Abstract:
A multi-source power converter is proposed to permit bidirectional DC to AC conversion from n (n≧2 and nεN) DC voltage sources to an AC load with a reduced number of switches, and DC to DC conversion. Both single and three phases AC load are considered. The proposed topology consists in a single stage of conversion, and therefore a high efficiency can be expected for the system. Any type of DC sources can be used in the system (fuel-cell, battery, ultra-capacitor, photo-voltaic cells, DC bus, DC to DC or AC to DC converter, etc.). The AC load can be either single or three phases (single-phase AC grid/microgrid, three-phase electric machines, induction machine, synchronous machine, etc.). There is no requirement for the n DC voltage source values; they can be equal or different and they can be used individually or together by the converter to generate the AC output. If different DC voltage values are used, the converter can be controlled to generate a multi-level AC voltage. This permits to improve system&#39;s voltage and current power quality and to reduce electro-magnetic interferences (EMI). Therefore gains on both differential and EMI filters design can be expected.

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/718,456 filed on Oct. 25, 2012, which is incorporated herein in its entirety by reference. 
     
    
     FIELD 
       [0002]    The embodiments disclosed herein related to power converters. More particularly, the disclosed embodiments to switched power converters. 
       BACKGROUND 
       [0003]    Increased costs relating to fossil fuel consumption, including energy, environmental and other costs, have resulted in significant consumer, industrial and government demand for more efficient and less fossil fuel-dependent systems. Significant research and development resources are presently directed towards power sources and electric energy storage devices as batteries, ultra-capacitors, solar panels, fuel-cells, etc. Each of these devices typically supplies a DC power signal at a particular voltage level, which must be converted to other DC levels for use with some devices or inverted into an AC signal to couple to an AC power grid or to supply a power signal to an AC electric motor. 
         [0004]    Similarly, it can be desirable to convert a single or three phase AC power signal into a DC power signal, for example, to allow the electric energy in the signal to be stored or transmitted to another DC electrical system. 
         [0005]    Existing switching devices for conversion of typically contain a relatively high number of switches and other components, making them inefficient due to increased power loss, heat dissipation, and production costs. For example, some tie-grid inverters used to couple a DC power signal to an AC power grid employ isolating transformers as well as a multistage conversion process, which converts DC power to high frequency AC, back to DC, then to a final AC output voltage. Multi-stage converters typically require an energy storage element to decouple converter inputs and outputs. In electrified traction systems, the use of different DC sources operating at different voltage levels requires the use of DC to DC converters to adjust voltage levels. 
         [0006]    There is a need for more efficient power converters. 
       SUMMARY 
       [0007]    In a first aspect, some embodiments of the invention provide a power converter comprising a plurality of DC terminals, a plurality of converter cells, and an AC terminal. 
         [0008]    In another aspect, some embodiments of the invention provide a power converter that coverts power from a plurality of independent DC voltage sources and provides a power output signal to an AC terminal. 
         [0009]    In another aspect, some embodiments of the invention provide a power converter that converts power from an AC source and provides a power output signal to a plurality of independent DC voltage loads. 
         [0010]    In some embodiments, the converter cells comprises bidirectional switches. 
         [0011]    In some embodiments, the converter cells each comprise two bidirectional switches per cell. 
         [0012]    In some embodiments, the converter cells each comprise three bidirectional switches per cell. 
         [0013]    In some embodiments, the plurality of converter cells are each coupled between a plurality of DC terminals and an AC terminal. 
         [0014]    In some embodiments, one converter cell is coupled between ground and an AC terminal. 
         [0015]    In some embodiments, the AC load is a single phase load. 
         [0016]    In some embodiments, the AC source is a single phase source. 
         [0017]    In some embodiments, the AC source is a three phase source. 
         [0018]    In some embodiments, the AC load is a three phase load. 
         [0019]    In some embodiments, a plurality of independent DC voltage sources coupled to the corresponding DC terminals are of equal voltage magnitude. 
         [0020]    In some embodiments, a plurality of independent DC voltage sources coupled to the corresponding DC terminals are of different voltage magnitude. 
         [0021]    In some embodiments, the voltage provided at the AC terminals is a multi-level voltage. 
         [0022]    In some embodiments, only a selection of the bidirectional switches of the plurality of converter cells are conducting power to the AC terminals. 
         [0023]    In some embodiments, only a selection of the bidirectional switches of the plurality of converter cells are conducting power to the DC terminals. 
         [0024]    In some embodiments, a backup DC switch is coupled between an independent backup DC voltage source and a corresponding DC terminal to permit fault tolerant operation. 
         [0025]    In some embodiments, the backup DC switch comprises a bidirectional switch. 
         [0026]    In some embodiments, the power converter converts power from an AC source to an independent backup DC voltage source. 
         [0027]    In some embodiments, the switching states of the bidirectional switches in the converter cells are controlled by a computing platform. 
         [0028]    In some embodiments, at least one DC to DC converter is coupled between at least two independent DC voltage sources. 
         [0029]    In some embodiments, the power converter is used to convert power from at least one independent DC voltage source to at least one independent DC voltage source. 
         [0030]    In some embodiments, the power converter supplies a DC power signal from a plurality of independent DC voltage sources to the AC terminals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    Various embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0032]      FIG. 1  illustrates a power converter according to an example embodiment. 
           [0033]      FIG. 2A  illustrates a power converter according to another example embodiment. 
           [0034]      FIG. 2B  illustrates an example embodiment waveform of a power output signal. 
           [0035]      FIG. 2C  illustrates a second example embodiment waveform of a power output signal 
           [0036]      FIG. 2D  illustrates a third example embodiment waveform of a power output signal. 
           [0037]      FIG. 3  illustrates a power converter according to another example embodiment. 
           [0038]      FIG. 4  illustrates a power converter according to another example embodiment. 
           [0039]      FIG. 5  illustrates a power converter according to another example embodiment. 
           [0040]      FIG. 6A  illustrates a bidirectional switch according to a first example embodiment. 
           [0041]      FIG. 6B  illustrates a bidirectional switch according to a second example embodiment. 
           [0042]      FIG. 6C  illustrates a bidirectional switch according to a third example embodiment. 
           [0043]      FIG. 6D  illustrates a bidirectional switch according to a fourth example embodiment. 
           [0044]      FIG. 6E  illustrates a bidirectional switch according to a fifth example embodiment. 
           [0045]      FIG. 7  illustrates a power output signal of a power converter according to an example embodiment. 
           [0046]      FIG. 8A  illustrates a power converter operating in a first mode according to a first example embodiment. 
           [0047]      FIG. 8B  illustrates a power converter operating in a first mode according to a second example embodiment. 
           [0048]      FIG. 8C  illustrates a power converter operating in a first mode according to a third example embodiment. 
           [0049]      FIG. 8D  illustrates a power converter operating in a first mode according to a fourth example embodiment. 
           [0050]      FIG. 8E  illustrates a power converter operating in a first mode according to a fifth example embodiment. 
           [0051]      FIG. 8F  illustrates a power converter operating in a first mode according to a sixth example embodiment. 
           [0052]      FIG. 8G  illustrates a power converter operating in a first mode according to a seventh example embodiment. 
           [0053]      FIG. 9A  illustrates a power converter operating in a second mode according to a first example embodiment. 
           [0054]      FIG. 9B  illustrates a power converter operating in a second mode according to a second example embodiment. 
           [0055]      FIG. 9C  illustrates a power converter operating in a second mode according to a third example embodiment. 
