Patent Publication Number: US-7215303-B2

Title: AC-type plasma display panel capable of high definition and high brightness image display, and a method of driving the same

Description:
TECHNICAL FIELD 
   The invention relates to an alternating current (AC) plasma display panel (PDP) used in computers, televisions, and the like, and a related method for driving the PDP. 
   BACKGROUND ART 
   Research into displays in recent years has been stimulated by the demand for improved performance, particularly in relation to higher definition (high vision, etc) and flatter devices. 
   Leading the way in flat panel technology are liquid crystal displays (LCDs) and plasma display panels (PDPs). PDPs are particularly suitable for thin, large-screen applications, and 50-inch class models are already being developed. 
   Direct current (DC-type) and alternating current (AC-type) are the two broad categories of PDP, although AC PDPs are currently preferred for their particular suitability in large-screen applications. 
     FIG. 11A  shows a sectional view of a main part of an exemplary prior art surface discharge AC PDP.  FIG. 11B  shows a sectional view along the A—A axis of the prior art PDP. 
   A PDP is commonly composed of a matrix of colored luminous cells. A known surface discharge AC PDP, as disclosed, for example, in unexamined patent application publication 9-35628 published in Japan, has the structure shown in  FIGS. 11A and 11B . In this PDP, a front glass substrate  211  and a back glass substrate  221  are arranged parallel to and facing each other with barrier ribs  224  interposed therebetween. A parallel pair of discharge electrodes (scan electrode  212   a  and sustain electrode  212   b ) are arranged on the facing surface of front glass substrate  211 , and covered with a dielectric layer  213  and a protective layer  214 . An address electrode  222  is arranged on the facing surface of the back glass substrate  221  so as to extend in an orthogonal direction to electrodes  212   a  and  212   b . Colored luminous cells are formed by arranging a colored phosphor layer  225  within a space  230  defined between the interposed barrier ribs. Space  230  is filled with a discharge gas containing neon and xenon, for example. 
   In this PDP, a drive circuit applies a voltage to each of the electrodes. Since each cell can only express the states of “on” or “off”, however, one field is divided into a plurality of subfields, and then by controlling the on/off timing of each subfield and thereby varying the combination of “on” subfields, intermediate graduations may be expressed with respect to the colors red (R), green (G) and blue (B). 
   Image display in a PDP is generally achieved in each of the subfields by using the so-called address display period separated subfield (ADS) method. This method involves a setup period, an address period, and a sustain period that are conducted consecutively. In the setup period a pulse voltage is applied uniformly to all the scan electrodes. In the address period a pulse voltage is applied sequentially to the scan electrodes as well as to address electrodes selected from among the plurality of address electrodes, and as a result wall charge is stored in the cells to be turned on. Finally, a pulse voltage is applied between the scan and sustain electrodes in the sustain period in order to sustain the discharge. Ultraviolet (UV) light is generated as a result of the sustain discharge, and image display is achieved when the phosphor elements (red, green, blue) are excited to illumination through contact with the UV light. 
   An object of the prior art PDP is to enhance luminous efficiency while maintaining a low drive voltage, this being a long-held objective of PDP designers. Keeping the drive voltage at a low level helps to simplify the circuitry architecture and minimize any losses relating to inefficient power usage. 
   In view of these factors, the pressure of the gas enclosed within the PDP is generally maintained at approximately 40 kPa to 65 kPa, and the xenon (Xe) component of the gas is maintained at around 5 vol %. Furthermore, the size of a gap dp (surface discharge gap) between the scan electrode  212   a  and the sustain electrode  212   b  in each pair is established at a value close to the minimum discharge voltage shown on a Paschen curve (generally about 80 μm), thus maintaining an external sustain voltage Vsus in a range from 180V to 200V. 
   As shown in  FIGS. 11A and 11B , the discharge electrodes  212   a  and  212   b  are composed of transparent electrodes  2121   a  and  2121   b  and metal bus lines  2122   a  and  2122   b , which allows for the discharge to expand by way of the transparent electrodes. 
   While conventional technology has been effective in enhancing the luminous efficiency of PDPs, currently achievable efficiency levels of approximately 11 m/W are still only about one-fifth of that achievable by cathode ray tube (CRT) displays. 
   Increasing the xenon partial pressure of the enclosed discharge gas has also proved effective in enhancing luminous efficiency. U.S. Pat. No. 5,770,921, for example, achieves this result by establishing the xenon component at 10 vol % or greater. Still further improvements are desired, however. 
   DISCLOSURE OF INVENTION 
   An object of the invention is to provide a PDP, a PDP display apparatus, and a related drive method capable of greatly enhancing luminous efficiency in comparison with conventional levels, while at the same time maintaining a low discharge sustain voltage. 
   A PDP capable of achieving this object has a first substrate and a second substrate arranged to face each other with barrier ribs interposed therebetween. A first electrode and a second electrode are arranged on a facing surface of the first substrate, the first and second electrodes extending parallel to each other and being covered with a dielectric layer. A third electrode is arranged on a facing surface of the second substrate so as to extend orthogonally to the first and second electrodes. A discharge gas is enclosed within a discharge space defined between the interposed barrier ribs. In this PDP, the discharge gas is a gas mixture containing xenon, the xenon component comprising at least 5 vol % and less than 100 vol % and having a partial pressure of at least 2 kPa. Furthermore, in this PDP, the gap between the first and second electrodes is greater than a height of the discharge space. The height of the discharge space is here measured in a thickness direction of the PDP, and approximates the distance separating the third electrode from either the first or second electrodes. 
