Abstract:
A method for driving a PDP is provided in which power loss is reduced and light emission efficiency is improved while applying a voltage pulse train so as to generate display discharge whose number of times corresponds to luminance in cells to be lighted. A drive step of one pulse for generating one time of display discharge includes steps of supplying current to a pair of display electrodes of the cells to be lighted from a drive power source so as to charge capacitance between the display electrodes so that voltage between the display electrodes exceeds display discharge start voltage and cutting off a current path between the display electrode pair and the drive power source at least in a part of a period from start to end of the display discharge.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and a device for driving a plasma display panel (PDP). 
     Increasing number of pixels in a PDP due to a large screen or a high definition cause increase of power consumption. It is necessary to reduce the power consumption for reducing load of a driving device and for taking measures against heat. 
     2. Description of the Prior Art 
     As a color display device, a surface discharge AC type PDP is commercialized. The surface discharge type has electrodes (display electrodes X and display electrodes Y) to be anodes and cathodes in display discharge for ensuring luminance. The display electrodes X and Y are arranged on a front substrate or a back substrate in parallel, and address electrodes (third electrodes) are arranged so as to cross the display electrode pairs. There are two forms of display electrode arrangement. In one form, a pair of display electrodes is arranged for each row of a matrix display. In another form, display electrodes X and display electrodes Y are arranged alternately at a constant pitch. In the latter form, each of the display electrodes except both ends of the arrangement works for displays of two neighboring rows. Regardless of the arrangement form, the display electrode pairs are covered with a dielectric layer. 
     In a surface discharge type PDP display, one of the display electrodes (pair) assigned to each row is used as a scan electrode for row selection, so as to generate address discharge between the scan electrode and the address electrode, and address discharge between the display electrodes triggered by the address discharge between the scan electrode and the address electrode. In this way, addressing is performed for controlling electrification quantity (wall charge quantity) of the dielectric layer in accordance with display contents. After addressing, sustain voltage (also called drive voltage) Vs having alternating polarity is applied to the display electrode pair. The sustain voltage Vs satisfies inequality (1). 
     
       
           Vf   XY   −Vw   XY   &lt;Vs &lt;Vf   XY   (1)  
       
     
     Here, Vf XY  denotes discharge start voltage between the display electrodes, and Vw XY  denotes wall voltage between the display electrodes. 
     When the sustain voltage Vs is applied, cell voltage (sum of drive voltage applied to the electrode and the wall voltage) exceeds the discharge start voltage Vf XY  only in the cell having a predetermined quantity of the wall charge so that surface discharge is generated on the substrate surface for a display. As an application period is shortened, light emission can be observed as if it is continuous. 
     A discharge cell of the PDP is basically a binary light emission element. Therefore, a half tone is realized by setting an integral light emission quantity of each discharge cell in a frame period in accordance with a gradation value of input image data. The color display is a type of the gradation display, and a display color is determined by a combination of luminance values of three primary colors. As a gradation display, there is used a method in which a frame is made of plural subframes (subfields for an interlace display) having a luminance weight, and the integral light emission quantity is set by a combination of on and off of the light emission for each subframe. A general driving sequence is as follows. A subframe period that is assigned to each subframe includes a reset period for equalizing charge distribution of the screen, an address period for forming the charge distribution in accordance with display contents, and a display period (or a sustain period) for generating display discharge (or sustain discharge) of the number of times in accordance with the gradation value by applying a pulse train having alternating polarities. Though lengths of the reset period and the address period are constant regardless of the luminance weight, a length of the display period is longer as the luminance weight is larger. 
     In the conventional driving method, a sustain pulse Ps having a simple rectangular waveform with an amplitude Vs is applied to a display electrode X and a display electrode Y alternately in the display period as shown in FIG.  17 . In other words, the display electrode X and the display electrode Y are temporarily biased to potential Vs alternately. Thus, the pulse train having alternating polarities is applied across the display electrode X and the display electrode Y (refereed to as an interelectrode XY). The difference between a pulse base potential (usually the ground level GND) and the bias potential, which is the sustain voltage Vs, is set to a value within a drive margin. The drive margin is defined as a difference between the discharge start voltage Vf and the minimum applied voltage Vsm necessary for sustaining a lighted state. If the sustain voltage Vs is the voltage Vf and above, the discharge is generated also in cells that were not lighted in the addressing period. If the sustain voltage Vs is less than Vsm, a lighted cell becomes a non-lighted state. 