           [0056]      FIG. 9D  illustrates a power converter operating in a second mode according to a fourth example embodiment. 
           [0057]      FIG. 9E  illustrates a power converter operating in a second mode according to a fifth example embodiment. 
           [0058]      FIG. 9F  illustrates a power converter operating in a second mode according to a sixth example embodiment. 
           [0059]      FIG. 9G  illustrates a power converter operating in a second mode according to a seventh example embodiment. 
           [0060]      FIG. 10A  illustrates a power converter operating in a third mode according to a first example embodiment. 
           [0061]      FIG. 10B  illustrates a power converter operating in a third mode according to a second example embodiment. 
           [0062]      FIG. 10C  illustrates a power converter operating in a third mode according to third example embodiment. 
           [0063]      FIG. 10D  illustrates a power converter operating in a third mode according to a fourth example embodiment. 
           [0064]      FIG. 10E  illustrates a power converter operating in a third mode according to a fifth example embodiment. 
           [0065]      FIG. 10F  illustrates a power converter operating in a third mode according to a sixth example embodiment. 
           [0066]      FIG. 10G  illustrates a power converter operating in a third mode according to a seventh example embodiment. 
           [0067]      FIG. 10H  illustrates a power converter operating in a third mode according to an eighth example embodiment. 
           [0068]      FIG. 10I  illustrates a power converter operating in a third mode according to a ninth example embodiment. 
           [0069]      FIG. 10J  illustrates a power converter operating in a third mode according to a tenth example embodiment. 
           [0070]      FIG. 10K  illustrates a power converter operating in a third mode according to an eleventh example embodiment. 
           [0071]      FIG. 10L  illustrates a power converter operating in a third mode according to a twelfth example embodiment. 
           [0072]      FIG. 10M  illustrates a power converter according to a thirteenth example embodiment. 
           [0073]      FIG. 10N  illustrates various switching states of bidirectional switches operating in a first mode according to an example embodiment. 
           [0074]      FIG. 10O  illustrates various switching states of bidirectional switches operating in a second mode according to an example embodiment. 
           [0075]      FIG. 10P  illustrates various switching states of bidirectional switches operating in a third mode according to an example embodiment. 
           [0076]      FIG. 11A  illustrates a power converter with an embedded computing platform for controlling bidirectional switch states according to an example embodiment. 
           [0077]      FIG. 11B  illustrates a power converter with an embedded computing platform for controlling bidirectional switch states according to another example embodiment. 
           [0078]      FIG. 12A  illustrates a power converter operating as a DC to DC converter according to an example embodiment. 
           [0079]      FIG. 12B  illustrates various switching states of bidirectional switches of a power converter operating as a DC to DC converter according to an example embodiment. 
           [0080]      FIG. 13  illustrates a power converter operating as an inverter according to an example embodiment. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0081]    Some of the following embodiments describe power converters that convert power between a plurality of independent DC voltage sources to a single phase or three phase AC load. Other embodiments describe power converters that convert power between a single phase or three phase AC source to a plurality of independent DC voltage loads. The DC voltage sources and DC voltage loads are independent, meaning that no series connections between them when the power converter is operating as an inverter or rectifier, respectively. 
         [0082]    The independent DC voltage sources or loads can have the same magnitude, or they can be of different magnitudes. As an inverter, the power converter generates a controlled three-phase AC output. As a rectifier, the power converter can supply a plurality of independent DC voltage loads of different magnitudes. Independent DC voltage sources of different magnitudes can generate a multi-level power output signal at the AC terminals. Conversely, the power converter can convert power from an AC source to a plurality of independent DC voltage loads of different magnitudes. 
         [0083]    Each additional independent DC voltage source added to the power converter described herein requires one additional converter cell. For example, a power converter with n independent DC voltage sources would require n+1 converter cells. The use of n independent DC voltage sources may provide 2 n −1 different modes to generate a power output signal. For example, a power converter with two independent DC voltage sources can operate in three modes. In a first mode, the converter cells conduct power from only the first independent DC voltage source. In a second mode, the converter cells conduct power from only the second independent DC voltage source. In a third mode, the converter cells successively conduct power from both the first and second independent DC voltage sources. 
         [0084]    The power converter described in the following embodiments has several advantages, for example, it provides a single stage of power conversion with a reduced number of switches, which results in higher reliability, efficiency, and integration. Additionally, a multi-level power output signal, where power is converted in small voltage steps, produces higher power quality waveforms, reduces dv/dt stresses on the AC load and reduces electromagnetic interference issues. 
         [0085]    Reference is first made to  FIG. 1 , which illustrates a power converter  100  operating as a power inverter for converting power from a plurality of independent DC voltage sources  105 ,  110 , and  115 , to a single phase AC load  170 , according to an example embodiment. In this embodiment DC voltage source  105  represents a first independent DC voltage source, DC voltage source  110  represents a second independent DC voltage source, and DC voltage source  115  represents an n th  independent DC voltage source. 
         [0086]    Power converter  100  includes a plurality of DC terminals  150 ,  155 ,  160 , and  165 , wherein DC terminal  150  represents a first DC terminal coupled to the first independent DC source  105 , DC terminal  155  represents a second DC terminal coupled to independent DC source  110 , DC terminal  160  represents the n th  DC terminal coupled to the n th  independent DC source, and DC terminal  165  is coupled to ground  120 . 
         [0087]    A plurality of converter cells  125 ,  130 , and  135  coupled between each corresponding DC terminals  150 ,  155 , and  160  respectively, and AC terminal  145 . One converter cell  140  coupled between DC terminal  165  and AC terminal  145 . 
         [0088]    Each converter cell includes two bidirectional switches  125   a ,  125   b ,  130   a ,  130   b ,  135   a ,  135   b ,  140   a , and  140   b . The AC terminal  145  comprises two nodes, a positive node  145   a  and a negative node  145   b , wherein switches  125   a ,  130   a ,  135   a , and  140   a  are coupled to the positive node  145   a , and switches  125   b ,  130   b ,  135   b , and  140   b  are coupled to the negative node  145   b . AC terminal nodes  145   a  and  145   b  are coupled to a single phase AC load  170 . AC load  170  comprises a positive node  170   a , and a negative node  170   b.    
         [0089]    The switching state of each bidirectional switch in the converter cells is controlled by a controller (not shown), which may include a digital signal processor board, microcontroller, or field programmable gate array. 
         [0090]    Power converter  100  has various applications, for example, for converting power between independent DC voltage sources including photovoltaic cells or wind turbines and an AC utility grid, or for converting power between an integrated battery management system or uninterrupted power supply and an AC utility grid, as well as for converting power between a hybrid energy storage device and DC micro grid. It can also be used in an electrified traction system, for example, in an aircraft or vehicle. 
         [0091]    Reference is next made to  FIG. 2A , which illustrates an example embodiment of power converter  200  operating as an inverter for converting power from two independent DC voltage sources  205  and  210 , to a single phase AC load  270 . Power converter  200  comprises three DC terminals  250 ,  255 , and  265 , three converter cells  225 ,  230 , and  235 , and AC terminal  245 . DC terminals  250  and  255  are coupled to independent DC voltage sources  205  and  210 , respectively, and DC terminal  265  is coupled to ground  220 . 
         [0092]    AC terminal  245  is coupled to single phase AC load  270 . AC terminal  245  comprises two AC terminal nodes  245   a  and  245   b . AC load  270  comprises a positive node  270   a , and a negative node  270   b . AC terminal node  245   a  is coupled to the positive node  270   a  of single phase AC load  270 , and  245   b , is coupled to the negative node  270   b  of single phase AC load  270 . 
