Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-12-05
2003-04-08
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169400, C345S060000, C345S063000, C345S067000
Reexamination Certificate
active
06545423
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for setting an applied voltage in a plasma display panel (PDP) and a method for driving the PDP. The methods are suitable for driving a surface discharge type PDP. In the surface discharge type, display electrodes (a first electrode and a second electrode), which are an anode and a cathode in a display discharge for securing a luminance, are arranged in parallel on a substrate of the front or the back side.
In a PDP, it becomes difficult to equalize the structure of cells while a screen becomes large, and a smaller cell has more influence to the discharge characteristics of a cell structure having a delicate difference. In order to promote development of a PDP having a larger screen and a high resolution, a driving method is necessary that has a sufficient margin of voltage for permitting a variance of characteristics.
2. Description of the Prior Art
A typical surface discharge type PDP has a three-electrode structure in which an address electrode (a third electrode) is arranged to cross a pair of display electrodes. The basic structure of the three-electrode structure has a pair of display electrodes for each row of the screen. In other electrode structure, when the number of rows of the screen is n, n+1 display electrodes are arranged at a constant pitch, and neighboring electrodes constitute the electrode pair for generating the surface discharge. In any case, a cell that is a display element (a discharge cell) has three electrodes whose potential can be controlled independently.
A memory function of a dielectric layer covering the display electrode pair is utilized for display. Namely, a row scan addressing is performed for generating a charged state in accordance with the display contents, and then a sustaining voltage Vs having an alternating polarity is applied to the display electrode pair of each row. The second electrode is used as a scan electrode, and the third electrode is used as a data electrode for addressing.
The sustaining voltage Vs satisfies the following inequality.
Vf−Vw<Vs<Vf
Vf is a start voltage of a sustaining discharge.
Vw is a wall voltage between electrodes.
The application of the sustaining voltage Vs generates a surface discharge along the substrate surface only in cells having a wall charge when the cell voltage (an effective voltage that is a voltage to be applied to the electrode plus the wall voltage) exceeds the discharge start voltage Vf.
A discharge cell of a PDP is a binary light emission element. A drive system of a PDP reproduces a halftone by setting an integral light emission quantity of each discharge cell for each frame in accordance with the gradation value. A color display is a kind of gradation display, and its display color is determined by a combination of luminance values of three primary colors. For the gradation display, a method is used in which one field includes plural sub fields weighted by the luminance, and the integral light emission quantity is set by a combination of on and off of the light emission for each subfield. In order to perform 256-step gradation display for example, a frame is divided into eight subframes having 1, 2, 4, 8, 16, 32, 64 or 128 weight of the luminance. In general, the weighting of the luminance can be set as the number of light emissions. The field means a unit image of time series of image display. In an interlace format the field constitutes a frame, while in a non-interlace format the field corresponds a frame.
An address period for addressing, a sustaining period for generating display discharges plural times corresponding to the weight of the luminance, and a period for an initialization for equalizing the charged state of the entire screen before the addressing (a preparation period for addressing) are assigned to a subfield. When the sustaining period finishes, discharge cells having relatively much wall charge and discharge cells having almost no wall charge are mixed. Therefore, the initialization is performed as the preparation process for increasing the reliability of the addressing.
U.S. Pat. No. 5,745,086 discloses the initialization step in which a first and a second ramp voltages are applied to the discharge cell sequentially. The application of the ramp voltage having a gentle gradient can make the light quantity of the light emission substantially zero in the initialization because of the characteristics of a microdischarge that will be explained below, so that a contrast is prevented from dropping. In addition, the wall voltage can be set to any target value despite the variance of the cell structure.
If the gradient of the ramp voltage is gentle, plural charge adjustment microdischarges occur while the applied voltage rises. When the gradient becomes gentler, the discharge intensity becomes smaller, and the discharge period becomes shorter, so that the discharge form transfers to a continuous form. Hereinafter, the word “microdischarge” means both a cyclic charge adjustment discharge and a continuous charge adjustment discharge.
In the microdischarge, the wall voltage can be set only by a peak voltage value of a ramp wave. It is because during the microdischarge, even if a cell voltage Vc (i.e., the wall voltage Vw plus the applied voltage Vi) that is applied to the discharge space exceeds a discharge starting threshold value (hereinafter, referred to as the voltage Vt) due to an increase of the ramp voltage, the cell voltage is always maintained at the voltage near the voltage Vt due to a generation of the microdischarge. The microdischarge decreases the wall voltage by substantially the same level as the increase of the ramp voltage. When Vr is the final value of the ramp voltage, and Vw is the wall voltage when the ramp voltage reaches the final value Vr, the following relationship holds. Namely, since the cell voltage Vc is maintained to be the voltage Vt,
Vc=Vr+Vw=Vt,
and therefore,
Vw
=−(
Vr−Vt
)
Since the voltage Vt is a constant value determined by the electric characteristics of the discharge cell, the wall voltage can be set to any target value by setting the final value Vr of the ramp voltage. Specifically, even if there is a delicate difference of the voltage Vt between the discharge cells, the difference between the voltages Vt and Vw can be equalized for all discharge cells.
In the conventional driving method, an application of a first ramp voltage generates a wall charge between the first electrode and the second electrode (hereinafter, referred to as between X and Y electrodes) as well as between the second electrode and the third electrode (hereinafter, referred to as between A and Y electrodes). After that, a second ramp voltage is applied so that the wall voltages between X and Y electrodes and between A and Y electrodes can approach the target value. The amplitude of the first ramp voltage is set to such a value that the second ramp voltage can always cause the microdischarge.
The conventional initialization will be explained in detail with reference to FIG.
36
.
FIG. 36
shows a variation of the voltages between X and Y electrodes and between A and Y electrodes with respect to the second electrode. It should be noted that the wall voltages between X and Y electrodes and between A and Y electrodes are plotted by the inverted polarity. Thus, the cell voltage between X and Y electrodes and the cell voltage between A and Y electrodes can be read directly from the difference between the waveform of the applied voltage Vi and the waveform of the wall voltage Vw. Namely, the distance between the plot position of the applied voltage Vi and the plot position of the wall voltage Vw at any time point indicates the absolute value of the cell voltage. Concerning the wall voltage Vw, in the previous subfield that was displayed before the subfield to be initialized, the voltage change when the noted cell is lighted is drawn by the broken line, while the voltage change when the noted cell is not lighted is drawn b
Awamoto Kenji
Hashimoto Yasunobu
Sakita Koichi
Staas & Halsey , LLP
Tran Thuy Vinh
Wong Don
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