Abstract:
A device operating in accordance with the invention receives data respective of an image to be displayed, determines the illumination load requirement for at least one illumination period according to the image data and adjusts the operation of the illumination driver according to the illumination load requirement such that a driving current is maintained between an electrode charging phase and an illumination phase according to the illumination load requirement. The invention seeks to negate the driving electrode inductance between the driving circuit and the load by maintaining an electrical current within the driving electrode between the charging phase and the conductive phase.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to the operation of gas discharge and other breakover conduction elements used within illumination and display devices providing circuits and methods for anticipating a current draw and while applying a pulse, maintaining a current flow through an electrode between charging and conductivity phases to mitigate inductive effects caused by large current flows during the conductivity phase which are impeded by electrode inductance. 
         [0003]    2. Description of the Related Art 
         [0004]    Breakover conduction elements are well known in the field of electronics and include gas discharge devices and solid state devices. A breakover conduction element typically has at least two terminals with a breakover voltage thereacross. In an OFF state, a breakover conduction element has high impedance and exhibits a capacitive characteristic. Upon exceeding a breakover voltage characteristic, the breakover conduction element transitions to a highly conductive low impedance ON state. A breakover conduction element remains conductive until a voltage thereacross is removed and/or a current therethrough is removed; typically both. A gas discharge device, such as a fluorescent lamp or plasma display illumination cell, or an AC diode (DIAC) has two terminals and a predetermined breakover conduction voltage there between. The breakover conduction element within gas discharge device is a dischargeable gas comprising at least one of helium, neon, argon, xenon, krypton, mercury and sodium. 
         [0005]    In a three electrode surface discharge plasma display panel (PDP), each illumination cell has three terminals; two sustain terminals and an addressing terminal. Each terminal is typically coated by a dielectric, and therefore has capacitance wherein wall charges indicative of a memory state are set. The sustain terminal dielectric coatings are exposed to a dischargeable gas which has high impedance and exhibits a capacitive characteristic. Upon exceeding a breakover voltage characteristic, the dischargeable gas becomes conductive and rapidly forms a plasma discharge. As current flows through the plasma discharge, electrical charge is transferred between the dielectric surfaces covering the terminals and molecular and atomic excitation yields the emission of visible and/or ultraviolet light photons. Adjacent to the plasma discharge, a phosphor material may convert ultraviolet photons to visible light. Coating the dielectric surfaces with a protective layer comprising at least one of; MgO, CaO, BaO, SrO or other suitable material aids in lowering the breakover voltage and reducing the gas discharge formation time. As the electrical charge is transferred between dielectric surfaces, the voltage across dischargeable gas falls to zero, the current flow ends and the dischargeable gas returns to a high impedance state. Thus, the breakover conduction element, i.e. the dischargeable gas, has high impedance, transitions to low impedance when the applied voltage thereacross exceeds a predetermined breakover conduction voltage of the dischargeable gas, and, once the charge is transferred, the voltage thereacross and the current therethrough decreases and falls below a predetermined threshold, and transitions to the high impedance state. 
         [0006]    Additionally, some breakover conduction elements, such as a silicon controlled rectifier or a plasma display illumination cell, have a third terminal referred to as a trigger terminal or addressing terminal. In using these devices, a voltage is typically applied across the main terminals while the device is in the OFF state. When sufficient voltage is applied to the third terminal, in reference to one of the main terminals, conductivity may be induced and the element switches into the ON state and behaves as previously described. During addressing operations within a PDP, a voltage is applied across sustain electrodes close to the breakover voltage. Thus, applying a data pulse to selected column electrodes is sufficient to trigger the breakover conduction (i.e. gas discharge) necessary to set ON state wall charges. 
         [0007]    In a large area display device, illumination cells are disposed at intersecting points of row and column driving electrodes. Long electrodes, coupling the illumination cells to a driving circuit, have additional resistive and inductive characteristics as described in U.S. Patent Application 61/476,382, herein incorporated by reference. Large electrode inductances present a problem in that under prior art driving conditions; long, parallel and magnetically coupled driving electrodes exhibit inductance due to pulsed currents flowing in a common direction. As voltage is applied across breakover conduction cells (discharge cell, memory cell, illumination cell, etc.), exceeding the breakover voltage of the dischargeable element, conduction initiates so rapidly that the voltage across the cell terminals drops sharply as instantaneous current flow is impeded by the inductance of the driving electrodes. As the voltage droops, the conduction is reduced. Albeit in a short period of time, the driving electrode current increases relatively slowly to supply the current requirement of the cell or plurality of cells in the ON state. 
