Saturday, December 24, 2011

Main Power Circuit of PLC High Voltage Pulse Modulator

The PLC-based high-voltage pulse modulator schematic is to obtain a high-voltage pulse, a hard tube of tetrode T, TM-702F, is used to change the DC high voltage. The high-voltage DC unit charges the capacitor of high-voltage C, 0.5 μF/40 kV, throughout resistor R1 and the silicon of high-voltage heap D. The momentary current of charging is restricted by resistor R1, 20–40 kohm. The capacitor discharges throughout the tetrode in series with the load of plasma when the tetrode is activated. Resistor R2 is used to secure the power system adjacent to an unexpected short circuit. An inductance of hollow T is used to limit the current peak caused by the rising rate of steeper the voltage at the starting of the pulse. The grids and cathode of the tube of tetrode are controlled by autonomous circuits. Based on the tetrode cutoff curve offered by the manufacturer in order to guarantee that the tetrode totally closes, the potential on the control grid of tetrode G1 have to be less than −750 V. The potential on control grid G2 is +1700 V in the power system. The potential at the grid G1 is organized by a driving unit of TTL based on PLC. The potential at the control grid G1 raises from −840 to +200 V when the triggering signal takes cause. The tetrode stays turned on as long as the pulse of TTL is present. The voltage and current output are noticed by the current monitor and capacitance divider, correspondingly.

A pulse increase time that is as short as possible is desired to reach uniform PIII. The potential at the control grid G1 must be optimized. This is +200 V in the system, and the increase time of the pulse is around 1 μsec. A bigger G1 potential may guide to harmful oscillation of electrical. The potential at the grid G2 has a minor persuade on the increase time of the pulse, and no clear difference is detected with the G2 potential varying from +1500 - +1900 V. Because of the plasma load and cable capacitance, a long fall time is persuaded after the tetrode off. This will direct to the energy no uniformity of occurrence ions, and surface spiting of pleasured samples may occur. This drop time may last some hundreds of microseconds because of the plasma sheath high resistance. The series solid-state switch or hard tube can be used to discharge the same capacitor quickly. A drop time of less than 1 μsec has been reported. Nevertheless, this technique may build the modulator circuit more complex and take extra hardware cost.

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