Resonant Charging Circuit for a Semiconductor-Based Marx Generator for an Electroporation Device

Electroporation devices for the treatment of plant material in a continuous flow may employ a Marx circuit featuring parallel charging and a complete discharge of the capacitors during each pulse. Thereby, the use of semiconductor switches rather than spark gap switches allows for a modern design without the need of recurring maintenance of the spark gap switches. However, the use of semiconductor switches demands for an adaptation of the circuit to the properties of these switches being able to operate at lower voltage and current but higher pulse repetition rate compared to spark gap switches. For such a design, resonant charging turned out to be of advantage because it combines a fast energy transfer from a DC-link capacitor as part of the power supply to the stage capacitors of a Marx generator with a charging circuit having no active switches. Moreover, it enables an increase in the charging voltage by a factor of approximately two with respect to the DC-link voltage. An 8-stage Marx circuit with a stage voltage of 1 kV has been set up to study resonant charging. Its charging path has been equipped with current-compensated chokes having a low inductance during charging and a high inductance for transient insulation of the stages during the pulse generation. The charging circuit has been designed for a peak current of 70 A and a charging time of 800
$\mu \mathrm{s}$
. A separate inductance between the DC-link capacitor of the power supply and the generator serves as the inductive component for the resonance circuit. As pulse switches IGBTs have been employed. In the course of the charging process, they serve as opening switches allowing for fine-tuning of the charging voltage in repetitive operation. The generator has been operated either with a ground connection at its negative output terminal or grounded at its center to deliver a ground-symmetric output voltage. In both cases, the magnetic energy stored inside the current-compensated chokes has been degraded to prevent the cores from saturation. In the latter case, measurements revealed oscillations in a resonant circuit comprising the stray capacitance of the current-compensated chokes. A series diode avoids these oscillations. The paper describes selected details of the circuit design and presents measurements of the charging process.