Abstract:
A method and a circuit for controlling a power recovery stage of a plasma display panel including a resonant circuit of at least one inductive element and one capacitive element, wherein the capacitive element is precharged to half a supply voltage of the display panel.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to plasma display panels which comprise two parallel plates each supporting two electrode networks and between which is present a gas originating from light discharges at regions of intersection between the electrodes of the different plates. 
         [0002]    The present invention more specifically relates to the control of the electrode networks of the screen in a so-called sustain phase in which the electrodes of each network are excited by an A.C. voltage. 
       DISCUSSION OF THE RELATED ART 
       [0003]      FIG. 1  generally and very schematically shows the structure of a plasma display panel  1  (PDP) of the type to which the present invention applies. Two parallel plates designated with general reference numeral  2  each support electrodes generally perpendicular from one plate to the other and parallel to each other on a same plate. Each line L of the screen is defined by two parallel electrodes  3  and  4  and each column C of the screen is defined by an electrode  5  in the other direction, supported by the other plate. The intersection of a line L and of a column C defines a pixel P of the screen. For the lighting of a pixel, a light discharge is organized between electrodes  3  and  4  of a line L, addressed by the corresponding column  5 . 
         [0004]    The control of screen  2  is performed by means of a circuit  6  for driving column electrodes (COL DRV) and circuits  7  for driving a first network of so-called scan line electrodes (for example,  3 ) (SCAN DRV). Circuit  7  is connected to a first power supply circuit  8  (PW CT). The second network of so-called line sustain electrodes (for example,  4 ) is directly connected to a circuit  8 ′ of power supply (PW REF) (generally, a reference voltage). Circuits  6 ,  7 ,  8 , and  8 ′ are controlled and synchronized by a unit  9  (CU), generally a calculator or a circuit in wired logic. 
         [0005]    The present invention more specifically relates to a portion of power supply circuits  8 ,  8 ′. 
         [0006]      FIG. 2  very schematically shows in the form of blocks a conventional example of architecture of circuits  8  and  8 ′ associated with the control circuits of the scan and sustain electrodes of the type to which the present invention applies. In  FIG. 2 , two lines  3  and  4  of the display panel have been symbolized by their equivalent capacitor having its two electrodes respectively connected to scan circuit  7  and to power supply circuit  8 ′. Typically, the control circuits of each scan electrode (contained in block  7 ) are formed of switches operating in an on/off manner to bring, onto one of the electrodes of the considered line, different voltage levels in different operating phases. For simplification, the different control signals have not been illustrated in the drawings. In practice, each scan electrode is associated with several switches of circuit  7  which are used to select that of the screen lines which will receive the different voltages. All the control circuits of the different electrodes have their respective power supply terminals interconnected to common power supply circuit  8 . 
         [0007]    Circuit  8  on the scan electrode side comprises an addressing stage  11  (ADD/ER) in charge of bringing an addressing voltage to electrodes selected by control circuit  7 . Addressing stage  11  may be completed by an erasing stage at a different voltage. Circuit  8  also comprises an electrode pre-bias or precharge stage  12  (BIAS). It further comprises a power recovery stage  13  (PR) to which the present invention more specifically applies, intended to impose a voltage on electrodes  3  by carrying off excess charges or by bringing missing charges during the so-called sustain operating phase. 
         [0008]    On the sustain electrode side, circuit  8 ′ amounts to a power recovery stage  13 ′ identical to stage  13  on the scan electrode stage, to which are interconnected all lines  4 . 
