Abstract:
A PDP driving circuit is comprised of a power recovery unit for recovering an electric power from a capacitive load by resonance operation with PDP which is the capacitive load Cp, and reusing the recovered power. The power recovery unit includes a recovery inductor for resonating with the capacitive load, a recovery switch element for connecting the recovery capacitor to the capacitive load and recovery inductor, and forming a channel for passing resonance current, a counterflow preventive diode for blocking flow of current in the recovery switch element in reverse polarity direction, and a protective diode for forming a closed current channel including the recovery inductor and recovery switch element when the counterflow preventive diode is changed from ON state to OFF state.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a plasma display panel driving circuit, as well as plasma display apparatus, used for wall-mounted television sets and large-screen monitors. 
     2. Related Art 
     An alternating-current surface discharge type plasma display panel (hereinafter called “PDP”) as a typical AC type is constituted by arranging a front plate containing a glass substrate formed by disposing a scan electrode and a sustain electrode which carry out surface discharge and a back plate containing a glass substrate formed by disposing data electrodes oppositely in parallel so that both electrodes set up a matrix and that a discharge space is formed in a gap, and by sealing the perimeter portion with sealing materials such as glass frit. Between both substrates of the front plate and the back plate, discharge cells divided by bulkheads area provided, and in a cell space between these bulkheads, a phosphor layer is formed. In the PDP of such configuration, ultraviolet rays are generated by gas discharge, and with these ultraviolet rays, phosphors of red (R), green (G), and blue (B) colors are excited to emit light, thereby achieving a color display. 
     In such plasma display apparatus, various techniques are proposed for saving the power consumption. 
     For example, a so-called electric power recovery circuit is proposed, that is, in consideration that the PDP is a capacitive load, an inductor is included in component elements of a resonance circuit, and the inductor and the capacity load of the PDP are resonated in LC, the electric power accumulated in the capacity load of the PDP is recovered in a capacitor for power recovery, and the recovered power is reused for driving the PDP (see, for example, patent document 1). 
     In this technique, the electric power recovered from the PDP can be reused in sustain pulse voltage to scanning electrode and sustain electrode in sustain period, and the power consumed in sustain period is saved, so that the power consumption can be reduced. 
     That is, in the sustain period generating circuit, a resonance circuit having an inductor, or an electric power recovery circuit is installed. The electric power accumulated in the capacitive load of the PDP (the capacitive load generated in the scanning electrode) is recovered, and the recovered electric power is reused as driving power of scanning electrode, and the power consumption is saved. In the sustain pulse generating circuit, a power recovery circuit is provided. Hence, the electric power accumulated in the capacitive load of the PDP (capacitive load generated in the sustain electrode) is recovered, and the recovered power is reused as driving power of sustain electrode, and the power consumption is saved. 
     The power recovery circuit recovers and supplies electric power by LC resonance between the capacity load of PDP and recovery inductor by using the recovery inductor of inductance element. When recovering the electric power, the electric power accumulated in the capacitive load generated in the scanning electrode is moved to the recovery capacitor by way of a counterflow preventive diode and a switching element. When supplying the electric power, the electric power accumulated in the recovery capacitor is supplied to the PDP by way of the counterflow preventive diode and switching element. Thus, the scanning electrode of PDP is driven in sustain period. Therefore, in the power recovery circuit, without supply of electric power from the power source in sustain period, the scanning electrode is driven by LC resonance, so that the power consumption is theoretically zero. 
     The above operation of recovery circuit is assumed without consideration of parasitic components of diode or wiring. To be precise, operation of recovery circuit is influenced by various parasitic components, such as parasitic capacity component parallel between drain terminal and source terminal of switching element, or anode terminal and cathode terminal of diode element, and parasitic inductance component in series to the pattern portion wiring between elements. 
     Effects of such parasitic components are serious problems when the diode element is switched from ON to OFF state. In recovery operation, the resonance current flows, but when the counterflow preventive diode is changed from ON to OFF, a reverse current due to parasitic capacity of diode flows (which is called recovery current). 
     By this recovery current, energy is accumulated in the recovery inductor, and when the counterflow preventive diode is completely turned off, the product of inductance value of recovery inductor and time change value of recovery current becomes a surge voltage, which is generated in the recovery inductor terminal. 
     This surge voltage is applied to the counterflow preventive diode, and the withstand voltage of the counterflow preventive diode is required to have a sufficient allowance more than the surge voltage as compared with the actual working voltage. 
     To solve this problem, it is proposed to use a protective diode element in the recovery circuit (see, for example, patent document 2).  FIG. 14  shows its configuration. In the diagram, the recovery circuit includes recovery capacitor Cr, recovery switches Q 3 , Q 4 , counterflow preventive diodes D 3 , D 4 , recovery inductors L 1 , L 2 , and protective diodes D 105 , D 106 . Switching elements Q 1 , Q 2  constitute a sustain circuit for supplying sustain voltage Vsus. To simplify the explanation, in  FIG. 14 , only the portion relating to recovery operation is described out of the configuration of scan circuit and sustain circuit.  FIG. 14  is a circuit diagram for explaining the operation when the sustain circuit is grounded. 
     At the time of generating a surge voltage in the counterflow preventive diode D 3 , the protective diode D 105  conducts, and the energy accumulated in the inductor L 1  is consumed in the channel of inductor L 1  to protective diode D 105  to switching element Q 1 , and thereby generation of surge is suppressed. 
     Similarly at the time of generating a surge voltage in the counterflow preventive diode D 4 , the protective diode D 106  conducts, and the energy accumulated in the inductor L 2  is consumed in the channel of inductor L 2  to switching element Q 2  to protective diode D 106 , and thereby generation of surge is suppressed. 