   According to this configuration, it is possible to achieve a high luminous efficiency when the PDP is driven. This is due to the high Xe partial pressure and consequent high levels of xenon present in the discharge space. 
   The above result is achieved as follows and is disclosed in U.S. Pat. No. 5,770,921 mentioned above. High levels of Xe in the discharge space help to generate more UV light, leading to an increased peak of an excitation wavelength (173 nm) formed by radiation from Xe molecules. The conversion efficiency of the phosphors emitting visible light is improved as a result. 
   Also, the fact that the gap between the first and second electrodes is greater than the height of the discharge space means that when a sustain pulse of alternating polarity is applied between the first and second electrodes, the discharge path is lengthened to form a positive column discharge. A positive column discharge is known to achieve a high luminous efficiency and is, therefore, desirable. 
   Also, the fact that a discharge is initiated between the third electrode and either the first or second electrodes when a sustain pulse is applied in the sustain discharge (the gap separating the third electrode from either the first or second electrodes being shorter than the gap between the first and second electrodes) allows the voltage applied in initiating the discharge to be maintained at a low level. 
   In other words, when a sustain pulse, during which the second electrode is negative, is applied in order to sustain the discharge, a discharge is initiated between the second and third electrodes, even at a low applied voltage, and the initiated discharge expands in the direction of the first electrode. Likewise, when a sustain pulse, during which the first electrode is negative, is applied in order to sustain the discharge, a discharge is initiated between the first and third electrodes, even at a low applied voltage, and the initiated discharge expands in the direction of the second electrode. Thus it is possible to sustain the discharge at a relatively low voltage, despite the large gap separating the first and second electrodes. 
   As described above, the present invention is able to greatly enhance luminous efficiency in comparison with known PDPs, while at the same time maintaining the discharge voltage at a low level. 
   Increasing the gap between the first and second electrodes, therefore, leads to improved luminous efficiency, although cell pitch and drive voltage place limitations on the practical size of this gap. Despite these limitations, however, a gap several times the height of the discharge space can still be achieved. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective overview showing a structure of a surface discharge AC PDP according to an embodiment of the present invention; 
       FIG. 2  shows a structure of a display apparatus connected to a drive circuit  100  of the PDP; 
       FIG. 3  shows an exemplary method for-dividing a field when the display apparatus is driven; 
       FIGS. 4A˜4D  show a timing, within a single subfield, of pulses applied by the drive circuit to each of the electrodes; 
       FIG. 5  shows a cross-section of the PDP in a length direction of an address electrode; 
       FIGS. 6A˜6C  and  FIGS. 7A˜7C  show discharge patterns of the PDP; 
       FIG. 8  is a characteristic diagram showing a relationship between a surface discharge gap and a discharge voltage; 
       FIG. 9  shows a relationship between xenon partial pressure and luminous efficiency with respect to both the PDP of the present invention and a prior art PDP; 
       FIG. 10  shows a relationship between xenon partial pressure and luminous efficiency in the PDP of the present invention; and 
       FIGS. 11A˜11B  are cross-sectional views of a main section of the prior art PDP. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   PDP Structure and the Related Drive Method 
     FIG. 1  is a perspective overview showing a structure of a surface discharge AC PDP according to the present embodiment. 
   The PDP of the present invention is formed from a front panel  10  and a back panel  20  that are positioned parallel to and facing each other with a space defined therebetween. Front panel  10  includes a front glass substrate  11 , and back panel  20  includes a back glass substrate  21 . The facing surface of front glass substrate  11  has arranged thereon first electrodes (scan electrodes)  12   a , second electrodes (sustain electrodes)  12   b , a dielectric layer  13 , and a protective layer  14 . A facing surface of back glass substrate  21  has arranged thereon third electrodes (address electrodes)  22 . 
   The space between the front and back panels is partitioned by barrier ribs  24  formed in a stripe pattern, and the interposed barrier ribs define discharge spaces  30 . A discharge gas is enclosed within discharge spaces  30 . 
   Phosphor layers  25  are formed between adjacent barrier ribs  24  on back panel  20 . The phosphor layers correspond respectively to the colors red, green, and blue, and are arranged repeatedly in the stated order so as to face into discharge spaces  30 . 
   The electrodes  12   a ,  12   b , and  22  are metal electrodes formed in a stripe pattern, and may be constructed, for example, by applying an Ag paste in a stripe pattern and firing the paste. The first and second electrodes extend in a direction orthogonal to barrier ribs  24 , while third electrode  22  extends in a direction parallel to the barrier ribs. 
   The gap between the first and second electrodes (surface discharge gap) is greater than a height of discharge spaces  30  (i.e. in a thickness direction of the PDP, hereafter “vertical discharge gap”). This configuration will be described below in greater detail. 
   Dielectric layer  13  is composed of a dielectric material and is arranged to cover the entire surface of front glass substrate  11  on which electrodes  12   a  and  12   b  are arranged. Dielectric layer  13  is generally formed, for example, from a low melting lead glass or bismuth glass. 
   Protective layer  14  is a thin layer formed from magnesium oxide (MgO) and other materials having a high secondary electron emission coefficient, and covers the entire surface of dielectric layer  13 . 
   Barrier ribs  24  are composed of a glass material and are mounted onto the facing surface of back glass substrate  21 . 
