Note: Descriptions are shown in the official language in which they were submitted.
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PATENT
Attorney Docket No. 0267-001-1 109
Title: FLEXIBLE PRINTEDCIRCUITBOARDFORUSE
WITH METAL OXIDE VARISTOR, APPLICABLE
TO A SURGE PROTECTOR
Inventor: Robert Romeo
CROSS-REFERENCE TO RELATED APPLICATION
Reference is made to U.S. Application Serial No. 08/514,202, filed
August ] 1, 1995, entitled APPARATUS FOR AND METHOD OF
SUPPRE,SSING POWER SURGES UTILIZING ELECTRICAL
STRIPL:[NES, invented by Albert Zaretsky, and assigned to the assignee of
the present invention. Such application is incorporated herein by reference.
BACKGROUND OF THE INVENTION
Currently, most transient voltage suppressor devices (TVSS devices)
use a metal oxide varistor (MOV). The MOV can redirect most of the surge
voltage to ground during voltage spikes. Redirecting surge voltage to
ground can be achieved in different ways. One way is to redirect the
voltage using an MOV from the phase (hot) line to ground. Another way is
using the MOV to redirect the voltage from the phase line to the neutral
line. Typically, the neutral line is connected to the ground line at the
service entrance in buildings. Thus, by redirecting to the neutral line, the
surge will find its way to ground. The MOV functions similarly to (and
may include) a zener diode, and breaks down using avalanche principles at
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a desired voltage level. When used across an electric transmission line, the
MOV starts conducting at a voltage above the normal operating building
voltage, such as occurs from lightning or switching of power supply
systems. and redirects the surge to ground/neutral.
All MOVs have rated clamping levels according to test
specifications outlined by the Underwriter Laboratories (UL) or the IEEE.
These or~g~ni~tions test the devices by transmitting pulses of known
current waveforms through the MOV, and measuring the resultant current
through the device. A typical UL waveform applied across the MOV
device has characteristics such as 8msec x 20msec, 3000 Amps, 6000 Volts
in combination. The clamping voltage is the measured voltage across the
MOV device under these conditions. It is desirable to have as small of a
clampinjg voltage level as possible.
Surge protection devices that incorporate MOV devices also have
voltage clamping levels. The design of a printed circuit board incorporating
MOV devices affects the voltage clamping levels. A low voltage clamping
level for a surge protector means that with a given surge, less voltage is
applied l;o the equipment being protected by the surge protector. Reducing
the clamping voltage level is therefore an important consideration in the
design of surge protectors. The wiring from the power source to the MOV
affects the clamping voltage level due to electrical impedance in the wires,
specifically wire inductance and capacitance between the wires. This can
be seen in FIG. 1, which shows a prior art wiring diagram for a surge
protector. MOV 10, housed in a module 12, is coupled through wires 14,
16 to a terminal board 18 having pins 20, 22, 24, 26, 27. The pins are
electrically connected to a power source, such as 120 VAC, with lines L 1
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and neutral N. A load 29 is coupled in parallel with MOV 10. The wires
14, 16 have a self-inductance, shown as phantom coils 28, 32 in FIG. 1.
The wires also have a capacitance between them, shown as phantom
capacitor 30 in FIG. 1. The electrical impedance Z "seen" by MOV 10 in
FIG. 1 is represented by the well-known relationship:
Z = ~ L/C
where L is the inductance of the wires and C is the capacitance between the
wires. Typically, L is relatively high due to the type of wiring required, and
C is relatively very low due to the spacing between the wires. Accordingly,
Z is typically relatively high.
The design of the MOV and the printed circuit board itself is
therefore an important consideration in surge protector design. U.S. Patent
No. 5,303,116, issued April 12, 1994 to Grotz is one prior art attempt at
optimizing surge protector design. The Grotz attempt, however, relies upon
a standard, relatively thick, rigid printed circuit board. (Col. 2, line 20).
Such boards are usually on the order of 0.062 inch thick. Therefore,
electrical conductors on opposed sides of the printed circuit boards are
separated by a considerable distance. This separation distance allows for
the generation of significant magnetic flux encircling each conductor.
Another disadvantage with conventional rigid printed circuit boards
is the dii'ficulty they present with mounting of the MOVs and other
components, and with fitting the entire assembly into cramped locations.
In certain applications, multiple layers of printed circuit boards may be
needed. This increases the wiring length to the MOV devices and further
penalize, transient suppression performance due to the increased
impedance. In conventional wiring implementation, it is often impossible
to place ;311 components in one place, adjacent to the power source terminal
,. . . . .