     Patent document 1: Japanese Patent Publication No. 7-109542 
     Patent document 2: Japanese Patent No. 3369535 
     The above explanation refers to an operation not in consideration of parasitic inductance component of wiring. Actually, as shown in  FIG. 14 , parasitic inductance components L 3  to L 6  present in the wiring between recovery switches Q 3 , Q 4  and counterflow preventive diodes D 3 , D 4 . In the conventional circuit, therefore, surge absorbing effect is not obtained in parasitic inductance components (L 3  to L 6 ). 
     Actually, due to effects of parasitic inductance components, a surge voltage is generated between terminals of counterflow preventive diode, and the required withstand voltage for counterflow preventive diode is raised. Elevation of withstand voltage between terminals leads to increase of semiconductor element loss of recovery circuit such as increase of forward voltage drop and decline of switching speed. To reduce the parasitic inductance, it is desired to increase the thickness and shorten the distance of wiring pattern. 
     In configuration of semiconductor elements, there are various limits from the aspects of substrate area, heat releasing efficiency of cooling plates for fixing semiconductor elements, and others, and it is practically next to impossible to design thick and short wiring pattern while satisfying these limits, and it has been difficult to keep the parasitic inductance always at low level. 
     In the prior art, as described herein, a higher withstand voltage is required in the semiconductor elements of recovery circuit when driving the PDP, and the loss of semiconductor elements is increased, and hence the recovery efficiency drops. Moreover, since the loss of semiconductor elements is increased, a plurality of semiconductor elements must be connected in parallel, which causes to increase the cost and increase the mounting area. 
     The invention is devised in the light of these problems, and it is hence an object thereof to present a PDP driving circuit capable of reducing the mounting area and enhancing the recovery efficiency by lowering the withstand voltage of the counterflow preventive diodes and protective diodes in the power recovery circuit, and thereby curtaining the number of component elements, and a plasma display apparatus using the same. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention provides a driving circuit for driving a plasma display panel which is a capacitive load. The driving circuit includes a pulse generation unit for generating a specified pulse voltage and applying to the capacitive load, and a power recovery unit for recovering an electric power from the capacitive load by resonant operation with capacitive load, and supplying the recovered power to a capacitive load. The power recovery unit has a larger capacity than the capacitive load, and includes a recovery capacitor for accumulating the recovered power, a recovery inductor for resonating with the capacitive load, a recovery switch element for forming a channel of passing current accompanied by resonance of capacitive load and recovery inductor by connecting the recovery capacitor to the capacitive load and recovery inductor, a counterflow preventive diode for blocking the current flowing in the recovery switching element in reverse polarity direction, and a protective diode. The protective diode forms a closed current channel including recovery inductor and recovery switch element when the counterflow preventive diode element is changed from ON to OFF state. 
     A second aspect of the invention provides a plasma display apparatus. The plasma display apparatus includes a plasma display panel having a plurality of scan electrodes and sustain electrodes, and the specified driving circuit for driving the plasma display panel. 
     According to the invention, in the power recovery circuit, when the counterflow preventive diode element is switched from ON state to OFF state by the protective diode, a closed current channel including recovery inductor and recovery switch element is formed, and the voltage applied to the protective diode is reduced, and the withstand voltage of the protective diode can be lowered. As a result, loss of protective diode is decreased, the number of elements connected in parallel can be reduced, and the mounting area can be decreased. 
     Further, the invention also suppresses the surge voltage generated when the counterflow preventive diode is changed from ON state to OFF state, and the withstand voltage of counterflow preventive diode can be lowered, and normal bias voltage drop Vf of counterflow preventive diode is lowered, and the recovery rate of electric power accumulated in the capacitive load of plasma display panel is improved, and the power consumption can be saved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of PDP driving circuit configuration in an embodiment of the invention. 
         FIG. 2  is a perspective view of configuration of plasma display panel (PDP). 
         FIG. 3  is an electrode configuration of PDP. 
         FIG. 4  is a diagram of drive voltage waveform applied to each electrode of PDP. 
         FIG. 5  is a diagram of parasitic inductance of wiring in PDP driving circuit. 
         FIG. 6  is a comparative explanatory diagram of drive waveform and, voltage and current waveform of component elements of PDP driving circuit in an embodiment and PDP driving circuit in a prior art. 
         FIG. 7  is a comparative explanatory diagram of drive waveform and, voltage and current waveform of component elements of PDP driving circuit in an embodiment and PDP driving circuit in a prior art. 
         FIG. 8  is a comparative explanatory diagram of drive waveform and, voltage and current waveform of component elements of PDP driving circuit in an embodiment and PDP driving circuit in a prior art. 
         FIG. 9  is a diagram showing the relation of timing of control signal upon start and voltage of constant voltage power source V 1  in a prior art. 
         FIG. 10  is a diagram showing the relation of timing of control signal upon start and voltage of constant voltage power source V 1  in an embodiment. 
         FIG. 11  is a diagram showing other example of configuration of power recovery circuit. 
         FIG. 12  is a diagram showing another example of configuration of power recovery circuit. 
         FIG. 13  is a block diagram of configuration of plasma display apparatus incorporating a PDP driving circuit in an embodiment. 
         FIG. 14  is a block diagram of configuration of a PDP driving circuit in a prior art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the invention is described specifically below by referring to the accompanying drawings. 
     1. PDP Driving Circuit 
       FIG. 1  is a block diagram of PDP driving circuit in an embodiment of the invention. The PDP driving circuit shown in  FIG. 1  is a circuit for applying a drive voltage to electrodes of a plasma display panel (PDP), and driving the PDP. Prior to specific description of configuration and operation of PDP driving circuit, the configuration and operation of PDP are explained below. In  FIG. 1 , the PDP  10  is shown as a capacitive addition Cp. 
     1.1 Plasma Display Panel (PDP) 
       FIG. 2  is a perspective view that indicates a structure of plasma display panel (PDP) driven by the PDP driving circuit of the embodiment. On a glass front plate  20  which is a first substrate, a plurality of display electrodes are formed in a pair of stripe scan electrode  22  and stripe sustain electrode  23 . To cover the scan electrode  22  and sustain electrode  23 , a dielectric layer  24  is formed, and a protective layer  25  is formed on the dielectric layer  24 . 
