FIELD OF INVENTION:
This invention relates to film foil self healing inductive type capacitor.
ABSTRACT
Film foil self healing inductive type capacitor having self-healing property (which can self heal) is disclosed. To provide self-healing in a inductively wound film foil capacitor at a reasonable cost
BACKGROUND:
Film capacitors can be divided into three basic construction types: film/foil capacitors, metallized film capacitors, and mixed technology capacitors.
Film/foil capacitors consist of two metal foil electrodes made of aluminum foil separated by a piece of plastic film which serves as the dielectric medium. The plastic film can be polyester, polypropylene or polycarbonate (PEN and PPS). There are other types of plastic films but these films are used in specialized applications. The thickness of the plastic film typically ranges from 2 µm to 24 µm and usually the dielectric film thickness is derived from voltage rating of the capacitor as is illustrated in the subsequent description. The aluminum foil thicknesses range from 4 µm to 9 µm
A film/foil capacitor is made by alternating two pieces of aluminum foil with two layers of plastic film. These interleaved layers are wound around a spindle in a manner that prevents the metal layers from touching.
Film/foil capacitors can be wound in two different ways - inductive and non-inductive. Normally it is cheaper to fabricate the inductively wound capacitor than the non-inductive capacitor.
Inductively wound capacitors have the aluminum foils centered between the layers of the dielectric films. Connection to the foils is accomplished by inserting the wires on the foils during winding. The location of the lead wires on the foils determines the amount of inductance for the capacitor.
In case of non-inductive capacitors, orientation of the foils is done in such a manner so that one side of each foil extends beyond the dielectric film in either one or the other direction. Therefore this type of winding is also known as the extended foil.

Connections in this type of winding to each electrode is accomplished by first connecting the portions of all extended foils together with a metal end spray or soldering at either end and then welding a lead wire to the metal spray or soldered side. Thus the additional expense of the metal end spray / soldering increases the fabrication cost of the capacitor. The inductance of the capacitor is determined by the width of the dielectric film which in itself is a drawback as in heavy rated capacitor the width of the dielectric film will increase. The inductance offered by this heavy rated capacitor unit will increase too which is an undesired feature.
Film/foil capacitors in general are characterized by high insulation resistance, good capacitance stability, low losses even at high frequencies, and high pulse handling capabilities. Inductive capacitors are characterized by low cost, small lead spacing, lower operating frequency range and lower operating temperature range whereas non-inductive capacitors are characterized by wide operating temperature range, high pulse current rating and wide operating frequency range. However, these are more expensive to fabricate than the inductively wound film/foil capacitor.
Whatever winding technique may be used one major inherent drawback of these types of capacitor family is their vulnerability to high voltage. When high voltage is applied across the capacitor the dielectric which in principle is the insulator, can be punctured by the electric charges, leading to short-circuit. Pinholes (i.e. dielectric weak points) are inevitable in dielectric film used in film foil or for that matter in any other dielectric film based capacitor. Hence whenever film/foil capacitor is subjected to high voltage there is a possibility that the foils may be exposed to each other and may even touch each other if the dielectric medium is depleted to a large extent resulting in electric short-circuiting rendering the capacitor useless. So, reliability of film foil capacitor in high voltage spike applications is low.
To overcome the above described drawback the second type of construction that is available is popularly known as metallized film capacitors. They differ from film/foil capacitors in the sense that the aluminum foils are replaced by a thin layer of metal deposited onto the dielectric film itself. This is accomplished using vacuum deposition technology. These vacuum deposited thin metallically conductive layers serve as the electrodes in this type of capacitors. The metal layer is typically aluminum or zinc that is extremely thin and the resistance value is in the range of 1.2 ohms / square to 12.0 ohms / square. It is so thin that you could see through when a single layer of metallized