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strip, especially when the components are housed in modules. Wiring in
three-phase implementations is even more difficult, given the required low
impedance's between the product entrance terminal board and the
suppression components.
l hese limitations are known to exist in present devices and methods
It would therefore be advantageous to provide an alternate device and
technique to overcome the limitations set forth above. Accordingly, a
suitable alternative includes features more fully disclosed hereinafter.
SUMM:ARY OF THE rNVENTION
l he present invention limits magnetic flux impedance generated by
flexible printed circuit board wiring which is connected to a surge protector
One or more metal oxide varistors are mounted on the printed circuit board
A first ]metalization layer and a second metalization layer are laid down on
the printed circuit board. The first metalization layer supplies electricity to
the metal oxide varistor while the second met~li7~tion làyer connects it to a
ground/neutral plane. A minim~l thickness insulative layer separates the
first mel:alization layer from the second metalization layer.
BRIEF ]DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a schematic diagram of a prior art embodiment of a
metal oxide varistor surge protector connected to a power terminal board
using conventional discrete wiring.
FIG. 2 illustrates a perspective view of one embodiment of a flexible
printed circuit board of the present invention.
FIG. 3 illustrates a cross-sectional view of the flexible printed circuit
board oi'FIG. 2.
FIG. 4 illustrates an alternative embodiment of the present invention.
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F IG. 5 illustrates a further embodiment of the present invention.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
Overall Design
lF IG. 2 illustrates a surge protector 10 of one embodiment of the
present invention. The surge protector is connected to power lines L 1 and N
and in parallel with a load 29. Load 29 may be any electrical device
requiring an eleckic supply where there is a desire to protect against
electrical power spikes. These spikes primarily occur from lightning and
power supply switching. The surge protector 10 protects against any
voltage level above a desired threshold. It is important that the surge
protector respond quickly to any power spike. FIG. 2 shows a printed
circuit bloard 40 on which is mounted a metal oxide varistor (MOV) 10,
housed in module 12. The metal oxide varistor 10 breaks down when a
large voltage level is reached. This break down is accomplished using an
avalanche mechanism, as is well known in the art.
In FIG. 2, there is a f1rst electrical path 42 from a power source 60 to
an MOV 10 along the top surface of printed circuit board 40. There is also
a seconcl electrical path 44 from MOV 10 to a ground/neutral plane 18
along the bottom surface of the printed board 40. In this disclosure, the
term "first metalization layer" is synonymous with the term "electrical
path". T here can be one or more electrical paths associated with each
metalization layer. Each metalization layer is applied to printed circuit
boards using masking or other well known technologies. The present
invention relates to embodiments of the printed circuit board that can
quickly respond to electrical surges by actuating the MOV device. This
quick response is accomplished by limiting the impedance along the first
electrical path 42 and along the second electrical path 44. There are several
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techniques that can be used to limit the impedance along the first and
second e lectrical paths 42, 44.
The first technique is to ensure that the path material used has a high
electrical conductivity. There has been little recent improvement in this
material. Copper and other conductors are commonly used. Therefore,
chip layout design becomes more important when optimizing surge
suppressor performance.
Flexible Printed Circuit Board Configuration
The FIG. 2 embodiment of the present invention illustrates a
configuration where the printed circuit board 40 is a flexible printed circuit
board. T he flexible printed board has a major advantage in that it is
extreme]y thin. This thinness allows the first electrical path 42 to lie close
to the second electrical path 44. In determining the direction of generated
magnetic flux due to electrical current flow, as shown by dashed line 46 in
FIG. 2, the right hand rule is used. If the electric flow in path 42 is along
the direction shown by the arrow, then the generated magnetic flux extends
in a circumferential magnetic flux path 47. The flow of electricity along
electrical path 44 in the opposite direction as that in path 42 results in a
magnetic flux being generated in circumferential magnetic flux path 46.
To achieve the best results, i.e., lowest impedance, at any given point
on the printed circuit board, the directions of the circumferential magnetic
flux pathls 46, 47 should oppose and cancel each other. Assuming that the
electric paths 42, 44 have a similar configuration, are formed from a similar
material, and have a similar current, then the magnetic flux at points
equidista~nt from each electric path 42, 44 should be substantially equal.
One object of the present invention is to ensure that the electric paths are as
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close to each other as possible, so that the magnetic flux generated at each
point by the first electric path 42 will be as close to equal and opposed to
the magnetic flux generated by the second electrical path 44 as possible.
This teclmique of matching the electric path 42, 44 material and
configuration is called "strip line technique".
The canceling of the respective magnetic fluxes as described above
results in limiting the impedance in each electric path 42, 44, in accordance
with the relationship:
Z = ~ L/C
where, in this case, the inductance L is very low and the capacitance C is
very high due to the close spacing of the copper tracings on the printed
circuit board. Thus, the impedance Z is much lower than it would be if the
MOV were connected to the terminal board by discrete wiring.
