Integrated EMI filter-DC blocking capacitor

Details Integrated EMI filter-DC blocking capacitor Integrated EMI filter-DC blocking capacitor

2001-05-31

2002-09-24

Dinkins, Anthony (Department: 2831)

Electricity: electrical systems and devices

Electrostatic capacitors

Fixed capacitor

C361S306100, C361S303000

Reexamination Certificate

active

06456481

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to ceramic capacitors which provide DC blocking and EMI filter functions. More specifically, this invention relates to an integrated DC blocking capacitor and high frequency EMI filter in a monolithic ceramic housing.
There are two primary ceramic capacitor geometries in common use in the industry—the rectangular chip and the feedthrough (often called a discoidal capacitor). Ceramic capacitors are typically constructed by interleaving nonconductive layers of high dielectric constant ceramic material with metallic electrodes. The metallic electrodes are typically “laid-down” on the green ceramic material by silk screening processes. The device is then fired (sintered) to form a rugged monolithic structure (the “capacitor”). Monolithic ceramic capacitors are well known in the art for a variety of applications in both surface mount (chip capacitor) and leaded applications. Also well known in the art are stacked film capacitors, which are constructed in a very similar manner to ceramic chip capacitors. Layers of film dielectric are interleaved with conductive electrodes, thereby forming a chip-type capacitor.
The ceramic monolithic 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 through 4
illustrate a prior art conventional MLC chip capacitor
120
. The chip capacitor
120
is of standard construction, including a ceramic dielectric
122
that has disposed therein alternating lay up patterns for a first set of electrode plates
124
and a second set of electrode plates
126
separated by the ceramic dielectric
122
(FIGS.
2
through
4
). The first set of electrode plates
124
terminates in a first metallization band
128
exposed at one end of the chip capacitor
120
, and the second set of electrode plates
126
is conductively coupled to a second metallization band
130
disposed at an opposite end of the chip capacitor
120
. The chip capacitor
120
acts as a two terminal device. That is, it is connected from one circuit trace
132
to another circuit trace
134
, or from a circuit trace to ground, in order to decouple or filter signals from one line to a reference point. In the embodiment of
FIGS. 1 through 4
, the metallization bands
128
and
130
are soldered or otherwise conductively coupled to pads for the circuit traces
132
and
134
as shown.
FIG. 5
is an electrical schematic diagram of the chip capacitor
120
of
FIGS. 1-4
illustrating its DC blocking capacitor capability.
FIG. 6
is an exemplary illustration of a circuit board
136
having three of the chip capacitors
120
mounted thereon, together with other electronic components.
FIGS. 7 through 9
illustrate a prior art integrated chip capacitor
220
wherein three individual chip capacitors
120
have been incorporated into a single monolithic block
222
. In the descriptions that follow, functionally equivalent elements of the various illustrated embodiments are referred to by the same reference number in increments of
100
. Accordingly, the chip capacitor
220
is of a standard construction, including a ceramic dielectric
222
that has disposed therein alternating lay-up patterns of a first set of electrode plates
224
and a second set of electrode plates
226
separated by the ceramic dielectric
222
(FIGS.
8
and
9
). The first set of electrode plates
224
terminates in first metallization bands
228
exposed along one side of the chip capacitor
220
, and the second set of electrode plates
226
is conductively coupled to respective second metallization bands
230
disposed at an opposite side of the chip capacitor
220
. Each of the chip capacitors
120
within the monolithic block
222
is connected from one respective circuit trace
232
to another respective circuit trace
234
(FIG.
10
), or from a circuit trace to ground, in order to decouple or filter signals from one line to a reference point. Such monolithic ceramic chip capacitors, also known as “chips” or DC blocking capacitors, are used in a myriad of applications, for example, in RF bypass, energy storage, and many other applications.
Feedthrough terminal or discoidal capacitor assemblies are generally well known for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators, or the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. However, the feedthrough terminal pins are typically connected to one or more lead wires which effectively act as an antenna and thus tend to collect stray electromagnetic interference (EMI) signals for transmission into the interior of the medical device. In prior devices, such as those shown in U.S. Pat. Nos. 5,333,095 and 4,424,551 (the contents of which are incorporated herein), the hermetic terminal pin sub-assembly has been combined in various ways with a ceramic feedthrough capacitor filter to decouple electromagnetic interference (EMI) signals into the housing of the medical device.
With reference to
FIGS. 11-13
, in a typical prior art unipolar feedthrough filter assembly (as described in U.S. Pat. No. 5,333,095), a round/discoidal (or rectangular) ceramic feedthrough filter capacitor
320
is combined with a hermetic terminal pin assembly to suppress and decouple the undesired interference or noise transmission along a terminal pin (not shown). The feedthrough capacitor
320
is coaxial, having two sets of electrode plates
338
,
340
embedded in spaced relation within an insulative dielectric substrate or base
322
, formed typically as a ceramic monolithic structure. One set of the electrode plates (active)
338
is electrically connected in parallel to a cylindrical metallized area at an inner diameter cylindrical surface of the coaxial capacitor structure and then to a conductive terminal pin
342
utilized to pass the desired electrical signal or signals. A second or ground set of electrode plates
340
is coupled in parallel at an outer diameter surface of the discoidal capacitor
320
to a cylindrical ferrule of conductive material (
330
), which is electrically connected in turn to the conductive housing of the electronic device. The number and dielectric thickness spacing of the electrode plate sets
338
and
340
varies in accordance with the capacitance value measured in microfarads or picofarads and the voltage rating of the coaxial capacitor
320
. The ground electrode plates
340
are coupled in parallel together by a metallized layer
330
which is either fired, sputtered or plated onto the ceramic capacitor. The metallized band
330
, in turn, is coupled to the ferrule by conductive adhesive, soldering, brazing, welding, or the like. Similarly, the active electrode plates
328
are coupled in parallel together by a metallized layer
328
which is either glass fit fired or plated onto the ceramic capacitor. This metallized band
3

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