     Since cells of the PDP are capacitive load for a power source, current flows so as to charge capacitance (CP) of the cell when the sustain pulse Ps is applied. Usually, the display discharge is generated with some delay after the terminal voltage of the capacitance reaches the sustain voltage Vs, while discharge current (referred to as light emission current) flows simultaneously. In the conventional method, the discharge current is supplied to the cell from a power source circuit connected to the PDP. For this reason, a path for supplying the power is long and passes many circuit devices such as switching transistors, so there was a problem of a large power loss and thereby degrading efficiency of the light emission. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to reduce the power loss and to increase the efficiency of the light emission. 
     According to the present invention, capacitance between display electrodes is charged sufficiently for generating display discharge, and after that a current path between a power source and a cell is cut off. Values of charge voltage and charge period are set so that the cut-off timing and the display discharge are overlapped. When display discharge is generated in the cut-off period, the discharge current is supplied to a discharge gap from the charged capacitance. In this case, a path of the discharge current that flows more rapidly than the charge current to the capacitance is located within the cell, so a power loss is smaller than the conventional structure in which the discharge current is supplied from the power source. 
     FIG. 1 shows a basic drive voltage waveform and a discharge current waveform according to the present invention. The drive voltage waveform is characterized by a step-like waveform including a step for applying voltage Vo higher than sustain voltage Vs to the interelectrode XY, a succeeding step of high impedance and a step for applying the sustain voltage Vs. The high impedance step is a step for cutting off power supply from the power source to the cell. The time for applying the voltage Vo from the leading edge of the waveform is denoted by “To”, and the time of the high impedance step is denoted by “Td”. In this waveform, a lot of power is supplied to capacitance of the interelectrode XY in the early stages by applying the voltage Vo. After that, when discharge is generated, power is consumed for current flowing in discharge gas. If the external power supply is stopped before the discharge finishes, the power for the current flowing in the discharge gas is supplied from the capacitance of the interelectrode XY. After that, the application voltage is set to an appropriate value of voltage Vs before the discharge finishes, so that the wall charge quantity at the end of the discharge is controlled to be suitable for sustaining. 
     FIG. 2 is a graph showing dependence of efficiency on the voltage Vo. FIG. 3 is a graph showing drive voltage margin. The light emission efficiency depends on a rate of a part of the discharge current that is supplied from the capacitance. It is desirable to set the voltage Vo such that a peak of the discharge current appears during the period for cutting off the electric path. As shown in FIG. 3, sufficient drive margin can be secured even if the voltage Vo is altered. According to the drive waveform of the present invention, a power loss can be reduced without decreasing the drive margin, so that the light emission efficiency can be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a basic drive voltage waveform and a discharge current waveform according to the present invention. 
     FIG. 2 is a graph showing dependence of efficiency on the voltage Vo. 
     FIG. 3 is a graph showing drive voltage margin. 
     FIG. 4 shows a structure of a display device according to the present invention. 
     FIG. 5 is a plan view showing a cell arrangement of a display screen. 
     FIG. 6 is a perspective view showing a cell structure of a PDP. 
     FIG. 7 is a plan view showing a shape of a display electrode. 
     FIG. 8 shows a concept of a frame division. 
     FIG. 9 shows a first example of drive waveforms. 
     FIG. 10 shows a second example of the drive waveforms. 
     FIG. 11 shows a third example of the drive waveforms. 
     FIG. 12 shows a fourth example of the drive waveforms. 
     FIG. 13 shows a fifth example of the drive waveforms. 
     FIG. 14 shows dependence of the efficiency on the voltage Vo in the fifth example of the drive waveforms. 
     FIG. 15 shows an example of a driving circuit. 
