Tolerable voltage control circuit

Claim 2

A control circuit for controlling the firing of a switch element in accordance with a voltage impressed thereacross, said circuit comprising an amplifier switchable in response to voltage levels applied thereto, said amplifier being responsive to the voltage across said switch element, means for applying a threshold voltage level to said amplifier determinative of a safe voltage across said switch element to permit firing, means establishing a threshold hystEresis for said amplifier by removably applying an initial threshold voltage level lower than said safe voltage threshold level, and means responsive to said switchable amplifier to inhibit firing of said switch element until the voltage across said switch element reaches a level low enough to cause said switchable amplifier to switch, said amplifier being switched and said initial threshold being removed whenever the voltage across said switch element decreases to said initial threshold level, the threshold level for said amplifier reverting to said safe voltage level and said amplifier being prevented from further switching until the voltage across the switch element increases to said safe voltage threshold level.

Claim 3

A control circuit as recited in claim 1 wherein the means for establishing said threshold hysteresis provides the initial threshold voltage level for said amplifier in response to the output of said amplifier prior to the voltage across the switch element reaching a level low enough to cause said switchable amplifier to switch.

Claim 4

A control circuit as recited in claim 2 wherein said amplifier includes first and second inputs and an output, and said switch element includes first and second power terminals and a firing pulse receiving control terminal, said control circuit further including means responsive to the voltage at one power terminal of said switch element for presenting a voltage proportional thereto to the first input of said amplifier, and means responsive to the voltage at the other power terminal for presenting a voltage proportional thereto to the second input of said amplifier, wherein said means for establishing said safe voltage threshold level is a reference voltage independent of the voltage across said switch element, said reference voltage being applied to the second input of said amplifier, wherein said hysteresis means is a feedback resistor coupling positive feedback to the first input from the output of said amplifier, and said means to inhibit firing prevents firing pulses from being applied to said control terminal until the voltage at the first input of said amplifier becomes less than the voltage at said second input.

Claim 5

A tolerable firing voltage control circuit for controlling the firing of a pair of inverter phase switch elements in accordance with voltages impressed across the elements, said elements being connected in series between the positive and negative input power buses of the inverter, said control circuit comprising: first and second amplifiers switchable in response to voltage levels applied thereto, said first amplifier being responsive to the voltage across the switch element of said pair connected to the positive bus and said second amplifier being responsive to the voltage across the other switch element of said pair, means establishing a threshold voltage level for switching each amplifier at a predetermined maximum tolerable voltage for each switch element, said threshold voltage levels being fixed and independent of the voltage across each switch element, means establishing a threshold hysteresis for each amplifier by providing a voltage threshold level for each amplifier lower than the maximum tolerable level in response to voltages across said switch elements higher than the maximum tolerable level and removing said lower voltage threshold level when the voltage across a switch element decreases below said lower level to switch the amplifier, said maximum tolerable threshold remaining for permitting the switch element to be fired even if the voltage across the switch element rises above the lower voltage threshold level, and gate means responsive to the switching of each amplifier to permit firing of the related switch element so long as the voltage across the element remains below the predetermined maximum tolerable level.

Claim 6

A tolerable firing voltage control circuit for controlling the firing of a pair of inverter phase switch elements in accordance with voltages impressed across the elements, said elements being connected in series between the positive and negative input power buses of the inverter, said control circuit comprising: first and second amplifiers switchable in response to voltage levels applied thereto, said first amplifier being responsive to the voltage across the switch element of said pair connected to the positive bus and said second amplifier being responsive to the voltage across the other switch element of said pair, means establishing a threshold voltage level for switching each amplifier at a predetermined maximum tolerable voltage for each switch element, said threshold voltage levels being fixed and independent of the voltage across each switch element, means establishing a threshold hysteresis for each amplifier by providing a voltage threshold level for each amplifier lower than the maximum tolerable level in response to voltages across said switch elements higher than the maximum tolerable level and removing said lower voltage threshold level when the voltage across a switch element decreases below said lower level to switch the amplifier, said maximum tolerable threshold remaining for permitting the switch element to be fired even if the voltage across the switch element rises above the lower voltage threshold level, and gate means responsive to the switching of each amplifier to permit firing of the related switch element so long as the voltage across the element remains below the predetermined maximum tolerable level.