Abstract:
A hybrid DC/AC inverter for converting DC power to AC power feed to a grid voltage system has an input circuit, a half/full bridge switchable circuit and an output circuit. The input circuit has two input terminals for connecting to a DC source and outputs the DC power. The half/full bridge switchable circuit can be operated in a buck mode based on amplitudes of the DC power and the grid voltage. The output circuit is for connecting to the grid voltage system. According to comparison results between of the DC power and the grid voltage, the half/full bridge switchable circuit is selectively operated in the buck mode to reduce switching loss and power consumption.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a DC/AC inverter, particularly to a DC/AC inverter capable of reducing switching loss. 
         [0003]    2. Description of the Prior Art 
         [0004]    A DC/AC inverter is commonly applied in a solar energy system for converting DC power to AC power. The DC/AC inverters can be categorized as either half bridge configuration or full bridge configuration. With reference to  FIG. 6 , a half bridge DC/AC inverter comprises two capacitors C 1 , C 2 , a first switch S 1 , a second switch S 2 , a rectifying circuit  61 . 
         [0005]    The two capacitors C 1 , C 2  are connected in series and further respectively connected to two DC power input terminals DC+, DC−. The first switch S 1  and the second switch S 2  are connected in series and further respectively connected to two DC power input terminals DC+, DC−. The rectifying circuit  61  is provided between a first node where the two capacitors C 1 , C 2  are connected in series and a second node where the two switches S 1 , S 2  are connected in series. The rectifying circuit  61  comprises two diodes D 1 , D 2  connected in opposite directions, and two switching elements S 3 , S 4  respectively connected to the two diodes D 1 , D 2  in parallel. 
         [0006]    When grid voltage  62  is in positive cycles, the first switch S 1  is operated in a switching mode and the second switch S 2  is turned off. The inverter outputs a positive half DC voltage (+DC/2) when the first switch S 1  is turned on and the second switch S 2  is turned off. When the first, second and fourth switches S 1 , S 2 , S 4  are turned off and the diode D 2  and the third switch S 3  are turned on, the freewheeling current occurs. 
         [0007]    When the grid voltage  62  is in negative cycles, the second switch S 2  is operated in a switching mode and the first switch S 1  is turned off. In more detail, the inverter outputs a negative half DC voltage (−DC/2) when the second switch S 2  is turned on and the first switch S 1  is turned off. When the first, second and third switches S 1 , S 2 , S 3  are turned off and the diode D 1  and the fourth switch S 4  are turned on, the freewheeling current occurs. By alternately turning on and off the two switches S 1 , S 2 , the inverter produces the positive or negative half DC voltages (+DC/2, −DC/2). Thus, a half bridge inverter has capability of converting a high DC voltage to a low DC voltage (DC/2) to reduce switching loss and power consumption, particularly suitable for electric systems using the relative small grid voltage  62 . 
         [0008]    With reference to  FIG. 7 , a common full bridge inverter comprises a DC capacitor C 1 , first to fourth switches S 1 -S 4 , two switches S 1 , S 2 , and two diodes D 1 , D 2  respectively connected to the two switches S 1 , S 2  in parallel. During positive cycles of the grid voltage  62 , the first switch S 1  is operated in a switching mode, the fourth switch S 4  is turned on, and the second switch S 2  and the third switch S 3  are turned off. In more detail, the inverter outputs a positive DC voltage (+DC) when the first and fourth switches S 1 , S 4  are turned on and the second and third switches S 2 , S 3  are turned off. When the first, second and third switches S 1 , S 2 , S 3  are turned off and the diode D 2  and the fourth switch S 4  are turned on, the freewheeling current occurs. 
         [0009]    During negative cycles the grid voltage  62 , the second switch S 2  is operated in the switching mode, the third switch S 3  is turned on and the first switch S 1  and the fourth switch S 4  are turned off. The inverter outputs a negative DC voltage (−DC) when the second and third switches S 2 , S 3  are turned on and the first and fourth switches S 1 , S 4  are turned off. When the first, second and fourth switches S 1 , S 2 , S 4  are turned off, and the diode D 1  and the third switch S 3  are turned on, the freewheeling current occurs. Thus, a full bridge inverter is suitable for electric systems requiring the large grid voltage. 
