Single and dual low inductance capacitor and header therefor

A capacitor which includes a case containing a capacitor element with multiple tabs extending therefrom is provided with a header which carries a pair of terminal members. Each of the terminal members has a pair of parallel connector leads extending out of the header connected by terminal portions inside the case. Each of the terminal portions includes portions parallel and nonparallel to corresponding terminal portions of the other terminal member and each of the tabs is welded to the outside of a parallel terminal portion.

BACKGROUND OF THE INVENTION 
The present invention relates generally to minimizing impedance in 
capacitors and more particularly to low resistance, low inductance 
capacitors having low manufacturing costs and high capacitance to volume 
ratios. 
All capacitors have series equivalent circuits with impedance consisting of 
capacitance, resistance, and inductance parameters, and conventional 
electrolytic capacitors have resonant frequencies between 5 and 100 
kilohertz depending on size, voltage rating, and construction. At 
frequencies below the resonant frequency, the capacitor impedance is 
primarily affected by the capacitance and is minimized by maximizing 
capacitance. While the total resistance effect, or equivalent series 
resistance (ESR), has minimal influence on the impedance except at 
frequencies near the resonant frequency, the effect of ESR is noticeable 
in heat generation at all frequencies. The effect of the ESR is minimized 
by minimizing the ESR. Above the resonant frequency, the capacitor 
impedance is primarily affected by the total inductance, or equivalent 
series inductance (ESL) and is minimized by minimizng the ESL. 
In the past, the capacitors used in conventional D.C. power supplies, which 
rectified 60 hertz line voltage to supply the desired D.C. level operated 
in ranges below the resonant frequency and the effects of the ESR and ESL 
were minimal. With the increased acceptance of switching regulated D.C. 
power supplies which operate at frequencies between 10 and 100 kilohertz 
(with resulting 20 to 200 kilohertz ripple and harmonics), new low ESR and 
ESL capacitors are required because normal operation is in ranges at or 
above the resonant frequencies. 
Various other capacitors have been developed having relatively low ESR's 
and low ESL's such as those shown in the Puppolo, et al, U.S. Pat. No. 
3,822,397 and the Voyles, et al, U.S. Pat. No. 3,806,770. However, since 
the terminals have not been side by side, or juxtapositioned, along their 
entire lengths inside the case, significant impedance still occurs in 
these type capacitors. Further, the split terminal cover design of the 
Puppolo patent and the tantalum capacitor design of the Voyles patent are 
expensive to manufacture and have low capacitance to volume ratios. 
Also, in switching regulated power supplies with previous capacitors, 
additional capacitors having higher ESL's were required to filter out the 
20 to 200 kilohertz ripple and harmonics fed to the load and from the 
switching regulator back into the main 60 hertz power supply line. 
SUMMARY OF THE INVENTION 
The present invention provides a capacitor and header in which the header 
carries a pair of spaced apart terminal members which have fully 
interactive electromagnetic fields when carrying current. Each of the 
terminal members is provided with an input and an output connector lead 
which are interconnected by parallel and nonparallel terminal portions 
respectively parallel and nonparallel to corresponding terminal portions 
on the other terminal portion. 
The present invention further provides that by proximally spacing and 
juxtapositioning the output connector leads and the parallel terminal 
portions, and by welding the flat portions of the capacitor element tabs 
in parallel to the parallel terminal portions, the related electromagnetic 
fields in this area can be made to neutralize each other resulting in a 
surprising four fold reduction in ESL at the output connector leads. 
An unexpected side effect of the tabs to terminal member connection was 
that the ESR was significantly reduced. Hindsight evaluation showed that, 
by making side welds to lower ESL in the small gap between the capacitor 
element and the header, the conventional long tabs required for bottom 
welding the tabs to the connector leads could be eliminated. While the 
difference in the length of the tabs was initially not considered 
significant, testing of the capacitor showed an unexpected, significant 
effect on ESR in addition to the substantial effect on ESL. 
While the most intensive evaluation and testing was directed at improving 
the capacitor for operation in the switching regulated power supply, the 
question arose whether it would be possible to improve the power supply by 
changes in the capacitor. It was found that, by causing a portion of the 
terminal members (the nonparallel terminal portions) to diverge toward the 
input connector leads, it is possible to control the ESL so that the 
single capacitor of the present invention has two different ESL's 
depending on the direction of the power input. 
The power from the main power line can now be fed through the capacitor's 
input connector leads and out the output connector leads to the switching 
regulated power supply with a minimal ESL while the undesirable ripple and 
harmonics fed back from the switching regulated power supply are filtered 
out and prevented from reaching the main power line from the output to the 
input connector leads because of the higher ESL. Similarly, pulsed power 
from the switching regulated power supply to the load can be supplied with 
the minimal ESL and filtered by the higher ESL. Thus, filter capacitors 
may be eliminated from the overall power supply, reducing its cost. 
