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. 1
33
Embed
Basic of Aluminium Electrolytic Capacitors for Optimum Perfo
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
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.
1
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:
2
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
3
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.
4
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.
5
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.
6
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.
7
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