Abstract:
A method for operating an energy supply unit for a motor vehicle electrical system including at least one first subsystem and one second subsystem having different voltage levels, one first stator winding of an electric machine being connected via one first converter circuit to the first subsystem, and one second stator winding of the electric machine being connected via one second converter circuit to the second subsystem, and a DC-DC conversion taking place between the first subsystem and the second subsystem, while one of the converter circuits Is operated as an inverter and the other of the converter circuits is operated as a rectifier, and the first stator winding and the second stator winding are operated as a transformer.

Description:
RELATED APPLICATION INFORMATION 
       [0001]    The present application claims priority to and the benefit of German patent application no. 10 2013 205 413.0, which was filed in Germany on Mar. 27, 2013, the disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method for operating an energy supply unit for a motor vehicle electrical system, including at least one first subsystem and one second subsystem having different voltage levels. 
       BACKGROUND INFORMATION 
       [0003]    Motor vehicle electrical systems may be configured as so-called two-voltage or multi-voltage vehicle electrical systems including at least two subsystems. Such electrical systems are used, for example, when consumers having different power requirements exist in a particular motor vehicle. In this case, at least two of the subsystems have different voltage levels, for example, 14 V (a so-called low-voltage subsystem) and 48 V (a so-called high-voltage subsystem). The subsystems may be connected to each other, for example via a DC-DC converter. At least one of the subsystems has a generator system that feeds the subsystem. A second or additional subsystem connected via the mentioned DC-DC converter may then in turn be supplied from the subsystem having the generator system. 
         [0004]    Electric machines may be used, in particular, in hybrid vehicles in order to be motor operated as well as generator operated. The internal combustion engine may be assisted by a motor operation of the electric machine at low rotational speeds at which the former does not yet deliver its full torque. Upon deceleration of the motor vehicle, kinetic energy may then be converted into electrical energy by the generator operation of the electric machine. 
         [0005]    During generator operation, the electric machine generates, if necessary, a polyphase current which may be rectified for a motor vehicle electrical system. To enable both motor operation as well as generator operation of the electric machine, the electric machine may be equipped with an inverter circuit which may be composed, for example, of electrical switches, for example, in the form of MOSFETs, an associated control circuit and an intermediate capacitance. To ensure high performances in both motor as well as generator operation of the electric machine, the electric machine may be operated with, or it may supply, the comparatively high, first voltage of the high voltage subsystem. 
         [0006]    However, the use of both an inverter circuit and a DC-DC converter in this configuration is cumbersome and is associated with high costs. Moreover, the separate circuits of the inverter circuit and the DC-DC converter put a strain on the already severely limited installation space in a motor vehicle. 
         [0007]    It is therefore desirable to provide a simple, cost-efficient and space-saving option for enabling both a generator as well as a motor operation of an electric machine in conjunction with different subsystems of the motor vehicle electrical system. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a method for operating an energy supply unit for a motor vehicle electrical system having the features described herein. Advantageous embodiments are the subject matter of the further descriptions as well as the following description. 
         [0009]    According to the present invention an electric machine of the energy supply unit includes two stator windings, each of which are connected via a converter circuit to one subsystem of the motor vehicle electrical system. The converter circuits may, for example, include half bridges with switches, in particular, MOSFETs. In particular, each of the converter circuits is configured analogously to a conventional inverter circuit. 
         [0010]    According to the present invention the converter circuits are activated in such a way that the energy supply unit functions as a DC-DC converter and the stator windings as a transformer between the two subsystems. Depending on the need, one of the two converter circuits in this configuration is activated as an inverter in order to convert the subsystem d.c. voltage of the corresponding subsystem into an a.c. voltage. This a.c. voltage generates a current flow in the one associated stator winding of the two stator windings, which in turn induces an a.c. voltage in the other of the two stator windings. The other of the two converter circuits is actuated as a rectifier in order to rectify this induced a.c. voltage and feed it into the other subsystem. 