         [0008]    In a prior art circuit topology illustrated in  FIG. 1 , a pair of resonant driving circuits produce output waveforms SA and SB to drive respective electrodes of display  140 . Driving signals S 1 -S 4  operate switches S 1 -S 4  respectively of resonant driving circuit  120  to produce output waveform SA under a zero load condition. Under this condition, resonant current pulse  101  flows through resonant inductor Ler, through the coupling capacitance Ce of display  140  and is returned through the opposing resonant driving circuit  130 . Resonant driving circuit  130  is held a constant potential during the operation shown. The value of inductance Ler is chosen to limit the rise time of output SA between times t 1 -t 3  and determines the amplitude of current pulse  102 . During operation, S 1  closes at time t 1  to apply voltage Ver to inductance Ler and current I 101  (and therefore I 102 ) begins increasing. At time t 2 , the voltage of output SA equals voltage Ver and current I 101  (and therefore I 102 ) peaks at this moment. Between times t 2 -t 3  the voltage of output SA increases to voltage Vr as the current flow diminishes, reaching zero at time t 3 . At time t 3 , a small reverse current (not shown) is induced by the output voltage SA being greater than voltage Ver. This reverse current is momentary and limited as diode D 1  becomes reversed biased. Also at time t 3 , switch S 3  is closed to apply the voltage Vs to output SA, producing the small current pulse subsequent to time t 3  shown on waveform I 102  thus completing the resonant charging phase. Without any pixels being illuminated, there is no conductive phase following the application of voltage Vs. 
         [0009]      FIG. 2  shows a circuit model for a surface discharge AC plasma display wherein row electrodes E 1  and E 2  form a row of pixels. Each electrode has distributed resistances Re and distributed inductances Le. Electrode E 1  is driven from the left side, and E 2  is driven from the right side. As voltage is applied to electrode E 1 , the current flow is distributed as currents I 1  and I 2  returns through electrode E 2 . The effective inductance is thus the sum of inductances Le and the effective resistance is the sum of resistances Re. Along the row of pixels, electrodes E 1  and E 2  are coupled by distributed capacitances Ce. Address electrodes and barrier ribs (not shown), orthogonal to row electrode pairs, divide each row into a plurality of addressable discharge cells. Each discharge cell  205  comprises a dielectric barrier covering electrode E 1 , the dischargeable gas, and a dielectric barrier covering electrode E 2  to form a capacitance in series with a breakover conduction element. As an electrical element  205 , the dischargeable gas has a breakover conduction characteristic. Once a breakover voltage is reached, the device becomes highly conductive and will remain conductive until its current flow falls below a threshold. With the dielectric barrier covering the electrodes, wall charges build up on the dielectric surfaces as the voltage across the gas diminishes. Once the dielectric surface is charged, the discharge self-extinguishes. The wall charge is indicative of memory state. Thus, electrical element  205  is a memory based breakover illumination cell. 
         [0010]      FIG. 3  shows an illustration of the electrode current I 102  in response to applying voltage SA from  FIG. 1  to electrode E 1  and the voltage Vp which results at the terminating (far right) end of electrode E 1 . During the electrode charging phase, times t 1 -t 3 , voltage Vp rises as current pulse I 103  of current I 102  flows into electrode E 1 , charges the distributed capacitances Ce from  0 V to Vs, and exits through electrode E 2 . At time t 3 , the voltage across the dischargeable element (gas or DIAC) reaches the breakover conduction point, and the illumination discharge is triggered. At time, t 3 , the resonant driver current, I 102  is reaching zero and momentarily reverses current between times t 3  and t 4  as the output switch S 3  closes to apply voltage Vs. While the illumination discharge forms between time t 3  and t 4 , the distributed electrode inductances Le exhibit high impedance due to the negligible, or reverse, current flow. Thus, conductivity phase current must be drawn locally through the distributed capacitances Ce and the voltage Vp at the end of electrode E 1 , droops to voltage Vdroop. The voltage difference across the electorde, i.e. Vs-Vdroop, applies a forcing voltage to the electrode&#39;s distributed inductances Le, and current pulse I 104  begins to flow proportional to the droop voltage Vdroop, time and the electrode inductance (i.e. VT/L). Thus, the voltage Vs-Vdroop over the time t 3 -t 5  induces current pulse I 104 . The electrode inductances and resistances limit the discharge current I 104  between times t 4  and t 5 . It is not until time t 5 , after the discharge completes, that the electrode voltage reaches, and often overshoots, the applied voltage Vs. 
