GB2057765A - Capacitor comprising stripline connections - Google Patents

Abstract

A capacitor comprising a capacitor roll 3 whose anode and cathode foils are connected to external connections by way of a number of flexible electrical striplines. Each stripline has a pair of conductive strips 13, 15 separated by an electrically insulating strip 14, or conclusive strip being connected to the anode while the other is connected to the cathode. As a result of these stripline connections, a capacitor is obtained which has a comparatively low equivalent series impedance for frequencies beyond the resonant frequency.

Description

SPECIFICATION Capacitor comprising stripline connections The invention relates to a capacitor, comprising a capacitor roll which is accommodated in a housing which is closed by a lid, an anode foil and cathode foil of said capacitor roll each being electrically connected to at least two external connections by means of a plurality of internal connection tabs which are distributed over the length of the respective foils.

In a known capacitor of the described kind (make Philips type 2222 106 3, described in Philips Technical Information", dated Sep- tember 22, 1975, page 5), the connection tabs connected to the anode foil are situated at one end of the capacitor roll, whilst the connection tabs connected to the cathode foil are situated at the other end of the capacitor roll. Because the connection tabs which are situated at the one end of the capacitor roll and which are connected to the anode foil at mutually equal distances are electrically parallel connected to the connection tabs which are situated on the other side of the capacitor roll and which are connected to the cathode foil at mutually equal distances (measured in the longitudinal direction of the foil), a capacitor is obtained whose so-called equivalent series impedance is comparatively low.

It is a drawback of the known capacitor that this equivalent series impedance is still higher than desirable for high frequency applications.

This is mainly due to the component of the equivalent series impedance which is due to the equivalent series inductance.

It is an object of the invention at least to mitigate this drawback.

The invention accordingly provides a capacitor comprising a capacitor roll which is accommodated in a housing which is closed by a lid, an anode foil and a cathode foil of said capacitor roll each being electrically connected to at least two external connections by means of a plurality of internal connection strips which are distributed over the length of the respective foils, characterized in that the internal connection strips form electrical striplines each of which is formed by two flexible electrically-conductive strips which are separated by an electrically-insulating strip, one strip of each pair of electrically conductive strips forming part of one and the same stripline being connected to the anode foil of the capacitor roll whilst the other strip is connected to the cathode foil of the capacitor roll at a location which adjoins the location where the said one strip is connected to the anode foil.

It is to be noted that from United States Patent Specifications 3,654,524 and 3,611,051 capacitors are known which comprise a connection in the form of a stripline. A capacitor of this kind is formed by the stacking of a number of flattened capacitor rolls.

Between two of the capacitor rolls in the stack there are inserted the longer legs of two Lshaped metal terminals which are separated by an electrically insulating layer. These Lshaped terminals are connected to each capacitor roll by welding and form a connection in the form of a stripline. Because the two Lshaped terminals have to be welded to the anode foil and the cathode foil within the stack of capacitor rolls, the ratio between the widths of the foils and their overlap has to be very large. As a result, only a comparatively small volume of the stack of capacitor rolls effectively forms capacitance. The capacitance per unit of volume, therefore, is comparatively low in a capacitor of this kind.

However, in a capacitor in accordance with the invention, the overlap between anode foil and cathode foil may be maximum in theory.

In order to minimize the risk of short-circuits at the end faces of the capacitor roll, the value used in practice is slightly smaller than this maximum value.

Embodiments of the invention will be described in detail hereinafter with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a side elevation of a first embodiment of a capacitor in accordance with the invention, Figure 2 is a top view of the capacitor shown in Fig. 1, Figure 3 is a sectional view at an increased scale, taken along the line Ill-Ill in Fig. 2, Figure 4 is a sectional view, on an increased scale, taken along the line IV-IV in Fig. 1, Figure 5 is partly a side elevation and partly a longitudinal sectional view of a second embodiment of a capacitor in accordance with the invention, Figure 6 is a sectional view taken along the line VI-VI in Fig. 5, Figure 7 shows the striplines in a capacitor in accordance with the invention, Figure 8 shows a graph of the equivalent series impedance and the equivalent series resistance as a function of the frequency for a capacitor as shown in Fig. 1, Figure 9 shows a graph of the equivalent series impedance and the equivalent series resistance as a function of the frequency for a capacitor of the type shown in Fig. 1 but comprising two connections to the anode foil and two connections to the cathode foil, Figure 10 shows a graph of the equivalent series impedance and the equivalent series resistance as a function of the frequency for a comparable known capacitor comprising two connections of a known type to the anode foil and two connections of a known type to the cathode foil, and Figure 11 shows a graph of the equivalent series impedance and the equivalent series resistance as a function of the frequency for a capacitor as shown in Fig. 5.