An additional side effect of having four connector leads with two spaced 
together and two spaced apart is that a generally tripodal configuration 
is achieved which provides greater mounting stability on printed circuit 
boards. The ends of the tripodal configuration connector leads define 
three points of a plane which can be matched to the plane of the circuit 
board. 
An additional side effect inside the capacitor is that the parallel and 
nonparallel terminal portions effectively extend radially outward from the 
center of the header so that substantially covering them with the material 
of the header allows the terminal portions to provide the dual functions 
of rigidizing the header while providing interiorly extending ribs which 
engage the capacitor element to prevent radial movement thereof when 
subject to vibration. 
Since it is necessary that the connector leads be solderable to printed 
circuit boards without the need for special welding equipment, the 
terminal members may be made of copper, or aluminum with a suitable 
plating. 
The above and additional advantages of the present invention will become 
apparent to those skilled in the art from a consideration of the following 
detailed description of the preferred embodiment when taken in conjunction 
with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, therein is shown an electrolytic capacitor 10 
havng a capacitor case 12 which contains a capacitor element 14. 
The capacitor element 14 is made up of a first spacer film 16 abutting and 
encircling a first electrode film 18 which in turn encircles and abutts a 
second spacer film 20. The second spacer film 20 further encircles and 
abutts a second electrode film 22. The first and second spacer films 16 
and 20 are saturated in a conventional manner with a conventional 
capacitor electrolyte. 
The first electrode film 18, the cathode, has a first tab 24 electrically 
and conductively abutting it and extending from it towards the open end of 
the capacitor case 12. Similarly, the second electrode film 22, the anode, 
has a second tab 26 electrically conductively and abutting it and 
extending therefrom. The first and second tabs 24 and 26 are flat, 
rectangular shaped pieces of a conductive material, such as aluminum, 
which are deformable as well as weldable. 
The capacitor case 12 is sealed by a header 28 (shown removed) which is 
made of a nonconductive moldable material in the preferred embodiment. As 
evident to those skilled in the art, the header can be made of a 
conductive material such as aluminum if proper insulation for the anode 
parts is provided. 
The header 28 is held in place, to seal off the open end of the capacitor 
case 12, by being inserted in the capacitor case 12 and then having the 
case's edge rolled over a rim on the header 28 to prevent outward movement 
thereof. 
Also shown in FIG. 1 are a pair of parallel output connector leads 32 and 
34 which are proximally spaced and a pair of parallel input connector 
leads 36 and 38 which are distally spaced. These leads will be described 
in greater detail later. 
Referring now to FIG. 2, therein is shown the header 28 as seen from inside 
of the capacitor case 12. The first and second tabs 24 and 26 are shown 
respectively welded by welds 30 and 31 to respective first and second 
terminal members 40 and 42. The first and second terminal members have 
respective parallel terminal portions 44 and 46 which are spaced apart and 
parallel, and nonparallel terminal portions 48 and 50 which are spaced 
apart and nonparallel. The nonparallel terminal portions 48 and 50 diverge 
toward their respective connections to the input connector leads 48 and 
50. 
Also seen in FIG. 2 are respective header ribs 52 and 54 which are 
extensions of the header material around the portion of the first and 
second terminal members 40 and 42 which are inside of the capacitor case 
12. It will be noted that the header ribs 52 and 54 do not extend 
completely over the parallel terminal portions 44 and 46 respectively so 
that the first and second tabs 24 and 26 may be respectively welded in 
parallel directly to the case proximal sides of the terminal members. 
Referring now to FIG. 3, therein may be seen the second terminal member 42 
with the second tab 26 affixed by weld 31 on to the case-proximate-surface 
of the parallel terminal portion 46. Also therein may be seen the integral 
output and input connector leads 34 and 38 which are portions of the 
second terminal member 42. Similarly, the output and input connector leads 
32 and 36 are integral portions of the first terminal member 40. 
In the preferred embodiment, the terminal members are made of aluminum 
while the connector leads may be solid copper but are preferably 
splash-coated with copper. The reason for copper is that while aluminum is 
inexpensive compared to copper, it can only be welded and cannot be 
soldered as required in most customer electrical applications. Thus, the 
required connection of the tabs can be done in the factory during 
manufacture and the aluminum allows low cost terminals while the copper 
allows the capacitor 10 to be soldered on to electrical circuit boards in 
a customer's factory. 
In a typical application, the electroytic capacitor 10 is mounted on 
circuit boards of a switching regulated power supply system (not shown). 
As known to those skilled in the art, such a system consists of a full 
wave rectifier connected in parallel with a first filter capacitor, to a 
transistor switching circuit. The transistor switch circuit is connected 
through a series inductance with a second filter capacitor to a direct 
current load. 