         [0011]    According to the present invention, no additional separate DC-DC converter is required; the already existing parts and components of the converter circuits are used for rectification, inversion and transformation, thereby ultimately enabling a DC-DC conversion. Accordingly, the DC-DC conversion in terms of the present invention takes place via the same parts which are used for rectification and inversion and via the first and second stator winding. Thus, the method according to the present invention combines the advantages and functions of an inverter circuit and a DC-DC converter in one single circuit. 
         [0012]    Therefore, no additional components and parts are required and the costs may be reduced. The stator winding is merely doubled, which is easy to implement and at no major expense. Compared to a conventional electric machine which has only one stator winding, the stator winding already existing in this case may either be split or an additional stator winding may also be integrated. The costs of integration and space requirements are also reduced. A processing unit used for actuating, including, for example, a microcontroller, may be used for controlling the rectification and the inversion as well as for controlling the transformation. 
         [0013]    The present invention is particularly suited for electric machines, for example, a separately excited synchronous machine for use in motor vehicles. The principle may be employed in connection with a boost recuperation system (BRS) in the electric machine (boost recuperation machine). 
         [0014]    By way of example, the first subsystem is assumed in the following to be a high voltage-vehicle electrical system which is operated with a first subsystem d.c. voltage, and the second subsystem is assumed to be a low voltage-vehicle electrical system which is operated with a second subsystem d.c. voltage, the first subsystem d.c. voltage having a higher voltage value than the second subsystem d.c. voltage. 
         [0015]    Depending on the need, the first subsystem d.c. voltage of the first subsystem is converted downwards and transferred into the second subsystem, or the second subsystem d.c. voltage of the second subsystem is converted upwards and transferred into the first subsystem. 
         [0016]    The first and second converter circuits may each also be activated as a step-down converter and/or as a step-up converter. In this way, the voltage value of the subsystem d.c. voltages of the two subsystems may be flexibly adjusted independently of one another. Alternatively or in addition, these voltage values may also be adjusted using a specific winding ratio of the first and of the second stator winding. The winding ratio may be oriented to the ratio of the two d.c. voltages, similarly to a conventional transformer. For example, a first d.c. voltage value of 48 V and a second d.c. voltage value of 12 V result in a winding ratio of 4:1. 
         [0017]    By activating the two converter circuits, the deviations of the two subsystem d.c. voltages are compensated by nominal values which occur, for example, during varying states of charge of the battery. It is also advantageously feasible to use two identical stator windings and to implement a downward conversion of the first subsystem d.c. voltage completely by a step-down converter operation of the first converter circuit. This is useful, in particular, when a conventional electric machine having as few configuration changes as possible is intended to be used for a method according to the present invention. 
         [0018]    The electric machine is advantageously operated in a second operating mode as a generator. In this mode, an in particular multiphase current, or an in particular multiphase output voltage provided by the electric machine is rectified by the converter circuits. Electrical power may be transferred in the second operating mode by the electric machine into the first and/or the second subsystem. The electrical powers transferred into the first and the second subsystem may be adjusted separately from one another by activating the two converter circuits. Thus, for example, a desired braking torque may be set in the first motor vehicle electrical system, while in the second motor vehicle electrical system a desired battery voltage may be regulated. In addition, this enables the electric machine to transfer electrical power directly into the first as well as into the second motor vehicle electrical system. This is particularly advantageous during an emergency operation of the generator in the event of a battery failure. 
         [0019]    The electric machine may be operated in a third operating mode as a motor. In this case, the subsystem d.c. voltage of one of the subsystems is inverted by the converter circuits. Electrical power is advantageously transferred in such a case by the subsystem into the electric machine. The electric machine is supplied from one of the subsystems, in particular from the subsystem which is configured as a high voltage-vehicle electrical system having the higher subsystem d.c. voltage. The first subsystem is assumed to be the same, for example. Electrical power from the first subsystem is converted into mechanical power for driving the motor vehicle. The level of this mechanical power is adjusted via activation of the first converter circuit. 