   Although the above description relates to a dielectric layer being formed only on front glass substrate  11 , a dielectric layer may also be formed over third electrodes  22  on back glass substrate  21 , and phosphor layers  25  may then be formed over this dielectric layer. 
   The discharge gas is a gas mixture composed of xenon (Xe) and at least one of helium (He), neon (Ne), and argon (Ar), all of which are known in prior art PDPs. The xenon partial pressure is established at 2 kPa or greater to ensure a high level of xenon in the discharge space. Thus when the total pressure of the discharge gas is in a range from 40 kPa to 67 kPa inclusive, the xenon component of the gas mixture is 5 vol % or greater. 
   In order to achieve a high luminous efficiency, the xenon partial pressure should preferably be established at 6.7 kPa or greater, or even 10 kPa or greater. A xenon partial pressure of 16 kPa is considered the upper limit given the capacity of known drive circuits. This area will be covered in greater detail below. 
     FIG. 2  shows a structure of a display apparatus connected to drive circuit  100  of the PDP. As shown in  FIG. 2 , third electrodes  22  extend in an orthogonal direction to electrodes  12   a  and  12   b . Discharge cells are formed in the space between the front and back glass substrates, and one pixel is composed of three cells (red, green, blue) adjacent in a lengthwise direction of electrodes  12   a  and  12   b.    
   According to this structure, an expansion of the discharge from one cell into an adjacent cell can be prevented as a result of neighboring discharge cells being partitioned off by barrier ribs  24 . 
     FIG. 3  shows an exemplary method for dividing a field in order to express 256 brightness values, the horizontal axis marking time and the shaded areas representing the sustain periods. 
   According to this exemplary division method, one field is composed of eight subfields, the sustain period ratio of the eight subfields being 1, 2, 4, 8, 16, 32, 64, and 128, and the 256 brightness values being expressed through a combination of these eight bit binary values. Given that an image is composed of 60 fields per second according to the NTSC standard, the period of one field is established at 16.7 ms. 
   Each subfield is composed of consecutive setup, address, and sustain periods, and one field of image display is achieved by conducting eight times the operation (i.e. setup, address, and sustain periods) of a single subfield. 
     FIGS. 4A to 4D  show a timing, within a single subfield, of the pulses applied by drive circuit  100  to each of the electrodes. 
     FIG. 4A  shows a voltage waveform Vx applied to a first electrode  12   a ,  FIG. 4B  shows a voltage waveform Vy applied to a second electrode  12   b ,  FIG. 4C  shows a voltage waveform Va applied to a third electrode  22 , and  FIG. 4D  shows a waveform of an absolute value of the current resulting from the discharge. 
   It should be noted that although a pulse is applied sequentially to a plurality of first electrodes as well as to a plurality of selected third electrodes in the address period, for ease of understanding,  FIGS. 4A to 4D  refer only to a single first electrode  12   a , a single second electrode  12   b , and a single third electrode  22 . 
   In the setup period, a positive initializing pulse is applied simultaneously to all first electrodes  12   a , thus storing wall charge on both protective layer  14  and phosphor layers  25 , and initializing all the discharge cells. 
   In the address period, a positive data pulse is applied to selected third electrodes  22 , and a negative scan pulse is applied sequentially to first electrodes  12   a . As a result, a discharge is initiated between the first and third electrodes in the cells to be turned on (hereafter, the “on” cells), wall charge forms on the surface of protective layer  14 , and one full screen of pixel information is written in a subfield. 
   In the sustain period, an AC voltage is applied collectively between the first and second electrodes, which results in a selective plasma discharge occurring only in the cells storing wall charge. 
   Surface Discharge Gap and Vertical Discharge Gap 
     FIG. 5  shows a cross-section of the PDP in a lengthwise direction of third electrodes  22 . 
   As shown in  FIG. 5 , the surface discharge gap dss between the first and second electrodes is greater than the vertical discharge gap dsa between the facing surfaces of protective layer  14  and phosphor layers  25  (i.e. dss&gt;dsa). 
   In designing a surface discharge AC PDP, the size of the vertical discharge gap dsa should ideally be established so as to facilitate the address discharge. In practice, however, the size of the gap is determined by factors such as the pressure of the discharge gas and the volume of the discharge space required to maintain a stable discharge. 
   In known PDPs, the surface discharge gap dss, on the other hand, is commonly established in accordance with Paschen&#39;s Law, which results in the gap dss being smaller than the gap dsa. 
   Thus, when the surface discharge gap dss is established to be larger than the vertical discharge gap dsa, as in the present embodiment, the length of the discharge in the sustain period is increased in comparison with prior art PDPs. 
   Although the practical size of the surface discharge gap dss is limited by cell pitch, a gap several times that of the vertical discharge gap dsa can still be achieved. 
   Specifically, the distance dst between the outer edge of the first and second electrodes (see  FIG. 5 ) is limited by cell pitch, which in turn effectively limits the size of the surface discharge gap dss. However, the size of the gap dss can be maximized within these limits by using only metal electrodes without providing transparent electrodes, and narrowing the width of the first and second electrodes as much as possible. A surface discharge gap dss several times that of the vertical discharge gap dsa can thus be achieved. 
   The gap dss is also restricted by the drive voltage, since even slight increases in the surface discharge gap dss lead to increases in the drive voltage. Despite this, the PDP of the present embodiment can still be driven with a gap dss five to six times that of the vertical discharge gap dsa. 
   Since the surface discharge gap dss should preferably be made as large as possible in order to achieve a longer discharge, it is advantageous for the gap dss to be at least 1.2 times, if not 1.5 times or even two or three times, the size of the vertical discharge gap dsa. 
   Chart 1 shows exemplary design parameters of the PDP according to the present embodiment. 
   
