VOLTAGE TRANSFORMER FOR CHARGING ENERGY STORES

Abstract: A voltage transformer is provided for charging two separate energy stores (1, 7) to two different predetermined charging voltages. An inductive energy storing device (2) is coupled to the battery voltage and is adapted to be periodically grounded under the control of a multivibrator (5) to generate a charging voltage higher than the battery voltage. The charging voltage is applied directly to a first of the energy stores (1) but is only applied to the second energy store (7), by way of a control device (8), when the charging voltage on the first energy store (1) has reached a predetermined level.

Description: VOLTAGE TRANSFORMER FOR CHARGING ENERGY STORES. State of the Art The present invention relates to a voltage transformer of the kind described in the precharacterising part of claim 1. Such a device is already known for use in connection with an air bag protection device in a motor vehicle. The energy store is used for providing energy to detonate one or more air bag gas capsules for inflating the air bag or bags in the event of loss of battery power during a collision condition. It is also necessary to provide energy during such conditions for driving additional electronic circuitry of the motor vehicle and the usual practice has been to provide a separate voltage charging source for this purpose since the energy store for driving the electronics is generally to be charged to a smaller voltage than that for detonating the air bag capsules. i is an object of the present invention to provide a voltage transformer which overcomes this disadvantage of the known arrangement. Advantages of the Invention The aforegoing object is achieved by adopting the feature set forth in_ the characterising part of claim 1. This has the advantage that rapid charging of two relatively large, separate energy stores with high, different voltages, and without mutual discharging, is achieved. B means of the features of the sub-claims, a voltage transformer can be achieved which, from a battery voltage in the range of 6-24v, can charge two separate energy storage devices to different voltages in a time of less than seven seconds. Drawing The invention is described further hereinafter, by way of example only, with reference to the accompanying drawing which is a circuit diagram of one embodiment of a voltage transformer in accordance with the present invention. Description of the Exemplary Embodiment The voltage transformer shown in the drawing operates on the known conversion principle of the so-called "flyback regulator" wherein energy is stored in a coil coupled to the battery voltage supply and a higher voltage is generated by periodically shorting the coil to earth by means of a controlled power stage. The battery voltage U BA χ protected from interference pulses, is supplied by way of a diode D-^ to one end of a coil L. The diode Di protects the circuit against reverse connection of the battery. The opposite end of the coil L is connected by way of a diode D£ to the one ends of a pair of storage capacitors CT , C2 whose other ends are earthed and which, together, form a first energy store 1 whose purpose is to provide energy for detonating one or more inflation capsules of air bags, used for the protection of the occupants of motor vehicles in a collision condition, in the event of loss of battery power. The diode D^ and coil L, together with a capacitor C^, form an inductive energy-storing device 2. In the present embodiment, each of the capacitors C^, C2 has a capacity of 4700 μF and is rated at 63V. The combined capacity is therefore 9.4 mF. A resistor R^ and further diode D3 lead from the battery input to the capacitors C*^, C2 and form an auxiliary charging circuit 3 in a conventional manner. Coupled to the point P at the s.econd end of the coil L is a MOSFET transistor Ti forming a power output stage 4. When rendered conductive, the transistor T acts to connect the point P to ground. The controlling terminal of the transistor T*^ is connected in a known manner to the output F of an astable multivibrator device 5 which is of known construction and is therefore not described further herein. The output F of the astable multivibrator 5 is also connected to a regulating device 6 which is described in more detail hereinafter but which acts under certain specified conditions to suppress the controlling action of the astable multivibrator 5 and prevents switching of the MOSFET transistor T^ • A second energy store 7, consisting of a further capacitor C^, is coupled to the first energy store 1 by way of a charge control circuit 8 which only allows the second energy store 7 to be charged when the charging voltage of the first energy store 1 has reached a predetermined level, in this case 30 volts. Until then, charging of the second energy store is prevented by means of a non-conductive transistor 2* The purpose of the second energy store is to provide energy for temporarily energising electronic control circuits within a motor vehicle during a collision condition and when there is a loss of battery power for such electronics. In this example, the capacitor C forming the second energy store 7 has a capacity of 6.8mF and a rated voltage of 40 volts. The device, as so far described, operates as follows: The battery voltage (typically 12-16 volts) protected against spikes, is applied to the coil L. The astable multivibrator 5 operates continuously and acts to cause the transistor T j to repeatedly connect and disconnect the point P to earth, thereby causing a high voltage to be induced across the coil L. This high voltage is rectified by the diode D and begins to charge the capacitors C^, C£- Initially, the charging voltage on the capacitors Cχ » c 2 cannot reach the second energy storing capacitor C^ since the transistor T2 is in a non-conductive state. However, upon the charging voltage on the capacitors C- j _, C reaching 30V, a Zener diode Z breaks down, thereby switching on a transistor T3 and thence switching on the transistor T£. With the transistor T2 conductive, the capacitor C can then charge via a resistor R . Charging of the capacitor C above a further voltage level (in this case 24V) is, however, prevented by the presence of a 24-volt Zener diode Z2- The charge on the first capacitors C* j _, C2 meanwhile continues to rise. When it reaches a predetermined level (in this case 38 volts), a further Zener diode Z3 breaks down and the regulator 6 is tripped. The regulator 6 includes a comparator 10 which compares a signal developed from the charging signal on the first capacitors C--_, C£. With a fixed reference voltage so as to generate an output at point F causing the pulsed output of the astable multivibrator to be suppresed, the voltage on the capacitors C-^, C therefore ceases to rise and the charging voltage is held at that level. Thus, the upstream capacitors C^, C first charge to 30 volts; then the capacitor C4 begins to charge to 24 volts and only then does the charging of the first capacitor Ci , C2 continue to rise to the maximum permitted level of 38 volts. The overall charging time to reach the specified charging levels can be achieved in a time t ^seven seconds. The first capacitors C*-_, C are charged in a time Ti three seconds. This, after battery failure, is still sufficient to detonate the inflation capsule(s) with a steady discharge of 10mA after t - seven seconds. It will be noted that discharging of one of the energy reserves does not discharge the other, i.e. there is no mutual discharging of the energy reserves.

