Abstract:
To generate a rising or falling edge simultaneously on the electrodes Ys and Ysa of a plasma display cell, the invention provides for the use of the power recovery circuit of the control device in order to apply, to one of the electrodes Yas and Y, the rising edge applied to the other of the electrodes by a dedicated circuit.

Description:
This application claims the benefit, under 35 U.S.C. 119 of French Patent Application 03/09729, filed Aug. 7, 2003. 
   FIELD OF THE INVENTION 
   The present invention relates to the generation of a rising or falling edge on the sustain and address-sustain electrodes of the cells of a plasma display. 
   BACKGROUND OF THE INVENTION 
   Simultaneously applying a same voltage rising or falling edge on the sustain electrode, hereafter denoted Ys, and the address-sustain electrode, hereafter denoted Yas, of a plasma display cell is a known technique. This case is illustrated in  FIG. 1  which shows an example of voltage signals applied on the electrodes Ys and Yas of a display cell during a phase where the electrical charges in the display cells are equalized. This equalization phase, known as the reset phase, conventionally comprises an operation for forming electrical charges, known as priming, followed by an operation for adjusting the charges, also known as the “erase” of these charges, after which, ideally, the internal voltages within the cells are substantially the same. The electrical charges are reset in the discharge regions between coplanar electrodes, called coplanar discharge regions, and in the discharge regions between non-coplanar electrodes, called non-coplanar discharge regions. 
   SUMMARY OF THE INVENTION 
   The invention proposes a reduction in the power losses within the device for controlling the PDP during the application of a rising or falling edge to the electrodes Ys and Yas of the PDP cells by using power recovery means already present in the control device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood upon reading the following description presented as a non-limiting example and with reference to the appended figures, among which: 
       FIG. 1 , already described above, is an example of voltage signals applied to the electrodes Ys and Yas of a cell in which a voltage rising edge is simultaneously applied to the 2 electrodes Ys and Yas of the cell; 
       FIG. 2  is a circuit diagram of the control device of the invention; and 
       FIG. 3  illustrates the operation of the device in  FIG. 2  for simultaneously bringing the electrodes Ys and Yas to the potential Vs. 
       FIG. 4  illustrates a part of the circuit diagram of the control device, according to a specific embodiment of the invention. 
   

   The invention relates to a control device for a plasma display panel designed to generate a voltage rising or falling edge simultaneously on a sustain electrode Ys and on an address-sustain electrode Yas of a cell of the said plasma display panel, the voltage generated going, during the said rising or falling edge, from an initial value to a final value, power recovery means being connected between the said sustain electrode and an address-sustain electrode in order to recover power during the sustain phase of the discharges in the display cells, characterized in that it comprises first means for taking the voltage of one of the said sustain electrode and address-sustain electrode from the initial value to the final value, the said first means cooperating with the power recovery means in order to simultaneously bring the other of the said sustain electrode and address-sustain electrode to the final voltage. 
   The use of the power recovery means of the control device allows the use of a second dedicated circuit for applying the final voltage to the other of the said sustain electrode and address-sustain electrode to be eliminated and, at the same time, an additional consumption of power in the device to be avoided. 
   Advantageously, the said first means comprise, in the case of a rising edge, a switch and a diode connected in series between a voltage source for supplying the said final voltage value and earth, with the diode anode on the earth side, and an inductor connected, by a first end, to the point situated between the switch and the diode and, by a second end, to one of the said sustain electrode and address-sustain electrode. These means have the advantage of consuming very little power. 
   As shown in  FIG. 1 , a reset operation is generally carried out first of all in the coplanar discharge regions (phase  1 ) and then in the non-coplanar discharge regions (phase  2 ). During phase  1 , the priming and erase operations are performed by applying a voltage ramp to the electrodes Yas, the potential on the electrodes Ys and on the column electrodes of the PDP (plasma display panel) being held constant. More precisely, the formation of electrical charges in the discharge regions is obtained by applying a rising voltage ramp to the electrodes Yas and the adjustment of the latter is then obtained by applying a falling voltage ramp also to these same electrodes. In the same way, the operation for resetting the electrical charges in the non-coplanar discharge regions (phase  2 ) consists in applying a rising voltage ramp and then a falling voltage ramp to the electrodes Ys and Yas of the cells. 
   As can be seen in this figure, a voltage rising edge between zero volts and a voltage Vs is applied simultaneously to the two electrodes Ys and Yas of the cells at the time t 1 . 
   Currently, this rising edge is generated and applied separately to the two electrodes Ys and Yas which requires the use of 2 individual circuits to generate this edge. Each of these circuits introduces power losses. 