         [0009]      FIG. 3  very schematically shows a conventional example of power recovery stages  13  and  13 ′ of the type to which the present invention applies, connected to a panel  2  of electrodes  3  and  4  of a plasma screen. Electrode panel  2  has been symbolized by the different lines  3  and  4  between which equivalent line capacitors C 1  have been shown. Panel  2 , having all its scan electrodes  3  and respectively all its sustain electrodes  4  interconnected during the power recovery phase, is equivalent to a capacitance Cp. The interconnection of all the scan electrodes is performed by means of control circuit  7 , and other switches in all or nothing of the pre-bias and addressing stages  12  and  11  (not shown in  FIG. 3 ) respectively connect the then common terminal  23  of electrodes  3  to an access terminal  31  at stage  13 . The interconnection of all sustain electrodes  4  is structural and their common terminal  24  is connected to an access terminal  31 ′ of stage  13 ′. 
         [0010]    Each stage  13  or  13 ′ or so-called Weber-type power recovery circuit is intended to impose a voltage on terminal  23  or  24  by evacuating excess charges and by bringing to these electrodes missing charges during the sustain phase. 
         [0011]    Stage  13  mainly comprises an inductive element L connecting, by a bi-directional switch  32 , an electrode  33  of a capacitor Cs to terminal  31  connectable to the common terminal (during the power recovery phase) of electrodes  3 . Output terminal  31  of stage  13  is connected by a switch M 1  to a terminal  25  of application of a positive voltage Vs and, by a switch M 3 , to a terminal  26  of application of a reference voltage (typically, the ground). In practice, each switch M 1  or M 3  is in parallel with a diode (typically, the intrinsic diode of a MOS transistor forming the switch) having its anode connected to terminal  31 , respectively  26 . Switch  32  is typically formed of two switches M 5 , M 6  in antiparallel and each in series with a diode D 5 , D 6 , respectively. The respective junction points of transistor M 5  and of diode D 5  and of transistor M 6  and of diode D 6  are connected to terminal  25  of application of positive supply voltage Vs and to ground terminal  25  by blocking diodes D 1  and D 2 . 
         [0012]    The same structure can be found on the side of circuit  13 ′ where a bi-directional switch  32 ′ connects an electrode  33 ′ of a capacitor Cs′ having its other electrode at the reference voltage to a first terminal of an inductive element L′ having its other terminal connected to output terminal  31 ′ of stage  13 ′, connectable to common terminal  24  of electrodes  4 . Terminal  31 ′ is connectable to terminals  25  and  26  by switches M 2  and M 4  (typically, MOS transistors) and bi-directional switch  32 ′ has the same structure as switch  32  of stage  13  (switches M 7  and M 8  in antiparallel and each in series with a diode D 7  and D 8  between inductance L′ and terminal  33 ′, the respective midpoints of the series connections being connected by blocking diodes D 1 ′ and D 2 ′ to terminal  25  and to ground  26 ). 
         [0013]    Switches M 1 , M 2 , M 3 , M 4  form an H bridge, switches M 1  and M 4  being intended to be turned on at the same time, the same holding true for switches M 2  and M 3 . 
         [0014]    An example of a circuit for driving a plasma display panel is described in International application WO03/102907. 
         [0015]    The display panel control can be temporally divided in frames and in display sub-frames during which are present different operation phases. In each display sub-frame, a first so-called pre-bias or precharge phase uses stage  12  to excite the display panel cells to pre-excite the contained gas and thus lower the addressing voltage under which the discharge will be performed subsequently. During the pre-bias phase, the excitation is for example performed under a voltage on the order of 400 volts. 
         [0016]    This pre-bias phase is followed by a so-called stabilization phase, then by an erasing phase aiming at bringing the scan and sustain electrodes to an erasing voltage, generally the ground. The erasing and pre-bias phases result in suppressing the charges to avoid unwanted lightings. 
         [0017]    A so-called addressing phase which aims at bringing a corresponding addressing voltage of a given level on electrodes  3  according to the respective states of addressing transistors of circuits  7  can then be observed. The period during which the addressing level is applied to the electrodes depends on the rank of the line in the display panel. 
