Abstract:
Improved electrical power conversion system configured to transfer power between a DC voltage differential occurring between input DC terminals and lower DC voltage differential made up of the output differential voltages between a positive output DC terminal and a system neutral terminal and a negative output DC terminal and the system neutral terminal. The system actively controls the output differential voltages to account for variations in the electrical loading placed on the system. The system also actively controls the neutral voltage differential between the neutral terminal and Earth Ground. The output differential voltages are controlled to be maintained within an acceptable range for the types of electrical loads powered by the system (e.g. computers, servers, LED lighting) and to the extent the differentials vary, the system corrects the variances at frequencies which do not adversely affect system circuit protection or the electrical loading on the system. Similarly, control of the neutral voltage differential is performed to maintain the differential constant (preferably at about 0 volts) and corrects variances at frequencies which do not adversely affect system circuit protection or the electrical loading on the system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 61/945,529, filed Feb. 27, 2014, the contents of which are incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     The present invention relates generally to the field of electrical power conversion. The present invention relates more specifically to the conversion of Direct Current (“DC”) electrical power from an input voltage source to at least one output voltage which is controlled in reference to a neutral voltage where the neutral voltage is controlled in reference to the actual ground voltage located proximate to the conversion system (“Earth Ground”). 
     DC uninterruptible power supplies (“UPSs”) are typically used in data centers which store data and provide computing for many uses such as local and remote computing, internet data handling, intranet data handling, cloud computing, storage, etc. UPSs are also used for other applications such as DC micro-grid applications having a narrow range of DC voltage capability. In addition to having a narrow range of DC voltage capability, these applications must be reliable and operate as efficiently as possible to reduce the power consumption and heat generated by such systems. Recently, there has been a trend to move data centers near power plants due to the amount of power used by data centers and also move data centers in geographic areas having cooler ambient temperatures and wind patterns which reduce the energy needed to cool (remove energy from) the data centers. Accordingly, improving the efficiency of electrical systems and data centers results in substantial cost savings as a result of increasing the quantity of computing for a given amount of energy and reducing the amount of cooling required for a given amount of computing. 
     Information technology loads such as a data servers, network switches, data storage, etc., are required to be grounded to the actual ground proximate to the device (“Earth Ground”). The neutral and ground of the device are coupled together at the device, and then the neutral/ground are coupled to the Earth Ground (e.g. ground terminal at a data warehouse) through power buses/conductors which inherently have impedance. This impedance is the result of the resistance, capacitance and inductance of the electrical path between the neutral/ground terminal at the device and the Earth Ground (“Impedance to Earth Ground”). Typically power is supplied to the devices by UPSs and other power supply sources along with conductors coupled to neutral and Earth Ground. Accordingly, depending upon the operation and components of the device, the voltage of the neutral/ground at the device can differ from the Earth Ground which causes a current flow from the neutral/ground to Earth Ground. This current flow generates a voltage difference related to the Impedance to Earth Ground and the current. There can be substantial energy lost as a result of this voltage imbalance and current flow between neutral/ground and Earth Ground. Additionally, this voltage difference can impact the operation of circuit protecting devices such as the circuit breaker used to protect for overload currents, ground faults, and short circuits in the power conductors/buses of a data center. 
     SUMMARY 
     One embodiment of the invention relates to a direct current (DC) voltage to DC voltage converter. The DC voltage to DC voltage converter includes a positive DC input terminal, a negative DC input terminal, a center point terminal, a positive DC output terminal, a negative DC output terminal, a neutral terminal, a positive DC output voltage controller, a negative DC output voltage controller and a neutral terminal voltage controller. The center point terminal is coupled to the positive DC input terminal by at least a first capacitance and coupled to the negative DC input terminal by at least a second capacitance. The neutral terminal is coupled to the positive DC output terminal by at least a first inductor and coupled to the negative DC output terminal by at least a second inductor. The positive DC output voltage controller is coupled to the positive DC input terminal, the positive DC output terminal, and the neutral terminal, to control the voltage of the positive DC output terminal relative to the neutral terminal based upon the current flow between the input and positive DC output terminals and the voltage between the positive DC output terminal and the neutral terminal. The negative DC output voltage controller is coupled to the negative DC input terminal, the negative DC output terminal, and the neutral terminal, to control the voltage of the negative DC output terminal relative to the neutral terminal based upon the current flow between the input and negative DC output terminals and the voltage between the negative DC output terminal and the neutral terminal. The neutral terminal voltage controller is coupled to the center point terminal, the neutral terminal and Earth Ground to minimize the difference in voltage potential between the neutral terminal voltage and the potential of Earth Ground, where Earth Ground is connectable to the ground of a building. 
     Another embodiment of the invention relates to an electrical power conversion circuit which generates output DC power at separate terminals referenced to a neutral terminal. The electrical power conversion circuit includes a first terminal to which a DC current is to be applied at a first voltage, a second terminal to which a DC current is to be applied at a second voltage different from the first voltage by an input voltage differential, a neutral terminal, a positive DC terminal, a negative DC terminal, a positive DC voltage controller and a negative DC voltage controller. The neutral terminal is coupled to the first terminal by a first inductor and a first capacitor and coupled to the second terminal by the first inductor and a second capacitor. The positive DC terminal is coupled to the first terminal by at least a second inductor and a first switch having a first control input, and coupled to the neutral terminal by the second inductor and a second switch having a second control input. The negative DC terminal is coupled to the second terminal by at least a third inductor and a third switch having a third control input, and coupled to the neutral terminal by the third inductor and a fourth switch having a fourth control input. The positive DC voltage controller to which a first signal representative of a first voltage differential between the positive DC terminal and the neutral terminal, and a second signal representative of a current flow between the positive DC terminal and the first terminal are applied, the positive DC voltage controller including first and second control outputs coupled to the first and second control inputs, respectively, to control the differential voltage between the positive DC terminal and the neutral terminal within a predetermined range. The negative DC voltage controller to which a third signal representative of a second voltage differential between the negative DC terminal and the neutral terminal, and a fourth signal representative of a current flow between the negative DC terminal and the second terminal are applied, the negative DC voltage controller including third and fourth control outputs coupled to the third and fourth control inputs, respectively, to control the differential voltage between the negative DC terminal and the neutral terminal within a predetermined range, wherein the input voltage range is greater than the sum of the first and second voltage differentials. 
