Abstract:
A method for operating an electronically controlled inverter and an inverter are provided. The inverter includes semiconductor switches, inductors and a first capacitor. The semiconductor switches of the inverter are controlled by a microcontroller alternately as elements of a buck converter and as elements of an inverting Cuk converter with a continuous connection of a neutral conductor at the output to a positive pole at the input side.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2008/061066 filed Aug. 25, 2008, and claims the benefit thereof. The International Application claims the benefits of Austrian Application No. A1473/2007 AT filed Sep. 20, 2007. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a method for operating an electronically controlled inverter comprising semiconductor switches, inductors and a capacitor. In addition, the invention relates to an arrangement for executing the method. 
       BACKGROUND OF INVENTION 
       [0003]    Electronically controlled inverters are known for example from the US publication: C. M. Penalver, et. al. “Microprocessor Control of DC/AC Static Converters”; IEEE Transactions on Industrial Electronics, Vol. IE-32, No. 3, August 1985, pp 186-191. They are used, for example, in solar systems to convert the direct current generated by the solar cells in such a way as to enable it to be fed into the public alternating voltage network. This is the only way to guarantee virtually unrestricted use of solar-produced energy. 
         [0004]    The plurality of possible applications for inverters has resulted, inter alia, in the modification of the basic types of boost converters, boost-buck converters and buck converters for special applications. An example of this cited here is a publication in the journal EDN of 17 Oct. 2002 “Slave converters power auxiliary outputs”, Sanjaya Maniktala; which describes different possible combinations of basic inverter types. 
       SUMMARY OF INVENTION 
       [0005]    An object is to further develop the inverters known from the prior art. 
         [0006]    The object is achieved with a method of the type described in the introduction, in which the inverter&#39;s semiconductor switches are controlled by means of a microcontroller alternately as elements of a boost-buck converter and as elements of an inverting Cuk converter having a continuous connection of a neutral conductor at the output to a positive pole at the input side. 
         [0007]    The combination of the functions of a boost-buck converter and a Cuk converter according to the invention results in a particularly low-loss inverter, which consequently also has a high degree of efficiency and is therefore particularly suitable for use in solar systems. Hereby, the through-switching of the positive pole to the neutral connector of an alternating voltage network ensures that a current source can be connected on the input side, said current source having a negative potential with respect to ground. This is, for example, the case with photovoltaic generators with back-contact cells (e.g. monocrystalline silicon cells). 
         [0008]    In an advantageous version of the method, the inverter&#39;s semiconductor switches are controlled by means of a microcontroller in such a way that a direct voltage applied to the input side during a negative half-wave of an alternating voltage applied to the output side is converted by means of a boost-buck converter and that the direct voltage applied to the input side during a positive half-wave of the alternating voltage applied to the output side is converted by means of a Cuk converter. This provides a low-loss method for feeding current from a direct current source into an alternating voltage network. 
         [0009]    To execute the method according to the invention, an inverter is provided comprising a microcontroller, which is suitably programmed to control the semiconductor switches. Hereby, advantageously, this is a common microcontroller suitable for forming pulse-width modulated signals in dependence on a controller output signal. 
         [0010]    Hereby it is of advantage for the inverter to comprise a first inductor, the first side of which is connected to the negative pole of an direct voltage and the second side of which is connected via a first semiconductor switch to the positive pole of the direct voltage, for the second side of the first inductor to be connected via the series circuit of a second semiconductor switch and a third semiconductor switch to the first terminal of a second inductor, the second terminal of which is connected to a conductor of the alternating voltage, for the connection of the second and third semiconductor switches to be connected via the first capacitor and a fifth semiconductor switch to the neutral conductor of the alternating voltage and for the connection of the first capacitor and fifth semiconductor switch to be connected via a fourth semiconductor switch to the first terminal of the second inductor. This circuit arrangement can be achieved with few circuit elements thus keeping the losses low hence achieving higher circuit efficiency. 
