Output circuit having electronic overload protection activated by voltage drop across output transistor

• US 5,896,263 A
• Filed: 06/27/1997
• Issued: 04/20/1999
• Est. Priority Date: 06/27/1997
• Status: Expired due to Fees

• Current Assignee: Allen-Bradley Company (Rockwell Automation, Inc.)
• Original Assignee: Allen-Bradley Company LLC (Rockwell Automation, Inc.)
• Inventors: Terdan, Dale R., O'Connell, John R.
• Primary Examiner(s): Gaffin, Jeffrey
• Assistant Examiner(s): HUYNH, KIM NGOC
• Application Number:
  US08/884,305
• Time in Patent Office: 662 Days
• Field of Search: 361/57, 361/86, 361/87, 361/79, 361/93, 361/101, 361/111
• US Class Current: 361/101
• CPC Class Codes: H03K 17/0822 in field-effect transistor switches

Abstract

An output circuit has multi-stage protection circuitry for protecting against different fault current conditions. The protection circuitry operates by detecting voltage changes across the output transistor of the output circuit. The output circuit comprises first, second and third switching devices. The first switching device is connected to the output device and power conditions the output signal. The second and third switching devices override the output signal and turn off the first switching device when a magnitude of current through the first switching device exceeds predetermined magnitudes for predetermined amounts of time. A protection circuit is also disclosed which has a sensing device that senses a current flow condition by sensing a voltage drop produced across two terminals of the first power conditioning transistor, the two terminals being the terminals through which the current flows. Advantageously, the output circuit utilizes highly flexible two-stage protection circuitry. The protection circuitry separately identifies surge current and overcurrent conditions. In addition to the flexibility provided by the use of two-stage protection circuitry, flexibility is also provided by the fact that component values may be easily varied to vary the magnitudes/durations of current within each stage which activate the protection circuitry. Moreover, the protection circuitry has a very fast response time. As a result, an output transistor which is protected by the protection circuitry of the present invention suffers much less stress in the event of a short circuit condition.

Claims

1. A protection circuit for protecting an output circuit against a current flow condition, said output circuit having a first transistor which provides power conditioning for an output signal that controls an output device, said protection circuit comprising:

a sensing device which is connected to said first transistor, said sensing device sensing said current flow condition, said current flow condition involving a current which flows through said first transistor and which is supplied to said output device; and

a second transistor which is connected to said sensing device and to said first transistor, said second transistor responding to said sensing device during said current flow condition by overriding said output signal and turning off said first transistor;

wherein said sensing device senses said current flow condition by sensing a voltage drop produced across two terminals of said first transistor, said two terminals being the terminals through which said current flows; and

wherein, when said second transistor turns off said first transistor, a bias current flows through said second transistor that causes said second transistor to turn on, the turning on of said second transistor causing said first transistor to turn off, said bias current being produced by said voltage drop across said two terminals of said first transistor, and wherein substantially none of said bias current flows through said output device.

2. A protection circuit according to claim 1,

werein said sensing device comprises a diode and a capacitor;

wherein said sensing device senses said current flow condition by sensing said voltage drop produced across said two terminals of said first transistor in combination with a voltage drop produced across said diode;

wherein said second transistor overrides said output signal and turns off said first transistor in response to said capacitor reaching a predetermined level of charge.

3. A protection circuit for protecting an output circuit against first and second current flow conditions, said output circuit having a first transistor which provides power conditioning for an output signal that controls an output device, said protection circuit comprising:

a first sensing device which is connected to said first transistor, said first sensing device sensing said first current flow condition, said first current flow condition involving a current which flows through said first transistor and which is supplied to said output device; and

a second transistor which is connected to said sensing device and to said first transistor, said second transistor responding to said first sensing device during said first current flow condition by overriding said output signal and turning off said first transistor;

a second sensing device which is connected to said first transistor, said second sensing device sensing said second current flow condition, said second current flow condition involving said current; and

a third transistor which is connected to said second sensing device and to said first transistor, said third transistor responding to said second sensing device during said second current flow condition by overriding said output signal and turning off said first transistor;

wherein said first sensing device senses said first current flow condition by sensing a voltage drop produced across two terminals of said first transistor, said two terminals being the terminals through which said current flows; and

wherein said second sensing device senses said second current flow condition by sensing said voltage drop produced across said two terminals of said first transistor.