     Another embodiment of the invention relates to an electrical power conversion circuit which generates output DC power at separate terminals referenced to a neutral terminal, wherein the voltage differential between the neutral terminal and Earth Ground is controlled. The electrical power conversion circuit includes a positive DC input terminal, a negative DC input terminal, a center point terminal, means for coupling the positive DC input terminal to the center point terminal, means for coupling the negative DC input terminal to the center point terminal, a positive DC output terminal, a negative DC output terminal, a neutral terminal, an Earth Ground terminal connectable to the ground of a building, means for coupling the neutral terminal to the center point terminal, means for coupling the positive DC output terminal to the positive DC input terminal and the neutral terminal, means for coupling the negative DC output terminal to the negative DC input terminal and the neutral terminal, means for controlling a positive voltage differential between the positive DC output terminal and the neutral terminal based upon a signal representative of the current flow between the positive DC input and output terminals and a signal representative of the positive voltage differential, means for controlling a negative voltage differential between the negative DC output terminal and the neutral terminal based upon a signal representative of the current flow between the negative DC input and output terminals and a signal representative of the negative voltage differential and means for controlling a neutral voltage differential between the neutral terminal and the Earth Ground terminal based upon a signal representative of the voltage differential between the center point terminal and the Earth Ground terminal and a signal representative of the voltage differential between the neutral terminal and Earth Ground terminal. 
     In another embodiment, the converter also includes a first capacitor and a first inductor coupled between the neutral terminal and the positive DC output terminal. 
     In another embodiment, the converter also includes a second capacitor and a second inductor coupled between the neutral terminal and the negative DC output terminal. 
     In another embodiment, the neutral terminal voltage controller includes a voltage sensor coupled between the neutral terminal and Earth Ground, and the output of the voltage sensor. 
     Alternative exemplary embodiments relate to other features and combinations of features as may be generally recited in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This application will become more fully understood from the following detailed description, taken in conjunction with the accompanying figures, wherein like reference numerals refer to like elements. 
         FIG. 1  is a system diagram illustrating supply of electrical power to computers (e.g. data servers) in a data center, where the electrical power is converted from alternating electrical power (“AC”) to DC power which can be more efficiently used by the computers; 
         FIG. 2  illustrates an embodiment of a DC to DC converter without control circuitry for controlling a neutral relative to Earth Ground; 
         FIG. 3  illustrates an embodiment of a DC to DC converter without control circuitry for controlling a neutral relative to Earth Ground; 
         FIG. 4  illustrates an embodiment of a DC to DC converter without control circuitry for controlling a neutral relative to Earth Ground; 
         FIG. 5  illustrates a first embodiment of a DC to DC converter with a neutral controlled relative to Earth Ground and DC voltages controlled relative to the neutral; 
         FIG. 5A  illustrates a second embodiment of a DC to DC converter with a neutral controlled relative to Earth Ground and DC voltages controlled relative to the neutral; 
         FIG. 6  illustrates a third embodiment of a DC to DC converter which has a neutral controlled relative to Earth Ground and DC voltages controlled relative to the neutral; 
         FIG. 7  illustrates a circuit diagram for a DC neutral point controller; 
         FIG. 8  illustrates a circuit diagram for a positive DC output voltage controller; 
         FIG. 9  illustrates a circuit diagram for a negative DC output voltage controller; and 
         FIG. 10  illustrates a fourth embodiment of a DC to DC converter which has a neutral controlled relative to Earth Ground and DC voltages controlled relative to the neutral. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments illustrated and described are representative of the operative sub-systems of a direct current un-interruptible power system (DC UPS). Embodiments of the direct current (DC) converter utilize a neutral output control that allows for return current to be sourced back to the converter. Preferred embodiments of the system are configured to manage and/or control the system neutral relative to the DC output voltages and Earth Ground based, in part, upon the return current. When DC power is distributed in power system for a facility such as a data center, the system provides output DC voltage to power buses including a neutral potential, a positive DC voltage relative to the neutral, a negative DC voltage relative to the neutral, and Earth Ground as a reference for the neutral. Selecting the components of the system provides system modifications to accommodate operation across a number of voltage ranges and system power requirements. In addition to the value of the DC voltage conversion and neutral control to a DC UPS system, this type of conversion and control would be valuable for use in other DC power systems wherein the final DC to DC conversion is configured to accommodate the DC voltage needs for any of the loads on the system e.g. LED lighting powered from a common DC power supply which distributes power (e.g. at 1.5 volts or multiple thereof) to a plurality of LED lights (without individual power conversion or supply circuits), computers powered from a common DC power supply which distributes power (e.g. at 12 volts) to a plurality of computers or servers.) 
