Patent Publication Number: US-2023163697-A1

Title: Uninterruptible power supply having short circuit load capability

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to uninterruptible power supplies (UPSs). In particular, the present disclosure relates to silicon carbide (SiC) UPSs having short circuit load capability. 
     BACKGROUND 
     A UPS system provides emergency power to a load when a primary input power source fails. Such systems generally include an energy storage device (e.g, a battery) and an inverter, which converts power provided by the energy storage device to power suitable for the load, such as by converting direct current (DC) to alternating current (AC). The inverter generally includes semiconductor switches (e.g.. metal-oxide-semiconductor field-effect transistor (MOSFET) switches), which are controlled to provide a desired power and/or current output. 
     As UPS systems operate continuously, increasing efficiency is a critical requirement for UPS products. Using silicon carbide (SiC) semiconductors for the inverter, the efficiency can be improved with potentially lower system-level costs However, during a short circuit event at the load. SiC semiconductors experience a relatively large current and may overheat due to their relatively small chip size. Accordingly, to meet short circuit current demands, UPS systems generally include an increased number of SiC chips, which may significantly increase the cost of the UPS system. A UPS system that can meet a given short circuit current demand using a fewer number of SiC chips is therefore desirable. 
     BRIEF DESCRIPTION 
     In one aspect, an inverter is provided. The inverter includes a DC bus having positive and negative rails. The inverter further includes an inverter arm coupled between the positive and negative rails of the DC bus. The inverter arm includes a first silicon carbide transistor having a first current-conducting terminal connected to a central node of the inverter arm, and a second silicon carbide transistor having a second current-conducting terminal connected to the central node of said inverter arm. The inverter further includes at least one silicon transistor having a third current-conducting terminal connected to the central node of the inverter arm. The inverter further includes a gate driver circuit configured to switch the first silicon carbide transistor and the second silicon carbide transistor to convert DC from said DC bus into AC, and to switch said at least one silicon transistor, when the inverter arm is subjected to a load-side short circuit current, to freewheel the load-side short circuit current. 
     In another aspect, an inverter circuit is provided. The inverter circuit includes a DC bus having positive and negative rails. The inverter circuit further includes an inverter arm coupled between the positive and negative rails of said DC bus. The inverter arm includes a first silicon carbide transistor having a first current-conducting terminal connected to a central node of the inverter arm, and a second silicon carbide transistor having a second current-conducting terminal connected to the central node of the inverter arm. The inverter circuit further includes at least one silicon transistor having a third current-conducting terminal connected to the central node of the inverter arm. The first silicon carbide transistor and the second silicon carbide transistor are configured to be switched by a gate driver circuit to convert DC from said DC bus into AC, and the at least one silicon transistor is configured to be switched by the gate driver circuit, when the inverter arm is subjected to a load-side short circuit current, to freewheel the load-side short circuit current. 
     In another aspect, a method for operating an inverter is provided. The inverter includes a DC bus having positive and negative rails, an inverter arm including a first silicon carbide transistor having a first current-conducting terminal connected to a central node of the inverter arm and a second silicon carbide transistor having a second current-conducting terminal connected to the central node of the inverter arm, and at least one silicon transistor having a third current-conducting terminal connected to the central node of the inverter arm. The method includes switching the first silicon carbide transistor and the second silicon carbide transistor to convert DC from the DC bus into AC. The method further includes switching the at least one silicon transistor, when the inverter arm is subjected to a load-side short circuit current, to freewheel the load-side short circuit current. 
     The foregoing and other aspects, features, details, utilities, and advantages of the present disclosure will be apparent from reading the following description and claims, and from reviewing the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG.  1    is a schematic diagram of an example inverter having a three-level topology; 
         FIG.  2    is a schematic diagram of an of another example inverter having a three-level topology; and 
         FIG.  3    is a flow diagram of an example method for operating an inverter. 
     
    
    
     Unless otherwise indicated, the drawings provided herein are meant to illustrate features of embodiments of this disclosure. These features are believed to be applicable in a wide variety of systems comprising one or more embodiments of this disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the embodiments disclosed herein. 