The first embodiment of a capacitor in accordance with the invention which is shown in the Figs 1 to 4 comprises a capacitor roll 3 of a known kind which is accommodated in a cylindrical aluminium housing 1. A capacitor roll of this kind may be composed of an anode foil of formed aluminium, a first separator foil of paper, an aluminium cathode foil, and a second insulating separator foil. The first separator foil may be impregnated with an electrolyte. The embodiments of the invention will be described with reference to an electrolytic capacitor, but it is by no means restricted thereto and can be used in all socalled wound capacitors. The housing 1 is closed by a lid 5 of an electrically-insulating material such as, for example, a hard rubber.

The capacitor roll 3 is secured in the housing 1 in a customary manner by means of a circumferential ridge 7. The anode foil 9 as well as the cathode foil 11 of the capacitor roll 3 (see Fig. 7) are connected, by means of four flexible, metal connection strips 1 3 and 15, respectively, to an L-shaped metal connection terminal 1 7 and an L-shaped metal connection terminal 19, respectively. Both strips in each of the four pairs of connection strips (13, 15) are separated by a respective strip 14 of an electrically-insulating material and, in conjunction with this strip 14, form a so-called electrical stripline having a comparatively low inductance. These four striplines extend between the L-shaped connection terminals 1 7 and 1 9 and the capacitor roll 3.As appears from Fig. 7, the stris are separated by electrically-insulating material only in the vicinity of the L-shaped terminals 1 7 and 1 9 and the capacitor roll 3. The connection strips belonging to one and the same pair are situated substantially one opposite the other (adjacently) in the rolled-out condition of the roll.

The connection strips of the anode foil and of the cathode foil are regularly distributed over the length of the relevant foil, i.e. the locations where the connection strips are connected to the relevant foils are situated at approximately the same distance from each other. The L-shaped terminals 1 7 and 1 9 are electrically insulated from each other by means of a plate 21 of an electrically-insulating material and together form a T-shaped construction which again contains a stripline having a low inductance. The latter stripline is formed by the longer, co-extensive, legs of the L-shaped terminals 1 7 and 1 9 and the plate 21.

Each of the L-shaped terminals 1 7 and 1 9 of the capacitor shown in Figs. 1 to 4 is provided with two threaded studs 23 and 25, respectively, for external connections. The studs 23 and 25 are symmetrically situated with respect to the plate 21 (see Fig. 2), the interconnected studs 23 being connected to the anode foil whilst the interconnected studs 25 are connected to the cathode foil. Because the L-shaped terminals 1 7 and 1 9 are situated completely within the housing 1 of the capacitor, the studs 23 and 25 extend through the lid 5 for external access.

As will be illustrated hereinafter with reference to Figs. 8 to 10, the described capacitor not only has a comparatively low equivalent series resistance, but in combination therewith also a comparatively low equivalent series inductance. This makes the capacitor extremely suitable for high frequency applications.

In the second embodiment of a capacitor in accordance with the invention which is shown in Figs. 5 and 6, the T-shaped construction formed by the assembly of two L-shaped terminals 1 7 and 1 9 and the insulating plate 21 is arranged in a location other than that in the capacitor shown in Figs. 1 to 4. Because the capacitor shown in Figs. 5 and 6 resembles the capacitor shown in Figs. 1 to 4, corresponding reference numerals are used for corresponding parts. The T-shaped construction of the terminals shown in Figs. 5 and 6 is situated partly within and partly outside the housing 1, because the longer legs of the Lshaped terminals 1 7 and 1 9 extend through the lid 5.On the lower surface (as viewed in Fig. 5) of the shorter legs of the L-shaped terminals 1 7 and 19, two respective connection terminals 23 and 25 are secured. The upper surfaces of the T-shaped construction, therefore, can be mounted directly against a power supply strip. Otherwise, the construction of the capacitors shown in the Figs. 1 and 5 is the same.

