Apparatus for protecting a transistor in the event of a shorted load condition

• US 4,816,963 A
• Filed: 12/21/1987
• Issued: 03/28/1989
• Est. Priority Date: 12/21/1987
• Status: Expired due to Fees

• Current Assignee: General Motors Corporation (General Motors Company)
• Original Assignee: General Motors Corporation (General Motors Company)
• Inventors: Eden, Gary M.
• Primary Examiner(s): Leung, Philip H.
• Assistant Examiner(s): Jennings, Derek S.
• Application Number:
  US07/135,943
• Time in Patent Office: 463 Days
• Field of Search: 361/87, 361/93, 361/91, 361/101, 330/207 P, 330/298
• US Class Current: 361/101
• CPC Class Codes: H03K 17/0822 in field-effect transistor switches
• CPC Class Codes: H03K 17/0826 in bipolar transistor switches

Abstract

A transistor is protected from excessive power dissipation in the event of a shorted load condition by alternately turning the transistor off for a relatively long period of time during which a capacitor is discharged through a resistor network and turning the transistor on for a relatively short period of time during which the capacitor is charged through a diode by the voltage developed across a resistor connected in circuit with the transistor.

Claims

1. For a circuit including a load and a transistor for driving the load such that when the transistor is turned on the load is energized and when the transistor is turned off the load is deenergized, apparatus for protecting the transistor from excessive power dissipation should the load become shorted, comprising:

a comparator connected to the transistor and responsive to a command voltage applied to a first input and a reference voltage applied to a second input to turn on the transistor when the command voltage exceeds the reference voltage and to turn off the transistor when the reference voltage exceeds the command voltage;

means for providing the command voltage at the first input of the comparator such that the command voltage is at a turn on level when it is desired to turn on the transistor and is at a turn off level lower than the turn on level when it is desired to turn off the transistor;

a sensor resistor connected in circuit with the transistor and the load so as to develop a sensor voltage across the sensor resistor such that when the transistor is turned on the sensor voltage is at a normal level when the load is not shorted and is at an abnormal level higher than the normal level when the load is shorted and such that when the transistor is turned off the sensor voltage is at a base level lower than the normal level;

means for providing the reference voltage at the second input of the comparator including;

a capacitor across which the reference voltage is developed,

means for normally maintaining the reference voltage at a nominal level intermediate the turn on and turn off levels of the command voltage such that the transistor is turned on when the command voltage is at the turn on level and is turned off when the command voltage is at the turn off level,

a diode connected between the sensor resistor and the capacitor such that the diode is reverse biased when the sensor voltage is at the normal level which is below the nominal level of the reference voltage and is forward biased when the sensor voltage is at the abnormal level which is above the nominal level of the reference voltage so as to rapidly charge the capacitor to increase the reference voltage toward the abnormal level until it rises above the command voltage whereupon the transistor turns off and the sensor voltage drops to the base level thereby to reverse bias the diode, and

means for discharging the capacitor to slowly decrease the reference voltage toward the nominal level until it falls below the command voltage whereupon the transistor briefly turns on to rapidly increase the reference voltage back above the command voltage thereby to turn off the transistor,

so that as long as the load is shorted and the command voltage is at the turn on level the transistor is alternately turned on for a relatively short period of time and is turned off for a relatively long period of time to protect the transistor from excessive power dissipation.