         [0093]    Converter cells  225 ,  230 , and  235 , each comprise two bidirectional switches  225   a ,  225   b ,  230   a ,  230   b ,  235   a , and  235   b , and each converter cell is coupled between a corresponding DC terminal,  250 ,  255 ,  265 , and the AC terminal  245 . Specifically bidirectional switches  225   a ,  230   a , and  235   a  are coupled to AC terminal node  245   a , and bidirectional switches  225   b ,  230   b , and  235   b  are coupled to AC terminal node  245   b.    
         [0094]    The switching states of each bidirectional switch in converter cells  225 ,  230 , and  235 , are controlled by an embedded computing platform (not shown), which may include a digital signal processor board, microcontroller, or field programmable gate array. 
         [0095]    The power converter of  FIG. 2A  can operate in three modes. For example, in a first mode converter cells are only conducting power from independent DC voltage source  205 , in a second mode converter cells are only conducting power from independent DC voltage source  210 , and in a third mode converter cells are successively conducting power from both independent DC voltage sources  205  and  210 . 
         [0096]    Now referring to  FIG. 2B , which illustrates power output signal  275  at AC terminal  245  for power converter  200  operating in a third mode across 5 time intervals t 1  to t 5 . Independent DC voltage sources  205  and  210  are of equal magnitude and power output signal  275  is a multi-level voltage. 
         [0097]    During t 1 , bidirectional switch  225   a  is closed and  225   b  is open, bidirectional switches  230   a  and  230   b  are open, bidirectional switch  235   a  is open and  235   b  is closed, and the power output signal at AC terminal  245  is equal to the voltage magnitude of independent DC source  205 . 
         [0098]    During t 2 , bidirectional switches  225   a  and  225   b  are open, bidirectional switch  230   a  is closed and  230   b  is open, bidirectional switch  235   a  is open and  235   b  is closed, and the power output signal at AC terminal  245  is equal to the voltage magnitude of independent DC source  210 . 
         [0099]    During t 3 , bidirectional switches  225   a  and  225   b  are open, bidirectional switches  230   a  and  230   b  are closed, bidirectional switches  235   a  and  235   b  are open, and the power output signal at AC terminal  245  is equal to zero. 
         [0100]    During t 4 , bidirectional switches  225   a  and  225   b  are open, bidirectional switch  230   a  is open and  230   b  is closed, bidirectional switch  235   a  is closed and  235   b  is open, and the power output signal at AC terminal  245  is equal to the voltage magnitude of independent DC voltage source  210 , but of negative polarity. 
         [0101]    During t 5 , bidirectional switch  225   a  is open and  225   b  is closed, bidirectional switches  230   a  and  230   b  are open, bidirectional switch  235   a  is closed and  235   b  is open, and the power output signal at AC terminal  245  is equal to the voltage magnitude of independent DC voltage source  205 , but of negative polarity. 
         [0102]    Now referring to  FIG. 2C , which illustrates power output signal  280  at AC terminal  245  for power converter  200  operating in a third mode across 5 time intervals t 1  to t 5 . Independent DC voltage source  205  is greater in magnitude than  210 , and power output signal  280  is a multi-level voltage. 
         [0103]    During t 1 , bidirectional switch  225   a  is closed and  225   b  is open, bidirectional switches  230   a  and  230   b  are open, bidirectional switch  235   a  is open and  235   b  is closed, and the power output signal at AC terminal  245  is equal to the voltage magnitude of independent DC voltage source  205 . 
         [0104]    During t 2 , bidirectional switches  225   a  and  225   b  are open, bidirectional switch  230   a  is closed and  230   b  is open, bidirectional switch  235   a  is open and  235   b  is closed, and the power output signal at AC terminal  245  is equal to the voltage magnitude of independent DC voltage source  210 . 
         [0105]    During t 3 , bidirectional switches  225   a  and  225   b  are open, bidirectional switches  230   a  and  230   b  are closed, bidirectional switches  235   a  and  235   b  are open, and the power output signal at AC terminal  245  is equal to zero. 
         [0106]    During t 4 , bidirectional switches  225   a  and  225   b  are open, bidirectional switch  230   a  is open and  230   b  is closed, bidirectional switch  235   a  is closed and  235   b  is open, and the power output signal at AC terminal  245  is equal to the voltage magnitude of independent DC voltage source  210 , but of negative polarity. 
         [0107]    During t 5 , bidirectional switch  225   a  is open and  225   b  is closed, bidirectional switches  230   a  and  230   b  are open, bidirectional switch  235   a  is closed and  235   b  is open, and the power output signal at AC terminal  245  is equal to the voltage magnitude of independent DC voltage source  205 , but of negative polarity. 
         [0108]    Now referring to  FIG. 2D , which illustrates power output signal  285  at AC terminal  245  for power converter  200  operating in a third mode across 5 time intervals t 1  to t 5 . Independent DC voltage source  210  is greater in magnitude than  205 , and power output signal  285  is a multi-level voltage. 
         [0109]    During t 1 , bidirectional switches  225   a  and  225   b  are open, bidirectional switches  230   a  is closed and  230   b  is open, bidirectional switch  235   a  is open and  235   b  is closed, the power output signal at AC terminal  245  is equal to the voltage magnitude of independent DC voltage source  210 . 
         [0110]    During t 2 , bidirectional switch  225   a  is closed and  225   b  is open, bidirectional switches  230   a  and  230   b  are open, bidirectional switch  235   a  is open and  235   b  is closed, and the power output signal at AC terminal  245  is equal to the voltage magnitude of independent DC voltage source  205 . 
         [0111]    During t 3 , bidirectional switch  225   a  and  225   b  are closed, bidirectional switches  230   a ,  230   b ,  235   a , and  235   b  are open, and the power output signal at AC terminal  245  is equal to zero. 
         [0112]    During t 4 , bidirectional switch  225   a  is open and  225   b  is closed, bidirectional switches  230   a  and  230   b  are open, bidirectional switch  235   a  is closed and  235   b  is open, and the power output signal at AC terminal  245  is equal to the voltage magnitude of independent DC voltage source  205 , but of negative polarity. 
         [0113]    During t 5 , bidirectional switches  225   a  and  225   b  are open, bidirectional switch  230   a  is open and  230   b  is closed, bidirectional switch  235   a  is closed and  235   b  is open, and the power output signal at AC terminal  245  is equal to the voltage magnitude of independent DC voltage source  210 , but of negative polarity. 
         [0114]    Now referring to  FIG. 3 , which illustrates power converter  300  operating as an inverter for converting power from a plurality of independent DC voltage sources to a three phase AC load  370 , according to an example embodiment. Power converter  300  comprises a plurality of DC terminals  350 ,  355 ,  360 , and  365 , a plurality of converter cells  325 ,  330 ,  335 , and  340 , and AC terminal  345 . 
         [0115]    DC terminals  350 ,  355 , and  360  are each coupled to an independent DC voltage source  305 ,  310 , and  315 , respectively. DC source  305  represents a first independent DC voltage source, DC source  310  represents a second independent DC voltage source, and DC source  315  represents an n th  independent DC voltage source. Each additional independent DC voltage source added to power converter  300  requires on additional converter cell. DC terminal  365  is coupled to ground  320 . 