         [0020]    The second converter circuit is activated so that no power is transferred into the second subsystem. The second converter circuit may advantageously also be activated in such a way that during motor operation electrical power is transferred from the first motor vehicle electrical system into both the electric machine as well as into the second subsystem. 
         [0021]    In one advantageous embodiment of the present invention, the second converter circuit is connectable, via a switch element, for example, to either the second subsystem or to the first subsystem. If the second converter circuit is connected to the first subsystem, the second stator winding may then also be used for transferring electrical energy between the first motor vehicle electrical system and the electric machine. During motor and generator operation, a maximum transferrable electrical power may then be transferred, respectively, between the first motor vehicle electrical system and the electric machine. 
         [0022]    If the second converter circuit is connected to the second motor vehicle electrical system, the maximum transferrable electrical power may no longer be transferred between the first motor vehicle electrical system and the electric machine; instead, additional electrical power may be transferred into the second motor vehicle electrical system. 
         [0023]    A processing unit according to the present invention, for example, a control unit of a motor vehicle, is programmed, in particular, to carry out a method according to the present invention. 
         [0024]    The implementation of the method in the form of software is also advantageous, since this entails particularly low costs, in particular if a performing control unit is also used for other tasks and is therefore present anyway. Suitable data media for providing the computer program are, in particular, diskettes, hard-disk drives, flash memories, EEPROMs, CD-ROMs, DVDs and the like. It is also possible to download a program from computer networks (Internet, Intranet etc.). 
         [0025]    Further advantages and embodiments of the present invention result from the description and the appended drawing. 
         [0026]    It is understood that the features cited above and those to be explained below are applicable not only in each specified combination, but also in other combinations or alone, without departing from the scope of the present invention. 
         [0027]    The present invention is schematically represented in the drawing based on exemplary embodiments and is described in greater detail below with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  schematically shows one specific embodiment of a multi-voltage vehicle electrical system having an energy supply unit according to the related art. 
           [0029]      FIG. 2  shows one specific embodiment of a multi-voltage vehicle electrical system having an energy supply unit which is configured to carry out one specific embodiment of a method according to the present invention. 
           [0030]      FIG. 3  shows in circuit diagram-like manner one specific embodiment of an energy supply unit which is configured to carry out one specific embodiment of a method according to the present invention. 
           [0031]      FIG. 4  shows in a circuit diagram-like manner another specific embodiment of an energy supply unit which is configured to carry out another specific embodiment of a method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Corresponding elements are denoted by identical reference numerals. For the sake of clarity, these will not be repeatedly explained. 
         [0033]      FIG. 1  schematically shows one specific embodiment of a multi-voltage vehicle electrical system having an energy supply unit of a motor vehicle electrical system according to the related art. In this example, the motor vehicle is configured as a hybrid vehicle. Connected downstream from a conventional electric machine  50  having a simple stator winding is an inverter circuit  150 . Electric machine  50  is intended in this example to be configured as a three-phase electric machine  50 . Inverter circuit  150  is used to rectify a multiphase current, in this example, a three-phase current which is provided by electric machine  100  during a generator operation. In addition, inverter circuit  150  makes it possible to convert a rectified current into a three-phase current in order to operate electric machine  50  in a motor mode. 
         [0034]    During the generator operation of electric machine  50 , inverter circuit  150  provides a first subsystem d.c. voltage of, for example, 48 V for a first subsystem N 1  of the motor vehicle electrical system. With the aid of this first subsystem d.c. voltage, it is possible to operate multiple electrical consumers, which are represented symbolically in  FIG. 1   a  and designated as V 1  and V 2 . Such an electrical consumer may, for example, be an electric drive of the hybrid vehicle or an energy store represented as V 2 . 