     
       
         
             
             
             
           
             
                 
               CHART 1 
             
             
                 
                 
             
           
          
             
                 
               Single Pixel Size 
               1080 × 1080 μm 2   
             
             
                 
               Surface Discharge Cap (dss) 
               400 μm 
             
             
                 
               Vertical Discharge Cap (dsa) 
                90 μm 
             
             
                 
               Barrier Rib Height 
               120 μm 
             
             
                 
               First/Second Electrode Width 
               100 μm 
             
             
                 
               Gas Composition 
               Ne (80%), Xe (20%) 
             
             
                 
               Gas Pressure 
               80 kPa 
             
             
                 
                 
             
          
         
       
     
   
   According to the above design parameters, the surface discharge gap dss between the first and second electrodes is 400 μm, which is more than four times the vertical discharge gap dsa (90 μm), and five times the surface discharge gap dss (80 μm) of the prior art PDP shown in  FIGS. 11A and 11B . 
   Applied Pulses and Resultant Discharge Patterns in Each Period 
   The following refers to  FIGS. 4A to 4D  in describing both the pulses applied in each of the setup, address, and sustain periods, and the patterns of discharge resulting from the pulses. Although the waveform of the pulses applied by drive circuit  100  are basically the same as in prior art PDPs, novelty lies in the discharge patterns arising from these pulses. 
   In  FIG. 4A , the broken line represents the wall voltage generated on phosphor layers  25  over third electrodes  22 , and on dielectric layer  13  and protective layer  14  over first electrodes  12   a . The broken line in  FIG. 4B  represents the wall voltage generated on phosphor layers  25  over third electrodes  22 , and on dielectric layer  13  and protective layer  14  over second electrodes  12   b . The polarity of stored wall charge is shown above the respective broken lines. 
   The wall voltage is generated by wall charge stored on protective layer  14  and phosphor layers  25  subsequent to the ignition of the discharge. 
   Also, the difference between the applied voltage (solid lines) and the wall voltage (broken lines) is equivalent to the voltage applied within the discharge space between the address electrode and each first and second electrode. 
     FIGS. 6A to 6C  and  FIGS. 7A to 7C  show the discharge patterns of the PDP, and will be referred to during the following description. 
   Setup Period: 
   In the first half of the setup period, a decreasing ramp voltage based on the potential of third electrodes  22  is applied to both the first and second electrodes. Protective layer  14 , which has a comparatively high secondary electron emission coefficient, thus becomes the cathode, and a weak discharge is readily initiated within a first vertical discharge space  30   a  (i.e. the discharge space between the first and third electrodes) and a second vertical discharge space  30   b  (i.e. the discharge space between the second and third electrodes). Initializing charge is formed within the first and second vertical discharge spaces as a result of this discharge. 
   In a middle period of the setup period, an increasing ramp voltage based on the potential of third electrodes  22  and having a relatively large amplitude is applied to both the first and second electrodes. A discharge occurs in the first and second vertical discharge spaces as a result, which in turn leads to negative charge being stored on protective layer  14  over the first and second electrodes. 
   In the latter half of the setup period, a decreasing ramp voltage based on the potential of third electrodes  22  is applied to first electrodes  12   a . A discharge occurs in the first vertical discharge spaces  30   a  as a result, which in turn leads to the elimination of some of the negative wall charge stored on the surface of protective layer  14 . 
   For the duration of the ramp voltage there is a continuous flow of current resulting from the discharge, and in the first vertical discharge spaces  30   a  a voltage approximating the magnitude of the discharge sustain voltage Vs is constantly applied. Consequently, when the setup period is completed, the difference between the applied voltage and the wall voltage is approximately equal to the discharge sustain voltage Vs within the discharge spaces. In  FIGS. 4A to 4D , the voltage applied in the first vertical discharge spaces  30   a  at the completion of the setup period is Vs x-a . 
