Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
1999-09-01
2001-10-16
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S321000, C327S326000, C361S111000, C361S056000
Reexamination Certificate
active
06304126
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of protection circuits that protect against electric surges that may affect components and/or electronic devices such as, for example, mobile phones, computers, printers. It is particularly desired to protect such devices against electrostatic surges which can, for example, correspond to current peaks, that have short durations but very high intensities, for example, several tens of amperes.
2. Discussion of the Related Art
Conventionally, as is shown in
FIG. 1A
, to protect inputs
1
and
2
of a device
3
connected to external components, an avalanche diode
5
or an equivalent device is disposed between these inputs. Resistors
7
and
8
correspond to connection resistances or to resistors voluntarily inserted in the circuit.
Theoretically, as is shown in
FIG. 1B
, an avalanche diode requires a voltage V
Z
between its terminals to be conductive. However, when the current through this diode significantly increases, as is shown in
FIG. 1B
, voltage V across the avalanche diode increases to reach a value V
S
equal to V
Z
=R
Z
I, where I is the current associated with the surge. To take a practical example, considering an avalanche diode of a surface area of approximately 0.4 mm
2
, in conventional technologies, the equivalent dynamic resistance R
Z
will be on the order of 0.4 ohm. If the current peak is on the order of 30 amperes, the effective voltage across an avalanche diode having a breakdown voltage of 6 volts will be 6+0.4×30=18 volts. The effective value is triple the nominal voltage and a destruction of the components of the circuit to be protected can result therefrom.
It is known that to reduce the dynamic resistance of an avalanche diode, its surface should be increased. If the surface is increased by a factor 4, to reach for example a surface of 1.6 mm
2
, the equivalent resistance will be of 0.1 ohms only. The overvoltage linked to the current flow will be of 3 volts only, that is, the voltage across the avalanche diode will reach, for 30 amperes, a value on the order of 9 volts, which remains acceptable.
However, this has the disadvantage that the cost of a diode increases with its surface and that a significant surface has to be provided so that the increase of the voltage with the current becomes negligible.
Further, an avalanche diode inevitably exhibits in the blocked (non-conducting) state a certain stray capacitance. In a conventional case, for a diode of 0.4 mm
2
, there will be a stray capacitance on the order of 250 pF. If the diode surface is multiplied by 4, the straye capacitance will be multiplied by four to reach a value on the order of one nanofarad. As a result, the protection system exhibits a high capacitance which cannot be chosen independently from the choice of the resistance. This can be a disadvantage in some applications.
SUMMARY OF THE INVENTION
Thus, an object of the present invention is to overcome the disadvantages of prior art systems such as described hereabove.
Another object of the present invention is to provide a protection structure such that the voltage thereacross remains close to the expected nominal value.
Another object of the present invention is to provide such a structure which occupies a reduced silicon surface.
Another object of the present invention is to provide such a structure which exhibits a selected filtering characteristic.
To achieve these and other objects, the present invention provides a structure for protection against electric surges, connected between two input terminals and two output terminals, the output terminals being connected to the inputs of a circuit to be protected, a first input terminal being connected to a first output terminal via an impedance, the second input terminal being connected to the second output terminal, the input terminals being interconnected by a first avalanche diode, the output terminals being interconnected by a second avalanche diode of same polarity as the first avalanche diode.
According to an embodiment of the present invention, the two avalanche diodes are identical or at least substantially identical.
According to an embodiment of the present invention, the two avalanche diodes are bidirectional.
According to an embodiment of the present invention, the impedance is a resistor connected between the first input and output terminals.
According to an embodiment of the present invention, the impedance is a tripole impedance, a third terminal of which is connected to the second input and output terminals, the elements of the tripole forming, with the avalanche diodes, a filtering structure.
According to an embodiment of the present invention, the tripole includes two series resistors and a parallel capacitor.
The present invention also provides monolithic implementations of the above structure. In one embodiment a monolithic implementation of the implementation of the protection structure includes, in a substrate of a first type of conductivity, two areas of a second type of conductivity forming Zener junctions with the substrate, and one metallization formed on an upper surface of the substrate with interposition of an insulating layer.
In another embodiment, a monolithic implementation of the protection structure is formed from a substrate of a first conductivity type, the substrate having a front surface and a rear surface. The structure includes a region of a second and opposite conductivity type formed on the front surface of the substrate. The region of the second and opposite conductivity type has three main areas, each of the three main areas being covered with a respective first metallization. Two of the first metallizations are respectively connected to one of the first input terminal and the first output terminal, and the structure further includes a second metallization covering the rear surface of the substrate.
According to an advantage of the present invention which will be discussed in further detail hereafter, the structure of the present invention is particularly well adapted to being combined with structures ensuring determined filtering effects.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.
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Patent Abstracts of Japan, vol. 7, No. 36 (E-158), & JP 57 190359, Nov. 22, 1982.
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Callahan Timothy P.
Galanthay Theodore E.
Morris James H.
Nguyen Long
STMicroelectronics S.A.
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