Patent Publication Number: US-2022224250-A1

Title: Bidirectional power converter having intermediate circuit

Description:
This nonprovisional application is a continuation of International Application No. PCT/EP2020/073216, which was filed on Aug. 19, 2020, and which claims priority to German Patent Application No. 10 2019 126 571.1, which was filed in Germany on Oct. 2, 2019, and which are both herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a transformerless power converter including a three-phase alternating current input and output, a first power converter stage having a first input and output and a second input and output, an intermediate circuit, a second power converter stage having a first input and output and a second input and output and a direct current input and output, the first input and output of the first power converter state being electrically connected to the three-phase alternating current input and output, and the second input and output of the second power converter stage being electrically connected to the intermediate circuit, the first input and output of the second power converter stage being electrically connected to the intermediate circuit, and the second input and output of the second power converter stage being electrically connected to the direct current input and output, the first power converter stage being a controlled six-pulse bridge circuit made up of controllable power converter valves arranged in half-bridges, the intermediate circuit including a capacitor half-bridge, i.e., a first capacitor and a second capacitor, the first capacitor being connected to a first connection of the second input and output of the first power converter stage and to a first connection of the first input and output of the second power converter stage, on the one hand, and to the second capacitor, on the other hand, the second capacitor being connected to a second connection of the second input and output of the first power converter stage and to a second connection of the first input and output of the second power converter stage, on the one hand, and to the first capacitor, on the other hand, the connecting point between the first capacitor and the second capacitor being connected to a neutral conductor connection of the three-phase alternating current input and output. 
     The invention further relates to a method for rectifying two-phase current from a two-phase alternating current source with the aid of the transformerless power converter, the power converter being connected to the two-phase alternating current source via a second outer conductor and a third outer conductor connection of the three-phase alternating current input and output; and a method for rectifying single-phase current from a single-phase alternating current source with the aid of the transformerless power converter, the power converter being connected to the single-phase alternating current source via a third outer conductor connection of the three-phase alternating current input and output. 
     Description of the Background Art 
     With the aid of the first power converter stage of the transformerless power converter, a direct voltage is generated from a single-phase, a two-phase or a three-phase current, which is supplied to the second power converter stage via the intermediate circuit. With the aid of the second power converter stage, the direct voltage of the intermediate circuit may be set to the voltage which is to be present at the direct current input and output of the power converter. 
     Power converters are used in charging devices for electric vehicles. Known transformerless power converters for charging electric vehicles on single-phase alternating current grids, two-phase alternating current grids or three-phase alternating current grids currently require a large number of power electronic stages, so that charging devices for universal use on single-, two- or three-phase alternating current grids are expensive. There is therefore demand for a cost-effective and compact charging device for electric vehicles. Compensating currents which may flow into the vehicle due to parasitic leakage capacitances of the battery or other components need to be suppressed so as not to adversely influence the functioning of protective equipment against electrical hazard for people. Circuit parts required for single- or multiphase operation should be used in different functions, depending on the operating mode, in order to minimize the complexity for the charging device. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a power converter that has a first half-bridge made up of controlled power converter valves, which is connected to the first connection and the second connection of the output of the first power converter stage, and the connecting point between the power converter valves of the first power converter valve half-bridge is connected to the connecting point of the capacitor half-bridge via a first throttle. The voltage over the first and/or second capacitor of the capacitor half-bridge of the intermediate circuit may be regulated with the aid of this first power converter valve half-bridge. Leakage currents in the vehicle may be avoided thereby when charged on a single- or two-phase alternating current grid with the aid of the three-phase rectifier. The regulation ensures that the voltage over the first capacitor and the voltage over the second capacitor are the same. 
     The circuit complexity may be reduced if the first power converter valve half-bridge is formed by a first of the half-bridges of the controlled six-pulse bridge circuit. It is then not necessary to provide special power converter valves only for the purpose of the invention. The existing infrastructure of the controlled six-pulse bridge circuit may be used. 
     If the first power converter valve half-bridge can be formed by a first of the half-bridges of the controlled six-pulse bridge circuit, it is advantageous if the power converter includes a first switch, which may be used to switch back and forth between an operation on a three-phase alternating current source and a source having fewer phases. 
