In vehicles, a primary electrical power supply source, which typically has a potential between 9 V and 18 V, is provided by a dynamo and a battery. Individual safety-relevant systems such as, for example, an airbag system, have their own electrical energy storage devices. It is thus ensured that, in the event of a failure of the primary power source, the safety-relevant systems are still able to perform their primary functions. To do so, a sufficient amount of energy must be storable in the energy storage devices. Since the capacity of the energy storage device cannot be arbitrarily increased for space and cost reasons, the energy storage devices are charged to an elevated potential, for example, 22 V to 45 V, to allow a higher amount of usable energy to be thus stored than would be possible at the typical supply potentials of 9 V to 18 V. In addition, voltages >10 V, for example, are needed for igniting airbag systems for reasons of usage restrictions.
When a vehicle or an airbag system is restarted, the energy storage device must be recharged. FIG. 6 schematically shows a device for charging the energy storage device. A main current path connects a power source E, which provides a supply current Iv, to energy storage device CE. A voltage transformer device 20, which is able to raise storage potential VE of energy storage device CE to above supply potential VB of power source E, is situated in the main current path. An external diode D1 and an inductance L are situated in the main current path between power source E and voltage transformer device 20 to connect voltage transformer device 20. Since diode D1, inductance L, and voltage transformer device 20 do not have a high resistance, a high supply current Iv would flow along the main current path when switching on even an empty energy storage device CE. This may be disadvantageous for several reasons:                1. Considerable mass displacements occur, which may interfere with sensitive circuits.        2. The maximum current which a device may require is often limited by predefined specifications.        3. The thermal and thermal-mechanical loads reduce the useful life of the components due to the high currents.        
Therefore, resistors R14 through R19 are situated in the main current path downstream from voltage transformer device 20. These resistors limit maximum supply current Iv during switch-on, due to the increased resistance of the main current path. The efficiency of voltage transformer device 20 is disadvantageously reduced by resistors R14 through R19 because electrical power is dissipated in these resistors. This efficiency may be increased by a capacitor C4 connected in parallel to resistors R14 through R19.
Due to the switched-mode operation of a transistor T2 of voltage transformer device 20 and the associated interference signals in the range >150 kHz, the main current path must be filtered by a low-pass filter having a capacitor C2 and resistors R13 through R10, which is situated between power source E and voltage transformer device 20 in the main current path.
The additional resistors R10 through R19 result not only in reduced efficiency of the device for charging energy storage device CE and in the associated problems of heat dissipation, but also in increased space requirement of the overall circuit and increased assembly costs due to the multiple individual elements.
USPTO Backgrounds
USPTO Backgrounds