Abstract:
The present invention generally relates to power electronic switching circuits and in particular to inverter modules employing two or more controlled switches that can be used with reactive loads. An inverter circuit is provided which comprises first and second input terminals for being connected to a DC power source; first and second output terminals for outputting an AC voltage; at least one metal oxide semiconductor field effect transistor, MOSFET, having a parasitic body diode. The inverter circuit further comprises at least one disabling element for disabling said body diode. This may result in an improved efficiency of the inverter circuit in combination with a reactive power capability. Further, a semiconductor switching device is disclosed, comprising at least one metal oxide semiconductor field effect transistor, MOSFET, and at least one insulated gate bipolar transistor, IGBT, wherein said MOSFET and said IGBT are connected in parallel.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention generally relates to power electronic switching circuits and in particular to inverter modules employing two or more controlled switches. 
         [0002]    Efficiency is becoming increasingly important in the field of power electronics and in many applications, such as inverter modules for the solar market, the efficiency optimi-zation turned out to represent a major design goal. Photovoltaic solar panels com-monly use PWM, pulse width modulation, inverters to convert DC power generated by the solar cell into AC power which can be fed into the power grid. Typical further appli-cations of these inverters include their use in uninterruptable power supplies, UPS, fuel cells and wind turbines. Further, PWM inverters may be used to provide compensation for reactive loads, harmonic cancellation of supply grids, or as variable speed drives for induction motors. The most commonly used inverters are one phase and three phase transformerless inverters. 
         [0003]    The most common switching elements used in inverter designs are field effect transis-tors, FET, such as metal oxide semiconductor field effect transistors, MOSFET, bipolar transistors, such as insulated gate bipolar transistors, IGBT, bipolar junction transis-tors, BJT, and gate turn-off thyristor, GTO. Traditionally, MOSFETs have been used for low DC voltage or low power inverter designs. IGBTs have been used in medium to high power or high voltage inverter designs. GTOs have been used in very high power inverter designs. 
         [0004]    In order to obtain low losses in an inverter, it is desirable to use transistors that have low switching losses and to use antiparallel/free-wheeling diodes across each transis-tor with good recovery behavior. MOSFETs are generally known to have very good switching performance but the internal antiparallel body diode exhibits poor recovery behavior. This diode can conduct current even if another current path is available, such as a parallel connected free-wheeling diode. When a MOSFET switch turns off, the current can transfer from the MOSFET channel into the parasitic body diode. When the control MOSFET turns on, the recovery charge stored in the body diode during conduction is swept out. Abrupt reverse recovery of the body diode can cause higher switching losses and high frequency ringing, which places higher stresses on the com-ponents and can couple to the output and can cause noise and electromagnetic inter-ference, EMI, associated problems. To compensate, inverter designs using MOSFETs have traditionally required the addition of both series and free-wheeling ultra fast di-odes. The addition of these diodes significantly increases the cost of the inverter de-sign and adds conduction losses. For these reasons IGBTs have been a more practi-cal choice for inverters operating above 100 to 200 VDC. IGBTs typically have poorer switching performance than MOSFETs, but require the addition of fewer diodes to pro-vide rapid recovery behavior, since the internal series diode present in IGBTs allows the designer to add a single diode to the free-wheeling path. The use of IGBTs can reduce the cost of an inverter design but may lower the inverter efficiency at higher frequencies. 
         [0005]    Furthermore, when developing highly efficient inverter topologies which can handle reactive power, the intrinsic diode of the MOSFET will cause high reverse recovery losses when reactive power has to be managed.  FIG. 12  shows an inverter circuit be-ing formed only by IGBTs which, however, is not fast enough for most novel solar ap-plications. 
         [0006]    In  FIG. 13  a combined IGBT and MOSFET inverter architecture is shown. This com-bined circuit, however, has the disadvantage that in the case of reactive power the intrinsic body diode causes losses which deteriorate the efficiency to an unacceptable extent. 
         [0007]    Consequently, for future solar inverters a further improvement of efficiency and the ability to handle reactive power is needed. 
         [0008]    In particular, the arrangement shown in  FIG. 13  can only be used when dealing with a reactive load, if a fast recovery epitaxial diode field effect transistor, a so-called FRED-FET, is used. However, these components usually have a higher Rds-on and therefore the losses are significant. Another disadvantage is that their poor reverse recovery properties lead to an unsatisfactory performance in power factor compensation and bi-directional usage. 
         [0009]    Accordingly, there is a need to provide an inverter power module that effectively con-trols and minimizes the effect of the body diode of MOSFETs and specifically, there is a need to provide means to reduce the adverse effects of EMI and the power loss due to the parasitic body diode. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention relates to inverter circuits that can be used with reactive loads. An inverter circuit is provided which comprises first and second input terminals for be-ing connected to a DC power source; first and second output terminals for outputting an AC voltage; at least one metal oxide semiconductor field effect transistor, MOSFET, having a parasitic body diode. The inverter circuit further comprises at least one dis-abling element for disabling said body diode. This results in high efficiency of the in-verter circuit in combination with a reactive power capability. 
