Abstract:
An isolating circuit for a DC/AC converter includes an input, an output, an energy storage element and a switch element. The DC/AC converter includes an energy storage isolated from mains during a freewheeling phase. The output of the isolating circuit is configured to be connected to the DC/AC converter, and the energy storage element is connected to the input and serves for storing energy received from the input. The switching element is connected between the energy storage element and the output of the isolating circuit and is operative to connect the energy storage element to the output during the freewheeling phase, and to isolate the energy storage element from the output outside the freewheeling phase of the DC/AC converter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of copending International Application No. PCT/EP2009/006577, filed Sep. 10, 2009, which is incorporated herein by reference in its entirety, and additionally claims priority from German Application No. DE 102008048841.0, filed Sep. 25, 2008, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Embodiments of the invention relate to the conversion of electric DC voltage to electric AC voltage by using a DC/AC converter, in particular to an isolating circuit for a DC/AC converter for isolating the same from a DC voltage energy source, such as a photovoltaic plant, a fuel cell, a battery or similar. 
         [0003]    Starting from a DC voltage potential of a DC voltage source, it is necessitated to generate alternating current for feeding the energy into an existing alternating voltage mains, which is adapted, with respect to polarity or phase and amplitude, to the potential curve of the alternating voltage, for example a 50 or 60 Hz sinusoidally implemented mains voltage. DC/AC converters are used, for example, in the field of photovoltaics and are implemented without transformers in order to obtain high levels of efficiency. However, it is a disadvantage of the transformerless circuits that the potential of the mains is looped through the transformerless DC/AC converter to the DC voltage side and hence to the solar generator. Therewith, the solar generator is no longer potential-free (floating) and cannot be grounded either, as desired, for example, for thin-film modules. 
         [0004]      FIG. 1  shows a single-phase DC/AC converter in an H4 bridge circuit as described, for example, in the introduction of DE 102 21 592 A1, to which reference is made with respect to more details of the mode of operation. As a DC voltage source, the circuit shown in  FIG. 1  comprises a solar generator SG having DC voltage terminals  10 ,  12 . For converting the solar generator DC voltage U SG  into an alternating current suitable for feeding into mains  14 , the single-phase transformerless DC/AC converter shown in  FIG. 1  comprises a buffer capacitor C 1  connected in parallel to a full bridge  16  consisting of 4 switch units S 1  to S 4 . 
         [0005]    The individual switch units S 1  to S 4  can be implemented as high-frequency switches able to realize, for example, switching operations having frequencies of up to several 100 kHz. Such switches can be implemented as MOS field-effect transistors or as IGBTs (insulated gate bipolar transistors). 
         [0006]    A bridge tap occurs centrally in the parallel branches of the bridge circuit  16  at the connecting nodes  18  and  20  between switch units S 1  and S 2  or between switch units S 3  and S 4 . Connecting nodes  18  and  20  are connected to AC voltage terminals  22  and  24 , which are themselves connected to mains  14 , via choke inductances L 1  or L 2 . The bridge voltage U br  is applied between connecting nodes  18  and  20 . 
         [0007]    For converting the solar generator voltage U SG  into the alternating current necessitated for mains supply, switch units S 1  to S 4  are opened and closed in a predetermined high-frequency timing pattern in a synchronized manner in order to generate bridge voltages distinguishable from each other in a time-discrete manner, whose average value is tuned to the externally applied alternating voltage U mains . During operation of the DC/AC converter, the bridge voltage U br  takes on the voltage U plus  in the case of closed switches S 1  and S 4 , and the voltage U minus  in the case of closed switch units S 2  and S 3 . 
         [0008]    The single-phase DC/AC converters  1  in a H4 bridge circuit described in Fig. are, for example, clocked in a bipolar manner, wherein the two output chokes L 1  and L 2  are provided to prevent potential jumps at the solar generator SG. Such potential jumps are unwanted, since the solar generator SG has a large capacity towards ground and a large capacitive charge-reverse current would flow at a potential jump. By the bipolar clocking performed across the diagonal and the usage of symmetrical output chokes, half the amplitude of the mains voltage U mains  is superimposed on the solar generator voltage U SG . Since this is an impressed voltage, the solar generator SG floats with sinusoidal potential to ground. 