     FIG. 16 is a timing chart of switching. 
     FIG. 17 shows a conventional drive voltage waveform. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be explained more in detail with reference to embodiments and drawings. 
     FIG. 4 shows a structure of a display device according to the present invention. The display device  100  comprises a surface discharge type PDP  1  having a color display screen of n rows and m columns, and a drive unit  70  for controlling light emission of cells. The display device  100  is used as a wall-hung television set or a monitor of a computer system. 
     The PDP  1  comprises a pair of substrate structures  10  and  20 . The substrate structure means a structure of a glass substrate on which electrodes and other elements are arranged. The PDP  1  includes display electrodes X and Y that constitute electrode pairs for generating display discharge and are arranged in the same direction, and address electrodes A that are arranged so as to cross the display electrodes X and Y. The display electrodes X and Y extend in the row direction (horizontal direction) of the screen and are covered with a dielectric layer and a protection film. The display electrode Y is used as a scan electrode. The address electrode A extends in the column direction (vertical direction) and is used as a data electrode. In FIG. 4, suffixes (1, n) of the reference numerals of the display electrodes X and Y indicate arrangement orders of the corresponding “rows”, while the suffixes (1-m) of the reference numerals of the address electrodes A indicate arrangement orders of the corresponding “columns”. The row is a set of cells of the number of columns (m) having the same arrangement order in the column direction, while the column is a set of cells of the number of rows (n) having the same arrangement order in the row direction. In addition, the letters R, G and B in parentheses indicate the light emission color of the cell corresponding to the element having the letter. 
     The drive unit  70  includes a controller  71 , a power source circuit  73 , an X driver  81 , a Y driver  84  and an A driver  88 . The drive unit  70  is supplied with frame data Df that indicate three luminance levels of red (R), green (G) and blue (B) colors along with various kinds of synchronizing signals from external equipment such as a TV tuner or a computer. The frame data Df are memorized temporarily in a frame memory of the controller  71 . The controller  71  converts the frame data Df into subframe data Dsf for gradation display, which are sent to the A driver  88 . The subframe data Dsf are a set of display data of one bit per cell. The value of each bit indicates on or off of the light emission for a cell in a corresponding subframe, more specifically whether the address discharge is necessary or not. In the case of interlace display, each of fields in a frame is made of plural subfields, and the light emission control is performed for each of the subfield. However, the contents of the light emission control are the same as the case of progressive display. 
     FIG. 5 is a plan view showing a cell arrangement of a display screen. 
     In the display screen, a discharge space  30  is divided into plural columns by partitions  29  that meander regularly, so that column spaces  31  having wide portions (the portion in which the width in the row direction is large)  31 A and narrow portions (the portion in which the width is small)  31 B arranged alternately. In other words, each of the partitions  29  is meandered at a constant pitch and constant amplitude in a plan view, so that the distance between the neighboring partitions  29  becomes smaller than a predetermined value at a constant pitch in the column direction. The predetermined value means a value that can suppress the discharge and is determined by discharge conditions such as a gas pressure. The structure in which the column space  31  between the neighboring partitions is continuous over all rows has some advantages of easy drive by priming for each row, uniformity of film thickness of fluorescent material layers and easy exhaust treatment in a manufacturing process. Since surface discharge is hard to be generated in the narrow portion  31 B, the wide portion  31 A substantially contributes to the light emission. Therefore, cells are arranged on alternate columns in each row. Noticing two neighboring rows, the column positions of the arranged cells alternate in every column. In other words, the cells are arranged zigzag in both the row direction and the column direction. Each of the cells C is a structure within one wide portion  31 A in the display screen. In FIG. 5, five representative cells C are denoted by circles indicated by chain lines (the area of each circle is a bit larger than the real scale to be seen easily). In the PDP  1 , three cells of R, G and B colors constitute one pixel, and the arrangement form of three colors in the color display is a triangle (delta) arrangement form. The delta arrangement has an advantage in high definition compared with an inline arrangement since the width of the cell in the row direction is larger than one third of the pixel pitch. In addition, the rate of non-lighted areas in the screen is small, so that high luminance display can be realized. It is not necessary that the horizontal direction is the row direction. The vertical direction can be the row direction while the horizontal direction can be the column direction. 