     Referring to  FIG. 1 , an embodiment of a DC UPS is illustrated. The DC UPS shown includes: 
     Alternating current input source  2 , which for a majority of data centers would be 480 volt (hereinafter “volt” or “v”), three-phase, alternating current (AC); 
     Alternating current input connection or distribution  4 , which would typically include an AC load center including appropriate circuit protection (e.g. three-phase circuit breakers) coupled between the AC supply and AC wiring or an AC power busway; 
     Direct Current Un-interruptible Power System (DC UPS)  6 , which in a preferred embodiment for a data center would be constructed in modular form to include multiple systems each supported within a rack (represented by the rectangle indicated at the arrow from no.  6 ) having form factor and frame construction the same as or similar to the racks supporting the servers in the data center; 
     Alternating current to direct current conversion stage  8 , which, for purposes of the preferred embodiment, would be the type of a conversion system commonly used in data centers to convert the AC supply to DC voltage to power the energy storage with DC power; 
     Internal DC power bus  10   a  and  10   b;    
     Energy storage system  12 ; 
     Earth Ground terminal  32 ; 
     Direct current to direct current conversion stage  14  (see details discussed below in  FIGS. 5 and 6  for preferred embodiment) which has its ground terminal  16  coupled to terminal  32  with a ground impedance  18 ; 
     Output power conductors (+190 volts DC, neutral, −190 volts DC)  20   a ,  20   b ,  20   c  of stage  14 , wherein neutral conductor  20   b  is coupled to terminal  32  by impedance  24 ; 
     Distribution system  26 , which would typically include a load center including appropriate circuit protection (e.g. two phase DC-rated circuit breakers for each branch circuit) coupled between the conductors  20   a ,  20   b  and  20   c , and DC wiring or DC power busways which define each branch circuit  27   n   1 ,  27   n   2 ,  27   nn;    
     Output power conductors (+190 volts DC, neutral, −190 volts DC)  29   a ,  29   b ,  29   c  for each branch circuit wherein the neutral conductor  29   b  is coupled to terminal  32  by an impedance  30 ; 
     DC loads  28   n   1 ,  28   n   2 ,  28   n , which in the preferred embodiments would be the DC to DC conversions circuits which convert the +/−190 volts DC to 12 volts DC for providing 12 volts DC power to computers and servers supported by server racks. 
     The component and circuit symbols used in  FIG. 1  are industry standard symbols. 
     In the preferred embodiment, stage  8  is a conversion circuit which converts 3-phase 480 volt alternating current (AC) power to DC power with a voltage in the range of 500 to 1000 volts with a typical nominal voltage of 720 volts. DC power is applied to the energy storage unit  12  via conductors  10   a  and  10   b . Storage unit  12  operates to store electrical energy at the nominal voltage generated between conductors  10   a  and  10   b , and also operates to remove AC components from the DC power supplied from stage  8  to conversion stage  14 . Unit  12  is preferably in the form of batteries arranged in series to match the DC output voltage from stage  8 . The storage unit  12  batteries may be lead acid; lithium-based, nickel-based, or could take the form of capacitors wherein the capacity of the unit  12  components are sized to provide enough energy to power the data center when the 480 volt AC power is temporarily lost or until backup generation is available to power the data center. It is contemplated that the storage unit will use a storage media which is selected by the system integrator of a data center based upon the cost and current state of the technology for commercially available electrical energy storage having the voltage and power capacities suitable for a particular data center. 
       FIGS. 5 ,  5 A,  6 , and  10  illustrate preferred embodiments for conversion stage  14  which converts the DC power from stage  8  into DC power at +/−190 volts applied to terminals  20   a  and  20   c , respectively, in reference to neutral terminal  20   b  and ground terminal  32 . As discussed above, the +/−190 volt terminals are connected to a distribution system  26  which includes a load center having circuit protection e.g. 2 or 3 pole, DC circuit breakers for each branch circuits  27   n   1 ,  27   n   2 ,  27   nn . These branch circuits may be made up of electrical wiring and/or power busways which supply electrical power to DC loads  28   n   1 ,  28   n   2 ,  28   nn  such as the power supplies for servers in server racks. In the present embodiment, the power supplies include DC to DC converters which convert the +/−190 volt DC power into 12 volt DC power usable by a typical computer server. In a preferred embodiment a plurality of such DC to DC converters would be connected to the power bus bars of a server rack to provide shared-power sources for all of the servers in the rack which are powered by a particular power bus in the rack. Alternatively each server may have its own DC to DC converter as a source of power. By way of example only, the DC to DC converters used to provide 12 volt DC power to the servers can be a single chip DC to DC converter of the type used for certain electric automobile applications when combined with appropriate output power conditioning/filtering. 
       FIG. 2  illustrates a DC to DC buck converter implemented utilizing dual insulated-gate bipolar transistor (IGBT) technology. The buck converter does not include a controlled neutral, and operates using the following components coupled as shown in  FIG. 2 : 
     Positive DC input terminal  34 , 
     Negative DC input terminal  38 , 
     Capacitive energy storage element  36 , 
     Upper leg switching semiconductor device  40 , 
     Lower leg switching semiconductor device  42 , 
     Inductive element  44 , 
     Capacitive energy storage element  46 , 
     Positive DC output terminal  48 , and 
     Negative DC output terminal  50 . 
     The component and circuit symbols used in  FIG. 2  are industry standard symbols. 
     Depending upon the particular electrical architecture used in a data center or for other applications of the system e.g. LED lighting power, the DC to DC converter shown in  FIG. 2  may be used in place of a preferred converter having output voltages and a neutral controlled relative to each other and ground. When used as the system DC to DC converter for stage  14 , terminal  34  would be connected to  10   a , terminal  38  to  10   b , terminal  48  to  20   a  and terminal  50  to  20   c . Because the circuit of  FIG. 2  operates only on the voltage differential between input terminals  34  and  38 , additional circuitry would be required to generate a neutral point for the system relative to an earth ground and based upon the system requirements. 
       FIG. 3  illustrates a DC to DC buck converter implemented utilizing single IGBT and diode technology. Referring to  FIG. 3 , the DC to DC converter does not included a controlled neutral, and operates using the following components coupled as shown in  FIG. 3 : 
     Positive DC input terminal  52 , 
     Negative DC input terminal  56 , 
     Capacitive energy storage element  54 , 
     Upper leg switching semiconductor device  58 , 
     Lower leg diode semiconductor device  60 , 
     Inductive element  62 , 
     Capacitive energy storage element  64 , 
     Positive DC output terminal  66 , and 
     Negative DC output terminal  68 . 
     The component and circuit symbols used in  FIG. 3  are industry standard symbols. 