As will be clarified with reference to Figs.

10 and 11, the capacitor shown in the Figs. 5 and 6 not only has a comparatively low equivalent series resistance, but in combination therewith also a comparatively low equivalent series inductance, which makes the capacitor very suitable for high frequency applications.

The characteristics of the capacitors associated with Figs. 8 to 11 can be summarized as follows: Figure 8 (capacitor of the type shown in Figs. 1 to 4): capacitance: 27000 mF operating voltage: 20 V operating temperature: 25"C inductance: 1.82 nH number of pairs of connection tabs (13, 15): four Figure 9 (capacitor of the type shown in the Figs. 1 to 4): capacitance: 27000 mF operating voltage: 20 V operating temperature: 25"C inductance: 2.54 nH number of pairs of connection tabs (13, 15): two Figure 10 (known capacitor of the type shown in the Figs. 1 to 4, however, without striplines between capacitor roll and L-shaped terminals): capacitance: 9000 mF operating voltage: 20 V operating temperature: 25"C inductance: 25.32 nH number of pairs of connection tabs: two Figure 11 (capacitor of the type shown in Figs. 5 and 6): capacitance 10000 mF operating voltage: 6 V operating temperature: 25"C inductance: 2.00 nH number of pairs of connection tabs (13, 15): four For the equivalent series impedance Z shown in the graphs of the Figs. 8 to 11 as a function of the frequency, the following is applicable: Z= ESR + ECR + ESL, in which ESR is the equivalent series resistance, ESC is the equivalent series capacitance, and ESL is the equivalent series inductance.

Because the Z curve as well as the ESR curve is shown in Figs. 8 to 11 and because the contribution to the Z curve by the equivalent series capacitance ESC is negligibly small for frequencies higher than the resonant frequency (which is at the minimum in the Z curve), the difference between the Z curve and the ESR curve can be considered to be the contribution of the equivalent series inductance ESL to the equivalent series impedance Z for any frequency beyond the resonant frequency.

A comparison of Figs. 8 and 9 with Fig. 10 reveals that in a capacitor in accordance with the invention, the gradient of the Z curve and hence the equivalent series inductance ESL beyond the resonant frequency is substantially less, thanks to the striplines between the capacitor roll and the L-shaped terminals.

Figs. 8 and 9 also demonstrate that, when the number of pairs of connection tabs is halved, the equivalent series resistance ESR increases.

A comparison of Fig. 11 with Fig. 10 reveals about the same for the capacitor shown in the Figs. 5 and 6.

It will be clear that the use of the described loyv-inductance stripline connections is not restricted to electrolytic wound capacitors, but can also be advantageously used for nonelectrolytic (dry) wound capacitors.

Claims (3)

1. A capacitor, comprising a capacitor roll which is accommodated in a housing which is closed by a lid, an anode foil and a cathode foil of said capacitor roll each being electrically connected to at least two external connections by means of a plurality of internal connection strips which are distributed over the length of the respective foils, characterized in that the internal connection strips form electrical striplines each of which is formed by two flexible electrically-conductive strips which are separated by an electrically-insulating strip, one strip of each pair of electrically conductive strips forming part of one and the same stripline being connected to the anode foil of the capacitor roll whilst the other strip is connected to the cathode foil of the capacitor roll at a location which adjoins the location where the said one strip is connected to the anode foil.

2. A capacitor as claimed in Claim 1, characterized in that the said external connections comprise two L-shaped metal terminals which are separated by an insulating layer and which are electrically-connected to the capacitor roll via said striplines.

3. A capacitor substantially as hereinbefore described with reference to the accompanying drawings.