PLASMA DISPLAY PANELS

Abstract: A plasma display panel comprises anodes (3) and cath­ odes (4) arranged in an X-Y matrix form with discharge gaps between them. Trigger electrodes (6) are arranged adjacent the cathodes (4) for inducing discharges. An insulating layer separates the cathodes (4) and the trigger electrodes (6). A trigger circuit connected to the trigger electrodes (6) gener­ ates at least one trigger voltage waveform (V T ) which has a first level (+V A ) during a first period of time (trigger interval) which is sufficient to induce discharge. The waveform (V T ) then changes abruptly to a second level (-V A ) for a second, relatively short period of time, after which is changes during a third period of time (inactive interval) to a level (V E ) inter­ mediate the first and second levels (+V A , -V A ).

Description: This invention relates to plasma display panels. Plasma display panels (PDPs) of X-Y matrix type are known as a means for displaying characters or images. An X-electrode group which comprises a data electrode may comprise anodes to which are applied high and low voltages corresponding to the display data (i.e. data to be displayed). A Y-electrode group which may be scanning electrodes may comprise cathodes which are scanned in a line sequential manner, a negative voltage pulse being applied. Japanese Patent disclosure number 56-128470 discloses a previously proposed plasma display panel having one or more trigger electrodes in addition to the X-Y electrodes. The trigger electrodes are arranged in the vicinity of the cathodes and are separated from them by an insulating dielectric layer. When a high voltage is applied to the trigger electrodes in synchronism with cathode scanning, a trigger discharge, which acts as an inducing discharge, is generated between the trigger electrodes and the cathodes. The inducing discharge permits easy and rapid discharge between the anodes and cathodes so that a drive voltage can be decreased, variations among the discharge cells can be averaged and flickering can be reduced. The trigger electrodes may be divided into a plurality of phases, in which case the cathodes may be divided into groups associated with each trigger electrode phase. When scanning drivers for the cathodes are commonly used among the phases and phase sequential scanning of the trigger electrodes is associated therewith, the number of cathode scanning elements can be decreased to one/(number of phases). Such a system is referred to as a "trigger matrix system" since third matrix electrodes are added to the system. Such trigger matrix systems are subject to erroneous discharges due to the fact that differences in firing potential and discharge maintaining voltages among the discharge cells, which can be 10 volts or more, can cause misfiring. An erroneous discharge generated in one cell gives rise to a trigger effect which causes sequential generation of erroneous discharges in non-selected cells of other phases. Also, there is a difference of about 10 volts between the discharge self-maintaining voltages at two ends of the panel due to the resistance of the cathode lines. for this reason, there is a margin of variation in the power supply voltage for preventing erroneous discharges, which is as small as several volts, and stable operation of the display panel over a long period of time cannot easily be achieved. Since the potential of the trigger electrodes of the non-selected (inactive) phases falls to a low voltage (such as ground potential), and erroneous discharge may be generated between the surface of the trigger delectric layer of the region of a non-selected phase and the anodes to which data voltages (such as high voltage pulses) are applied. Light emission due to the erroneous discharge is called "rain discharge" because it appears as a plurality of stripes along a number of anode lines in a vertical display direction. This causes considerable degradation in the display definition. Also, in such previously proposed display devices, the trigger system requires a large amount of power because the trigger electrodes are capacitive loads and are driven by high voltage and high frequency pulses. In other words, assuming (for example) that there are four hundred cathodes and a frame frequency of 60kHz, the frequency of the cathode scanning is about 24 kHz, which is about 40 microseconds/line. Since a high voltage pulse of about 300 volts having the same frequency as that of the cathode scanning must be applied to the trigger electrodes, and since the trigger electrodes are capacitive loads as mentioned above, the trigger circuit requires several tens of watts of power. Since the trigger electrodes are mounted adjacent the cathodes with a very thin dielectric insulating layer separating them, the dielectric breakdown can easily occur. In a paper by Y Amano et al entitled "A new dc PDP with low voltage drive and high resolution", published at pages 169-174 of "Proceedings of the S.I.D. 23(1982) No. 3, Los Angeles, USA, there is described a plasma display panel comprising: pairs of discharge electrodes arranged in an X-Y matrix form in an envelope with discharge gaps between them; at least one trigger electrode for inducing discharges, the trigger electrode being arranged adjacent at least one of the discharge electrodes and separated therefrom by an insulating layer; and a trigger circuit connected to the trigger electrode and operative to produce at least one voltage waveform (V T ) which varies as a function of time. According to the present invention, there is provided a plasma display panel comprising: pairs of discharge electrodes arranged in an X-Y matrix form in an envelope with discharge gaps between them; at least one trigger electrode for inducing