Aluminum Electrolytic Capacitors

INTRODUCTION

Aluminium electrolytic capacitors are widely used in power supply

circuitry of electronic equipment as there after several advantages over

other types of capacitances. The selection of a capacitor for an

application without knowing the basics may result in unreliable

performance of the equipment due to expanitor problems. It may lead to

customer dissatisfaction and damage market to potential or the image of a

reputed company. The aluminium eletrolytic capacitors are suitable to be

used when a great capacitance value is required in a very small size. The

volume of an electrolytic capacitor is more than 10 times less than a film

one considering the same rated voltage and capacitance. The cost per F

is also less when compared with all other capacitors.

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Aluminum Electrolytic Capacitors

CONSTRUCTION

An aluminium electrolytic capacitor is composed of high-purity,

thin aluminium foil (0.05 to I mm thick) having a dieletric anidation on its

surface to prevent current flow in one direction. This outs as anode.

Another these two aluminium coils is an electrolytic impregnated paper,

which cuts as the dieletric. Since the capacitors is inversely propotional

to the dieletric thiclenen. And the dieletric thicknen is propotional to the

forming voltage, the relationship between capacitance and cerming

voltage is.

Capacitance X Forming Voltage = Constant.

Aluminium tabs attached to the anode and cathode coils act as the

positive and negative leads of the capacitor respectively. The entire

element is sealed into an aluminium can by using rubber, bakelite or

phenolic plastic. The construction of an aluminum electrolytic capacitor

is the following:

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Aluminum Electrolytic Capacitors

The anode (A):

The anode is formed by an aluminium foil of extreme purity. The

effective surface area of the coil is greatly enlarged (by a factor upto 200)

by electrochemical etching in order to achive the maximum possible

capacitance values.

The dieletric (O):

The aluminum foil (A) is covered by a very thin oxidised layer of

aluminium oride (O=AlO3. This oxide is obtained by means of an eletro

chemical process. The typical value of forming voltage is 1.2 nm/v. the

oxide with stands a high electric field strength and it has a high dielectric

constant. Aluminium oxide is therefore well suited as a capacitor

dieletric in a polar capacitor. The A12O3 has a high insulation resistance

for voltages lower than the forming voltage. The oxide layer consistitutes

a nonlinear voltage dependent resistance: the current increases more

steeply as the voltage increases

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Aluminum Electrolytic Capacitors

The electrolytic Paper, cathode (C,K)

The negative electrode is a liquid electrolyte absorbed in a paper.

The paper also acts as a spacer between the positive foil carrying the

dieletric layer and the opposite Al-foil ( the negative Coil) acting as a

contact medium to the eletrolyte. The cathode foil serves as a large

contact area for passing current to the operating eletrolyte. Bipolar Al

electrolytic capacitors are also available. In this designs both the anode

foil and cathode foil are anodized. The cathode foil has the same

capacitance rating as the anode foil. This construction allows for

operation of direct voltage of either polarity as well as operation of purely

alternating voltages. Since it causes internal heating the applied

atternating voltage must be kept considerably below the direct voltage

rating. Since we have the series connection of two capacitor elements,

the total capacitance is equal to only half the individual capacitance value.

So compared to polar capacitor, a bipolar capacitor requires upto twice

the volume for the same total capacitance.

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Aluminum Electrolytic Capacitors

TECHANICAL TERMS EXPLANATION

The technical terms are useful in evaluating or comparing the

capacitors from various manufacturers.

RATED CAPACITANCE: Rated capacitance one usually defined at

20 C for 100 Hz or 120 Hz. Circuit designers should take into account its

variation with frequency and temperature.

1. When temperature increases, the leakage current and capacitace

increases while ESR decreases.

2. when frequency increases, capacitance and impedance decrease

while tan (dissipation factor) increases.

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Aluminum Electrolytic Capacitors

3. When frequency decreases, the ripple – current heat generated

increases the equivalent series resistance (ESR).

RATED VOLTAGE (Rr): Rated voltage is the maximum voltage that

can be continously applied to the capacitor within specified operating

temperature range of the capacitor. The following should be taken into

account: In case an AC voltage is super imposed on on a DC Voltage, the

sum of the DC voltage and the peak value of AC should not exceed the

rated voltage (Vr) of the capacitor. If the DC voltages of both polarities

are likely to be encountered in an application, use DC bipolar capacitors.