         [0011]    An advantageous method for operating the advantageous inverter envisages that, during the negative half-wave of the alternating voltage, the first, second, third and fourth semiconductor switches are pulsed and the fifth semiconductor switch is permanently switched-on by means of a microcontroller and that hereby the first and second semiconductor switches and third and fourth semiconductor switches are in push-pull mode in each case and that, during the positive half-wave of the alternating voltage, the first and fifth semiconductor switches are pulse-switched in push-pull mode and that, during this period, the second and the fourth semiconductor switches are permanently switched-on and the third semiconductor switch is permanently switched-off. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention will now be described in more detail below in an exemplary way with reference to the attached figures. These show schematically: 
           [0013]      FIG. 1  circuit diagram of the inverter with boost-buck converter and Cuk converter when using general semiconductor switches 
           [0014]      FIG. 2  circuit diagram of the inverter with boost-buck converter and Cuk converter when using n-channel barrier layer MOSFETs 
           [0015]      FIG. 3  current flow during a switched-on phase of the Cuk converter 
           [0016]      FIG. 4  current flow during a switched-off of the Cuk converter 
           [0017]      FIG. 5-8  current flows during operation of the boost-buck converter with a negative half-wave of the alternating voltage 
           [0018]      FIG. 9  inverter signal patterns inverter with boost-buck converter and Cuk converter operation 
           [0019]      FIG. 10  alternative signal patterns of the inverter with boost-buck converter and Cuk converter operation 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0020]    A simple exemplary switching arrangement of an inverter with boost-buck converter and Cuk converter using few components is shown in  FIG. 1 . At the input side, a direct voltage U IN  is applied to an input capacitor Ci. The negative pole of this direct voltage U IN  is connected to the first side of a first inductor L 1 . The second side of the first inductor L 1  is connected via a first semiconductor switch S 1  to the positive pole of the direct voltage U IN . 
         [0021]    The second side of the first inductor L 1  is also connected via the series circuit of a second and a third semiconductor switch S 2 , S 3  to the first terminal of a second inductor L 2 , the second terminal of which is connected to the conductor L of an alternating voltage U OUT  on the output side. The connection of the second and third semiconductor switches S 2 , S 3  is connected via a first capacitor Cc and a fifth semiconductor switch S 5  to the neutral connector N of the alternating voltage U OUT . In addition, a direct connection is provided between the positive pole of the direct voltage U IN  and the neutral connector N of the alternating voltage U OUT . An interconnection point between the first capacitor Cc and fifth semiconductor switch is connected via a fourth semiconductor switch S 4  to the first terminal of the second inductor L 2 . An output capacitor Co is optionally connected between the conductor L and neutral connector N of the alternating voltage U OUT  as an output filter. 
         [0022]    Alternatively to this arrangement, the method according to the invention can also be executed with other switching arrangements, for example with a parallel circuit of a boost-buck converter and a Cuk converter. 
         [0023]    If, as shown in  FIG. 2 , semiconductor switches S 1 , S 2 , S 3 , S 4 , S 5  with inverse diodes are used (n-channel barrier layer MOSFETs or IGBTs), the flow directions of these diodes should be noted. Hereby, the flow direction of the inverse diode of the first semiconductor switch S 1  is specified as running from the negative pole to the positive pole of the direct voltage U IN . The flow directions of the inverse diodes of the second and third semiconductor switches S 2 , S 3  are switched from the first capacitor Cc to the inductors L 1 , L 2 . The inverse diode of the fourth semiconductor switch S 4  is switched from the second inductor L 2  to the first capacitor Cc in the conducting direction. The conducting direction of the inverse diode of the fifth semiconductor switch S 5  is finally specified as running from the connection line between the positive pole of the direct voltage U IN  and the neutral connector of the alternating voltage U OUT  to the first capacitor Cc. 
         [0024]    An arrangement of this kind prevents undesirable current flows through the inverse diodes in the individual switching phases of the inverter. 
         [0025]      FIGS. 3 to 8  show switching arrangements with general semiconductor switches S 1 , S 2 , S 3 , S 4 , S 5 . The switching states hereby also apply to semiconductor switches S 1 , S 2 , S 3 , S 4 , S 5  with inverse diodes. 
         [0026]      FIGS. 3 and 4  show the switching states of the semiconductor switches S 1 , S 2 , S 3 , S 4 , S 5  during a positive half-wave of the alternating voltage U OUT . The conversion of the direct voltage U IN  into an alternating voltage U OUT  is hereby performed by means of a Cuk converter. The second and fourth semiconductor switches S 2 , S 4  are permanently switched-on and the third semiconductor switch S 3  is permanently switched-off, as is also shown in  FIGS. 9 and 10 . The first and the fifth semiconductor switch S 1 , S 5  are pulse-switched in push-pull mode. A starting operation of the Cuk converter is characterized by the switching-off of the fifth semiconductor switch S 5  and the switching-on of the first semiconductor switch S 1 , as shown in  FIG. 3 . Current flows from the positive pole of the direct voltage U IN  via the first switching element S 1  and the first inductor L 1  to the negative pole of the direct voltage U IN . At the same time, current flows from the neutral connector N of the alternating voltage U OUT  via the first semiconductor switch S 1 , the second semiconductor switch S 2 , the first capacitor Cc, the fourth semiconductor switch S 4  and the second inductor L 2  to the conductor L of the alternating voltage U OUT . 
         [0027]    A switched-off phase of the Cuk converter starts with the switching-on of the fifth semiconductor switch S 5  and the switching-off of the first semiconductor switch S 1 , as shown in  FIG. 4 . In the input circuit, the current commutates from the first semiconductor switch S 1  to the series circuit comprising the fifth semiconductor switch S 5 , the first capacitor Cc and the continuously closed second semiconductor switch S 2 . In the output circuit, the current goes from the neutral connector N of the alternating voltage U OUT  via the fifth semiconductor switch S 5 , the fourth semiconductor switch S 4  and the second inductor L 2  to the conductor L of the alternating voltage U OUT . 
