BACKGROUND OF THE INVENTION

[0001] 1 Field of the Invention

[0002] The invention relates to a drive circuit for a firing cap, triggerable by an electric direct current firing pulse, of a vehicle restraint system. The drive circuit has a firing circuit which forms a series connection of a high side switch to the firing cap and of a low side switch. The firing circuit is connected between a supply voltage of a first potential, and a reference voltage of a second potential, in parallel with a capacitor which stores energy, and is activated by a drive signal which is fed simultaneously to the high side switch and the low side switch, in order to feed a firing current to the firing cap during the firing pulse.

[0003] FIG. 3 is a schematic view of a drive circuit that is currently manufactured and marketed by the present applicant. The firing energy which flows through the firing circuit during the firing pulse and which is composed of a firing cap 1 , a high side switch 2 and a low side switch 3 is supplied by a vehicle battery 8 and by a capacitor 4 which has a capacitance C and stores energy which is charged by a boost regulator 6 . Both the high side switch 2 and the low side switch 3 are closed during the firing pulse, initiated by a drive signal SA. The capacitor 4 that stores energy can be charged with energy both by the vehicle battery 8 and by the boost regulator 6 . When the battery voltage is disconnected, for example at a node KL15, the firing energy must be supplied by the capacitor 4 alone.

[0004] So that a firing current of, for example, 1 amp to 3 amps, which is sufficient to fire the firing cap 1 (depending on the type of firing cap 1 ) can flow through the firing circuit during a time of, for example, 0.5 ms to 5 ms, even when the battery 8 is disconnected, the capacitor 4 must be able to store sufficient energy, that is to say it must have a sufficiently high capacitance, and is therefore large.

[0005] A further problem which can occur with vehicle accidents is an energy excess situation in the clamped load dump condition, for which reason the breakdown voltage of the capacitor 4 has to be larger than the maximum voltage of the power supply (for example 40 V) in case this occurs.

[0006] As is known, the capacitance of a capacitor that is necessary to store a certain amount of energy is inversely proportional to the square of the voltage across the capacitor. This advantage is frequently exploited to reduce the capacitance and also the size of the capacitor 4 that stores the energy in a vehicle restraint system.

[0007] However, as mentioned, there is a high probability of the battery supply in a vehicle becoming disconnected when accidents occur, the previously described situation of excess energy in the clamped load dump condition can occur, in which case the dynamo 7 ensures that the supply voltage is excessively increased at the node KL15. For this reason, the voltage class of the capacitor 4 must be configured in such a way that it withstands the excess voltage in the event of the excess energy in the clamped load dump condition.

[0008] In normal states, the voltage of a vehicle battery is in the range between 6 V and 18 V, which is too low for an optimized supply voltage for the capacitor 4 . For this reason, the boost regulator 6 which increases a supply voltage V _bb of the capacitor 4 to a value of up to 40 V is provided. The boost regulator 6 must therefore generate a boost voltage which lies in the region of the voltage that occurs in the situation of excess energy in the clamped load dump condition.

[0009] The firing cap 1 may be essentially considered as a resistor with a resistance value between 1 and 5 ohms. A firing event is triggered by closing the high side and low side switches 2 and 3 when a non-illustrated electronic regulator unit or ECU applies the firing signal S _A . If the voltage drop across the firing cap 1 is calculated during a firing event (during the firing pulse), it has approximately a value that is less than approximately 8 V. However, the voltage across the capacitor 4 that stores energy is much higher and generates a constant current across the firing cap 1 in the conductive state of the firing circuit. The constant current in turn ensures that there is a high voltage drop at the high side switch 2 and at the low side switch 3 . Here, the energy which is consumed by the high side switch 2 and/or the low side switch 3 during the firing event depends on the firing current strength and on the voltage drop as well as on the switch-on time of the high side switch 2 and of the low side switch 3 . The switch-on time of the transistors 2 and 3 of the firing circuit depends on the triggering time, that is to say the period of time until the firing cap 1 has fired. In the most extreme case, the length of the firing pulse, that is to say of the current through the firing circuit is equal to the length of the firing signal S _A at the driver transistors 2 and 3 and is, for example, 0.5 to 5 ms, the firing signal S _A being determined by the ECU. Since, as in the present case, the high side switch 2 and the low side switch 3 are integrated in an integrated circuit 5 as relative small transistors, the power loss which is consumed in the transistors 2 and 3 , that is to say the heat which is generated cannot be absorbed by a heat sink (for example by a radiator).

[0010] For this reason, a technical problem which has to be solved in a drive circuit with regard to a firing cap which can be triggered by an electrical direct current firing pulse is to configure an integrated circuit 5 with comparatively cheap high side and low side switches 2 , 3 in such a way that as many pairs as possible of high side switches and low side switches for a vehicle restraint system are integrated in a common integrated circuit, and nevertheless the power loss which is to be consumed by each pair of high side and low side switches even in the most extreme case can be kept sufficiently small. At the same time, the size of the capacitor 4 is to be reduced as far as possible.

[0011] In the prior art, the silicon area of the driver transistors 2 and 3 , and thus their thermal capacity has hitherto been increased so that the driver transistors 2 and 3 can absorb a high amount of energy. However, if it is desired to accommodate a large number of driver transistors in a highly complex integrated circuit, high costs are incurred per area unit of the silicon chip in order to implement the driver transistors in high-voltage BICMOS technology. Therefore, this hitherto problem was solved at a high cost by integrating a large silicon area of the driver transistors using a complex wafer manufacturing technology.

