Abstract:
The present invention relates to an amplifier designed to generate a rectangular voltage signal with soft switching on a capacitive load, comprising a half-bridge structure consisting of two switches, which is connected between a supply terminal and a reference terminal, the mid-point of said structure being connected to said capacitive load, and a soft-switching circuit with energy recovery. To limit the conduction losses in the amplifier, the soft-switching circuit with energy recovery comprises no active component, such as a diode or transistor. According to the invention, it is formed only from an inductive elements connected in series with a capacitive element between the terminals of said capacitive load. Advantageously, the inductive element comprises a saturable inductor in order to limit the conduction losses of the amplifier.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an amplifier designed to generate a rectangular voltage signal with soft switching on a capacitive load, comprising a half-bridge structure consisting of two switches, said structure being connected between a supply terminal and a reference terminal, the mid-point of said structure being connected to said capacitive load, and a soft-switching circuit with energy recovery. The invention relates more particularly to amplifiers designed to generate high-voltage rectangular sustain or address signals in a plasma display panel.  
       BACKGROUND OF THE INVENTION  
       [0002]     Conventionally, a plasma display panel comprises a plurality of cells organized in lines and columns. In the coplanar technology currently employed, each cell is provided with three electrodes:  
         [0003]     one column electrode, used essentially to address the cells; the column electrodes of all the cells of the panel are connected to a column driver; and  
         [0004]     two line electrodes, one being called the line scanning electrode used to individually address each line of cells, and the other being called the line common electrode; all the line scanning electrodes are connected, on one side of the panel, to a line driver and the line common electrodes are connected together on the other side of the panel.  
         [0005]     In this type of panel, each cell is addressed by applying a specific high-voltage signal between its line scanning electrode and its column electrode in order to modify its state of charge. After the addressing operation, the cell can have two states of charge, namely a first state called the “excited state”, which will turn the cell on during the sustain phase of the cells to follow, and a second state in which the cell will remain turned off. The cell sustain phase that follows the address phase is a period during which high-voltage rectangular signals are applied to the line scanning electrodes and to the line common electrodes. During this phase, the previously excited cells are turned on.  
         [0006]     To generate such voltage signals, the display panel is provided with power amplifiers. The panel comprises in particular a column amplifier to generate the address signal to be applied to the column electrode of the cells and a sustain amplifier to generate the sustain signal applied to the line scanning electrode and to the line common electrode of the cells.  
         [0007]     These amplifiers have in common the requirement to generate signals having high-voltage transitions on a very high capacitive load, equal to the equivalent capacitor of all of the cells of the panel or to the capacitance of a large part of them.  
         [0008]     The cell sustain operation involves an enormous transfer of energy between the amplifier and the cells of the panel, which energy must necessarily be recovered. The same applies to the operation of addressing the cell columns.  
         [0009]     For this purpose, an energy-recovery sustain amplifier, called a Weber amplifier after the name of its inventor, has been developed. It is shown schematically in  FIG. 1 . This amplifier  10  in fact comprises two identical amplifiers, one  11  designed to supply the line scanning electrode Y of the cells via a line driver  12  and the other  13  designed to supply their line common electrodes Z. The cells are shown in the figure by their equivalent capacitor Cp. This equivalent capacitor is in practice composed of the capacitor Cp 1  present between the line scanning electrodes Y and the line common electrodes Z of the panel, of the capacitor Cp 2  present between the line scanning electrodes Y and the column electrodes of the panel and finally of the capacitor Cp 3  present between the line common electrodes Z and the column electrodes of the panel.  