4. A protection circuit according to claim 3, wherein said first current flow condition is a surge current condition and said second current flow condition is an overcurrent condition.

5. A method of protecting an output transistor against a current flow condition, said output transistor being used for power conditioning an output signal that controls an output device, the method comprising:

sensing said current flow condition using a voltage which appears across two terminals of said output transistor, said current flow condition involving a current which flows through said output transistor and which is supplied to said output device, and said two terminals being the terminals through which said current flows;

turning on a protection transistor in response to said current flow condition being sensed, said turning on step including the step of providing said protection transistor with a bias current, said bias current being produced by said voltage across said two terminals of said output transistor, and substantially none of said bias current flowing through said output device; and

turning off said output transistor in response to said protection transistor being turned on.

6. An output circuit for power conditioning an output signal which controls an output device, said output circuit comprising:

a first transistor connected to said output device, said first transistor turning on when said output signal assumes a first state and turning off when said output signal assumes a second state, and said first transistor power conditioning said output signal, said first transistor having a voltage appearing across two terminals thereof, said voltage having a magnitude which increases as said magnitude of said current through said first transistor increases;

a second transistor connected to said first transistor, said second transistor overriding said output signal and turning off said first transistor when a magnitude of current through said first transistor exceeds a first predetermined magnitude for a first predetermined amount of time; and

a third transistor connected to said first switching device, said third transistor overriding said output signal and turning off said first transistor when the magnitude of current through said first transistor exceeds a second predetermined magnitude for a second predetermined amount of time;

a first sensing circuit, said first sensing circuit being connected to said first and second transistors, said first sensing circuit sensing whether the magnitude of current through said first transistor has exceeded said first predetermined magnitude for said first predetermined amount of time and responding by causing said second transistor to override said output signal and turn off said first transistor;

a second sensing circuit, said second sensing circuit being connected to said first and third transistors, said second sensing circuit sensing whether the magnitude of current through said first transistor has exceeded said second predetermined magnitude for said second predetermined amount of time and responding by causing said third transistor to override said output signal and turn off said first transistor;

and wherein said first and second sensing devices sense the magnitude of said current through said first transistor by sensing the magnitude of said voltage drop across said two terminals of said first transistor.

7. An output circuit according to claim 6, wherein said first predetermined magnitude and said first predetermined amount of time corresponds to a surge current condition, and wherein said second predetermined magnitude and said second predetermined amount of time correspond to an overcurrent condition.

8. An output circuit according to claim 6,

wherein said first sensing circuit comprises a diode and a capacitor;

wherein said first sensing circuit senses whether the magnitude of current through said first transistor has exceeded said first predetermined magnitude for said first predetermined amount of time by sensing said voltage drop across said two terminals said first transistor in combination with a voltage drop produced across said diode; and

wherein said second transistor overrides said output signal and turns off said first transistor in response to said capacitor reaching a predetermined level of charge.

9. An output circuit according to claim 6, wherein a voltage supply input of said second transistor is connected to a voltage supply by way of a first voltage reference circuit, and wherein a voltage supply input of said third transistor is connected to said voltage supply by way of a second voltage reference circuit.

10. An output circuit according to claim 6, further comprising a comparator circuit which disables said output circuit when a magnitude of a voltage supply which supplies said first transistor drops below a predetermined level.

11. An output circuit according to claim 6,

wherein, when said second transistor turns off said first transistor, a first bias current flows through said second transistor that causes said second transistor to turn on, the turning on of said second transistor causing said first transistor to turn off, said first bias current being produced by said voltage drop across said two terminals of said first transistor, and wherein substantially none of said first bias current flows through said output device;

and

wherein, when said third transistor turns off said first transistor, a second bias current flows through said third transistor that causes said third transistor to turn on, the turning on of said third transistor causing said first transistor to turn off, said second bias current being produced by said voltage drop across said two terminals of said first transistor, and wherein substantially none of said second bias current flows through said output device.

12. An output circuit for power conditioning an output signal which controls an output device, said output circuit comprising:

A. a first transistor connected to said output device, said first transistor turning on when said output signal assumes a first state and turning off when said output signal assumes a second state, said first transistor power conditioning said output signal;

B. a surge current protection stage, including

(1) a surge current sensing circuit which is connected to said first transistor and which senses a surge current condition, and

(2) a second transistor connected to said surge current sensing circuit and said first transistor, said second transistor overriding said output signal and turning said first transistor off in response to said surge current condition; and

C. an overcurrent protection stage, including

(1) an overcurrent sensing circuit which is connected to said first transistor and which senses an overcurrent condition, and

(2) a third transistor connected to said first transistor, said third transistor overriding said output signal and turning said first transistor off in response to an overcurrent condition.