         [0009]      FIG. 2  illustrates the DC voltages of the solar generator to ground, wherein the DC/AC converter is illustrated in a simplified manner in  FIG. 2  and provided with reference numeral  26 . 
         [0010]    The disadvantage of bipolar clocking as described above based on  FIG. 1  is that the obtainable efficiency is only very low. Higher efficiency could be obtained with unipolar clocking or with the so-called single-phase chopping, since here unipolar voltages are generated at the output of the bridge  16  and hence the current ripple in the choke is significantly reduced compared to bipolar clocking, however such clocking methods have disadvantages that do not allow usage in the conversion of a DC voltage, for example a DC voltage provided by a solar generator. In unipolar clocking or single-phase chopping of the bridge, the solar generator SG would show clock-frequent potential jumps to ground, which would result in large capacitive output currents, so that these just described, basically advantageous clocking types cannot be used. 
         [0011]    The problems just described with respect to the efficiency of single-phase DC/AC converters in H4 bridge circuit can be solved by the circuits described based on  FIGS. 3 and 4 , namely the Heric® circuit according to DE 102 21 592 A1 shown in  FIG. 3  and by the H5 circuit according to DE 10 2004 030 912 B3 shown in  FIG. 4 . In the following, only the basic structure of these two known circuits according to the stated publications will be discussed, and regarding a more detailed discussion of the functional principle of these circuits, reference is made to the stated publications. 
         [0012]    In addition to the circuit shown in  FIG. 1 , the circuit shown in  FIG. 3  comprises two parallel connecting paths between bridge taps  18  and  20 , wherein one switch S 5  or S 6  as well as a rectifier diode D 1  or D 2  connected in series are provided in each of them, wherein the rectifier diodes in the individual connecting paths are mutually switched in opposite forward direction. In addition to the circuit described in  FIG. 1 , in the circuit according to  FIG. 4 , switch S 5  is provided between direct current terminal  10  and bridge  16 . Due to their structure, the circuits described based on  FIGS. 3 and 4  allow switching of a so-called freewheeling path. 
         [0013]    In the circuit according to  FIG. 3 , the positive freewheeling current flows across the transistor or switch S 5  and the diode D I , and the negative freewheeling current runs across the transistor or switch S 6  and the diode D 2 . During freewheeling, the solar generator is turned off by switches or transistors S 1  to S 4 , so that the same does not experience any potential jumps. 
         [0014]    The situation is similar in the H5 circuit shown in  FIG. 4 . Here, the positive freewheeling current flows across the transistor S 1  and the freewheeling diode of transistor S 3 , and the negative freewheeling current runs across the transistor S 3  and the freewheeling diode of transistor S 1 . Here, during freewheeling, the solar generator SG is isolated by switches or transistors S 2 , S 4  and S 5 . 
         [0015]    By the circuits described based on  FIGS. 3 and 4 , levels of efficiency that are 1 to 2% higher compared to the levels of efficiency obtainable with the circuit shown in  FIG. 1  can be obtained. 
         [0016]      FIG. 5  shows the voltage of the solar generator to ground in the single-phase transformerless DC/AC converters described based on  FIGS. 1 ,  3  and  4 . As can be seen, in the potential of the solar generator to ground, half the mains voltage amplitude is superimposed. In all cases, the solar generator floats with a sinusoidal potential to ground and cannot be grounded since this would result in a direct path between solar generator SG and mains  14 . 
         [0017]    This may be acceptable for many implementations of solar generators, however, solar generators exist where grounding is desired, in particular when such solar generators use thin-film modules or rear-side contacted solar cells. In thin-film modules, grounding is desired for preventing premature aging of the thin-film modules. Further, grounding of the solar generator may be mandatory in some countries due to national standards. 
