Protection circuit

Claim 2

A protective circuit for protecting an output transistor of an amplifier driving an inductive load, said output transistor having a collector electrode connected to a terminal for a source of operating voltage, an emitter electrode, and a base electrode receptive of an input signal, said amplifier further including a first output terminal coupled to the emitter electrode of said output transistor, and a second output terminal, said inductive load being connected between these terminals, comprising: limiting means coupled between the emitter and base electrodes of said output transistor, responsive to power dissipation of greater than a given value of said output transistor, for limiting the amplitude of the input signal coupled to the base electrode of said output transistor, and for providing a control signal at such times, said limiting of the amplitude of said input signal, when it occurs, resulting in a reduction in current flow to said inductive load and a corresponding relatively high-level voltage spike produced by said inductive load which can, if sufficient current is still passing through said output transistor, damage said transistor; and circuit means responsive to said control signal for suppressing the production of said voltage spike, said means comprising means for placing a low impedance between said output terminals.

Claim 3

A protective circuit as set forth in claim 1, wherein said circuit means comprises a gate controlled rectifier having a gate electrode and a conduction path, said conduction path being connected between said two output terminals, and said gate electrode being connected to respond to said control signal.

Claim 4

A protective circuit as set forth in claim 2, wherein said limiting means comprises a current path connected between said base electrode and said first output teminal, and means responsive to said power dissipation of greater than a given value for switching said path from a high to a low impedance condition, for diverting the input signal from said base electrode to said current path, and wherein said protective circuit includes means responsive to said flow of current through said current path for producing said control signal.

Claim 5

A protective circuit as set forth in claim 3, wherein said circuit means further includes: a first switching transistor having a base electrode receptive of said control signal, for switching said first switching transistor into saturation, an emitter electrode connected to said first output terminal, and a collector electrode coupled to said operating voltage terminal; and a second switching transistor having a base electrode coupled to the collector electrode of said first switching transistor, the saturation of said first switching transistor causing said second switching transistor to in turn saturate, an emitter electrode coupled to said operating voltage terminal, and a collector electrode coupled to said gate electrode of said gate controlled rectifier for supplying a turn-on signal at said gate electrode, when said second transistor is switched into saturation.

Claim 6

A circuit for protecting an output transistor of an amplifier for driving an inductive load, said transistor having collector, emitter, and base electrodes, comprising: a terminal for a source of voltage connected to said collector electrode; first and second output terminals across which said inductive load is connected, the first coupled to said emitter electrode, and the second to a point of reference potential, respectively; protective means responsive to the current flowing through the collector-to-emitter path of said output transistor, for placing a low impedance path between said base electrode and said first output terminal, for limiting the power dissipation of said output transistor to below a predetermined level, whereby, when said protective means is activated said current is immediately reduced, resulting in inductive kickback from said inductor, which manifests itself as a voltage spike having a level at least as great as said source of voltage, which may, if of sufficient duration, damage said transistor; and means for suppressing the production of said voltage spike, said suppression means comprising means responsive to the operation of said first protective means for providing a low impedance current path between said two output terminals.

Claim 7

A circuit for protecting an output transistor of an amplifier for driving an inductive load, said transistor having collector, emitter, and base electrodes, comprising: a terminal for a source of voltage connected to said collector electrode; first and second output terminals across which said inductive load is connected, the first coupled to said emitter electrode, and the second to a point of reference potential, respectively; protective means responsive to the current flowing through the collector-to-emitter path of said output transistor, for placing a low impedance path between said base electrode and said first output terminal, for limiting the power dissipation of said output transistor to below a predetermined level, whereby, when said protective means is activated said current is immediately reduced, resulting in inductive kickback from said inductor, which manifests itself as a voltage spike having a level at least as great as said source of voltage, which may, if of sufficient duration, damage said transistor; and means for suppressing the production of said voltage spike, said suppression means comprising means responsive to the operation of said first protective means for providing a low impedance current path between said two output terminals.

Claim 8

The circuit of claim 5, wherein said protective means includes; a first transistor having a collector electrode coupled to the base electrode of said output transistor, an emitter electrode coupled to said first output terminal, and a base electrode; means coupled between said emitter electrode of said first transistor and point of reference potential, for providing a turn-on signal at the base of said first transistor, whenever the power dissipation of said output transistor reaches a predetermined level.

