Abstract:
A drive circuit of plasma display panel (PDP) unit including at least a pair of electrodes and discharge cells connected thereto, further comprises a first path having a first switch and a first coil for applying voltage to the discharge cells; a second path having a second switch and a second coil for expelling the voltage applied to the discharge cells; and electric charge accumulating devices connected to the first path and the second path, wherein the inductance value of the second coil is larger than the inductance value of the first coil.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to technology relating to matrix type plasma display panel (PDP) and more particularly to technology relating to plasma display panel whose drive power supply is automatically adjusted. 
     2. Description of the Related Art 
     Since before, it has been well known that reactive power can be decreased in sustain period because if the resonance frequency of the sustain circuit is reduced in a drive circuit of the PDP unit, a current peak value at the time of resonance is decreased, so that effective current is also decreased. 
     The sustain circuit mentioned here refers to a circuit for supplying electric power to a discharge cell in order to sustain discharge in a stable condition and the sustain period refers to a period in which light emission of the discharge cell is repeated by the sustain circuit so as to maintain the light emission condition. 
     Conventional technology about the driving circuit of the PDP unit has been disclosed in Japanese Patent Application Laid-Open No. 2000-293135. 
     However, if the resonance frequency of the sustain circuit is decreased, there is a trouble that it leads to changing discharge condition thereby affecting discharge margin and discharge intensity. 
     The above-mentioned Japanese Patent Application Laid-Open No. 2000-293135 does not describe any special matter about reduction of reactive power by operating the resonance frequency of the sustain circuit. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a drive circuit of the PDP unit capable of operating the resonance frequency of the sustain circuit so as to reduce reactive power in a circuit in the sustain period without affecting discharge margin and discharge intensity. 
     A drive circuit of PDP unit including at least a pair of electrodes and discharge cells connected thereto, is further provided: a first path having a first switch and a first coil for applying voltage to the discharge cells; a second path having a second switch and a second coil for expelling the voltage applied to the discharge cells; and electric charge accumulating devices connected to the first path and the second path, wherein the inductance value of the second coil is larger than the inductance value of the first coil. 
     Consequently, according to the present invention, the resonance frequency at the time of fall, which does not affect discharge condition, can be reduced while maintaining a conventional resonance frequency at the time of leading, which affects the discharge condition. Thus, the peak value of current upon resonance can be reduced and effective current can be reduced, so that reactive power in a circuit in the sustain period can be reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing an outline of the PDP unit according to an embodiment of the present invention; 
     FIG. 2 is a schematic diagram showing drive sequence of the PDP according to an embodiment of the present invention; 
     FIG. 3 is a conceptual diagram showing the structure of the PDP drive circuit according to an embodiment of the present invention; 
     FIG. 4 is a conceptual diagram showing drive sequence of the PDP according to an embodiment of the present invention; and 
     FIG. 5 is a schematic diagram showing the relation between sustain voltage and sustain current in leading period and falling period. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of a plasma display panel (PDP) unit of the present invention will be described with reference to the accompanied drawings. 
     FIG. 1 is a block diagram showing an outline of the PDP unit. 
     The PDP unit comprises an input terminal  21 , an A/D converter  22 , a display data generating portion  23 , a frame memory  24 , an address driver  2 , a control portion  5 , an X-electrode driver  3  and a Y-electrode driver  4 . 
     A video signal inputted from the input terminal  21  is converted to digital video data by the A/D converter  22 , processed to display data by the display data generating portion  23 , and then supplied to the frame memory  24 . The display data generating portion  23  computes emission time corresponding to the intensity of illumination of video data and corrects the data by reallocation so as to generate display data. The frame memory  24  is composed of, for example, a VRAM, which accumulates display data of a screen sent from the display data generating portion  23  and supplies it to the address driver  2  following synchronous signal from a control portion  5  which will be described later. The address driver  2  is composed of a driving circuit having a DC power supply and switching device, and generates pixel data pulse to each discharge cell on the display panel based on the display data inputted from a frame memory  24  and applies this to a column electrode Dj for every display line. 