       SUMMARY 
       [0018]    According to an embodiment, an isolating circuit for a DC/AC converter, wherein the DC/AC converter has an energy storage isolated from mains during a freewheeling phase, may have: an input; an output connectable to the DC/AC converter; an energy storage element connected to the input and operative to store energy received from the input; and a switching element connected between the energy storage element and the output, wherein the switching element is operative to connect the energy storage element to the output during the freewheeling phase of the DC/AC converter, and to isolate the energy storage element from the output outside the freewheeling phase of the DC/AC converter. 
         [0019]    According to another embodiment, a system may have: a solar generator connected to a reference potential; a DC/AC converter implemented to convert a DC voltage provided by the solar generator into an AC voltage and to provide it to an output of the DC/AC converter, wherein the DC/AC converter is further implemented to isolate an energy storage of the DC/AC converter from the output of the DC/AC converter during a freewheeling phase; and an inventive isolating circuit. 
         [0020]    According to another embodiment, a DC/AC converter circuit for converting a received DC voltage into an AC voltage may have: an input; an output; an energy storage; a switching network connected between the energy storage and the output and operative to isolate the energy storage from the output during a freewheeling phase and to connect the energy storage to the output outside the freewheeling phase; and an inventive isolating circuit connected between the input and the energy storage. 
         [0021]    According to another embodiment, a method for converting a DC voltage provided by a solar generator connected to a reference potential into an AC voltage may have the steps of: outside a freewheeling phase of a DC/AC converter, when an energy storage connected to the input of the DC/AC converter is connected to an output of the DC/AC converter, isolating the solar generator from the DC/AC converter and temporarily storing the energy provided by the solar generator; and during the freewheeling phase of the DC/AC converter, during which the energy storage of the DC/AC converter is isolated from the output of the DC/AC converter, charging the energy storage of the DC/AC converter. 
         [0022]    According to embodiments of the invention, the intermediate circuit capacitor C 1  of the DC/AC converter (see  FIGS. 1 to 4 ) is charged by a grounded solar generator during the freewheeling phase of the DC/AC converter, since the intermediate circuit capacitor C 1  is isolated from mains potential during that time. Outside the freewheeling phases, when the intermediate circuit capacitor is connected to mains via the bridge transistors or bridge switches, the grounded solar generator is isolated, which prevents a short-circuit. According to embodiments of the invention, this isolation is performed with two additional transistors or switches. In order for the solar generator to provide energy during isolation, a further input capacitor C 01  is provided as energy storage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Embodiments of the present invention will be detailed subsequently referring to the appended drawings, in which: 
           [0024]      FIG. 1  is a circuit diagram of a single-phase DC/AC converter in H4 bridge circuit; 
           [0025]      FIG. 2  is an illustration of the definition of the DC voltages of the solar generator to ground; 
           [0026]      FIG. 3  is a schematic diagram of a conventional DC/AC converter; 
           [0027]      FIG. 4  is a schematic diagram of a DC/AC converter in H5 circuit; 
           [0028]      FIG. 5  is the DC voltage curves of the solar generator to ground when using the single-phase transformerless DC/AC converters according to  FIGS. 1 ,  3  and  4 ; 
           [0029]      FIG. 6  is the schematic diagram of an embodiment of the invention consisting of an energy storage, an isolator and a DC/AC converter, wherein in  FIG. 6(   a ) the negative pole of the solar generator is grounded, and wherein in  FIGS. 6(   b ) the positive pole of the solar generator is grounded; 
           [0030]      FIG. 7(   a ) is an embodiment of the isolating circuit with a capacitor as buffer storage and two electronic switches; 
           [0031]      FIG. 7(   b ) is the isolating circuit shown in  FIG. 7(   a ) having a further diode for suppressing a back current into the capacitor and having a solar generator whose negative pole is grounded; 