DC bipolar capacitors should not be used for AC applications.

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Aluminum Electrolytic Capacitors

When capacitors are connected is series to achieve a higher

operational voltage, the voltage distribution on each of the capacitors will

not be the same even for capacitors for same voltage rating. This is due

to normal DC leakage distribution.

SURGE VOLTAGE (Vp) : Surge voltage is the maximum over

voltage including DC peak AC and transients to which the capacitor can

be subjected for short periods (not exceeding 30 secs every 5 mints). Its

value varies between capacitors from different manufacturers and is

related to the rated voltage as follows:

Vp = 1.15Vr for capacitors having Vr = 200V

Vp = 1.10 Vr for capacitors having Vr>200V

EQUIVALENT SERIES RESISTANCE (ESR): the ESR is the

resistance that the capacitors offers to an alternating current flow. It

arises due to resistance from various components including the

electrolyte, paper coil etc. the ESR to an alternating current generate

heat with an the capacitor. It is specified for 100 Hz at 20C. It decreases

with the increase in temperature and frequency.

LEAKAGE CURRENT : When the rated voltage is applied to a

capacitor, there is initially a high current flow, which exponentially

decreases as the capacitor gets charged. Even after the capacitor is fully

charged, there will be a constant small value of current flowing into the

capacitor. This is formed as the leakage current. It is due to the

aluminium oxide which acts as the dielectric. The curve gradient of the

exponential current decrease is a measure of the quality of the capacitor.

The steeper the curve gradient, the better the capacitor.

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Aluminum Electrolytic Capacitors

The leakage current increases with temperature. After long periods

of storage (at a high temperature), the leakage current may exceed the

rated value. This is particularly important to check high voltage

capacitors, where during the first turn – on, the circuit may trip or, the

worst case, cause failure due to increased value of leakage current.

Circuit designers should take into account this phenomenon while

designing. To bring down the leakage current value, reanodise the

capacitor after long periods of storage.

Reanodisation means applying the rated voltage to the capacitors

for one to two hars through a series resistor. The value of the resistor can

be 100 ohm for Vr 100V DC and 1 kohm per Vr>1000 V.

DISSIPATION FACTOR (tanδ). It is defined as the ratio of the

ESR to the capacitive reactance.

Dissipation factor (DF) = tanδ + ESR / Xc. Where capacitive

reactance Xc =1/(2fc). Therefore, tanδ = ESR ( 2 fc)

The dissipation factor, also called the loss angle tangent (tanδ), is a

very important parameter for capacitors. It increases with frequency .

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Aluminum Electrolytic Capacitors

INDUCDUCTANCE (XL)

A small value of inductance, usually of the order of a few

nanohenries (nH), is present in the electrolytic capacitor, its reactance is

denoted by XL.

IMPEDENCE (Z): The impedence of the capacitor given by

Z=ESR2 +(XL-XC)2

impedence is dominated by capacitive reactance XL of low frequencies.

At the series resonance frequencies, the inductive reactance is equal

to capacitive reactance so the impedence:

Z=ESR

Above the series resonance frequency, the capacitor behaves like

an inductor, which means the impedence is dominated by inductive

reactances

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Aluminum Electrolytic Capacitors

Capacitive reactance predominates at low frequencies. With increase in

frequency the capacitive reactance Xc=1/wC0 decreases until it reaches

the order of magnitude of electrolyte resistance Re(A). At even higher

frequencies ,the resistance of the electrolyte predominates :Z=Re(A-B)=

When the capacitor’s resonance frequency is reached (W0) ,capcitive and

inductive reactance mutually cancel each other 1/wCe=wL,

w0=SQR(1/LCe). Above this frequency ,the inductive reactance of the

winding and its terminal(XL=Z=wL) becomes effective and leads to an

increase in impedance. Generally speaking it can be estimated that

Ce=0.01Co.

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Aluminum Electrolytic Capacitors

Re is the most temperature dependent component of electrolytic

capacitor equivalent circuit.The electrolyte resistivity will decrease if the

temperature rises.

