Transient Voltage Sharing in Series Coupled High Voltage Switches

For switching voltages in excess of the maximum blocking voltage of a switching elment (for example, thyristor, MOSFET or bipolar transistor), such elements are often coupled in series and additional circuitry has to be provided to ensure equal voltage sharing Between each such series element and system ground there exist a certain parasitic capacitance which may draw a significant current during high speed voltage transients. The "open" switch is modelled as a ladder network. Analysis reveals an exponential progression in the distribution of the applied voltage across the elements. Overstressing thus occurs in some of the elements at levels of the total voltage which are significantly below the design value. This difficulty is overcome by grading the voltage sharing circuitry, coupled in parallel with each element, in a prescribed manner, as presented here.