Current limit controller for an air bag deployment system

Abstract: An air bag deployment system (10) generates the proper current to activate a squib (74) and deploy an air bag. A separate capacitor (48) powers circuitry in a drive limit controller (54) for providing a variable voltage drive to a MOS device (68) to regulate squib (74) firing current. A switch circuit (38) supplies gate current to a transistor (46) such that an electrical connecting path allows a capacitor (22) to charge a capacitor (48). During squib (74) firing and air bag deployment, the switch circuit (38) provides electrical isolation between the decaying voltage stored on the capacitor (22) that powers the squib (74) current and the capacitor (48) that provides current regulation to the squib (74) from the drive limit controller (54).

Description: Background of the Invention The present invention relates to a circuit in an air bag deployment system for activating air bag deployment, according to the preamble of claim 1. The air bag deployment system for automotive vehicles generally employs an accelerometer as the sensor that is sensitive to vehicle motion, especially deceleration. The vehicle battery or ignition system voltage powers the deployment circuit and the firing circuit. The deployment circuit and the firing circuit are to a large extent controlled by a microprocessor on an integrated circuit chip that monitors the accelerometer, but an external harness leads to a squib at the site of each air bag. The squib is a wire that is heated for igniting explosive material that causes the air bag to inflate. Firing of the squib typically requires a pair of Field Effect Transistors (FETs) conducting current, one on each side of the squib. Charge pumps are used to boost the voltage for the firing circuits above that supplied by the vehicle battery. Charged capacitors can be used to supply power to the electronics if the vehicle battery becomes disconnected or damaged during a crash event. To forestall the possibility that an inadvertent electrical connection to the firing circuit might cause deployment of the air bag or disable the system, it is desirable to monitor the system to preclude any such event. During normal vehicle operating conditions the air bag deployment system provides continuous diagnostic checks. One such diagnostic check monitors the resistivity parameter of the squib and another monitors the generated voltage of about twenty-four volts on a charged boost storage capacitor that stores the energy to fire the squib. Also, to assure readiness of the firing circuit to fire, it is desirable to test the FETs (and their drive circuits) for operability. When the accelerometer records a reading that the microprocessor interprets as a crash event, the microprocessor supplies the pair of FETs with a conduction current for firing the squib. The energy to initiate and sustain the firing comes from the generated voltage on the charged boost storage capacitor. Typically, the boost storage capacitor drops to about ten volts as the specified firing current is supplied to the squib. The charge pump actively tries to boost the voltage and replenish the charge dissipated by the squib. The prior art document having publication number WO 90 02440 and titled 'VOLTAGE TRANSFORMER FOR CHARGING ENERGY STORES', on which the preamble of claim 1 is based, discloses first and second capacitors that provide energy for detonating the air bags and a third capacitor that provides energy for temporarily energizing electronic control circuits during a collision condition. In this document the first and second capacitors are initially charged to a predetermined level of 30 volts. After reaching the 30 volt level, the third capacitor receives charge from the battery through a Zener diode. After the third capacitor reaches a regulated voltage of 24 volts, the first and second capacitors continue to charge until reaching a predetermined level of 38 volts. Thus, in a collision condition the electronic control circuits are supplied with 24 volts and the air bags are supplied with 38 volts. Another prior art document having publication number EP-A-0 594 234 and titled 'CURRENT SOURCE FOR INFLATABLE RESTRAINT SYSTEM' discloses a switch device that is forward biased to allow current to flow to the squib for deploying the air bag. A current control circuit is coupled to the sense element to regulate and limit the current in the switch device. It would be desirable to have an improved circuit to generate the proper current to activate the squib and deploy the air bag. The circuitry generating the squib firing current should provide a constant current for the approximately two milliseconds while the air bag is deployed and provide monitoring for diagnostic checks to assure readiness to fire and operability. According to the invention, a cricuit according to claim 1 has been provided. Brief Description of the Drawings FIG. 1 illustrates in partial block form and partial schematic form, a portion of an air bag deployment system in accordance with an embodiment of the present invention; FIG. 2 is a schematic diagram of circuitry provided by example for a drive limit controller shown in FIG. 1; and FIG. 3 is a schematic diagram of a switch circuit for the air bag