RIPPLE CURRENT (Ir):- It is the superimposed alternating ripple

current defined of 100 hz at 85ºC . The ripple current is limited by the

internal temperature rise within the capacitor as follows: power dissipated

P = Irip2 *(ESR)

=TS

Where ΔT is the difference between ambient temperature and

capacitor surface temperature, S is capacitor surface (cm2) and in is

dissipation factor or thermal gradient (watt/cm2 0C).

Therefore, Irip =√(ΔTSμ/ESR)

Frequency dependence of the ripple current:-

The ESR and thus the tanδ depend on the frequency of the applied

voltage. It means that the allowed ripple current is a function of the

frequency too.

Temperature dependence of the ripple current:-

The data sheet specifies that the maximum current at the upper

category temperature of each temperature.

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Aluminum Electrolytic Capacitors

SUPERIMPOSED AC, RIPPLE VOLTAGE:-

A superimposed alternating ac voltage ,or ripple voltage, may be

applied to aluminium electrolytic capacitors provided that :

1. The sum of the direct voltage and superimposed alternating voltage

does not exceed the rated voltage;

2. The rated ripple current is not exceeded;

3. No polarity reversal will occur.

MAXIMUM PERMISSIBLE OPERATING

TEMPERATURE (upper category temperature):-

The upper category temperature is the maximum permissible

temperature at which the capacitor may be operated, measured on the

can.If the above limit is trespassed the capacitor may fail prematurely.

MINIMUM PERMISSIBLE OPERATING

TEMPERATURE (lower category temperature):-

The minimum category temperature is the minimum permissible

temperature at which the capacitor may be operated measured on the

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Aluminum Electrolytic Capacitors

can.The conductivity of the electrolyte reduces with decreasing

temperature,causing electrolyte resistance, impedance and ESR

increasing.For this reason, minimum permissible operating temperature

are specified for aluminium electrolytic capacitors.

STORAGE TEMPERATURE:-

Storage at high temperature (eg:- upper category temperature) will

reduce the leakage current stability, life and reliability of electrolytic

capacitors.Store capacitors at atemperature of 5 to35 ºC and a humidity

75% maximum.

SAFETY VENT:-

An overpressure device (safety vent) ensuring that the gas can

escape when the pressure reaches a certain value.

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Aluminum Electrolytic Capacitors

CAPACITOR BANK DESIGN

In some applications the required capacitance may not be achieved

by using a single Al electrolytic capacitor. This may be the case if:

- The required electrical charge is too high to be stored in a single

capacitor,

- The voltages that are to be applied are higher than can be attained

by the permissible operating voltage ratings,

- Charge-discharge and ripple current loads would generate more

heat than could be safely dissipated by a single capacitor, and

- The requirements on the electrical characteristics (e.g. series

resistance, dissipation factor or inductance) are so high that it

would be too difficult or even impossible to implement them in a

single capacitor. In these cases, banks of capacitors connected in

parallel or in series or in combined parallel and series circuits will

be used.

Parallel connection of Al electrolytic capacitors

If one of the capacitors in a parallel circuit fails as a result of an

internal short circuit, the entire bank is discharged through the defective

capacitor. In the case of large banks with high energy content this may

lead to extremely abrupt and severe discharge phenomena. It is therefore

advisable to take measures to prevent or limit the short-circuit discharge

current. In smoothing capacitor banks, for example, this is achieved by

installing individual fuses; the principle is shown in figure

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Aluminum Electrolytic Capacitors

This principle is not suitable for capacitor banks designed for

impulse discharges. Here, the capacitors should be protected during the

charging process by means of appropriate resistors. The capacitors are

then connected in parallel immediately before they are to be discharged.

The principle is shown in figure

Series connection of Al electrolytic capacitors

When designing series circuits with Al electrolytic capacitors, care

must be taken to ensure that the load on each individual capacitor does

not exceed its maximum permissible voltage. Here, the fact that the total

dc voltage applied is divided up among the individual capacitors in

proportion to their individual dielectric insulation resistances must be

taken into consideration. Since the dielectric insulation resistance of the

individual capacitors may differ quite strongly, the voltage distribution

may also be non-uniform, which may lead to the permissible voltage of

individual capacitors being exceeded. For this reason, forced balancing of

the voltage distribution is recommended.