film is held up against a light.
The capacitors are produced by winding in the same manner as a non-inductive capacitor is wound such that the vacuum deposited thin metal layers are oriented in such a manner that they seem to extend to either side along with the portion of the dielectric film on which the extended thin metallized film is deposited. The said thin metal layers deposited on the dielectric extend into the end metal spray which are used to make the lead connections as described above in the non-inductive wound capacitor unit construction.
The main advantages of these metallized capacitors are their reduced physical size due to the thin metal deposit layer instead of the metal foils and more importantly their inherent feature of self healing in case of shorting due to over voltage spikes.
The self-healing property is inherent to capacitors with metallized films and is their single biggest advantage over film/foil capacitors. Self-healing is a phenomenon where in the event when high voltage is applied across the capacitor unit and the electrodes are exposed to each other because of the depletion of the dielectric medium film as described above, instead of the capacitor shorting, the capacitor repairs itself. This repairing of the capacitor is basically due to the thinness of the vacuum deposited metalized layers serving as the electrodes in this case.
In a film/ foil capacitor when the foils are exposed to each other, the foils would touch and short together leading to failure of the capacitor unit. When a capacitor with metalized films has the thin metalized layers of foils exposed to each other, they also will touch each other, but here the combination of the foil's thinness and the high energy density at the fault area causes the thin metalized deposited layers of foils to vaporize. Insulating halos are formed around the puncture and the capacitor stays in operation. The cause of short-circuit is effectively burned away.
During self-healing only a fraction of the energy stored in the capacitor is dissipated with the self-healing lasting less than 10 seconds. There is also marginal reduction in terms of the capacitance capability of the metallized capacitor unit in description but that is a small price to pay as the capacitor is still in operation.
Metallized film capacitors are characterized by small size, wide operating frequency

range, and low losses, low to medium pulse handling capabilities and self-healing. These metallized film capacitors provide self-healing and other advantages but are more expensive to fabricate. At the same time their inherent feature of non-inductive type winding i.e. use of end metal sprays to take out lead connection makes them complicated to fabricate and expensive.
Another type that needs to be described in relation to the present specification is known as the mixed technology capacitors. They are basically a combination of either film/foil, or metallized film capacitor types. Normally these are high voltage capacitors with internal series connections. Mixed technology capacitors have the characteristics of both capacitor types, such as high pulse carrying capabilities and self-healing properties. But despite of all their advantages they still are expensive to fabricate and definitely complicated to produce because of the combination of both film/foil and metallized type of capacitors.
SUMMARY OF THE INVENTION:
The object of the invention is therefore to provide an inductively wound film/ foil capacitor with self-healing properties.
Another object of the invention is to provide for inexpensive inductively wound self-healing capacitors fabricated without using the end metal spray technology or using the soldering.
Yet another object of the invention is to provide for an inductively wound self healing capacitor that allows for internal series construction during the fabrication process.
Accordingly the instant invention provides for self-healing inductively wound film foil capacitor comprising a pair of aluminum foils (AL1, AL2), a pair of plain films (PL1 and PL2), a pair of protective film (PR1 and PR2), single sided metallized film (SML) wherein said aluminum foils are planer but do not run parallel to each other. The said aluminum foils (AL1, AL2) are distanced so as to maintain a separation margin in the range of 0.5 mm to 2 mm depending on the voltage specification of the said film foil capacitor unit.
The said single sided metallized film (SML) is placed between the said aluminum foil (AL2) and plain film (PL1) acting as the main dielectric, said aluminum foils AL1 and

AL2, plain films (PL1 and PL2), protective films (PR1 and PR2) and the single sided metallized film (SML) being inductively wound to achieve self-healing in a cost effective manner under all operating voltage condition.
The inductive winding of the said film foil capacitor pertains to insertion of lead wire connections (LW1, LW2) on said planarly arranged pair of aluminum foils (AL1, AL2). The exposure of the said capacitor to electric breakdown leads to vaporization of the said thin metallized deposit of the single sided metallized film (SML) initiating self healing of the said capacitor unit thereby preventing the said planarly arranged aluminum foils (AL1, AL2) from short circuiting.
The said thin metallized deposit of the single sided metallized film (SML) and the first aluminum foil (AL1) is so disposed so as to form a capacitor unit (CI) with the plain film (PL1) serving as the dielectric, while the said metallized deposit forms a capacitor unit (C2) with the plain film portion of the SML serving as the dielectric in the said capacitor unit (C2) during normal operating voltage conditions.
The cost-effectiveness in the operation of this invention is achieved by the planar disposition of the aluminum foils and the placement of the SML in the capacitor unit. The plain film of the said SML bears metallized deposits on both sides to form double sided metallized film (DSML) which along with an additional plain film (PL3) serving as a dielectric medium forms yet another cost effective capacitor unit thereby improving the current carrying capacity and the rated AC voltage.
BRIEF DESCRIPTION OF THE DIAGRAM:
Fig 1 illustrates a film foil self healing inductive type capacitor depicting internal series construction.
Fig 2 illustrates a film foil self healing inductive type capacitor having double sided metallized film depicting internal series construction.
DETAILED DESCRIPTION OF THE INVENTION IN RELATION TO THE DRAWINGS:
The present invention pertains to the product and processes relating to film foil self

