Abstract:
Disclosed is an electric circuit, in particular for a medium voltage power converter. The circuit has at least four semiconductor switches which form a series connection and which are connected to poles of a direct current voltage (Ud). A diode is connected in parallel in an inverse direction to each semiconductor switch. A capacitor is connected in parallel to the two semiconductors in the middle of the series connection. The circuit is provided with a pole of an output potential (Ua) which is connected centrally in the series connection. The circuit has a control device for successively controlling the semiconductor switches. The time interval between the transition of two of the semiconductor switches into their respective controlling states is minimal.

Description:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/EP00/03275 which has an International filing date of Apr. 12, 2000, which designated the United States of America, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to an electric circuit, in particular for a medium-voltage power converter, having at least four semiconductor switches that form a series circuit and are connected to poles of a direct voltage, a diode being connected in antiparallel with each of the semiconductor switches, and a capacitor being connected in parallel with the two middle semiconductor switches of the series circuit, having a pole of an output voltage that is connected in the middle to the series circuit, and having a control unit for sequentially driving the semiconductor switches. The invention likewise relates to a corresponding method for operating an electric circuit, in particular for a medium-voltage power converter. 
   2. Related Art 
   Such an electric circuit is generally known and is used, in particular, in medium-voltage power converters. The direct voltage is connected to the outer two taps of the semiconductor switches forming a series circuit, and the output voltage is present at their common, middle tap. The semiconductor switches are switched by a control unit one after another into their conducting and their blocking states. The resulting AC voltage includes of a sequence of pulses. 
   Asymmetric voltage splits between the various semiconductor switches can occur because of the ever present differences between the performance quantities of the semiconductor switches connected in series, and this can lead to overloadings of the individual semiconductor switches. Likewise, as a consequence of switching over the semiconductor switches the voltage jump of the individual pulses of the output voltage is very large, and this can lead to overvoltage peaks. These disadvantages are removed or at least lessened in the case of known power converters with substantial outlay on circuitry. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to create an electric circuit and a method of the type mentioned at the beginning hereof which generate low overvoltage peaks buth without requiring a special outlay. 
   According to the present invention, the object is achieved in the case of an electric circuit of the type mentioned at the beginning hereof by virtue of the fact that a time period between a transition of two semiconductor switches into a respectively conducting state is very short. The object is achieved correspondingly in the case of a method of the type mentioned at the beginning hereof. 
   It is possible with the aid of the circuit according to the present invention to design individual pulses of an output voltage in a stair-step fashion. The result is thus a stair-stepped variation in the edges of the output voltage, in particular. This means at the same time that only a portion of the total direct voltage is switched at one and the same instant in the case of the output voltage. The voltage jumps of the individual stair steps of the output voltage are therefore smaller than in the case of the known circuit. This leads to lower overvoltage peaks and rates of rise of voltage in the case of the output voltage. 
   The outlay required by the present invention on circuitry is limited substantially by comparison with the known art to a different drive of the semiconductor switches. 
   In an advantageous embodiment of the present invention, the time period between the transition of two semiconductor switches is selected as a function of the switching time of one of the semiconductor switches and/or the resonant frequency of the load, if appropriate including the cables present. The time period between the transition of two semiconductor switches is preferably between approximately 0.01 microseconds and approximately 10 microseconds, preferably approximately 2 microseconds. A reduction is thereby achieved in the resulting rate of voltage rise and in the overvoltage peaks at the load. 
   The time period between two successive stair steps of the output voltage is fixed in this way. Such a short time interval between the stair steps according to the present invention is attended by the substantial advantage that because of the short time interval the capacitors via which a current flows during this interval are loaded only slightly. It is therefore not necessary to provide large capacitors, and so the outlay on circuitry in this regard remains low and can even be reduced by comparison with the known art. 