     In a first variant, the switch may be a changeover switch, whose shared middle connection is connected to the connecting point of the first power converter valve half-bridge and which, depending on the switch position, is connected to either the first throttle or possibly to a first outer conductor connection of the three-phase alternating current input and output via a further throttle. 
     In a second variant, the switch may also be a changeover switch, whose shared middle connection is connected to the connecting point of the first power converter valve half-bridge via the first throttle and which, depending on the switch position, is connected to either the connecting point of the capacitor half-bridge or to a first outer conductor connection of the three-phase alternating current input and output. 
     In a third variant, the first switch may be an on/off switch, which connects a first outer conductor connection of the three-phase alternating current input and output to the connecting point of the capacitor half-bridge, the first outer conductor connection being connected to the first input and output of the controlled six-pulse bridge circuit via the first throttle. 
     Other variants are possible. In each variant, a conductive connection from the connecting point of the first power converter valve half-bridge to the first outer conductor connection of the three-phase alternating current input and output should be established by the first switch in the case of a supply on a three-phase alternating current source, in order to also supply the power converter with current from the first outer conductor connection, while a conductive connection from the connecting point of the first power converter valve half-bridge to the connecting point of the capacitor half-bridge via the throttle exists, due to the first switch, in the case of a supply on a single-phase or two-phase alternating current source, so that the power converter valves of the first power converter valve half-bridge may be used to regulate a voltage over the second capacitor of the capacitor half-bridge, in particular keep it constant, by a control of the first power converter stage. Due to the constant voltage over the second capacitor, a constant voltage is present at the connecting point of the capacitor half-bridge, which is connected to the neutral conductor. This makes it possible to avoid output ripple currents. Since the neutral conductor may be connected to the PE connection of the vehicle and a charging station or charging column on the side of the alternating current input and output, impermissible reactive currents due to parasitic capacitances of the vehicle may thus be avoided. 
     The quality of the current at the direct current input and output of the power converter when charging on a single-phase alternating current grid may be further improved if the power converter includes a second half-bridge made up of two controlled power converter valves, which is connected to the first connection and the second connection of the second input and output of the first power converter stage, and whose connecting point between the two controlled power converter valves is connected to the second connection of the second input and output of the first power converter stage via a series circuit made up of a second throttle and a third capacitor. With the aid of this second power converter valve half-bridge, the voltage over the capacitor half-bridge of the intermediate circuit may be regulated, in particular held constant. Ripples in the current at the direct current output of the power converter may be avoided thereby, if the three-phase rectifier is used for charging on a single-phase alternating current grid. 
     It is possible that the second power converter valve half-bridge is operated as an active ripple filter, using the third capacitor as a storage capacitor. 
     The power converter may be operated not only as a rectifier but also as an inverter. In particular, an operation as a single-phase inverter or as a three-phase inverter is possible. The second power converter stage may then be operated as a direct current converter, in particular as a step-up converter, which supplies the intermediate circuit with a voltage. This voltage is converted by the first power converter stage operated as an inverter into an alternating voltage. The first power converter stage, i.e., the controlled six-pulse bridge circuit, may be line-commutated or self-commutated, in particular in isolated operation, thus forming a grid. Even in the case of an inverter mode, it is necessary to both keep the power factor constant and prevent leakage currents due to voltage ripples via parasitic capacitances. Even in inverter mode, the voltage ripples over the capacitor half-bridge of the intermediate circuit may therefore be avoided in single-phase mode. The voltage ripples may be avoided using the same means as in rectifier mode. 
     The circuit complexity may be reduced if the second power converter valve half-bridge is formed by a second of the half-bridges of the controlled six-pulse bridge circuit. It is then not necessary to provide special power converter valves only for the purpose of the invention. The existing infrastructure of the controlled six-pulse bridge circuit may be used. 
     If the second power converter valve half-bridge is formed by the second of the half-bridges of the controlled six-pulse bridge circuit, it is advantageous if the power converter includes a second switch, which may be used to switch back and forth between an operation on a three- or two-phase alternating current source and a source having one phase. 
     In a first variant, the second switch may be a changeover switch, whose shared middle connection is connected to the connecting point of the second power converter valve half-bridge and which, depending on the switch position, is connected to the second throttle or possibly to a second outer conductor connection of the three-phase alternating current input and output via a further throttle. 
     In a second variant, the second switch may be a changeover switch, whose shared middle connection is connected to the connecting point of the second power converter valve half-bridge via the second throttle and which, depending on the switch position, is connected to either the third capacitor or a second outer conductor connection of the three-phase alternating current input and output. 