         [0116]    AC terminal  345  is coupled to a three phase AC load  370 . AC terminal  345  comprises three AC terminal nodes,  345   a ,  345   b , and  345   c . Each AC terminal node is coupled to a separate phase of AC load  370 , where  370   a  represents phase A,  370   b  represents phase B, and  370   c  represents phase C of AC load  370 . 
         [0117]    Converter cells  325 ,  330 ,  335 , are each coupled between a corresponding DC terminal  350 ,  355 , and  360 , respectively, and AC terminal  345 . Converter cell  340  is coupled between DC terminal  365  and AC terminal  345 . Each converter cell  325 ,  330 ,  335 , and  340 , comprises three bidirectional switches  325   a ,  325   b ,  325   c ,  330   a ,  330   b ,  330   c ,  335   a ,  335   b ,  335   c ,  340   a ,  340   b ,  340   c , wherein bidirectional switches  325   a ,  330   a ,  335   a , and  340   a  are coupled to AC terminal node  345   a , bidirectional switches  325   b ,  330   b ,  335   b , and  340   b , are coupled to AC terminal node  345   b , and bidirectional switches  325   c ,  330   c ,  335   c , and  340   c , are coupled to AC terminal node  345   c.    
         [0118]    The switching states of each bidirectional switch in converter cells  325 ,  330 ,  335 , and  340  are controlled by an embedded computing platform (not shown), which may include a digital signal processor board, microcontroller, or field programmable gate array. 
         [0119]    Power converter  300  has various applications, for example, hybrid electric or electric vehicles that employ DC sources to power a three phase motor. In an example embodiment, a first independent DC source may include a high power density source such as an ultra-capacitor and a second independent DC source may include a high energy density source battery. The independent DC source may comprise, for example, a rectified AC voltage generated by an AC electric machine operating as a generator. The AC load may include a three phase electric motor (e.g. synchronous or induction). 
         [0120]    Although power converter  300  in  FIG. 3  illustrates inverter mode of operation, a rectifier mode of operation can be inferred. Using the above example embodiment, a three phase electric motor may supply power to the plurality of independent DC sources through power converter  300  during regenerative braking. 
         [0121]    Reference is next made to  FIG. 4 , which illustrates power converter  400  operating as a rectifier for converting power from a single phase AC source  470  to a plurality of independent DC loads  405 ,  410 , and  415 . Power converter  400  comprises a plurality of DC terminals  450 ,  455 ,  460 , and  465 , a plurality of converter cells  425 ,  430 ,  435 , and  440 , and an AC terminal  445 . 
         [0122]    DC terminals  450 ,  455 , and  460 , are coupled to independent DC loads  405 ,  410 , and  415  respectively, where DC load  405  represents a first independent DC load, DC load  410  represents a second independent DC load, and DC load  415  represents an nth independent DC load. DC terminal  465  is coupled to ground  420 . 
         [0123]    AC terminal  445  comprises two AC terminal nodes,  445   a  and  445   b , and is coupled to a single phase AC source  470 . AC source  470  comprises a positive node  470   a , and a negative node  470   b.    
         [0124]    Converter cells  425 ,  430 ,  435 , and  440  are each coupled between a corresponding DC terminal  450 ,  455 ,  460 , and  465 , respectively, and the AC terminal  445 . Each converter cell  425 ,  430 ,  435 , and  440  comprise 2 bidirectional switches  425   a ,  425   b ,  430   a ,  430   b ,  435   a ,  435   b ,  440   a , and  440   b , where bidirectional switches  425   a ,  430   a ,  435 , and  440   a , are coupled to AC terminal node  445   a , and bidirectional switches  425   b ,  430   b ,  435   b , and  440   b  are coupled to AC terminal node  445   b . AC terminal node  445   a  is coupled to a positive terminal  470   a  of AC source  470 , and AC terminal node  445   b  is coupled to a negative terminal  470   b  of AC source  470 . 
         [0125]    Power converter  400  has various applications for charging a plurality of DC loads using a multi-level power output signal, where each DC load is charged to a different voltage level. For example, charging hybrid electric vehicles that contain a plurality of DC loads, (e.g. an ultra-capacitor and lithium ion battery), using a single phase AC source. 
         [0126]    Although power converter  400  illustrates a rectifier for converting power between a single phase AC source and a plurality of DC loads, a rectifier for converting power from a three phase AC source and a plurality of DC loads can be inferred by using converter cells comprising three bidirectional switches per converter cell. 
         [0127]    Now referring to  FIG. 5 , which illustrates power converter  500  for converting power from a plurality of independent DC voltage sources  505 ,  510 ,  515 , or a plurality of independent backup DC voltage sources  590 ,  591 ,  592  for fault tolerant operation, and a single phase AC load  570 . 
         [0128]    Power converter  500  comprises a plurality of DC terminals  550 ,  555 ,  560 , and  565 , a plurality of converter cells  525 ,  530 ,  535 , and  540 , a plurality of backup DC switches  575 ,  580 , and  585 , and an AC terminal  545 . 
         [0129]    DC terminals  550 ,  555 , and  560 , are coupled between converter cells  525 ,  530 , and  535 , and independent DC voltage sources  505 ,  510 , and  515 , and backup DC switch  575 ,  580 , and  585 , respectively. DC terminal  565  is coupled between ground  520  and converter cell  540 . 
         [0130]    DC voltage source  505  represents a first independent DC voltage source, DC voltage source  510  represents a second independent DC voltage source, and DC voltage source  515  represents an n th  independent DC voltage source. 
         [0131]    Backup DC switches  575 ,  580 , and  585 , each comprise a single bidirectional switch that is coupled between DC terminals  550 ,  555 , and  560 , and independent backup DC voltage sources  590 ,  591 , and  592 , respectively. 
         [0132]    Converter cells  525 ,  530 , and  535 , are coupled between DC terminals  550 ,  555 , and  560 , respectively, and AC terminal  545 . Converter cell  540  is coupled between DC terminal  565  and AC terminal  545 . AC load  570  comprises a positive node  570   a , and a negative node  570   b.    
         [0133]    AC terminal  545  comprises two AC terminal nodes,  545   a , which is coupled to the positive node  570   a  of single phase AC load  570 , and  545   b , which is coupled to the negative node  570   b  of single phase AC load  570 . Each converter cell comprises two bidirectional switches  525   a ,  525   b ,  530   a ,  530   b ,  535   a ,  535   b ,  540   a , and  540   b , where bidirectional switches  525   a ,  530   a ,  535   a , and  540   a  are coupled to AC terminal node  545   a , and bidirectional switches  525   b ,  530   b ,  535   b , and  540   b  are coupled to AC terminal node  545   b.    
         [0134]    The switching states of each bidirectional switch in converter cells  525 ,  530 ,  535 ,  540 , and backup DC switches  575 ,  580 , and  585 , are controlled by an embedded computing platform (not shown), which may include a digital signal processor board, microcontroller, or field programmable gate array. 
         [0135]    Although power converter  500  illustrates an inverter for converting power from a plurality of independent DC voltage sources or a plurality of independent backup DC voltage sources, to a single phase AC load, a rectifier for converting power from a single phase AC source can be inferred. Similarly an inverter or rectifier for converting power between a three phase AC source or load to a plurality of DC sources or loads or backup DC sources or loads can be inferred by using converter cells comprising three bidirectional switches per converter cell. 
         [0136]    Power converter  500  has various applications where fault tolerant operation is advantageous, for example, uninterruptable power supplies and integrated battery management systems for safety critical or mission critical systems. It can be also used to drive a three phases traction motor in an electrified vehicle. 