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Aluminum Electrolytic Capacitors

The safest method of achieving this is to use electrically isolated

voltage sources for the individual capacitors as shown in figure. If this is

not possible, external balancing resistors RSymm can be connected to the

individual capacitors. The balancing resistances must be equal to one

another, and must be substantially lower than the dielectric insulation

resistance of the capacitor.

Experience has shown that it is preferable to choose balancing

resistance values that will cause a current of approximately 20 times the

leakage current of the capacitor to flow through the resistors. The

equation for calculating the resistance value is:

The balancing measures described above may be omitted in cases

where the total dc voltage to be applied is substantially lower than the

sum of the rated voltages of the capacitors to be used. Experience has

shown that this is possible for n = 2 to 3 single capacitors in series

without any considerable risk if the total voltage does not exceed 0,8 · n ·

UR. However, this solution can only be implemented if the series circuit

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Aluminum Electrolytic Capacitors

consists of matching capacitors (same type, same capacitance), so that the

dielectric insulation resistance of the capacitors, which is the only factor

determining the voltage distribution in this case, will not vary too greatly

from one capacitor to the next.

RSymm 50 M. · µF 1

CR

------- ?=

Combined parallel and series connection

The recommendations given above apply similarly to combinations

of parallel and series circuits. If balancing resistors are to be used, it is

advisable to allocate a separate resistor to each capacitor

Combined parallel / series connection (voltage balancing by shunt

resistors)

The alternative solution, parallel connection of the series capacitors

in the individual branch and the use of one balancing resistor for each

capacitor group, is shown in figure

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Aluminum Electrolytic Capacitors

Combined parallel / series connection (group voltage balancing)

This solution is less complicated, but it has one serious

disadvantage:

If a capacitor in one of the series branches fails and causes a short-

circuit, the total voltage will be applied to the remaining capacitors. This

will lead to a voltage overload and may destroy the remaining capacitors.

In the balancing arrangement shown in figure, only the series branch with

the defective capacitor is subject to this risk, whereas in the more simple

configuration shown in figure, the voltage overload affects all series

branches due to the internal cross-connections, thus causing more severe

damage. For the same reason, internal parallel connections should not be

used in parallel groups connected in series without balancing resistors.

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Aluminum Electrolytic Capacitors

FEATURES AND ADVANTAGES

The main features of aluminum electrolytic capacitors are

1. high mechanical stability

2. They are polarized

3. Charge/discharge proof

4. Long life.

5. Keyed polarity

Advantages:- Large value of capacitance is available in a very small size.

The volume of electrolytic capacitor is more then 10 times less than a

film one considering the same rated capacitance and voltage. Cost per µF

of this capacitor is less when compared to all other types.

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Aluminum Electrolytic Capacitors

CONCLUSION

The advantages of aluminum electrolytic capacitors that have lead

to their wide application range are their high volumetric efficiency (that is

capacitance per unit volume); which enables the production of capacitors

upto 1F capacitance. And the fact that an aluminium electrolytic

capacitor provides a high ripple current capability together with a high

reliability and excellent price/performance ratio. So these are widely used

in electrical and electronics industries. They find their wide applications

in telecommunication and computer industries also.

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Aluminum Electrolytic Capacitors

REFERENCES

Electronics for you, July 2004

http://www.arcotronics.com

http://www.omega-electronics.com

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Aluminum Electrolytic Capacitors

ABSTRACT

Aluminium electrolytic capacitors are widely used for the power

supply circuitry of electronic equipment as these after several advantages

over other types of capacitors. The capacitors are electrical components

that store electrical charge according to the equation Q=CV, where Q is

the charge in coulombs (C), C is the capacitance in Farads (F) and V is

the Voltage (V)

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Aluminum Electrolytic Capacitors

ACKNOWLEDGEMENT

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Aluminum Electrolytic Capacitors

CONTENTS

INTRODUCTION

CONSTRUCTION

TECHANICAL TERMS EXPLANATION

CAPACITOR BANK DESIGN

FEATURES AND ADVANTAGES

CONCLUSION

REFERENCES

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