         [0011]    The present invention may be used for highly efficient solar inverters, uninterruptable power sources, UPS, bidirectional chargers, for instance for use with electrical vehicles and highly efficient and high-speed motor drives for use in electrical vehicles. 
         [0012]    This disabling element which can be used in bridge or in neutral point clamped, NPC, configurations, can comprise a bypass diode for providing an alternative conduction path for a reactive load current or an inductor for decoupling the MOSFET from the reactive load. 
         [0013]    Together with the bypass electrode additional protection components can be used. As protection element either a suppressor diode connected with a high voltage FRED can be used which protects which the IGBT from reverse current when the parasitic capac-ity of the MOSFET is charged at switch-off. Instead of such a suppressor diode a ca-pacitor paralleled with a resistor can also be used as protection component. 
         [0014]    When decoupling the MOSFET by means of an inductor, also a bypass diode for the reverse path can be advantageous. 
         [0015]    The present invention also relates to a semiconductor switching device, comprising at least one metal oxide semiconductor field effect transistor, MOSFET, having a drain terminal and a source terminal, and at least one insulated gate bipolar transistor, IGBT, having a collector terminal and an emitter terminal, wherein said MOSFET and said IGBT are connected in parallel by coupling the collector terminal of the IGBT to the drain terminal of the MOSFET, and by coupling the emitter terminal of the IGBT to the source terminal of the MOSFET. This structure results in two basic improvements in efficiency: 
         [0016]    Firstly, the efficiency at high load range is enhanced by allocating the static losses of the switch to the IGBT and the dynamic losses to the MOSFET. Secondly, the effi-ciency at light load range can be enhanced by assigning both the static and the switch-ing losses to the MOSFET. The MOSFET turns on fast and is delayed at switch-off so that the switching losses are assigned to the MOSFET. The MOSFET will also carry the current at low power, thus eliminating one pn junction voltage drop of the IGBT, whereas the IGBT will take the majority at maximum load condition. At low load the IGBT is not conducting at all, so resistential tail current losses are eliminated for the whole power range. Therefore it is possible to improve both the overall and the maximum load efficiency. 
         [0017]    The idea of connecting the IGBT and the MOSFET in parallel can be applied to all IGTB based topologies where the switching losses are significantly high, and in all MOSFET topologies which have significant static losses. For solar applications, NPC topologies play a significant role. 
         [0018]    The invention minimizes the effect of parasitic body diodes by preventing significant current flow through the internal body diode of MOSFETs present in an inverter circuit. This ensures minimal body diode reverse recovery effects on every switching cycle. The switching circuit is made more robust because of the reduced power loss, which in turn allows for the use of smaller components, smaller filters, less heat sinking and potentially lower total costs. 
         [0019]    The above brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention may be ob-tained by reference to the following detailed description, advantageous embodiments in connection with the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The foregoing features and other features of the present invention will now be de-scribed with reference to the drawings. In the drawings, the same components have the same reference numerals. The accompanying drawings are incorporated into and form a part of the specification for the purpose of explaining the principles of the invention. The drawings are not to be construed as limiting the invention to only the illustrated and described examples of how the invention can be made and used. Further features and advantages will become apparent from the following and more particular description of the invention which is illustrated in the accompanying drawings, wherein: 
           [0021]      FIG. 1  shows a bridge configuration inverter only using MOSFETs and being suitable also for reactive loads; 
           [0022]      FIG. 2  shows an NPC inverter with mixed chip configuration comprising bypass di-odes for reactive power; 
           [0023]      FIG. 3  shows an NPC inverter with split-up bypass diode paths; 
           [0024]      FIG. 4  shows an NPC inverter with decoupled output and MOSFETs for all switches; 
           [0025]      FIG. 5  is a diagram of the inverter efficiency comparing the inverter topologies of  FIGS. 3 and 4 ; 
           [0026]      FIG. 6  shows an NPC inverter with decoupling inductors at the output and bypass diodes analog to  FIG. 3 ; 
           [0027]      FIG. 7  shows a semiconductor switching device comprising a MOSFET in parallel with an IGBT; 
           [0028]      FIG. 8  shows the configuration of  FIG. 2  using the switching devices of  FIG. 7  instead of the pure MOSFETs; 
           [0029]      FIG. 9  is a diagram of the inverter efficiency of the arrangement of  FIG. 9  in compari-son to the arrangement of  FIG. 2 ; 
           [0030]      FIG. 10  shows a high voltage NPC circuit using the parallel structure of  FIG. 7 ; 
           [0031]      FIG. 11  shows a high voltage NPC inverter circuit with decoupling inductors and by-pass diodes; 
           [0032]      FIG. 12  shows a bridge configuration using only IGBTs according to the state of the art; 
           [0033]      FIG. 13  shows a bridge configuration inverter using a combination of MOSFETs and IGBTs according to the state of the art. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]      FIG. 1  shows a highly efficient inverter topology having the ability to handle a reactive load which uses only MOSFET switches in a full bridge configuration. The circuit shown in  FIG. 1  is coupled to a 400 Volt DC input as it results for instance from a solar photovoltaic panel. The output can be connected to the 220 Volt AC 50 Hz power grid and tolerates reactive power, even if it is not only necessary for line impedance meas-urements or for the protection of the components at special conditions. The circuit al-lows for full reactive power and enlarges the use as a cos φ compensation of the power grid and also for fully bidirectional applications, such as high-efficiency battery chargers. An excellent efficiency level is reached in both directions when using silicon carbide Schottky diodes. 