         [0137]    Referring now to  FIG. 6 , which illustrates various example embodiments of bidirectional switches. Bidirectional switches used in converter cells or backup DC switches can be any bidirectional switches.  FIG. 6A  illustrates a bidirectional switch  600  according to a first example embodiment.  FIG. 6B  illustrates a bidirectional switch  605  according to a second example embodiment.  FIG. 6C  illustrates a bidirectional switch  610  according to a third example embodiment.  FIG. 6D  illustrates a bidirectional switch  615  according to a fourth example embodiment.  FIG. 6E  illustrates a bidirectional switch according to a fifth embodiment. 
         [0138]    In some cases, as illustrated, bidirectional switches  605 ,  610  and  620  include insulated-gate bipolar transistors or IGBTs. In some other cases, bidirectional switches  605 ,  610  and  620  include metal-oxide-semiconductor field-effect transistors or MOSFETs. 
         [0139]    In one case, as illustrated, bidirectional switch  610  of  FIG. 6C  includes two IGBTs connected at respective emitters to provide a common emitter connection. In other cases, bidirectional switch  610  may include two IGBTs connected at respective collectors to provide a common collector connection. In some further cases, bidirectional switch  610  may include MOSFETs connected at respective drains or sources to provide a common drain or a common source connection. 
         [0140]    Bidirectional switch  600  may be configured in a variety of different ways. For example, in one case, as illustrated, bidirectional switch  600  includes a single pole, single throw (SPST) relay or contactor. In some other cases, bidirectional switch  600  includes a single pole, double throw (SPDT) relay or contactor. 
         [0141]    Solid-state bidirectional switches are typically used in power converters that require high switching frequency. Solid-state bidirectional switches include solid-state components, such as IGBTs, MOSFETs etc. Examples of solid-state bidirectional switches include switches  605 ,  610  and  620 . Solid-state bidirectional switches have high switching speeds, such as, for example, switching speeds in the range of microseconds or less. Other bidirectional switches are typically used as relays or contactors. Such bidirectional switches have relatively lower switching speeds, such as, for example, switching speeds in the range of milliseconds or more. 
         [0142]    Reference is made to  FIG. 8A , which illustrates power converter  800  operating as an inverter in a first mode for converting power from independent DC voltage sources  805  and  810  to a three phase AC load  855 , according to an example embodiment. In this mode, although two independent DC voltage sources  805 ,  810  are coupled to converter cells, the converter cells are conducting power from only one independent DC voltage source  805 . Power converter  800  comprises three DC terminals  840 ,  845 , and  850 , three converter cells  820 ,  825 , and  830 , and an AC terminal  835 . Each converter cell comprises three bidirectional switches  820   a ,  820   b ,  820   c ,  825   a ,  825   b ,  825   c ,  830   a ,  830   b , and  830   c . The AC terminal  835  comprises three AC terminal nodes,  835   a ,  835   b , and  835   c . AC load  855  comprises three phase nodes,  855   a ,  855   b , and  855   c  which represent V an , V bn , and V cn , respectively. AC terminal nodes  835   a ,  835   b , and  835   c  are each coupled to corresponding AC load nodes  855   a ,  855   b , and  855   c , respectively. 
         [0143]    Converter cells  820 ,  825 , and  830 , are coupled between DC terminals  840 ,  845 , and  850 , respectively, and AC terminal  835 . DC terminals  840  is coupled to a first independent DC voltage source  805 , DC terminal  845  is coupled to a second independent DC voltage source  810 , and DC terminal  850  is coupled to ground  815 . 
         [0144]    Converter cells  820 ,  825 , and  830 , each comprise three bidirectional switches  820   a ,  820   b ,  820   c ,  825   a ,  825   b ,  825   c ,  830   a ,  830   b , and  830   c , wherein bidirectional switches  820   a ,  825   a ,  830   a  are coupled to AC terminal node  835   a , bidirectional switches  820   b ,  825   b ,  830   b  are coupled to AC terminal  835   b , and bidirectional switches  820   c ,  825   c , and  830   c  are coupled to AC terminal  835   c.    
         [0145]    In this embodiment, bidirectional switches  820   a ,  830   b , and  830   c  are closed, and bidirectional switches  820   b ,  820   c ,  825   a ,  825   b ,  825   c , and  830   a , are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  is equal to the independent DC voltage source  805 , V bn  and V cn  are equal to zero. 
         [0146]    Now referring to  FIG. 8B , which illustrates an example embodiment of power converter  800  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0147]    In this embodiment, bidirectional switches  820   c ,  830   a , and  830   b  are closed, and bidirectional switches  820   a ,  820   b ,  825   a ,  825   b ,  825   c , and  830   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  and V bn  are equal to zero, and V cn  is equal  805 . 
         [0148]    Now referring to  FIG. 8C , which illustrates an example embodiment of power converter  800  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0149]    In this embodiment, bidirectional switches  820   b ,  820   c , and  830   a  are closed, and bidirectional switches  820   a ,  825   a ,  825   b ,  825   c ,  830   b ,  830   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  is equal to zero, V bn  and V cn  are equal to independent DC voltage source  805 . 
         [0150]    Now referring to  FIG. 8D , which illustrates an example embodiment of power converter  800  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and the corresponding AC load nodes. 
         [0151]    In this embodiment, bidirectional switches  820   a ,  820   c , and  830   b  are closed, and bidirectional switches  820   b ,  825   a ,  825   b ,  825   c ,  830   a , and  830   c  are open. The power output signal at AC terminal nodes and the corresponding AC load nodes are as follows: V an  and V cn  are equal to independent DC voltage source  805 , and V bn  is equal to zero. 
         [0152]    Now referring to  FIG. 8E , which illustrates power converter  800  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0153]    In this embodiment, bidirectional switches  820   a ,  820   b , and  830   c  are closed, and bidirectional switches  820   c ,  825   a ,  825   b ,  825   c ,  830   a , and  830   b  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  and V bn  are equal to independent DC voltage source  805 , and V cn  is equal to zero. 
         [0154]    Now referring to  FIG. 8F , which illustrates power converter  800  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0155]    In this embodiment, bidirectional switches  820   a ,  820   b , and  820   c  are closed, and bidirectional switches  825   a ,  825   b ,  825   c ,  830   a ,  830   b , and  830   c , are closed. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an , V bn , and V cn  are equal to independent DC voltage source  805 . 
         [0156]    Now referring to  FIG. 8G , which illustrates power converter  800  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0157]    In this embodiment, bidirectional switches  820   b ,  830   a , and  830   c  are closed, and bidirectional switches  820   a ,  820   c ,  825   a ,  825   b ,  825   c , and  830   b  are open. The power output signal at AC terminal nodes and corresponding AC load nodes are as follows: V an  and V cn  are equal to zero, and V bn  is equal to independent DC voltage source  805 . 
         [0158]    Reference is made to  FIG. 7 , which illustrates the power output signal  860   a ,  860   b , and  860   c  at corresponding AC terminal nodes  835   a ,  835   b , and  835   c , generated by power converter  800  operating in a first mode for each example embodiment illustrated in  FIGS. 8A to 8G . 