           [0032]      FIG. 7(   c ) is the isolating circuit shown in  FIG. 7(   a ) having a further diode for suppressing a back current into the capacitor and having a solar generator whose positive pole is grounded; 
           [0033]      FIG. 8  is the DC voltage curves of the solar generator to ground when using the isolating means according to embodiments of the invention, wherein  FIG. 8(   a ) shows the DC voltage curves for a solar generator whose negative pole is grounded, and wherein  FIG. 8(   b ) shows the DC voltage curves for a solar generator whose positive pole is grounded; 
           [0034]      FIG. 9(   a ) is a further embodiment of the invention having a capacitor as a buffer storage and two electronic switches, two choke coils and a freewheeling diode; 
           [0035]      FIG. 9(   b ) is the embodiment shown in  FIG. 9(   a ) having a further diode for suppressing a back current in the capacitor and having a solar generator whose negative pole is grounded; 
           [0036]      FIG. 9(   c ) is the embodiment shown in  FIG. 9(   a ) having a further diode for suppressing a back current into the capacitor and having a solar generator whose positive pole is grounded; 
           [0037]      FIG. 10  is the usage of the isolating means according to  FIG. 7(   a ),  FIG. 7(   b ) and  FIG. 7(   c ) having a conventional DC/AC converter circuit according to  FIG. 3  ( FIG. 10(   a ),  FIG. 10(   b ) and  FIG. 10(   c )); 
           [0038]      FIG. 11  is the usage of the isolating means according to  FIG. 7(   a ),  FIG. 7(   b ) and  FIG. 7(   c ) having a conventional DC/AC converter circuit according to  FIG. 4  ( FIG. 11(   a ),  FIG. 11(   b ) and  FIG. 11(   c )); 
           [0039]      FIG. 12  is the usage of the isolating means according to  FIG. 9(   a ),  FIG. 9(   b ) and  FIG. 9(   c ) having a conventional DC/AC converter circuit according to  FIG. 3  ( FIG. 12(   a ),  FIG. 12(   b ) and  FIG. 12(   c )); and 
           [0040]      FIG. 13  is the usage of the isolating means according to  FIG. 9(   a ),  FIG. 9(   b ) and  FIG. 9(   c ) having a conventional DC/AC converter circuit according to  FIG. 4  ( FIG. 13(   a ),  FIG. 13(   b ) and  FIG. 13(   c )). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]    In the following description of the embodiments of the invention, the same elements or equal elements are provided with the same reference numbers. Elements already described based on  FIGS. 1 to 5  will not be described again in detail and in this regard reference is made to the above statements. 
         [0042]      FIG. 6(   a ) shows an embodiment of the invention where an isolating means  30  is connected between the solar generator SG and the DC/AC converter  26 . In the embodiment shown in  FIG. 6 , the negative pole  32  of the solar generator SG is grounded. The further embodiments also describe a solar generator SG whose negative pole  32  is grounded. It should be noted that the present invention is not limited to such an implementation. Rather, the positive pole  34  of the solar generator can also be grounded, as shown in  FIG. 6(   b ). The present invention is also not limited to a connection of one of the poles of the solar generator SG to ground, but rather the solar generator SG can be connected to any predetermined reference potential, for example by providing an additional voltage source for connecting potentials of the solar generator differing from zero to ground, wherein the voltage source can either be part of the solar generator or an additional external voltage source. 
         [0043]      FIG. 6(   a ) and  FIG. 6(   b ) show schematically the isolating means  30  according to embodiments of the invention which allows to decouple the solar generator SG from mains  14 , wherein the isolating means  30  additionally comprises one or several switches S, as well as at least one energy storage element, for example in the form of a capacitor C. Optionally, further choke elements L or rectifier diodes D can additionally be provided. The isolating means  30  allows the intermediate circuit capacitor C 1  of the DC/AC converter  26  to be charged by the grounded solar generator SG during the freewheeling phase of the DC/AC converter, since the same is isolated from mains potential during the freewheeling phase. During the phase when the intermediate capacitor is connected to mains, switches S isolate the solar generator SG, which prevents a short-circuit. 