discharges, the trigger electrode being arranged adjacent at least one of the discharge electrodes and separated therefrom by an insulating layer; and a trigger circuit connected to the trigger electrode and operative to produce at least one voltage waveform which varies as a function of time; characterised in that the trigger circuit produces a voltage waveform which has a first voltage level during a first period of time which is sufficient to induce discharge, which is then switched to a substantially different second voltage for a second period of time which is shorter than said first period of time, the second voltage being insufficient to initiate an inducing discharge, and which is then returned to a third voltage between the levels of the first and second voltages. Preferred embodiments of the present invention described hereinbelow seek to solve the above-mentioned problems by providing plasma display panels in which power consumption of the trigger electrode drive circuitry is decreased and erroneous discharge and dielectric breakdown is prevented so as to obtain a high definition display and stable operation over long periods of time. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of a plasma discharge panel in which the present invention can be embodied; Figure 2 is a partial sectional view of the display panel of Figure 1; Figure 3A to 3E are waveform charts for explaining a method of driving a trigger electrode of a previously proposed plasma display panel; Figures 4A to 4G are waveform charts for explaining the method of driving the trigger electrode of the previously proposed plasma display panel; Figure 6 is a diagram illustrating potential relationships between anodes and cathodes of a plasma display panel embodying the present invention; Figure 7 is a diagram for explaining the principles of operation of a trigger circuit of a plasma display panel embodying the present invention; Figure 8A and Figure 8B illustrate output waveforms of the trigger circuit shown in Figure 7; Figure 9 is a detailed electrical schematic diagram of the trigger circuit illustrated in Figure 7; Figures 10A to 10C are voltage and discharge waveform charts for illustrating results obtained when the rising speed of a trigger voltage is changed; Figures 11A to 11C illustrate voltage and discharge waveform charts obtained when the falling speed of the trigger voltage is changed; Figures 12A to 12C illustrate voltage and discharge waveforms charts obtained when an intermediate level return speed of the trigger voltage is changed; Figures 13A to 13C illustrate trigger voltage and discharge waveforms obtained when a cathode bias voltage is changed; Figures 14A to 14C illustrate trigger voltage and discharge waveforms obtained when an anode bias voltage is changed; Figure 15 is a graph showing the relationship between the cathode and anode bias voltages and the lowest trigger voltage; Figure 16 is a graph showing the relationship between the trigger voltage and a minimum main discharge voltage; Figure 17 is a drive circuit diagram illustrating an embodiment of a trigger matrix system or method; Figures 18A to 18J are operational waveform charts for the circuit of Figure 17; Figure 19 is a drive circuit diagram illustrating another embodiment of the trigger matrix method; and Figures 20A to 20H are operational waveform charts for the circuit of Figure 19. Figures 1 and 2 are, respectively, a schematic plan view end a partial sectional view of a plasma display panel (PDP) in which the present invention may be embodied. The plasma display panel includes a front glass panel 1, a rear glass panel 2, anodes 3 (which may be data electrodes) and cathodes 4 (which may be scanning electrodes). The anodes 3 and cathodes 4 are sandwiched between the glass panels 1 and 2 with a suitable space and discharge gas therebetween and are arranged in an X-Y matrix form to oppose each other with small discharge gaps between them. Trigger electrodes 6, which are divided into a plurality of phases (for example eight phases in the particular example illustrated) are arranged under and along and parallel to the cathodes 4. An insulating or dielectric layer 5 is disposed between the trigger electrodes 6 and the cathodes 4. Alternative anodes 3 are connected to an upper anode driver or drive circuit 7A and the other alternate anodes 3 are connected to a lower anode driver or drive circuit 7B. A high level state voltage (for example "1") or a low level data voltage ("0") corresponding to display data (i.e. data to be displayed) is supplied to the anodes 3 in synchronism with cathode scanning by a shift register with parallel outputs and switching output elements of the drivers 7A and 7B acccording to a serial display data input. A negative voltage is supplied to the cathodes 4 by a cathode scanning circuit 8 from the upper edge to the lower edge in a line sequential fashion. A discharge is generated between a selected cathode 4 and an anode 3 to which a high voltage is applied. The trigger electrodes 6 are driven by a trigger circuit 9. In a previously proposed triggering method illustrated by the waveforms of Figures 3A to 3E, a trigger voltage V T (Figure 3D) in the form of high level pulses is supplied to one of the selected trigger electrodes 6 corresponding to an activated trigger phase in synchronism with the timing of cathode scanning pulses K1, K2, K3 ... illustrated in Figures 3A, 3B and 3C. Trigger discharges, which are inducing discharges, as indicated at D T in Figure 3E, are generated between the trigger electrodes 6 and the opposing cathode 4. This causes the breakdown voltage between the cathode 4 and anode 3 to be decreased due to spatial ions caused by the trigger discharge, so that a main discharge between the