Claims: CLAIMS 1. A voltage transformer for charging an energy store to a predetermined charging voltage, having an inductive energy storing device (2) which is coupled to a battery voltage and is adapted to be periodically grounded under the control of a multivibrator (5) to generate a charging voltage higher than said battery voltage, the higher voltage being used to charge an energy store (1) , characterised in that said higher voltage is used to charge two separate energy reserves (1,7) to different voltages in such a manner that mutual discharging does not occur. 2. A voltage transformer according to claim 1, wherein the second energy store (C4) is coupled to the first energy store (1) by way of a control device (8) which only connects the charging voltage to the second energy store when the charge on the first energy store has reached a predetermined charging voltage. 3. A voltage transformer according to claim 2, wherein the control device (8) comprises a first transistor T disposed between the first and second energy stores (1,7), the transistor Ϊ£ being arranged to be non-conductive until such predetermined charging voltage has been reached. 4. A voltage transformer according to claim 3, wherein the control device (8) further comprises the series combination of a resistor (R4) and first Zener diode (Zi) connected between the first energy store (1) and earth, and a second transistor T3 whose control terminal is connected to the junction of said resistor (1X ) atl d the Zener diode (Zi) such that, when the charging voltage reaches the breakdown voltage of the first Zener diode (Z χ ) , the second transistor (T3) opens the " first transistor (T2) so as to connect the charging voltage to the second energy store (7). 5. A voltage transformer according to claim 4, wherein a second Zener diode (Z2) in the control device (8) ensures that the maximum charging voltage on the second energy store (7) is less than that on the first energy store (1). 6. A voltage transformer according to any of claims 1 to 5, including a regulator device (6) which causes the multivibrator to be suppressed when the charging voltage on the first energy store (1) reaches a second predetermined voltage, higher than said first mentioned predetermined voltage. 7. A voltage transformer according to any of claims 1 to 6, wherein the first energy store is used to provide energy for detonating one or more air bag protection devices in a motor vehicle when battery voltage has been lost, and the second energy store is used to provide energy for other electronic control circuits of the vehicle under the same conditions.

Inventors: DROBNY WOLFGANG, NITSCHKE WERNER, PFEUFER REINHARD, REHAGE RAINER, DROBNY, WOLFGANG, NITSCHKE, WERNER, PFEUFER, REINHARD, REHAGE, RAINER
Applicants: BOSCH GMBH ROBERT, ROBERT BOSCH GMBH
Application year: 1988
Main classification: H02M3
IPC codes: B60R16, B60R21, H02J1, H02J7, H02M3