Switching-off phenomena in an L-C-R network

Voltage transients on power leads, protected by L-C-R filters, due to a switching-on event, were discussed in our previous paper. In this paper the transient effects in L-C-R circuits at switching-off events are investigated, with emphasis on the overvoltage transients developed across the terminals of the switching device, and transients across the power line leads of the output ports of the L-C-R filter. In addition, the gradual transition of the solid state device from “on” to “off”, states and the consequent time dependent behavior of its dynamic impedance are investigated. It is shown that a gradual variation of the switch resistance results in a drastic reduction of the overvoltage amplitude. The analysis demonstrated that an oscillatory process may be developed during the switch-off event, primarily due to the series resonant circuit composed of the series filter inductor and the switch parasitic capacitance, however, under certain circuit conditions, the transient process may be a-periodic. Results of this investigation can be applied for the optimization of L-C-R filter networks, and for the proper selection of switching elements