Abstract:
A solid-state circuit breaker for a DC power system which may operate unidirectional and bidirectional and does not require an external power supply to provide current interruption protection during an event of a short circuit fault.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to solid-state circuit breakers and, more particularly, to a family of semiconductor switches with self-powered auxiliary circuits that interrupt short circuit fault currents in DC power systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    DC power systems are gaining more popularity with increased DC loads (IT equipment, variable speed motor drives, LED lighting, etc.), renewable power generation (photovoltaic, wind, etc.), and distributed energy resources (batteries, fuel cells, microturbines, small gas generators). This is especially true for data centers, residential/commercial buildings, shipboard power systems, and high voltage DC transmission systems (HVDC), for example as described in “Low-Voltage DC Distribution System for Commercial Power Systems With Sensitive Electronic Loads,” D. Salomonsson and A. Sannino, IEEE Trans. Power Delivery, Vol. 22, No. 3, pp. 1620-1626, July 2007. DC power architectures improve energy efficiency by eliminating several conversion stages between AC/DC required in AC power systems. However, one major technical challenge for DC systems is to provide circuit breaker (CB) protection in the event of short circuit faults. 
         [0003]    Electromechanical DC circuit breaker products are available from companies like ABB, Eaton, and Schneider Electric, but suffer from slow action, typically with a response time of 30-100 milliseconds, and limited lifetime due to arcing. DC solid-state circuit breaker (SSCB) solutions were also developed to provide a much faster response time, typically with a response time of tens to hundreds of microseconds, and a much longer lifetime. Power semiconductor switching devices such as silicon IGBTs or thyristors, and more recently silicon carbide (SiC) JEFTs (Junction Effect Transistors) or SITs (Static Induction Transistors) are often employed in these solutions. For further reference see: “Solid-state circuit breakers for Medium Voltage DC power,” Kempkes, M., Roth, I., Gaudreau, M., IEEE Electric Ship Technologies Symposium (ESTS), 2011, Page(s) 254-257; U.S. Patent Application Publication 2014/0029152 A1; U.S. Patent Application Publication 2013/0154391; and “SiC-SIT Circuit Breakers With Controllable Interruption Voltage for 400-V DC Distribution Systems,” Y. Sato et al., IEEE Tran. Power Electronics, Vol. 29, No. 5, May 2014, Page(s):2597-2605. 
         [0004]    One challenge of these “smart” SSCBs is that they typically rely on complex and expensive over-current sensing circuitry, signal processing and data communication functions, and one or more external power supplies, which may or may not be available during the same short circuit fault event. A simple, stand-alone, two-terminal SSCB as reliable as the conventional electromechanical AC circuit breaker without requiring external power supply is highly desirable. 
         [0005]    An objective of this invention is to develop a family of unidirectional and bidirectional SSCBs that do not require an external power supply to provide current interruption protection during the event of a short circuit fault in a DC power system. All known solutions rely on sensing over-current through a switch during a short circuit fault event and require one or more separate power supply to power up the control electronics of the SSBC. A preferred embodiment of this invention does not sense an over-current through a switch but rather a voltage across the switch to detect a short circuit fault. The sudden increase in the voltage across the switch, “desaturation,” provides the power to turn and hold off the switch until the short circuit condition is removed. In a preferred embodiment, the stand-alone SSCB of this invention does not require or draw any power in the conduction state for normal operation. The stand-alone SSCB uses a very small amount of leakage power to cut off a fault current when a short circuit condition is present. The power is drawn from the high voltage built across the switch using a DC-DC converter. For example, one or more isolated low wattage flyback DC-DC converters can be used for this purpose. These DC-DC converters must offer very fast dynamic response but relatively relaxed efficiency and voltage regulation requirements. This invention is simpler and more reliable than the prior art. In addition, the embodiments of the invention require a smaller die size of the core semiconductor switches than the prior art, resulting in considerable cost reduction. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention comprises a solid-state circuit breaker for a DC power system. The invention comprises multiple embodiments which may be designed to operate unidirectional and bidirectional and do not require an external power supply to provide current interruption protection during an event of a short circuit fault. This invention may be designed with silicon and/or wide bandgap (WBG) semiconductor switches, with the understanding that WBG semiconductors, such as SiC and GaN, are more appropriate for voltage ratings over 300 volts. 
         [0007]    In an embodiment of this invention, the solid-state circuit breaker of the invention includes a solid-state switch and a voltage sensing and power supply circuit that can be connected between a power supply and a load. The voltage sensing and power supply circuit preferably includes a DC-DC converter and a voltage sensing element. When the load is not shorted, the solid-state switch conducts a normal current. However, when the load is shorted, a voltage across the solid-state switch increases rapidly causing a voltage change across the voltage sensing element and a large reverse biasing voltage through the DC-DC converter, turning off the solid-state switch and providing current interruption protection. 