     Depending upon the particular electrical architecture used in a data center or for other applications of the system e.g. LED lighting power, the DC to DC converter shown in  FIG. 3  may be used in place of a preferred converter having output voltages and a neutral controlled relative to each other and ground. When used as the system DC to DC converter for stage  14 , terminal  52  would be connected to  10   a , terminal  56  to  10   b , terminal  66  to  20   a  and terminal  68  to  20   c . Because the circuit of  FIG. 3  operates only on the voltage differential between input terminals  52  and  56 , additional circuitry would be required to generate a neutral point for the system relative to earth ground  32  and based upon the system requirements. 
       FIG. 4  illustrates a bi-polar DC/DC buck converter that symmetrically bucks voltage from a higher voltage DC source to a lower output voltage and also does not include a controlled neutral. Referring to  FIG. 4 , the DC to DC converter shown includes: 
     Positive DC input terminal  70 , 
     Capacitive energy storage elements  71 , 
     Center-point of capacitive energy storage elements  72 , 
     Negative DC input terminal  74 , 
     Center-point of upper leg switching semiconductor devices  76 , 
     Center-point of stacked semiconductor devices  78 , 
     Center-point of lower leg switching semiconductor devices  80 , 
     Capacitive energy storage elements  81 , 
     Center-point of capacitive energy storage elements  82 , 
     Positive DC output terminal  84 , and 
     Negative DC output terminal  86 . 
     The component and circuit symbols used in  FIG. 4  are industry standard symbols. 
     Depending upon the particular electrical architecture used in a data center or for other applications of the system e.g. LED lighting power, the DC to DC converter shown in  FIG. 4  may be used in place of a preferred converter having output voltages and a neutral controlled relative to each other and ground. When used as the system DC to DC converter for stage  14 , terminal  70  would be connected to  10   a , terminal  74  to  10   b , terminal  84  to  20   a  and terminal  86  to  20   c . Because the circuit of  FIG. 4  operates only on the voltage differential between input terminals  52  and  56 , additional circuitry would be required to generate a neutral point for the system relative to earth ground  32 , and based upon the voltage at terminal  82  and the system requirements. 
     The converters illustrated in  FIGS. 2-4  are configured to operate from a DC input bus at a voltage higher than the desired DC voltage. This voltage is then stepped down through the converter to a lower DC voltage by switching power devices and inductors. 
     Unlike the converters illustrated in  FIGS. 2-4 , the DC to DC converters described in detail below provide voltage balancing that allows the system to maintain positive and negative DC voltages relative to ground during fault events. This is accomplished by converting the DC power applied to  10   a  and  10   b  to power in the form of a positive DC voltage applied to  20   a , negative DC voltage applied to  20   c  wherein the differential voltage between neutral  20   b  and  20   a , neutral  20   b  and  20   c  and neutral  20   b  and ground  32  are controlled so the differential voltages remain constant e.g. +190 volts between  20   a  and  20   b , −190 volts between  20   b  and  20   c  and 0 volts between neutral  20   b  and ground  32 . 
     A constant differential is defined in terms of the operational requirements of the system. For example, in a data center, proper operation of the circuit protection (e.g. circuit breakers) for overload, short circuit and ground fault protection ideally requires that the differential not vary if possible, but if it varies, the amplitude of the variation is relatively small and the frequency of the variation is at a frequency sufficiently high not to adversely affect the proper operation of the system circuit protection. This type of balancing also provides improved performance and efficiency in electrical loads (e.g. server power supplies) powered by the converter. In addition to the computers/servers in a data center, these loads can include micro-grid components, LED system applications, or electric vehicle fast chargers. 
       FIG. 5  illustrates a first embodiment of a preferred DC to DC converter  14  which is contemplated for use in the system discussed in reference to  FIG. 1 . Unlike the prior art buck converters illustrated in  FIGS. 2-4 , the converter of  FIG. 5  has a circuit design which provides a neutral potential terminal controlled in reference to ground  32  and the +/− output DC voltages. The first embodiment of converter/stage  14  includes the following components coupled as shown in  FIG. 5 . 
     Positive DC input terminal  10   a,    
     Upper leg capacitive energy storage element  90 , 
     Center point  91 , 
     Center point voltage to ground sensor  92 , 
     Lower leg capacitive energy storage element  94 , 
     Negative DC input terminal  10   b,    
     Neutral point regulator circuit (i.e. neutral voltage controller)  98 , 
     Neutral leg switching devices for neutral point regulator circuit  100   a  and  100   b,    
     Upper leg switching devices for neutral point regulator circuit  102 , 
     Connection node for neutral point regulator circuit  104 , 
     Neutral regulator current sensor  105 , 
     Lower leg switching devices for neutral point regulator circuit  106 , 
     Inductive element for neutral point regulator  108 , 
     Node for neutral point voltage to ground sensor connection  110 , 
     Neutral point voltage to ground sensor  112 , 
     Capacitive energy storage element to ground for neutral point regulator circuit  114 , 
     Upper half—upper leg switching device for output stage  116 , 
     Connection node for output stage  118 , 
     Upper half—lower leg switching device for output stage  120 , 
     Connection node for neutral system  122 , 
     Lower half—upper leg switching device for output stage  124 , 
     Connection node for output stage  125 , 
     Lower half—lower leg switching device for output stage  126 , 
     Upper leg output control current sensor  128 , 
     Lower leg output control current sensor  130 , 
     Upper leg output inductive element  132 , 
     Lower leg output inductive element  134 , 
     Capacitor  135 , 
     Connection node for neutral system output side  136 , 
     Capacitor  137 , 
     Upper leg output secondary inductive element  138 , 
     Lower leg output secondary inductive element  140 , 
     Output positive rail voltage to neutral point sensor  142 , 
     Output negative rail voltage to neutral point sensor  144 , 
     Output positive phase/leg voltage controller  146 , 
     Output negative phase/leg voltage controller  148 , 
     Positive DC output terminal  20   a,    
     Neutral terminal  20   b , and 
     Negative DC output terminal  20   c.    