         [0011]    The substantial voltage drops along electrodes E 1  and E 2  reduce the current peak I 104 , slowing the discharge at each pixel. In a gas discharge device, the efficacy of a gas discharge is reduced by the impeded current flow, the brightness is reduced, and the brightness becomes non-uniform across the gas discharge device&#39;s illumination area. Thus, there is a need for reducing inductive effects presented by the current requirements of breakover conduction elements. 
       SUMMARY OF THE INVENTION 
       [0012]    The invention contained herein provides circuits and operating methods that address the aforementioned problems.  FIG. 4  illustrates the driving method of the invention. In a first step, a display operating in accordance with the invention receives data respective of an image to be displayed. In a second step, the display determines the illumination load requirement for at least one illumination period according to the image data. In a third step, the display modulates timing and/or voltage to adjust the operation of the illumination driver according to the illumination load requirement such that a driving current is maintained between an electrode charging phase and an illumination phase according to the illumination load requirement. The invention provides driving circuits and methods for driving a data dependent load which, during the application of a voltage pulse, may be characterized as having; a capacitive characteristic during a charging phase and a conductive characteristic after exceeding a breakover conduction voltage. The invention seeks to negate the driving electrode inductance between the driving circuit and the load by maintaining an electrical current within the driving electrode between the charging phase and the conductive phase. 
         [0013]    As is well known in the art, PDPs are operated using a subfield driving method wherein an image frame is divided into brightness weighted illumination periods. In other display technologies, a field sequential driving method divides an image frame into color specific illumination periods. Regardless of the driving method, each illumination period has an illumination requirement based upon number of light emitting elements being illuminated such that the accumulated illumination of all the illumination periods within a frame time corresponds to the desired image. Emissive technologies such as PDPs and LEDs and OLEDs arrays, have a current requirement based upon the illumination requirement and the area of the light emitting element. 
         [0014]    For a memory based illumination technology, such as a PDP, a subfield contains at least, an addressing period and an illumination period. During the addressing period, each row electrode coupled to a plurality of cells is selected, and wall charges are set (ON or OFF) indicative of each cell&#39;s illumination requirement for the respective subfield. During the illumination period, only cells bearing wall charges are illuminated by illumination pulses. Since the illumination power is proportional to the number of cells being illuminated, the illumination load requirement for each subfield is determined by accumulating the number of cells to be illuminated and thus a loading ratio or value for each subfield may be anticipated and the operation of the driving circuit may be modulated according to the anticipated current draw. Thus, the conductivity phase current can be induced before the conductivity phase current draw begins, allowing full conductivity to occur sooner. 
         [0015]    According to the invention, a controller anticipates an illumination load as the image data is received and arranged into subfield data. Subsequently, the illumination load value is utilized within respective illumination periods to alter the operation of the driver circuit; either by controlling voltages, timing or both. The driving circuit applies a current pulse for charging the display&#39;s electrode capacitance to the breakover conduction voltage with excess energy such that a current flow can be maintained between the charging phase and the conduction phase according to the anticipated current draw, with greater energy than is required to charge the electrode capacitance to the desired operating voltage. The excess energy (i.e. current) is thus available to minimize the initial voltage droop of the conduction phase, while not overshooting the desired operating voltage at the completion of applying the pulse. 
         [0016]    Although the invention is widely applicable, the description contained herein presents embodiments of the invention described in reference to multi-electrode dielectric barrier discharge devices used for illumination and addressable matrix gas discharge devices, such as PDPs. Large area PDP&#39;s benefit from the methods contained herein due to their large electrode capacitance, high discharge current, variable load and large electrode inductance. 
         [0017]    In a first embodiment of the invention, a controller anticipates an expected current draw and, according to the expected load, modulates the output switch timing of the resonant driving circuit topology of  FIG. 1 . Using an illumination load value derived from the received image data, a timing controller advances the turn-on timing of the driving circuit&#39;s output switch S 3 . During the resonant electrode charging phase, the circuit applies a voltage to the resonant inductance Ler to induce the prior art charging current  101 . Under minimal load, the closing of switch S 3  occurs at approximately the same time as current  101  returns to zero, as in the prior art. According to the invention, as the illumination load increases, the timing of output switch S 3  is advanced to apply the output voltage Vs to maintain a current flow between an electrode charging phase and an illumination phase of each pulse. Thus, illumination along the electrode occurs while a current flow is maintained within the electrode, according to the expected load such that the application of excess energy is prevented. 