     In a third variant, it is furthermore possible that the second switch is an on/off switch, which connects a second outer conductor connection of the three-phase alternating current input and output to the third capacitor, the second outer conductor connection being connected to the first input and output of the controlled six-pulse bridge circuit via the second throttle. 
     Further variants also exist for the design of the second switch and the integration into the power converter. In each variant, a conductive connection from the connecting point of the second power converter valve half-bridge to the second outer conductor connection of the three-phase alternating current input and output should be able to be established by the second switch in the case of a supply on a three-phase alternating current source, in order to also supply the power converter with current via the second outer conductor connection, while a conductive connection from the connecting point of the second power converter valve half-bridge to the connecting point of the second connection of the second input and output of the first power converter stage via the series circuit made up of the second throttle and the third capacitor exists, due to the first switch, in the case of a supply on a single-phase alternating current source, so that the power converter valves of the first power converter valve half-bridge may be used to regulate a voltage in the intermediate circuit by a control of the first power converter stage. 
     The second power converter stage may be a DC-DC converter, in particular, a synchronous converter. 
     The second power converter stage may comprise at least one power converter valve half-bridge made up of two power converter valves, a first of the two power converters being connected to the first connection and the second of the two power converters to the second connection of the first input and output of the second power converter stage, on the one hand, and both power converters being connected to a connecting point between the two power converters, on the other hand. The connecting point and the second connection of the first input and output may be connected to the second input and output of the second power converter stage via a low pass. If the second power converter stage includes two power converter valve half-bridges of this type, the capacitors of the low-pass filter may be combined into one component. 
     The power converter valves used in the invention may be transistors, power transistors, thyristors, IGBTs, FETs, MOSFETs, or the like. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes, combinations, and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
         FIG. 1 a    shows a power converter, in which the first power converter valve half-bridge and the second power converter valve half-bridge are formed by power converter valves outside a first power converter stage, and in which the second power converter stage is formed by a synchronous converter including one power converter valve half-bridge; 
         FIG. 1 b    shows a variant of the power converter from  FIG. 1 a   , in which, in contrast to the power converter from  FIG. 1 a   , the second power converter stage is formed by a synchronous converter including two power converter valve half-bridges; 
         FIG. 2 a    shows a power converter, in which the first power converter valve half-bridge is formed by a power converter valve half-bridge of the first power converter stage, and the second power converter valve half-bridge is formed by power converter valves outside a first power converter stage, and in which the second power converter stage is formed by a synchronous converter including one power converter valve half-bridge; 
         FIG. 2 b    shows a variant of the power converter from  FIG. 2 a   , in which, in contrast to the power converter from  FIG. 2 a   , the second power converter stage is formed by a synchronous converter including two power converter valve half-bridges; 
         FIG. 3 a    shows a power converter, in which the first power converter valve half-bridge is formed by power converter valves outside a first power converter stage, and the second power converter valve half-bridge is formed by a power converter valve half-bridge of the first power converter stage, and in which the second power converter stage is formed by a synchronous converter including one power converter valve half-bridge; 
         FIG. 3 b    shows a variant of the power converter from  FIG. 3 a   , in which, in contrast to the power converter from  FIG. 3 a   , the second power converter stage is formed by a synchronous converter including two power converter valve half-bridges; 
         FIG. 4 a    shows a power converter, in which the first power converter valve half-bridge and the second power converter valve half-bridge are formed by a power converter valve half-bridge of the first power converter stage, and in which the second power converter stage is formed by a synchronous converter including one power converter valve half-bridge; 
         FIG. 4 b    shows a variant of the power converter from  FIG. 4 a   , in which, in contrast to the power converter from  FIG. 4 a   , the second power converter stage is formed by a synchronous converter including two power converter valve half-bridges; 
         FIG. 5 a    shows a variant of the power converter from  FIG. 4 a   ; and 
         FIG. 5 b    shows a variant of the power converter from  FIG. 4   b.    
     
    
    
     DETAILED DESCRIPTION 
     The drawings each show a power converter including an intermediate circuit, which has a capacitor half-bridge and a first power converter valve half-bridge, for regulating the voltage at a connecting point of the capacitor half-bridge, and which has a second power converter valve half-bridge for regulating a voltage in the intermediate circuit. 