   In an advantageous development of the invention, a provision is made of corresponding further semiconductor switches, diodes and capacitors, it being possible for the control unit to control the semiconductor switches of the series circuit into the conducting state one after another. The individual pulses of the output voltage can thereby be designed as finely as desired in terms of the stair steps. At the same time, overvoltage peaks can be further reduced because voltage jumps become ever smaller. 
   It is particularly advantageous if the capacitance of the capacitor or the capacitors is very small. As already mentioned, this further reduces the outlay on implementing the output voltage of fine stair steps according to the present invention. 
   In an advantageous refinement of the present invention, the time period between the transition of two neighboring semiconductor switches into the respectively conducting state is a long one, in particular between approximately 100 microseconds and approximately 500 microseconds, preferably approximately 250 microseconds. 
   In pictorial terms, this long time period generates an offset in the stair-step output voltage. This offset entails the advantage that current harmonics which would arise per se at the usual operating frequencies of the semiconductor switches from approximately 500 Hz to approximately 1000 Hz are strongly damped or reduced to a lesser extent. The diminished current harmonics are achieved in this case in essence only by driving the semiconductor switches appropriately, and so to this extent there is no need for special outlay on circuitry. 
   The present invention also includes a method for operating an electric circuit in particular for a medium-voltage power converter, the circuit being provided with a plurality of semiconductor switches that form a series circuit and are connected to poles of a direct voltage, a diode being connected in antiparallel to each of the semiconductor switches, a capacitor being connected in parallel in each case starting from the two middle semiconductor switches, the circuit being provided with a pole of an output voltage that is connected in the middle to the series circuit, the semiconductor switches being controlled into their conducting state one after another, and the time period between the transition of two semiconductor switches into the respectively conducting state being very short. 
   The stair-step course of the output voltage already described is generated with the aid of the abovementioned method according to the present invention. As likewise already mentioned, this essentially requires only one appropriate drive of the semiconductor switches, in which case it is possible to generate either quickly successive stair steps or offsets that are spaced further apart, depending on the time intervals between these drives. 
   Further features, possible applications and advantages of the present invention emerge from the following description of exemplary embodiments of the invention, which are illustrated in the figures of the drawing. Here, all the features described or illustrated form the subject matter of the present invention per se or in any desired combination, independently of their combination in the patent claims or their back referral and independently of their formulation and/or representation in the description and/or in the drawing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a schematic circuit diagram of a first exemplary embodiment of an electric circuit according to the present invention; 
       FIG. 2  illustrates a schematic timing diagram of the output voltage of the circuit of  FIG. 1 ; 
       FIG. 3  illustrates a schematic circuit diagram of a second exemplary embodiment of an electric circuit according to the present invention; 
       FIG. 4  illustrates a schematic circuit diagram of a third exemplary embodiment of an electric circuit according to the present invention; 
       FIG. 5  illustrates a schematic timing diagram of the output voltage of the circuit of  FIG. 3 ; and 
       FIG. 6  shows a schematic timing diagram of the output voltage of the circuit of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Illustrated in  FIG. 1  is an electric circuit  10  that may be used, in particular, in a medium-voltage power converter and with which, for example, an electric motor is fed as a load. 
   The direct voltage Ud that is polarized according to the data in  FIG. 1  is applied on the input side to the circuit  10 . The direct voltage Ud can be in a range from approximately 1 kV to approximately 100 kV, preferably at approximately 16 kV. The circuit  10  transforms the direct voltage Ud into an AC voltage Ua that includes a sequence of pulses. 
   Three such circuits  10  with the aid of which one phase of a three-phase AC voltage is generated in each case are normally present in a medium-voltage power converter. 
   Twelve semiconductor switches  11  to  22  are connected in series in the circuit  10  between the positive pole and the negative pole of the direct voltage Ud. The semiconductor switches  11  to  22  are driven by a control unit that is not illustrated in  FIG. 1 . 
   A diode  23  to  34  is connected in antiparallel to each of the semiconductor switches  11  to  22 . 