         [0028]      FIGS. 5 to 8  show the switching states during a negative half-wave of the alternating voltage U OUT . Hereby, the voltage conversion is performed by means of a boost-buck converter. The first, second, third and fourth semiconductor switches S 1 , S 2 , S 3 , S 4  are pulsed and the fifth semiconductor switch S 5  remains permanently switched-on, wherein the first and second semiconductor switches S 1 , S 2  and third and fourth semiconductor switches S 3 , S 4  are switched in push-pull mode in each case. 
         [0029]    In the zero crossover from the positive to the negative half-wave, the first semiconductor switch S 1  is switched-on and the second and the fourth semiconductor switch S 2 , S 4  is switched-off, as shown in  FIG. 5 . In this switching status, the inverter accepts energy from a direct voltage source on the input side. To this end, a current path is established between the positive pole of the direct voltage U IN  via the first semiconductor switch S 1  and the first inductor L 1  and the negative pole of the direct voltage U IN . 
         [0030]    Hereby, the first inductor L 1  stores energy, which, as shown in  FIG. 6 , in the next step, after the opening of the first semiconductor switch S 1  with the second and third semiconductor switches S 2 , S 3  now closed, is output via the second inductor L 2  to an alternating voltage network on the output side or a load. 
         [0031]    The electric circuit produced thereby runs from the positive pole of the direct voltage U IN  via the alternating voltage network or the load, the second inductor L 2 , the third and second semiconductor switches S 3 , S 2  and the first inductor L 1  to the negative pole of the direct voltage U IN . Hereby, the second inductor L 2  stores energy. At the same time, the first capacitor Cc is charged due to the fact that the fifth semiconductor switch S 5  is also closed. 
         [0032]    In the next switching operation, as shown in  FIG. 7 , the third semiconductor switch S 3  is open and the fourth semiconductor switch S 4  is closed. An electric circuit is formed via the second inductor L 2 , the fourth and fifth semiconductor switches S 4 , S 5  and the alternating voltage network, wherein the second inductor L 2  outputs the stored energy to the alternating voltage network. 
         [0033]    At the same time, a further electric circuit runs from the positive pole of the direct voltage U IN  via the fifth and second switching elements S 5 , S 2 , the first capacitor Cc and the first inductor L 1  to the negative pole of the direct voltage U IN . 
         [0034]    With switching status shown in  FIG. 8 , a switching cycle is concluded during the negative half-wave. The first semiconductor switch S 1  is closed and thereby a current path is established between the positive pole of the direct voltage U IN  via the first semiconductor switch. S 1  and the first inductor L 1  to the negative pole of the direct voltage U IN . The inverter accepts electrical energy from the direct voltage source. 
         [0035]    At the same time, the second inductor L 2  outputs energy to the alternating voltage network, since the corresponding electric circuit is still closed via the fourth and the fifth semiconductor switches S 4 , S 5 . The electric circuit is only interrupted again on the opening of the fourth semiconductor switch S 4 . 
         [0036]      FIGS. 9 and 10  each show the exemplary course of the control signals for the semiconductor switches S 1 , S 2 , S 3 , S 4  and S 5 , wherein the two diagrams show conceivable different switching variants during the period of the negative half-wave of the alternating voltage U OUT . 
         [0037]    With the switching variant shown in  FIG. 9 , during a negative half-wave, the simultaneous operation of a boost converter and a buck converter take place. The first semiconductor switch S 1  with the function of a boost converter element and the third semiconductor switch S 3  with the function of a buck converter element are continuously pulse-switched. Hereby, the second semiconductor switch S 2  functions as a synchronous rectifier, which is switched synchronously in push-pull mode with the first semiconductor switch  51 . 
         [0038]    Alternatively to this,  FIG. 10  shows a switching variant in which, during the negative half-wave, the inverter works as either a buck converter or as a boost converter. 
         [0039]    During the time intervals in which the alternating voltage U OUT  is lower than the direct voltage U IN , the third semiconductor switch S 3  and, in push-pull mode also the fourth semiconductor switch S 4 , is pulse-switched. During this, the first semiconductor switch S 1  remains continuously switched-off and the second semiconductor switch S 2  remains continuously switched-on. 
         [0040]    In the time interval in which the alternating voltage U OUT  is higher than the alternating voltage U IN , the first semiconductor switch S 1  and, in push-pull mode also the second semiconductor switch S 2 , is pulse-switched. Hereby, the third semiconductor switch S 3  remains continuously switched-on and the fourth semiconductor switch S 4  remains continuously switched-off.