healing inductive type capacitor. To obtain the said objective a metallized film is introduced between the main dielectric film (plain film) and the aluminum foils while these elements are inductively wound in the said capacitor unit construction. The aluminum foils are planar, existing in the same plane, but do not run parallel to each other. The main advantages in introducing this metallized layer in the capacitor unit is to provide self healing of the capacitor in case of electrical breakdown in a cost effective manner. It also achieves internal series construction in normal operating voltage condition and increases the rated AC voltage of the said capacitor thus reinforcing its reliability.
Since the film/foil self healing inductive type capacitor is with internal series construction, the voltage across the capacitor will be divided internally and the AC voltage rating will be increased making it an affordable and reliable capacitor.
To better explain the invention in relation to the drawings, the legend is provided below:
AL1, AL2 - Aluminum foil 1, Aluminum foil 2
PL1, PL2 - Plain film 1 and Plain film 2
PR1, PR2 - Protective Film 1 and Protective Film 2
LW1, LW2 - Lead Wire 1 and Lead Wire 2
SML - Single-sided metallized film
DSML - Double sided metallized film comprising of metallized deposits ML1
and ML2 deposited on either side.
The instant invention alternates a pair of aluminum foils (AL1, AL2) with a pair of plain film (PL1, PL2) as the basic building blocks of the capacitor unit. The plain film (PL1) and metallized film with both side margin (SML) is the main dielectric medium. These interleaved layers of aluminum foils (AL1 and AL2) and plain film (PL1), incorporate the protective films (PR1, PR2) along with the single sided metallized film (SML).
These various elements of the invention are wound around a spindle in such a manner that the aluminum foils and the SML are prevented from coming in contact with each other. Further this novel arrangement also prevents all other elements of the said capacitor unit from coming in direct contact with each other under normal operating conditions. These said elements are inductively wound such that the said aluminum foils (AL1, AL2) are planar, existing in the same plane, but do not run parallel to each

other. The said aluminum foils (AL1, AL2) are distanced so as to maintain a separation margin in the range of 0.5 mm to 2 mm depending on the voltage specification of the said film foil capacitor unit.
These are disposed amid the layers of the plain film (PL1, PL2) and single side metallized film which act as the main dielectric medium. The connection to the aluminum foils is provided in such a manner that the lead wires (LW1, LW2) are inserted against the said planar aluminum foils (AL1, AL2) during the process of inductive winding.
In normal voltage conditions the said thin single sided metallized film (SML) along with AL1 will serve as a pair of electrode of a capacitor unit with the PL1 serving as the dielectric thus completing the first capacitor unit. In the same arrangement, a second capacitor unit is formed by the said SML and AL2 serving as the electrodes with the plain film on which the metal layer has been deposited serving as the dielectric medium. This novel arrangement facilitates the internal series construction.
In case of an electric breakdown such as when an overvoltage occurs across the said capacitor unit it results in the depletion of the main dielectric medium unit. The excess energy i.e. the voltage is absorbed by the said metallized film (SML) leading to its vaporization of the vacuum deposited metallized portion of the SML thus preventing the said main aluminum foils (AL1, AL2) of the said capacitor unit from shorting. The capacitor unit thus remains operational but at a marginally reduced functionality.
The above described process achieves the self-healing in an inductively wound cost effective film/foil capacitor without incurring the cost of expensive process of deposition of end metal sprays normally used in self-healing capacitors. Thus the inductively wound self healing film foil capacitor provides the benefit of metallized film capacitor at a cost that is almost in the range of 10% to 30% less than that of non inductive metallized film capacitor.
Besides the above, another feature of the instant invention is that the plain film on which the metal was deposited acts as an additional dielectric medium since it remains even after the depletion of the metallized portion to obtain the said self-healing. In case of the electric breakdown as mentioned above, the said plain film forms highly compressed plasma, which pushes the layers of the dielectric apart thereby providing

additional protection by shielding the aluminum foils from coming in contact and shorting.
In another embodiment instead of the single sided metallized film (SML), the capacitor unit has a double sided metallized film (DSML). The double sided metallized film (DSML) comprises of a pair of metallized deposits ML1 and ML2 deposited on either side of the plain film along with an additional plain film (PL3) serving as one of the main dielectric.
When this embodiment is subjected to normal operating voltage conditions, the arrangement works in the following manner:
a. AL1 along with ML1 are the two electrodes of a capacitor with PL1 serving as the
dielectric medium to complete the capacitor unit CI
b. ML2 along with AL2 act the two electrodes of a capacitor with the newly
introduced plain film PL3 serving as the dielectric medium to complete the
capacitor unit C2)
In the event of electrical breakdown ML1 and ML2 are vaporized to prevent AL1 and AL2 from coming in contact. Thus the capacitor avoids shorting and is able to achieve self-healing. Also because of 3 dielectric media present in this embodiment the aluminum foils AL1 and AL2 are further shielded from shorting. By using double sided metallized film and a capacitor unit provided with greater reliability and improved current carrying capability, AC voltage rating of the capacitor is elevated.
Also if an electric breakdown occurs, the dielectric forms highly compressed plasma, which pushes the layers of the dielectric apart and as the plain film on which the said metallized film is placed also acts as an additional layer of dielectric in the said capacitor unit, it provides extra protection for the main aluminum foils in case of an electric breakdown.
These elements are wound in an inductive manner to form capacitor in order to increase reliability as specified in the description above, and also AC rated voltage of the capacitor has been elevated because of the internal series construction that has been achieved. The said metallized film placed on the said plain film will act as an electrode to form a capacitor unit along with one of the said first aluminum foil and the said main dielectric medium dispensed between them and another capacitor unit with the second