         [0044]      FIG. 7(   a ) shows a simple example of a possible implementation of the isolating means according to embodiments of the invention, wherein the isolating means  30  is connected between the direct current terminals  10 ,  12  of the solar generator SG and the input terminals  36  and  38  of the DC/AC converter  26 . In the embodiment shown in  FIG. 7 , the isolating means  30  comprises the two switches S 01  and S 02 , which can be implemented, for example, as electronic switches or transistors, as well as the capacitor C 01  as energy storage. Energy storage C 01  is connected in parallel to terminals  10 ,  12 , i.e. the input of the isolating means  30 , and switch S 01  is connected in series between a first input terminal  10  and a first output terminal  36  of the isolating circuit  30 . Switch S O2  is connected between a second terminal  12  of the input of the isolating circuit  30  and a second terminal  38  of the output of the isolating circuit  30 . Switches S 01  and S 02  are controlled in the DC/AC converter  26  during the freewheeling phase, so that the energy storage capacitor C 1  of the DC/AC converter, which is isolated from mains during this freewheeling phase can be charged by the energy temporarily stored in the energy storage C 01  of isolating means  30 . Outside the freewheeling phase of the DC/AC converter  26 , i.e. during the time when the capacitor C 1  of the DC/AC converter  26  is connected to mains, switches S 01  and S O2  are open to prevent the short-circuit between grounded solar generator SG and grounded mains. At the same time, the energy storage element C 01  allows the energy provided by the solar generator SG to be also temporarily stored by the energy storage element C 01  of the isolating means  30  outside the freewheeling phase of the DC/AC converter  26  for a later release to the DC/AC converter. 
         [0045]      FIGS. 7(   b ) and  7 ( c ) show modifications of the embodiment of  FIG. 7(   a ) where switches S 01  and/or S 02  are realized by transistors. Such transistors may have inverse diodes that still allow back current into capacitor C 01  during isolation of capacitor C 01  from mains  14 . In order to prevent unwanted back current into the capacitor C 01  due to the inverse diodes of the transistors, in such an implementation, diodes D 01  and D 02  are additionally provided. In the circuit according to  FIG. 7(   b ) having a solar generator SG whose negative pole is grounded, the diode D 02  is connected between switch (transistor) S 02  and node  38 . In the circuit according to  FIG. 7(   c ) having a solar generator SG whose positive pole is grounded, the diode D 01  is connected between switch (transistor) S 01  and node  36 . Alternatively, the diode D 01  or D 02  can also be arranged before switch S 01  or S 02 , which means between capacitor C 01  and switch S 01  or S 02 . 
         [0046]      FIG. 8  shows the DC voltage curves of the solar generator SG to ground when using the isolating means as described, for example, based on  FIG. 7 .  FIG. 8(   a ) shows the DC voltage curves for a solar generator whose negative pole is grounded, and  FIG. 8(   b ) shows the DC voltage curves for a solar generator whose positive pole is grounded.  FIG. 8  shows the potentials of the solar generator again to ground, and a comparison with  FIG. 5  shows that by using the isolating means according to embodiments of the invention, the sinusoidal portion of U plus  ( FIG. 8(   a )) or U minus  ( FIG. 8(   b )), as would conventionally occur (see  FIG. 5) , has been substantially eliminated. Further, the potential of the negative pole ( FIG. 8(   a )) or the positive pole ( FIG. 8(   b )) is on zero, since the same is grounded. 
         [0047]      FIG. 9(   a ) shows an isolating circuit according to a further embodiment of the invention, again having a capacitor C 01  as a buffer storage and the two electronic switches S 01  and S 02  that have already been described based on  FIG. 7 . Additionally, the isolating circuit  30 ′ according to  FIG. 9  comprises the two choke coils L 01  and L 02  as well as the freewheeling diode D 03 . Choke coil L 01  is connected in series between the switch S 01  and the first terminal  36  of the output of the isolating circuit  30 ′, and the second choke coil S 02  is connected in series between the switch S 02  and the second terminal  38  of the output of the isolating means  30 ′. Freewheeling diode D 03  is connected between the node  40  between switch S 01  and choke coil L 01  and the node  42  between switch S 02  and choke coil L 02 . 