   The setup pulse waveform is substantially the same as that disclosed in unexamined patent application publication 12-267625 published in Japan. By utilizing such a waveform, the initialization can be conducted in a comparatively short period of time, which thus allows for the sustain period to be extended. 
   Address Period: 
   In the address period, a bias voltage Vab and a negative pulse voltage are applied to first electrodes  12   a , the pulse voltage being applied while sequentially scanning first electrodes  12   a . At the same time, a discharge is selectively initiated in the “on” cells by applying a positive data pulse (voltage Va) to third electrodes  22  corresponding to the “on” cells. 
   Also in the address period, a positive voltage based on the potential of first electrodes  12   a  is applied continuously to second electrodes  12   b.    
   As a result, a voltage (VS x-a +Va) is applied in the first vertical discharge space  30   a  of the “on” cells at time t 1 , initiating a discharge in these discharge spaces. 
   The voltage (Vs x-a ) is substantially the same as the discharge sustain voltage applied in the first vertical discharge spaces  30   a , thus allowing the discharge to be initiated at a comparatively low data pulse voltage Va. 
   Also, because of the positive voltage, which is based on the potential of first electrodes  12   a , being continuously applied to second electrodes  12   b  as described above, the discharge generated in the first vertical discharge space  30   a  of the “on” cells expands towards second electrodes  12   b , initiating a discharge in the second vertical discharge space  30   b  of the “on” cells. 
   As a result, in the “on” cells positive charge is stored on protective layer  14  over first electrode  12   a , and negative charge is stored on protective layer  14  over second electrode  12   b , as shown in  FIG. 6A . 
   In contrast, no data pulse is applied to third electrodes  22  corresponding to the “off” cells, and as a result no discharge occurs in these cells. Thus at the completion of the setup period, the charge stored on protective layer  14  over the first and second electrodes in the “off” cells remains substantially unchanged. 
   Sustain Period: 
   In the sustain period, first and second sustain pulses having opposite polarities and an amplitude Vsus, are applied alternately between the first and second electrodes. 
     FIGS. 6A to 6C  and  FIGS. 7A to 7C  are simplified cross-sectional views of the PDP of the present embodiment showing the state of the applied voltage, the wall charge, and the discharge plasma when the first sustain pulse is applied (note: protective layer  14  is not depicted in these drawings). 
   The following describes in detail, with reference to  FIGS. 6A to 6C  and  FIGS. 7A to 7C , the way in which a discharge initiated in the second vertical discharge space  30   b  of an “on” cell expands in the sustain period to the first vertical discharge space  30   a  of the “on” cell. 
   As shown in  FIGS. 4A to 4D , an external sustain voltage Vsus is applied to first electrode  12   a  at time t 3  and second electrode  12   b  is grounded. 
   Consequently, the polarity of the first sustain pulse applied at time t 3  is such that second electrode  12   b  is negative and first electrode  12   a  is positive. 
   The negative polarity of second electrode  12   b  results from the negative wall charge stored on dielectric layer  13  over second electrode  12   b  in the “on” cells in the address period. Thus the discharge initiated by applying the first sustain pulse is such that second electrode  12   b  (i.e. on the side of the second vertical discharge space  30   b ) is negative. 
   The discharge generated in the second vertical discharge space  30   b  expands toward first electrode  12   a  as a result of positive wall charge stored on the surface of phosphor layer  25 . By way of note, the storage of positive wall charge on the phosphor layer results from third electrode  22  having a low potential relative to the high positive voltage applied to second electrode  12   b  in the address period, which leads to the third electrode attracting positive charge. 
     FIG. 6B  shows the initiation of a discharge in the second vertical discharge space  30   b . Large amounts of positive and negative charge generate from this discharge, and the generated charge is attracted to the second and third electrodes, respectively, thereby forming wall charge. The wall voltage generated by the wall charge serves to eliminate the voltage applied in the second vertical discharge space  30   b  and terminate the discharge within this discharge space. 
   Because the dielectric constant of phosphor layer  25  over third electrode  22  is smaller than that of dielectric layer  13  over second electrode  12   b , wall charge is stored at a faster rate on phosphor layer  25 . 
   As a result, the part of the discharge nearest the anode (i.e. nearest first electrode  12   a  during the first sustain pulse) is attracted to and moves along the surface of phosphor layer  25 , depositing negative charge as it proceeds (see FIG.  6 B/C). 
