Multi-Level Choppers for High Voltage Applications

SummaryIn the field of High Voltage Power Conversion, the circuit designer is often confromed to a serious problem: there arc no semiconductors capable of sustaining the desired voltage (traction applications for example). The first solution inovolves plain series connection of several switches with synchronous cotrol signals, thus obtaining the equivalent of a high voltage switch. Static and dynamic balancing of the voltage across the switches requires all the semiconductors to switch exactly at the same time: this requires selecting semiconductors with the same turn-on and turn-off times or using control strategies Capable of compensating the difference in switching time. In other respects, the output dV/dt at each commumtion is the sum of the dV/dt genceratd by each switch, which is a serious stress for the environment and especially the firing circuits! The technique presented in this paper tolerates semiconductors switching at different times, thus allowing high voltage conversion with standard dV/dr (Fig. 1.a). More, it will be shown that the control signals can be significantly phase-shifted to improve the harmonic spectrum or the output voltage (Fig. 1 .b).