         [0010]     The amplifier  11  designed to supply the electrodes Y conventionally comprises switches M 1  and M 2  that are connected as a half-bridge structure and mounted in series between a supply terminal, which receives a very high sustain voltage VS, and a reference terminal (connected here to ground GND). These switches are operated so as to generate, on the electrode Y of the cells of the panel, a rectangular signal that alternates between the voltage VS and the potential present on the reference terminal. As shown in the figure, these switches are generally MOS transistors with their antiparallel diodes. To recover and re-inject the capacitive energy and to produce soft switching between VS and ground, the amplifier  11  includes a resonant inductor L connected in series with a switching module MC and a storage capacitor C 1 . These three elements are connected between the electrode Y and the reference potential. The switching module has two current conduction paths connected in parallel, each permitting the current to flow in just one direction. The first current path comprises a switch M 3  connected in series with a diode D 3  in order to permit current to flow towards the storage capacitor C 1  when the switch M 3  is closed, and thus to produce the falling edge of the output signal of the amplifier. The second current path comprises a switch M 4  connected in series with a diode D 4  in order to permit current to flow towards the resonant inductor L when the switch M 4  is closed, and thus to produce the rising edge of the output signal.  
         [0011]     As regards the amplifier  13 , this comprises the same components as the amplifier  11 , these being connected in the same way between the line common electrode Z and the reference terminal. To distinguish the components of the amplifier  11  from those of the amplifier  13  in the rest of the present description, the components of the amplifier  11  are denoted by M 1 , M 2 , L, MC, M 3 , M 4 , D 3 , D 4  and C 1 , while those in the amplifier  13  are denoted by M 1 ′, M 2 ′, L′, MC′, M 3 ′, M 4 ′, D 3 ′, D 4 ′ and C 1 ′.  
         [0012]      FIG. 2  shows the sustain voltage signals to be generated on the electrodes Y and Z and the resultant voltage across the terminals of the panel cells according to one mode of operation well known for obtaining good sustaining of the electrical discharges in the cells. According to this mode of operation, the transitions in the voltage signal generated on the electrode Y are synchronized with those in the voltage signal generated on the electrode Z so that the voltage across the terminals of the panel cells permanently alternates between +VS and −VS. This mode of operation has been given merely as an example in order to make the operation of the Weber circuit understood. Of course, other modes of operation exist, especially one in which the voltage transitions on the electrode Y of the cells are offset with respect to those on the electrode Z. The invention will be applicable to all these modes of operation. The equivalent capacitor Cp varies depending on the mode of operation selected.  
         [0013]     To obtain one or other of the voltage signals shown in  FIG. 2 , the amplifier  10  is operated as illustrated in  FIG. 3 . This figure shows more particularly the voltages for controlling the switches M 1  to M 4 , the output voltage of the amplifier that results therefrom, and the current iL flowing through the resonant inductor L. In this figure, it is assumed that in the initial state the switches M 2 , M 3  and M 4  are open and the switch M 1  is closed. The voltage on the electrode Y is therefore equal to VS. After switch M 1  has been opened and switch M 3  has been closed, the voltage on the electrode Y starts to drop. During this phase, the resonant circuit composed of the inductor L and the equivalent capacitor Cp is closed by the diode D 3 , the switch M 3  and the storage capacitor C 1 , with the following initial conditions:  
         [0014]     the current iL flowing through the inductor L is zero;  
         [0015]     the voltage on the electrode Y is equal to VS; and  
         [0016]     the voltage across the terminals of the storage capacitor is equal to VS/2.  
         [0017]     Since the capacitance of the storage capacitor C 1  is much higher than that of the capacitor Cp, it may be considered that the voltage across its terminals is constant and equal to VS/2. As the current through the inductor L rises, the output of the amplifier and the voltage across the terminals of the capacitor Cp decrease along a sinusoidal portion until the voltage on the electrode Y reaches VS/2 (the point where the current iL stops rising). This first phase corresponds to a transfer of energy from the capacitor Cp to the inductor L. A reverse transfer takes place during the next phase. During this phase, the current iL decreases and the voltage on the electrode Y continues to decrease along another sinusoidal portion until reaching 0 volts (the reference potential). The diode D 3  prevents the current from flowing in the reverse direction. Closing the switch M 2  then allows the voltage on the electrode Y to be maintained at 0 volts. The transition of the voltage on the electrode Y from zero volts to VS is performed in the same manner, by closing the switch M 4 .  