     The control portion  5  is constituted of, for example, CPU, which outputs a synchronous signal to the A/D converter  22 , the display data generating portion  23  and the frame memory  24 . The X-electrode driver  3  and the Y-electrode driver  4  are each constituted of a driving circuit having a DC power supply and switching devices as shown in FIG.  3 . The X-electrode driver  3  applies sustain discharge pulse IPx to an electrode Xj and sustain discharge pulse IPy to an electrode Yj based on the synchronous signal from the control portion  5 . 
     The operation of the PDP unit having such a structure will be described below. 
     A video signal inputted from the input terminal  21  as an analog signal is converted to digital video data by the A/D converter  22 , processed to display data by the display data generating portion  23 , and supplied to the frame memory  24 . The frame memory  24  accumulates display data sent from the display data generating portion  23  and supplies it to the address driver  2  following synchronous signal from the control portion  5 . 
     A synchronous signal is separated from the video signal inputted from the input terminal  21  by a sync separation circuit (not shown), and then the control portion  5  outputs the synchronous signal to the A/D converter  22 , the display data generating portion  23  and the frame memory  24  on the basis of this separated synchronous signal. The control portion  5  drives the PDP by controlling ON/OFF of a switching device of the PDP driving circuit  1 , which is a panel driving device shown in FIG.  3 . Here, the PDP drive circuit  1  comprises the address driver  2 , the X-electrode driver  3 , and the Y-electrode driver  4 . 
     The drive sequence of the PDP unit will be described with reference to FIG.  2 . 
     A set of a sub-field ( 1  SF) has a reset period, an address period, and a sustain period. A set of a field, which is a drive sequence of the PDP, has several sub-fields, repeated N times, and thereafter a main erase process for resetting to a condition in which wall charge is erased by applying erase pulse respectively to all cells. 
     In the reset period, all the discharge cells of the PDP unit are gotten into luminous discharge cell condition. In the subsequent address period, the address driver  2  forms wall charge selectively to each discharge cell based on video signal so as to generate pixel data pulse which sets up luminous discharge cell or non-luminous discharge cell and apply this pulse to a column electrode of every display line. In the sustain period, sustain discharge pulse IPx and sustain discharge pulse IPy are generated alternately and applied to the column electrode X and column electrode Y alternately. As a result, in a luminous discharge cell in which the above-described wall charge remains, discharge light emission is repeated and then that light emission is sustained. 
     The drive circuit of the PDP unit of this embodiment reduces reactive power in the circuits of the X-electrode driver  3  and the Y-electrode driver  4  in the sustain period. 
     FIG. 3 is a conceptual diagram showing the structure of the PDP drive circuit  1  of this embodiment. 
     The PDP drive circuit  1 , which works as a panel drive device, is comprised of the address driver  2 , the X electrode driver  3 , and the Y electrode driver  4 . The X electrode driver  3  includes a reset pulse driver portion and a first sustain driver portion. The Y electrode driver  4  includes a reset pulse driver portion, a scan driver portion, and a second sustain driver portion. 
     The reset pulse driver portion applies a reset pulse respectively to all the column electrodes X 1 -Xn, Y 1 -Yn at the same time in the reset period. Consequently, all the discharge cells in the PDP unit are simultaneously discharged and excited to generate charged particles. After this discharge is stopped, a predetermined quantity of wall charges are accumulated on a dielectric layer of the discharge cells, so that luminous discharge cell condition is attained. 
     The scan driver portion applies a scan pulse SP to the electrode Yj in the address period so as to set the electrode Yj to a predetermined positive potential (Vh-Voff). This application is carried out synchronously with application of pixel data pulse DPj from the address driver  2 . As a result, of the cells of the column electrode on which the scan pulse SP is applied, discharge occurs in only a cell onto which pixel data pulse of positive voltage is applied at the same time. 
