In the field of High Voltage Power Conversion, the circuit designer is often confronted to a serious problem: there are no semiconductors capable of sustaining the desired voltage (traction applications for example). The first solution involves plain series connection of several switches with synchronous control 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 using control strategies capable of compensating the difference in switching time. In other respects, the output dV/dt at each commutation is the sum of the dV/dt generated 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 conversions with standard dV/dt. More, it will be shown that the control signals can be significantly phase-shifted to improve the harmonic spectrum of the output voltage.
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