cathode and anode is induced. This method of triggering requires a large amount of power and a bulky power supply. When the trigger voltage V T (Figure 3D) falls, a small discharge R T illustrated in Figure 3E is generated betwen the cathode 4 or anode 3 and a surface of the insulating layer or dielectric layer 5. The discharge R T serves to neutralise or discharge negative charges (electrons) which become present in the surface of the insulating layer 5 due to the trigger discharge D T , which increases the potential of the surface of the layer 5 in the positive direction. Since the small discharge R T prepares the circuit for the generation of succeeding trigger discharges, it is referred to herein as a recovery discharge. It can be assumed that, since the recovery discharge serves mainly to prepare the circuit for the generation of succeeding trigger discharges, it need not be generated for each cathode line. In one activated phase of a trigger electrode covering several or several tens of cathode lines, the recovery discharge can be generated once in the trigger electrode after generating trigger discharges corresponding to the number of cathodes. This is sufficient for the recovery function. For this purpose, a triggering method illustrated in the waveform chart of Figures 4A to 4G is proposed. As shown in Figure 4B, which represents the waveform of the trigger voltage V T , when one trigger electrode 6 of one phase is provided for n cathodes 4, a constant trigger voltage V T equal to 2V A (where V A is an anode drive voltage) is applied to the trigger electrode 6 for a time until sequential cathode scanning operations K1, K2...Kn (Figures 4C to 4F) are completed. Trigger discharges illustrated in Figure 4G are generated n times between the cathodes to which a negative cathode voltage V E is applied and the corresponding trigger electrode 6. This interval corresponds to a trigger interval for one trigger phase. When the scanning operations of the cathodes during this phase are completed, the trigger voltage V T is lowered to a low potential V E during the following inactive interval as shown in Figure 4B and the trigger interval of another phase is started. When the trigger voltage V T falls from 2V A to V E , a recovery discharge R T is generated for the overall region of the corresponding trigger phase so as to discharge or neutralise negative charges occurring in the surface of the insulating layer 5 above the trigger electrode 6 so that this trigger phase will recover and be ready for the next trigger discharge. During the triggering or trigger interval, when a positive data voltage pulse illustrated in Figure 4A is applied to the anode 3, discharge emission is caused between the cathode 4 of the triggered cell and the anode. It is to be noted that Figure 4B shows a pre-charge interval prior to the triggering interval, the pre-charge interval being a charging interval of a pre-charge capacitor for producing the trigger voltage V T through a voltage doubler. Using this method, which is called a batched or block trigger or triggering method, since the frequency of the trigger pulses is decreased considerably the power consumption will be reduced greatly. For example, if the capacitance of one trigger electrode 6 is C T , and if the last or last four of the eight trigger electrodes 6 in Figure 1 corresponds to one phase, the capacitance of each phase will be 4C T , and if the trigger voltage is 300 volts and the frame frequency is 60 Hz, the power consumption W for two phases for the trigger electrodes 6 is expressed by the following equation: W = (1/2). 4C T V²T x 60 (Hz) x 2 (phases) If C T = 5nF, then W = 0.1 watts. The power consumption of a batched triggering method can be about 1/100 of the power consumption of conventional triggering systems. However, in the batched or block trigger method, since the voltage difference (Figures 4A and 4B) between the anode 3 to which the data voltage is applied and the trigger electrode 6 at the low level V E becomes large (V AT ) during the inactive interval, an erroneous discharge is undesirably generated. The erroneous discharge is the above-mentioned so-called "rain discharge" which appears as stripes along the anode lines and thus degrades considerably the display definition. During the trigger interval, a voltage difference V TK (Figures 4B and 4C) between the trigger electrode 6 and the selected cathode 7 will be 300 volts or more and when this voltage is applied to the insulating layer 5a dielectric breakdown occurs. It should be noted that V TK = V T (which is the trigger voltage) minus V E (which is the cathode voltage) and V E is at a potential level of logic "L" or, in other words, at ground potential. An embodiment of the present invention can solve this problem by utilising a trigger voltage having a waveform illustrated in Figure 5. After the recovery discharge R T is generated at the trailing edge of a trigger voltage pulse V T , the potential of the trigger voltage is increased to the intermediate potential level V E . When the recovery discharge is generated, positive charges are produced in the surface of the insulating layer 5. In this state, if the anode voltage rises, an erroneous discharge would be generated. However, since the trigger voltage V T is increased to the intermediate potential V E , the erroneous discharge will be prevented. An up magnitude V UP shown in Figure 5 can be slightly larger than a pulse amplitude of V A - V B of the anode voltage, where V A is the anode drive voltage and V B is a bias voltage which is applied to the inactive anode 3 and is lower than V A by about 50 volts. When the trigger voltage falls and the recovery discharge R T has once been generated, and assuming that the voltage between the