     The principles of a power converter according to the invention are now explained on the basis of the power converter from  FIG. 1 a   . The power converter according to the invention according to  FIG. 1 a   , like the power converters in the other figures, is a bidirectional three-phase alternating current rectifier, which includes a three-phase alternating current input and output L 1 , L 2 , L 3 , N having outer conductor connections L 1 , L 2 , L 3  and a neutral conductor connection N, a first power converter stage  1 , an intermediate circuit  3 , which comprises a capacitor half-bridge C 1 , C 2 , a second power converter stage  2  and a direct current output  24 ,  25 . 
     A first input and output of first power converter stage  1  of the power converter from  FIG. 1 a    is connected to outer conductor connections L 1 , L 2 , L 3  of three-phase alternating current input and output L 1 , L 2 , L 3 , N via a throttle I 1 , I 2 , I 3  in each case (like the inputs of first power converter stages  1  of the power converter from  FIGS. 1 b  through 4 b   ). This three-phase alternating current input and output L 1 , L 2 , L 3 , N may be connected not only to a three-phase alternating current grid. With the aid of three-phase alternating current input and output L 1 , L 2 , L 3 , N, the power converter may also be connected to a two-phase alternating current grid or to a single-phase alternating current grid and be operated on these grids. 
     A second input and output  13 ,  14  of first power converter stage  1  is connected to intermediate circuit  3  (like outputs  13 ,  14  of first power converter stages  1  of the power converter from  FIGS. 1 b  through 5 b   ). The intermediate circuit of the power converter is also connected to first input and output  21 ,  22  of second power converter stage  2 , whose second input and output  23 ,  24  forms the second input and output of the power converter. 
     First power converter stage  1  is formed by a controlled six-pulse bridge circuit made up of controllable power converter valves Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , Q 6  arranged in half-bridges Q 1 , Q 2 ; Q 3 , Q 4 ; Q 5 , Q 6 . A controller for controlling power converter valves Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , Q 6  is preferably provided but not illustrated. Connecting points between power converter valves Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , Q 6  of a half-bridge Q 1 , Q 2 ; Q 3 , Q 4 ; Q 5 , Q 6  are connected to outer conductor connections L 1 , L 2 , L 3  via the first input and output of first power converter stage  1  and throttles I 1 , I 2 , I 3 . Connections outside half-bridges Q 1 , Q 2 ; Q 3 , Q 4 ; Q 5 , Q 6  are connected to connections  13 ,  14  of first power converter stage  1 . 
     Capacitor half-bridge C 1 , C 2  is connected to first connection  13  of second input and output  13 ,  14  of first power converter stage  1  and to first connection  21  of first input and output  21 ,  22  of second power converter stage  2 , on the one hand, and to second connection  14  of output  13 ,  14  of first power converter stage  1  and to second connection  22  of first input and output  21 ,  22  of second power converter stage  2 , on the other hand. A connecting point  31  between the two capacitors C 1 , C 2  is connected to neutral conductor connection N. 
     During an operation of the power converter on a single-phase alternating current source or on a two-phase alternating current source, undesirable leakage currents may occur in a device connected to the power converter if the voltage over first capacitor C 1  and the voltage over second capacitor C 2  are not the same. 
     A voltage present between connecting point  31  of the capacitor half-bridge and neutral conductor N, on the one hand, and second connection  14  of the output of first power converter stage  1  and second connection  22  of the output of second power converter  2 , on the other hand, is stabilized by the first power converter valve half-bridge. In the power converter from  FIG. 1 a   , the first power converter valve half-bridge is formed by power converters Q 7 , Q 8 . The connecting point between the two power converters Q 7 , Q 8  is connected to connecting point  31  of capacitor half-bridge C 1 , C 2  and neutral conductor connection N via a throttle I 4 . Connections outside power converter valve half-bridge Q 7 , Q 8  are connected to first connection  13  of second input and output  13 ,  14  of first power converter stage  1  and to first connection  21  of first input and output  21 ,  22  of second power converter stage  2 , on the one hand, and to second connection  14  of second input and output  13 ,  14  of first power converter stage  1  and to second connection  22  of first input and output  21 ,  22  of second power converter stage  2 , on the other hand. 
