Abstract:
The invention relates to a device that comprises a plurality of energy stores and a driver circuit. Said driver circuit is supplied by at least two of the energy stores via a selecting circuit in such a manner that the driver circuit is always supplied by the energy store that has the highest voltage as compared to the remaining energy stores.

Description:
CLAIM FOR PRIORITY 
   This application claims priority to International Application No. PCT/DE01/01619 which was published in the German language on Dec. 6, 2001. 
   TECHNICAL FIELD OF THE INVENTION 
   The invention relates to a device and a method for supplying power to a firing means of a vehicle-occupant restraint system. 
   BACKGROUND OF THE INVENTION 
   What are referred to as electric firing circuits form the essential component of a controller for vehicle-occupant restraint systems, for example airbag controllers. Such firing circuits contain, as their essential component, at least one firing cap which causes the airbag propellant to explode when firing occurs by supplying electrical energy. Further components or at least two electronic switches which are frequently implemented as part of a customized circuit, for example in the form of a MOS field-effect transistor. The switches which are usually opened are closed when the respective airbag is fired, by means of a control signal which is correspondingly adapted by a driver circuit for controlling the switches (MOS field-effect transistor). The electronic switches have, in the connected-through state, an electrical contact resistance which is, inter alia, a function of the gate voltage of the respectively used MOS field-effect transistor. In order to achieve low power losses it is favorable to keep the contact resistance during the firing to a minimum. However, for this purpose the gate voltage of the MOS field-effect transistor must be as high as possible. This factor in itself requires the supply voltage of the driver circuit for the electrical circuits to be kept high for as long as possible, the supply voltage then being several volts above the firing voltage. 
   For this reason, a switching regulator which increases the battery voltage of the vehicle to the level of the firing voltage is usually provided in the airbag controller. A step-up converter, which has a capacitor (preferably electrolytic capacitor) for smoothing the switching voltage and for buffering the energy necessary to operate a control device (ECU), is usually used to step up the voltage. The energy reserve stored in the capacitor (acting as an energy store) is fed via a diode to a charge pump whose output voltage is several volts higher than the firing voltage. The voltage which is stepped up by the charge pump is then used to supply the driver circuit for the switches. 
   However, energy is removed from the energy store (capacitor) during the firing process and, as a result, the voltage across the energy store drops. This can lead to a situation in which sufficient voltage to ensure complete through-connection of the MOS field-effect transistors is no longer available. As a result of this, under certain circumstances the firing process can in turn only be carried out incompletely, which itself can lead to faults in the overall functioning of the airbag. 
   SUMMARY OF THE INVENTION 
   The invention improves the through-connection of the power supply of a vehicle-occupant restraint system. 
   In one embodiment of the invention, alongside the energy store at least one further energy store is additionally made available in the step-up converter in a controller for a vehicle-occupant restraint system. During the firing of the firing means (firing caps), the various energy stores are loaded to differing degrees so that the voltages at the individual energy stores drop at different speeds. According to one embodiment of the invention, the driver circuit is supplied via a selector circuit from at least two such energy stores such that the driver circuit is always supplied from the energy store with the voltage which is the highest in relation to the remaining energy stores. This measure ensures that the highest voltage is always used to supply the driver circuit so that the driver circuit is supplied in an optimum way using the means available. A further advantage is that if an energy store is already loaded by the firing process, and its voltage thus sinks, this store is not yet additionally loaded with supplying the driver circuit. 
   In another embodiment, it is possible, for example, for a switching device which connects the energy store with the respective highest voltage to the driver circuit to be provided as the selector circuit. However, a diode matrix is preferably used as it requires a small outlay in terms of circuitry and provides a high level of operational reliability. 
   For example, the energy stores in the firing circuits of front airbags of the vehicle-occupant restraint system and/or in the firing circuits of side airbags of the vehicle-occupant restraint system and/or in a step-up converter for generating a firing voltage for the vehicle-occupant restraint system and/or in a step-up converter for generating a supply voltage for the control device are used as suitable energy stores, a single step-up converter being preferably used both to generate the firing voltage and to generate the supply voltage. As a result, energy stores which are already present in the system are used so that the efficiency of the overall system is increased, and in the process the outlay in terms of circuitry can be kept low. 