anode and trigger electrodes in the inactive state is V R as illustrated in Figures 4A and 4B, then unless a voltage greater than V R is applied to the trigger phase in the inactive interval erroneous discharges will never occur. In practice, as illustrated in Figure 5, the voltage V A (which is the same voltage as the anode drive voltage and might be 180 volts) is applied to the trigger electrode 6 during the trigger interval of the active phase and, after completing all of the trigger discharges between the trigger electrodes 6 and the cathodes 4 included in the active phase, the trigger voltage V T is decreased instantaneously to -V A so as to cause generation of the recovery discharge R T . Thereafter, the trigger voltage is returned to the intermediate potential V E , which is ground potential. In this manner, a negative pulse is applied instantaneously so as to generate the recovery discharge at the end of the trigger interval. The method of trigger voltage generation illustrated in Figure 5 is referred to hereinafter as a refresh trigger method. The surface of the insulating layer 5 is positively charged by the recovery discharge and the potential of the trigger electrode 6 of the inactive phase is stepped up during the following inactive interval. For this reason, a positive electric field is generated along the surface of the insulating layer 5. Such electric field, which is formed in the inactive region, prevents diffusion of plasma ions generated by main discharges of displays occurring in the adjacent activated trigger electrode region of the active phase. For this reason, erroneous discharges at the inactive discharge cells, which might be triggered by ions diffusing into the inactive phase, will be prevented. Also, in the block trigger method illustrated in Figures 4A to 4G, a voltage of 2V A (which may be 300 volts or more) is applied during the trigger interval and, therefore, dielectric breakdown possibly can occur. On the other hand, in the refresh trigger method illustrated in Figure 5, since the trigger voltage V T varies with respect to the ON potential V E of the activated cathode by a maximum amount of plus or minus V A , the maximum voltage applied to the insulating layer 5 will not be grater than 200 volts. Thus, the insulating layer 5 will not suffer from dielectric breakdown, which will prolong the life of the display panel. The insulating layer 5 can be thin, because the application voltage is decreased, and the required trigger voltage therefore can be decreased. Figure 6 is a schematic diagram of a power supply for a plasma display panel in accordance with this embodiment of this invention. An anode drive voltage source V A (which might be 150 to 180 volts) is used as a main discharge voltage. With reference to the voltage source V A , a negative bias voltage source V BA of 50 to 90 volts is used and a bias potential V B for an anode off mode is derived from the negative side thereof. Also, the negative side of the anode drive voltage source V A is at a reference potential V E and, in a cathode on mode, this potential is applied to the cathodes 4. The potential V E is at a logic level "L" of the cathode scanning circuit 8 and can be, for example, ground potential. Also, with reference to the potential V E , a positive bias voltage source V BK is used and a cathode potential V K is derived from the positive side thereof. The trigger voltage V T is supplied to the trigger electrodes 6 and is produced by doubling the anode drive voltage V A . Figure 7 illustrates in principle the operation of a trigger circuit and Figures 8A and 8B are waveform charts of the trigger voltage. In this embodiment, trigger circuits for two phases (A and B phases) are provided and the eight trigger electrodes 6 illustrated in Figure 1 are divided into a pair of phases, namely upper and lower phases, with four trigger electrodes in each phase. As shown in the trigger waveforms of Figures 8A and 8B, the trigger circuits are driven in a time divisional manner with the phase alternating over a frame period 1V which is a vertical interval. As is illustrated in Figure 7, the trigger circuit 9 comprises two pairs of serial switches SW1, SW2 and SW3 and SW4 which are connected between a power supply line V A and a reference voltage line V E . The connection points or junctions a and b of the respective series-connected pairs of switches SW1, SW2 and SW3, SW4 are coupled by a pre-charge capacitor C P and the output from the junction b between the switches SW3 and SW4 is supplied as the trigger voltage to the trigger electrodes 6. The circuit of Figure 7 operates in the following manner. The switches SW2 and SW3 are turned on, with the other switches turned off, and the capacitor C P is pre-charged to the voltage V A . At this time, the potential at the connection point a of Figure 7 is V A and the output level to the trigger electrodes 6 of the B phase is V E , as illustrated in Figure 8B. This interval corresponds to the inactive interval. The switches SW2 and SW3 are then turned off, and the switch SW4 is turned on, whereupon the output becomes V A . This interval corresponds to the trigger interval of the B phase, and in the region of the B phase, trigger discharges are generated in a cathode sequence. In this trigger interval, charges accumulated in the capacitor C P will not vary and the potential at the connection point a becomes 2V A . Then, at the end of the trigger interval, the switch SW1 only is turned on. Since the potential at the connection point a is connected to V E the output level of the connection point b has a potential of -V A which is lower than V E by the voltage charged in the capacitor C P . At this time, therefore, the recovery discharge due to the refresh triggering