Abstract:
A DC voltage converter has a primary side and a secondary side coupled galvanically to the primary side. The primary side has at least one inductor, and the secondary side has at least two secondary capacitors connected in series. A controllable electronic switching device is situated between the primary side and the secondary side. In a first operating mode, depending on the switching position, the secondary capacitors are charged one after the other via the inductor, and the respective charging process ends approximately at the zero crossing of the respective charging current.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a DC-DC converter having a primary side and a secondary side that is coupled galvanically to the primary side. 
         [0003]    2. Description of Related Art 
         [0004]    To supply electric machines of hybrid drives, high voltage batteries or traction batteries are used, to which an inverter is postconnected. A nominal voltage of high voltage batteries is approximately 100 V-300 V. Based on the battery&#39;s internal resistance, a voltage at an intermediate circuit of the inverter, depending on the operating type, as a motor or as a generator, of the electric machine, and depending on the transmitted electric power, amounts to between ca. 50 V and 400 V. A high intermediate voltage leads to cost savings and space savings in the inverter, in wiring harnesses used in the motor vehicle and in the electric machine. In order to achieve these, a single-phase or multi-phase boost chopper is used for increasing the voltage. The classical boost chopper has an inductor which generates an intermittently increased voltage, together with a capacitor, a diode and using a switch. The disadvantage of using such a boost chopper in a hybrid drive is that a very high induction value of the inductor is required, which leads to high costs and to the requirement of a large installation space. Furthermore, semiconductors are used as switches which, during switching, bring about current step changes, which leads to high electrical losses and, with that, to a large required semiconductor surface, which also requires corresponding installation space and generates high costs. In addition, the current step changes lead to a high electromagnetic load in the environment. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    It is therefore an object of the present invention to bring about the increase in a DC voltage in a cost-effective manner, and while saving installation space. 
         [0006]    The object is attained, according to the present invention, in that the primary side has at least one inductor and the secondary side has at least two secondary capacitors connected in series, a controllable or regulatable electronic switching device being situated between the primary side and the secondary side, which in a first operating mode, depending on the switching position, charges the secondary capacitors one after the other via the inductor, and ends the respective charging process approximately at the zero crossing of the respective charging current. In the first operating mode, a DC voltage present on the primary side is increased using the DC voltage converter, and is output on the secondary side. In this context, it is especially provided that the primary side is assigned to a high voltage battery and the secondary side is assigned to an electric machine. The electric machine is preferably a drive assembly of a hybrid drive. Then a motor drive comes about for the first operating mode. Because of the ending of the respective charging process, approximately at the zero crossing of the respective charging current, it is prevented that the switching device generates current step changes upon switching. This, in turn leads to only slight losses being created on the switching device. In addition, based on the procedure according to the present invention, for preventing current step changes from occurring during switching by switching at zero crossings, the electromagnetic load on the environment is considerably reduced. For a durable DC voltage increase, the secondary capacitors are loaded and unloaded in a cyclical manner. 
         [0007]    According to one advantageous refinement of the present invention, it is provided that the inductor and the switching rate of the switching device are dimensioned in such a way that the respective charging current has an approximately sinusoidal half-wave curve. In order to achieve this, a resonant behavior of the inductor within the DC voltage converter is of advantage. Based on the design of the inductor having resonance, only a very slight inductance value of the inductor is required, and the inductor may therefore be designed to be very small. The switching rate gives the frequency of switching of at least one switching element. If the charging current has an approximately sinusoidal half-wave curve, it follows that there is a zero crossing of the charging current at each switching. 
         [0008]    According to one refinement of the present invention, it is provided that the primary side has two input terminals to which a primary capacitor is connected. The use of an additional primary capacitor leads to the primary capacitor, being charged first in a DC voltage conversion. Subsequently, the secondary capacitors are charged using the voltage stored in the primary capacitor, via the inductor and the switching device, whereby the DC voltage conversion is able to be generated very effectively and cyclically. 