         [0018]    This addressing phase is followed by the sustain phase to which the present invention more specifically applies. During this phase, a pulse train of constant duty cycle and of amplitude Vs (on the order of 200 volts) is applied onto terminal  23 . Recovery stages  13  of electrodes  3 , and  13 ′ of electrodes  4 , are used to ease the charge of electrodes  3  (respectively  4 ) to level Vs and ease the discharge of these same electrodes during respective low levels of the pulses. The turning on and off of the switches (M 1 , M 4 , M 3 , M 2 ) are alternated at the rate of the pulses of level Vs to be applied onto terminals  31  and  31 ′. This results, under the effect of the resonance circuit formed of inductance L and of capacitor Cs, in the application of an A.C. voltage between the scan and sustain lines. Typically, the frequency of the pulse trains is on the order of from 200 to 250 kHz and the average frequency on an image sub-frame of the sustain periods ranges between 0 and 85 kHz. 
         [0019]    During the sustain or power recovery phase, equivalent capacitor Cp of the display screen is alternately charged and discharged by the resonant circuits of the scan and sustain electrodes by exploiting a Vs/2 charge level of capacitors Cs and Cs′. 
         [0020]    Each power recovery pulse starts with a turning-on of switches M 6  and M 4  to supply a current to inductance L and bring voltage Vp across electrodes  3  to voltage level Vs, followed by a resonance phase after which transistor M 1  and transistor M 4  of the H bridge are turned on, and is ended by a discharge phase by the turning-on of transistor M 5  (transistor M 4  remaining on), enabling disappearing of voltage Vp through resonant circuit L-Cs. The next pulse is performed on the side of sustain electrodes  4  (transistors M 8 , M 3 , then M 2 , M 3 , then M 3 , M 7 ) and so on. The resonance phase is especially used to decrease losses in the transistors of the H bridge and ease the transition between levels to avoid the current peaks in the display panel. 
         [0021]    At the beginning of the resonance phase, the blocking diodes D 1  and D 2  (respectively, D 1 ′ and D 2 ′) of the stage are used to carry off the latching current of diodes D 5  and D 6  (respectively D 8  and D 7 ) which block at the end of the charge or discharge phase, and to limit overvoltages to protect the switches. 
         [0022]    A disadvantage of known circuits is that they require high-voltage components, especially for switches M 5  and M 6  and diodes D 5  and D 6 . In practice, voltage Vs is on the order of 200 volts and each transistor M 5  or M 6  sees half this supply voltage when capacitor Cs is charged to value Vs/2. A problem is however posed at the beginning when capacitor Cs is discharged and where one of the two transistors (M 5 ) and one of the diodes (D 6 ) then sees a voltage on the order of 200 volts, which requires use of transistors and diodes that can withstand several hundreds of volts. This increases the size of the transistors and increases their losses. 
         [0023]    Another problem is linked to a dissymmetry between the charge and discharge phases due to different impedances of switches M 5  and M 6 . 
         [0024]    Such problems can of course be encountered on the side of switches M 7  and M 8  and of diodes D 7  and D 8 . 
         [0025]    Another disadvantage is that the voltage Vp obtained between electrodes  3  and  4  at resonance is not equal to voltage Vs. This results in possible peaks on switchings. 
         [0026]    Another disadvantage of known circuits is that the blocking diodes take a long time to carry off latching currents. 
       SUMMARY OF THE INVENTION 
       [0027]    The present invention aims at overcoming all or part of the disadvantages of known plasma display panel control circuits. 
         [0028]    The present invention more specifically aims at enabling use of lower-voltage components in power recovery circuits. 
         [0029]    The present invention also aims at decreasing losses by enabling a decrease in the size of power recovery circuit control switches. 
         [0030]    The present invention also aims at preserving the architecture of conventional circuits and, in particular, at requiring no modification of the other plasma display panel control stages. 
         [0031]    The present invention aims at more rapidly evacuating recovery currents in the diodes of the bi-directional switches of power recovery circuits. 