is generated between the surface of the insulating layer 5 and the anode 3 or cathode 4. Then, the switches SW1 and SW3 are again turned on and the trigger potential returns to V E . This operation is repeated for each frame cycle. The trigger waveform of the other phase (phase A) is reversed with respect to that of phase B and is illustrated in Figure 8A. Using refresh timing, even when the switch SW1 is turned on, since the other switches SW2 to SW4 are off the accumulated charge in the pre-charge capacitor CP will not be discharged. Therefore, the amount of power consumed by the pre-charge capacitor C P is very small. The capacitance of the capacitor C P is selected to be sufficiently larger than that of the trigger electrodes 6 to enable voltage doubling from V A to -V A without loss of power. Figure 9 is a detailed circuit diagram of a practical form of circuit embodiment corresponding to the circuit shown in principle in Figure 7. Transistors Q1, Q2, Q3 and Q4 respectively correspond to the switches SW1, SW2, SW3 and SW4. Diodes D1 and D2 act as protective diodes. Since the transistors Q2 and Q3 are turned on at the same time for pre-charging, an ON signal S1 to the transistor Q3 is supplied to the transistor Q2 through a level shift transistor Q5. During the trigger interval the transistor Q4 is turned on in response to a signal S2 supplied through a level shift transistor Q6 and the voltage V A is applied to the trigger electrodes 6. An ON signal S3 is supplied to the transistor Q1 at the end of the trigger interval and refresh triggering is performed at the trailing edge of the trigger voltage. Desired and preferred voltage waveforms and the timing of the trigger voltage will now be described. Figures 10A to 10C illustrate a trigger voltage waveform V T and a trigger discharge D T and show different values for a resistor R8 (an emitter resistor of the transistor Q4) illustrated in Figure 9, which varies the rising speed of the trigger voltage at the initiation of triggering. Figure 10A illustrates a case where the resistor A is equal to 51 ohms and the rising speed of the voltage V T is 170 volts/26 microseconds. Figure 10B illustrates a case where the resistor R8 is 300 ohms and the rising speed of the voltage V T is 170 V/67 microseconds. Figure 10C illustrates a case where the resistor R8 is 500 ohms and the rising speed is 170 V/108 microseconds. In each case the trigger voltage V T is set at 300 V p-p , the anode bias voltage V BA is set at 50 volts and the cathode bias voltage V BK is set at 45 volts. As is illustrated by the waveforms of Figures 10A to 10C, when the rising speed of the trigger voltage is lowered a stronger trigger discharge D T will be obtained. When the rising speed is high or steep, the leading discharge D P will be large and will be generated between the trigger electrode 6 and the cathode 4 or anode 3 and the positive charges accumulated in the insulating layer 5 are discharged. Thus, this makes the generation of the following trigger discharge difficult. The trigger voltage for the selected active phase must rise before starting of the cathode selection or scanning. Thus, the trigger voltage presumably reaches a high level within about 2H, where H is an interval of about 40 microseconds and represents a scanning interval of the cathode line, so as to start rising from about 3H before every phase switching time. The rise time can be adjusted by adjusting the resistance R8 of the emitter electrode of the transistor Q4 shown in Figure 9. Figures 11A to 11C illustrate the voltage waveform V T and the recovery charge R T when the falling speed of the trigger voltage V T varies. The falling speed of V T can be adjusted by adjusting the resistance of a base resistor R3 of the transistor Q1 illustrated in Figure 9. The resistance of the resistor R3 is increased as shown in Figures 11A to 11C, thereby decreasing the rising speed. The resistance of the resistor R3 is 200 ohms in Figure 11A, 510 ohms in Figure 11B and 1.2 kilohms in Figure 11C. This increase in the resistance of R3 decreases the rising speed. In this case, the trigger voltage is 320 V p-p and the anode and cathode bias voltages V BA and V BK are the same as those mentioned in the description of Figures 10A to 10C. As is apparent from Figures 10A to 10C, the higher the falling speed of the refresh trigger, the greater the strength of the recovery discharge R T becomes. Thus, when the recovery discharge R T is strongest, the trigger discharges D T can be performed completely during the next trigger interval after the next inactive interval. This is because negative charges occurring on the insulating layer 5 due to the trigger discharges are neutralised and discharged by the strong recovery discharge due to refreshing so as to increase the surface potential of the insulating layer whereby the next trigger discharge can therefore easily be generated. Figures 12A to 12C illustrate the voltage waveform and the trigger discharge when the return speed for returning the trigger voltage V T to the intermediate potential V E is varied. The return speed of the trigger voltage can be varied by changing the resistance of an emitter resistor R4 of the transistor Q2 shown in Figure 9. The resistance of the resistor R4 is 0 ohms in Figure 12A, 100 ohms in Figure 12B and 500 ohms in Figure 12C. As the resistance of the resistor R4 increases as illustrated in Figures 12A to 12C, the return speed of V T is decreased. In Figures 12A to 12C the trigger voltage is 300 volts V p-p and the anode and cathode bias voltages are the same as those used in Figures 10A to 10C and 11A to 11C. As can be seen from the waveform charts shown in Figures 12A to 12C, the return speed does not influence the trigger discharge V T or