aluminum foil along with the plain film having the metallized film deposited on it, acting as the dielectric medium in this unit but only until a voltage spike occurs in which case it will vaporize to save the aluminums foil from coming in contact with each other and hence provide the said self-healing. Therefore the said AC rating is elevated because of this internal series construction. This type of capacitors are more expensive than the normal inductively wound film/foil capacitors but less expensive than the metallized non inductively wound film capacitors. Introduction of single-sided metallized deposit or double-sided metallized deposit on the plain filmland the planer arrangement of aluminum foils drastically decreases the possibility of failure during high voltage spikes at almost 25% less cost of the metallized capacitor.
An embodiment of the present invention incorporates a pair of aluminum foils arranged in single plane running short of parallel. These are alternatively wound with a couple of plain films serving as the main dielectric medium and two protective films encompassing a single sided metallized film. These elements are inductively wound and have the said aluminum foils arranged between the layers of the said main dielectric films in a manner that the lead connections to these capacitor is accomplished by inserting the said lead wires on the said aluminum foils arrangement during winding.
In normal voltage conditions the said thin single sided metallized film will serve as a proper electrode of a capacitor unit either as a cathode or an anode with one of the said aluminum foils across the said main dielectric medium (one of the said plain film) acting as the other electrode depending on the voltage polarity applied across the said lead wires, thereby completing a single said capacitor unit.

We claim:
1. A film foil self-healing inductively wound capacitor comprising
- a pair of aluminum foils (AL1, AL2),
- a pair of plain films (PL1 and PL2),
- a pair of protective film (PR1 and PR2),
- single sided metallized film (SML) wherein
said aluminum foils are planer but do not run parallel to each other,
said single sided metallized film (SML) is placed between the said
aluminum foil (AL2) and plain film (PL1) acting as the main dielectric,
said aluminum foils AL1 and AL2, plain films (PL1 and PL2), protective films
(PR1 and PR2) and the single sided metallized film (SML) being inductively
wound to achieve self-healing in a cost effective manner under all operating
voltage condition.
2. A film foil self-healing inductively wound capacitor as claimed in claim 1 wherein the said inductive winding of the said film foil capacitor pertains to insertion of lead wire connections (LW1, LW2) on said planarly arranged pair of aluminum foils (AL1, AL2).
3. A film foil self-healing inductively wound capacitor as claimed in claim 1 wherein the said aluminum foils (AL1, AL2) are distanced so as to maintain a separation margin based on the voltage specification of the said film foil capacitor unit.
4. A film foil self-healing inductively wound capacitor as claimed in claim 4 wherein the said separation margin is in the range of 0.5 mm to 2 mm
5. A film foil self-healing inductively wound capacitor as claimed in claim 1 wherein exposure of the said capacitor to electric breakdown leads to vaporization of the said thin metallized deposit of the single sided metallized film (SML) initiating self healing of the said capacitor unit thereby preventing the said planarly arranged aluminum foils (AL1, AL2) from short circuiting.
6. A film foil self-healing inductively wound capacitor as claimed in claim 1 wherein the said thin metallized deposit of the single sided metallized film (SML) and the first aluminum foil (AL1) is so disposed so as to form a capacitor unit (CI) with the plain film (PL1) serving as the dielectric, while the said metallized deposit forms a capacitor unit (C2) with the plain film portion of the SML serving as the dielectric in the said capacitor unit (C2) during normal operating voltage conditions.
7. A film foil self-healing inductively wound capacitor as claimed in claim 1 wherein the plain film of the said single sided metallized film (SML) bears metallized deposits on both sides to form double sided metallized film (DSML) which along with an additional plain film (PL3) serving as a dielectric medium forms yet another cost effective capacitor unit thereby improving the current carrying capacity and the rated AC voltage.