Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
2000-07-14
2003-01-21
Martin, David S. (Department: 2841)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S794000, C361S776000, C361S828000, C361S785000, C200S292000, C335S088000, C335S119000
Reexamination Certificate
active
06510058
ABSTRACT:
CROSS REFERENCES TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a printed circuit board (PCB) configuration, which improves the electromagnetic compatibility/electromagnetic interference (EMC/EMI) performance of electromechanical relay circuits.
2. Description of the Related Art
Relay devices frequently switch large currents into inductive loads. Contact bounce and circuit interruption result in high back-EMF voltages and large currents across the relay contact. The relay contact essentially forms a spark gap with quenching performance that varies with the movement of the relay armature. Consequently, a whole spectrum of high-energy noise is created that is well known to be disruptive to logic and microprocessor circuits.
The inductive load circuits controlled by relay contacts are normally located outside the enclosure of the controlling logic, and some distance away. Because of the proximity of the relays to logic and microprocessor circuits, inductive and capacitive coupling mechanisms may exist which will introduce radio-frequency noise onto the relay circuits. The wiring for the logic circuits form antennas that will radiate any noise energy that may be present onto the relay circuits.
Referring to
FIGS. 1 and 2
, in classical analysis of EMC problems, there is always a source
2
, a victim
4
, and a coupling mechanism
6
. The mechanism of inductively coupled noise and the current methods used to minimize its impact is discussed hereinbelow. Analogies can be drawn using an air-core radio-frequency (RF) transformer as the coupling mechanism
6
. Analysis shows that the strength of the inductive coupling depends upon the mutual inductance. A circuit loop on a PCB will behave like a single-turn transformer. The source device circuit loop
3
will couple to the circuit loop
5
of the victim. Various techniques are currently employed to deliberately reduce the coupling effect of the PCB transformer.
Referring to
FIGS. 3 and 4
, the disruptive effects of relay contact circuits on logic and microprocessor circuits have been controlled by segregation of relay contact circuits
8
and electronic logic circuits
10
. Segregation simply separates the sensitive logic circuits
10
from the larger currents switched by the relay circuits
8
and the resultant magnetic flux
12
. Wiring loop
13
acts like a coupling transformer for magnetic flux
12
.
Referring to
FIG. 5
, at times segregation includes separate PCBs
14
and
16
, and partitioning the system with a magnetic shield
18
made of a ferrous material between the relay circuits
8
and the logic circuits
10
. The magnetic shield
18
provides a low-reluctance path for the magnetic flux
12
, containing it largely within the shield
18
sub-enclosure. Segregation approaches have been successful, but they require a product to be large and bulky, and accessibility for product service can be compromised.
EMI filters have been used to reduce noise that is directly conducted by the signal wiring. Such devices are presently available off-the-shelf as line filters for power supply applications. These devices are often bulky because relay contact circuits are normally required to handle large currents. EMI filters can be connected to the wiring loop
13
shown in FIG.
3
.
Referring to
FIGS. 6 and 7
, presently available metal oxide varister (MOV) devices can be used that reduce inductive kickback voltages resulting from interrupting the current to user-connected equipment. They also define the path of the inductive-discharge currents to limit the disruptive effects to nearby electronic circuits. MOV devices are compact and cost effective, but have a finite service life. However, by diverting the energy away from the relay contacts, the service life of the relay can be increased.
FIG. 6
illustrates a circuit without an MOV device in which a large discharge current
20
passes through the relay contact
9
. The noise generated in the spark gap (relay contact
9
) couples inductively to nearby logic circuits.
FIG. 7
illustrates a circuit using an MOV device
22
in which discharge current
21
is much smaller than discharge current
20
because the energy is diverted through the MOV device
22
. Smaller discharge current
21
results in a smaller noise current and less potential disruptions of nearby logic circuits.
Referring to
FIG. 8
, placing the forward and return traces for the relay contacts on closely spaced parallel conductors
24
reduces the inductive area of the PCB circuit and thus reduces inductive coupling in comparison to wiring loop
13
shown in
FIG. 4. A
further enhancement of this technique places the two paths of the circuit on opposite sides of the PCB. This reduces the inductive area to the thickness of the PCB.
Referring to
FIG. 9
, the use of multilayer boards
25
has been found to greatly reduce the EMI generated by PCBs. Top wiring or etched layer
26
contains the signal wiring traces
28
and associated logic circuits
10
. The ground and power planes
30
are separated by insulation, and a bottom wiring/etched layer
31
can be included. The ground and power planes
30
allow the return currents
32
to form directly adjacent to each signal line
28
, with each return circuit
32
finding the path of least impedance that closely mirrors each signal trace
28
. The signal traces
28
and these mirror currents
32
form the smallest inductive area and thus minimize the effects of inductive coupling and electromagnetic disturbances. The path of least impedance for rapid changing currents is the path that forms the smallest inductive area, directly under the signal trace. The mirror currents automatically form the paths that achieve minimum inductive coupling. If the signal path must cross a gap in the planes, the mirror currents are forced to form a larger area and generate much more inductive noise.
Referring to
FIG. 10
, when a toggling logic output drives a logic input, there is a finite return current
34
. The return current
34
moves through the ground path
36
. If a cable
38
is connected to the driven gate, even on the logic chip ground lead, it will become an antenna radiating RF energy. The ground path
36
forms an inductor with small but finite inductance. This distributed inductance forms an autotransformer. The finite currents changing in the finite inductance produce voltages
39
on the connected cable that may radiate several milliwatts of power.
The autotransformer coupling mechanism described above is generally termed common impedance coupling. The 3 or 5-volt logic transitions are not the problem. If the toggling output of a logic gate was directly connected to twisted-pair of unshielded cable it would produce less radiated noise than in the above example. This is because the return line currents are always the precise equal and opposite of the signal line currents. The balanced (equal and opposite) fields produced by these differential currents on the twisted-pair cable are forced to cancel each other and will not produce strong EMI.
By reducing the effect of the inductive and electric field coupling mechanisms, both electrical immunity and EMC/EMI performance are greatly enhanced.
BRIEF SUMMARY OF THE INVENTION
A first aspect of the invention provides a multilayered printed circuit board for mounting a relay thereon and having a first layer with an electrically conductive plane for electrical connection to a common armature contact of the relay. The electrically conductive plane is sized to substantially cover the mounting footprint of the relay. A second layer parallel to and electrically separate from the first layer has an electrically conducting first section for electrical connection to a normally-open contact of the relay and an electrically conducting second section for electrical connection to a normally-closed contact of the relay. The first and the second sections are electrically separate from each other and
Bui Hung
Comoglio Rick F.
Martin David S.
Sensormatic Electronics Corporation
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