         [0032]    To achieve all or part of these objects, as well as others, the present invention provides a method for controlling a power recovery stage of a plasma display panel comprising a resonant circuit of at least one inductive element and one capacitive element, comprising at least one step of precharge of the capacitive element to half a supply voltage of the display panel. 
         [0033]    According to an embodiment of the present invention, switches of activation of the power recovery stage are inhibited during the precharge step, at least until the voltage across the capacitive element has reached a first threshold depending on the supply voltage. 
         [0034]    According to an embodiment of the present invention, the step of precharge of the capacitive element is inhibited when the voltage thereacross reaches a second threshold depending on the supply voltage greater than the first one. 
         [0035]    According to an embodiment of the present invention, the precharge of the capacitive element is obtained by means of a controllable current source. 
         [0036]    The present invention also provides a control circuit of a power recovery stage of a plasma display panel having a resonant circuit of at least one inductive element and at least one capacitive element, comprising: 
         [0037]    a controllable current source between a terminal of application of a supply voltage and the capacitive element; and 
         [0038]    at least one first comparator for comparing the voltage across the capacitive element with respect to a first threshold to activate switching elements of the recovery circuit. 
         [0039]    According to an embodiment of the present invention, a second comparator compares the voltage across the capacitive element with respect to a second threshold greater than the first one to control said current source. 
         [0040]    The present invention also provides a power recovery stage of a plasma display panel comprising a resonant circuit of at least one capacitor in series with a bi-directional switch and at least one inductive element between the junction point of a first branch of an H bridge and the ground, said junction point being connected to first electrodes of the display panel and the bi-directional switch being formed of an antiparallel association of two switches, each in series with a diode. 
         [0041]    According to an embodiment of the present invention, two blocking circuits each comprise a zener diode in series with a diode between the terminal of the inductance connected to the bidirectional switch and two terminals of application of the supply voltage. 
         [0042]    The present invention also provides a plasma display panel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]    The foregoing objects, features, and advantages of the present invention, as well as others, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, in which: 
           [0044]      FIGS. 1 to 3 , previously described, are intended to show the state of the art and the problem to solve; 
           [0045]      FIG. 4  very schematically shows in the form of blocks an embodiment of a power recovery circuit according to the present invention; 
           [0046]      FIG. 5  is a detailed electric diagram of an embodiment of the power recovery circuit of  FIG. 4 ; 
           [0047]      FIG. 6  illustrates, in a timing diagram, the operation of the circuit of  FIG. 5 ; 
           [0048]      FIGS. 7A ,  7 B,  7 C,  7 D,  7 E,  7 F,  7 G,  7 H,  7 I,  7 J, and  7 K are timing diagrams illustrating the power recovery phase in a plasma display screen according to an embodiment of the present invention; and 
           [0049]      FIG. 8  shows a variation of a power recovery circuit according to the second aspect of the present invention. 
       
    
    
       [0050]    The same elements have been designated with the same reference numerals in the different drawings and the timing diagrams have been drawn out of scale. For clarity, only those elements and operation steps which are useful to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the generation of the control signals adapted to the operation of the switches has not been described in detail, the present invention being compatible with the use of conventional circuits for generating such signals. Similarly, the full operation of a plasma display panel (especially, that of the other scan or sustain electrode control stages) has not been described in detail, the present invention being here again compatible with conventional systems. 
       DETAILED DESCRIPTION 
       [0051]    A feature of an embodiment of the present invention is to precharge the capacitor of the resonant circuit of a Weber-type power recovery stage of a plasma display panel to half the supply voltage. 
         [0052]      FIG. 4  very schematically shows in the form of blocks an embodiment of a power recovery stage  43  of a plasma display screen according to the present invention. For simplification, only one stage  43  on the scan electrode side ( 3 ,  FIGS. 1 ,  2 , and  3 ) has been shown. The other stage on the sustain electrode side has an identical structure. 
