1. Field of the invention
The invention relates to the plasma panels of coplanar sustaining type, and it particularly relates to means for containing, in predetermined zones, the discharges in the gas.
2. Discussion of the Background
Plasma panels are flat screen display devices, now well known, which make possible the display of alphanumeric, graphic or other images, either in color or black and white. Generally, the plasma panels include two insulating plates limiting a space occupied by a gas (generally a mixture with a neon base). These plates support conductive electrodes arranged in columns and in lines, so as to be crossed and to define a cell matrix, each cell forming an image surface element or pixel (one cell being approximately the gaseous space between two crossed electrodes). The operating principle is the selective generation (at the intersection of electrodes in a line and electrodes in a column, i.e. at the level of the pixels selected) of electric discharges in the gas. The display of the data is assured by a light emission which accompanies these discharges.
Some plasma panels operate continuously, but most often it is preferred to use panels of the so-called "alternating" type, whose operation is based on an excitation under alternating conditions of the electrodes. In this case, the electrodes are covered by a dielectric material layer, and they are no longer in direct contact with the gas or with the discharge. One of the advantages of this plasma panel type called "alternating" is to offer a memory effect which makes it possible to address the useful data only to the pixels whose state (lit or extinguished) it is desired to change. For the other pixels, their state is maintained simply by repetition of alternate electric discharges, called maintenance discharges, discharges which are obtained only at the level of the pixels which are in the lit state.
Of the plasma panels of alternating type, some use only two electrodes to define a pixel: an electrode arranged in columns called a column electrode which is crossed with an electrode arranged in a line called a line electrode. These two electrodes assure both the addressing functions and the sustaining functions.
In order to particularly improve the luminance of the plasma panels and also to make possible the display of several colors, it is preferable to use plasma panels of the energized type under alternating conditions as described above and which further have coplanar sustaining. In this latter plasma panel type called "coplanar sustaining," each pixel of the matrix consists of at least three electrodes, more precisely at the crossing between an addressing electrode with two parallel sustaining electrodes forming a sustaining electrode pair. In this plasma panel type, the sustaining of the discharges, i.e. the repetition of the alternate discharges mentioned previously, is assured between the two sustaining electrodes of the same pair, and the addressing of a given pixel is made by discharge generation between two crossed electrodes of which one is the addressing electrode and of which the other is one of the two electrodes of the sustaining electrode pair. The addressing electrode performs only an addressing function, and it is arranged most often in the direction of the columns. The sustaining electrodes are parallel and arranged most often in the direction of the lines, and of the two electrodes of the same sustaining electrode pair: one is called addressing-sustaining electrode and it performs an addressing function in cooperation with the addressing electrode, and it performs, on the other hand, a sustaining function in cooperation with the second sustaining electrode of the same pair; the second sustaining electrode is called "only sustaining electrode," and it performs only a sustaining function of discharges.
The operation of a plasma panel of the coplanar sustaining type, with three electrodes per pixel, is known, for example, in European patent document EP-A-0135382.
The coplanar sustaining plasma panels offer many advantages but also raise some difficulties particularly concerning the separation or the limitation of the discharges throughout the electrodes.
To define the sustaining discharge zone better at the level of a pixel, it is known to give the sustaining electrodes a shape such that they each exhibit a protuberance or a projecting surface capable of promoting the discharge: in the same sustaining electrode pair, the projecting surfaces of an electrode are oriented toward those of the other electrode so that, at the level of a pixel, the projecting surfaces of the two electrodes are opposite one another, aligned on the same axis identical or parallel to the axis of the addressing electrode which crosses them, so that the distance between the projecting parts of the two electrodes is smaller than the distance between the electrodes themselves (of the same pair), which tends to delimit the zone of the beginning of the sustaining discharges between the two projecting surfaces. However, it can be difficult to obtain a correct containment of the discharges in the assigned zone, which particularly results in a limitation on the range of operating voltages applied between the two electrodes of the same sustaining electrode pair.
FIG. 1 shows, diagrammatically and partially, a coplanar sustaining plasma panel of the prior art, a panel which is represented mainly by addressing electrodes and sustaining electrodes, and which makes it possible to better understand the problem being presented. Plasma panel 1 of FIG. 1 comprises addressing electrodes X1, X2, arranged in columns, and sustaining electrode pairs p1, p2 arranged in lines. To simplify the figure, only two addressing electrodes X1, X2 and only two sustaining electrode pairs p1, p2 are shown, and consequently only four pixels PX1 to PX4 are shown.
Sustaining electrode pairs p1, p2 each comprise an addressing-sustaining electrode Y1, Y2 and a sustaining-only electrode E1, E2.
Addressing electrodes X1, X2 are perpendicular to sustaining electrode pairs p1, p2, and, in the example shown in FIG. 1, addressing electrodes X1, X2 are shown in a plane having less depth than the plane in which sustaining electrode pairs p1, p2 are arranged. Furthermore, sustaining electrode pairs p1, p2 appear to be seen through addressing electrodes X1, X2 in the part where they are crossed with the latter, and, for more clarity of the figure, addressing electrodes X1, X2 are shown in dotted lines. It should be noted that such an arrangement corresponds to the most common standard structure, in which the discharges in the gas are masked partially by the addressing electrodes or seen through the latter when the latter are transparent.