deployment system shown in FIG. 1. Detailed Description of the Drawings FIG. 1 is a block diagram of a portion of an air bag deployment system 10 in accordance with an embodiment of the present invention. A battery 12 provides a positive voltage of about twelve to fourteen volts at terminal 14 in the preferred embodiment. The negative terminal of battery 12 is connected to ground. Input terminal 18 of boost pump circuit 16 receives the voltage of about twelve to fourteen volts and provides a voltage of about twenty-four volts at output terminal 20. A capacitor 22 with a value in the range of five-hundred microfarads (uf) to tenthousand microfarads is connected between terminal 24 and ground. Terminal 24 receives a boost voltage generated by boost pump circuit 16 at terminal 20 and stores that boost voltage on capacitor 22 (the opposite side of which is coupled to electrical ground). Terminal 28 of mechanical arm 26, terminal 34 of voltage monitor 32, terminal 40 of switch circuit 38 and the collector of NPN transistor 46 are connected to terminal 20 of boost pump circuit 16. Transistor 46 is a three-terminal semiconductor device with a base, a collector as a conducting electrode, and an emitter as a conducting electrode. Terminal 36 of voltage monitor 32 and terminal 50 of capacitor 48 are connected to the emitter of transistor 46. Capacitor 48, with a value of about 0.1 uf in the preferred embodiment, is connected between terminal 50 and ground. Terminal 44 of switch circuit 38 is connected to the base of transistor 46. Terminal 41 of switch circuit 38 is coupled for receiving a diagnostics check signal DISABLE. Terminal 43 of switch circuit 38 is coupled for receiving a signal NBIAS. High side current circuit 52 of FIG. 1 comprises drive limit controller 54, N-channel Metal Oxide Semiconductor (MOS) device 68, and resistor 70. Terminal 56 of drive limit controller 54 is coupled to terminal 50 for receiving a Charged Gate Drive (CGD) voltage of about twenty to thirty volts in the preferred embodiment. Terminal 58 of drive limit controller 54 provides a variable voltage output to the gate of MOS device 68 when the air bag is deployed. The drain of MOS device 68 is connected to output terminal 60 of high side current circuit 52. Terminal 60 receives a voltage from terminal 30 of mechanical arm 26 that indicates the air bag should be deployed. Resistor 70 is connected across terminals 64 and 66 of drive limit controller 54 and in series with MOS device 68. The source of MOS device 68 is coupled to terminal 64. Terminal 66 of drive limit controller 54 is connected to terminal 72 of high side current circuit 52. Squib 74 is a low resistive wire with a first terminal connected to terminal 72 of high side current circuit 52. The second terminal of squib 74 is connected to the drain of MOS device 76. The gate of MOS device 76 is connected to terminal 78 and the source of MOS device 76 is connected to ground. When squib 74 is fired, i.e. the squib is conducting about 2.1 amps of current, MOS device 68 as the high side FET and MOS device 76 as the low side FET are switched on for conducting the squib current. Future industry trends are for currents less than the 2.1 amps as the firing current to the squib. FIG. 2 is a schematic diagram of circuitry provided as an example for drive limit controller 54 shown in FIG. 1. A current source 82 is connected between terminal 56 and the collector of NPN transistor 84. A current source 88 is connected between terminal 56 and the collector of NPN transistor 86. The common bases of the transistors 84 and 86 are connected to the collector of transistor 86, thus forming a current mirror for transistors 84 and 86. The emitter of transistor 84 and the emitter of transistor 86 are connected to terminals 66 and 64, respectfully, of drive limit controller 54. In another example drive limit controller 54 could be a current source connected between terminal 56 and terminal 58 should current limiting not be required for MOS device 68 during air bag deployment. FIG. 3 is a schematic diagram of switch circuit 38 for the air bag deployment system shown in FIG. 1. The gate of N-channel MOS device 92 is connected to terminal 41. Terminal 41 receives a signal DISABLE for diagnostics purposes. MOS device 92 has the source connected to ground and the drain connected to the source of MOS device 94, forming a node for terminal 44. Terminal 44 is coupled for providing a signal to the base of transistor 46 (see FIG. 1). The common gates of P-channel MOS devices 94 and 96 are connected to the drain of MOS device 96. The sources for both MOS devices 94 and 96 are connected to terminal 40. The drain of N-channel MOS device 98 is connected to the drain of MOS device 96. The source of MOS device 98 is connected to the drain of N channel MOS device 100. The source of MOS device 100 is connected to ground and the gate is connected to terminal 43. Inverter 102 has an input and an output, with the input connected to terminal 41 and the output connected to the gate of MOS device 98. In operation, the embodiment shown in FIG. 1 of an air bag 5 deployment system 10 sets a current limit and regulates that current into squib 74 when deployment is initiated. The vehicle battery 12 provides a voltage of about