   A capacitor  35  is connected to the associated diodes  28 ,  29  in parallel with the two semiconductor switches  16 ,  17 . A capacitor  36  is connected to the associated diodes  27  to  30  in parallel with the four semiconductor switches  15  to  18 . A capacitor  37  is connected to the associated diodes  26  to  31  in parallel with the six semiconductor switches  14  to  19 . A capacitor  38  is connected to the associated diodes  25  to  32  in parallel with the eight semiconductor switches  13  to  20 . Finally, a capacitor  39  is connected to the associated diodes  24  to  33  in parallel with the ten semiconductor switches  12  to  21 . 
   As specified in  FIG. 1 , it is assumed that the capacitors  35  to  39  are charged to the following voltages: the capacitor  35  is charged to ⅙ of the direct voltage Ud, the capacitor  36  is charged to ⅓ of the direct voltage Ud, the capacitor  37  is charged to ½ of the direct voltage Ud, the capacitor  38  is charged to ⅔ of the direct voltage Ud, and, finally, the capacitor  39  is charged to ⅚ of the direct voltage Ud. This charging of the capacitors  35  to  39  is achieved by appropriate measures in terms of circuitry. 
   A timing diagram of a pulse of the AC voltage Ua is illustrated against time t in  FIG. 2 . It is assumed that at an instant t 0  all the semiconductor switches  11  to  16  are blocked, and all the semiconductor switches  17  to  22  are conducting. It is also assumed that the output voltage Ua is equal to − 3/6 Ud at the instant t 0 , and an output current Ia flows to the load via the diodes  29  to  34 . It is likewise assumed that complementary semiconductor switches are always switched, that is to say, for example, the semiconductor switch  11  is blocked and then the semiconductor switch  22  is switched to be conducting, or the semiconductor switch  11  is switched to be conducting and then the semiconductor switch  22  is blocked. 
   When at the instant t 1 , for example, the semiconductor switch  11  is controlled to be conducting and the semiconductor switch  22  is blocked, and the semiconductor switches  12  to  16  remain blocked while the semiconductor switches  17  to  21  remain conducting, a current flows from the positive pole of the direct voltage Ud via the semiconductor switch  11 , via the capacitor  39  and via the diodes  33 ,  32 ,  31 ,  30  and  29  to the pole of the output voltage Ua. 
   It is sensible to start with driving the semiconductor switch  11  when the capacitor  39  has a voltage lower than ⅚ Ud and the absolute value of the deviation is the largest of all the capacitors, since this capacitor  39  is charged when the semiconductor switch  11  is closed. It is possible in this way to control the symmetry of the capacitor voltages via the sequence of driving the semiconductor switches and via the time delay. 
   Because of the capacitor  39  charged to ⅚ of the direct voltage Ud, only − 2/6 Ud of the direct voltage − 3/6 Ud still remain as output voltage Ua at this instant t 1 . This is illustrated in  FIG. 2  by the corresponding stair step at the instant t 1 . 
   After the instant t 1 , the abovementioned current would charge the capacitor  39  from ⅚ to the total direct voltage Ud. Before this is the case, however, at an instant t 2  the semiconductor switch  21  is also blocked in addition to the blocked semiconductor switch  22 , and the next semiconductor switch  12  is also controlled to be conducting in addition to the closed semiconductor switch  11 , the other semiconductor switches  13  to  16  remaining blocked, and the semiconductor switches  17  to  20  remaining conducting. The consequence of this is that a current flows from the positive pole of the direct voltage 3/6 Ud via the semiconductor switches  11  and  12 , via the capacitor  38  and via the diodes  32 ,  31 ,  30  and  29  to the pole of the output voltage Ua. 
   Because of the capacitor  38  charged to ⅔ of the direct voltage Ud, only −⅙ Ud of the total direct voltage Ud present still remain as output voltage Ua at this instant t 2 . This is illustrated in  FIG. 2  by the corresponding further stair step at the instant t 2 . 