         [0053]    The drawing shows inductance L and capacitor Cs forming the resonant circuit as well as bi-directional switch  32  formed of two MOS transistors M 5  and M 6  each in series with a diode D 5 , respectively D 6 , in antiparallel between an electrode  33  of capacitor Cs and a first electrode of inductance L. The other terminal of inductance L is connected to a terminal  31  forming a midpoint of a first branch of an H bridge connectable to the electrode panel (here, a first transistor M 1  in series with a second transistor M 2  between two terminals  25  and  26  of application of supply voltage Vs). For simplification, the blocking diodes (D 1  and D 2 ,  FIG. 3 ) have not been shown. 
         [0054]    The different switches are, for example, formed of MOS transistors which, according to a preferred embodiment of the present invention, are sized to withstand a voltage approximately equivalent to half the supply voltage Vs, plus a value corresponding to the expected voltage peaks, for example, on the order of from 10 to 15% of voltage Vs. 
         [0055]    According to the embodiment shown in  FIG. 4 , electrode  33  of capacitor Cs is connected to terminal  25  of application of voltage Vs by a controllable current source  41 . Source  41  is connected by a circuit  42  (COMP Vcs/Vs) of comparison of voltage Vcs across capacitor Cs with respect to at least one threshold depending on supply voltage Vs. The function of circuit  42  is to activate current source  41  only during starting periods when a precharge of capacitor Cs is required. This functionality will be subsequently detailed in relation with  FIG. 5 . 
         [0056]    Switches M 1 , M 2 , M 5 , and M 6  (as well as the switches of the other portion of the H bridge (M 2 , M 4 ,  FIG. 3 )) are controlled in synchronized fashion by a circuit  45  (CTRL) having outputs S connected to the control terminals (gates) of the different switches. 
         [0057]      FIG. 5  shows an example of a detailed diagram of a power recovery stage of a plasma display panel according to the present invention. The different switches have been shown in the form of MOS transistors, conversely to the previous drawings. 
         [0058]    According to this embodiment, controllable current source  41  is formed of a transistor M 41  is series with a resistor R 41  between terminal  25  and the anode of a diode D 41  having its cathode connected to electrode  33  of capacitor Cs. A biasing resistor Rp connects the gate of transistor M 41  to terminal  25  and a zener diode DZ 41  connects this gate to the anode of diode D 41  (the anode of diode DZ 41  being connected to the anode of diode D 41 ). Such a current source structure is conventional. Source  41  is made controllable by means of a switch (MOS transistor M 10 ) connecting the anode of diodes DZ 41  and D 41  to ground  26 . When transistor M 10  is blocked, source  41  charges capacitor Cs. When transistor M 10  is on, the current source is grounded. Diode DZ 41  sets the value of the current by limiting the voltage across its resistive elements. Further, it limits the gate-source voltage of transistor M 10 . Other controllable current source structures can be envisaged. 
         [0059]    The gate of transistor M 10  is connected to the output of a first comparator  421  of circuit  42  having the function of comparing a voltage proportional to voltage Vcs across capacitor Cs with a threshold VB. Threshold Vth of comparator  421  is set, for example, by a resistive bridge formed of two resistors R 1  and R 2  supplied in series by voltage Vs. Threshold voltage Vth is applied, for example, to the inverting input of comparator  421 , the non-inverting input of which receives a voltage representative of the voltage across capacitor Cs, obtained by a second resistive bridge formed of three resistors R 3 , R 4 , and R 5  in series between terminal  33  and the ground. The non-inverting input of comparator  421  is connected to the junction points of resistors R 4  and R 5 . The output of comparator  421  is connected to the gate of transistor M 10  (possibly via circuit  45 ). Resistors R 1  to R 5  set a threshold VB=VsR 2 (R 3 +R 4 +R 5 )/(R 2 +R 1 )R 5  for voltage Vcs. 
