Protection circuit for switching transistors serially connected between a D.C. voltage source

A circuit for selectively switching a plurality of voltage levels to an output terminal comprising 2n semiconductor switches having main current paths series connected between first and second voltage source terminals having a potential difference of 2 nE volts between them, where n is an integer greater than 1, each of said switches having a control electrode responsive to a control signal having first and second states respectively causing the main current path of the switch to be open and closed, each of said switches having an overvoltage rating across the main current path thereof less than 2 nE volts, an output terminal between switches n and (n+1), diode means connecting a terminal between the main current paths of switches k and (k+1) to a predetermined voltage, where k selectively equals 1, 2,-(2n-1) but is not equal to n, the control signals causing the main current paths of n of the switches to be simultaneously open and of the other n switches to be simultaneously closed such that during time interval 1 switches 1-n are closed, during time interval f switches f-(n+f-1) are closed, and during time interval g switches (n+1)-2n are closed, where f is each of the intervals except 1 and g and g is at least 3, the diode means being poled and the predetermined voltages being such that during time intervals 1 and g the voltages at the first and second voltage source terminals are respectively coupled to the output terminal and during time interval f voltages of the same polarity and magnitude are coupled to the output terminal from a common terminal between switches h and (h+1) and from a common terminal between switches (h+n) and (h+1+n) where h is selectively every integer from 1 to (n-1), the voltage magnitude having a value between the value of the voltage difference between the first and second terminals, a separate means for detecting an overvoltage across the main current path of each switch m, where m is selectively every integer from 1 to 2n, and means responsive to the 2n detecting means for deriving an indication of an overvoltage subsisting across one of the switches responsive to the second state of the control signal.

A circuit for selectively switching a plurality of voltage levels to an output terminal comprising 2n semiconductor switches having main current paths series connected between first and second voltage source terminals having a potential difference of 2 nE volts between them, where n is an integer greater than 1, each of said switches having a control electrode responsive to a control signal having first and second states respectively causing the main current path of the switch to be open and closed, each of said switches having an overvoltage rating across the main current path thereof less than 2 nE volts, an output terminal between switches n and (n+1), diode means connecting a terminal between the main current paths of switches k and (k+1) to a predetermined voltage, where k selectively equals 1, 2,-(2n-1) but is not equal to n, the control signals causing the main current paths of n of the switches to be simultaneously open and of the other n switches to be simultaneously closed such that during time interval 1 switches 1-n are closed, during time interval f switches f-(n+f-1) are closed and during time interval g switches (n+1)-2n are closed, where f is each of the intervals except 1 and g and g is at least 3, the diode means being poled and the predetermined voltages being such that during time intervals 1 and g the voltages at the first and second voltage source terminals are respectively coupled to the output terminal and during time interval f voltages of the same polarity and magnitude are coupled to the output terminal from a common terminal between switches h and (h+1) and from a common terminal between switches (h+n) and (h+1+n) where h is selectively every integer from 1 to (n-1), the voltage magnitude having a value between the value of the voltage difference between the first and second terminals, a separate means for detecting an overvoltage across the main current path of switch m, where m is selectively every integer from 1 to 2n, said detecting means for switch m including a series circuit comprising a voltage breakdown means series connected with a current responsive signalling device, the series circuit being in shunt with the main current path of switch m, a terminal applying a bias potential between the breakdown means and the current responsive signalling means;

• the bias voltage, the voltages at the first and second terminals, the predetermined voltage magnitude and polarity, the diode means, the overvoltages, and the breakdown means being such that for the voltage across the main current path of switch m being (a) less than the overvoltage across the main current path of switch m, the breakdown means of detector means m does not break down and current flows through the signalling device of detector means m, and (b) greater than the overvoltage across the main current path of switch m, the breakdown means of detector m breakdowns and current does not flow through the signalling device.