         [0010]    Therefore, the use of a full bridge inverter will bring a great benefit of high power converting efficiency in the electric systems that the grid voltage is greater than the half DC voltage. If the grid voltage is smaller than the half DC voltage, the full bridge inverter causes much power loss and the half bridge inverter will be more preferable. 
         [0011]    With reference to  FIG. 8  showing the signal (VInverter) of the inverter and the main voltage (VGrid), since amplitudes of the positive or negative DC voltages generated by the inverter are always greater than that of the grid voltage, either the half bridge inverter or the full bridge inverter encounters the problem of switching loss. 
         [0012]    To overcome the shortcomings, the present invention provides a hybrid DC/AC inverter to mitigate or obviate the aforementioned problems. 
       SUMMARY OF THE INVENTION 
       [0013]    The main objective of the present invention is to provide a hybrid DC/AC inverter provided between a DC source and a grid voltage system for converting DC power to AC power by properly choosing different switching modes based on states of the received DC power and the grid voltage. 
         [0014]    The hybrid DC/AC inverter has an input circuit, a half/full bridge switchable circuit and an output circuit. The input circuit has two input terminals for connecting to a DC source. The half/full bridge switchable circuit can be operated in a buck mode when the grid voltage is smaller than the DC power to convert the DC power to AC power, thereby reducing switching loss. The output circuit is for connecting to the grid voltage system. Other objectives, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a circuit diagram of a first preferred embodiment of a hybrid DC/AC inverter of the present invention; 
           [0016]      FIG. 2  shows an output voltage produced by the hybrid DC/AC inverter of the present invention and a grid voltage output from a grid voltage system; 
           [0017]      FIG. 3  is a circuit diagram of a second preferred embodiment of a hybrid DC/AC inverter of the present invention; 
           [0018]      FIG. 4  is a circuit diagram of a third preferred embodiment of a hybrid DC/AC inverter of the present invention; 
           [0019]      FIG. 5  is a circuit diagram of a fourth preferred embodiment of a hybrid DC/AC inverter of the present invention; 
           [0020]      FIG. 6  is a circuit diagram of a conventional half bridge inverter; 
           [0021]      FIG. 7  is a circuit diagram of a conventional full bridge inverter; and 
           [0022]      FIG. 8  shows a voltage signal produced by the conventional inverter and a grid voltage. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    With reference to  FIG. 1 , a hybrid DC/AC inverter of the present invention is connected between a DC source  10  and a grid voltage system  50  and has an input circuit  20 , a half/full bridge switchable circuit  30  and an output circuit  40 . 
         [0024]    The input circuit  20  includes a positive input terminal +DC, a negative input terminal −DC and two capacitors C 1 , C 2 . The two capacitors C 1 , C 2  have their first ends connected together to form a first node V 1  and their second ends respectively connected to the positive input terminal +DC and the negative input terminal −DC. 
         [0025]    The half/full bridge switchable circuit  30  includes a full bridge switching unit  31 , a half bridge rectifying unit  32  and a controller  33 . 
         [0026]    The full bridge switching unit  31  is connected to the positive input terminal +DC and the negative input terminal −DC and comprises first to fourth switches S 1 -S 4 . The first switch S 1  and the second switch S 2  have their one ends connected together to form a second node V 2  and their the other ends respectively connected to the positive input terminal +DC and the negative input terminal −DC. Similarly, the third switch S 3  and the fourth switch S 4  have their one ends connected together to form a third node V 3  and their the other ends respectively connected to the positive input terminal +DC and the negative input terminal −DC. A fourth diode D 4  and a third diode D 3  are respectively connected in parallel to the first and second switches S 1 , S 2 . The half bridge rectifying unit  32  shares the third switch S 3  and the fourth switch S 4  with the full bridge switching unit  31  and further includes a fifth switch S 5 , a sixth switch S 6 , a first diode D 1 , and a second diode D 2 . The fifth switch S 5  and the sixth switch S 6  are connected in series between the first node V 1  and the second node V 2 . The two diodes D 1 , D 2  are connected in parallel to the fifth switch S 5  and the sixth switch S 6  respectively, wherein cathodes of the two diodes D 1 , D 2  are respectively connected to the first node V 1  and the second node V 2 . 