         [0018]     The main drawback of this circuit is that it comprises a large number of active components (switches and diodes) that generate conduction losses during operation of the circuit that are due to the high rms value of the current iL and to the reverse recovery losses of the diodes when they are turned off.  
       SUMMARY OF THE INVENTION  
       [0019]     The invention relates to an amplifier, able to be used as a column address or sustain amplifier, capable of generating rectangular signals with soft switching, which comprises a small number of active components capable of introducing energy losses.  
         [0020]     The invention relates to an amplifier designed to generate a rectangular voltage signal with soft switching on a capacitive load, said amplifier comprising:  
         [0021]     a half-bridge structure consisting of two switches, which is connected between a supply terminal and a reference terminal, the mid-point of said structure being connected to said capacitive load; and  
         [0022]     a soft-switching circuit with energy recovery, which is connected to said half-bridge structure, said soft-switching circuit with energy recovery being formed by an inductive element connected in series with a capacitive element between the terminals of said capacitive load. According to the invention, the inductive element comprises a first inductor connected in series with a second inductor capable of operating in saturated mode, to reduce even further the conduction losses in the amplifier. The first and second inductors are advantageously produced in one and the same coil.  
         [0023]     According to the invention, this amplifier is included in a plasma display panel, either to generate a sustain signal on the line scanning electrodes or the line common electrodes of the cells of the panel, in order to sustain electrical discharges in the cells of the panel that were previously placed in an excited state, or to generate an address signal on the column electrodes of the cells of the panel. The capacitive load then corresponds to the equivalent capacitor of the cells of the panel.  
         [0024]     The invention also relates to an amplifier designed to generate first and second rectangular voltage signals with soft switching on the line scanning electrodes and the line common electrodes for the cells of the plasma display panel, said amplifier comprising:  
         [0025]     a first half-bridge structure having two switches, which is connected between a supply terminal and a reference terminal, the mid-point of said structure being connected to said line scanning electrodes;  
         [0026]     a second half-bridge structure having two switches, which is connected between a supply terminal and a reference terminal, the mid-point of said structure being connected to said line common electrodes; and  
         [0027]     a soft-switching circuit with energy recovery, which is connected between said mid-points of said first and second half-bridge structures, said soft-switching circuit with energy recovery being formed from an inductive element connected in series with a capacitive element. According to the invention, the inductive element comprises a first inductor connected in series with a second inductor capable of operating in saturated mode in order to further reduce the conduction losses in the amplifier. The first and second inductors are advantageously produced in one and the same coil. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0028]     The invention will be better understood on reading the description that follows, given by way of non-limiting example and with reference to the appended drawings in which:  
         [0029]      FIG. 1 , already described, shows a diagram of a sustain amplifier of the prior art;  
         [0030]      FIG. 2 , already described, shows timing diagrams illustrating the voltage signals generated by the sustain amplifier of  FIG. 1  in a known mode of operation of the amplifier;  
         [0031]      FIG. 3 , already described, shows control signals for controlling elements of the amplifier of  FIG. 1  and signals illustrating the operation of said amplifier;  
         [0032]      FIG. 4  shows a diagram of a first amplifier according to the invention, capable of generating a rectangular voltage signal with soft switching on one or other of the electrodes of the cells of a plasma display panel in order to sustain the cells or to address the columns of cells;  
         [0033]      FIG. 5  shows timing diagrams illustrating the control signals for controlling the amplifier of  FIG. 4  in order to generate a signal to be applied to the electrodes Y or Z of the cells during the sustain phase of the cells;  
         [0034]      FIG. 6  shows timing diagrams illustrating the control signals for controlling the amplifier of  FIG. 4  in order to generate a signal to be applied to the column electrodes of the cells during the address phase of the cells;  
         [0035]      FIG. 7  shows a diagram of a second amplifier according to the invention capable of generating a rectangular voltage pulse with soft switching on one or other of the electrodes of the cells of a plasma display panel in order to sustain the cells or to address the columns of cells;  
         [0036]      FIG. 8  shows timing diagrams illustrating the control signals for controlling the amplifier of  FIG. 7  in order to generate a signal to be applied to the electrodes Y or Z of the cells during the sustain phase of the cells; and  
         [0037]      FIG. 9  shows a sustain amplifier according to the invention, in which the soft-switching circuit with energy recovery is common to the electrodes Y and Z of the cells of the panel. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0038]     According to the invention, the circuit used for generating soft switching, while still recovering energy, is limited to an inductive element L 2  connected in series with a capacitive element C 2 , the combination being connected to the terminals of the equivalent capacitor Cp of the cells of the panel. This circuit is shown in  FIG. 4  for generating a rectangular signal to be applied to the electrode Y or Z of the cells during their sustain phase or to the column electrode of the cells during their address phase.  