   This method is continued until the semiconductor switches  11  to  16  are controlled to be conducting and the semiconductor switches  17  to  22  are blocked, and thus the positive pole of the direct voltage 3/6 Ud is connected directly to the pole of the output voltage Ua via the semiconductor switches  11  to  16 . This then effects in  FIG. 2  at an instant t 6  a last stair step to the total direct voltage 3/6 Ud. The total direct voltage 3/6 Ud is therefore present as output voltage Ua at the instant t 6 . 
   Overall, the output voltage Ua has therefore risen from a first level, specifically −Ud/2, in six stair steps to a second level, specifically the direct voltage Ud/2. 
   Thereafter, the semiconductor switches  11  to  16  are controlled again into their blocked state, and the semiconductor switches  17  to  22  are controlled again into their conducting state. The consequence of this is that the output voltage Ua goes back again to Ud/2 in a stair-step fashion. The stair steps correspond in this case in the reverse direction to the stair steps shown in  FIG. 2 . 
   Overall, a pulse has thereby been generated in the output voltage Ua of the circuit  10 . In this case, the switch-on edge and the switch-off edge of this pulse are of stair-step design. 
   The sequence of the driving of the semiconductor  3  switches  11  to  22  in  FIG. 1  is designed, insofar as it relates to the generation of positive and negative edges, as a function of the charge state of the associated capacitors  35  to  39 . Here, this sequence has no influence on the stair-step shape of the voltage generated. 
   Overall, the above-described driving of the semiconductor switches  11  to  22  from the direct voltage Ud on the input side can be used to generate the output voltage Ua in the shape of pulses, the switch-on and switch-off edges of these pulses respectively being of stair-step design. 
   The time interval between the individual instants at which the semiconductor switches  11  to  22  are reversed one after another is very short. In particular, this time interval is selected as a function of the switching time of the semiconductor switches  11  to  22  used and/or of the resonant frequency of the load, if appropriate including the cables present. In particular, this time interval can be selected such that the overvoltage peaks at the load are minimized. For example, the time interval is in a range between approximately 0.01 microseconds and approximately 10 microseconds. As is also specified in  FIG. 2 , this period is preferably 2 microseconds. 
   The capacitances of the capacitors  35  to  39  can be selected to be relatively small on the basis of the existing time intervals between the individual instants at which the semiconductor switches  11  to  22  are reversed one after another. They can be calculated in this case using the following equation:
 
 C =( I ×delta  t )/delta  U   a
 
   Here, C is the capacitance to be calculated, I is the charging current through the respective capacitor, delta t is the time interval between the individual instants at which the semiconductor switches  11  to  22  are reversed one after another, for example 2 microseconds, and delta U is approximately 10% of the nominal voltage of the associated capacitor. 
   The sequence of the driving of the individual series-connected semiconductor switches of a half group should preferably be determined by which capacitors have the voltage deviating most from their nominal value. It is possible in this way respectively to introduce a current flow through the capacitors that recharges the capacitors such that the asymmetric voltage is counteracted. 
   In each of the previously described stair steps, only ⅙, that is to say approximately 17%, of the total direct voltage Ud is passed on to the output voltage Ua. The result of this is that possible overvoltage peaks, for example, in a downstream electric motor, are caused only by these stair steps. The electric motor need therefore not be designed for overvoltage peaks that would occur upon the switching of the total direct voltage Ud. 
     FIGS. 3 and 4  illustrate electric circuits  50  and  60  that largely correspond to the electric circuit  10  of  FIG. 1 . Identical components are therefore marked with identical reference numerals. 
   The timing diagram of  FIG. 5  belongs to the circuit  50  of  FIG. 3 , and the timing diagram of  FIG. 6  belongs to the circuit  60  of  FIG. 4 . The timing diagrams of  FIGS. 5 and 6  are similar to the timing diagram of  FIG. 2 . Identical features are therefore provided with identical designations. 