         [0027]    The controller  33  connects to and controls the first to sixth switches S 1 -S 6 . The controller  33  also selectively drives the first to sixth switches S 1 -S 6  to activate the full bridge rectifying unit  31  or the half bridge rectifying unit  32  based on comparison results between a DC voltage of the DC source  10  with a grid voltage of the grid voltage system  50 . 
         [0028]    In this embodiment, the output circuit  40  has a filter circuit  41  comprised of two inductors L 1 , L 2 . First ends of the two inductors L 1 , L 2  are respectively connected to the second node V 2  and the third node V 3 . Second ends of the two inductors L 1 , L 2  are connected to the grid voltage system  50  as two AC output terminals AC 1 , AC 2 . 
         [0029]    With reference to  FIG. 2 , when amplitude of the grid voltage (Vgrid) is smaller than that of the half DC voltage (DC/2) during the periods t 0 -t 1  and t 2 -t 3  in positive cycles, the sixth switch S 6  is operated in a switching mode, and the third switch S 3  is turned on, enabling the half bridge rectifying unit  32  to operate. When the grid voltage (Vgrid) becomes greater than the positive half DC voltage (DC/2) during the period t 1 -t 2 , the second switch S 2  is operated in a switching mode, and the third switch S 3  is turned on, enabling the full bridge switching unit  31  to operate. When the first, second, fourth to sixth switches S 1 , S 2 , S 4 -S 6  are turned off and the fourth diode D 4  and the third switch S 3  are turned on, the half/full bridge switchable circuit  30  outputs a freewheeling current during the positive cycles. 
         [0030]    When amplitude of the grid voltage (Vgrid) is smaller than that of the half DC voltage (−DC/2) during the periods t 3 -t 4  and t 5 -t 6  in negative cycles, the fifth switch S 5  is operated in a switching mode, the fourth switch S 4  is turned on, enabling the half bridge rectifying unit  32  to operate. When amplitude of the grid voltage (Vgrid) becomes greater than that of the half DC voltage (−DC/2) during the period t 4 -t 5  in negative cycles, the first switch S 1  is operated in a switching mode, and the fourth switch S 4  is turned on, enabling the full bridge switching unit  31  to operate. When the first, second, third, fifth and sixth switches S 1 , S 2 , S 3 , S 5 , S 6  are turned off and the third diode D 3  and the fourth switch S 4  are turned on, the half/full bridge switchable circuit  30  outputs a freewheeling current during the negative cycles. 
         [0031]    In summary, when the amplitude of the grid voltage (Vgrid) is smaller than that of the half DC voltage (DC/2) in both the positive and negative cycles, the half bridge rectifying unit  32  is enabled and operates in a buck mode, wherein the third and sixth switches S 3 , S 6  are operated for positive cycles, and the fourth and fifth switches S 4 , S 5  are operated for negative cycles. 
         [0032]    With reference to  FIG. 3 , a second embodiment of the hybrid DC/AC inverter differs from the first embodiment in removing the original diodes in parallel to the first and second switches S 1 , S 2 . Further, the output circuit  40  further includes a third by-pass diode D 3 , a fourth by-pass diode D 4 , a seventh switch S 7  and a eighth switch S 8 . The seventh switch S 7  is connected between the third switch S 3  and the third node V 3 . The eighth switch S 8  is connected between the fourth switch S 4  and the third node V 3 . The third by-pass diode D 3  has its anode connected to the second node V 2 , and its cathode connected to a node between the third and seventh switches S 3 , S 7 . The fourth by-pass diode D 4  has its cathode connected to the second node V 2 , and its anode connected to a node between the fourth and eighth switches S 4 , S 8 . 