At the level of each pixel, each of the electrodes of each sustaining electrode pair p1, p2 is provided with a setback or protuberance or projecting surface. These surfaces are referenced SA1, SA2 for addressing-sustaining electrodes Y1, Y2, and referenced SE1, SE2 for the sustaining-only electrodes E1, E2. These projecting surfaces SA1, SA2, SE1, SE2 all are formed in the same manner for each pixel, and by taking, for example, first pixel PX1, formed at the crossing of first addressing electrode X1 and first pair p1, first addressing-sustaining electrode Y1 and first sustaining electrode E1, respectively, these electrodes comprise a projecting surface SA1 and a projecting surface SE1 which are oriented toward one another, opposite and aligned on same axis x1 which constitutes the axis of first addressing electrode X1. A similar arrangement is found at the level of other pixels PX2, PX3, PX4.
As an example, first pixel PX1 has the ends opposite projecting surfaces SA1, SE1 which are, at a distance D, less than the distance which is necessary to trigger a discharge between these two projecting parts SA1, SE1, taking into account potential difference V which is applied to these two projecting surfaces, i.e., which is applied between the two electrodes of each maintenance electrode pair p1, p2.
In operation, after the addressing has been made with a discharge between, for example, first addressing electrode X1 and first addressing-sustaining electrode Y1, assuming that first pixel PX1 has been selected, the alternate sustaining discharges produce the light emitted by pixel PX1.
The electrodes as well as the projecting parts are insulated by dielectric layers, and during a sustaining discharge, electric charges are placed on the dielectric layers and create an internal electric field which is opposed to the electrical field induced between the two electrodes of the same pair, by the voltage pulses of opposite polarity which are applied to the two electrodes of same sustaining electrode pair p1, p2. The internal field created by these charges increases until it brings about the end of the discharge, i.e., the extinguishing of the pixel. But the cell or pixel preserves in memory the internal field previously acquired, and for the following sustaining discharge, this internal field promotes the triggering of the discharge, by being added to the internal electric field, which results from the application to the sustaining electrodes of the sustaining voltage pulses whose polarities are reversed relative to the preceding occurrence. Thus, when the sustaining pulses are applied to the addressing-sustaining electrodes and sustaining-only electrodes which constitute these pairs p1, p2, all addressing-sustaining electrodes Y1, Y2 are brought to a first polarity while sustaining electrodes E1, E2 are brought to the opposite polarity. By assuming that at a given moment when a sustaining discharge is made at the level of first pixel PX1, for example, addressing-sustaining electrodes Y1, Y2 are at a polarity +V, sustaining-only electrodes E1, E2 are brought to opposite polarity -V, and the ionization of the gas creates positive and negative charges referenced by + signs and - signs. Positive charges +are placed mainly on projecting surface SE1, but also on a part of sustaining-only electrode El close to this projecting surface, and negative charges - are fixed mainly on the edges of projecting surface SA1, but also on a part of addressing-sustaining electrode Y1 close to this projecting surface SA1; these positive and negative charges being produced until the end of the discharge.
With the distance D which separates two projecting surfaces SA1, SE1 in a pixel PX1 being less than a distance D1 which separates addressing-sustaining electrode Y1 from sustaining-only electrode E1, the potential difference between these two electrodes determines the equipotential lines referenced a, b, c, which correspond respectively, for example, to +V/2, to zero volt, to -V/2, and which are much closer between the parts opposite projecting surfaces SA1, SE1 than along the electrodes outside of these parts opposite, i.e., for example, in the direction of second projecting surfaces SA2, SE2 of second pixel PX2. As a result, the forces exerted on these positive and negative charges +, - can be insufficient to prevent these charges from extending in the direction of second pixel PX2 during the ionization of the gas.
As a result, for the following sustaining discharge, the polarity of the voltage pulses applied to addressing-sustaining electrodes Y1, Y2 and sustaining-only electrodes E1, E2 are reversed. The charges thus accumulated promote the triggering of the discharge between projecting surfaces SA1, SE1 opposite, belonging to first pixel PX1, but these charges also can promote the creation of discharges along two electrodes Y1 and E1 to project beyond the zone reserved for adjacent pixel PX2.
A solution to this problem of the migration of charges consists in using barriers of insulating material, to insulate the pixels from one another materially. Such a structure is described in an article of G. W. DICK published in PROCEEDINGS OF THE SIDE, Vol. 27/3, 1986, p. 183-187. It should be noted that in the structure described in this document, the sustaining electrodes have a constant width, i.e., they do not comprise a projecting surface opposite in a maintenance electrode pair.
One of the drawbacks of this solution based on the barriers used to contain the sustaining discharge in predetermined zones is that it significantly complicates the production.
It should be noted that another drawback of structures of the type shown in FIG. 1 resides in the fact that the light emitted by a pixel has a greater intensity at the level of the projecting parts opposite than for the remainder of the pixel, and the addressing electrode is arranged exactly in front of this part forming a light source of greater intensity from which a loss of the light output results.