     On a back plate  30  which is a second substrate, a plurality of stripe data electrodes  32  covered with a dielectric layer  33  are formed so as to make overhead crossing with scan electrodes  22  and sustain electrodes  23 . A plurality of bulkheads  34  are disposed parallel to the data electrodes  32  on the dielectric layer  33 , and a phosphor layer  35  is formed on the dielectric layer  33  between these bulkheads  34 . The data electrodes  32  are disposed at positions between adjacent bulkheads  34 . 
     The front plate  20  and back plate  30  are disposed facing each other across a microscopic discharge space so that the scan electrodes  22  and sustain electrodes  23  may be orthogonal to the data electrodes  32 , and the perimeter portion is sealed with a sealing material such as glass frit. The discharge space is packed with a mixed gas of, for example, neon (Ne) and xenon (Xe) as discharge gas. The discharge space is partitioned into a plurality of compartments by the bulkheads  34 , and phosphor layers  35  emitting light of each color of red (R), green (G), and blue (B) are sequentially disposed in each compartment. Discharge cells are formed at portions where scan electrodes  22  and sustain electrode  23  intersect with data electrodes  32 , and one pixel is composed of three adjacent discharge cells forming phosphor layers  35  emitting light of each color. A region forming the discharge cells composing the pixel is an image display region, and a surrounding area of image display region is a non-display region not displaying image such as glass frit region. 
       FIG. 3  shows an electrode configuration in the PDP  10 . In the row direction, n rows of scan electrodes SC 1  to SCn (scan electrodes  22  in  FIG. 2 ) and n rows of sustain electrodes SU 1  to SUn (sustain electrodes  23  in  FIG. 2 ) are arrayed alternately, and in the column direction, m columns of data electrodes D 1  to Dm (data electrodes  32  in  FIG. 2 ) are arrayed. Discharge cells Ci,j including a pair of scan electrode SCi and sustain electrode SUi (i=1 to n), and one data electrode Dj (j=1 to m) are formed in the discharge space. The total number of discharge cells C is (m×n) pieces. 
     In the PDP  10  having such configuration, an ultraviolet ray is generated by gas discharge, and phosphors of each color of R, G, and B are excited by the ultraviolet ray, and light is emitted, thereby a color display is produced. The PDP  10  displays in gradation by dividing one field period into a plurality of subfields, combining subfields of colors and driving. Each subfield is composed of reset period, address period, and sustain period. To display image data, a signal waveforms that varies in accord with reset period, address period, and sustain period, respectively, is applied to each electrode. 
     1.1.1 Drive Voltage Waveform of PDP 
       FIG. 4  shows each drive voltage waveform to be applied to each electrode of the PDP  10 . As shown in  FIG. 4 , each subfield has reset period, address period, and sustain period. Each subfield operates almost similarly except that the number of sustain pulses in the sustain period is different in order to vary the weight in light emission period, and the principle of operation is nearly the same in the subfields, and operation of only one subfield is explained below. 
     First, in the reset period, for example, a positive pulse voltage is applied to all scan electrodes SC 1  to SCn, and a necessary wall charge is accumulated on the protective layer  25  and phosphor layer  35  on the dielectric layer  24  covering the scan electrodes SC 1  to SCn and sustain electrodes SU 1  to SUn. In this reset period, priming (detonator for discharge=exciting particle) is generated in order to decrease discharge delay and generate address discharge stably. 
     Specifically, in first half of reset period, the data electrodes D 1  to Dm, and sustain electrodes SU 1  to SUn are held at 0 (V), and in scan electrodes SC 1  to SCn, a slant waveform voltage ascending slowly from voltage Vi 1  below discharge start voltage toward voltage V 12  exceeding discharge start voltage is applied to data electrodes D 1  to Dm. In the ascending process of slant waveform voltage, a first feeble reset discharge occurs between scan electrodes SC 1  to SCn and sustain electrodes SU 1  to SUn, and data electrodes D 1  to Dm. Negative wall voltages are accumulated in the upper parts of scan electrodes SC 1  to SCn, and positive wall voltages are accumulated in the upper parts of data electrodes D 1  to Dm and upper parts of sustain electrodes SU 1  to SUn. Herein, the wall voltages in the upper parts of electrodes refer to voltages generated by wall charges accumulated on the dielectric layer covering the electrodes. 
     In latter half of reset period, sustain electrodes SU 1  to SUn are held at positive voltage Ve, and in scan electrodes SC 1  to SCn, a slant waveform voltage descending slowly from voltage V 13  below discharge start voltage toward voltage V 14  exceeding discharge start voltage is applied to sustain electrodes SU 1  to SUn. In this process, a second feeble reset discharge occurs between scan electrodes SC 1  to SCn and sustain electrodes SU 1  to SUn, and data electrodes D 1  to Dm. Negative wall voltages in the upper parts of scan electrodes SC 1  to SCn, and positive wall voltages in the upper parts of sustain electrodes SU 1  to SUn are weakened, and the positive wall voltages in the upper parts of data electrodes D 1  to Dm are adjusted to values suited to address operation. As a result, reset operation is terminated (hereinafter, the drive voltage waveform applied to each electrode in the reset period is called the “reset waveform.”). 
     Consequently, in address period, scanning is performed by applying negative scanning pulses sequentially to all scan electrodes SC 1  to SCn. While scanning the scan electrodes SC 1  to Scn, positive write pulse voltages are applied to data electrodes D 1  to Dm on the basis of display data. Thus, address discharge occurs between scan electrodes SC 1  to SCn and data electrodes D 1  to Dm, and a wall charge is formed on the surface of protective layer  25  on the scan electrodes SC 1  to SCn. 