         [0039]     In the case of a sustain operation, the voltage VS is the voltage, of around 200 volts, for sustaining the cells and, in the case of addressing the cell columns, the voltage VS is an address voltage of around 60 volts. The capacitance of the capacitor Cp varies depending on the mode of operation (with synchronized or unsynchronized transitions) and depending on the type of operation (address phase or sustain phase). The capacitor Cp corresponds, depending on the case, to the equivalent capacitor of the panel seen from the electrode Y or Z, or from the column electrode. For example, in the case of a synchronized-transition mode, the capacitance of the capacitor Cp is equal:  
         [0040]     with the electrode Y, the equivalent capacitance of the capacitors Cp 2  and Cp 1 /2, and  
         [0041]     with the electrode Z, the equivalent capacitance of the capacitors Cp 3  and Cp 1 /2  
         [0042]     For the address phase, the capacitance of the capacitor Cp depends on the images to be displayed.  
         [0043]     Unlike the circuit of the prior art described above, this energy recovery circuit comprises neither switching transistors nor diodes. Moreover, the inductance of the inductive element L 2  is very much greater than that of the inductor L or L′ of the Weber circuit of  FIG. 1 , being 100 to 1000 times greater. Likewise, the capacitance of the capacitive element is very much greater than the capacitance of the capacitor Cp, being at least 10 times greater.  
         [0044]     The operation of the amplifier is described more particularly with reference to  FIG. 5 . It is made up of eight successive phases, numbered  1  to  8  in the figure.  
         [0045]     During phases  1 ,  2 ,  5  and  6 , the switches M 1  and M 2  are open. These phases correspond to transition phases in which there is a transition in the voltage at the output of the amplifier. Phases  1 ,  4 ,  5  and  8  correspond to phases in which energy is stored in the inductor L 2 , and phases  2 ,  3 ,  6  and  7  are phases in which this energy is recovered from the cells of the panel.  
         [0046]     More particularly, the inductor L 2  resonates with the load capacitor C 2  and the panel capacitor Cp during phases  1 ,  2 ,  5  and  6 . Since C 2  is very much greater than Cp, it may be considered that L 2  resonates with Cp. At the end of phases  1  and  5 , the inductor L 2  is charged with a maximum current iL 2   max . It then decreases during the next phase. This current changes direction at the end of phases  3  and  7 .  
         [0047]     During phases  3  and  7 , the switching transistors M 1  and M 2  may either be open or closed. If they are open, their intrinsic diodes then allow the current to flow. The areas shaded grey in the timing diagrams of  FIG. 6  indicate those periods in which it does not matter whether the state of the switching transistors of the amplifier is open or closed.  
         [0048]     With this circuit, the losses are essentially limited to the conduction losses of the current iL 2  and possibly to losses associated with the electrical discharge currents in the gas of the cells, depending on the characteristics and the operating mode of the panel.  
