Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-04-28
2001-06-19
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169400, C345S063000, C345S068000, C345S077000, C345S204000, C345S208000
Reexamination Certificate
active
06249087
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for driving a plasma display panel (PDP).
As a display device for a television set with a large screen, a surface discharge type AC plasma display panel is commercialized. This surface discharge type has first and second display electrodes that are arranged in parallel on a front side or a backside substrate as anodes and cathodes of display discharge for securing intensity. In the surface discharge type, three kinds of fluorescent material for color display, which is red, green and blue fluorescent material, can be disposed separately from the pair of display electrodes in the direction of the thickness of the panel. Thus, a deterioration of the fluorescent layer due to an ion impact upon discharge is reduced so that a long life color screen can be realized.
If the screen becomes larger, it is more difficult to make a cell structure uniform. If the cell becomes smaller, a small difference of the cell structure affects the discharge characteristics more largely. Therefore, in order to promote a wide screen and a high definition of the screen, a driving method is necessary that can permit a variation of the discharge characteristics and has a large margin of voltage.
2. Description of the Prior Art
As an electrode matrix structure of the surface discharge type plasma display panel, a “three-electrode structure” is known widely, in which an address electrode is arranged to cross a pair of display electrodes. The three-electrode structure basically has a pair of display electrodes for each row. An arrangement distance of the display electrodes in each row (a surface discharge gap length) is set to several dozens of microns so that the discharge can be generated by application of a voltage at approximately 150-200 volts. An electrode gap between neighboring rows is set to a value that is sufficiently larger than (several times of) the surface discharge gap length. The arrangement distance of the display electrode in each row is different from that between the rows. In another three-electrode structure, display electrodes whose number is one larger than the number n of the screen rows are arranged at an equal pitch, and the surface discharge is generated by neighboring electrodes as an electrode pair.
The display utilizes a memory function of a dielectric layer that covers display electrodes. Namely, addressing is performed for forming a charged state corresponding to a display contents in the line scanning format, and then a sustaining voltage Vs having alternating polarity is applied to the display electrode pair of each row. One of the display electrodes (a second display electrode) is used as a scanning electrode for addressing, and the address electrode is used as a data electrode.
The sustaining voltage Vs satisfies the following equation (1).
Vf−Vw<Vs<Vf
(1)
Here, Vf is a discharge starting voltage and Vw is a wall voltage between display electrodes.
When the sustaining voltage Vs is applied, a cell voltage Vc (a sum of the applied voltage and the wall voltage, which is also referred to as an effective voltage Veff) exceeds the discharge starting voltage Vf in the cell having the wall charge, so that the surface discharge is generated along the surface of the substrate. By shortening the application period of the sustaining voltage Vs, an apparent continuous lighting state is obtained.
Since the cell of the plasma display panel is a binary light emitting element, middle tones are reproduced by setting the number of discharge times per one field in each cell in accordance with a gradation level. Color display is one of the gradation displays, and the display color of the display depends on a combination of intensity of three fundamental colors. The word “field” means a unit image of a sequential image display in this specification. In the television, it means each field of an interlace format frame, while in a non-interlace format such as a computer output, it means a frame itself. For the gradation display, one field includes plural subfields having weights of intensity, and the total number of discharge of one field is set by combining on and off of each subfield. If the application period (drive frequency) of the sustaining voltage Vs is constant, the application time of the sustaining voltage Vs is different between different weights of intensities.
In general, an addressing preparation period is assigned to the subfield along with an addressing period and a sustaining period. At the end of the sustaining period, cells with remaining wall charge and cells without remaining wall charge are mixed. Therefore, the charged states of all cells are uniformed in the addressing preparation period so that the reliability of the addressing is improved. Fundamentally, all cells are set to non-charged state in the addressing preparation period for a writing format addressing, while a constant quantity of wall charge is formed in all cells for an erasing format addressing. However, there is a little variation of discharge characteristics between cells in fact. Therefore, if the charge quantity of all cells is made uniform, the voltage margin of addressing is narrowed by the variation of the characteristics.
A method of performing a preparation process is proposed in U.S. Pat. No. 5,745,086 and Japanese unexamined patent publication No. 10-157107. The method includes a charge forming step and a charge adjusting step for enlarging the voltage margin of the addressing. In the charge forming stage, wall voltage having the same polarity is generated in all cells. It is not required to control the charge quantity strictly. In the charge adjusting step, a slowly increasing voltage having a small gradient (a ramp voltage used here) is applied so as to decrease the wall voltage to an appropriate value.
The principle of the charge adjusting will be explained as follows. When applying an appropriate mild ramp voltage as the conventional driving method shown in the Japanese unexamined patent publication No. 10-157107, the cell voltage Vc reaches the discharge starting voltage Vf, and after that a weak discharge occurs periodically so that the wall voltage drops gradually. The cell voltage alters a little with the drop of the wall voltage and the increase of the application voltage. However, it is kept substantially at the discharge starting voltage Vf. In addition, if an extremely gentle ramp voltage is applied as the conventional method shown in the U.S. Pat. No. 5,745,086, the cell voltage Vc is close to the discharge starting voltage Vf and does not exceed the same while a continuous current flows so that the wall voltage drops gradually. In this specification, the discharge for decreasing the wall voltage gradually is referred to as a “charge adjusting discharge,” which includes a state of generating a periodical minute discharge, a mixing state of discrete discharge and continuous discharge, and a state of continuous discharge. When the application of the ramp voltage is finished, the cell voltage Vc drops to the value Vwr of the wall voltage at the end of the charge adjusting discharge. This value Vwr corresponds to the difference between the discharge starting voltage Vf and the maximum value Vr of the applied ramp voltage as shown in the equation (2).
Vwr=Vf−Vr
(2)
It is obvious from the equation (2) that the value Vwr of the wall voltage does not depend on the value of the wall voltage at the start of the application of the ramp voltage, but depends on the setting of the maximum value Vr of the applied voltage. Therefore, in the charge forming stage, a wall voltage is generated in the range that can generate the charge adjusting discharge after that.
In the addressing after the above-mentioned charge adjusting, a pulse voltage that has the same polarity as the ramp voltage applied in the charge adjusting step is applied for generating an address discharge. Using the peak value (amplitude) Vp of the pulse voltage, the cell voltage Vc when applyin
Awamoto Kenji
Hashimoto Yasunobu
Kishi Tomokatsu
Sakita Koichi
Takayama Kunio
Fujitsu Limited
Philogene Haissa
Staas & Halsey , LLP
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