     Specifically, in address period, the scan electrodes SC 1  to SCn are once held at voltage Vscn. Next, in address operation of discharge cells Cp, 1  to Cp,m (p being an integer of 1 to n), scan pulse voltage Vad is applied to scan electrode SCp, and positive address pulse voltage Vd is applied to data electrode Dq (Dq being a data electrode selected on the basis of video signal out of D 1  to Dm) corresponding to video signal to be displayed on the p-th row of data electrodes D 1  to Dm. Thus, address discharge occurs at discharge cells Cp,q corresponding to the intersection of data electrode Dq provided with address pulse voltage and scan electrode SCp provided with scan pulse voltage. By this address discharge, positive voltages are accumulated in the upper part of scan electrode CSp of discharge cells Cp,q, and negative voltages are accumulated in the upper part of the sustain electrode SUp, and the address operation is terminated. Hereinafter, similar address operation is executed up to the discharge cells Cn,q on the n-th row, and the address operation is completed. 
     In successive sustain period, for a specific period, a sufficient voltage for sustaining discharge is applied between scan electrodes SC 1  to SCn and sustain electrodes SU 1  to SUn. As a result, a discharge plasma is generated between scan electrodes SC 1  to SCn and sustain electrodes SU 1  to SUn, and the phosphor layer is excited and illuminated for a specific period. At this time, in the discharge space not provided with address pulse voltage in address period, discharge does not occur, and the phosphor layer  35  is not excited or illuminated. 
     Specifically, in sustain period, the scan electrodes SC 1  to SCn are once returned to 0 (V), and then the sustain electrodes SU 1  to SUn are returned to 0 (V) Consequently, positive sustain pulse voltage Vsus is applied to scan electrodes SC 1  to SCn. At this time, the voltage between upper part of scan electrode SCp and upper part of sustain electrode SUp in the discharge cells Cp,q causing address discharge is provided with wall voltages accumulated in the upper part of scan electrode SCp and upper part of sustain electrode SUp in address period, in addition to positive sustain pulse voltage Vsus, and hence becomes larger than discharge start voltage, and a first sustain discharge is generated. In the discharge cells Cp,q causing sustain discharge, negative voltages are accumulated in the upper part of scan electrode SCp so as to cancel the potential difference of scan electrode SCp and sustain electrode SUp upon generation of sustain discharge, while positive voltages are accumulated in the upper part of sustain electrode SUp. In this way, the first sustain discharge is terminated. After first sustain discharge, scan electrodes SC 1  to SCn are once returned to 0 (V), and then Vsus is applied to sustain electrodes SU 1  to SUn. At this time, the voltage between upper part of scan electrode SCp and upper part of sustain electrode Sup in the discharge cells Cp,q causing first sustain discharge is provided with wall voltages accumulated in the upper part of scan electrode SCp and upper part of sustain electrode SUp in first sustain discharge, in addition to positive sustain pulse voltage Vsus, and hence becomes larger than discharge start voltage, and a second sustain discharge is generated. Similarly, thereafter, sustain pulses are alternately applied to scan electrodes SC 1  to SCn and sustain electrodes SU 1  to SUn, and sustain discharge is executed consecutively in the discharge cells Cp,q causing address discharge by the number of times of sustain pulse. 
     1.2 Scan Electrode Driving Circuit and Sustain Electrode Driving Circuit 
     Back to  FIG. 1 , the PDP driving circuit in the embodiment is provided with scan electrode driving circuit  501  and sustain electrode driving circuit  6 . 
     1.2.1 Scan Electrode Driving Circuit 
     The scan electrode driving circuit  501  includes sustain pulse generating circuit  5101 , reset waveform generating circuit  52 , scan pulse generating circuit  53 , and separation switches S 9 , S 10 . 
     (Sustain Pulse Generating Circuit) 
     The sustain pulse generating circuit  5101  has constant voltage power source V 1  for issuing direct-current voltage Vsus, switching elements (sustain switches) Q 1 , Q 2 , and power recovery circuit  50 . The sustain switches Q 1 , Q 2  are composed of generally known elements for performing switching action such as MOSFET. 
     The power recovery circuit  50  has recovery inductors L 1 , L 2 , recovery capacitor Cr, switching elements (recovery switches) Q 3 , Q 4 , counterflow preventive diodes D 3 , D 4 , and protective diodes D 5 , D 6 . The sustain pulse generating circuit  5101  generates sustain pulses to be applied to scan electrodes SC 1  to SCn by on/off operation of switching elements Q 1 , Q 2 , Q 3 , Q 4 . 
     One end of recovery capacitor Cr is connected to the ground. Between other end of recovery capacitor Cr, and connection point of sustain switch Q 1  and sustain switch Q 2 , recovery capacitor Cr, counterflow preventive diode D 3 , recovery switch Q 3 , and recovery inductor L 1  are connected in series. Between other end of recovery capacitor Cr, and connection point of switching element Q 1  and switching element Q 2 , counterflow preventive diode D 4 , recovery switch Q 4 , and recovery inductor L 2  are connected in series. The anode of protective diode D 5  is connected to the connection point of counterflow preventive diode D 3  and recovery switch Q 3 , and its cathode is connected to the constant voltage power source V 1 . The cathode of protective diode D 6  is connected to the connection point of counterflow preventive diode D 4  and recovery switch Q 4 , and its anode is connected to the ground. 
     The power recovery circuit  50  uses recovery inductors L 1 , L 2  which are inductance elements, and recovers and reuses the electric power by LC resonance between capacity load of PDP (capacitive load generated in scan electrodes SC 1  to SCn in  FIG. 3 ) and inductance of recovery inductor L 1  or L 2 . 
     The switching element Q 1  supplies electric power to scan electrodes SC 1  to SCn of the PDP  10  through switching elements S 9  and S 10  from the constant voltage power source V 1 , and clamps the scan electrodes SC 1  to SCn at voltage value Vsus. The switching element Q 2  clamps the scan electrodes SC 1  to SCn at grounding potential by way of switching elements S 9  and S 10 . By these operations, the scan electrodes SC 1  to SCn are driven. 