         [0049]     To calculate the values of the components, since it may be considered that the capacitance of the capacitor C 2  is very much greater than that of the panel capacitor Cp, it may be assumed that the fluctuation in the voltage across the terminals of the storage capacitor C 2  is small. The voltage VC 2  across the terminals of the capacitive element C 2  is constant and equal to VS×t 1 /T, t 1 /T defining the duty cycle of the signal, T being the period of the output signal of the amplifier and t 1  being the time interval between two consecutive instants when the voltage across the terminals of L passes through zero, encompassing phases  7  and  8  when the switch M 1  is closed.  
         [0050]     Moreover, during the transitions of duration Δt, it may be considered with a first approximation that iL 2  is constant and equal to iL 2   max ; Δt can then be taken equal to Cp×VS/iL 2   max , where iL 2   max  depends on VS and LZ. More precisely:  
           iL2   max     =     T   ·     VS   L2     ·       x   ⁡     (     1   -   x     )       2         ,       
 
 where x=t 1 /T and iL 2   max  determines the slope of the transitions. 
 
         [0051]     To generate a square signal as shown in  FIG. 5 , t 1 =T/2 and the positive and negative slopes of the current iL 2  are symmetrical.  
         [0052]     To generate a rectangular signal for an operation to address the columns of cells, t 1 ≠T/2, as shown in  FIG. 6 . The current iL 2  is still centred on 0, but the positive and negative slopes of this current are no longer symmetrical. In the example shown in  FIG. 6 , where t 1 &gt;T/2, the negative slope of the current iL 2  is greater, in absolute value, than the positive slope since it is produced with a voltage across the terminals of the inductor equal to VC 2  which is higher than that during the positive slope.  
         [0053]     This amplifier has in particular the following advantages over the Weber amplifier:  
         [0054]     the structure of the amplifier is simplified, as it comprises fewer passive components. This therefore makes it possible to reduce the actual cost of the circuit, to reduce the size of the circuit on silicon, and to improve the reliability thereof; and  
         [0055]     there are few conduction losses during the transitions, since the corresponding current flows only through passive components.  
         [0056]     On the downside, since the rms current iL 2  flowing through the circuit is very high, the conduction losses outside the voltage transition phases are correspondingly higher.  
         [0057]     An improved embodiment, allowing these conduction losses to be reduced, is shown in  FIG. 7 . In this embodiment, the inductive element L 2  comprises two inductors connected in series, namely an inductor L 2   1  of low value and an inductor L 2   2  of high value capable of operating in saturated mode. In saturated mode, an inductor behaves like an air inductor (with no magnetic material). The inductor L 2   2  acts in the present case like an automatic switch. Before saturation, little current flows through it and, after saturation, a high current flows through it. In the rest of the description, L 2  denotes both the inductive element L 2  and the inductance of this inductor.  
         [0058]     In unsaturated mode, the inductor L 2  acts as an inductor with an inductance L 2   2  (L 2   1  being very small compared with L 2   2 ) and in saturated mode as an inductor of inductance L 2   1  (L 2   2  is close to 0). The operation in unsaturated or saturated mode depends on the current iL 2  flowing through L 2 .  
         [0059]     The operation of the amplifier of  FIG. 7  is illustrated with reference to  FIG. 8 . This figure shows the control signals for controlling the transistors M 1  and M 2 , the voltage signal generated by the amplifier, and the current iL 2  flowing through the inductors L 2   1  and L 2   2 .  
         [0060]     The half-period of operation of the current iL 2  is divided into four consecutive operating phases denoted  1  to  4 .  
         [0061]     During phase  1 , the switch M 1  is closed and the switch M 2  is open. The output voltage of the amplifier is equal to VS and the current flowing through the inductor L 2  is controlled by the inductor L 2   2  of higher inductance. Thus, the current flowing through one of the switches of the half-bridge structure is much smaller, which allows the conduction losses to be reduced. The voltage across the terminals of the inductor L 2  is essentially that across the terminals of the inductor L 2   2 .  