         [0159]    Reference is made to  FIG. 9A , which illustrates power converter  900  operating in a second mode as an inverter for converting power from two independent DC voltage sources  905  and  910  to a three phase AC load  955 , according to an example embodiment. In this mode, the converter cells are conducting power from only one independent DC voltage source  910 . Power converter  900  comprises three DC terminals  940 ,  945 , and  950 , three converter cells  920 ,  925 , and  930 , and an AC terminal  935 . Each converter cell comprises three bidirectional switches  920   a ,  920   b ,  920   c ,  925   a ,  925   b ,  925   c ,  930   a ,  930   b , and  930   c . The AC terminal  935  comprises three AC terminal nodes,  935   a ,  935   b , and  935   c . AC load  955  comprises a three phase load with nodes,  955   a ,  955   b , and  955   c  which represent V an , V bn , and V in , respectively. AC terminal nodes  935   a ,  935   b , and  935   c  are each coupled to corresponding AC load nodes  955   a ,  955   b , and  955   c , respectively. 
         [0160]    Converter cells  920 ,  925 , and  930 , are coupled between DC terminals  940 ,  945 , and  950 , respectively, and AC terminal  935 . DC terminals  940  is coupled to a first independent DC voltage source  905 , DC terminal  945  is coupled to a second independent DC voltage source  910 , and DC terminal  950  is coupled to ground  915 . 
         [0161]    Converter cells  920 ,  925 , and  930 , each comprise three bidirectional switches  920   a ,  920   b ,  920   c ,  925   a ,  925   b ,  925   c ,  930   a ,  930   b ,  930   c , wherein bidirectional switches  920   a ,  925   a ,  930   a  are coupled to AC terminal node  935   a , bidirectional switches  920   b ,  925   b ,  930   b  are coupled to AC terminal  935   b , and bidirectional switches  920   c ,  925   c , and  930   c  are coupled to AC terminal  935   c.    
         [0162]    In this embodiment, bidirectional switches  925   a ,  930   b , and  930   c  are closed, and bidirectional switches  920   a ,  920   b ,  920   c ,  925   b ,  925   c , and  930   a , are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  is equal to the independent DC voltage source  910 , V bn  and V cn  are equal to zero. 
         [0163]    Reference is now made to  FIG. 9B , which illustrates an example embodiment of power converter  900  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0164]    In this embodiment, bidirectional switches  925   a ,  925   c ,  930   a , and  930   b  are closed, and bidirectional switches  920   a ,  920   b ,  920   c ,  925   b , and  930   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  and V bn  are equal to zero, and V cn  is equal to the independent DC voltage source  910 . 
         [0165]    Reference is now made to  FIG. 9C , which illustrates an example embodiment of power converter  900  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0166]    In this embodiment, bidirectional switches  925   a ,  925   b ,  925   c  are closed, and bidirectional switches  920   a ,  920   b ,  920   c ,  930   a ,  930   b , and  930   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an , V bn , and V cn  are equal to the magnitude of independent DC voltage source  910 . 
         [0167]    Reference is now made to  FIG. 9D , which illustrates an example embodiment of power converter  900  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0168]    In this embodiment, bidirectional switches  925   a ,  925   b ,  930   a , and  930   c  are closed, and bidirectional switches  920   a ,  920   b ,  920   c ,  925   c ,  930   a , and  930   b  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  and V cn  are equal to zero, and V bn  is equal to the magnitude of independent DC voltage source  910 . 
         [0169]    Reference is now made to  FIG. 9E , which illustrates an example embodiment of power converter  900  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0170]    In this embodiment, bidirectional switches  920   b ,  920   c , and  925   a  are closed, and bidirectional switches  920   a ,  925   b ,  925   c ,  930   a ,  930   b , and  930   c , are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  is equal to the independent DC voltage source  910 , V bn  and V cn  are equal to the independent DC voltage source  905 . 
         [0171]    Reference is now made to  FIG. 9F , which illustrates an example embodiment of power converter  900  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0172]    In this embodiment, bidirectional switches  920   c ,  925   a , and  930   b  are closed, and bidirectional switches  920   a ,  920   b ,  925   b ,  925   c ,  930   a , and  930   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  is equal to the magnitude of independent DC voltage source  910 , V bn  is equal zero, and V cn  is equal to the magnitude of independent DC voltage source  905 . 
         [0173]    Reference is made to  FIG. 10A , which illustrates power converter  1000  operating in a third mode as an inverter for converting power from two independent DC voltage sources  1005  and  1010  to a three phase AC load  1055 , according to an example embodiment. Power converter  1000  comprises three DC terminals  1040 ,  1045 , and  1050 , three converter cells  1020 ,  1025 , and  1030 , and an AC terminal  1035 . Each converter cell comprises three bidirectional switches  1020   a ,  1020   b ,  1020   c ,  1025   a ,  1025   b ,  1025   c ,  1030   a ,  1030   b , and  1030   c . The AC terminal  1035  comprises three AC terminal nodes,  1035   a ,  1035   b , and  1035   c . AC load  1055  comprises three phase nodes,  1055   a ,  1055   b , and  1055   c  which represent V an , V bn , and V cn , respectively. AC terminal nodes  1035   a ,  1035   b , and  1035   c  are each coupled to corresponding AC load nodes  1055   a ,  1055   b , and  1055   c , respectively. 
         [0174]    Converter cells  1020 ,  1025 , and  1030 , are coupled between DC terminals  1040 ,  1045 , and  1050 , respectively, and AC terminal  1035 . DC terminals  1040  is coupled to a first independent DC voltage source  1005 , DC terminal  1045  is coupled to a second independent DC voltage source  1010 , and DC terminal  1050  is coupled to ground  1015 . 
         [0175]    Converter cells  1020 ,  1025 , and  1030 , each comprise three bidirectional switches  1020   a ,  1020   b ,  1020   c ,  1025   a ,  1025   b ,  1025   c ,  1030   a ,  1030   b ,  1030   c , wherein bidirectional switches  1020   a ,  1025   a ,  1030   a  are coupled to AC terminal node  1035   a , bidirectional switches  1020   b ,  1025   b ,  1030   b  are coupled to AC terminal  1035   b , and bidirectional switches  1020   c ,  1025   c , and  1030   c  are coupled to AC terminal  1035   c.    
         [0176]    In this embodiment, bidirectional switches  1020   a ,  1025   b , and  1030   c  are closed, and bidirectional switches  1020   b ,  1020   c ,  1025   a ,  1025   c ,  1030   a , and  1030   b  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  is equal to the magnitude of independent DC voltage source  1005 , V bn  is equal to the magnitude of independent DC voltage source  1010 , and V cn  is equal zero 
         [0177]    Reference is now made to  FIG. 10B , which illustrates an example embodiment of power converter  1000  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0178]    In this embodiment, bidirectional switches  1020   b ,  1025   a , and  1030   c  are closed, and bidirectional switches  1020   a ,  1020   c ,  1025   b ,  1025   c ,  1030   a , and  1030   b  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  is equal to the magnitude of independent DC voltage source  1010 , V bn  is equal to the magnitude of independent DC voltage source  1005 , and V cn  is equal to zero. 
         [0179]    Reference is now made to  FIG. 10C , which illustrates an example embodiment of power converter  1000  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0180]    In this embodiment, bidirectional switches  1020   b ,  1025   c , and  1030   a  are closed, and bidirectional switches  1020   a ,  1020   c ,  1025   a ,  1025   b ,  1030   b , and  1030   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  is equal to zero, V bn  is equal to the magnitude of independent DC voltage source  1005 , and V cn  is equal to the magnitude of independent DC voltage source  1010 . 