         [0009]    According to one refinement of the present invention, two inductors are provided, the one inductor being connected to the one input terminal and to the switching device, and the other inductor being connected to the other input terminal and to the switching device. The two inductors make possible a symmetrization of the circuit structure of the DC voltage converter. Furthermore, its simultaneous action as a filter for electromagnetic compatibility is of advantage. 
         [0010]    According to one advantageous refinement of the present invention, it is provided that the switching device has electronic power semiconductors as switching elements. Because of the switching at zero crossings of the charging current, when semiconductors are used in the switching device, only a small semiconductor surface is required, whereby costs and installation space of the DC voltage converter may also be saved. 
         [0011]    According to one refinement of the present invention, it is provided that diodes are connected in parallel to the switching elements. The use of the diodes in parallel to the switching elements leads to the switching elements being able to develop their interrupted action only in one current flow direction. Consequently, it is possible to maintain the current flow in one direction, via the diode, for instance, from the secondary side to the primary side at one place, whereas the reverse direction is only able to be used if necessary by closing the switching element. 
         [0012]    According to one refinement of the present invention, it is provided that at least two switching elements are connected in series while developing a connecting point, and to that connecting point one of the inductors being connected to the series connection of one of the secondary capacitors. The use of a plurality of switching elements at one connecting point leads to different circuit paths being able to have current applied to them within the DC voltage converter. If, in addition to the switching elements, diodes are used that are connected in parallel to them, it is possible to establish a circuit direction by switching the switching elements. A circuit then closes using a switch, via one of the diodes as well as the inductor. 
         [0013]    In one advantageous refinement of the present invention, it is provided that the switching device, in a second operating mode, charges the primary capacitor via the at least one inductor, using a successive discharge of the secondary capacitors, the respective charging current being switched off by the switching device approximately at a zero crossing. The second operating mode leads to the charging current being led from the secondary side to the primary side. In the process, the DC voltage present at the secondary side is correspondingly lowered going towards the primary side. This second operating mode is particularly advantageous if the DC voltage converter is to be used optionally as a step-up converter, that is, for increasing the DC voltage present at the primary side, or as a step-down converter, that is, for decreasing the direct voltage present at the secondary side. This may be used when the high voltage battery is connected to the primary side and the electric machine is connected to the secondary side. In the first operating mode, in the operation as motor, the high voltage battery applies current to the electric machine, whereby the latter functions as an electric drive. In the second operating mode, the electric machine applies current to the high voltage battery, whereby the latter is loaded, which is denoted as operation as a generator. 
         [0014]    On the secondary side of the DC voltage converter, the potential is shifted at the switching rate of the switching device with respect to the potential on the primary side. From this it comes about that an intermediate circuit voltage supply at an inverter that is postconnected to the secondary circuit has to be set up free of potential. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  shows a circuit diagram of a DC voltage converter. 
           [0016]      FIG. 2  shows a charging current at a first secondary capacitor in a first operating mode. 