         [0060]    A second comparator  422  of circuit  42  compares voltage Vth provided by bridge R 1 -R 2  with second data representative of the voltage across capacitor Cs different from the first one. This second voltage is taken at the junction point of resistors R 3  and R 4  connected to the non-inverting input of comparator  422  having its inverting input connected to the junction point of resistors R 1  and R 2 . In fact, this amounts to comparing voltage Vcs with a threshold VA (=VsR 2 (R 3 +R 4 +R 5 )/(R 5 +R 4 )(R 2 +R 1 )) lower than threshold VB. The output of comparator  422  is connected to switch control circuit  45 . 
         [0061]    The function of the two thresholds is to distinguish the activation of the power recovery circuit (transistors M 5 , M 6  and H bridge formed of transistors M 1 , M 3  and M 2 , M 4  shown in dotted lines) from the activation or deactivation of current source  41  by the control of transistor M 10 . 
         [0062]    In  FIG. 5 , blocking diodes DI and D 2  have been shown between transistors M 5 , respectively M 6  and diode D 5 , respectively D 6 . 
         [0063]      FIG. 6  illustrates, in a timing diagram showing voltage Vcs across capacitor Cs along time, the operation of the circuit of  FIG. 5 . Initially (time t 0 ), capacitor Cs is discharged. Circuit  42  provides, on the one hand, a signal of blocking of transistor M 10  by comparator  421 , while comparator  422  notifies block  45  that voltage Vcs is smaller than first threshold VA so that switches M 5 , M 6 , M 1 , and M 3  are all off. Voltage Vcs across the capacitor increases substantially linearly due to the charge by means of current source  41 . At a time t 1  when voltage level Vcs reaches threshold VA, comparator  422  switches and control block  45  activates the power recovery phase. Current source  41  is not inhibited yet, whereby the voltage across capacitor Cs keeps on increasing. 
         [0064]    Preferably, threshold VA is selected so that latching voltage VRM of diodes D 5  and D 6  is smaller than level Vs−VA plus the overvoltages linked to switchings. 
         [0065]    At a time t 2  when threshold VB is reached by voltage Vcs, comparator  421  turns on transistor M 10 , which removes the charge of capacitor Cs by current source  41 . Since the operation of the power recovery circuit has started at time t 1 , the increase in voltage Vcs to level Vs/2 (time t 3 ) is carried on by the resonant circuits. 
         [0066]    Preferably, the difference between threshold VB and level Vs/2 is selected according to the manufacturing tolerances of the different components and especially to technological dispersions on manufacturing of the resistors of the dividing bridges and of the transistors. Threshold VB must be sufficiently lower than value Vs/2 so as not to be crossed in case of a technological dispersion between components. This tolerance range Tol is illustrated in  FIG. 6  by dotted lines. With such notations, VB&lt;Vs/2−Tol/2. 
         [0067]      FIGS. 7A to 7K  illustrate the operation of a power recovery stage once the precharge of capacitor Cs has been obtained. These timing diagrams respectively illustrate examples of shapes of the control signals (on or off stage) of transistors M 1 , M 2 , M 3 , M 4 , M 5 , M 6 , M 7 , M 8 , and M 10  (and M 10 ′) as well as the corresponding shapes of voltage Vp across equivalent capacitor Cp of the plasma display panel and of currents IL and IL′ in the inductances of the power recovery stages. 