     The first sustain driver portion and the second sustain driver portion generate sustain discharge pulse IPx and sustain discharge pulse IPy alternately in the sustain period and apply it to the column electrodes X 1 -Xn and the column electrodes Y 1 -Yn alternately. Consequently, discharge light emission is repeated in the light emission discharge cell in which the wall charge remains, and the light emission is sustained. 
     A first sustain driver portion of the X-electrode driver  3  contains a power supply B 1 , switching devices S 1 -S 4 , diodes D 1 , D 2 , coils L 1 , L 2 , and a capacitor C 1 , which works as a charge accumulating device. According to this embodiment, the inductance value of the coil L 2  is larger than the inductance value of the coil L 1 . 
     A second sustain driver portion of the Y-electrode driver  4  contains a power supply B 3 , switching devices S 11 -S 15 , diodes D 3 , D 4 , coils L 3 , L 4 , and a capacitor C 2 , which works as a charge accumulating device. According to this embodiment, the inductance value of the coil L 4  is larger than the inductance value of the coil L 3 . 
     The operation of the PDP drive circuit  1  having the above described structure will be described below. 
     The column electrode Xj is an electrode at the column j (one electrode composing the j display line) in the column electrodes X 1 -Xn and the column electrode Yj is an electrode at the column j (the other electrode composing the j display line) in the column electrodes Y 1 -Yn. The display panel cell is located between the column electrode Xj and Yj which form a pair and acts as a capacitor Co. The power supply B 1  outputs a sustain voltage Vs 1 . The power supply B 2  outputs a reset voltage Vr 1 . 
     The power supply B 3  outputs a sustain voltage Vs 1 . The power supply B 4  outputs a reset voltage Vr 1 . The power supply B 5  generates a voltage Voff while the power supply B 6  generates a scan pulse voltage Vh. 
     The operation of the PDP drive circuit  1  having such a structure will be described with reference to a time chart shown in FIG.  4 . The drive sequence of this PDP describes the operation in a single sub-field. Subsequently, the reset period, address period, and sustain period will be described separately. 
     First, in the reset period, a switching device S 8  of the X electrode driver  3  is turned ON, and at the same time, switching devices S 16  and S 22  of the Y electrode driver  4  are turned ON. The other switching devices are kept OFF. When the switching device S 8  is turned ON, current flows from the electrode Xj to a negative terminal of the power supply B 2  through a resistor R 1  and a switching device S 8 . When the switching device S 16  is turned ON, current flows into the electrode Yj from a positive terminal of a power supply B 4  through a switching device S 16 , a resistor R 1 , and a switching device S 22 . The potential of the electrode Xj is decreased gradually depending upon time constant of a capacitor Co and the resistor R 1  so that a reset pulse RPx is generated. The potential of the electrode Yj is increased gradually depending on time constant of the capacitor Co and the resistor R 1  so that a reset pulse RPy is generated. Then, the potential of the reset pulse RPx is saturated to the voltage level −Vr 1 , and the potential of the reset pulse RPy is saturated to the voltage level Vr 1 . This reset pulse RPx is applied to all the column electrodes X 1 -Xn at the same time and the reset pulse RPy is applied to all the column electrodes Y 1 -Yn simultaneously. 
     When these reset pulses RPx, RPy are applied at the same time, all discharge cells of the PDP are discharged and excited at the same time so as to generate charged particles. After this discharge is stopped, a predetermined quantity of wall charge is accumulated on dielectric layers of all discharge cells, so that luminous discharge cell condition is attained. When the reset pulses RPx and RPy are saturated after a predetermined time interval elapses, the switching device S 8  and the switching device S 16  are turned OFF before the reset period is terminated. At the same time, the switching devices S 4 , S 14  and S 15  are turned ON and the electrodes Xj, Yj are grounded. The reset period is terminated. 
     Next, in the address period, the address driver  2  forms wall charge selectively to each discharge cell based on display data outputted by the display data generating portion  23 , pixel data pulses DP 1 -DPm generate and the pulses set the cells the luminous discharge cells or non-luminous discharge cells. This process is applied to the column electrodes D 1 -Dm for every display line. Pixel data pulses DPj, DPj+1 are applied to the electrodes Yj, Yj+ 1 . When the address period is started, the switching devices S 14  and S 15  are turned OFF, and the switching devices S 17  and S 21  are turned ON, and simultaneously the switching device S 22  is turned OFF. If the switching devices S 17  and S 21  are turned ON, positive potential (Vh-Voff) is applied to the electrode Yj. 