     The component and circuit symbols used in  FIG. 5  are industry standard symbols. 
       FIG. 5A  illustrates a second embodiment of a preferred DC to DC converter  14  which is contemplated for use in the system discussed in reference to  FIG. 1 . In particular,  FIG. 5A  illustrates a dual half-bridge implementation of the converter. The second embodiment of converter/stage  14  includes the following components coupled as shown in  FIG. 5A . 
     Positive DC input terminal  10   a,    
     Upper leg capacitive energy storage element  90 , 
     Center point  91 , 
     Center point voltage to ground sensor  92 , 
     Lower leg capacitive energy storage element  94 , 
     Negative DC input terminal  10   b,    
     Neutral point regulator circuit (i.e. neutral voltage controller)  98 , 
     Neutral leg switching devices for neutral point regulator circuit  100   a  and  100   b,    
     Upper leg switching devices for neutral point regulator circuit  102 , 
     Connection node for neutral point regulator circuit  104 , 
     Neutral regulator current sensor  105 , 
     Lower leg switching devices for neutral point regulator circuit  106 , 
     Inductive element for neutral point regulator circuit  108 , 
     Node for neutral point voltage to ground sensor connection  110 , 
     Neutral point voltage to ground sensor  112 , 
     Capacitive energy storage element to ground for neutral point regulator circuit  114 , 
     Upper half—upper leg switching device for output stage  116 , 
     Connection node for output stage  118 , 
     Upper half—lower leg switching device for output stage  120 , 
     Lower half—upper leg switching device for output stage  124 , 
     Connection node for output stage  125 , 
     Lower half—lower leg switching device for output stage  126 , 
     Upper leg output control current sensor  128 , 
     Lower leg output control current sensor  130 , 
     Upper leg output inductive element  132 , 
     Lower leg output inductive element  134 , 
     Capacitor  135 , 
     Connection node for neutral system output side  136 , 
     Capacitor  137 , 
     Upper leg output secondary inductive element  138 , 
     Lower leg output secondary inductive element  140 , 
     Output positive rail voltage to neutral point sensor  142 , 
     Output negative rail voltage to neutral point sensor  144 , 
     Output positive phase/leg voltage controller  146 , 
     Output negative phase/leg voltage controller  148 , 
     Positive DC output terminal  20   a,    
     Neutral terminal  20   b , and 
     Negative DC output terminal  20   c.    
     The component and circuit symbols used in  FIG. 5A  are industry standard symbols. 
     Circuit  98  (shown and described in detail in reference to  FIG. 7 ), controller  146  (shown and described in detail in reference to  FIG. 8 ) and controller  148  (shown and described in detail in reference to  FIG. 9 ) operate to control the voltage differential between neutral  20   b  and ground  32  in conjunction with maintaining the voltage differential between the output DC voltages between  20   a  and  20   b  and  20   b  and  20   c , respectively. There are 3 non-set point inputs to circuit  98 . The first is the output of a voltage sensor  92  which operates as a center point sensor. In particular, voltage sensor  92  generates a signal representative of the voltage differential between ground  32  and the center point voltage  91  between the positive DC voltage  10   a  and the negative DC voltage  10   b  and applies this signal to circuit  98 . The second input is the output of a voltage sensor  112  which operates as a neutral point sensor. In particular, voltage sensor  112  generates a signal representative of the voltage differential between ground  32  and the neutral voltage  20   b  between the positive DC voltage  20   a  and the negative DC voltage  20   c  and applies this signal to circuit  98 . The third input is the output of current sensor  105 . In particular, current sensor  105  generates a signal representative of the current flow through inductor  108  and applies this signal to circuit  98 . 
     There are 2 non-set point inputs to controller  146 . The first is the output of a voltage sensor  142  which generates a signal representative of the voltage differential between positive DC voltage  20   a  and neutral  20   b  and applies this signal to controller  146 . The second input is the output of current sensor  128  which applies a signal to controller  146 . There are also 2 non-set point inputs to controller  148 . The first is the output of a voltage sensor  144  which generates a signal representative of the voltage differential between positive DC voltage  20   c  and neutral  20   b  and applies this signal to controller  148 . The second input is the output of current sensor  130 , which applies a signal to controller  148 . 
     Circuit  98  includes 4 outputs  1 ,  2 ,  3  and  4  which are connected to the respective gates of switches  100   a ,  100   b ,  102  and  106 . The switches are controlled by circuit  98  based upon i) the 2 non-set point input voltages from  92  and  112 ; ii) logic which provides control appropriate for a particular application; and iii) an input from current sensor  105  which generates a signal representative of the current in inductor  108 .  FIG. 7 , described in detail below, illustrates a preferred embodiment of such control useable for purposes of providing DC power in a data center. 
     Controllers  146  and  148  include 2 outputs each,  6  and  7 , and  8  and  9 , respectively, which are connected to the respective gates of switches  116 ,  120 ,  124 , and  126 . Switches  116  and  118  are controlled by controller  146  based upon i) the 2 non-set point input voltages from  142  and  128 ; ii) the power rating input (see  FIG. 8 ,  268 ); iii) a signal representative of a current limit threshold (see, e.g.  FIG. 8 ,  269 ); and iv) logic which provides control appropriate for a particular application.  FIG. 8 , described in detail below, illustrates a preferred embodiment of such logic useable for purposes of providing DC power in a data center. Switches  124  and  126  are controlled by controller  148  based upon i) the 2 non-set point input voltages from  144  and  130 ; ii) the power rating input (see  FIG. 9 ,  288 ); iii) a signal representative of a current limit threshold (see, e.g.  FIG. 9 ,  289 ); and iv) logic which provides control appropriate for a particular application.  FIG. 9 , described in detail below, illustrates a preferred embodiment of such control useable for purposes of providing DC power in a data center. 