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     The present disclosure is directed to an inverter circuit that may be used in a UPS or other system that provides AC power to a load. The inverter circuit includes a DC bus having positive and negative rails, which may be coupled to a DC source. The inverter circuit further includes an inverter arm coupled between the positive and negative rails of the DC bus. The inverter arm includes two SiC transistors, which are coupled at a central node of the inverter arm, which in turn may be coupled to a load. The inverter arm further includes at least one silicon transistor connected to the central node of the inverter arm. The inverter circuit further includes a gate driver circuit configured to, during normal operation of the inverter, switch the first and second SiC transistors to convert DC from the DC bus into alternating current AC at the central node. When the inverter arm is subjected to a load-side short circuit current, the gate driver controls the at least one silicon transistor to freewheel the load-side short circuit current, which reduces the current load on the SiC transistors to increase the overall short circuit capacity of the inverter circuit. Assuming an inverter system made up of one or more of this inverter circuit has a given current requirement, utilizing silicon transistors to increase the short circuit current capacity and reduce the peak short circuit current of each individual inverter circuit reduces the number of inverter circuits, and associated components such as inductors, needed to meet the system-level current capacity. Accordingly, the overall cost of the system in terms of material and/or space may be reduced. Further, because the peak currents are reduced, the system may have improved reliability when electromagnetic interference (EMI) noise is present. 
     In the following specification and the claims, reference will be made to a number of terms, which shall be defined to have the following meanings. 
     The singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, “approximately”, and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. 
       FIG.  1    illustrates an example inverter  100 . Inverter  100  includes a direct current (DC) bus  102  including a positive rail  104  and a negative rail  106 . Inverter  100  further includes an inverter arm  108  coupled between positive rail  104  and negative rail  106 . Inverter arm  108  includes a first SiC transistor  110  and a second SiC transistor  112 . In certain embodiments, first SiC transistor  110  and second SiC transistor  112  are coupled to define a central node  114  therebetween, and include a first current-conducting terminal  116  and a second current-conducting terminal  118 , respectively. Central node  114  may be coupled to a load to provide, for example, alternating current (AC) to the load. SiC diodes  120  are coupled between respective source and drain terminals of first SiC transistor  110  and second SiC transistor  112 . In certain embodiments, SiC diodes  120  may be external diodes and/or body-diodes respectively integrated internally into first SiC transistor  110  and second SiC transistor  112 . 
     Inverter  100  further includes a first silicon transistor  122  and a second silicon transistor  124 . First silicon transistor  122  is coupled to central node  114  of inverter arm  108 , and second silicon transistor  124  is coupled in anti-series (i.e., in series with an opposite-facing polarity) to first silicon transistor  122 . First silicon transistor  122  and second silicon transistor  124  are unidirectional voltage blocking devices, and therefore are connected in anti-series to block voltage in either direction during normal operation of inverter  100 . Silicon diodes  126  are coupled between respective collector terminals and emitter terminals of first silicon transistor  122  and second silicon transistor  124  to enable bidirectional current flow. When activated, first silicon transistor  122  and second silicon transistor  124  form a current path from central node  114  to an external node, such as a DC bus mid-point. In some embodiments, first silicon transistor  122  and second silicon transistor  124  are insulated gate bipolar transistors (IGBTs). Because first silicon transistor  122 , second silicon transistor  124 , and silicon diodes  126  are made from silicon, they may be relatively inexpensive compared to similar components made from other materials, such as SiC. In alternative to the example embodiment shown in  FIG.  1   , in certain embodiments, inverter  100  includes more or fewer than two silicon transistors. 
     Inverter  100  further includes a gate driver circuit  128  coupled to respective gates of first SiC transistor  110 , second SiC transistor  112 , first silicon transistor  122 , and second silicon transistor  124 . During normal operation of inverter  100 , gate driver circuit  128  is configured to switch first SiC transistor  110 , second SiC transistor  112  to convert DC from the DC bus into AC. When a short circuit condition occurs at the load, gate driver circuit  128  is configured to switch first silicon transistor  122  and second silicon transistor  124  to freewheel the load-side short circuit current. In other words, at least some of the short circuit current passes through first silicon transistor  122  and second silicon transistor  124  rather than being dissipated through first SiC transistor  110  and second SiC transistor  112 . First silicon transistor  122  and second silicon transistor  124  are activated only during short circuit conditions, and are deactivated by gate driver circuit  128  during normal operation of inverter  100 . 
