High voltage transient protection circuit for voltage regulators

An improved shunt voltage regulator of the type possessing an input terminal, an output terminal and a common terminal with a shunt resistor connected across said input and said output terminals, a shunt transistor having an emitter, a collector and a base, said emitter and said collector operatively connected across said output and said common terminals in parallel with a voltage divider having an intermediate voltage tap, said base connected to the output of an error amplifier having one input connected to said intermediate voltage tap and another input connected to a voltage reference, wherein the improvement comprises:

• a first NPN transistor having an emitter, a collector and a base, said emitter and said collector of said first transistor operatively connected across said output terminal and said common terminal;
• and
• means connected between said input terminal and said base of said first transistor for biasing said first transistor into conduction in response to a voltage transient applied at said input terminal whose magnitude exceeds a selected value, whereby said output terminal is protectively coupled to said common terminal during said voltage transient, said means including
• a second NPN type transistor having an emitter, a collector and a base, said second emitter connected to said first base;
• a third PNP type transistor having an emitter, a collector and a base, said third collector connected to said second base forming a first control node, said third base connected to said second collector;
• a first resistor connected between said first control node and said second emitter;
• a second resistor connected between said third emitter forming a second control node thereat and said third base;
• and
• a plurality of zener diodes connected in series, anode to cathode, possessing an end anode terminal, an end cathode terminal and at least one intermediate diode connection terminal, said end anode terminal connected to said input terminal of said shunt voltage regulator, said end cathode terminal connected to said first control node and a selected said at least one intermediate diode connection terminal connected to said second control node.

An improved series pass voltage regulator of the type possessing an input terminal, an output terminal, a common terminal and a series pass transistor having an emitter, a collector and a base, said emitter and said collector operatively connected across said input and said output terminals, a voltage divider having an intermediate voltage tap connected across said output and said common terminals, a current source connected between said input terminal and said base, an error amplifier having an output connected to said base, one input connected to said intermediate voltage tap and another input connected to a voltage reference, wherein the improvement comprises:

• a first NPN transistor having an emitter, a collector and a base, said emitter and said collector of said first transistor operatively connected between said base of said series pass transistor and said common terminal;
• and
• means connected between said input terminal and said base of said first transistor for biasing said first transistor into conduction in response to a voltage transient applied at said input terminal whose magnitude exceeds a selected value, whereby said base of said series pass transistor is protectively coupled to said common terminal during said voltage transient, said means including
• a second NPN type transistor having an emitter, a collector and a base, said second emitter connected to said first base;
• a third PNP type transistor having an emitter, a collector and a base, said third collector connected to said second base forming a first control node, said third base connected to said second collector;
• a first resistor connected between said first control node and said second emitter;
• a second resistor connected between said third emitter forming a second control node thereat and said third base;
• and
• a plurality of zener diodes connected in series, anode to cathode, possessing an end anode terminal, an end cathode terminal and at least one intermediate diode connection terminal, said end anode terminal connected to said input terminal of said series pass voltage regulator, said end cathode connected to said first control node and a selected said at least one intermediate diode connection terminal connected to said second control node.