     In operation, circuit  98 , controller  146  and controller  148  control the gates of the respective switches based upon the respective inputs and set points to i) maintain the voltage differential between neutral  20   c  and ground  32  constant (as defined above) at 0 volts, and ii) maintain the voltage differential between positive voltage  20   a  and neutral  20   b , and negative voltage  20   c  and neutral  20   b  at a constant (as defined above) system voltage differential (e.g. +190 volts and −190 volts). 
       FIG. 6  illustrates a third embodiment of a preferred DC to DC converter/stage  14  which is contemplated for use in the system discussed in reference to  FIG. 1 . Unlike the prior art buck converters illustrated in  FIGS. 2-4 , the converter of  FIG. 6  has a circuit design which provides a neutral potential terminal controlled in reference to ground  32  and the +/− output DC voltages. This embodiment of converter/stage  14  includes the following components coupled as shown in  FIG. 6 : 
     Positive DC input terminal  10   a,    
     Upper leg capacitive energy storage element  158 , 
     Center point voltage to ground sensor  160 , 
     Center point  161 , 
     Lower leg capacitive energy storage element  162 , 
     Negative DC input terminal  10   b,    
     Neutral point regulator circuit (i.e. neutral voltage controller)  166 , 
     Neutral point voltage to ground sensor  168 , 
     Neutral leg switching devices for neutral point regulator circuit  170   a  and  170   b,    
     Upper leg switching devices for neutral point regulator circuit  172 , 
     Connection node for neutral point regulator circuit  174 , 
     Neutral regulator current sensor  175 , 
     Lower leg switching devices for neutral point regulator circuit  176 , 
     Inductive element for neutral point regulator circuit  178 , 
     Node for neutral point voltage to ground sensor connection  180 , 
     Capacitive energy storage element to ground for neutral point regulator circuit  182 , 
     Upper half—upper leg switching device for output stage  184 , 
     Connection node for output stage  186 , 
     Upper half—lower leg diode device for output stage  188 , 
     Connection node for neutral system  190 , 
     Lower half—upper leg diode device for output stage  192 , 
     Connection node for output stage  194 , 
     Lower half—lower leg switching device for output stage  196 , 
     Upper leg output control current sensor  198 , 
     Lower leg output control current sensor  200 , 
     Upper leg output inductive element  202 , 
     Lower leg output inductive element  204 , 
     Capacitor  205 , 
     Connection node for neutral system output side  206 , 
     Capacitor  207 , 
     Upper leg output secondary inductive element  208 , 
     Lower leg output secondary inductive element  210 , 
     Output positive rail voltage to neutral point sensor  212 , 
     Output negative rail voltage to neutral point sensor  214 , 
     Output positive phase/leg voltage controller  216 , 
     Output negative phase/leg voltage controller  218 , 
     Positive DC output terminal  20   a,    
     Neutral terminal  20   b , and 
     Negative DC output terminal  20   c.    
     The component and circuit symbols used in  FIG. 6  are industry standard symbols. 
     Circuit  166  (also shown and described in detail in reference to  FIG. 7 ), controller  216  (also shown and described in detail in reference to  FIG. 8 ) and controller  218  (also shown and described in detail in reference to  FIG. 9 ) operate to control the voltage differential between neutral  20   b  and ground  32  in conjunction with maintaining the voltage differential between the output DC voltages between  20   a  and  20   b  and  20   b  and  20   c , respectively. There are 2 non-set point inputs to circuit  166 . The first is the output of a voltage sensor  160  which operates as a center point sensor. In particular, voltage sensor  160  generates a signal representative of the voltage differential between ground  32  and the center point voltage  161  between the positive DC voltage  10   a  and the negative DC voltage  10   b  and applies this signal to circuit  166 . The second input is the output of a voltage sensor  168  which operates as a neutral point sensor. In particular, voltage sensor  168  generates a signal representative of the voltage differential between ground  32  and the neutral voltage  20   b  between the positive DC voltage  20   a  and the negative DC voltage  20   c  and applies this signal to circuit  166 . The third input is the output of current sensor  175 . In particular, current sensor  175  generates a signal representative of the current flow through inductor  178  and applies this signal to circuit  166 . 
     There are 2 non-set point inputs to controller  216 . The first is the output of a voltage sensor  212  which generates a signal representative of the voltage differential between positive DC voltage  20   a  and neutral  20   b  and applies this signal to controller  216 . The second input is the output of current sensor  198  which applies a signal to controller  216 . There are also 2 non-set point inputs to controller  218 . The first is the output of a voltage sensor  214  which generates a signal representative of the voltage differential between positive DC voltage  20   c  and neutral  20   b  and applies this signal to controller  218 . The second input is the output of current sensor  200  which applies a signal to sensor  218 . 
     Circuit  166  includes 4 outputs  1 ,  2 ,  3  and  4  which are connected to the respective gates of switches  170   a ,  170   b ,  172  and  176 . The switches are controlled by circuit  166  based upon i) the 2 non-set point input voltages from  160  and  168 ; ii) logic which provides control appropriate for a particular application; and (iii) an input from current sensor  175  which generates a signal representative of the current in inductor  178 .  FIG. 7 , described in detail below, illustrates a preferred embodiment of such control useable for purposes of providing DC power in a data center. 
     Controllers  216  and  218  include 1 output each,  6  and  9 , respectively, which are connected to the respective gates of switches  184  and  196 . Switch  184  is controlled by controller  216  based upon i) the 2 non-set point input voltages from  198  and  212 ; ii) the power rating input (see  FIG. 8 ,  268 ); iii) a signal representative of a current limit threshold (see, e.g.  FIG. 8 ,  269 ); and iv) logic which provides control appropriate for a particular application.  FIG. 8 , described in detail below, illustrates a preferred embodiment of such control useable for purposes of providing DC power in a data center. Switch  196  is controlled by controller  218  based upon i) the 2 non-set point input voltages from  200  and  214 ; ii) the power rating input (see  FIG. 9 ,  288 ); iii) a signal representative of a current limit threshold (see, e.g.  FIG. 9 ,  289 ); and iv) logic which provides control appropriate for a particular application.  FIG. 9 , described in detail below, illustrates a preferred embodiment of such control useable for purposes of providing DC power in a data center. 