         [0033]    The operations of the second embodiment are similar to that of the first embodiment. When the of the grid voltage (Vgrid) is smaller than the half DC voltage (DC/2) in positive cycles, the third switch and sixth switch S 3 , S 6  are operated in a switching mode, and the seventh switch S 7  is turned on, enabling the half bridge rectifying unit  32  to operate. When the third and sixth switches S 3 , S 6  are turned off and the third diode D 3  and the seventh switch S 7  are turned on, the output circuit  40  outputs a freewheeling current. When the grid voltage (Vgrid) becomes greater than the half DC voltage (DC/2) in positive cycles, the second switch and third switch S 2 , S 3  are operated in a switching mode, and the seventh switch S 7  is turned on, enabling the full bridge switching unit  31  to operate. When the second and third switches S 2 , S 3  are turned off and the third diode D 3  and the seventh switch S 7  are turned on, the output circuit  40  outputs a freewheeling current. 
         [0034]    When amplitude of the grid voltage (Vgrid) is smaller than that of the half DC voltage in negative cycles, the fourth switch and fifth switch S 4 , S 5  are operated in a switching mode, and the eighth switch S 8  is turned on, enabling the half bridge rectifying unit  32  to operate. When the fourth and fifth switches S 4 , S 5  are turned off and the fourth diode D 4  and the eighth switch S 8  are turned on, the output circuit  40  outputs a freewheeling current. When amplitude of the grid voltage (Vgrid) becomes greater than that of the half DC voltage in negative cycles, the first switch and fourth switch S 4 , S 4  are operated in a switching mode, and the eighth switch S 8  is turned on, enabling the full bridge switching unit  31  to operate. When the fourth and first switches S 4 , S 1  are turned off and the fourth diode D 4  and the eighth switch S 8  are turned on, the output circuit  40  outputs a freewheeling current. 
         [0035]    In summary, when the amplitude of the grid voltage (Vgrid) is smaller than that of the half DC voltage (DC/2) in either positive or negative cycles, the half bridge rectifying unit  32  is enabled and operates in a buck mode, wherein the third, sixth and seventh switches S 3 , S 6 , S 7  are operated for positive cycles, and the fourth, fifth, eighth switches S 4 , S 5 , S 8  are operated for negative cycles. 
         [0036]    With reference to  FIG. 4 , a third embodiment of the hybrid DC/AC inverter differs from the first embodiment in removing the original diodes in parallel to the first and second switches S 1 , S 2 . The output circuit  40  further includes a third diode D 3 , a fourth diode D 4 , a seventh switch S 7  and a eighth switch S 8 . The seventh switch S 7  is connected between the third node V 3  and the cathode of the third diode D 3 . The anode of the third diode D 3  is connected to the second node V 2 . The eighth switch S 8  is connected between the second node V 2  and the cathode of the fourth diode D 4 . The anode of the fourth diode D 4  is connected to the third node V 3 . 
         [0037]    When amplitude of the grid voltage (Vgrid) is smaller than that of the half DC voltage (DC/2) in positive cycles, the third and sixth switches S 3 , S 6  are operated in a switching mode, enabling the half bridge rectifying unit  32  to operate. When the third and sixth switches S 3 , S 6  are turned off and the third diode D 3  and the seventh switch S 7  are turned on, the output circuit  40  outputs a freewheeling current. When the grid voltage (Vgrid) becomes greater than the positive half DC voltage (DC/2) in positive cycles, the second and third switches S 2 , S 3  are operated in a switching mode, enabling the full bridge switching unit  31  to operate. When the third and second switches S 3 , S 2  are turned off and the third diode D 3  and the seventh switch S 7  are turned on, the output circuit  40  outputs a freewheeling current. 