twelve to fourteen volts that is increased to about twenty to thirty volts by boost pump circuit 16. Capacitor 22 stores the boosted charge for supplying the current of about 2.1 amps to fire squib 74 when the air bag (not shown) is to be deployed. The deployment of the air bag is typically in response to an accelerometer output monitored by a microprocessor (not shown) for evaluating the deceleration and determining if a crash has occurred. Mechanical arm 26 safeguards the possibility that an inadvertent electrical connection could fire and cause deployment of the air bag. To initiate the air bag deployment, both high side MOS device 68 and low side MOS device 76 conduct current through the current path activated by the closing of mechanical arm 26. In particular, current from capacitor 22 flows through mechanical arm 26, high side MOS device 68, resistor 70, squib 74, low side MOS device 76, and into ground. High side current circuit 52 shown in FIG. 1 is powered by the electrically stored charge on capacitor 48. Resistor 70 monitors the current of about 2.1 amps conducting through squib 74 for modulating the gate voltage of MOS device 68. Drive limit controller 54 provides a variable gate voltage of about eighteen volts to high side MOS device 68, such that MOS device 68 functions as a current source in maintaining a relatively constant current of about 2.1 amps to squib 74 in the preferred embodiment. The example shown in FIG. 2 for drive limit controller 54 provides a current source 82 with a current of about eight microamps into the collector of transistor 84. Current source 88 provides a current of about one microamp into the collector of transistor 86. A voltage of about fifty-six millivolts appears across terminals 64 and 66 in accordance with the squib firing current of about 2.1 amps through resistor 70 (see FIG. 1). The current limit to squib 74 is provided in accordance with the following equation in the preferred embodiment: l(lim) = 26 millivolts * ln(182/188) / V(R70) where 182 is the current supplied by current source 82, 188 is the current supplied by current source 88, and V(R70) is the voltage across resistor 70 at terminals 64 and 66. Thus, in accordance with a ratio of currents supplied by the current sources 82 and 88 and the voltage across resistor 70, a voltage is provided from the drain of transistor 84 to the gate of MOS device 68 for regulating the drain-to-source conduction current of power FET MOS device 68 at about 2.1 amps in the preferred embodiment. The voltage across resistor 70 summed with the base-emitter voltage (Vbe) for transistor 86 matches the Vbe for transistor 84 in the current mirror when in current regulation. For instance, when the conduction current of MOS device 68 changes about a value, such as 2.0 amps, the current change in serially connected resistor 70 causes a voltage change across resistor 70. An increased current will increase the voltage at terminal 64 which when combined with the Vbe for transistor 86 provides for an increased Vbe for transistor 84. The voltage at the collector of transistor 84 decreases in accordance with the increased Vbe for transistor 84. The gate of MOS device 68 is connected to the collector of transistor 84, thus decreasing the gate-to-source voltage of MOS device 68. Therefore, an increasing conduction current above the 2.0 amps value in MOS device 68 is sensed in resistor 70 and the current mirror transistor 84 in drive limit controller 54 provides a decreasing gate voltage to MOS device 68, thus regulating the squib current to a value of about 2.0 amps. In the preferred embodiment, current sources 82 and 88 are supplied at terminal 56 with current stored in capacitor 48 (see FIG. 1) while squib 74 is supplied with current stored in capacitor 22. During squib deployment capacitor 48 provides about one microamp of current to current source 88 and about eight microamps of current to current source 82 while capacitor 22 provides a current of about 2.1 amps to squib 74. For example, the separate storage capacitor 48 maintains a voltage in the range of eighteen to twenty four volts during deployment of the air bag compared with the voltage on capacitor 22 starting at about twenty-four volts and dropping to about ten volts. Thus, providing a separate capacitor 48 for supplying energy to drive limit controller 54 allows for the generation of a voltage for driving the gate of MOS device 68 such that a relatively constant current of about 2.1 amps is maintained for squib 74 during air bag deployment. As part of the embodiment shown in FIG. 1, capacitor 22 15 connected at terminal 24 is coupled to capacitor 48 connected at terminal 50 through NPN transistor 46. Thus, during normal operation or standby, switch circuit 38 keeps transistor 46 conducting current such that the gate drive capacitor 48 is charged to about the same voltage as capacitor 22, i.e. twenty-four volts. Diagnostic checks disable the charging of capacitor 48 from capacitor 22 through transistor 46, or generate an isolation path of capacitor 48 from capacitor 22. While isolated, the voltage across capacitor 48 can be monitored to be above a minimal value of about 18 volts. The voltage across capacitor 48 remains relatively constant during the two milliseconds of time