recovery discharge R T . However, when the return timing of the trigger voltage V T to the intermediate potential V E is delayed, the above-mentioned undesirable rain discharge can be generated between the trigger electrode 6 and the anodes 3 to which the data voltage is applied. Therefore, the return speed is preferably maintained as high as posible. Figures 13A to 13C illustrate waveform charts for the trigger discharge when the cathode bias voltage V BK illustrated in Figure 6 is varied. Under these conditions, the rising speed of the trigger voltage V T is kept constant. The trigger voltage is 310 V p-p and the anode bias voltage V BA is fixed at 90 volts. The cathode bias voltage V BK illustrated in Figures 13A to 13C is progressively lowered, the voltage being, for example, 70 volts in Figure 13A, 50 volts in Figure 13B and 30 volts in Figure 13C. It is apparent from Figures 13A to 13C that the higher the cathode bias voltage is, the stronger the trigger discharge D T becomes. This is because the amplitude of the drive voltage applied to the cathodes 4 is large and the leading discharge D P at the initiation of the trigger interval of the active phase is small. Also, as can be seen from Figures 13A to 13C, when the cathode bias voltage V BK becomes lower the lead discharge emission becomes large. Since the lead discharge discharges the positive charges accumulated on the surface of the insulating layer 5 in the region of the activated trigger phase, the voltage V BK is preferably set to be high and the leading discharge is preferably set to be small. Figures 14A to 14C illustrate waveform charts of the trigger discharge when the anode bias voltage V BA varies as, for example, between 100 volts in Figure 14A, 90 volts in Figure 14B and 80 volts in Figure 14C. In this case, the falling speed of the trigger voltage V T is kept constant. The trigger voltage is set at 310 V p-p ' and the cathode bias voltage V BK is fixed at 45 volts. The anode off potential V B with respect to the reference potential V E is 100 volts, 90 volts and 80 volts in Figures 14A, 14B and 14C respectively, and, as can be seen, the recovery discharge R T becomes stronger when the anode off voltage is V B is higher. This is the reason that the recovery discharge is mainly generated between the surface of the insulating layer 5 and the anodes 3. Therefore, when the anode off potential V B increases so as to enlarge the difference between the potential V B and the peak voltage -V A , the refresh pulse becomes large and a stronger recovery discharge can be generated. As is illustrated in Figure 14A, the next trigger discharge can be ensured. It should be noted that an increase in the anode off potential V B corresponds to a decrease in the anode bias voltage V BA illustrated in Figure 6. Therefore, when the potential V B is higher, an anode driver element having a lower breakdown voltage can be employed. Figure 15 is a graph showing the relationship between the necessary minimum value (p-p value) of the trigger voltage V T and the anode and cathode bias voltages. As is illustrated in Figure 15, the higher the cathode bias voltage V BK and the anode off voltage V B are, the lower will be the minimum trigger voltage. The cause of a decrease in the trigger voltage is that, when the cathode bias voltage is increased, the leading discharge is suppressed so that positive charges cannot be discharged from the insulating layer surface and a potential difference between the cathodes enabled by the sequence scanning and the surface of the insulating layer becomes large. Also, when the anode bias voltage is increased, a stronger recovery discharge is generated and the potential of the insulating layer becomes higher, thereby allowing easy generation of the next trigger discharge. As described above, the rising time of the beginning of the trigger interval is set to be lower and the falling time at the end thereof is set to be higher. Furthermore, the cathode and anode biases are set to be higher, thus providing satisfactory operation. As shown in Figure 8A in the trigger drive waveform of this embodiment, the rise and fall times are set to be about 3H and about 1H, respectively, where H is about 40 microseconds. Figure 16 is a graph in which the batched or block trigger method is represented by the trigger waveform V T illustrated in Figures 4A to 4G and is compared with the refresh trigger method represented by the trigger waveform V T illustrated in Figure 5. The trigger voltage V T is plotted against the abscissa and the minimum main discharge voltage V A (which is the anode drive voltage without erroneous discharge) is plotted along the ordinate. As described previously, the trigger method proposed by the present application has a main advantage of decreasing the main discharge voltage by the trigger discharge. However, when the trigger voltage V T is gradually lowered as shown in Figure 16, the trigger discharge will not be generated below a given level and the main discharge voltage required abruptly increases. In terms of a voltage region of V T in which the trigger effect is available, for the block batch trigger method it can be obtained when the trigger voltage V T is 320 volts or higher, as indicated by the dotted line. However, in the refresh trigger method the trigger effect can be obtained when a slightly lower trigger voltage of 300 volts or higher is applied, as indicated by the solid line. The minimum main discharge voltage V A under the condition that the trigger discharge is effective is 140 volts or higher in the block trigger method. In the refresh trigger method, when the voltage V A is at a value of 138 volts or higher, a stable discharge display can be provided. An example in which the refresh trigger method