13. An output circuit according to claim 12,

wherein said surge current sensing circuit comprises a diode and a capacitor;

wherein said surge current sensing circuit senses said surge current condition by sensing a voltage drop across a voltage supply input and an output of said first transistor in combination with a voltage drop produced across said diode; and

wherein said second transistor overrides said output signal and turns off said first transistor in response to said capacitor reaching a predetermined level of charge.

14. A programmable controller system having an output module which utilizes the output circuit according to claim 12.

15. An output circuit according to claim 12,

wherein, when said second transistor turns off said first transistor, a first bias current flows through said second transistor that causes said second transistor to turn on, the turning on of said second transistor causing said first transistor to turn off, said first bias current being produced by said voltage drop across said two terminals of said first transistor, and wherein substantially none of said first bias current flows through said output device;

and

wherein, when said third transistor turns off said first transistor, a second bias current flows through said third transistor that causes said third transistor to turn on, the turning on of said third transistor causing said first transistor to turn off, said second bias current being produced by said voltage drop across said two terminals of said first transistor, and wherein substantially none of said second bias current flows through said output device.

17. An output circuit according to claim 15, wherein said voltage supply input of said second transistor is connected to said second terminal of said voltage supply by way of a first voltage reference circuit, and wherein said voltage supply input of said third transistor is connected to said second terminal of said voltage supply by way of a second voltage reference circuit.

18. An output circuit according to claim 15,

wherein said first current flow condition is sensed by sensing a voltage drop across said voltage supply input and said output of said first transistor in combination with a voltage drop across said first diode;

and

wherein said second current flow condition is sensed by sensing the voltage drop across said voltage supply input and said output of said first transistor in combination with said voltage drop across said first diode in combination with a voltage drop across said second diode.

19. An output circuit according to claim 15, wherein said voltage supply input of said input device is connected to a comparator circuit, said comparator circuit comparing the magnitude of said voltage supply with a predetermined threshold level and disabling said input device when said magnitude of said voltage supply is too low.

20. A programmable controller system having an output module which utilizes the output circuit according to claim 15.

16. An output circuit for power conditioning an output signal which controls an output device, said output circuit comprising:

an input device, said input device being adapted for receiving said output signal, said input device having a voltage supply input and an output, said voltage supply input being connected to a first terminal of a voltage supply;

a first resistor, said first resistor having first and second terminals, said first terminal being connected to a second terminal of said voltage supply, one of said first and second voltage supply terminals being a ground terminal;

a second resistor, said second resistor having a first terminal connected to said output of said input device and a second terminal connected to said second terminal of said first resistor;

a first transistor, said first transistor having a signal input connected to said second terminal of said second resistor, a voltage supply input connected to said second terminal of said voltage supply, and an output connected to a first terminal of said output device, said output device having a second terminal connected to said first terminal of said voltage supply, and said first transistor power conditioning said output signal;

a first diode, said first diode having first and second terminals, said first terminal being connected to said output of said first transistor;

a third resistor, said third resistor having a first terminal connected to said second terminal of said first diode and a second terminal connected to said output of said input device;

a second diode, said second diode having first and second terminals, said first terminal being connected to said first terminal of said third resistor;

a fourth resistor, said fourth resistor having a first terminal connected to said second terminal of said second diode and a second terminal connected to said output of said input device;

a first capacitor, said first capacitor having a first terminal connected to said second terminal of said voltage supply and a second terminal connected to the first terminal of said third resistor;

a second transistor, said second transistor having a voltage supply input, a signal input connected to said second terminal of said first capacitor, and an output connected to said signal input of said first transistor;

a second capacitor, said second capacitor having a first terminal connected to said second terminal of said voltage supply and a second terminal connected to the first terminal of said fourth resistor; and

a third transistor, said third transistor having a voltage supply input, a signal input connected to said second terminal of said second capacitor, and an output connected to said signal input of said first transistor; and

wherein said second transistor turns off said first transistor in response to a first current flow condition sensed by said first diode, said third resistor, and said first capacitor; and

wherein said third transistor turns off said first transistor in response to a second current flow condition sensed by said first and second diodes, said fourth resistor, and said second capacitor.