     (Reset Waveform Generating Circuit) 
     The reset waveform generating circuit  52  includes switching elements S 21 , S 22  composed of generally known elements for making switching action such as MOSFET, constant voltage power source V 2  of voltage value Vset of second power source higher than the potential of the constant voltage power source V 1 , and constant voltage power source V 3  of negative voltage value Vad of third power source. Electric power is supplied to scan electrodes SC 1  to SCn from the constant voltage power source V 2  through switching element S 21 , and electric power of negative potential is supplied to scan electrodes SC 1  to SCn from constant voltage power source V 3  through switching element S 22 , and thereby reset waveform is generated. The switching element S 21  is disposed in a direction of cutting off the current of the body diode flowing in the main discharge channel from the constant voltage power source V 2 , and the switching element  322  is disposed in a direction of cutting off the current of the body diode flowing from the main discharge channel X to the constant voltage power source V 3 . 
     The reset waveform generating circuit  52  generates, in first half of reset period, voltage V 12  exceeding discharge start voltage from voltage Vi 1  below discharge start voltage to data electrodes D 1  to Dm, that is, slant waveform ascending slowly toward Vset, and in second half of reset period, generates voltage V 14  exceeding discharge start voltage from voltage V 13  below discharge start voltage to sustain electrodes SU 1  to SUn, that is, slant voltage slowly descending toward Vad, and applied to scan electrodes SC 1  to SCn. 
     (Scan Pulse Generating Circuit) 
     The scan pulse generating circuit  53  includes switching elements S 31 , S 32  composed of generally known elements for making switching action such as MOSFET, constant voltage power source V 4  of voltage value Vscn, counterflow preventive diode D 31  for preventing flow of current into the constant voltage power source V 4 , capacitor C 31 , and scan IC (IC 31 ) for making switching action, and generates negative scan pulses in address period, and applies sequentially to scan electrodes SC 1  to SCn. The scan IC (IC 31 ) is a circuit for selecting among scan electrodes SC 1  to SCn for applying voltage for address discharge. 
     These switching elements S 1 , S 2 , S 5 , S 6 , S 21 , S 22 , S 31 , S 32 , and scan IC (IC 31 ) are changed over and controlled on the basis of subfield control signal generated in the subfield processing circuit  3 . 
     1.2.2 Sustain Electrode Driving Circuit 
     The sustain electrode driving circuit  6  includes constant voltage power source V 5  for issuing direct-current voltage Vsus, switching elements S 7 , S 8  composed of generally known elements for making switching action such as MOSFET, and power recovery circuit  50   b . The power recovery circuit  50   b  has recovery inductors L 11 , L 12 , recovery capacitor Cr 2 , switching elements Q 31 , Q 41 , counterflow preventive diodes D 31 , D 41 , and protective diodes D 51 , D 61 . Operation of sustain electrode driving circuit  6  is same as that of sustain pulse generating circuit  5101 . The sustain electrode driving circuit  6  cooperates with the sustain pulse generating circuit  5101 , and applies a specified voltage to the PDP  10 . 
     1.2.3 Operation of Power Recovery Circuit 
     Operation of power recovery circuit  50  in the embodiment is explained while referring to  FIG. 5  to  FIG. 8 . 
       FIG. 5  is a circuit diagram adding actually existing parasitic inductance components L 3  to L 6  of wiring to the electric power circuit  50  in  FIG. 1 . Parasitic inductance L 3  shows the sum of parasitic inductance of wiring between counterflow preventive diode D 3  and recovery switch Q 3 , and wiring between recovery switch Q 3  and protective diode D 5 . Parasitic inductance L 4  shows the parasitic inductance of wiring between recovery switch Q 3  and recovery inductor L 1 . Parasitic inductance L 5  shows the parasitic inductance of wiring between recovery inductor L 2  and recovery switch Q 4 . Parasitic inductance L 6  shows the sum of parasitic inductance of wiring between recovery switch Q 4  and counterflow preventive diode D 4 , and wiring between recovery switch Q 4  and protective diode D 6 . 
       FIG. 6  to  FIG. 8  show drive waveform of sustain pulse generating circuit  5101 , and voltage waveform and current waveform of elements in power recovery circuit.  FIG. 6  ( a ),  FIG. 7  ( a ), and  FIG. 8  ( a ) show waveforms corresponding to the conventional power recovery circuit shown in  FIG. 12 , and  FIG. 6  ( b ),  FIG. 7  ( b ), and  FIG. 8  ( b ) show waveforms corresponding to the power recovery circuit of the embodiment shown in  FIG. 1 . 
     In  FIG. 6  and  FIG. 8 , the waveforms represent, sequentially from the top, gate signal of recovery switch Q 3 , gate signal of sustain switch Q 1 , gate signal of recovery switch Q 4 , gate signal of sustain switch Q 2 , voltage V_Cp of PDP  10 , sum iL of current (iL 1 ) flowing in recovery inductor L 1  and current (iL 2 ) flowing in recovery inductor L 2 , voltage across counterflow preventive diode D 3  (reverse bias is positive direction) V_D 3 , voltage across recovery switch Q 3  V_Q 3 , and current iLP flowing in parasitic inductors L 3 , L 4  (or L 5 , L 6 ). 
     In  FIG. 7  ( a ), V_D 105  and V_D 106  respectively show waveforms of voltages across protective diodes D 105  and D 106  in the prior art (reverse bias is positive direction). In  FIG. 7  ( b ), V_D 5  and V_D 6  are respectively voltages across protective diodes D 5  and D 6  (reverse bias is positive direction). 
     Operation of power recovery circuit  50  in the embodiment in the modes of period T 1  to T 4  is explained below in comparison with the prior art. 