         [0181]    Reference is now made to  FIG. 10D , which illustrates an example embodiment of power converter  1000  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0182]    In this embodiment, bidirectional switches  1020   c ,  1025   b , and  1030   a  are closed, and bidirectional switches  1020   a ,  1020   b ,  1025   a ,  1025   c ,  1030   b , and  1030   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  is equal to zero, V bn  is equal to the magnitude of independent DC voltage source  1010 , and V cn  is equal to the magnitude of independent DC voltage source  1005 . 
         [0183]    Reference is now made to  FIG. 10E , which illustrates an example embodiment of power converter  1000  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0184]    In this embodiment, bidirectional switches  1020   c ,  1025   a , and  1030   b  are closed, and bidirectional switches  1020   a ,  1020   b ,  1025   b ,  1025   c ,  1030   a , and  1030   b  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  is equal to the magnitude of independent DC voltage source  1010 , V bn  is equal to zero, and V cn  is equal the magnitude of independent DC voltage source  1005   
         [0185]    Reference is now made to  FIG. 10F , which illustrates an example embodiment of power converter  1000  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0186]    In this embodiment, bidirectional switches  1020   a ,  1025   c , and  1030   b  are closed, and bidirectional switches  1020   b ,  1020   c ,  1025   a ,  1025   b ,  1030   a , and  1030   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  is equal to the magnitude of independent DC voltage source  1005 , V bn  is equal to zero, and V cn  is equal to the magnitude of independent DC voltage source  1010 . 
         [0187]    Reference is now made to  FIG. 10G , which illustrates an example embodiment of power converter  1000  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0188]    In this embodiment, bidirectional switches  1020   a ,  1025   b , and  1025   c  are closed, and bidirectional switches  1020   b ,  1020   c ,  1025   a ,  1030   a ,  1030   b , and  1030   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  is equal to the magnitude of independent DC voltage source  1005 , and V bn  and V cn  are equal to the magnitude of independent DC voltage source  1010 . 
         [0189]    Reference is now made to  FIG. 10H , which illustrates an example embodiment of power converter  1000  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0190]    In this embodiment, bidirectional switches  1020   b ,  1020   c , and  1025   a  are closed, and bidirectional switches  1020   a ,  1025   b ,  1025   c ,  1030   a ,  1030   b , and  1030   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  is equal to the magnitude of independent DC voltage source  1010 , V bn  and V cn  are equal to the magnitude of independent DC voltage source  1005 . 
         [0191]    Reference is now made to  FIG. 10I , which illustrates an example embodiment of power converter  1000  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0192]    In this embodiment, bidirectional switches  1020   b ,  1025   a , and  1025   c  are closed, and bidirectional switches  1020   a ,  1020   c ,  1025   b ,  1030   a ,  1030   b , and  1030   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  and V cn  are equal to the magnitude of independent DC voltage source  1010 , V bn  is equal to the magnitude of independent DC voltage source  1005 . 
         [0193]    Reference is now made to  FIG. 10J , which illustrates an example embodiment of power converter  1000  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0194]    In this embodiment, bidirectional switches  1020   a ,  1020   c , and  1025   b  are closed, and bidirectional switches  1020   b ,  1025   a ,  1025   c ,  1030   a ,  1030   b , and  1030   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  and V cn  are equal to the magnitude of independent DC voltage source  1005 , and V cn  is equal to the magnitude of independent DC voltage source  1010 . 
         [0195]    Reference is now made to  FIG. 10K , which illustrates an example embodiment of power converter  1000  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0196]    In this embodiment, bidirectional switches  1020   c ,  1025   a , and  1025   b  are closed, and bidirectional switches  1020   a ,  1020   b ,  1025   c ,  1030   a ,  1030   b , and  1030   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  and V bn  are equal to the magnitude of independent DC voltage source  1010 , and V cn  is equal to the magnitude of independent DC voltage source  1005 . 
         [0197]    Reference is now made to  FIG. 10L , which illustrates an example embodiment of power converter  1000  with a different bidirectional switch configuration to produce a different power output signal at AC terminal nodes and corresponding AC load nodes. 
         [0198]    In this embodiment, bidirectional switches  1020   a ,  1020   b , and  1025   c  are closed, and bidirectional switches  1020   c ,  1025   a ,  1025   b ,  1030   a ,  1030   b , and  1030   c  are open. The power output signal generated at AC terminal nodes and the corresponding AC load nodes are as follows: V an  and V bn  are equal to the magnitude of independent DC voltage source  1005 , and V cn  is equal to the magnitude of independent DC voltage source  1010 . 
         [0199]    Now referring to  FIG. 10M , which illustrates power converter  1000  with all bidirectional switches in an open position. 
         [0200]    Now referring to  FIG. 10N , which illustrates various bidirectional switch configurations of power converter  1000  of  FIG. 10M  operating in a first mode. The line-to-line power output signals, V ab , V bc , and V ca , correspond to the voltage potential between AC terminal nodes  1035   a  and  1035   b ,  1035   b  and  1035   c , and  1035   c  and  1035   a , respectively. The output power signal between the AC terminal nodes and between the corresponding AC load nodes,  1055   a - 1055   b ,  1055   b - 1055   c ,  1055   c - 1055   a , ranges between the magnitude of independent DC voltage source  1005  in positive and negative polarity. 
         [0201]    Now referring to  FIG. 10O , which illustrates various bidirectional switch configurations of power converter  1000  of  FIG. 10M  operating in a second mode. The line-to-line power output signals V ab , V bc , and V ca , generated between AC terminal nodes and corresponding AC load nodes ranges between the magnitude of independent DC voltage source  1010  in positive and negative polarity. 
         [0202]    Now referring to  FIG. 10P , which illustrates various bidirectional switch configurations of power converter  1000  of  FIG. 10M  operating in a third mode. The line-to-line power output signals V ab , V bc , and V ca , generated between AC terminal nodes and corresponding AC load nodes ranges between the magnitudes of independent DC voltage sources  1005  and  1010  in positive and negative polarity, as well as the corresponding difference between both independent DC voltage sources  1005  and  1010 . In this example embodiment DC terminal  1050  is coupled to ground  1015 . In other embodiments terminal  1050  may be a non-zero voltage. For example, terminal  1050  may be coupled to a DC voltage source. In such embodiments, the other voltages in power converter  1000  will be referenced to such non-zero voltage. 
         [0203]    Now referring to  FIG. 11A , which illustrates an example embodiment of power converter  1100  operating as an inverter for converting power from two independent DC voltage sources  1105  and  1110 , to a single phase AC load  1170 . Power converter  1100  comprises three DC terminals  1150 ,  1155 , and  1165 , three converter cells  1125 ,  1130 , and  1135 , and AC terminal  1145 . DC terminals  1150  and  1155  are coupled to independent DC voltage sources  1105  and  1110 , respectively, and DC terminal  1165  is coupled to ground  1120 . Although this example embodiment illustrates a single phase AC load, power converter  1100  can operate as an inverter for converting power between independent DC voltage sources and a three phase AC load by using converter cells that comprise three bidirectional switches per cell. 
         [0204]    AC terminal  1145  is coupled to single phase AC load  1170 . AC terminal  1145  comprises two AC terminal nodes  1145   a  and  1145   b . AC load  1170  comprises a positive node  1170   a , and a negative node  1170   b . AC terminal node  1145   a  is coupled to the positive node  1170   a  of single phase AC load  1170 , and  1145   b , is coupled to the negative node  1170   b  of single phase AC load  1170 . 