     By alternately closing and opening power converter valves Q 7 , Q 8  of first power converter valve half-bridge Q 7 , Q 8  with a variable pulse duty factor, the voltage between connecting point  31  and first connection  13  of output  13 ,  14 , which falls over capacitor C 2 , may be set. Power converter valves Q 7 , Q 8  of first power converter valve half-bridge Q 7 , Q 8  are opened and closed in such a way that the voltage over first capacitor C 1  and second capacitor C 2  is the same, or at least the voltage over C 2  is constant as much as possible. Leakage currents may be avoided thereby. 
     A voltage falling over capacitor half-bridge C 1 , C 2 , i.e., at second input and output  13 ,  14  of first power converter stage  1  and at first input and output  21 ,  22  of second power converter stage  2 , may be stabilized with the aid of an active filter for the purpose of avoiding voltage ripples. The active filter is formed by the second power converter valve half-bridge comprising power converter valves Q 9 , Q 10  and the series circuit made up of third capacitor C 3  and throttle I 5 . Connections outside the power converter valve half-bridge are connected to first connection  13  of second input and output  13 ,  14  of first power converter stage  1  and to first connection  21  of first input and output  21 ,  22  of second power converter stage  2 , on the one hand, and to second connection  14  of second input and output  13 ,  14  of first power converter stage  1  and to second connection  22  of first input and output  21 ,  22  of second power converter stage  2 , on the other hand. A connecting point between the two power converter valves Q 9 , Q 10  is connected to second connection  14  of second input and output  13 ,  14  of first power converter stage  1  via series circuit C 3 , I 5  and to second connection  22  of first input and output  21 ,  22  of second power converter stage  2 . By closing and opening power converter valves Q 9 , Q 10 , the voltage over the second power converter valve half-bridge, and thus the voltage over second input and output  13 ,  14  of first power converter stage  1  and over first input and output  21 ,  22  of second power converter stage  2 , may be set. For this purpose, capacitor C 3  and throttle I 5  may be charged while power converter valve Q 9  is open and power converter valve Q 10  is closed and discharged while power converter valve Q 9  is closed and power converter valve Q 10  is open. Voltage ripples may be prevented or at least mitigated thereby. 
     Second power converter stage  2  is formed by a DC-DC converter, namely by a first synchronous converter. Its power converter valve half-bridge made up of two power converter valves Q 11  and Q 12  is connected by its outer connections to first input and output  21 ,  22  of second power converter  2 . A connecting point between power converter valves Q 11  and Q 12  and second connection  22  of the first input and output are connected to second input and output  23 ,  24  via a low pass made up of a throttle I 6  and a capacitor C 4 . 
     The power converter illustrated in  FIG. 2 a    differs from the power converter illustrated in  FIG. 1 a    in that the first power converter valve half-bridge is not formed by components outside first power converter stage  1 , but rather by a first of power converter valve half-bridges Q 1 , Q 2  of controlled six-pulse bridge circuit Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , Q 6 , which is connected to first outer conductor connection L 1  of three-phase alternating current input and output L 1 , L 2 , L 3 , N in the case of three-phase supply. Unlike the power converter from  FIG. 1 a   , the connecting point of power converter valve half-bridge Q 1 , Q 2  is not connected directly to outer conductor connection L 1  via a throttle. The connecting point is connected to a shared middle connection of a changeover switch S 1 . Depending on the control of changeover switch S 1 , a connection may be established from this middle connection to outer conductor connection L 1 , or a connection may be established via throttle I 4  to connecting point  31  in the capacitor bridge circuit. In the case of a connection to outer conductor connection L 1 , power converter valves Q 1 , Q 2  may be used for connection to a three-phase current source. In the case of a connection to throttle I 4 , the power converter valves (in a single-phase or two-phase connection to a power source) may be used to stabilize the voltage between connecting point  31  or neutral conductor connection N and second connection  14  of second input and output  13 ,  14  of first power converter stage  1  and second connection  22  of first input and output  21 ,  22  of second power converter stage  2 . 