for air bag deployment in accordance with the small currents supplied from capacitor 48 to drive limit controller 54. During air bag deployment transistor 46 becomes reverse biased, thus blocking a conductive path between capacitors 22 and 48. Switch circuit 38 in FIG. 3 functions in the following way. When the input signal DISABLE at terminal 41 is high, MOS device 92 is conducting such that the voltage at terminal 44 is about zero volts. A gate voltage with the value of zero volts turns off transistor 46 (see FIG. 1), thus preventing an electrical conduction path between capacitor 22 and capacitor 48. The input signal DISABLE is high during diagnostic checks for determining the voltages on capacitors 22 and 48. When the input signal DISABLE is low, MOS devices 96, 98, and 100 connected serially are conducting about twenty-five microamps of current. MOS device 94 is sized with a channel width about twenty times greater than the channel width for MOS device 96 in the preferred embodiment, such that MOS device 94 is conducting about four-hundred microamps of current. The approximate four-hundred microamps of current supplied by MOS device 94 is the base current into terminal 44 such that transistor 46 (see FIG. 1) provides the electrical conduction path allowing capacitor 48 to be charged from capacitor 22. In standby or normal operation, capacitor 48 is charged to a voltage of about twenty to thirty volts in preparation for the discharge of the air bag. By now it should be appreciated that the present invention 25 for air bag deployment system 10 generates the proper current to activate squib 74 and deploy the air bag. A capacitor 48, separate from capacitor 22 that supplies energy for firing the squib, powers circuitry in drive limit controller 54 providing a variable voltage drive to MOS device 68 for regulating current at about 2.1 amps for firing the squib. Switch circuit 38 supplies gate current to transistor 46 such that an electrical connecting path allows capacitor 22 to charge capacitor 48. The charge stored on capacitor 48 is monitored through diagnostic checks to assure readiness to fire and operability. Switch circuit 38 also provides electrical isolation between capacitors 22 and 48 such that the voltage to drive limit controller 54 remains above eighteen volts during air bag deployment. An integrated circuit suitable for manufacture can be built to include boost pump circuit 16, voltage monitor 32, switch circuit 38, transistor 46, and drive limit controller 54. While specific embodiments of the present invention have been shown and described, further modifications and improvements will occur to those skilled in the art. For instance, one embodiment has capacitors 22 and 48 supplying two loops of high side current circuit 52 for potentially firing two squibs 74 and deploying two air bags. However, air bag deployment system 10 could power high side current circuit 52 in a single loop system and fire a single squib 74 and deploy a single air bag. Also, squib firing currents below the 2.1 amps are anticipated.

Claims: A circuit in an air bag deployment system for activating air bag deployment having a first capacitor (22) and a second capacitor (48), the circuit comprising: a first transistor (46) having an emitter coupled to the second capacitor; and a switch circuit (38) coupled for receiving an input signal (41) having a first value and having a second value that causes the first transistor to be nonconductive and electrically isolate the first capacitor from the second capacitor, characterized in that : the first transistor (46) has a collector coupled to the first capacitor; the first value of the input signal causes the first transistor to match a voltage on the second capacitor with a voltage received by the first capacitor. The circuit of claim 1, wherein the switch circuit (38) further includes: a second transistor (92) having a control terminal coupled for receiving the input signal, a first current conducting terminal coupled to ground, and a second current conducting terminal coupled to a base of the first transistor; a third transistor (94) having a first current conducting terminal coupled to a terminal of the first capacitor (22) and a second current conducting terminal coupled to the second current conducting terminal of the second transistor; a fourth transistor (96) having a first current conducting terminal coupled to the first current conducting terminal of the third transistor and a control terminal and a second current conducting terminal commonly coupled to a control terminal of the third transistor; a fifth transistor (98) having a control terminal coupled for receiving an inverted input signal and a first current conducting terminal coupled to the second current conducting terminal of the fourth transistor (96); and a sixth transistor (100) having a control terminal coupled to a terminal (43) for receiving a signal, a first current conducting terminal coupled to the second current conducting terminal of the fifth transistor (98), and a second current conducting terminal coupled to ground.

Inventors: BENNETT PAUL T, GRAY RANDALL C, BENNETT, PAUL T., GRAY, RANDALL C.
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1997
Main classification: B60R21
IPC codes: B60R21