         [0205]    Converter cells  1125 ,  1130 , and  1135 , each comprise two bidirectional switches  1125   a ,  1125   b ,  1130   a ,  1130   b ,  1135   a , and  1135   b , and each converter cell is coupled between a corresponding DC terminal,  1150 ,  1155 ,  1165 , and the AC terminal  1145 . Specifically bidirectional switches  1125   a ,  1130   a , and  1135   a  are coupled to AC terminal node  1145   a , and bidirectional switches  1125   b ,  1130   b , and  1135   b  are coupled to AC terminal node  1145   b.    
         [0206]    The switching states of each bidirectional switch in converter cells  1125 ,  1130 , and  1135 , are controlled by an embedded computing platform  1175   a , which may include a digital signal processor board, microcontroller, or field programmable gate array. Embedded computing platform  1175   a  comprises several outputs  1175   c , which are coupled to bidirectional switches  1125   a ,  1125   b ,  1130   a ,  1130   b ,  1135   a , and  1135   b , and controls the switching state of each bidirectional switch as either open or closed. 
         [0207]    Now referring to  FIG. 11B , which illustrates an example embodiment of power converter  1100  operating as an inverter for converting power from two independent DC voltage sources  1105  and  1110 , to a single phase AC load  1170 , where the switching states of each bidirectional switch in converter cells  1125 ,  1130 , and  1135  are controlled by embedded computing platform  1175   b  which may include a digital signal processor board, microcontroller, or field programmable gate array. Embedded computing platform  1175   b  comprises an input  1175   d , and several outputs  1175   c , which are coupled to bidirectional switches  1125   a ,  1125   b ,  1130   a ,  1130   b ,  1135   a , and  1135   b , in order to vary the switching state as either open or closed. Embedded computing platform input  1175   b  may receive instructions on varying the switching states from an external computer (not shown) through its input  1175   d.    
         [0208]    Reference is now made to  FIG. 12A , which illustrates an example embodiment of power converter  1200  operating as a DC to DC converter for converting power from one independent DC voltage source  1210  to a second independent DC voltage source  1205 , when a three phase AC load  1255  is stationary (i.e. not drawing power or supplying power). The AC load can be the windings of an AC electric machine. 
         [0209]    Power converter  1200  comprises three DC terminals  1240 ,  1245 , and  1250 , three converter cells  1220 ,  1225 , and  1230 , and an AC terminal  1235 . Each converter cell comprises three bidirectional switches  1220   a  (not shown),  1220   b  (not shown),  1220   c ,  1225   a  (not shown),  1225   b ,  1225   c  (not shown),  1230   a  (not shown),  1230   b  (not shown), and  1230   c . The AC terminal  1235  comprises three AC terminal nodes,  1235   a ,  1235   b , and  1235   c . AC load  1255  comprises three phase nodes,  1255   a ,  1255   b , and  1255   c  which represent V an , V bn , and V cn , respectively. AC terminal nodes  1235   a ,  1235   b , and  1235   c  are each coupled to corresponding AC load nodes  1255   a ,  1255   b , and  1255   c , respectively. 
         [0210]    Converter cells  1220 ,  1225 , and  1230 , are coupled between DC terminals  1240 ,  1245 , and  1250 , respectively, and AC terminal  1235 . DC terminals  1240  is coupled to a first independent DC voltage source  1205 , DC terminal  1245  is coupled to a second independent DC voltage source  1210 , and DC terminal  1250  is coupled to ground  1215 . 
         [0211]    The switching states of each bidirectional switch in converter cells  1225 ,  1230 , and  1235 , are controlled by an embedded computing platform  1275 , which may include a digital signal processor board, microcontroller, or field programmable gate array. Embedded computing platform  1275  comprises outputs  1275   a ,  1275   b , and  1275   c , which are coupled to bidirectional switches  1230   a ,  1225   b ,  1220   c , respectively, and controls the switching state of each bidirectional switch as either open or closed. 
         [0212]    In this embodiment, bidirectional switch  1225   b  is closed, and bidirectional switches  1220   c  and  1230   c  are active, which means the switch position is varied by the embedded computing platform  1275 , bidirectional switches  1220   a ,  1220   b ,  1230   a , are  1230   b  are not active, and bidirectional switches  1225   a ,  1225   c  are open. When bidirectional switches  1225   b  and  1220   c  are both closed, independent DC voltage source  1210  can supply a DC power signal to charge independent DC voltage source  1205 . 
         [0213]    Reference is now made to  FIG. 12B , which illustrates various example embodiments of different bidirectional switch positions of power converter  1200  operating as a DC to DC converter. 
         [0214]    Reference is now made to  FIG. 13 , which illustrates an example embodiment power converter  1300  operating as an inverter for converting power from two independent DC voltage sources  1305  and  1310  to a three phase AC load  1355 , wherein a DC to DC converter  1360  is coupled between the independent DC voltage sources  1305  and  1310  to allow one independent DC voltage source to supply power to a second independent voltage source. Power converter  1300  comprises three DC terminals  1340 ,  1345 , and  1350 , three converter cells  1320 ,  1325 , and  1330 , and an AC terminal  1335 . Each converter cell comprises three bidirectional switches  1320   a ,  1320   b ,  1320   c ,  1325   a ,  1325   b ,  1325   c ,  1330   a ,  1330   b , and  1330   c . The AC terminal  1335  comprises three AC terminal nodes,  1335   a ,  1335   b , and  1335   c . AC load  1355  comprises three phase nodes,  1355   a ,  1355   b , and  1355   c  which represent V an , V bn , and V cn , respectively. AC terminal nodes  1335   a ,  1335   b , and  1335   c  are each coupled to corresponding AC load nodes  1355   a ,  1355   b , and  1355   c.    
         [0215]    Converter cells  1320 ,  1325 , and  1330 , are coupled between DC terminals  1340 ,  1345 , and  1350 , respectively, and AC terminal  1335 . DC terminals  1340  is coupled to a first independent DC voltage source  1305 , DC terminal  1345  is coupled to a second independent DC voltage source  1310 , and DC terminal  1350  is coupled to ground  1015 . 
         [0216]    Converter cells  1320 ,  1325 , and  1330 , each comprise three bidirectional switches  1320   a ,  1320   b ,  1320   c ,  1325   a ,  1325   b ,  1325   c ,  1330   a ,  1330   b ,  1330   c , wherein bidirectional switches  1320   a ,  1325   a ,  1330   a  are coupled to AC terminal node  1335   a , bidirectional switches  1320   b ,  1325   b ,  1330   b  are coupled to AC terminal  1335   b , and bidirectional switches  1320   c ,  1325   c , and  1330   c  are coupled to AC terminal  1335   c.    
         [0217]    The switching states of each bidirectional switch in converter cells  1320 ,  1325 , and  1330 , are controlled by an embedded computing platform (not shown), which may include a digital signal processor board, microcontroller, or field programmable gate array. 
         [0218]    DC to DC converter  1360  can be a unidirectional or bidirectional switching power converter and may have electrical isolation. DC to DC converter  1360  comprises two positive terminals,  1360   b  and  1360   d , and two negative terminals  1360   a  and  1360   c . Each positive and negative terminal of the DC to DC converter is coupled to the corresponding polarity of the independent DC voltage sources. 
         [0219]    In this embodiment, power converter  1300  can produce a power output signal at the AC terminals and using a DC to DC converter  1360  to permit one independent DC voltage source to supply a DC power signal to a second independent DC voltage source. 
         [0220]    The present invention has been described here by way of example only. Various modifications and variations may be made to these exemplary embodiments without departing from the spirit and scope of the invention, which is limited only by the appended claims.