         [0038]    When amplitude of the grid voltage (Vgrid) is smaller than that of the half DC voltage (−DC/2) in negative cycles, the fourth and fifth switches S 4 , S 5  are operated in a switching mode enabling the half bridge rectifying unit  32  to operate. When the fourth and fifth switches S 4 , S 5  are turned off and the fourth diode D 4  and the eighth switch S 8  are turned on, the output circuit  40  outputs a freewheeling current. When amplitude of the grid voltage (Vgrid) becomes greater than that of the half DC voltage (−DC/2) in negative cycles, the first and fourth switches S 1 , S 4  are operated in a switching mode, enabling the full bridge switching unit  31  to operate. When the fourth and first switches S 4 , S 1  are turned off and the fourth diode D 4  and the eighth diode S 8  are turned on, the output circuit  40  outputs a freewheeling current. 
         [0039]    In summary, when the amplitude of the grid voltage (Vgrid) is smaller than that of the half DC voltage (DC/2) in either positive or negative cycles, the half bridge rectifying unit  32  is enabled and operates in a buck mode, wherein the third and sixth switches S 3 , S 6  are operated for positive cycles, and the fourth and fifth switches S 4 , S 5  are operated for negative cycles. 
         [0040]    With reference to  FIG. 5 , a fourth embodiment of the hybrid DC/AC inverter differs from the first embodiment in removing the original diodes in parallel to the first and second switches S 1 , S 2 . The output circuit  40  further including a third diode D 3 , a fourth diode D 4 , a seventh switch S 7  and a eighth switch S 8 . The seventh switch S 7  is connected between the third switch S 3  and the third node V 3 . The third by-pass diode D 3  has its anode connected to the second node V 2 , and its cathode connected to a node between the third and seventh switches S 3 , S 7 . The eighth switch S 8  is connected between the second node V 2  and the cathode of the fourth diode D 4 . The fourth diode D 4  has its anode connected to the third node V 3 . 
         [0041]    When the grid voltage (Vgrid) is smaller than that of the half DC voltage (DC/2) in positive cycles, the third switch and sixth switch S 3 , S 6  are operated in a switching mode, and the seventh switch S 7  is turned on, enabling the half bridge rectifying unit  32  to operate. When the third and sixth switches S 3 , s 6  are turned off and the third diode D 3  and the seventh switch S 7  are turned on, the output circuit  40  outputs a freewheeling current. When the grid voltage (Vgrid) becomes greater than the half DC voltage (DC/2) in positive cycles, the second switch and third switch S 2 , S 3  are operated in a switching mode, and the seventh switch S 7  is turned on, enabling the full bridge switching unit  31  to operate. When the third and second switches S 3 , S 2  are turned off and the third diode D 3  and the seventh switch S 7  are turned on, the output circuit  40  outputs a freewheeling current. 
         [0042]    When amplitude of the grid voltage (Vgrid) is smaller than that of the half DC voltage in negative cycles, the fourth switch and fifth switch S 4 , S 5  are operated in a switching mode, enabling the half bridge rectifying unit  32  to operate. When the fourth and fifth switches S 4 , S 5  are turned off and the fourth diode D 4  and the eighth switch S 8  are turned on, the output circuit  40  outputs a freewheeling current. When amplitude of the grid voltage (Vgrid) becomes greater than that of the half DC voltage in negative cycles, the firth switch and fourth switch S 1 , S 4  are operated in a switching mode, enabling the full bridge switching unit  31  to operate. When the fourth and first switches S 4 , S 1  are turned off and the fourth diode D 4  and the eighth switch S 8  are turned on, the output circuit  40  outputs a freewheeling current. 
         [0043]    In summary, when the amplitude of the grid voltage (Vgrid) is smaller than that of the half DC voltage (DC/2) in either positive or negative cycles, the half bridge rectifying unit  32  is enabled and operates in a buck mode, wherein the third, sixth and seventh switches S 3 , S 6 , S 7  are operated for positive cycles, and the fourth and fifth switches S 4 , S 5  are operated for negative cycles. 
         [0044]    Even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and features of the invention, the disclosure is illustrative only. Changes may be made in the details, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.