This results in more responsive electrical flow along the electrical
paths 42, 44 to and from the MOV. It is worth noting that each printed
circuit board may have a plurality of MOVs, so that limiting the generated
magnetic flux not only eases the electric flow along the electrical paths 42,
44 generating the magnetic flux, but along any electric paths in the vicinity
that supply electricity to and from other MOVs as well.
T:he FIG. 2 configuration limits the distance between the electrical
paths 42, 44 by using a flexible printed circuit board instead of a rigid
printed circuit board. Rigid printed circuit boards have a common thickness
of about 0.062 inch, while flexible printed circuit boards have a thickness of
about 0.() 10 inch. In one embodiment, the insulative material used for the
board is plastic, such as Mylar. Using the thinner flexible printed circuit
board permits the first electrical path 42 to be placed closer to the second
electrica]l path 44. Placing the electrical paths 42, 44 closer to each other
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limits the generated magnetic field in the vicinity of the electrical paths 42,
44 by cancellation, as described above. The capacitance between the paths
is increased. Both flexible and rigid printed circuit boards are formed from
electrically insulative material, but the material of the printed circuit boardsdoes not shield magnetic fluxes.
FIG. 3 shows the printed circuit board 40 of FIG. 2 in cross section.
Met:~11i7:~tion layers 42 and 50 are laid down on top of board 40, while
met~lli7:~tion layers 44, 48 are laid down on the bottom of board 40.
Layers 42, 44, 48 and 50 are commonly of copper but may be of any
electrically conductive metal or other material.
Another advantage of the flexible printed circuit board over rigid
printed circuit boards is that the flexible printed circuit board may be bent
or twisted if needed. This bending or twisting permits flexible printed
circuit boards to be packaged in smaller packages than a similarly sized
rigid printed circuit board. Thus, great freedom in mechanical design of the
product iis achieved, without penalizing transient suppression performance
because of long lengths of wiring or multiple levels of PC boards and the
required wiring connections between them. The need for manual wiring is
also redwced or elimin~ted, and use of the printed circuit board facilitates
controlled transient tr~n.smi.~.~ion parameters by facilitating the adjustment
of board thickness and copper trace widths.
F][G. 4 illustrates an alternative embodiment (mating boards) of the
present invention. In this embodiment, there are provided a plurality of
flexible printed circuit boards 60, 62, with their respective electrical paths
78, 80, 74, 76 facing each other. A thin, flexible electrically insulative layer64 is placed between circuit boards 60 and 62. The electrically insulative
layer 64 is preferably formed from plastic, but may be formed from any
other material that permits magnetic flux to pass though, but limits passage
.rdll./llr~ C~
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of electric voltage across. In the FIG. 4 embodiment, the first circuit board
layer 60 has an electric paths 78, 80 across the inner facing surface 79 of
board 6(); the second circuit board layer 62 has an electric paths 74, 76
across the inner facing surface 81 of board 62. The first inner facing surface
79 of the first circuit board 60 faces the first inner facing surface 81 of the
second circuit board 62. The spacing between the two surfaces 79, 81 is
determined by a thickness of the electrically insulative layer 64. The
thickness of the first circuit board 60 and the second circuit board 62
therefore do not make a difference in the spacing between the electric paths
74, 78 and 76, 80.
B,oth the flexible printed circuit board configuration and the mating
printed circuit board configuration enable the first and second electric paths
to be very close to each other. Therefore, the description under the flexible
printed circuit board segment applies to both configurations. All
embodirnents of the present invention permit very close electric path
spacing, while ensuring proper insulation between the two electric paths. In
addition, the insulator in all embodiments does not effect the cancellation of
magnetic fluxes caused by substantially even electric flow through the two
electric paths.
Another embodiment of the invention is shown in FIG. 5. Here, a
portion 91 of a met~lli7~tion layer consisting, for example, of copper, is laid
down near two other portions 93 and 95. Portion 91 may be connected to
the phase line, portion 93 may be connected to the neutral line and portion
95 may be connected to the ground line. As can be seen, all three portions
are on the same side. Each portion is connected to a separate met~lli7Ation
layer 10 l, 103 through leads (not shown), and the met~lli7~tion layers are
ra~ r~ a~
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separated by one or more layers 97, 99 of a plastic substrate, to form a
multilayer board.
~ Ihile only a few embodiments of the present invention have been
described in this disclosure, it is understood that many changes and
modifications that would be obvious to one having ordinary skill in the art
upon consideration of the disclosure and the drawings are within the scope
of the present invention.
I ~wor~ r~!lle~