   In contrast, the positive voltage, which is based on the potential of second electrode  12   b , applied to first electrode  12   a  helps guide the expanding discharge toward first electrode  12   a .  FIG. 6C  shows the part of the discharge nearest the anode expanding toward first electrode  12   a , eliminating the positive charge stored on the surface of phosphor layer  25  as it proceeds. 
   As shown in  FIG. 7A , the anode side of the discharge reaches first electrode  12   a  at time t 4 , thus generating a discharge in the first vertical discharge space  30   a.    
     FIG. 7B  shows the discharge immediately before termination, and  FIG. 7C  shows the discharge having been terminated as a result of wall charge stored on dielectric layer  13  and phosphor layer  25 . 
   Subsequent to the discharge described above, negative and positive wall charge forms on the surface of dielectric layer  13  and phosphor layer  25 , respectively, in the first vertical discharge space  30   a . As a result, negative charge is stored on dielectric layer  13  over first electrode  12   a , and positive charge is stored on phosphor layer  25  and on dielectric layer  13  over second electrode  12   b.    
   As shown in  FIG. 7C , almost all of the wall charge has been eliminated from the second vertical discharge space  30   b  within which the discharge originated. 
   Large amounts of UV light emits from the positive column discharge as a result of the long discharge connecting the first and second vertical discharge spaces. Here, “positive column discharge” is used to refer to any filament-shaped discharge generated in a long discharge space between electrodes. 
   The distribution of wall charge in  FIG. 7C  is the opposite of that at time t 3  (see  FIG. 6A ). In  FIGS. 4A to 4D , the second sustain pulse at time t 5  is applied in the same manner as the first sustain pulse at time t 3 , although the function of the first and second electrodes is reversed. Thus an external sustain discharge Vsus is applied to second electrodes  12   b  and first electrodes  12   a  are grounded. 
   Repetitions of an identical sustain discharge can be achieved as a result. 
   The surface discharge patterns occurring in the sustain period according to the present embodiment differ to those of the prior art PDP shown in  FIGS. 11A and 11B . Specifically, the discharge according to the present embodiment is generated via the vertical discharge gap, and is, therefore, somewhat similar to a discharge formed between electrodes positioned facing one another (i.e. as opposed to electrodes positioned on a flat plane). 
   Furthermore, the timing at time t 3  of (i) the application of the external sustain voltage Vsus to first electrodes  12   a  and (ii) the grounding of second electrodes  12   b  should preferably be such that second electrodes  12   b  (i.e. on the side of the second vertical discharge space  30   b ) are negative when the discharge is initiated. This timing can be realized as follows. 
   One method is to firstly apply the external sustain voltage Vsus to first electrodes  12   a  (i.e. no discharge generated), and then to initiate a discharge by grounding second electrodes  12   b . A further method involves grounding second electrodes  12   b  and then applying the external sustain voltage Vsus to first electrodes  12   a  for the desired duration of the discharge. The latter method allows for a reduction in the discharge current, which serves to reduce the load on the drive circuit. 
   Effects of the PDP of the Present Embodiment 
   As described above, by establishing the xenon partial pressure in the PDP of the present embodiment at 2 kPa or greater, the level of xenon in discharge spaces  30  is increased (note: at total discharge gas pressures of 40 kPa or greater, the xenon component of the discharge gas is 5 vol % or greater). Moreover, by establishing the surface discharge gap dss to be greater than the height of discharge spaces  30 , a longer discharge can be sustained at a low discharge voltage, which allows for luminous efficiency to be enhanced while maintaining a low discharge voltage. The reasons and supporting material for these effects are detailed below. 
   Firstly, the reasons for being able to maintain a low discharge voltage will be described. 
   When the surface discharge gap dss between the first and second electrodes is large, the discharge firing voltage Vfss required to sustain a discharge between the first and second electrodes when the third electrode  22  is not utilized is greatly increased according to Paschen&#39;s Law. 
   Increases in the discharge firing voltage Vfss lead to corresponding increases in the external sustain voltage Vsus. Given that the sum total of wall charge on dielectric layer  13  over the first and second electrodes is Vwss, the voltage occurring in the discharge spaces equals the external sustain voltage Vsus+Vwss. Thus, to sustain the discharge between the first and second electrodes in the sustain period, formula 1, as given below, should be satisfied.
 