         [0035]    Since most electrical consumers in the hybrid vehicle, such as a starter motor of an internal combustion engine, a car radio or an on-board computer are operated with a lower voltage than the first subsystem d.c. voltage, the first subsystem d.c. voltage is reduced by a DC-DC converter to a second subsystem d.c. voltage, for example, 14 V, for a second subsystem N 2 . Electrical consumers which are operated with the second d.c. voltage are represented symbolically in  FIG. 1  and designated as V 3 , V 4  and V 5 . 
         [0036]    The voltage levels 48 V and 14 V used are merely examples. The present invention may also be used in conjunction with other voltages or voltages varying over time. 
         [0037]      FIG. 2  schematically shows one specific embodiment of a multi-voltage vehicle electrical system having an energy supply unit  1 , which is configured to carry out one specific embodiment of a method according to the present invention. Connected downstream from an electric machine  100  having a double stator winding are two converter circuits  200 . Converter circuits  200  serve both as inverter circuits for supplying a first subsystem N 1  with a first subsystem d.c. voltage U 1 , for example 48 V, via which the electrical consumers V 1  and V 2  are operated, and for supplying a second subsystem N 2  having a second subsystem d.c. voltage U 2 , for example 12 V, via which the consumers V 3 , V 4  V 5  are operated. Energy supply unit  1  makes it possible in a first operating mode to transfer electrical power between the two subsystems N 1  and N 2 , to transfer in a second generator operation electrical power from electric machine  100  to subsystem N 1  and/or N 2  and, in a third motor operation, to transfer electrical power from first motor vehicle electrical system N 1  to electric machine  100  and, if necessary, also to second motor vehicle electrical system N 2 . 
         [0038]    Energy supply unit  1  and a specific embodiment of a method according to the present invention for operating energy supply unit  1  are described with reference to  FIG. 3 . 
         [0039]    Electric machine  100  in this example is configured as a three-phase electric machine. Electric machine  100  includes a first stator winding  101  and a second stator winding  102 . Each of stator windings  101  and  102  includes three stator inductances or phases L 1a , L 1b , L 1c  and L 2a , L 2b , L 2c . The stator inductances of stator windings  101  and  102  are each connected to a delta circuit. Electric machine  100  further includes an excitation winding L 3 . 
         [0040]    First stator winding  101  and second stator winding  102  are each connected to a converter circuit W 1  and a second converter circuit W 2 . The two converter circuits W 1  and W 2  as a whole are labeled with reference numeral  200 . First converter circuit W 1  is connected to terminals P 1a  and P 1b  of first subsystem N 1 , between which first subsystem d.c. voltage U 1  is applied. Second converter circuit W 2  is connected to terminals P 2a  and P 2b  of second subsystem N 2 , between which second subsystem d.c. voltage U 2  is applied. 
         [0041]    Converter circuits W 1  and W 2  are configured, in particular, analogously to an inverter circuit  150  according to the related art. In this configuration, each of converter circuits W 1  and W 2  includes in each case three half bridges B 1a , B 1b , B 1c  and B 2a , B 2b , B 2c . Each of the half bridges includes two switches S 1a  through S 1f  and S 2a  through S 2f , which in this example are configured as MOSFETs. Each of half bridges B 1a , B 1b  and B 1c  of first converter circuit W 1  is connected in each case via a center tap M 1a , M 1b  and M 1c  to a phase connection E 1a , E 1b  and E 1c  of first stator winding  101 . The same applies to center taps M 2a , M 2b  and M 2c  of second converter circuit W 2  and phase connections E 2a , E 2b  and E 2c  of second stator winding  102 . 
         [0042]    Shown in addition to energy supply unit  1  is a processor unit which is configured, in particular, as a control unit  300  of the vehicle, which is programmed to carry out a specific embodiment of a method according to the present invention. Control unit  300  controls the activation of electric machine  100  and converter circuits W 1  and W 2  in general and of the individual parts and the switching of individual switches S 1a  to S 1f  and S 2a  to S 2f  in particular. 
         [0043]    In a transformational operating mode (first operating mode), electrical power is transferred between first subsystem N 1  and second subsystem N 2  via first and second converter circuits W 1  and W 2  and via first and second stator windings  101  and  102 . 
