Aluminum electrolytic capacitor

Details Aluminum electrolytic capacitor Aluminum electrolytic capacitor

2000-01-26

2001-10-23

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Liquid electrolytic capacitor

C361S511000, C361S512000, C361S509000, C361S528000

Reexamination Certificate

active

06307733

ABSTRACT:

This invention relates to an aluminum electrolytic capacitor including an anode and a cathode which are formed of aluminum foil.
BACKGROUND OF THE INVENTION
Many of prior art aluminum electrolytic capacitors generally have the following structure. As shown in
FIG. 1
, a capacitor includes a capacitor element
2
having an anode
14
including aluminum anode lead tabs
12
and an aluminum anode sheet
11
connected together. The anode sheet
11
is formed of aluminum foil. Each anode lead tab
12
has a smooth surface, and the aluminum sheet
11
has its surface roughened and has an anodized film formed thereon. Each aluminum anode lead tab
12
is connected to the aluminum anode sheet
11
by welding at suitable locations
13
. Alternatively, the tabs
12
and the anode sheet
11
may be connected together by needling the stack of the tabs
12
and the anode sheet
11
, bending and pressing resulting burs down against the opposite surface of the stack. (This technique is referred to as “needling” hereinafter.) Further, the capacitor element
2
has a cathode
24
including aluminum cathode lead tabs
22
and an aluminum cathode sheet
21
connected together. The cathode sheet
21
is formed of aluminum foil. Each tab
22
has a smooth surface, and the aluminum cathode sheet
21
has its surface roughened. Each aluminum cathode lead tab
22
is connected to the aluminum sheet
21
by “needling” or welding at suitable locations
23
. The anode
14
and the cathode
24
are rolled together into a cylindrical shape, with separator sheets
1
sandwiching either the anode
14
or the cathode
24
. The roll is then impregnated with an electrolytic solution to thereby form the capacitor element
2
.
Then, as shown in
FIG. 2
, the capacitor element
2
is encapsulated in a cylindrical aluminum casing
3
. The casing
3
has an opening, which is hermetically sealed with a sealing structure
6
including a plate
4
of synthetic resin and a rubber plate
5
disposed on the resin plate
4
. The sealing structure
6
has an explosion preventing valve
7
including an aperture formed through the resin sheet
4
and a thinned portion formed in the rubber sheet
5
. Metallic terminal members
10
and
20
extend through the sealing structure
6
. The anode lead tabs
12
extending out of the capacitor element
2
are joined together and connected to the terminal member
10
within the casing
3
, and the cathode lead tabs
22
are joined together and connected to the terminal member
20
within the casing
3
. The lower portion of the capacitor element
2
is fixed to the inner surface of the casing
3
with a fixing material
8
.
Referring to FIG.
6
(
a
), the anode
14
of the above-described prior art capacitor includes the aluminum anode sheet
11
with relatively thick anodic oxide films
31
on its opposite surfaces, which result from electrolytic processing at a high voltage above a capacitor rating voltage applied thereto. The aluminum cathode sheet
21
of the cathode
24
has thin oxide films
32
on its opposite surfaces, which result from spontaneous oxidation of the sheet
21
or from electrolysis with a low voltage of several volts. The aluminum sheets for the anode lead tabs
12
and the cathode lead tabs
22
has a thickness of about 200 &mgr;m. The surfaces of the anode and cathode lead tabs
12
and
22
are not roughened by, e.g. etching. The surfaces of each anode lead tab
12
is covered with an electrochemically formed oxide film, while the surfaces of each cathode lead tab
22
is covered with an oxide film formed through spontaneous oxidation.
The above-described electrolytic capacitor charges and discharges in the following manner. As shown in FIG.
6
(
a
), the electrostatic capacitance of the electrolytic capacitor can be considered to be a series combination of the capacitance exhibited between the anode sheet
11
and the separator
1
impregnated with the electrolytic solution, with the oxide film
31
interposed therebetween, and the capacitance exhibited between the aluminum cathode sheet
21
and the separator
1
with the oxide film
32
interposed therebetween. Since the oxide film
32
is considerably thin relative to the oxide film
31
, the capacitance associated with the oxide film
32
should be far larger than the capacitance associated with the oxide film
31
. On the other hand, a very large leakage current is associated with the oxide film
32
. Accordingly, when a voltage V is applied between the anode sheet
11
and the cathode sheet
21
as shown in FIG.
6
(
a
), the voltage Va across the oxide film
31
is larger than the voltage Vc across the oxide film
32
. The apparent capacitance per unit area of the capacitance associated with the oxide film
31
is Ca (&mgr;F/cm
2
), and the apparent capacitance per unit area of the capacitance associated with the oxide film
32
is Cc (&mgr;F/cm
2
). The amounts of charge stored in these capacitances are Qa and Qc, respectively.
When the two terminals of the above-described electrolytic capacitor charged to the voltage V are connected together, the two capacitances Ca and Cc are connected in parallel as shown in FIG.
6
(
b
), so that the voltage between the two terminals becomes Vc′ due to dicharge of the charge on the smaller capacitance Cc, and charge of Qa−Qc remains. Since the overall capacitance is Ca+Cc and the stored charge is Qa−Qc, the remaining voltage Vc′ is expressed by the following expression (1).
Vc
′
=
(
Qa
-
Cc
)
/
)
⁢
(
Ca
+
Cc
)
=
CaVa
-
CcVc
Ca
+
Cc
(
1
)
If the voltage applied across the cathode oxide film
32
during discharging is excessive, an oxide film may be further grown on the cathode sheet
21
, which may cause undesirable things, such as generation of gas, to occur within the capacitor casing
3
. Then, the remaining voltage Vc′ expressed by the expression (1) must be equal to or smaller than the maximum voltage V′ which can be applied across the cathode oxide film
32
without growing any additional oxide film during discharging. In other words, the condition expressed by the following expression (2) must be met during discharge.
V
′
≥
CaVa
-
CcVc
Ca
+
Cc
(
2
)
Since Va=V−Vc, the following expression (3) can be derived from the expression (2).
V
′
≥
V
1
+
(
Cc
/
Ca
)
-
Vc
(
3
)
A ripple waveform resulting from rectifying an AC voltage and a rectangular charge and discharge voltage waveform contain portions in which the voltage rapidly changes from the maximum value to the minimum value in short time intervals. If the condition expressed by the expression (3) is met, no oxide film growth takes place on the cathode sheet
21
even when such rapidly changing current or voltage is applied to the cathode sheet
21
. In prior art, a major attempt to improve the ripple insensitivity and the charge-and-discharge insensitivity of an electrolytic capacitor has been to fulfil the condition expressed by the expression (3). For example, it has been done to use a cathode sheet having a large capacitance per unit area, or a sheet with an additional oxide film intentionally pre-formed on it having a high withstand voltage. The term “ripple insensitivity” is used in this specification to represent a property of a sheet of, for example, aluminum, that an oxide film does not grow or hardly grow on the sheet when ripple current above an allowable magnitude is applied to the sheet. The term “charge-and-discharge insensitivity” used in this specification is a measure indicating how an oxide film does not grow when a large voltage difference occurs between charging and discharging of a capacitor.
There is a limit to the prior art improvement of the ripple insensitivity and the charge-and-discharge insensitivity of an electrolytic capacitor. The inventors have conducted experiments on electrolytic capacitors which have been judged to be sufficiently ripple and charge-and-discharge insensitive. They have made analysis of such electrolytic capacitors used in circuits in which ripple current

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