     FIG. 6 is a perspective view showing a cell structure of the PDP. 
     The PDP  1  includes a front glass substrate  11  whose inner surface is provided with the display electrodes X and Y, a dielectric layer  17  and a protection film  18 , and a back glass substrate  21  whose inner surface is provided with the address electrodes A, an insulator layer  24 , partitions  29  and the fluorescent material layers  28 R,  28 G and  28 B. Each of the display electrodes X and Y includes a transparent conductive film  41  constituting a surface discharge gap and a metal film  42  as a bus conductor. The display electrodes X and Y are arranged alternately at a constant pitch (with the surface discharge gap) in the column direction. The gap direction of the surface discharge gap, i.e., the opposing direction of the display electrodes X and Y is the column direction. 
     FIG. 7 is a plan view showing a shape of the display electrode. 
     Each of the display electrodes X and Y includes a transparent conductive film  41  that extends in the row direction meandering in the column direction and a band-like metal film  42  that extends in the row direction meandering along the partition  29  so as to avoid the wide portion  31 A. The transparent conductive film  41  has a curved band-like shape and is patterned in a shape having a gap forming portion arching from the metal film  42  toward the wide portion  31 A in each column. In each of the wide portions  31 A, the gap forming portion of the display electrode X and the gap forming portion of the display electrode Y face each other, so that a drum-like surface discharge gap is formed. In the pair of gap forming portions facing each other, the opposing sides are not parallel. The width of the band-like transparent conductive film  41  may alter regularly. 
     This electrode shape enables reduction of the interelectrode capacitance without increasing the surface discharge gap (the minimum distance between electrodes) compared with a linear band-like shape. In addition, since the distance between the transparent conductive film  41  and the metal film  42  is large in the middle of the wide portion  31 A in the row direction, the intensity of the electric field in the gap between the transparent conductive film  41  and the metal film  42  decreases, so that a discharge interference between rows can be prevented. In addition, as a side effect, shading effect of the metal film  42  is reduced so that the light emission efficiency increases. 
     FIG. 8 shows a concept of a frame division. In a display using the PDP  1 , a frame F of the input image data is divided into q subframes SF so that a color is reproduced by on-off control of lighting. In other words, each frame F is replaced with a set of q subframes SF. The subframes SF are provided with weights, e.g., 2 0 , 2 1 , 2 2 , . . . 2 q−1  in order so as to set the number of times of the display discharge in each subframe SF. Though the subframe arrangement is in the weight order in FIG. 8, other order can be adopted. Redundant weighting can be adopted for reducing quasi contour. In accordance with this frame structure, a frame period Tf that is a frame transfer period is divided into q subframe periods Tsf, and one subframe period Tsf is assigned to each subframe SF. In addition, the subframe period Tsf is divided into a reset period TR for initialization, an address period TA for addressing and a display period TS for sustaining. The lengths of the reset period TR and the address period TA are constant regardless of the weight, while the length of the display period TS is longer as the weight is larger. Therefore, the length of the subframe period Tsf is also longer as the weight of the corresponding subframe SF is larger. The driving sequence is repeated for each subframe. The order of the reset period TR, the address period TA and the display period TS is common to each of the q subframes SF. 
     Hereinafter, drive waveforms in the display period TS, which are relevant to the present invention, will be exemplified. 
     FIG. 9 shows a first example of the drive waveforms. In this example, three kinds of potential, which are positive voltage, lower positive voltage and the ground voltage are set for each of the display electrodes X and Y. The application time of the highest voltage is short, and a high impedance period shown by the broken line is provided at the switching time from the high voltage to the low voltage. Similar drive can be performed by negative low voltage, negative high voltage and the ground level. The application time of the low voltage is short, and a high impedance period may be provided at the switching time from the low voltage to the high voltage. There are two absolute values of potential difference except zero volts at the interelectrode XY in this example. This example has an advantage that only a single output polarity is required in the power source. 