         [0048]    Similar to  FIGS. 7(   b ) and  7 ( c ),  FIGS. 9(   b ) and  9 ( c ) show modifications of the embodiment of  FIG. 9(   a ), where switches S 01  and/or S 02  are realized by transistors. Such transistors can possibly have inverse diodes that still allow a back current into the capacitor C 01  during an isolation of the capacitor C 01  from mains  14 . In order to prevent the unwanted back current into the capacitor C 01  due to the inverse diodes of the transistors in such an implementation, diodes D 01  or D 02  are additionally provided. In the circuit according to  FIG. 9(   b ) having a solar generator SG whose negative pole is grounded, the diode D 02  is connected between switch (transistor) S 02  and node  42 . In the circuit according to  FIG. 9(   c ) having a solar generator SG whose positive pole is grounded, the diode D 01  is connected between switch (transistor) S 01  and node  40 . Alternatively, diode D 01  or D 02  can also be arranged before switch S 01  or S 02 , i.e. between capacitor C 01  and switch S 01  or S 02 . Again, in an alternative implementation, diode D 01  or D 02  can also be arranged after choke coil L 01  or L 02 , i.e. between choke coil L 01  or L 02  and node  36  or  38 . 
         [0049]    As in the embodiments described based on  FIG. 7 , in the embodiments described based on  FIG. 9 , transistors S 01  and S 02  are also only controlled during the freewheeling phase of the DC/AC converter  26 , and by pulse width modulation, the current in choke coils L 01  or L 02  can be regulated. Compared to the implementations described based on  FIG. 7 , the circuits according to  FIG. 9  are advantageous, since here the input voltage at the capacitor C 01  can be regulated independently of the voltage of the capacitor C 01  in the DC/AC converter  26 . 
         [0050]    Based on  FIG. 10 , examples are described, according to which the isolating means according to  FIG. 7(   a ),  FIG. 7(   b ) or  FIG. 7(   c ) is combined with the circuit according to  FIG. 3  (see  FIG. 10(   a ),  FIG. 10(   b ) or  FIG. 10(   c )). Based on  FIG. 11 , examples are described, according to which the isolating means according to  FIG. 7(   a ),  FIG. 7(   b ) or  FIG. 7(   c ) is combined with the circuit according to  FIG. 4  (see  FIG. 11(   a ),  FIG. 11(   b ) or  FIG. 11(   c )). 
         [0051]    Based on  FIG. 12 , examples are described, according to which the isolating means according to  FIG. 9(   a ),  FIG. 9(   b ) or  FIG. 9(   c ) is combined with the circuit according to  FIG. 3  (see  FIG. 12(   a ),  FIG. 12(   b ) or  FIG. 12(   c )). Based on  FIG. 13 , examples are described, according to which the isolating means according to  FIG. 9(   a ),  FIG. 9(   b ) or  FIG. 9(   c ) is combined with the circuit according to  FIG. 4  (see  FIG. 13(   a ),  FIG. 13(   b ) or  FIG. 13(   c )). 
         [0052]      FIGS. 10 and 12  show the coupling of the isolating means according to  FIGS. 7  or  FIG. 9  with the DC/AC converter circuit according to  FIG. 3 . During the freewheeling phase in the DC/AC converter, i.e. when the current flows through switches S 5  or S 6 , the four bridge transistors S 1  to S 4  are turned off and there is no conductive connection between capacitor C 1  and mains  14 . During this time, the capacitor C 1  can be recharged via switches S 01  and S 02 . Thereby, its potential to ground jumps from the floating mains potential to the fixed solar generator potential. 
         [0053]      FIGS. 11 and 12  show the combination of the isolating means according to  FIG. 7  or  FIG. 9  with the DC/AC converter according to  FIG. 4 . Freewheeling of the DC/AC converter is performed via transistors S 1  and S 3 . During this phase, transistors S 2 , S 4  and S 5  are turned off and capacitor C 1  is potential-free. By switching on transistors S 01  and S 02  of the isolating means, the capacitor C 1  can be recharged in this phase. Thereby, the potential jumps to that of the solar generator. 
         [0054]    Based on  FIGS. 9 ,  12  and  13 , embodiments have been described where two choke coils are provided. The present invention is not limited to this embodiment in practice for symmetry reasons. Alternatively, in these embodiments, only one choke coil can be provided. 
         [0055]    While this invention has been described in terms of several advantageous embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.