is applied to a trigger matrix drive, which is a method for commonly using a cathode drive means among a plurality of trigger phases, will next be described. As described above, in the refresh trigger method the trigger electrode 6 in the active phase generates the leading discharge D P and the recovery discharge R T at the beginning and at the end of the trigger interval, respectively, and these discharges are generated in the entire trigger region in the selected phase. Additionally, the discharge timings of the leading and recovery discharges are involved in the active interval of trigger electrodes in the adjacent phase. Therefore, in the trigger matrix, in which the cathode driver is commonly used among a plurality of trigger phases so as to reduce the number of drive elements, commonly driven cathode lines in different phases are activated at boundary portions of the inactive trigger regions adjoining an active phase which results in the generation of erroneous discharges. Figure 17 illustrates a trigger matrix drive circuit which can solve this problem. The circuit comprises four phase triggers T1 to T4 each having four cathodes. The trigger electrodes 6 of each phase are driven in a phase sequential manner as indicated by T1 to T4 in Figures 18A to 18D. The cathodes 4 located at boundary portions of the trigger electrodes 6 are not commonly driven together with those in the adjacent phases, but are independently and selectively driven by an addressable driver 20 at a predetermined timing as indicated by waveforms KA1, KA4, KA5 and KA8 illustrated in Figures 18G to 18J, respectively. In the middle portion of each trigger phase, since these cathodes are not selected in an overlapping manner by sequential triggering, the cathode lines are commonly connected among four phases and, as indicated by waveforms KM2 and KM3 of Figures 18E and 18F, they are commonly driven by a matrix driver 21 over four phases. Figure 19 shows another drive circuit. This circuit also comprises a four phase trigger in which each phase has four cathodes. As described previously, the overlapping selection of the cathodes associated with the leading and recovery discharges occurs between each two adjacent trigger phases. For this reason, in this embodiment, the cathodes in every other trigger phase are commonly driven. As shown in Figure 19, a first line of a first phase and that of a third phase are commonly driven and a first line of a second phase and that of a fourth phase are commonly driven. The four phase trigger electrodes are selected and are triggered in a phase sequential manner as indicated by waveforms T1 to T4 of Figures 20A to 20D. The corresponding cathodes in every other phase are sequentially and commonly driven by a matrix driver 22 by waveforms KM1 to KM8 illustrated in Figures 20E to 20H. It is to be noted that, in the above embodiments, the anodes 3 and the cathodes 4 correspond to data and scanning electrodes, respectively. However, instead, the display data can be supplied to the cathodes and the anodes can be scanned in a line sequential manner. In this case, the trigger electrodes are arranged along the anodes 3 and a group of a plurality of anodes is covered by one phase of the trigger electrodes. Furthermore, in the above embodiments, the trigger electrodes are divided into two or four phases so as to perform time divisional drive. However, it is possible for a single phase drive to be performed without time division. In this case, in order to hold intervals for the leading and recovery discharges at the start and end of each trigger interval, a blanking period may be provided for each frame. In summary of the foregoing disclosure, there is provided a plasma display panel having cathodes 4, anodes 3 and trigger electrodes 6 wherein the trigger electrodes correspond to a plurality of discharge electrodes (e.g. the cathodes 4) which are scanned sequentially. An insulating layer 5 separates the trigger electrodes 6 and the discharge electrodes and a constant trigger voltage is supplied until the plurality of discharge electrodes corresponding to the trigger electrode have been activated. The trigger voltage is quickly changed and is then returned to an intermediate level (V E ) and the frequency of the trigger voltage pulses is reduced to one over the number of block-discharged electrodes so that the power consumption of the trigger electrodes is greatly reduced. Since the trigger voltage is returned to an intermediate level (V E ), erroneous discharges will not be generated between inactive trigger electrodes (6) and the other discharge electrodes to which data voltage pulses are supplied, thus producing a high definition display. The trigger voltage swings in positive and negative directions with reference to the intermediate level (V E ), and an ON potential low level of the discharge electrodes (e.g. the cathodes 4), which sequentially fall to a low level by scanning activation, may be set to be the same as the intermediate level. Thus, the absolute value of the trigger voltage applied to the insulating or dielectric layer 5 between the cathodes 4 and trigger electrodes 6 can be substantially one-half a required value and plus or minus one-half with reference to the intermediate level and the breakdown voltage of the insulating layer 5 can be decreased. Therefore, stable operation will be provided over long periods of time without dielectric breakdown and, in addition, the insulating layer 5 can be thin and the trigger voltage can be decreased. Although the invention has been described above by way of example with respect to methods or techniques of applying voltages to trigger electrodes of a display panel, it is more generally applicable to applying voltages to various electrodes.