     &lt;Mode 1&gt;: Period from T 1  to T 2   
     Voltage V_Cp of PDP  10  is zero, the sustain switch Q 2  is on, and is turned off at timing T 1 , and the recovery switch Q 3  is turned on. As a result, capacity Cp of PDP  10  is charged from the recovery capacitor Cr in the channel of diode D 3  to parasitic inductor L 3  to recovery switch Q 3  to parasitic inductor L 4  and to recovery inductor L 1 . This flowing resonance current iL is inverted after a certain peak and becomes zero, and mode  1  is terminated at this timing (T 2 ). 
     In this period, the voltage across terminals V_D 106  of the conventional protective diode D 106  is applied up to Vsus as shown in  FIG. 8  ( a ), whereas the voltage across terminals V_D 6  of the protective diode D 6  of the embodiment is applied only to Vsus/2 as shown in  FIG. 8  ( b ). This is because the connection point of the conventional protective diode D 106  is the terminal of recovery inductor L 2 , whereas the connection point of the protective diode D 6  of the invention is connected to the connection point of recovery switch Q 4  and counterflow preventive diode D 4 . 
     That is, in this mode, the connection point potential of recovery inductor L 2  and protective diode D 106  in the prior art is Vsus, but the connection point potential of protective diode D 6  of the embodiment is Vsus/2, lower by the portion of Vsus/2 of the voltage across terminals of recovery switch D 4  than Vsus. 
     Therefore, the withstand voltage of the protective diode D 6  of the embodiment can be reduced to half of the conventional protective diode D 106 . It is hence possible to use a protective diode element having a lower forward voltage drop Vf, and heat loss occurring in protective diode can be reduced. Besides, the cost of the protective diode element is lowered. When a plurality of diodes are connected in parallel, since the number of pieces of parallel connection can be decreased, and the substrate mounting area is smaller, the manufacturing cost can be reduced. 
     &lt;Mode 2&gt;: Period from T 2  to T 3   
     Voltage V_Cp of PDP  10  is charged nearly to Vsus, and the sustain switch Q 1  is off, and is turned on at the timing T 2 . At this timing, voltage V_Cp of PDP  10  is fixed at Vsus, but resonance current iL inverted at point T 2  charges the parasitic capacity (not shown) of the counterflow preventive diode D 3 , and turns off the counterflow preventive diode D 3 . 
     At this time, in the voltage waveform V_D 3  across both ends of conventional counterflow preventive diode D 3 , a surge voltage (see broken line region A in  FIG. 7  ( a )) is generated. By contrast, in the voltage waveform V_D 3  across both ends of the counterflow preventive diode D 3  of the power recovery circuit  50  of the embodiment, surge voltage is suppressed (see broken line region A in  FIG. 7  ( b )). The reason is as follows why the surge voltage is suppressed. 
     When the voltage of parasitic capacity of counterflow preventive diode D 3  is charged up to Vsus/2, the protective diode D 5  is in forward bias and conducts, and a loop channel is formed in the sequence of recovery inductor L 1  to parasitic inductor L 4  to recovery switch Q 3  to parasitic inductor L 3  to protective diode D 5  to sustain switch Q 1  to recovery inductor L 1 . Inverting this loop channel, the resonance current iL returns, and the energy accumulated in inductors L 1 , L 3 , L 4  is consumed in the loop channel. As a result, the change (di/dt) of current iLP flowing in parasitic inductors L 3 , L 4  is smaller than in the prior art (see broken line region B in  FIG. 7 ), and thereby generation of surge voltage is suppressed. 
     &lt;Mode 3&gt;: Period from T 3  to T 4   
     Voltage V_Cp of PDP  10  is Vsus, and the sustain switch Q 1  is on, and is turned off at the timing T 3 , and the recovery switch Q 4  is turned on. From capacitive load Cp of PDP  10 , an electric charge is regenerated in recovery capacitor Cr by way of recovery inductor L 2  to parasitic inductor L 5  to recovery switch Q 4  to parasitic inductor L 6  to counterflow preventive diode D 4 . The resonance current iL flowing at this time is inverted at a certain peak, and is inverted again to become zero, and mode  3  is terminated at this timing (T 4 ). 
     In this period, the voltage across terminals V_D 105  of the conventional protective diode D 105  is applied up to Vsus as shown in  FIG. 8  ( a ), whereas the voltage across terminals V_D 5  of the protective diode D 5  of the embodiment is applied only to Vsus/2 as shown in  FIG. 8  ( b ). This is because the connection point of the conventional protective diode D 105  is the terminal of recovery inductor L 1 , whereas the connection point of the protective diode D 5  of the invention is connected to the connection point of recovery switch Q 3  and counterflow preventive diode D 3 . 
     That is, in this mode, the connection point potential of recovery inductor L 1  and protective diode D 105  in the prior art is Vsus, but the connection point potential of protective diode D 5  of the embodiment is Vsus/2, lower by the portion of Vsus/2 of the voltage across terminals of recovery switch D 3  than Vsus. 
     Therefore, the withstand voltage of the protective diode D 5  can be reduced to half of the conventional protective diode D 105 . It is hence possible to use a protective diode element having a lower forward voltage drop Vf, and heat loss occurring in protective diode can be reduced. Besides, the cost of the protective diode element is lowered. When a plurality of diodes are connected in parallel, since the number of pieces of parallel connection can be decreased, and the substrate mounting area is smaller, the manufacturing cost can be reduced. 
     &lt;Mode 4&gt;: Period from T 4  to Next T 1   
     Voltage V_Cp of PDP  10  is discharged nearly to zero (GOD potential), and the sustain switch Q 2  is off, and is turned on at the timing T 4 . At this timing, voltage V_Cp of PDP  10  is fixed at zero (GND potential), but resonance current iL inverted at point T 4  charges the parasitic capacity (not shown) of the counterflow preventive diode D 4 , and turns off the counterflow preventive diode D 4 . 
     At this time, as shown in broken line region A in  FIG. 8 , in the terminal voltage waveform V_D 4  of the conventional counterflow preventive diode D 4 , a surge voltage is generated, but in the terminal voltage waveform V_D 4  of the counterflow preventive diode D 4  of the embodiment, a surge voltage is suppressed. 
