Discharge Voltage Droop

Recall from FIG.1 (Electrode Inductance) that each electrode has distributed resistances and inductances.  When the pixels discharge, the discharge current must be provided by the electrodes.  This is important: the plasma discharge LEADS the discharge current.  Relative to the speed of a plasma discharge, the sourcing sustainer and sinking sustainer are far, far away.  As the sustain pulse voltage rises and current flows through the sustain electrodes, the voltage at each pixel rises to the gas breakdown voltage.  The plasma discharge forms very quickly, in less than 50nS.  At the moment the discharges form, the energy recovery inductor current is falling to zero and the sustain output FET's are beginning to turn ON.  With minimal current flow through the electrodes, the electrodes exhibit high impedance, given the large electrode inductance.  Thus, the discharge current is initially sourced through the distributed capacitances Ce which act as bypass capacitors.  Since the capacitances Ce are small relative to the discharge current, there is a large voltage droop which occurs along each electrode.  FIG. 2 illustrates the voltage droop which steadily increases along the electrode from the driving end (SA) to the far end of electrode E1.  FIG. 2 is exemplary of the droop which occurs at the end of electrode E1.  Note that if sustainer SB is held at ground (right side), the left end of electrode E2 would have a pull-out voltage due to the inductances along electrode E2.

FIG. 2 illustrates the current I102 flowing into electrode E1.  Waveform Vp illustrates the voltage at the far end of electrode E1.  During the energy recovery rise between times t1-t3, current I103 flows into electrode E1 and out electrode E2.  Between times t3 and t4, the electrode current becomes slightly negative as the energy recovery current I103 goes to zero.  Also between times t3 and t4, the cells begin discharging.  The zero, negative or small current flow at time t4 is insufficient to source the discharging cells, thus the voltage at the end of electrode E1 droops from the desired sustain voltage Vs to Vdroop despite the electrode begin driven by voltage Vs at the driving end.  I104 is NOT an indirect result of the discharging cells.  Current pulse I104 is produced by the voltage drop across the electrode inductance, i.e. Vs-Vdroop.  Thus I104 increases while the discharge is occurring, and peaks when the voltage at the far end reaches voltage Vs.  However, with current flowing, additional discharge activity occurs as the voltage overshoots.

FIG. 2

Thus, the discharge current flowing into electrode E1 is a delayed current which "re-charges" the distributed capacitances Ce and sources discharge current beyond the capacity of capacitances Ce.  Loading occurs because the plasma discharge is impeded by the voltage droop across the cell.  Efficacy is reduced by the weakened discharge.  

Another viewpoint is that the electrode voltage droop presents a forcing voltage applied to the electrode inductance.  As an inductive response to the discharge, the electrode's discharge current is a steadily increasing current flow through the electrode due to the droop voltage applied to the electrode and the time duration of the voltage droop applied across the length of the electrode.  Thus, the electrode discharge current begins to flow after the discharge begins, and peaks when the voltage at the far end (V2) equals the applied voltage Vs.  Keep in mind that this is the current peak and the energy stored in the electrode inductance must be dissipated before current flow will stop.  Much of this excess energy is consumed by excess discharge activity in cells receiving an over voltage.  Thus, some cells will receive more wall charge than other cells and will therefore appear to have a lower operating voltage.  Most importantly, more power is consumed than is necessary.