Claims: A plasma display panel comprising: pairs of discharge electrodes (3,4) arranged in an X-Y matrix form in an envelope (1,2) with discharge gaps between them; at least one trigger electrode (6) for inducing discharges, the trigger electrode (6) being arranged adjacent at least one of the discharge electrodes (4) and separated therefrom by an insulating layer (5); and a trigger circuit (9) connected to the trigger electrode (6) and operative to produce at least one voltage waveform (V T ) which varies as a function of time; characterised in that the trigger circuit (9) produces a voltage waveform (V T ) which has a first voltage level (V A ) during a first period of time which is sufficient to induce discharge, which is then switched to a substantially different second voltage (-V A ) for a second period of time which is shorter than said first period of time, the second voltage being insufficient to initiate an inducing discharge, and which is then returned to a third voltage (V E ) between the levels of the first and second voltages (V A , -V A ). A plasma display panel according to Claim 1, comprising means (8) for activating the discharge electrodes in a line sequential manner. A plasma display panel according to Claim 1 or Claim 2, wherein the trigger circuit comprises first and second voltage reference sources (V A , V E ), first and second switches (SW1, SW2) connected in series between the first and second voltage reference sources, a capacitor (C p ), a third switch (SW3), the second switch (SW2), the capacitor (C p ) and the third switch (SW3) being connected in series between the first and second voltage reference sources (V A , V E ), a fourth switch (SW4) connected in parallel with the combination of the second switch (SW2) and the capacitor (C p ), and switch actuating means connected to the first, second, third and fourth switches (SW1 to SW4) to open and close them so as to produce the at least one trigger voltage (V T ). A plasma display panel according to Claim 3, wherein the first, second, third and fourth switches (SW1 to SW4) comprise first, second, third, and fourth transistors (Q1 to Q4). A plasma display panel according to Claim 4, including a fifth transistor (Q5) connected to the second transistor (Q2) to control it, and a sixth transistor (Q6) connected to the fourth transistor (Q4) to control it. A plasma display panel according to Claim 5, wherein a resistor (R1) is connected between a base of the second transistor (Q2) and the first voltage reference source (V A ) and the base of the second transistor (Q2) is connected to a collector of the fifth transistor (Q5). A plasma display panel according to Claim 5 or Claim 6, wherein a resistor (R2) is connected between an emitter of the fifth transistor (Q5) and the second voltage reference source (V E ). A plasma display panel according to any one of Claims 4 to 7, including a resistor (R3) connected to a base of the first transistor (Q1). A plasma display panel according to any one of Claims 4 to 8, including a resistor (R4) connected between the first voltage reference source (V A ) and an emitter of the second transistor (Q2). A plasma display panel according to any one of Claims 4 to 9, including a resistor (R5) connected between an emitter of the third transistor (Q3) and the second voltage reference source (V E ). A plasma display panel according to any one of Claims 4 to 10, including a resistor (R6) connected between a base of the fourth transistor (Q4) and the first voltage reference source (V A ). A plasma display panel according to any one of Claims 4 to 11, including a resistor (R8) connected between an emitter of the fourth transistor (Q4) and the first voltage reference source (V A ).

Inventors: SHIONOYA, TOSHIO C/O SONY CORPORATION, TSUBOI, TAKASHI C/O SONY CORPORATION
Applicants: SONY CORPORATION
Application year: 1985
Main classification: G09G3
IPC codes: H01J17, G09G3