrolytic capacitor is exposed to high temperatures such as direct sunlight, heating elements, etc., the life of the capacitor may be adversely affected. When capacitors are stored under humid conditions for long periods of time, the humidity will cause the lead wires/terminals to oxidize and thus impairing solderability. Therefore aluminum electrolytic capacitors should be stored at room temperature, in a dry place and out of direct sunlight.

A voltage treatment/reformation process to electrolytic capacitors may have to be applied after a capacitor has been stored for more than 2 or 3 years. If aluminum electrolytic capacitors are stored above room temperature for long periods of time, the anode foil may react with the electrolyte increasing the leakage current. After storage, the application of even normal voltages to these capacitors may result in higher than normal leakage currents.
In most cases the leakage current will return to normal
levels in a short period of time. However in extreme cases, the amount of gas generated may cause the safety vent to open.

Capacitors that are stored for long periods of time should be subjected to a voltage treatment/reforming process (Note 1) which will reform the dielectric and return the leakage current to the initial level. Leakage current increase during storage will vary with the working voltage of the capacitors, normally in this order:

Low voltage capacitors < Middle voltage capacitors < High voltage capacitors.

Note 1: In the reformation process the applied voltage is gradually increased up to the rated voltage without exceeding the initial specified leakage current of the capacitor. After reaching the rated voltage, continue applying this voltage for 30 to 60 minutes.

Figure 28. Effect of Storage Time and Temperature

FOCUS MARKETS
NEWS AND EVENTS

ABOUT United Chemi-Con

United Chemi-Con, a wholly owned subsidiary of Nippon Chemi-Con, was established in the United States in 1970. The company is the largest manufacturer and supplier of aluminum electrolytic capacitors in North America, with more than 8,000 unique products available.


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CONTACT INFO

United Chemi-Con Inc
1701 Golf Road, 1-1200
Rolling Meadows, IL 60008
United States

Tel:847.696.2000
Fax:847-696-9278
E-mail:info@chemi-con.com