         [0044]    Described by way of example below is the transfer of electrical power from first subsystem N 1  to second subsystem N 2 . The same applies to the transfer of electrical power in the other direction. First subsystem d.c. voltage U 1  of first subsystem N 1  is converted into a three-phase a.c. voltage operated with the aid of first converter circuit W 1  which is operated or activated as an inverter. For this purpose control unit  300  advantageously activates switches S 1a  through S 1f  of first converter circuit W 1 . This three-phase a.c. voltage generates a current flow in first stator winding  101 , which in turn induces a three-phase a.c. voltage in second stator winding  102 . This induced three-phase a.c. voltage is rectified with the aid of second converter circuit W 2 , which is activated as a rectifier, and fed into second subsystem N 2 . For this purpose control unit  300  advantageously activates switches S 2a  through S 2f  of second converter circuit W 2 . The clocked, advantageous activation of the individual switches of converter circuits W 1  and W 2  enables second subsystem d.c. voltage U 2  to be adjusted. 
         [0045]    An excitation current of the excitation winding L 3  of electric machine  100  is advantageously equal to zero so that no synchronous generated voltage is induced in stator windings  101  and  102 . The transfer of electrical energy may be carried out in conjunction with rotating as well as with stationary electric machine  100 . 
         [0046]    In the second generator operating mode, mechanical power is converted into electrical power and, depending on the need, delivered to first subsystem N 1  with first subsystem d.c. voltage U 1  and/or to second subsystem N 2  with second subsystem d.c. voltage U 2 . The level of these two transferred electrical powers is regulated by activating switches S 1a  through S 1f  of first converter circuit W 1  and switches S 2a  through S 2f  of second converter circuit W 2 , as well as the current through excitation winding L 3 . 
         [0047]    In the third motor operating mode, the phases of electric machine  100  are energized with the aid of clocked advantageous switching of switches S 1a  through S 1f  of first converter circuit W 1 , as a result of which electrical power from first subsystem N 1  is converted into mechanical power. The level of converted electrical power may be adjusted by such activation of switches S 1a  through S 1f . 
         [0048]    Depending on the need, switches S 2a  through S 2f  of second converter circuit W 2  are activated in such a way that in addition to the conversion of electrical power into mechanical power, electrical power is likewise transferred from first subsystem N 1  into second subsystem N 2 . The level of this transferred power is regulated by the activation of switches S 2a  through S 2f  of second converter circuit W 2 . Alternatively, switches S 2a  through S 2f  may also be activated in such a way that no electrical power is transferred from first subsystem N 1  into second subsystem N 2 . 
         [0049]      FIG. 4  shows in a circuit diagram-like manner another embodiment of an energy supply unit  1 * which is configured to carry out another specific embodiment of a method according to the present invention. Energy supply unit  1 * from  FIG. 4  is similar in configuration to energy supply unit  1  from  FIG. 3 . For the sake of clarity, therefore, not all reference numerals are shown again in  FIG. 4 . 
         [0050]    Energy supply unit  1 * from  FIG. 4  differs from energy supply unit  1  from  FIG. 3  by converter circuit W 2 *. The half bridges B 2a , B 2b , B 2c  of energy supply unit  1 * are configured identically to those of energy supply unit  1 . Converter circuit W 2 *, however, includes two switch elements S a * and S b * which are activated by control unit  300 . With the aid of these switch elements S a * and S b * converter circuit W 2 * may be connected either to terminals P 2a  and P 2b  of second subsystem N 2  or to terminals P 1a  and P 1b  of first subsystem N 1 . 
         [0051]    If converter circuit W 2 * is connected to first motor vehicle electrical system N 1 , a maximum transferrable electrical power may be converted in the motor and in the generator operation mode. 
         [0052]    If converter circuit W 2 * is connected to second motor vehicle electrical system N 2 , energy supply unit  1 * is activated similar to  FIG. 3 .