Claim 9

The circuit of claim 6, wherein said suppression means includes; switching means responsive to the turn-on of said first transistor, for providing a control signal; and a gate controlled rectifier having a gate electrode responsive to said control signal, and anode and cathode electrodes coupled between said first output terminal and reference potential.

Claim 10

The circuit of claim 7, wherein said switching means includes; a second transistor having an emitter electrode connected to said first output terminal, a collector electrode coupled to said voltage source terminal, and a base electrode coupled to the emitter electrode of said first transistor, said second transistor saturating in response to turn-on of said first transistor; a third transistor having an emitter electrode coupled to said voltage source terminal, a base electrode coupled to the collector of said second transistor, and a collector electrode, said third transistor saturating in response to said second transistor saturating; and voltage divider means connected between said collector electrode of said third transistor and said first output terminal, for providing said control signal to the gate electrode of said gate controlled rectifier.

Claim 11

In a circuit for protecting an output transistor of an amplifier for driving an inductive load, said output transistor having collector, emitter, and base electrodes, said circuit including a terminal for a source of voltage at said collector electrode, a first output terminal coupled to said emitter electrode, a second output terminal coupled to a source of reference potential, said inductive load being connected across said first and second output terminals, said base electrode being receptive of an input signal, and limiting means for limiting the power dissipation of said output transistor to a predetermined level, said limiting means being coupled between said base electrode and said first output terminal and responsive to the current flowing through the collector-to-emitter path of said output transistor, for providing a low impedance current path between said base electrode and said first output terminal to shunt input signals away from said base electrode, whenever said current exceeds a given value, whereby, when said limiting means is activated, the result is a substantially instantaneous reduction in current from said transistor to said inductor, further resulting in inductive kickback from said inductor producing a voltage spike, which if of high enough level and time duration, may damage said transistor, the improvement comprises; means for providing a control signal in response to operation of said limiting means; and means for suppressing said voltage spike comprising means for placing a low impedance current path between said two output terminals in response to said control signal.

Claim 12

In a circuit for protecting an output transistor of an amplifier for driving an inductive load, said output transistor having collector, emitter, and base electrodes, said circuit including a terminal for a source of voltage at said collector electrode, a first output terminal coupled to said emitter electrode, a second output terminal coupled to a source of reference potential, said inductive load being connected across said first and second output terminals, said base electrode being receptive of an input signal, and limiting means for limiting the power dissipation of said output transistor to a predetermined level, said limiting means being coupled between said base electrode and said first output terminal and responsive to the current flowing through the collector-to-emitter path of said output transistor, for providing a low impedance current path between said base electrode and said first output terminal to shunt input signals away from said base electrode, whenever said current exceeds a given value, whereby, when said limiting means is activated, the result is a substantially instantaneous reduction in current from said transistor to said inductor, further resulting in inductive kickback from said inductor producing a voltage spike, which if of high enough level and time duration, may damage said transistor, the improvement comprises; means for providing a control signal in response to operation of said limiting means; and means for suppressing said voltage spike comprising means for placing a low impedance current path between said two output terminals in response to said control signal.

Claim 13

The circuit of claim 9, wherein said control signal providing means includes a resistor connected in series with and within said low impedance current path of said power dissipation limiting means, the voltage developed across said resistor, during the operation of said limiting means, being said control signal.

Claim 14

The circuit of claim 9, wherein said means responsive to said control signal for providing a low impedance current path between said two output terminals, is a crowbar circuit including: a first transistor having a base electrode, said base electrode being said input terminal, an emitter electrode connected to said first output terminal, and a collector electrode resistively coupled to said source of voltage; a second transistor having a base electrode resistively coupled to both said voltage source and said collector electrode of said first transistor, an emitter electrode resistively coupled to said voltage source terminal, and a collector electrode resistively coupled to said first output terminal; and a gate controlled rectifier having a gate electrode coupled to the collector electrode of said second transistor, and anode and cathode electrodes connected between said two output terminals.