   As a difference from the circuit  10  of  FIG. 1 , in the circuit  50  of  FIG. 3  a larger capacitor  51  is provided instead of the capacitor  37 . Moreover, in accordance with the timing diagram of  FIG. 5  a longer time interval is provided between the instants t 3  and t 4  than in the case of the timing diagram of  FIG. 2 . 
   In the case of  FIG. 5 , the time interval between, for example, the instants t 1  and t 2  is 2 microseconds, for example, as before. The time interval between the instants t 3  and t 4  is, however, greater by a factor of approximately 100. This time interval is, for example, in a range from approximately 100 microseconds to approximately 1000 microseconds. The time interval is preferably approximately 250 microseconds, as is also specified in  FIG. 5 . 
   The result of this is that the stair steps already known from  FIG. 2  are likewise present at the instants t 1  and t 2  in  FIG. 5 . However, because of the longer time interval, an offset  52  is present in  FIG. 5  between the instants t 3  and t 4 . 
   During this offset  52 , a current flows from the positive pole of the direct voltage Ud via the semiconductor switches  11 ,  12  and  13 , via the capacitor  51  and via the diodes  31 ,  30  and  29  to the pole of the output voltage Ua. Because of the longer time interval between the instants t 3  and t 4 , this current flows longer than between, for example, the instants t 1  and t 2 . This current flowing for a longer time interval imposes a higher load on the capacitor  51  of  FIG. 3  than the capacitor  37  of  FIG. 1 . For this reason, the capacitance of the capacitor  51  is selected to be larger than the capacitance of the capacitor  37 . It can, in turn, be calculated with the aid of the equation already specified, the larger value of the capacitance resulting from the larger delta t. 
   The output voltage Ua therefore rises in the case of the circuit  50  from the first level, specifically −Ud/2, via three stair steps to a second level, specifically the offset  52 , and from there in a further three stair steps to a third level, specifically to the direct voltage Ud/2. 
   When the circuit  50  of  FIG. 3  is used in a medium-voltage power converter, the semiconductor switches  11  to  22 , for example appropriate IGBTs, can usually be operated with an operating frequency from approximately 500 Hz to approximately 1000 Hz. In the case of two-stage inverters, the consequence of these operating frequencies is current harmonics in the output-side current of the AC voltage that are not insubstantial. 
   In accordance with  FIG. 5 , the intermediate circuit voltage is switched from positive to negative with a longer offset  52 , and this leads to a reduction in the voltage harmonics. In the case of the circuit  50  of  FIG. 3  with the associated offset  52  according to  FIG. 5 , the output-side current therefore has smaller current harmonics than in the case of the circuit  10  of  FIG. 1 . 
   As a difference from the circuit  10  of  FIG. 1 , in the circuit  60  of  FIG. 4  two larger capacitors  61  and  62  are provided instead of the capacitors  38  and  36 . The capacitance of the capacitors  61 ,  62  corresponds approximately to the capacitance of the capacitor  51  of  FIG. 3 . 
   Furthermore, in accordance with  FIG. 6  a longer time interval is provided in each case between the instants t 2  and t 3  and between the instants t 4  and t 5  than in the case of  FIG. 2 . This longer time interval corresponds approximately to that of  FIG. 5 . 
   The consequence of this is that two offsets  63  and  64  are present in accordance with  FIG. 6 , which is associated with the circuit  60 . 
   The output voltage Ua therefore rises in the case of the circuit  60  from the first level, specifically −Ud/2, via two stair steps to a second level, specifically the offset  63 , from there in a further two stair steps to a third level, specifically the offset  64 , and from there in a further two stair steps to a fourth level, specifically to the direct voltage Ud/2. 
   In the case of the circuit  60  of  FIG. 4  with the associated offsets  63  and  64  according to  FIG. 6 , the output-side current of the AC voltage therefore has still smaller current harmonics than in the case of the circuit  50  of  FIG. 3 . 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.