         [0068]    The precharge illustrated in  FIG. 6  occurs before time t 1  of activation of the stage at which, for example, transistor M 6  ( FIG. 7F ) is switched on, transistor M 4  ( FIG. 7D ) being already on, and transistor M 10  remaining off until time t 2  (threshold VA) slightly subsequent to time t 1 . From time t 1 , the current ( FIG. 7K ) in inductive element L charges equivalent capacitor Cp (voltage Vp,  FIG. 7J ) to reach a voltage Vs (neglecting the voltage drops in the on components) at a time t 4 . At this time t 4 , diode D 5  blocks, and an latching phenomenon which causes a current increase in inductance L can be observed. This power is carried off by diodes D 6  and D 2  for a time period (Δt,  FIG. 7K ) which is set by the voltage across the inductance. Time period Δt must of course be shorter than interval t 5 −t 4 . The blocking of transistor M 6  is illustrated at time t 4 , knowing that its control can be slightly delayed due to the automatic blocking of diode D 5 . Transistor M 1  ( FIG. 7A ) is turned on from time t 4 . Towards the end of the power recovery phase, transistor M 5  is turned on at a time t 5  ( FIG. 7E ) at the same time as (or slightly after) transistor M 1  is turned off. An operation of the resonant circuit in the other direction by a discharge of capacitor Cp until a time t 6  when the disappearing of current IL in inductance L blocks diode D 6  and a latching phenomenon opposite to that of diode D 5  can then be observed. Transistors M 4  and M 5  are turned off from time t 6  and transistor M 3  ( FIG. 7C ) is turned on at this time t 6  (in practice, slightly after the turning off of transistor M 4 ). 
         [0069]    From time t 0 ′, the operation described hereabove in relation with the power recovery stage on the scan electrode side is reproduced on the power recovery stage on the sustain electrode side by the second branch of the H bridge. This operation is illustrated in the right-hand portion of the timing diagrams of  FIGS. 7A to 7K  (times t 0 ′, t 1 ′, t 2 ′, t 4 ′, t 5 ′, and t 6 ′). 
         [0070]    If, between time t 6  and start t 0 ′ of the power recovery phase on the sustain electrode side, the voltage across capacitor Cs drops too much, a new precharge of said capacitor by current source  41  can be observed. 
         [0071]    An advantage of the present invention is that the voltage seen by transistors M 5  and M 6  is now limited to half the supply voltage (plus the switching overvoltages). 
         [0072]    Another advantage of the present invention is that in case of a dissymmetry due to the impedance difference between switches M 5 , M 6 , this dissymmetry is compensated for by the precharge system. 
         [0073]    Another advantage of the present invention is that the thresholds adapt to possible variations of supply voltage Vs. 
         [0074]      FIG. 8  shows an embodiment of a power recovery stage  43  according to a second aspect of the present invention. For simplification, the precharge elements of capacitor Cs at point  33  have not been shown. 
         [0075]    The same structure as in the previous power recovery stages can be found. 
         [0076]    According to this aspect of the present invention, a first zener diode DZ 1  is interposed between blocking diode D 1  (having its anode connected to the anode of diode D 5 ) and terminal  25  of application of supply voltage Vs. A second zener diode DZ 2  is interposed between the cathode of diode D 6  and that of blocking diode D 2  connected to ground  26 . 
         [0077]    The function of zener diodes DZ 1  and DZ 2  is to set a blocking voltage greater than that brought by diodes D 1  and D 2 . 
         [0078]    Conversely to the circuit of  FIG. 5  where the current flow in the latching phase (after time t 6 ,  FIGS. 7A to 7K ) runs (for negative phases) through diode D 6 , inductance L, transistor M 3 , and diode D 2 , this current is according to the embodiment of  FIG. 8  looped back by diode D 2  and diode DZ 2  without running through diode D 6 . 
         [0079]    The same operation occurs in positive phases through diode DZ 1 . 
         [0080]    An advantage of setting the blocking voltage by means of a zener diode DZ 1  or DZ 2  is that this enables decreasing time Δt of evacuation of the power stored in inductance L after the respective blockings of diodes D 5  and D 6 . This advantage is particularly substantial with the increase in display panel operating frequencies, which decreases the available intervals. 
         [0081]    Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. For example, the switches described as being MOS transistors may be replaced with insulated-gate bipolar transistors (IGBT). Further, the dimensions to be given to the different components of the starting circuit of the present invention are within the abilities of those skilled in the art based on the functional indications given hereabove. Further, the adaptation of the generally digital control circuits of the plasma display panel to take into account the thresholds detected by the present invention is also within the abilities of those skilled in the art by using conventional tools.