     The reason is as follows: when the voltage of parasitic capacity of counterflow preventive diode D 4  is charged up to Vsus/2, the protective diode D 6  is in forward bias, and a loop channel is formed in the sequence of recovery inductor L 2  to sustain switch Q 2  to protective diode D 6  to parasitic inductor L 6  to recovery switch Q 4  to parasitic inductor L 5  to recovery inductor L 2 . Inverting this loop channel, the resonance current iL returns, and the energy accumulated in inductors L 2 , L 5 , L 6  is consumed in the loop channel. As a result, the change (di/dt) of current iLP flowing in parasitic inductors L 6 , L 5  is smaller than in the prior art (see broken line region B in  FIG. 8 ), and thereby generation of surge voltage is suppressed. 
     In the embodiment, as described herein, the withstand voltage of protective diodes D 5 , D 6  of the PDP driving circuit can be reduced to half. Further, since the surge voltage of the counterflow preventive diodes D 3 , D 4  can be suppressed, the withstand voltage may be much lower than in the prior art. 
     In the above explanation, the recovery capacitor Cr is assumed to be charged to about Vsus/2, but it is possible to charge by other method, for example, a method of establishing a circuit and a period for charging the recovery capacitor Cr, in a period before start of recovery operation. 
     Besides, if any extra charging circuit is not provided, when charging by the regenerative power from the PDP  10 , the recovery capacitor Cr can be changed up to Vsus/2 by starting recovery operation while gradually elevating the Vsus voltage (for example, elevating gradually from about Vsus/2). 
     The reason why the start mode is necessary is as follows: the withstand voltage of protective diode depends on the differential portion of Vsus of supply voltage and voltage across terminals of recovery switch, and usually withstand voltage of Vsus/2 is enough, but when the supply voltage is Vsus or when the voltage across terminals of recovery switch is nearly zero, the withstand voltage decreasing effect of protective diode is not obtained. In other words, the starting method is not particularly specified as far as the start mode is controlled so that the difference of supply voltage and voltage across terminals of recovery switch may be Vsus/2 or less. 
     The start mode of the embodiment is more specifically described below. 
     First, the conventional start mode is explained.  FIG. 9  is a diagram explaining the relation of control signal timing and voltage of constant voltage power source V 1  upon start of PDP driving circuit. In the diagram, solid line X denotes the output voltage of constant voltage power source V 1  and solid line Y represents the voltage of recovery capacitor Cr. 
     As shown in  FIG. 9 , the output of constant voltage power source V 1  gradually climbs up after start and reaches specified voltage (Vsus). In the prior art, a drive signal of sustain pulse generating circuit was applied at a later timing (t 1 ) than the timing (t 2 ) of the output of constant voltage power source V 1  reaching the specified voltage (Vsus). In initial state, the voltage of recovery capacitor Cr is zero, but by the subsequent on/off operation of switching elements Q 1  to Q 4 , the recovery capacitor Cr is charged to Vsus/2. At this time, from the moment of output of constant power source V 1  reaching specified voltage (Vsus) (t 2 ), till the moment of voltage of recovery capacitor Cr reaching Vsus/2 (t 4 ), voltage Vsus is applied between both ends of protective diode. This causes a problem in that withstand voltage of protective diode cannot be lowered. 
     This problem can be solved by the following starting operation. 
       FIG. 10  is a diagram explaining the relation of control signal timing and voltage of constant voltage power source V 1  upon start of PDP driving circuit of the embodiment. 
     As stated above, when the PDP driving circuit is started (the power is turned on), the output of constant voltage power source V 1  climbs up gradually and reaches specified voltage (Vsus). At this time, in the embodiment, before the output of constant voltage power source V 1  reaches Vsus/2, that is, at timing (t 1 ) before the output of constant voltage power source V 1  reaches Vsus/2 (t 3 ), driving signal of sustain pulse generating circuit  5101  is applied. 
     When the driving signal of sustain pulse generating circuit  5101  is applied, the switching elements Q 1  to Q 4  start on/off operation at specified timing. 
     In initial state, since the voltage of recovery capacitor Cr is zero, if the switching element Q 3  is turned on, the capacitive load Cp of PDP  10  will not be charged. At the turn-on timing of switching element Q 1 , a potential equal to the voltage of constant voltage power source V 1  is supplied to the capacity load Cp. Then, after turning off the switching elements Q 1 , Q 2 , the switching element Q 4  is turned on. At this time, the voltage accumulated in the capacitive load Cp of PDP  10  is regenerated into the recovery capacitor Cr. This regenerative voltage is determined by the ratio of capacity of capacitive load Cp of PDP and capacity of recovery capacitor Cp. Generally, the capacity of recovery capacitor Cr is about 10 to 30 times of capacity of PDP, and by repeating the turn-on operation of switching element Q 4  by 10 to 30 times, about half voltage (Vsus/2) of voltage of constant voltage power source V 1  can be accumulated in the recovery capacitor Cr. 
     The rise time of constant voltage power source V 1  depends on the output power capacity of power source circuit and capacity of electrolytic capacitor connected to the constant voltage power source V 1 , but generally 500 msec to 1 sec or more is needed. By contrast, the turn-on period of switching element Q 4  is about 5 μsec, and if this recovery operation is repeated 30 times, charging operation of recovery capacitor Cr is completed only in about 200 μsec, and about half voltage (Vsus/2) of constant voltage power source V 1  can be charged in the recovery capacitor Cr. 
     By this starting operation, before the output of the constant voltage power source V 1  reaches the voltage Vsus, the voltage of recovery capacitor Cr reaches Vsus/2. Hence voltage larger than Vsus/2 is not applied between both ends of protective diode. That is, as the difference of voltage of constant voltage power source V 1  and voltage of recovery capacitor Cr is always held below Vsus/2 whether upon start or in stationary operation, the withstand voltage of protective diode can be lowered. 