 Vfss&lt;Vsus+Vwss   Formula 1:
 
   As described above in relation to the discharge patterns of the present embodiment, a discharge is initiated between either the first and third electrodes (first vertical discharge space  30   a ) or the second and third electrodes (second vertical discharge space  30   b ) in order to sustain the discharge between the first and second electrodes. This allows the discharge firing voltage Vfss, and consequently the external sustain voltage Vsus, to be maintained at considerably low levels. 
   As described above in relation to the discharge patterns when the sustain pulse is applied, the first electrode  12   a  is negative when a discharge is to be initiated in the first vertical discharge space  30   a , and the second electrode  12   b  is negative when the discharge is to be initiated in the second vertical discharge space  30   b , which thus allows for further reductions in the discharge firing voltage. The reasons for this will be described after first defining a number of terms. 
   The discharge space between the first and third electrodes is defined as a first vertical discharge space  30   a , and the discharge space between the second and third electrodes is defined as a second vertical discharge space  30   b.    
   The discharge firing voltage applied between the first and second electrodes (i.e. within the gap dss) is given as Vfss. 
   The discharge firing voltage applied within in the first/second vertical discharge space when the first/second electrode has a low potential with respect to the third electrode  22  is given as Vfsa. 
   The discharge firing voltage applied within the first/second discharge space when the third electrode  22  has a low potential with respect to the first/second electrode is given a Vfas. 
   Thus Vfsa and Vfas are discharge firing voltages having opposite polarities. In comparison to Vfsa, which is the discharge voltage when protective layer  14 , having a high secondary electron emission coefficient, is on the cathode side, Vfsa is the discharge firing voltage when phosphor layers  25 , having a low secondary electron emission coefficient, are on the cathode side. Thus, Vfsa&lt;&lt;Vfas. 
   Having protective layer  14  on the cathode side is advantageous as it allows the discharge to be initiated at a lower discharge firing voltage. 
   The effects of the present invention will now be described with reference to the data in  FIGS. 8 to 10 . 
     FIG. 8  is a characteristic diagram showing the relationship between a discharge gap d (i.e. surface discharge gap) and the discharge voltage. The Q curve represents a discharge generated between the first and second electrodes when the third electrode  22  is utilized, as per the present embodiment. In contrast, the P curve represents a discharge generated between the first and second electrodes when the third electrode  22  is not included. 
   The P curve follows Paschen&#39;s Law. The discharge voltage has a minimum value at a relatively small discharge gap, and increases markedly with increases in the size of the discharge gap. 
   With respect to the Q curve, on the other hand, only slight increases in the discharge voltage result, even from substantial increases in the size of the discharge gap d. Thus the discharge voltage applied to the first and second electrodes can be maintained at levels substantially the same as the discharge voltage applied in the vertical discharge spaces. This is because the vertical discharge gap dsa remains fixed, and the discharge voltage is determined in relation to the fixed gap dsa. 
   Furthermore, according to  FIG. 8 , although the Q curve is higher than the P curve in regions where the discharge gap d is small, the Q curve is lower than the P curve beyond a certain gap length dc. In other words, the discharge voltage is lower when the discharge is conducted using both the third electrode  22  and phosphor layers  25 . The gap length dc is referred to as the critical length. 
   The critical length is substantially the same as the vertical discharge gap dsa. 
   Consequently, when the surface discharge gap dss is larger than the vertical discharge gap dsa, the PDP can be driven at a discharge voltage that is lower than the discharge voltage estimated from the P curve. 
   This result proves that the PDP of the present embodiment can be driven at a discharge voltage substantially lower than the discharge voltage estimated for the discharge gap d according to Paschen&#39;s Law. 
     FIG. 9  shows changes in luminous efficiency relative to changes in xenon partial pressure, comparing the PDP of the present embodiment (i.e. discharge gap larger than height of discharge space) with the prior art PDP in  FIGS. 11A and 11B  (i.e. discharge gap smaller than height of discharge space). The results are based on a fixed discharge gas pressure of 67 kPa and a variable xenon partial pressure. 
   In  FIG. 9 , the X curve represents the prior art PDP, and the Y curve represents the PDP of the present embodiment. The xenon partial pressure is given as a percentage of the total discharge gas pressure, which is 67 kPa in the given example. 
   Although both curves show improvements in luminous efficiency as a result of increases in the xenon partial pressure, these improvements are substantially greater with respect to the Y curve. 