           [0017]      FIG. 3  shows a charging current at a second secondary capacitor in a first operating mode. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]      FIG. 1  shows a DC voltage converter  1  as a circuit diagram. DC voltage converter  1  has a primary side  2  and a secondary side  3 , between which a switching device  4  is situated. DC voltage converter  1  has two input terminals  5  and  6 , which connect a high voltage battery, that is not shown, to primary side  2 , whereby a primary voltage is present at the terminals. On secondary side  3  an inverter, that is not shown, which is preconnected to an electric machine of the hybrid drive of a motor vehicle, is connected via two output terminals  7  and  8 , at which a secondary voltage is present. Starting from input terminal  5 , a line  9  runs to a node  10 . From node  10 , a line  11  runs to an inductor  12 , which is connected to a connecting point  14 , using a line  13 . Starting from node  10 , an additional line  15  runs to a primary capacitor  16 , which is connected to a node  18  via a second line  17 . Node  18  leads to input terminal  6  via line  19 . Via a third line  20 , node  18  is connected to an inductor  21 , which is connected to connecting point  23  using a line  22 . Connecting points  14  and  23  are the connecting points  14  and  23  of primary side  2  to switching device  4 . Switching device  4  has four switching elements  24 ,  25 ,  26  and  27 . Each of switching elements  24 ,  25 ,  26  and  27  has an input node  28  and an output node  29 . Switching elements  24 ,  25 ,  26  and  27  are developed as power semiconductors  30 , in this context. Each of power semiconductors  30  has a flow-through direction that goes from its input node  28  to its output node  29 . Diodes  31 ,  32 ,  33  and  34  are assigned to switching elements  24 ,  25 ,  26  and  27 . Diodes  31 ,  32 ,  33  and  34  are each connected via a line  35  to output node  29  and via a line  36  to input node  29  of switching element  24 ,  25 ,  26  and  27  that is assigned to them. Diodes  31 ,  32 ,  33  and  34  have a flow-through direction that runs counter to the flow-through direction of power semiconductor  30  assigned to them. Connecting point  14  is connected to output node  29  of switching element  24  via a line  37 . Furthermore, connecting point  14  is connected to input node  28  of switching element  25  via a line  38 . At output node  29  of switching element  25 , a line  39  is connected which goes to a node  40 , from which a line  41  goes to input node  28  of switching element  26 . Output node  29  of switching element  26  is connected via a line  42  to connecting point  23 , which is connected by a line  43  to input node  28  of switching element  27 . Secondary side  3  is connected by a line  44  to input node  28  of switching element  24 , by a line  45  to node  40  and by a line  46  to output node  29  of switching element  27 . Line  44  leads to a node  47 , which is connected to output terminal  7  via a line  48 . From node  47 , an additional line  49  leads to a first secondary capacitor  50 , which is connected to a node  52  via a line  51 . Node  52  is also connected to line  45 , and has another, third line  53 , which leads to a second secondary capacitor  54 . A line  55  connects secondary capacitor  54  to a node  56 , which is connected to line  46  and an additional line  57 . Line  57  connects node  56  to output terminal  8 . 
         [0019]      FIG. 2  shows a Cartesion coordinate system  60  having an abscissa  61 , that is associated with time t, and an ordinate  62 , that is associated with a charging current I 1 , which is present at secondary capacitor  50 . Four sinusoidal half-wave curves  63  are situated within the Cartesion coordinate system. Between the half-wave curves  63 , time spans  64  are present, in which charging current I 1  is equal to zero. 
         [0020]      FIG. 3  shows a Cartesion coordinate system  65  having an abscissa  66 , that is associated with time t, and an ordinate  67 , that is associated with a charging current I 2 , which is present at secondary capacitor  54 . Sinusoidal half-wave curves  68  are shown within coordinate system  65 . Between the sinusoidal half-wave curves  68 , time spans  69  are present, in which charging current I 2  is equal to zero. 
         [0021]    The sinusoidal half-wave curves  63  and  68  in  FIGS. 2 and 3  are offset in time with respect to each other in such a way that half-wave curves  68  lie within time spans  64  and half-wave curves  63  lie within time spans  69 . 
         [0022]    DC voltage converter  1  shown in  FIG. 1  raises the primary voltage applied between input terminals  5  and  6  by a fixed factor. This factor is preferably the factor of 2, other factors such as factors of 3, 4 and 5 also being conceivable. For those, however, changes would be required in the design of DC voltage converter  1 . At output terminals  7  and  8  a correspondingly raised secondary voltage is emitted. The raising of the primary voltage to the secondary voltage represents a first operating mode, which is used to increase the DC voltage of the high voltage battery and then make it available to the inverter of the electric machine, which is why the first operating mode is designated as the operation as a motor. In addition, a second operating mode using the DC voltage converter  1  shown, in which the secondary voltage is supplied and reduced to the primary voltage. This is used to charge the high voltage battery using the electric machine, which is why this second operating mode is designated as operation as a generator. 