[0012] A very important problem of a vehicle restraint system, which is relevant to safety, arises from the fact that a plurality of pairs of high side and low side switches are integrated in a single integrated circuit 5 , that is to say in a single chip. If both driver transistors 2 and 3 for the firing cap 1 are inadvertently activated simultaneously due to some chip-internal fault, this leads to a faulty firing of the firing cap 1 with the consequence that, for example, an airbag unfolds without the relative external reason, which can lead to serious accidents.

[0013] The safety-relevant problem that was mentioned above was solved in the prior art by mechanically closable switches (the so-called safety sensor switches, for example microswitches) which are mounted outside the integrated circuit 5 , or the problem was solved electrically by an additional triggering signal generated by mechanical components.

SUMMARY OF THE INVENTION

[0014] It is accordingly an object of the invention to provide a drive circuit for a firing cap of a vehicle restraint system that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which permits an improved drive circuit for a firing cap, triggerable by an electrical direct current firing pulse, of a vehicle restraint system, in such a way that the above-mentioned problems are solved and as a result both a cost-intensive process technology and unintended firing of the firing cap when there is an undesired malfunction of the driver transistors are avoided.

[0015] With the foregoing and other objects in view there is provided, in accordance with the invention, a drive circuit for a firing cap, triggerable by an electric direct current firing pulse, of a vehicle restraint system. The drive circuit contains a firing circuit connected between a supply voltage of a first potential, and a reference voltage of a second potential. The firing circuit contains a high side switch having a control electrode and a controlled path connected to the firing cap, and a low side switch having a control electrode and a controlled path connected to the firing cap. The high side switch, the firing cap and the low side switch are connected in series. The firing circuit is activated by a drive signal fed simultaneously to the control electrode of the high side switch and of the low side switch for feeding a firing current to the firing cap during the firing pulse. A power switching element having a controlled path is connected in series with the controlled path of the high side switch and of the low side switch to draw lost power from the firing circuit during the firing pulse. A capacitor for storing energy is connected in parallel with the firing circuit.

[0016] According to one essential aspect, the drive circuit according to the invention for the firing cap, triggerable by the electrical direct current firing pulse, of the vehicle restraint system, is distinguished by the fact that, in the firing circuit, a power element is also connected in series with the high side switch and with the low side switch in order to absorb lost power from the firing circuit during the firing pulse.

[0017] With this measure it is possible to reduce the supply voltage for the plurality of driver transistor pairs in the integrated circuit; and to prevent a situation of excess energy at the supply pins of an integrated circuit containing the driver transistor pairs in a clamped load dump condition.

[0018] The proposed additional power element is, in one exemplary embodiment, a discrete N-type channel power FET which is an inexpensive power element which can absorb the energy during a firing event.

[0019] If, as in a preferred exemplary embodiment, the gate voltage of the external N-type channel FET is generated by a constant voltage source (for example 15 V), the external N-type channel FET has the function of a source follower whose source voltage is determined by the value of the gate voltage, the actual driver current, and the characteristic values of the device.

[0020] As a result, the supply voltage for the integrated circuit which contains the high side switch and the low side switch for the firing cap is stabilized, in particular at load current crossovers as a source follower is not a regulator which is subject to the disadvantages of possible instability. The constant voltage source that generates the gate voltage for the N-type channel power FET may be integrated, for example, in an integrated circuit chip which also contains the boost regulator. Alternatively, the constant voltage source can also be implemented discretely.

[0021] According to one essential aspect, the proposed power element is a switching element that can be switched on and off and connected to a control device that switches on the power switching element at least during the firing pulse, and then switch it off again. As long as the external power switching element is switched off, unintentional firing of the firing cap caused by any fault whatsoever is prevented.

[0022] In addition, when an external power switching element is switched off, a fault of the integrated circuit chip which contains the high side switch and the low side switch can be sensed by a driver test in which a current whose strength is far less than that necessary to fire the firing cap flows through the driver transistors. Such driver tests can be carried out by switching on an individual high side switch or low side switch without there being the need to limit the driver current to an uncritical value so that a firing event is not triggered. In the best case, the integrated circuit chip can be tested with the high side and low side switches with a genuine firing instruction as long as the external power switching element is definitely switched off.

[0023] If, as preferred, the high side and low side switches do not contain a reverse diode, that is to say both transistors have a reverse current blocking function, the N-type channel power FET which is preferably used as an external power switching element contains a reverse diode.

[0024] In the already mentioned preferred exemplary embodiment in which a plurality of pairs of high side and low side switches for a number of firing caps are integrated together in one integrated circuit chip, the power switching element which is connected outside the integrated circuit is preferably common to all the firing circuits.

[0025] The drive circuit according to the invention is preferably used for an airbag restraint system in a motor vehicle.

[0026] In accordance with an added feature of the invention, the control device switches on the power switching element before a start of the firing pulse. The high side switch and the low side switch are integrated and form an integrated circuit chip, and the power switching element is connected externally to the integrated circuit chip.

[0027] In accordance with a further feature of the invention, the high side switch is a P-type channel FET, the low side switch is an N-type channel FET, the power switching element is an N-type channel power FET, and the first potential is a high potential and the second potential is a low potential. Preferably, the N-type channel power FET is wired to operate as a source follower during a switched-on period and/or the N-type channel power FET contains a reverse diode.

[0028] Additionally, the high side switch and the low side switch may both have a reverse current blocking function.

[0029] In accordance with another feature of the invention, the high side switch and the low side switch together define one of a plurality of pairs that are assigned to a respective firing circuit and integrated in an integrated circuit. The power switching element, which is connected outside the integrated circuit, is common to all the firing circuits.

[0030] Other features which are considered as characteristic for the invention are set forth in the appended claims.

[0031] Although the invention is illustrated and described herein as embodied in a drive circuit for a firing cap of a vehicle restraint system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

[0032] The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.