Arc Pulse Width Modulation
Using the Integrated Power Module (IPM), which is a high current solid-state switch, and high voltage nanosecond pulsers (NSPs), EHT developed a solid-state system to initiate, drive, and modulate an arc. By decoupling the high voltage pulse required to initiate breakdown and the high current required to sustain the arc, each component can be controlled and optimized individually.
The EHT NSP (20 kV, 120 W) has been previously used for generating pseudosparks and initiating discharges. Using a 1 pF voltage divider and current monitor, the load voltage and current were measured. The waveform shows the voltage (yellow) reaching a peak before the discharge occurs and current (green) begins to flow. Once current starts flowing, the voltage across the electrodes collapses, and the current is constant for the duration of the pulse. In this example, the current output of the NSP was resistively limited to about 30 A. For higher current applications, the NSP can be combined with the EHT IPM, which can drive higher currents in the arc for longer duration pulses.
The NSP and IPM were integrated into a system that can be connected to the electrode. The circuit diagram is shown above. This figure shows a simplified version of the NSP and IPM without many of their components, but the basic concept is illustrated. Diodes D1 and D3 will need to be constructed. These diodes prevent the NSP from driving the IPM and vice versa.
The arc was initiated with the high voltage of the NSP and sustained with the high current of the IPM. The figure shows the voltage across the solid-state switch in the IPM (yellow) and current in the load (magenta). At a IPM charge voltage of 800 V, 700 A was pulse width modulated (PWM) at 2.2 MHz in the plasma load at 70% duty cycle. In this system, an inductor (~500 nH) was added between the IPM and D3 to minimize the current ripple. This had the additional effect of slowing down the current rise out of the IPM to ~4 μs. Alternatively, the IPM could be triggered before the NSP. No current will flow until the arc is triggered by the EHT NSP. This system could be burst at 100 Hz – 1 kHz.