         [0023]    In operation as a motor, electric power is transmitted from the high voltage battery to the electric machine. In the process, the electric charge is transmitted from primary capacitor  16  to secondary capacitors  50  and  54  in two steps. In the first step first secondary capacitor  50  is first charged. In this case, switching element  26  is closed and switching elements  24 ,  25  and  27  are open. Secondary capacitor  50  is then charged by primary capacitor  16  via diode  31 , switching element  26  and inductors  12  and  21 . The inductances of inductors  12  and  21  are adjusted resonantly to the entire electrical system in such a way that charging current I 1  at first secondary capacitor  50  has positive sinusoidal half-wave curve  63 . When charging current I 1  reaches the value zero, switching element  26  is opened, at no, or hardly any current step change. In the second step the charging of secondary capacitor  54  takes place. For this purpose, switching element  25  is closed and switching elements  24 ,  26  and  27  remain open. Secondary capacitor  54  is then charged by primary capacitor  16  via switching element  25 , diode  34  and inductors  12  and  21 . Because of the resonant design of the inductances of inductors  12  and  21 , the positive sinusoidal half-wave curve  68  comes about for charging current I 2 . When charging current I 2  reaches the value zero, switching element  25  is opened, without a current step change taking place in the process. In this way, the operation as a motor is able to be generated durably by a cyclical, alternating switching of switching elements  26  and  25 . 
         [0024]    In operation as a generator, power is transmitted from the electric machine to the high voltage battery. In this context, electric charge is transmitted by secondary capacitors  50  and  54  to primary capacitor  16  in two steps. In the first step there is a charge transmission from first secondary capacitor  50  to primary capacitor  16 . For this purpose, switching element  24  is first closed and switching elements  25 ,  26  and  27  are maintained in the opened state. Primary capacitor  16  is then charged by secondary capacitor  50  via diode  33 , switching element  24  and inductors  12  and  21 . Based on the resonant design of the inductances of inductors  12  and  21 , there comes about in this charging of primary capacitor  16  charging current I 1  having negative sinusoidal half-wave curves that are not shown. When charging current I 1  reaches the value zero, switching element  24  is opened, without generating a current step change. In the second step, the electric charge is transmitted by second capacitor  54  to primary capacitor  16 . For this purpose, switching element  27  is first closed and switching elements  24 ,  25  and  26  are maintained open. Primary capacitor  16  is then charged by secondary capacitor  54  via switching element  27 , diode  32  and inductors  12  and  21 . Based on the resonant design of the inductances of inductors  12  and  21 , it turns out that charging current I 2  has negative sinusoidal half-wave curves, that are not shown. When charging current I 2  reaches the value zero, switching elements  27  is opened in the advantageous manner shown. Consequently, it turns out that charging currents I 1  and I 2  assume from operation as a generator the curve of charging currents I 1  and  1   2  from operation as a motor, but having a negative sign. 
         [0025]    In the DC voltage converter  1  provided, what is critical is particularly sudden voltage changes between a potential of the high voltage battery and the potential of a postconnected inverter intermediate circuit, which is preconnected to the electric machine. This comes about since, especially, the difference of the potentials during switching on a power semiconductors  30  changes suddenly. This sudden change in the potential difference leads to high frequency harmonics in the voltage curve of DC voltage converter  1 . These high frequency harmonics are able to lead to critical compensation currents via a capacitively coupled ground. To counter that, these compensating currents are able to be advantageously designed by suitable grounding concepts within the hybrid drive device. Moreover, it is conceivable that one may use time spans, in which all the switching elements  24 ,  25 ,  26  and  27  are open, for a pre-charge reversal of the voltage potentials. 
         [0026]    The electromagnetic load additionally created by the shifting of the potentials is in contrast to a topology-conditioned filtering, and, with that, a reduction in high frequency interference on the traction network side caused by an inverter operation.