         [0035]    When referring to  FIG. 14 , a neutral point clamped, NPC, inverter is shown which uses a mixed chip configuration, i.e. MOSFETs in combination with IGBTs. The NPC in-verter of  FIG. 14  reaches 99.2% European efficiency, EE, at 2 kW nominal power. However, it cannot be used with reactive power. As shown in  FIG. 2 , by adding two bypass diodes D 1 , D 2  in the output, reactive power can be handled. These bypass diodes D 1  and D 2  are for instance made from silicon carbide and have a breakdown voltage of 1200 V. 
         [0036]    As a protective circuit, a suppressor diode D 3 , D 4  connected in series with a high-voltage FRED D 5 , D 6  protects the IGBT from reverse current when the parasitic ca-pacity of the MOSFETs is charged at switch-off. It is also possible to design a bidirec-tional inverter with highest efficiency for the reverse conversion. For this purpose, sili-con carbide Schottky technology is recommended for 1200 V rectifier. 
         [0037]    As these high-voltage silicon carbide diodes are expensive, the circuit of  FIG. 3  repre-sents an improved topology for high efficiency compensation or bidirectional mode and only needs silicon carbide rectifiers with 600 V breakdown voltage. In this circuit of course the IGBT 1  and IGBT 2  can also be provided with a protective diode as shown in  FIG. 2 . Diodes D 1 , D 2 , D 4  and D 6  are formed by 600 V SiC Schottky diodes, whereas D 3  and D 5  are fast silicon diodes and D 7  and D 8  are small silicon diodes to protect the rectifier from overvoltage. 
         [0038]    If the arrangements of  FIGS. 2 and 3  is to be modified in a way that the design is only using MOSFETs, it is necessary to disable all the body diodes of the MOSFETs. This can be achieved according to the present invention by using separate outputs and in-dividual decoupling inductors L 1 , L 2  for each of the output paths. 
         [0039]    An NPC inverter with separated output and MOSFETs for all switches is shown in  FIG. 4 . This figure corresponds to the mixed chip solution of  FIG. 2  using 1200 V SiC di-odes for the reactive power option. In contrast to the mixed chip solution of  FIG. 2 , the circuit of  FIG. 4  using only MOSFETs can significantly augment the inverter efficiency. 
         [0040]    This is illustrated in  FIG. 5 . Here the inverter efficiency is shown in correlation with the input power. The circuit of  FIG. 4  is represented by a solid line  501  and the mixed solu-tion of  FIG. 3  is represented by the broken line  502 . The nominal power in this case was PN=2 kW. As can be derived from this figure, the circuit of  FIG. 4  mainly im-proves the efficiency at low power. The efficiency of the inverter module is increased from 99.2 percent to 99.4 percent. The reactive power is managed by fast 1200 V diodes D 3 , D 4 . Silicon carbide diodes are recommended for maximum efficiency in the reverse direction. 
         [0041]    In an analogous way as shown in  FIG. 3 , also the bypass diode paths D 3 , D 4  of  FIG. 4  can be split up in order to allow the use of diodes having lower breakdown voltages. This is shown in  FIG. 6 . Here diodes D 1 , D 2 , D 4  and D 6  are formed by 600 V SiC Schottky diodes, whereas the remaining diodes are fast 600 V silicon diodes. The sili-con carbide diodes represent the rectifying diodes, whereas the silicon diodes account for the reactive power option. 
         [0042]    According to a further aspect of the present invention, the efficiency at the high load range and at the light load range can be improved by using a MOSFET paralleled with an IGBT as the switching devices in any of the inverter structures. 
         [0043]      FIG. 7  shows the diagram of such a semiconductor switching device  700 . The IGBT and the MOSFET are connected to each other in parallel by coupling the collector ter-minal of the IGBT to the drain terminal of the MOSFET, and by coupling the emitter terminal of the IGBT to the source terminal of the MOSFET. 