     In operation, circuit  166 , controller  216  and controller  218  control the gates of the respective switches based upon the respective inputs and set points to i) maintain the voltage differential between neutral  20   c  and ground  32  constant (as defined above) at 0 volts, and ii) maintain the voltage differential between positive voltage  20   a  and neutral  20   b , and negative voltage  20   c  and neutral  20   b  at a constant (as defined above) system voltage differential (e.g. +190 volts and −190 volts. 
     Referring again to  FIGS. 5 ,  5 A and  6 , these figures show the circuitry of embodiments of DC to DC converters with neutral point balancing. By using high frequency control of the central semiconductor device units,  FIGS. 5 and 5A , elements  100   a ,  100   b ,  102 , and  106 , and  FIG. 6 , components  170   a ,  170   b ,  172 , and  176 , the input bus is balanced which maintains two independent voltage rails relative to neutral, and neutral point regulator maintains the neutral point to 0V relative to ground. 
     The neutral balancing circuitry allows for constant voltage to ground of the system. This voltage is maintained both under normal operating conditions and under fault conditions. By maintaining the neutral voltage, the system is allowed to respond in a similar way to an alternating current system when a fault occurs. This permits a protective device to clear and the system to return to normal operation after the fault has occurred. 
     In the preferred embodiments described in  FIGS. 5 ,  5 A,  6 , and  10 , the switches are insulated gate bipolar transistors. Alternative switching components, e.g., alternate semiconductor switches, field effect transistors, etc., can be used for components  100   a ,  100   b ,  102 ,  106 ,  116 ,  120 ,  124 , and  126  in  FIGS. 5 and 5A , and components  170   a ,  170   b ,  172 ,  176 ,  184 , and  196  in  FIG. 6 . In addition, the stacking of H-bridge converters can be further stacked to allow for higher voltage neutral point variants of the circuitry shown.  FIG. 10  is an example which allows for higher voltage differential input voltages and yet allows for lower voltage power electronic semiconductors to be used, and the neutral point regulator to allow for a prescribed output DC voltage with the same behavior as the similar stacked configurations. 
       FIG. 7  illustrates an embodiment of switch control logic  98  and  166 . The control logic illustrated in  FIG. 7  includes: 
     Voltage loop controller logic  230 , 
     Current loop controller logic  232 , 
     Switching frequency controller logic  234 , 
     Output switch controller node  256 , 
     Output switch controller node  258 , 
     Output switch controller node  260 , and 
     Output switch controller node  264 . 
     The component and circuit symbols used in  FIG. 7  are industry standard symbols. As depicted, the controller has an inner current loop  232 , an outer voltage loop  230 , which work together to maintain a zero volt reference at point  110  of  FIGS. 5 and 5A , and at point  180  of  FIG. 6 , and equivalents in other topologies such as the embodiment shown in  FIG. 10 . 
       FIG. 8  illustrates an embodiment of a positive leg controller  146  or  216 . The positive leg controller of  FIG. 8  includes: 
     Positive leg current sensor feedback  266 , coupled to  128  or  198 , 
     Kilowatt rated power output for discrete unit  268 , 
     Positive voltage feedback signal  270 , coupled to  142  or  212 , 
     Voltage droop controller logic  274 , 
     Outer voltage loop controller logic  276 , 
     Inner current loop controller logic  278 , 
     Switching frequency controller logic  280 , 
     Positive output switch primary control signal for gating  6 , and 
     Positive output switch NOT control signal for gating  7 . 
     The component and circuit symbols used in  FIG. 8  are industry standard symbols. 
       FIG. 9  illustrates an embodiment of a negative leg controller  148  or  218 . The negative leg controller of  FIG. 9  includes: 
     Negative leg current sensor feedback  286  coupled to  130  or  200 , 
     Kilowatt rated power output for discrete unit  288 , 
     Negative voltage feedback signal  290  coupled to  144  or  214 , 
     Voltage droop controller logic  294 , 
     Outer voltage loop controller logic  296 , 
     Inner current loop controller logic  298 , 
     Switching frequency controller logic  300 , 
     Negative leg output switch primary control signal for gating  9 , and 
     Negative leg output switch NOT control signal for gating  8 . 
     The component and circuit symbols used in  FIG. 9  are industry standard symbols. 
       FIG. 10  illustrates a fourth embodiment of DC to DC converter/stage  14  which has neutral  20   b  controlled relative to ground  32  with DC voltages  20   a  and  20   c  controlled relative to neutral  20   b . The converter of  FIG. 10  includes: 
     Positive DC input terminal  10   a,    
     Upper capacitive energy storage element  308 , 
     Upper center point node  310 , 
     Center point node  316 , 
     Center point voltage sensor  318 , 
     Lower center point node  322 , 
     Lower capacitive energy storage element  326 , 
     Negative DC input terminal  10   b,    
     Neutral leg switching device for neutral voltage controller circuit ( 2 )  330 , 
     Neutral leg switching device for neutral voltage controller circuit ( 1 )  332 , 
     3 Level neutral, voltage controller (i.e. neutral voltage controller)  334 , 
     Upper switching device for neutral voltage controller circuit ( 3 )  336 , 
     Upper connection node connection  338 , 
     Neutral leg switching device for neutral voltage controller circuit ( 4 )  340 , 
     Neutral leg switching device for neutral voltage controller circuit ( 5 )  342 , 
     Lower connection node connection  344 , 
     Lower switching device for neutral voltage controller circuit ( 6 )  346 , 
     Inductive element for neutral point voltage controller  348 , 
     Neutral point voltage to ground sensor connection  350 , 
     Neutral regulator current sensor  351 , 
     Capacitive energy storage element to ground for neutral point regulator circuit  352 , 
     Earth ground  32 , 
     Upper 3 level diode clamped switch array ( 7 ,  8 ,  9 ,  10 ,  11 ,  12 )  354 , 
     Central node  356 , 
     Lower 3 level diode clamped switch array ( 13 ,  14 ,  15 ,  16 ,  17 ,  18 )  358   
     Upper leg output control current sensor  360 , 
     Lower leg output control current sensor  362 , 
     Output positive phase voltage controller, drooping, outer voltage, inner current, 3 level controller  364 , 
     Upper leg output inductive element  366 , 
     Lower leg output inductive element  368 , 
     Upper leg capacitive energy storage element  370 , 
     Lower leg capacitive energy storage element  372 , 
     Upper leg output secondary inductive element  374 , 
     Lower leg output secondary inductive element  376 , 
     Output positive rail voltage to neutral point sensor  378 , 
     Output negative rail voltage to neutral point sensor  380 , 
     Positive DC output terminal  20   a,    
     Neutral terminal  20   b,    
     Negative DC output terminal  20   c , and 
     Output positive phase voltage controller, drooping, outer voltage, inner current, 3 level controller  388 . 