     FIG. 10 shows a second example of the drive waveforms. The drive waveforms in this example have three set potentials including positive voltage, negative voltage and the GND level. The positive voltage is applied to one of the display electrodes X and Y, while the negative voltage is applied to the other. The application time of the negative voltage is short, and the high impedance period is provided at the switching time from the negative voltage to the ground level. In the same way, it is possible to shorten the positive voltage the application time, and to provide the high impedance period at the switching time from the positive voltage to the ground level. There are two absolute values of the potential difference except zero volts at the interelectrode XY. This example has an advantage that the power source can be realized using a device having low withstand voltage. 
     FIG. 11 shows a third example of the drive waveforms. The drive waveforms in this example have positive high voltage, positive low voltage and the ground level. The positive high voltage is applied to one of the display electrodes. After a short time the other display electrode is separated from the power source to be the high impedance state, and then positive low voltage is applied. These can be replaced with negative low voltage, negative high voltage and the ground level. There are two absolute values of the potential difference except zero volts at the interelectrode XY. 
     FIG. 12 shows a fourth example of the drive waveforms. This example corresponds to a case where electrode potential setting in the third example is shifted to negative polarity side. These drive waveforms have positive voltage, the ground level and negative voltage. A pair of display electrodes X and Y is set to negative potential simultaneously. After that one of the display electrodes is set to positive potential, and after a short time the other display electrode is set to the high impedance state and then to the ground level. Alternatively, it is possible that the display electrodes X and Y are set to the positive voltage simultaneously, then one of the display electrodes is set to the negative potential, after a short time the other display electrode is set to the high impedance state and then to the ground level. There are two absolute values of the potential difference except zero volts at the interelectrode XY. In this example, compared with the above-mentioned second example, the period between the time of the high impedance state and the previous potential switching time is long, so the request of response to the switching device that is used for the electrode potential control is relieved. 
     FIG. 13 shows a fifth example of the drive waveforms. The drive waveforms in this example have positive voltage, the ground level and negative voltage. One of the display electrodes is set to negative potential, and then the other display electrode is set to positive potential. After a short time, the display electrode at the negative potential is set to the high impedance state, and then the display electrode at the high impedance state is set to the ground level. Alternatively, it is possible that one of the display electrodes is set to the positive potential, then the other display electrode is set to the negative potential, and after a short time the display electrode at the positive potential is set to the high impedance state, and then the display electrode at the high impedance state is set to the ground level. There are three absolute values of the potential difference except zero volts at the interelectrode XY. Until the polarity of the interelectrode XY voltage is reversed, there is a single pulse. From the leading edge of the pulse, there is a first level, a second level and a third level. Among them, the second level is the maximum voltage. In order to generate display discharge in the high impedance period, the first level must be lower than the third level. 
     Noting the voltage of the interelectrode XY and comparing this fifth example with the first through fourth examples explained above, the high impedance period is delayed from the leading edge of the pulse. This delay works to adjust the overlap of the display discharge generating time and the high impedance period. FIG. 14 shows dependence of the efficiency on the voltage Vo using the period Ts for keeping the first level as a parameter. As shown in FIG. 14, the fifth example has an advantage that high efficiency can be obtained even if the voltage Vo is low. 
     FIG. 15 shows an example of the driving circuit. FIG. 16 is a timing chart of the switching. Here, the case of generating the drive waveforms of the fourth example will be explained. 