         [0044]    This combination allows two basic improvements regarding the efficiency: first, the efficiency at high load range can be enhanced by assigning the static losses of the switch to the IGBT and the dynamic losses to the MOSFET. Secondly, the efficiency at light load can be enhanced by assigning both the static and the switching losses to the MOSFET. 
         [0045]    The MOSFET turns on fast and is delayed at switch-off so that the switching losses are assigned to the MOSFET. The MOSFET will also carry the current at low power, so eliminating one pn junction voltage drop of the IGBT. The IGBT, on the other hand, will take the majority at maximum load condition. At low load the IGBT is not conduct-ing at all, so the resistential tail current losses are eliminated for the whole power range. With the structure of  FIG. 7  it is therefore possible to improve both the overall and maximum load efficiency. At switch-on, the gate of the MOSFET is directly parallel with the IGBT gate, because the MOSFET will be faster and take over the switch-on losses. However, at switch-off the MOSFET has to be controlled in order to be delayed so that the IGBT can be released from switch-off losses. Semiconductor switching de-vices according to  FIG. 7  can be used in all IGBT based topologies where the switch-ing losses are significantly high and in every MOSFET topology having significant static losses. In particular, for solar applications including the reactive power feature, topologies which are shown in the following will be of interest. 
         [0046]    When referring back to  FIG. 2 , the inventive semiconductor switching device according to  FIG. 7  can replace MOSFET  1  and MOSFET  2 , respectively. This modified NPC inverter circuit is shown in  FIG. 8 . An advantageous application for such an inverter are applications with power ratings of more than  7  kW and DC voltages higher than 600 V. The inverter circuit of  FIG. 8  can for instance be implemented as an integrated power module. When using 1200 V diodes D 1  and D 2 , the active current will be trans-ported because the intrinsic characteristics of the diodes of the MOSFETs which are part of the switches S 1  and S 2 , are disabled. The suppressor diode connected with a high voltage FRED protects the IGBTs from reverse current when the parasitic capac-ity of the MOSFETs is charged at switch-off. 
         [0047]      FIG. 9  shows a comparison between the efficiency of the inverter structure of  FIG. 2  (solid line  901 ) and the efficiency of the circuit according to  FIG. 8  (broken line  902 ). As can be seen from  FIG. 9 , in particular for higher power the efficiency is augmented significantly. For  FIG. 9  an implementation of  FIG. 2  with a 45 mΩ CoolMOS MOSFET was compared to the arrangement of  FIG. 8  using a 99 mΩ CoolMOS in parallel with PTIGBT. The frequency was 16 kHz and the nominal power PN is 1 kW (CoolMOS is a trademark of INFINEON). 
         [0048]      FIG. 10  depicts a three level inverter circuit which is able to gain further efficiency by using higher voltage switches. This high voltage NPC can achieve circuit efficiencies of more than 99% at higher power levels. Because of the neutral clamping, it is possible to use the MOSFETs up to nearly the breakdown voltage. The handling of reactive power is limited with the circuit of  FIG. 10  because of the limitation of the body diode of the MOSFET. In order to further improve the circuit of  FIG. 10 , therefore, a circuit where the IGBTs which are connected in series between the neutral point and the out-put are connected in an antiparallel fashion, is shown in  FIG. 11 . In  FIG. 11  a MOSFET followed by a diode is shown. However, the switch in the neutral point clamp can of course also be an IGBT or a semiconductor switch according to  FIG. 7  containing both. 
         [0049]    In order to achieve a high efficiency in combination with reactive power capability, the present invention proposes inverter circuits which are based on the common idea to disable the body diode of the MOSFET in bridge or neutral point clamped configura-tions. This disabling can be implemented by a bypass diode and some protection components as this is shown in  FIGS. 2 and 8 . As protection elements a suppressor diode connected with a high voltage FRED can be provided which protects the IGBT from reverse current, when the parasitic capacity of the MOSFET is charged at switch-off. The suppressor diode can also be replaced by a capacitor which is paralleled with a resistor. 
         [0050]    Alternatively, the decoupling by means of inductors in connection with a bypass diode for the reverse path, is used in the embodiments shown in  FIGS. 1 ,  3 ,  4 ,  6  and  11 . 
         [0051]    While the invention has been described with respect to the physical embodiments con-structed in accordance therewith, it will be apparent to those skilled in the art that vari-ous modifications, variations and improvements of the present invention may be made in the light of the above teachings and within the purview of the appended claims with-out departing from the spirit and intended scope of the invention. 
         [0052]    In addition, those areas in which it is believed that those ordinary skilled in the art are familiar have not been described herein in order not to unnecessarily obscure the in-vention described herein. 
         [0053]    Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiments but only by the scope of the appended claims.