     The switching device S 21  is turned OFF synchronously with application of the pixel data pulse DPj from the address driver  2  and then switching device S 22  is turned ON. Consequently, a negative potential indicating the voltage −Voff at a negative terminal of the power supply B 5  is applied to as a scan pulse SP the electrode Yj through the switching device S 22 . Then, the switching device S 21  is turned ON synchronously with termination of the pixel data pulse DPj from the address driver  2 , and the switching device S 22  is turned OFF, so that a predetermined positive potential (Vh-Voff) is applied to the electrode Yj. After that, the scan pulse SP is applied to the electrode Yj+1 also synchronously with application of the pixel data pulse DPj+1 from the address driver  2  like the case of the electrode Yj. 
     In the discharge cell belonging to the column electrode onto which the scan pulse SP is applied, discharge occurs only in a discharge cell onto which pixel data pulse of positive voltage is applied at the same time, so that wall charge of the cell erases. On the other hand, no discharge occurs in a discharge cell on which pixel data pulse of positive voltage is not applied at the same time although the scan pulse is applied and therefore, wall charge of the cell remains. In this case, the discharge cell in which the wall charge remains turns to a luminous discharge cell, while a discharge cell in which the wall charge is erased turns to a non-luminous discharge cell. When the address period is switched over to the sustain period, the switching devices S 17  and S 21  are turned OFF and at the same time, the switching devices S 14 , S 15  and S 22  are turned ON. The switching device S 4  is kept ON. 
     Finally, the sustain period begins, the switching device S 4  is turned OFF while the switching device S 1  is turned ON. Consequently, current, whose origin is an electric charge accumulated in the capacitor C 1 , flows to the electrode Xj through a coil L 1 , a diode D 1 , and the switching device S 1  to charge the capacitor Co. At this time, the potential of the electrode Xj is raised gradually depending on time constant of the coil L 1  and capacitor Co. When half cycle of resonance cycle by the coil L 1  and capacitor Co elapses, the switching device S 1  is turned OFF while the switching device S 3  is turned ON. Consequently, the potential of the electrode Xj becomes an equal to the sustain voltage Vs 1  of the power supply B 1 . 
     Here, the half cycle (leading period) Tax of resonance cycle by the coil L 1  and capacitor Co is expressed in a following expression. 
     [Expression 1] 
     
       
           Tax =π×( L 1 ×Co )½ 
       
     
     If after a predetermined time elapses, the switching device S 3  is turned OFF and the switching device S 2  is turned ON, current flows to the capacitor C 1  through the coil L 2 , the diode D 2 , and the switching device S 2  based on electric charge accumulated in the capacitor Co, so that the capacitor C 1  is charged. At this moment, the potential of the electrode Xj drops gradually depending upon time constant of each of the coil L 2  and the capacitor Co. When the half cycle (falling period) of resonance cycle by the coil L 2  and capacitor Co elapses (when the potential of the electrode Xj reaches 0 V), the switching device S 2  is turned OFF while the switching device S 4  is turned ON. 
     Here, the half cycle (falling period) Tbx of the resonance cycle by the coil L 2  and the capacitor Co is expressed in a following expression. 
     [Expression 2] 
     
       
           Tbx =π×( L 2 ×Co )½ 
       
     
     Because the inductance value of the coil L 2  is larger than the inductance value of the coil L 1  as described above, it comes that Tax&lt;Tbx. 