   In order to be able to generate a sufficient high drive voltage when MOS field-effect transistors are used as switches, in particular as high-side switches, in one embodiment of the invention the driver circuit is supplied from the energy stores via a charge pump. For this purpose, a charge pump is connected between the selector circuit and the supply lines of the driver circuit. 
   In one aspect, the charge pump can have a further energy store for buffering energy and thus increasing the reliability of supply. 
   In another aspect, capacitors, and in particular electrolytic capacitors or gold capacitors owing to their high capacitances, are used as energy stores. In this context, capacitors have a relatively favorable price/capacitance ratio in comparison to other energy stores. However, specific types of accumulator batteries, for example polymer film accumulator batteries, can also be applied. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail below with reference to the exemplary embodiments illustrated in the FIGURE. 
       FIG. 1  illustrates a firing device in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the firing device which is shown as an exemplary embodiment, an airbag (not illustrated) is fired when necessary by means of a firing cap  1 . The firing cap is connected to a firing voltage by means of two MOS field-effect transistors  2  and  3  which are used as controlled switches, it being possible to tap the firing voltage to a firing capacitor  5  by means of a diode  4 . For this purpose, the firing cap  1  is connected via the drain-source path of the MOS field-effect transistor  2  to a terminal of the firing capacitor  5  which is connected to a reference potential  6 , and on the other hand to the other terminal of the firing capacitor  5  with intermediate connection of the drain-source path of the MOS field-effect transistor  3  and of the diode  4 . The firing capacitor  5  is fed by a step-up converter  7  which includes a storage capacitor  8 . The firing voltage  8  which is provided both for firing the firing cap (in conjunction with firing capacitor  5 ) and for operating a control device  9  in the present exemplary embodiment, can be tapped at the storage capacitor  8 . The control device  9  controls a driver circuit  10  as a function of a signal  11  of a crash sensor (not illustrated in the drawing). The driver circuit  10  is finally coupled to the gate terminals of the MOS field-effect transistors  2  and  3  in order to connect them through when necessary in accordance with the driving by the control device  9 . 
   In the exemplary embodiment, the two MOS field-effect transistors  2  and  3  are preferably of identical design so that the MOS field-effect transistor  3  correspondingly forms a high-side switch. In order to connect this transistor through sufficiently, a voltage is necessary which is greater than the firing voltage present at the firing capacitor  5  and the control device  9 . For this reason, the driver circuit  10  is supplied by a charge pump  12 . The charge pump  12  increases a voltage present at its input by a specific amount. In the exemplary embodiment the input voltage is approximately doubled in this context. 
   In particular, the present charge pump  12  comprises a capacitor  13  which is connected on the one hand to the input of the charge pump  12  and on the other hand to a node point. The node point is coupled here on the one hand to an auxiliary voltage  15  via a resistor  14  and to the reference potential  6  via the collector-emitter path of a bipolar transistor  16 . The bipolar transistor  16  is driven by means of a square-wave generator  17  so that the bipolar transistor  16  is continuously switched on and off. The input of the charge pump  12  is coupled to its output via a diode  18  in the conducting direction, the output being connected to the reference potential  6  via a storage capacitor  19 . The voltage at the input of the charge pump  12  is made available by a selector switch  20 . The selector switch  20  includes a diode matrix with three individual diodes  21 ,  22  and  23  which are connected in the conducting direction and are at the same time connected respectively to an energy store at the input end and to one another at the output end. 
   In addition to the storage capacitor  8  of the step-up converter  7 , the storage capacitors  24  and  25  of a front airbag controller  26  and of a side airbag controller  27  are also provided as energy stores. 
   As a result, the airbag controller (ECU) in the present exemplary embodiment has a total of at least three energy reserves, namely the reserve of the step-up converter  7 , the energy reserve of the front firing circuit (front airbag controller  26 ) and the energy reserve for the side firing circuit (side airbag controller  27 ) and if appropriate the reserve of the charge pump. If the voltage of the step-up converter  7  collapses during an impact owing to the connections of the car battery being torn off while at the same time the front airbags are fired, the driver circuits of the firing circuits can be fed from the side energy reserve which is not loaded and which can keep its voltage virtually at the initial firing voltage.