     By providing an additional current path from central node  114  when activated, first silicon transistor  122  and second silicon transistor  124  reduce a current load on first SiC transistor  110  and second SIC transistor  112  during short circuit events, and increase the overall capacity of inverter  100  to handle load-side short circuits. In some embodiments, first silicon transistor  122  and second silicon transistor  124  handle load-side short circuit current, for example, between 300% and 500% of a nominal current converted by the first SiC transistor  110  and second SiC transistor  112 . Because a system may include a certain number (N) of interleaved inverter arms  108  in order to achieve a specified load-side short circuit capacity, by including silicon transistors such as first silicon transistor  122  and second silicon transistor  124  for each inverter arm  108 , a fewer number (&lt;N) of inverter arms  108  and SiC transistors are needed, reducing the overall cost of the system. 
     During the short-circuit event, the freewheeling of short-circuit current through first silicon transistor  122 , second silicon transistor  124 , and silicon diodes  126 , a significantly lower switching frequency is used compared to the switching frequency used during normal operation, which reduces switching loss that may be associated with silicon diodes. Switching of first silicon transistor  122  and second silicon transistor  124  is disabled during normal operation of the inverter  100 , when the switching frequency is higher and greater switching losses could result. 
       FIG.  2    illustrates another example inverter  200 . Inverter  200  includes DC bus  102  and inverter arm  108 , which generally function as described with respect to  FIG.  1   . Inverter  200  includes a first silicon transistor  202  and a second silicon transistor  204  coupled to each other in anti-parallel and to central node  114  of inverter arm  108 . Similar to first silicon transistor  122  and second silicon transistor  124  described with respect to  FIG.  1   , when a short circuit condition occurs at the load, gate driver circuit  128  is configured to switch first silicon transistor  202  and second silicon transistor  204  to freewheel the load-side short circuit current. As with first silicon transistor  122  and second silicon transistor  124 , first silicon transistor  202  and second silicon transistor  204  are activated only during short circuit conditions, and are disabled during normal operation of inverter  100 . Because, first silicon transistor  202  and second silicon transistor  204  are bidirectional blocking devices that conduct in only one direction, first silicon transistor  202  and a second silicon transistor  204  are coupled in anti-parallel to achieve bidirectional current flow. 
       FIG.  3    is a flow diagram of an example method  300  for operating an inverter (such as inverter  100 ). The inverter includes a DC bus (such as DC bus  102 ) having positive and negative rails (such a positive rail  104  and negative rail  106 ), an inverter arm (such as inverter arm  108 ) including a first SiC transistor (such as first SiC transistor  110 ) having a first current-conducting terminal (such as first current-conducting terminal  116 ) connected to a central node (such as central node  114 ) of the inverter arm and a second SiC transistor (such as second SiC transistor  112 ) having a second current-conducting terminal (such as second current-conducting terminal  118 ) connected to the central node of the inverter arm, and at least one silicon transistor (such as first silicon transistor  122  and second silicon transistor  124 ) having a third current-conducting terminal connected to the central node of the inverter arm. In certain embodiments, the inverter arm is one of a plurality of interleaved arms of the inverter. 
     Method  300  includes switching  302  the first SiC transistor and the second SiC transistor to convert DC from the DC bus into alternating current (AC). Method  300  further includes switching  304  the at least one silicon transistor, when the inverter arm is subjected to a load-side short circuit current, to freewheel the load-side short circuit current. In some embodiments, method  300  further includes deactivating  306  the at least one silicon transistor when the load-side short circuit current is no longer present. 
     In certain embodiments, load-side short circuit current is between 300-500% of a nominal current converted by switching the first SiC transistor and the second SiC transistor. 
     In some embodiments, the at least one silicon transistor includes a first IGBT. In some such embodiments, the at least one silicon transistor further includes a second IGBT arranged in anti-series or anti-parallel with the first IGBT. In embodiments in which the silicon transistors are arranged in anti-series, a silicon diode may be coupled between collector and emitter terminals of each IGBT. 
     In certain embodiments, the first SiC transistor and second SiC transistor are MOSFETs. In such embodiments, a SiC diode may be coupled between source and drain terminals of each MOSFET. 
     Example embodiments of inverters for a UPS system, as well as methods for operating inverters in a UPS system, are described in detail. The circuits and methods are not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the method may also be used in combination with other components and are not limited to practice only with the circuits as described herein. Rather, the example embodiment can be implemented and utilized in connection with many other applications. 
     Although specific features of various embodiments of the disclosure may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the disclosure, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the embodiments, including the best mode, and also to enable any person skilled in the art to practice the embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.