         [0062]     At the start of phase  2 , the inductor L 2   2  saturates. The circuit is then controlled by the inductor L 2   1 , and the current iL 2  then increases linearly while the switch M 1  remains closed.  
         [0063]     Phase  3  then starts when the two switches M 1  and M 2  are open. The inductor L 2   1  then resonates with the capacitor Cp, and the output voltage over the amplifier starts to fall along a sinusoid portion. In the middle of phase  3 , the voltage across the terminals of the inductor L 2  is reversed and the current through it decreases. After this phase, the output voltage of the amplifier reaches 0 volts (a reference potential).  
         [0064]     At the start of phase  4 , the current through the inductor L 2  continues to fall linearly, whether the switch M 2  is open or closed owing to its intrinsic diode (the area shaded grey). M 2  must then be closed before the current becomes zero (end of the area shaded grey). After this phase, the inductor L 2   2  is no longer saturated. A phase symmetrical with phase  1  then starts.  
         [0065]     The choice of inductor L 2   2  is of paramount importance. A suitable magnetic material has to be chosen and the number of turns needed has to be calculated. The number of turns of L 2   2  may be defined in the manner below.  
         [0066]     During each operating phase, for example during phase  1  of  FIG. 8 :  
           V     L2   2       =     nAe   ⁢       Δ   ⁢           ⁢   B       Δ   ⁢           ⁢     t   ph1             ,       
 
 in which: 
        A e  is the effective cross section of the magnetic material;     ΔB is the change in magnetic induction during this phase; and     Δt ph1  is the duration of phase  1 .        
 
         [0070]     During this phase, the voltage across the terminals of L 2   2  is equal to VS/2 and the magnetic induction varies between +B sat  and −B sat  (or vice versa), hence:  
               VS   2     =         n   ⁢           ⁢     A   e     ⁢       2   ⁢     B   sat         Δ   ⁢           ⁢     t   ph1           ⁢     
     →   n     =       VS   ⁢           ⁢   Δ   ⁢           ⁢     t   ph1         4   ⁢     B   sat     ⁢     A   e                   (   1   )             
 
         [0071]     B sat  and A e  are variables that depend on the magnetic material employed. The number of turns of the inductor L 2   2  is thus calculated from Equation (1). Having chosen the material, it is necessary to ensure that the magnetization cycle is sufficiently rectangular so that the saturation is not “soft” and that the current iL 2  at the points of saturation is low (in order to reduce the intensity of the rms current). In addition, the area of this cycle must be small in order to avoid hysteresis losses.  
         [0072]     Advantageously, the inductors L 2   1  and L 2   2  are produced in one and the same coil, provided that the number of turns of the coil and the effective cross section of the magnetic material are adjusted accordingly. For example, if the number of turns n calculated as described above is not suitable for the coil L 2   1 , which corresponds to the inductance of the inductor L 2  when in saturated mode, it is possible to add an additional coil in series with L 2 . However, it is also possible to readjust the number of turns n and the cross section A e .  
         [0073]     For example, if the number of turns n calculated for phase  1  is too large for the following phases, it is sufficient to reduce this number and to consequently increase the cross section A e  so that Equation 1 is still satisfied.  
         [0074]     For example, if the number of turns calculated for phase  1  is 10 and if L 2   1  is four times too high for phases  2 ,  3  and  4 , it suffices to half the number of turns n and to double the cross section A e .  
         [0075]     Finally, another sustain amplifier embodiment is proposed in  FIG. 9 . In this embodiment, the circuit (L 2 , C 2 ) is placed in common between the two sustain amplifiers for the electrodes Y and Z of the panel. The circuit (L 2 , C 2 ) is then connected between the mid-points of the half-bridge structures (M 1 , M 2 ) and (M 1 ′, M 2 ′). With this structure, the sustain signals intended for the electrodes Y and Z necessarily have synchronous transitions in phase opposition.