     The illustrated circuit includes terminals XTP 1  and YTP 1  that are connected to the power source for generating the positive voltage, switches XSw 1  and YSw 1  for switching current path between output terminals XOUT and YOUT connected to the PDP  1  and the terminals XTP 1  and YTP 1 , rectifier elements XD 1  and YD 1  forming current paths from the switches XSw 1  and YSw 1  to the output terminals XOUT and YOUT, terminals XTP 2  and YTP 2  that are connected to the power source for generating the negative voltage, switches XSw 2  and YSw 2  for switching current paths between the terminals XTP 2  and YTP 2  and the output terminals XOUT and YOUT, rectifier elements XD 2  and YD 2  for forming current paths from the output terminals XOUT and YOUT to the switches XSw 2  and YSw 2 , terminals XTP 3  and YTP 3  that are connected to the ground line, switches XSw 3  and YSw 3  for switching current paths between the terminals XTP 3  and YTP 3  and the output terminals XOUT and YOUT, rectifier elements XD 3  and YD 3  for forming current paths from the switches XSw 3  and YSw 3  to the output terminals XOUT and YOUT, terminals XTP 4  and YTP 4  that are connected to the ground line, switches XSw 4  and YSw 4  for switching current paths between the terminals XTP 4  and YTP 4  and the output terminals XOUT and YOUT, rectifier elements XD 4  and YD 4  for forming current paths from the output terminals XOUT and YOUT to the switches XSw 4  and YSw 4 , terminals XTP 5  and YTP 5  that are connected to the power source for generating the positive voltage, rectifier elements XD 5  and YD 5  for forming current paths from the output terminals XOUT and YOUT to the terminals XTP 5  and YTP 5 , terminals XTP 6  and YTP 6  that are connected to the power source for generating the negative voltage, and the rectifier elements XD 6  and YD 6  for forming current paths from the terminals XTP 6  and YTP 6  to the output terminals XOUT and YOUT. 
     In the drive waveforms, a drive period of two pulses is divided into T 1 , T 2 , T 3 , T 4 , T 5 , T 6 , T 7  and T 8 . In the periods T 1  and T 5 , both the display electrodes X and Y are set to the negative potential. In the periods T 2  and T 6 , one of the display electrodes X and Y is set to the positive potential, and the other is set to the negative potential. In the periods T 3  and T 7 , the display electrodes that were set to the negative potential in the period T 2  or the period T 6  are set to the high impedance state. In the periods T 4  and T 8 , one of the display electrodes X and Y is set to the positive potential, and the other is set to the ground potential. 
     In the period T 1 , the switches XSw 2  and YSw 2  are closed so as to set the output terminals XOUT and YOUT to the negative potential. On this occasion, the switches XSw 4  and YSw 4  can be either closed or opened. In the period T 1  the switches XSw 1 , XSw 3 , YSw 1  and YSw 3  are opened. In addition, the switches XSw 2  and XSw 4  are opened till the period T 2 . 
     In the period T 2 , the switch XSw 1  is closed so as to set the output terminal XOUT to the positive potential. On this occasion, the switch XSw 3  for flowing current from the ground line to the output terminal XOUT can be either closed or opened. In the period T 2 , the switch YSw 2  is closed, so the output terminal YOUT is set to the negative potential. The switch YSw 4  can be either closed or opened. 
     In the period T 3 , the switches XSw 1 , XSw 2 , XSw 3  and XSw 4  maintain the state of the period T 2 . In the period T 3 , the switch YSw 2  is opened so as to shut off the power supply from the negative power source. In this state, the output terminal YOUT is lower than the ground level. Since the rectifier element YD 4  is connected, the output terminal YOUT is set to the high impedance state even if the switch YSw 4  is closed. In addition, if discharge is generated in this period T 3 , potential of the output terminal YOUT rises. If the potential rises largely, potential difference at the interelectrode XY becomes small, and the wall charge cannot be formed sufficiently, resulting in the drive margin failure. In the period T 3 , the switch YSw 4  for flowing current from the output terminal YOUT to the ground line is closed, so as to set potential of the output terminal YOUT below the ground level. 
     In the period T 4 , the switches XSw 1 , XSw 2 , XSw 3  and XSw 4  maintain the state of the period T 2 . The switches YSw 3  and YSw 4  are closed so as to fix the output terminal YOUT to the ground level. 
     In the periods T 5 -T 8 , the switching is performed with exchanging the relationship between the display electrode X and the display electrode Y in the periods T 1 -T 4 . 
     While the presently preferred embodiments of the present invention have been shown and described, it will be understood that the present invention is not limited thereto, and that various changes and modifications may be made by those skilled in the art without departing from the scope of the invention as set forth in the appended claims.