         [0018]    In a second embodiment of the invention, the functionality of the resonant driving circuit topology of  FIG. 1  is augmented with a circuit comprising an additional voltage source coupled to a switch and to a second inductor. Utilizing the illumination load value to modulate the voltage source and/or the switch timing, supplemental energy is applied during the charging phase to be consumed by the load during the load&#39;s conduction phase. 
         [0019]    A third embodiment applies the invention to a multi-phase resonant driving circuit, wherein the aforementioned second embodiment may be employed. In this embodiment, the charging phase comprises two concurrent charging phases and two concurrent conduction phases wherein reciprocal current flows produce canceling magnetic fields to reduce the electrode inductance. The lowered inductance enables higher and faster currents during the conductivity phase. In a PDP, these improvements exhibit increased brightness and efficacy. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  illustrates a prior art resonant driving circuit 
           [0021]      FIG. 2  illustrates an electrode model for a surface discharge PDP. 
           [0022]      FIG. 3  illustrates the voltage droop of prior art driving methods 
           [0023]      FIG. 4  illustrates a flow chart describing the method of the invention 
           [0024]      FIG. 5  illustrates maintaining an electrode current between a charging phase and a conduction phase. 
           [0025]      FIG. 6  illustrates an operation of the invention on the prior art topology of  FIG. 1  using time modulation according to an anticipated loading level. 
           [0026]      FIG. 7  illustrates a resonant driving circuit according to the invention. 
           [0027]      FIG. 8  illustrates a second resonant driving circuit according to the invention. 
           [0028]      FIG. 9  illustrates a third resonant driving circuit according to the invention. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0029]      FIG. 5  illustrates operation of the electrode structure from  FIG. 2 , according to the methods of the invention. Over the time period t 1  to t 5 , a current pulse I 102  is applied to electrode E 1  comprising charging component I 101  and a conduction component I 530 . Waveform Vp illustrates the voltage at the end of electrode E 1 . In combination with ON state wall charges, voltage Vs is greater than the breakover voltage Vbr of cell  205  and represents the desired output voltage. At a time t 1 , the resonant driving circuit  120  (S 1 , D 1 , Ler) applies the first current pulse portion I 101  to charge the distributed electrode capacitances Ce during time period t 1 -t 3  from zero volts to the sustain voltage Vs. The driving circuit provides an anticipated current pulse portion  1530  through a portion of the charging phase t 1 -t 3  and, into the conduction phase t 3 -t 5 , such that current flow is maintained (designated as I 101 +I 530 ) and sufficient to supply at least a portion of the accumulated current draw I 104 . Note that during the time t 2 -t 4 , the slope of current pulse I 102  is flat, i.e. the di/dt=0, substantially eliminating the inductive voltage drop component of Vdroop and therefore, the droop voltage Vdroop becomes predominantly resistive. Note that the amplitude of the conductivity phase current pulse I 104  is determined by the image data and proportional to the number of cells being illuminated. Thus, current pulse I 530  is to be controlled according to an illumination load value derived from the image data. 
         [0030]      FIG. 6  illustrates operating the topology of  FIG. 1  according to the invention under a variety of load conditions. Specifically, output SA waveforms are shown for proportions of 0% (all pixels off), 33%, 66% and 100% (all pixels illuminated) of cells being discharged. Current waveform I 102  illustrates the overlap of the charging phase current pulse I 101  and a combined pull-up and conductivity phase component I 104  for each loading condition (i.e. the plurality I 104 s) I0%, I330%, I66%, and I100% for each respective loading condition. Times t 1 -t 4  illustrate the timing of closing switch S 3 , with closure occurring at t 4 , t 3 , t 2  and t 1  for loading conditions 0%, 33%, 66% and 100% respectively. As can be seen in the illustration, as the closure of switch S 3  is advanced (made earlier) from time t 4  to t 1 , the output voltage SA is pulled to sustain voltage Vs up by switch S 3  at earlier times, while the resonant charging current pulse I 101  is substantially maintained. Referring to the current pulse waveform I 102  and top SA waveform driving a 0% load, the turn-on timing of  FIG. 1  switch S 3  occurs at time t 4  (same as time t 3  of  FIG. 1 ). Thus under 0% load, the current I 101  provides the substantial charging of output SA to voltage Vr and current I10% is supplied by switch S 1  to pull output SA up to voltage Vs. As the turn-on timing of switch S 3  advances (earlier) from t 4  to t 1 , output SA pulls up to voltage Vs earlier and the corresponding pull-up currents I33%, I66% and I100% increase. Referring to the current pulse waveform I 102  and SA waveform at 100% load, the cumulative current I 102  varies with time, reaching a resonant charging current I 101  peak at time t 1  coincidentally concurrent with the start of the 100% load pull-up current I104I100% supplied by switch S 3 . It should be noted that the waveforms shown are at the driving end of the electrodes receiving the signal. Fast turn-on of switch S 3  applies voltage Vs to the accumulated electrode inductances Le and thus applying a forcing voltage to the electrode inductance and thus the current I 104  increases to a peak and is maintained by the electrode inductances time that output SA reaches voltage Vs. Consequently, as the voltage across the illumination cell is driven above the breakover voltage of the dischargeable gas, the high speed breakover currents (i.e. gas discharge currents) flow freely, minimizing the voltage droop at the cells being discharged. At time t 2 , the electrode voltage SA equals supply voltage Vs, and the charging phase is concluded. Subsequently, between times t 2 -t 5 , the voltage across the illumination cells is driven above the breakover conduction voltage and current flows through the illumination cells as the illumination occurs. As the illumination cell capacitance is charged, the current decrease to zero. Thus, the current flow I 102  within the device&#39;s electrodes is maintained between the charging phase and the conductivity phase. 