     By such operation, the X electrode driver  3  applies sustain discharge pulse IPx to the electrode Xj. At the same time when the switching device S 4  for erasing the sustain discharge pulse IPx is turned ON, the Y electrode driver  4  turns ON the switching device S 11  and turns OFF the switching device S 14 . When the switching device S 14  is turned ON, the potential of the electrode Yj is at grounding potential of OV. When the switching device S 11  is turned ON while the switching device S 14  is turned OFF, current flows to the electrode Yj through a coil L 3 , a diode D 3 , the switching device S 11 , a switching device S 15  and a diode D 6  based on electric charge accumulated on the capacitor C 2  so that the capacitor Co is charged. At this time, the potential at the electrode Yj rises gradually depending upon time constant of the coil L 3  and the capacitor Co. 
     When the half cycle (leading period) Tay of the resonance cycle by the coil L 3  and the capacitor Co elapses, the switching device S 11  is turned OFF while the switching device S 13  is turned ON. Consequently, the potential of the electrode Yj becomes equal to the sustain voltage Vs 1  of the power supply B 3 . 
     Here, the half cycle (leading period) Tay of resonance cycle by the coil L 3  and capacitor Co is expressed in a following expression. 
     [Expression 3] 
     
       
           Tay =π×( L 3 ×Co )½ 
       
     
     If after a predetermined time elapses, the switching device S 13  is turned OFF and the switching device S 12  is turned ON, current flows to the capacitor C 2  through the switching device S 22 , the switching device S 15 , the coil L 4 , the diode D 4 , and the switching device S 12  based on electric charge accumulated in the capacitor Co, so that the capacitor C 2  is charged. 
     At this time, the potential of the electrode Yj drops gradually depending upon time constant of the coil L 4  and capacitor Co. When the half cycle of resonance cycle by the coil L 4  and the capacitor Co elapses (when the potential of the electrode Yj reaches 0V), the switching device S 12  is turned OFF while the switching device S 14  is turned ON. 
     Here, the half cycle (falling period) Tby of resonance cycle by the coil L 4  and capacitor Co is expressed in a following expression. 
     [Expression 4] 
     
       
           Tby =π×( L 4 ×Co )½ 
       
     
     Because the inductance value of the coil L 4  is larger than the inductance value of the coil L 3  as described above, it comes that Tay&lt;Tby. 
     By such operation, the Y electrode driver  4  applies sustain discharge pulse IPy of positive voltage to the electrode Yj. In the sustain period, the sustain discharge pulse IPx and the sustain discharge pulse IPy are generated alternately and applied to the column electrode X 1 -Xn and the column electrodes Y 1 -Yn alternately. As a result, the luminous discharge cell in which the wall charge remains repeats discharge light emission so as to sustain its light emission. 
     Next, the relation between the sustain voltage and sustain current in the leading period and the falling period will be described with reference to FIG.  5 . 
     FIG. 5 is a schematic diagram showing the relation between the sustain voltage and sustain current in the leading period and the falling period. 
     Its left side shows the conventional relation between the sustain voltage and sustain current, while its right side shows the relation of the present invention between the sustain voltage and the sustain current. Further, its upper side indicates changes in the sustain voltage while its lower side indicates changes in the corresponding sustain current. 
     If the leading period Ta is equal to the falling period Tb like the conventional drive circuit, a period from the start of the leading to the end of the falling becomes shorter. However, the absolute values at the peak of the sustain current in the leading period and the falling period are equal high values. 
     Conversely, if the falling period Tb is longer than the leading period Ta like the present invention, the period from the start of the leading to the end of the falling becomes longer. However, the absolute value at the peak of the sustain current in the falling period is lower than that in the leading period. Consequently, effective current upon resonance becomes smaller, so that reactive power in a circuit in the sustain period can be reduced. 
     According to the present invention, the resonance frequency in the falling period Tb, which does not affect discharge condition, can be reduced while maintaining the resonance frequency of the leading period Ta which affects discharge condition under the same condition as conventionally. Thus, the peak value of current in the falling period Tb can be reduced so as to reduce the effective current. Therefore, reactive power in the circuit in the sustain period can be reduced. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 
     The entire disclosure of Japanese Patent Application No. 2001-199364 filed on Jun. 29, 2001 including the specification, claims, drawings and summary is incorporated herein by reference in its entirety.