Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2001-03-28
2002-10-29
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S309000, C361S311000, C361S301400
Reexamination Certificate
active
06473291
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to chip capacitors having a four terminal capacitor lead frame, and related methods of construction. More specifically, the present invention relates to an improved capacitor lead frame design which acts as a transmission line that routes the output current or input current of an electronic device through the capacitor in a manner causing it to act as a four terminal network and further causing the capacitor equivalent series inductance (ESL) and capacitor equivalent series resistance (ESR) both to be dramatically reduced.
There are two primary ceramic capacitor geometries in common use in the industry—the rectangular chip and the feedthrough (often called a discoidal capacitor). The ceramic monolithic rectangular chip (MLC) capacitor (or “chip capacitor”) is produced in very high commercial volumes in highly automated facilities. Over the years the cost of ceramic chip capacitors has dropped a great deal. It is now common to purchase certain value chip capacitors for only a few pennies. The ceramic feedthrough capacitor is only produced in a small fraction of the chip capacitor volume. Accordingly, feedthrough capacitor production has not been nearly as automated. In addition, the feedthrough capacitor is inherently more expensive to produce due to drilling and centering the through hole, tighter dimensional control, reduced volumetric efficiency and difficulty in automating the manufacturing process. Typically the cost of a particular value chip capacitor is ten to twenty percent of the cost of an equivalent value discoidal feedthrough capacitor.
FIGS. 1 and 2
illustrate prior art conventional MLC chip capacitors
50
with flat and tombstone mounting. The chip capacitors
50
are of standard construction, including a ceramic dielectric
52
that has disposed therein alternating lay up patterns for a first set of electrode plates
54
and a second set of electrode plates
56
separated by the ceramic dielectric
52
(FIGS.
3
and
4
). The first set of electrode plates
54
terminate in a first metallization band
58
exposed at one end of the chip capacitor
50
,and the second set of electrode plates
56
is conductively coupled to a second metallization band
60
disposed at an opposite end of the chip capacitor
50
.The chip capacitors
50
act as two terminal devices. That is, they are connected from one circuit trace
62
to another circuit trace
64
or from a circuit trace to ground in order to decouple or filter signals from one line to a reference point. In the embodiments of
FIGS. 1 and 2
, the metallization bands
58
and
60
are soldered or otherwise conductively coupled to pads for the circuit traces
62
and
64
as shown.
FIG. 5
illustrates a typical prior art cylindrical chip-type capacitor
50
′ having axial leads
66
and
68
.
FIG. 6
illustrates a typical prior art chip capacitor
50
having radial leads
70
and
72
extending from the metallization bands
58
and
60
.
It has become common practice to use assembled stacks of monolithic ceramic capacitors
50
which form either vertical or horizontal capacitor arrays
74
. These are typically installed with a lead frame
76
as shown in
FIGS. 7-9
. A multiplicity of capacitors
50
are typically stacked up in order the equivalent series resistance (ESR) and also the equivalent series inductance (ESL) of the device. As shown in
FIG. 8
, the lead frame
76
for the capacitor array
74
includes a lead form
78
for stress relief adjacent to the top ends of mounting feet. An optional outer case
82
may be provided over the array
74
and secured in place by means of an epoxy backfill
84
. An equivalent circuit model of generic two terminal capacitor devices as discussed above is shown in FIG.
10
. Any of these prior art capacitors may be conformally coated, epoxy encapsulated or enclosed in a mounting case as discussed above.
Why Is Inductance Important?
The inductive properties of capacitors in electronic circuits is becoming increasingly important. As important as these properties are now, it is expected that inductive properties will be even more important as operating speeds increase, active device packaging densities increase, and frequencies of application continue to rise. All other things being equal, device power increases with the square of the frequency. In addition, as active devices increase in complexity, the pin count for power distribution and grounding increases. As the number of pins increase, the total lead length and thus associated inductances increase, requiring the increased use of decoupling capacitors to handle system issues associated with the greater inductance. As early as the mid 1980 s, bypass applications of capacitors were estimated to be up to 85% of the U.S. capacitor market (Rappaport, Andy, “Capacitors, ” Electronic Design News, Vol. 27 (20), Oct. 13, 1982, pp. 104-113, 115-116, 118). This percentage has likely increased.
Following are examples of some capacitor applications where capacitor equivalent series inductance (ESL) is very important:
1. Low ESL/Low ESR capacitors are increasingly needed as ripple current filters in the inputs and/or outputs of switch mode power supplies (SMPS), buck regulators, power converters and DC to DC converters. The switching frequency of such devices has been increasing over the last ten years which improves their efficiency and packaging density.
2. Low inductance capacitors are needed to match the fast turn-on/off times of silicon-controlled rectifiers, used in a variety of equipment, including: railway signaling units, spike suppression circuitry, power supply filters, and motor control equipment.
3. Capacitors are used to reduce the “Delta-I” noise in high speed computers (Chen, Howard C. and Schuster, Stanley E., “On-Chip Decoupling Capacitor Optimization for High Performance VLSI Design,”
Proceedings. VLSI Tech Systems Applications Conference
, 1995, pp. 99-103; Travis, Bill, “Use Local Bypass Capacitors to Meet Rigorous High-Speed-System Demands,” EDN [European Edition], Vol. 40 (1), Jan. 5, 1995, pp. 63-66, 68, 70; Martin, Arch G., “Multilayer Ceramic Capacitors Beat the Inductance Blues,” Electronic Design, Vol. 29 (11), May 28, 1981, pp. 99-102.). In a bypass application, decoupling capacitors are added a) to reduce switching noise between the power supply and the IC input/output (I/O) buffers, b) to reduce voltage drops across the power supply caused by interconnect inductances, and c) to reduce radiated EMI noise.
The amount of induced EMF can be given by the equation:
Induced EMF=L (di/dt)
To reduce the total EMF, one approach is to reduce inductance. This is usually accomplished first by attempting to put decoupling capacitors on the substrate itself in parallel with current-carrying conductors on the microprocessor. This approach is often not feasible which leads to the addition of one or more decoupling chips to be put nearby. This alternative may require signal fan-out connections on the surface of the module. This allows the uppermost internal layers to be dedicated to capacitor-to-chip power connections to reduce switching noise that occurs when many drivers on the IC switch simultaneously. Excessive switching noise can cause both signal delays and possible false switching of adjacent logic gates. The capacitors act as local, temporarily independent energy sources to refresh or switch logic gates.
There are two applications here in reality: a) main supply-line bypass applications to reduce fluctuations caused by amps of power switching through supply-line inductances, and b) local integrated circuit (IC) bypass applications where capacitors are mounted very near ICs or blocks of ICs to reduce fluctuations caused by very fast, local current wave fronts.
4. Radio frequency applications include EMI filters, resonant circuits, matching networks, and a number of coupling/decoupling uses. One specific application is the use of capacitors in resonant converters of high voltage power supplies for traveling wave tubes. Another
GB Aquisition Co., Inc.
Kelly Bauersfeld Lowry & Kelley LLP
Reichard Dean A.
Thomas Eric W.
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