   DESCRIPTION OF PREFERRED EMBODIMENTS 
   With reference to  FIG. 2 , the control device of the invention comprises a circuit  1  for locking the voltages applied to the electrodes Ys and Yas, a power recovery circuit  2  and a means  3  of applying a voltage Vs to the electrode Ys. According to the invention, the means  3  cooperates with the power recovery circuit  2  in order to simultaneously apply the voltage from the means  3  to the two cell electrodes Ys and Yas of the plasma display. The capacitance between the electrodes Ys and Yas of the panel is represented by the capacitor C 1  in the figure. Similarly, the capacitance between, on the one hand, the electrodes Ys and Yas and, on the other hand, the column electrodes referenced X of the panel is represented in the figure by the capacitors C 2  and C 3 . These capacitances are shown in the figure by dotted line. 
   The locking circuit  1  consists of four switches I 1 , I 2 , I 3  and I 4 . Two switches, I 1  and I 2 , are connected in series between a, power supply terminal receiving the voltage Vs and earth. The mid-point between these two switches is connected to the cell electrodes Ys of the display. The two other switches, I 3  and I 4 , are also connected in series between a power supply terminal receiving the voltage Vs and earth. The mid-point between these two switches is connected to the cell electrodes Yas of the display. 
   The means  3  comprises a switch I 7  connected in series with a diode D 3  between a power supply terminal receiving the voltage Vs and earth. The diode D 3  is oriented so as to prevent the current through the switch I 7  from flowing to earth. An inductor L 2  is also connected between the point situated between the switch I 7  and the diode D 3  on the one hand and the sustain electrode Ys on the other. The means  3  could, of course, just as well be connected to the address-sustain electrode Yas. 
   The power recovery circuit  2  is connected between the electrodes Ys and Yas of the display cells. This circuit is, for example, of the type described in the European Patent Application EP 0 704 834. It comprises an inductor L 1  connected in series with a two-way switch between the electrodes Ys and Yas. The two-way switch is formed by a switch I 5  in series with a diode D 1  that allows the current to flow in one direction when the switch I 5  is closed and by, connected in parallel, a switch I 6  connected in series with a diode D 2  that allows the current to flow in the opposite direction when the switch I 6  is closed. Thus, when one or the other of the switches I 5  and I 6  is closed, the inductor L is connected in parallel with the display capacitance shown by the capacitors C 1 , C 2  and C 3  in the  FIG. 2  and forms a resonant circuit with the latter. The complete operation of this power recovery circuit  2  with the locking circuit  1  is described in detail in the European Patent Application EP 0 704 834. This power recovery circuit  2  is generally used during the sustain phase of the discharges in the cells. Outside of this phase, the switches I 5  and I 6  are generally open. 
   According to the invention, when it is desired to simultaneously apply a voltage Vs to the electrodes Ys and Yas, the switch I 5  is closed in order to transmit the voltage Vs applied to the electrode Ys to the electrode Yas. 
   This phase of operation of the control device of the invention is illustrated in  FIG. 3 . When the voltage Vs is to be applied to the electrodes Ys and Yas, the switches I 7  and I 5  are closed. The duration of the closed state for the switch I 5  is equal to around twice that for the switch I 7 . 
   In more detail, at time t 2 , the switches I 5  and I 7  are closed. Advantageously, the switch I 5  can even be closed shortly before the switch I 7  in order to limit the switching losses in the switch I 5 . A current originating from the supply source of the voltage Vs is now delivered to the inductor L 2 . The current rises progressively in the inductor L 2  and is retransmitted to the electrode Ys and, via the switch I 5 , to the electrode Yas. The voltage on the electrodes Ys and Yas therefore rises progressively. The voltage rise on the electrode Ys happens shortly before that of the electrode Yas owing to the presence of the inductor L 1 . At a variable time t 3 , the switch I 7  is opened. The voltage across the terminals of the inductor L 2  is inverted and the current in the latter starts to decrease. The continuity of the current in the inductor L 2  is assured by the diode D 3 . This current continues to be delivered to the electrodes Ys and Yas. At a time t 4  corresponding to the cancellation of the current in the inductor L 2 , the switch I 5  is opened. The switches I 1  and I 3  are then closed and take over from the means  3  in supplying the voltage Vs. This closure of the switches I 1  and I 3  may indifferently be shortly before, at the same time as, or shortly after that of the switch I 5 . 
   In a less elaborate version, the means  3  could be eliminated and the switch I 1  be used to raise the voltage of the electrode Ys. However, this embodiment will result in greater power losses than those of the device in  FIG. 2 . These losses would nevertheless be smaller than those of a device comprising dedicated circuits for raising the voltage of the electrodes Ys and Yas. 
   It goes without saying that, as illustrated in  FIG. 4 , in the case of a falling edge and the application of a negative voltage Vs to the electrodes Ys and Yas of the display cells, the orientation of the diode D 3  would be inverted, namely its cathode would then be connected to earth. In this case, it is the switch I 6  of the power recovery circuit that would then be closed in place of the switch I 5 . 
   The advantages of this control device are manifold:
         a second circuit is not required to raise the voltage of the electrode Yas;   the means  3  and the power recovery circuit  2  result in little power loss during the application of the voltage Vs to the two electrodes Ys and Yas;   the means  3  does not interfere with the operation of the power recovery circuit  2  during the sustain phase of the discharges in the PDP cells.