     1.3 Modified Examples 
     Other example of power recovery circuit is shown in shown in  FIG. 11 . In the power recovery circuit  50  shown in  FIG. 11 , the recovery inductances L 1  and L 2  shown in  FIG. 1  are formed by one recovery inductance L 7 . This configuration is possible only in the condition that the period of mode  1  and period of mode  3  are identical. In the power recovery circuit  50  shown in  FIG. 1 , supposing the resonance current iL flowing in recovery conductor L 1  or L 2  to be corresponding to resonance current flowing in recovery inductance L 7  in power recovery circuit  55 , the operation can be explained same as in the power recovery circuit  50  shown in  FIG. 1 . 
     In this configuration, too, same as in  FIG. 1 , parasitic inductances L 3  to L 6  are present in the wiring of power recovery circuit  50 , and the halving effect of withstand voltage of protective diodes D 5 , D 6  and suppressing effect of surge voltage of counterflow preventive diodes D 3 , D 4  can be obtained. 
     Switching elements Q 1 , Q 2 , Q 3 , Q 4  may be generally known elements for making switching action such as MOSFET. In this case, body diodes are generated in reverse parallel to the switching action portions, and if the switching action is in cut-off state, a forward current for body diodes can be passed. Switching elements Q 1 , Q 2 , Q 3 , Q 4  may be also composed of generally known insulating gate type bipolar transistors (IGBTs) having features of low loss and easy control in high voltage operation. It is considered that a large current of hundreds of amperes flows when driving the PDP  10 . Since parasitic diodes are not generated in the IGBT, if the switching elements Q 1  and Q 2  are IGBTs, diodes corresponding to body diodes parasitically generated in the MOSFET are connected in reverse parallel to the switching elements Q 1 , Q 2 . 
     In the embodiment, types of switching elements are not particularly limited, and the switching elements Q 1 , Q 2  may be composed of IGBTs, and switching elements Q 3 , Q 4  may be composed of MOSFETs, or may be composed of other generally known elements making switching action. 
     In the power recovery circuit  50  shown in  FIG. 1 , the position of recovery inductors L 1 , L 2  may be replaced by the position of recovery switches Q 3 , Q 4  (see  FIG. 12 ). In this case, the anode of protective diode D 5  is connected to the connection point of counterflow preventive diode D 3  and recovery inductor L 1 , and the cathode of protective diode D 6  is connected to the connection point of counterflow preventive diode D 4  and recovery inductor L 2 . 
     1.4 Summary 
     According to the embodiment, in the power recovery circuit of sustain pulse generating circuit  5101 , by connecting the protective diode between the recovery switch and counterflow preventive diode, the withstand voltage of protective diode can be reduced to half. Therefore, loss of protective diode can be decreased, and the number of elements connected in parallel can be reduced. Moreover, since the withstand voltage of counterflow preventive diode can be reduced, and Vf of counterflow preventive diode can be decreased, adn the recovery rate of electric power accumulated in the capacitive load of PDP  10  is improved, and the power consumption can saved. 
     2. Plasma Display Apparatus 
       FIG. 13  is a block diagram of configuration of plasma display apparatus incorporating the PDP driving circuit of the embodiment. 
     The plasma display apparatus shown in  FIG. 13  includes AD converter  1 , video signal processing circuit  2 , subfield processing circuit  3 , data electrode driving circuit  4 , scan electrode driving circuit  5 , sustain electrode driving circuit  6 , and PDP  10 . 
     The AD converter  1  converts an input analog video signal into a digital video signal. The video signal processing circuit  2  converts from video signal of one field into subfield data for controlling each subfield, in order to emit light and display in the PDP  10  by combination of plural subfields different in weight in light emission period from the input digital video signal. 
     The subfield processing circuit  3  generates control signal for data electrode driving circuit, control signal for scan electrode driving circuit, and control signal for sustain electrode driving circuit, from the subfield data generated in the video signal processing circuit  2 , and issues respectively to the data electrode driving circuit  4 , scan electrode driving circuit  5 , and sustain electrode driving circuit  6 . 
     The PDP  10 , as described above, is composed of n rows of scan electrodes SC 1  to SCn (scan electrodes  22  in  FIG. 2 ) and n rows of sustain electrodes SU 1  to SUn (sustain electrodes  23  in  FIG. 2 ) arrayed alternately in the row direction, and m columns of data electrodes D 1  to Dm (data electrodes  32  in  FIG. 2 ) arrayed in the column direction. Discharge cells Ci,j including a pair of scan electrode SCi and sustain electrode SUi (i=1 to n) and one data electrode Dj (j=1 to m) are formed by (m×n) pieces in discharge space, and one pixel is composed of three discharge cells emitting light in red, green and blue colors. 
     The data electrode driving circuit  4  drives data electrodes Dj independently on the basis of the control signal for data electrode driving circuit. 
     The scan electrode driving circuit  501  incorporates a sustain pulse generating circuit  5101  for generating sustain pulses to be applied to scan electrodes SC 1  to SCn in sustain period, and can independently drive the scan electrodes SC 1  to SCn. The scan electrode driving circuit  501  independently drives the scan electrodes SC 1  to SCn on the basis of the control signal for driving the scan electrodes. 
     The sustain electrode driving circuit  6  incorporates a sustain pulse generating circuit  61  for generating sustain pulses to sustain electrodes SU 1  to SUn in sustain period, and can drive all of sustain electrodes SU 1  to SUn in the PDP  10  in batch. The sustain electrode driving circuit  6  drives the sustain electrodes SU 1  to SUn on the basis of control signal for driving the sustain electrodes. 
     INDUSTRIAL APPLICABILITY 
     The invention relates to PDP driving circuit having power recovery circuit and plasma display apparatus, and is particularly useful for PDP driving circuit and plasma display apparatus capable of reducing withstand voltage of diode elements in recovery circuit.