     The component and circuit symbols used in  FIG. 10  are industry standard symbols. 
     Circuit  334  (which would be configured to operate in accordance with the circuit in  FIG. 7  to control 2 additional switches), controller  364  (which would be configured to operate in accordance with the circuit in  FIG. 8  to control 4 additional switches) and controller  388  (which would be configured to operate in accordance with the circuit in  FIG. 9  to control 4 additional switches) operate to control the voltage differential between neutral  20   b  and ground  32  in conjunction with maintaining the voltage differential between the output DC voltages between  20   a  and  20   b  and  20   b  and  20   c , respectively. There are 3 non-set point inputs to circuit  334 . The first is the output of a voltage sensor  318  which operates as a center point sensor. In particular, voltage sensor  318  generates a signal representative of the voltage differential between ground  32  and the center point voltage  316  between the positive DC voltage  10   a  and the negative DC voltage  10   b  and applies this signal to circuit  334 . The second input is the output of a voltage sensor  350 . In particular, voltage sensor  350  generates a signal representative of the voltage differential between ground  32  and the neutral voltage  20   b  between the positive DC voltage  20   a  and the negative DC voltage  20   c  and applies this signal to circuit  334 . The third input is the output of a current sensor  351 . In particular, current sensor  351  generates a signal representative of the current through inductor  348  and applies this signal to circuit  334 . 
     Various modifications and configurations of DC to DC power transfer implemented by a reduction in voltage and increase in available current and voltage control have been described in detail above. However, as improvements and changes are made in the availability of semiconductors which can replace those herein, it is anticipated that those changes would fall within the scope of the claims set out below. One of the primary objectives of the circuitry is to transfer power with the DC to DC converter as efficiently as possible. Accordingly, it is contemplated that components and circuitry in the converter would be replaced with improved and/or more efficient substitutes. 
     In operation, the circuitry set out herein is configured to provide a multilevel power electronic half bridge which cancels or reduces disturbances on a neutral point of the bipolar output stage (e.g. at neutral  20   b ). Such a circuit is effective for many applications without complete cancellation as long as the amplitude and frequency of the disturbances are below those tolerable by the system utilizing the DC power generated by the circuit. Cancellation is possible for harmonics up to roughly one tenth of the switching frequency (e.g. 4-12 KHz and preferably 8 KHz) for the circuits of  FIGS. 5 ,  5 A,  6  and  10 ). In operation, higher frequency harmonics are then shunted to ground via a capacitor (i.e. capacitor  114  in  FIGS. 5 and 5A , capacitor  182  in  FIG. 6 , and capacitor  352  in  FIG. 10 ) connected between the neutral  20   a  and ground  32 . The result of this combination is that low frequency disturbances (e.g. less than 1/10  of the switching frequency) are handled by the power electronic stage and higher frequencies (e.g. greater than 1/10  the switching frequency) are managed with the capacitor connection resulting in a near zero voltage differential between neutral  20   a  and ground  32 . 
     As disclosed above and shown in  FIGS. 5 ,  5 A,  6 , and  10 , controllers  146  and  148  (FIG.  5 / 5 A embodiment), controllers  216  and  218  ( FIG. 6  embodiment) and controllers  364  and  388  ( FIG. 10  embodiment) control the bipolar output stage between DC voltage  20   a  and neutral  20   b  and the bipolar output stage between neutral  20   b  and DC voltage  20   c . These stages, which use variations of electronic half bridges which are independently switched by the respective controllers, improve the control and relationship of the differential voltages between  20   a  and  20   b , and  20   b  and  20   c . This independent control provides for robust disturbance rejection during unbalanced loading of the electrical system. More specifically, this arrangement permits the DC voltages at  20   a  and  20   c  to be independently balanced around neutral  20   b  within a frequency range which does not adversely affect the power supplied to the electrical loads (e.g. servers and computers) or the ability of the system to accomplish circuit protection. 
     It should be understood that the figures illustrate the exemplary embodiments in detail, and it should be understood that the present application is not limited to the details or methodology set forth in the description or illustrated in the figures. It should also be understood that the terminology is for the purpose of description only and should not be regarded as limiting. 
     The details of the circuits shown in  FIGS. 5 ,  5 A,  6 , and  10  are not dependent on size of the system. This topology can be utilized for very small power level systems as well as large power systems by paralleling and interleaving the units together on their outputs. So without respect to size, this topology can be utilized from low voltage in to medium voltage direct current voltages all depending on the semiconductor devices and configuration of the converter topologies chosen for the application. The neutral point regulator allows for the application of the any typical bi-polar topology to be utilized and yet still retain the characteristics of the not exceeding a prescribed DC voltage to ground on the output side of the converter. 
     Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only. The construction and arrangements, shown in the various exemplary embodiments, are illustrative only. Although only a few embodiments have been described in detail in this disclosure, many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described herein. Some elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. The order or sequence of any process, logical algorithm, or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present invention.