         [0031]      FIG. 7  illustrates a second embodiment of the invention, wherein a controller  710  receives an input signal to modulate the timing of a supplemental switching circuit  730  and/or the voltage level of voltage source Vs 1 .  FIG. 7  illustrates a circuit and driving method wherein switch S 5 , diode D 3  and inductor L 2  are added in parallel to the existing energy recovery circuit  720  to provide additional current between the charging and conducting phases. 
         [0032]    During the application of a rising transition, S 1  is closed to begin the resonant charging phase. Switch S 5  may be closed thereafter in response to the anticipated current draw as was described in  FIG. 6  relative to switch S 3 . The voltage of voltage source Vs 1 , the timing of switch S 5  and the inductance of inductor L 2  may all be predetermined to provide zero or minimal additive current during a 0% load condition, to provide substantial additive current for the 100% load condition and proportional current sourcing therebetween. If additional current remains following the discharge, any residual current will be channeled through S 3 , back to the supply Vs. In a preferred embodiment of the invention, voltage Vs 1  greater than voltage Ver but less than voltage Vs, so that the current flow through inductor L 2  diminishes to zero prior to the falling transition of the output SA. As shown in  FIG. 7 , using the closing times of switches S 1  and S 3  for reference, under zero or minimal load, switch S 5  may be closed at a time t 2  to provide a small current IL 2  during the application of pulse SA. For increased load, switch S 5  may be closed earlier, up to time t 1  to source additional current through inductor L 2 . The current IL 2  is proportional to the amplitude of voltage across inductor L 2  and the length of time a positive voltage (relative to the instantaneous electrode voltage) is applied there across. Thus, as the voltage SA increases and the load transitions into its conductivity phase, the current being sourced by inductance L 2  is conducted by the breakover conduction and any additional current requirement may be sourced through switch S 3 . 
         [0033]    While  FIG. 7  illustrates the second terminal of inductor L 2  connected to the output SA, similar operation may be attained by connecting the second terminal of inductor L 2  to the node where the first terminal of inductor Ler connects to diode D 1 . 
         [0034]    In another application of the embodiment, the turn-on timing of switch S 5  may be fixed, and optionally coincident with the turn-on timing switch S 1 , while the voltage VS 1  is modulated between voltage Ver and Vs dependent upon the illumination load value load. 
         [0035]    In another application of the embodiment, the operation of switch S 5  may applied to the, with the cathode of diode D 3  connected to switch S 5  and the voltage VS 1  set to a voltage relative to ground. 
         [0036]    In a third embodiment shown in  FIG. 8 , voltage Vr, switch S 6 , diode D 4  and inductor L 3  may be operated in like fashion during falling transitions of output SA for displays having an illumination current flow on both rising and falling sustain pulse transitions. 
         [0037]    In a fourth embodiment of the invention shown in  FIG. 9 , the invention is applied to a driving circuit wherein an energy recovery circuit  935  transfers capacitive energy between outputs SA and SB. 
         [0038]    It should be noted that these embodiments may easily be applied to other common technologies such as opposed discharge, tubular, spherical, multi-electrode and other illumination and display technologies wherein a current draw occurs subsequent to applying a voltage. 
         [0039]    It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.