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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