     The power converter illustrated in  FIG. 3 a    differs from the power converter illustrated in  FIG. 1 a    in that the second power converter valve half-bridge is not formed by components outside first power converter stage  1 , but rather by a second of power converter valve half-bridges Q 3 , Q 4  of controlled six-pulse bridge circuit Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , Q 6 , which is connected to second outer conductor connection L 2  of three-phase alternating current input and output L 1 , L 2 , L 3 , N in the case of three-phase supply. Unlike the power converter from  FIG. 1 a   , the connecting point of power converter valve half-bridge Q 1 , Q 2  is not connected directly to outer conductor connection L 2  via a throttle. The connecting point is connected to a shared middle connection of a changeover switch S 2 . Depending on the control of changeover switch S 2 , a connection may be established from this middle connection to outer conductor connection L 2  or, a connection may be established via the series circuit of throttle I 5  and capacitor C 3  to second connection  14  of second input and output  13 ,  14  of first power converter stage  1  and to second connection  22  of first input and output  21 ,  22  of second power converter stage  2 . In the case of a connection to outer conductor connection L 2 , power converter valves Q 1 , Q 2  may be used for connection to a three-phase or two-phase current source. If connected to series circuit I 5 , C 3 , the power converter valves (with a single-phase connection to a power source) may be used to stabilize the voltage between first connection  13  of second input and output  13 ,  14  of first power converter stage  1  and first connection  21  of first input and output  21 ,  22  of second power converter stage  2 , on the one hand, and to second connection  14  of second input and output  13 ,  14  of first power converter stage  1  and second connection  22  of first input and output  21 ,  22  of second power converter stage  2 , on the other hand. 
     In the case of the power converter illustrated in  FIG. 4 a   , both first power converter valve half-bridge Q 1 , Q 2  and second power converter valve half-bridge Q 3 , Q 4  are used to stabilize the voltage over capacitor C 2  or the voltage in intermediate circuit  3  in the case of a single- or two-phase supply of the power converter. For this purpose, first power converter valve half-bridge Q 1 , Q 2  and second power converter valve half-bridge Q 3 , Q 4  may be connected via changeover switches S 1  and S 2  either to outer conductor connections L 1 , L 2 , on the one hand, or to throttle I 4  or the series circuit made up of throttle I 5  and capacitor C 3 , on the other hand, as is possible in the case of the converter from  FIG. 2 a    or from  FIG. 3   a.    
     The power converter illustrated in  FIG. 5 a    differs from the power converter illustrated in  FIG. 4 a    in that the number of components used is further reduced. This is possible in that changeover switches S 1 , S 2  are replaced by on and off switches S 1 , S 2 , and the connection of switches S 1 , S 2  to the connecting points between power converter valves Q 1 , Q 2  or Q 3 , Q 4  was changed. In the case of the power converter from  FIG. 5 a   , these connecting points are connected to switches S 1 , S 2  via throttles I 1 , I 2 . 
     With switch S 1  closed, a connection is then established from the connecting point between power converter valves Q 1 , Q 2  via throttle I 1  to neutral conductor connection N or to connecting point  31 . Throttle I 1  takes on the function of throttle I 4  of the power converters from  FIGS. 1 a , 2 a , 3 a  and 4 a   . Power converter valves Q 1 , Q 2  are switched in single- or two-phase alternating current mode in such a way that the voltage at capacitor C 2  of capacitor half-bridge C 1 , C 2  is constant as far as possible. This is the case when the current through throttle I 1  is the inverse of the current through throttle I 3 . 
     With switch S 2  closed, a connection is established to capacitor C 3 , so that a connection exists from the connecting point between power converter valves Q 3 , Q 4  via throttle I 2  and the capacitor to second connection  14  of second input and output  13 ,  14  of first power converter stage  1  and to second connection  22  of first input and output  21 ,  22  of second power converter stage  2 . Throttle I 2  takes on the function of throttle I 5  of the power converters from  FIGS. 1 a , 2 a , 3 a    and  4   a.    
     The power converter from  FIG. 1 b    differs from the power converter from  FIG. 1 a    in that second power converter stage  2  has, in addition to the first synchronous converter, which is already provided in the second power converter stage of the power converter according to  FIG. 1 a   , a second synchronous converter connected in parallel to the first. This second synchronous converter has two power converter valves Q 13 , Q 14  in a half-bridge arrangement and a throttle I 6 , which is connected to the connecting point between power converter valves Q 13 , Q 14  and capacitor C 4 , which is used by both synchronous rectifiers. In the same way, the power converters according to  FIGS. 2 b , 3 b , 4 b  and 5 b    also differ from the power converters illustrated in  FIGS. 2 a , 3 a , 4 a    and  5   a.    
     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 to be included within the scope of the following claims.