The output of the full wave rectifier is filtered by the first filter 
capacitor and is chopped by the transistor switching circuit into a pulse 
train. The series inductance and the parallel second filter capacitor 
integrates the pulse train to provide the D.C. output. The frequency or 
duty cycle of the switching regulated power supply is varied to maintain 
the output at the desired level. 
The electrolytic capacitor 10 as the second filter capacitor between the 
series inductance and the load acts as a filter capacitor. It is connected 
with the input connector leads 38 and 36 respectively connected to the 
inductance and the transistor switching circuit. The output connector 
leads 32 and 34 are connected to the load. In this application, the low 
inductance output out of the output connector leads 32 and 34 provides a 
substantially constant, low voltage D.C. output while the higher 
inductance on the input adds to the series inductance to help filter out 
the undesirable harmonic and ripple components of the output. 
The electrolytic capacitor 10 as the first capacitor between the full wave 
rectifier and the transistor switch circuit is connected with the output 
connector leads 36 and 38 connected to the rectifier and the input 
connector leads 32 and 34 connected to the transistor switch circuit. In 
this application, the capacitor is used to convert the full wave rectified 
output to a high voltage D.C. output while the higher inductance filters 
out the high frequency harmonics and ripple from the transistor switch 
circuit from feeding back into the main power supply which is generally a 
60 cycle A.C. source. 
To achieve the minimum impedance at the output connector leads 32 and 34, 
the parallel terminal portions 44 and 46 of the terminal members 40 and 42 
as well as the output connector leads 32 and 34 were placed in parallel to 
neutralize the interaction of electromagnetic fields therebetween and 
minimize the capacitor output inductance. Basically, this works because 
the electromagnetic field of any given conductor may be neutralized by 
passing an equal current in the opposite direction through a second 
conductor parallel and close to the given conductor. 
The above further affords a means for controlling the inductance to a 
slightly higher level by causing the wires to be nonparallel and diverge 
by a controlled amount as desired by a particular application. Thus, in 
the preferred embodiment, the terminal members 40 and 42 are respectively 
provided with the nonparallel terminal portions 48 and 50. 
Further, it has been determined by welding the first and second tabs 24 and 
26 to the parallel terminal portions 44 and 46, that it is possible to 
maintain the flat, terminal proximate portions of the tabs parallel to 
each other so as to further minimize the inductance, as explained above, 
between the first and second tabs 24 and 26. 
In the past, it was a practice to weld the tabs directly to the bottom 
portions of the connector leads. This necessitated long enough tabs to 
allow the header to be pivoted up perpendicular to the capacitor case so 
that butt welding could take place. In the preferred embodiment, the welds 
30 and 31 are made from the sides of the capacitor element 14 to the 
case-proximate portions of the parallel terminal portions 44 and 46. This 
allows the first and second tabs 24 and 26 to be of a minimum length 
because the header 28 does not have to be pivoted up for welding. The 
Inventor found this change in length, which was initially considered 
significant to reduce ESL, also resulted in the equivalent series 
resistance (ESR) being a significantly lower 0.0032 ohms as compared to 
0.0039 ohms of a conventional electrolytic capacitor having the same 
capacitance value of 39,000 microfarads. 
An additional advantage of the above arrangement is the possibility of 
reducing the overall size of the capacitor to increase capacitance to 
volume ratio by providing the largest capacitance in the smallest volume. 
Less volume is required for the tabs and the inward extension of the 
terminal members can be minimized. 
By means of the parallel terminal portions 44 and 46, the parallel tab 
welding of the first and second tabs 24 and 26, and the parallel 
positioning of the output connector leads 32 and 34, it has been possible 
to reduce the equivalent series inductance (ESL) to 3.5 nanohenries from 
the 14 nanohenries in the conventional capacitor. This is an unexpected 
four fold reduction in ESL. 
The header ribs 52 and 54 are formed of header material covering over the 
case-interior portions of the first and second terminal members 40 and 42. 
When the header 28 is fully positioned in the capacitor case 12, the 
header ribs 52 and 54 will indent the capacitor element 14 as it is 
compressed against the closed end of the capacitor case 12. This 
indentation of the capacitor element 14 will prevent relative radial as 
well as axial movement between the capacitor element 14 and the header 28 
so as to prevent flexure and subsequent breakage of the first and second 
tabs 24 and 26. 
While the invention has been described in conjunction with the specific 
embodiment, it is to be understood that many alternatives, modifications, 
and variations will be apparent to those skilled in the art in light of 
the aforegoing description. Accordingly, it is intended to embrace all 
such alternatives, modifications, and variations which fall within the 
spirit and scope of the appended claims.