   This result proves clearly that enhancements in luminous efficiency gained through increases in the xenon partial pressure for a PDP having a discharge gap greater than the height of the discharge space are over and above similar improvements recorded in relation to the prior art PDP. 
   As shown in  FIG. 9 , particularly high luminous efficiency can be achieved when the xenon partial pressure is 10 vol % or greater (i.e. a xenon partial pressure of 6.7 kPa or greater). 
   Whereas known PDPs (i.e. Xe component approx. 5 vol %; discharge gap smaller than height of discharge space) can only achieve a luminous efficiency of approximately 1.01 m/W,  FIG. 9  shows that in the PDP of the present embodiment, increases in the xenon partial pressure are matched by equal improvements in luminous efficiency. Thus it is clear that a PDP having enhanced luminous efficiency can be achieve by establishing both the discharge gap to be greater than the height of the discharge space, and the xenon partial pressure to be at least 2 kPa (e.g. a xenon component of at least 3.3 vol %, given a total discharge gas pressure of 66.7 kPa). 
   Furthermore, although the results in  FIG. 9  were obtained by varying the xenon component at a fixed total discharge gas pressure, increasing the xenon partial pressure by varying the total pressure gives substantially the same improvements in luminous efficiency. 
     FIG. 10  shows the change in luminous efficiency when the xenon partial pressure is varied in a test PDP manufactured in accordance with the present embodiment. The relationship between xenon partial pressure (kPa) and luminous efficiency is shown. 
   Although the test PDP uses a gas mixture composed of neon and xenon, effects identical to those shown in  FIG. 10  can be achieved by replacing the neon with helium, argon, krypton, or a mixture of these gases. 
   The maximum achievable xenon partial pressure depends on the breakdown voltage of the drive circuit. 
   With respect to the test PDP, a luminous efficiency of 2.1 lm/W was achieved, for example, when an external sustain voltage Vsus of 340V was applied. Although it is anticipated that even higher luminous efficiency can be achieved with further increases in the xenon partial pressure, limitations regarding the withstanding voltage of known circuitry dictates that the external sustain voltage not exceed 340V. Practical operation of the PDP at xenon partial pressures in excess of 16 kPa is presently not considered feasible. 
   In view of the above restrictions, the xenon partial pressure should preferably be maintained at 16 kPa or below. 
   If the breakdown voltage of the drive ICs can be increased, xenon partial pressures in excess of 16 kPa, say, 30 kPa, for example, may become achievable. Since the luminous efficiency as shown in  FIG. 10  improves at an excellent rate with respect to increases in the xenon partial pressure, a high xenon partial pressure of 30 kPa would, according to the graph in  FIG. 10 , result in a luminous efficiency of around 3.5 lm/W. 
   Although practical operation of the PDP is not considered feasible at xenon levels in excess of 20 vol % when the total discharge gas pressure is around 66.7 kPa, the PDP can be driven at xenon levels in excess of 20 vol % by reducing the total pressure of the discharge gas. 
   As described above, by establishing the xenon partial pressure at 2 kPa or greater (or, alternatively, at 5 vol % of the total pressure), and by enlarging the gap between the first and second electrodes, it is possible to greatly enhance luminous efficiency while maintaining a low drive voltage in the AC PDP according to the present embodiment. 
   Furthermore, since it is readily feasible to achieve a surface discharge gap dss that is considerably larger than the vertical discharge gap dsa in high definition PDPs given the marked reductions in the gap dsa required in such PDPs, the PDP of the present embodiment is particularly suited to applications requiring high definition. 
   Variations 
   Although the above embodiment was described in relation to an AC PDP employing the address display period separated subfield (ADS) method, the same effects can be obtained in an AC PDP that uses other drive methods, an example of which is a method that involves the addressing being conducted sequentially line by line, and the discharge being sustained immediately after the addressing of each respective line. 
   Also, the waveform of voltages applied in the setup and address periods is not limited to those described in the above embodiment. For instance, the wall charge may be selectively formed in the discharge cells in accordance with the image data. 
   Furthermore, while the above embodiment was described in terms of band-like barrier ribs being formed parallel to the third electrodes, the same effects may be achieved, for example, by forming the barrier ribs in a grid. 
   INDUSTRIAL APPLICABILITY 
   The PDP drive method and display apparatus of the present invention are applicable in display apparatuses such as computers and televisions, and are particularly applicable in large-scale display apparatuses requiring both high definition and high brightness.