         [0008]    In one embodiment of this invention, the present invention is a unidirectional SSCB comprising a normally-on asymmetric semiconductor JFET, a voltage sensing and power supply circuit having a blocking diode, a first resistor, a second resistor, an isolated DC-DC converter, a capacitor, and a metal-oxide varistor (MOV). The JFET includes a drain, a source, and a gate terminal, wherein a main current flows from the drain to the source. The voltage sensing and power supply circuit, the capacitor, and the MOV are connected between the drain and the source of the JEFT. The input terminals of the isolated DC-DC converter are connected across the second resistor of the voltage sensing and power supply circuit. The output terminals of the isolated DC-DC converter are connected between the source and gate of the JFET through a second blocking diode. 
         [0009]    In an alternative embodiment, the unidirectional SSCB of this invention comprises a normally-on asymmetric gallium nitride (GaN) high electron mobility transistor (HEMT), a voltage sensing and power supply circuit having a blocking diode, a first resistor, a second resistor, an isolated DC-DC converter, a capacitor, and a metal-oxide varistor (MOV). The HEMT includes a drain, a source, and a gate terminal with a main current flowing from the drain to the source. The voltage sensing and power supply circuit, the capacitor, and the MOV are connected between the drain and source of the HEMT. The input terminals of the isolated DC-DC converter are connected across the second resistor of the voltage sensing and power supply circuit. Output terminals of the isolated DC-DC converter are connected between the source and gate of the HMET through a second blocking diode. 
         [0010]    In another embodiment, the present invention is a bidirectional SSCB comprising a first and second asymmetrical normally-on JFETs, a first and second voltage sensing and power supply circuits each having a blocking diode, a first resistor, a second resistor, a first isolated DC-DC converter and a second isolated DC-DC converter, a capacitor, and a metal-oxide varistor (MOV). Each of the JFETs includes a drain, a source, and a gate terminal. The drain terminals of the two JFETs are connected together to form a bidirectional switch with the main current flowing from the source of one of the JFETs to the source of the other JFET. The two voltage sensing and power supply circuits, the capacitor, and the MOV are connected between the source terminals of the two JEFTs. The input terminals of the first isolated DC-DC converter are connected across the second resistor of the first voltage sensing and power supply circuit. The output terminals of the first isolated DC-DC converter are connected between the source and gate of the first JFET through a third blocking diode. The input terminals of the second isolated DC-DC converter are connected across the second resistor of the second voltage sensing and power supply circuit. The output terminals of the second isolated DC-DC converter are connected between the source and gate of the second JFET through a third blocking diode. 
         [0011]    In another embodiment, this invention is a bidirectional SSCB comprising first and second asymmetrical normally-on GaN HEMTs, a first and second voltage sensing and power supply circuits each having a blocking diode and a first resistor, a second resistor, a first isolated DC-DC converters, a second isolated DC-DC converter, a capacitor, and a metal-oxide varistor (MOV). Each of the two HEMTs has a drain, source, and gate terminal. The drain terminals of the two HEMTs are connected together to form a bidirectional switch with the main current flowing from the source of the first HEMT to the source of the second HEMT or vice versa. The two voltage sensing and power supply circuits, the capacitor, and the MOV are connected between the source terminals of the two HEMTs. The input terminals of the first isolated DC-DC converter are connected across the second resistor of the first voltage sensing and power supply circuit. The output terminals of the first isolated. DC-DC converter are connected between the source and gate of the second HEMT through a third blocking diode. The input terminals of the second isolated DC-DC converter are connected across the second resistor of the second voltage sensing and power supply circuit. The output terminals of the second isolated DC-DC converter are connected between the source and gate of the first HEMT through a fourth blocking diode. 
         [0012]    In another embodiment, the present invention is a bidirectional SSCB comprising a symmetrical normally-on JFET, a first blocking diode and a second blocking diode, a voltage sensing and power supply circuit having a first resistor, a second resistor and a third resistor, and a diode bridge, an isolated DC-DC converter, a capacitor, and a metal-oxide varistor (MOV). The symmetrical JFET has a drain, a source, and a gate region with a main current flowing between the source and the drain of the JFET. The voltage sensing and power supply circuit, the capacitor, and the MOV are connected between the source and the drain of the JEFT. The input terminals of the isolated DC-DC converter are connected across the second resistor of the voltage sensing and power supply circuit through the diode bridge. A negative output terminal of the isolated DC-DC converter is connected to the gate of the JFET. A positive output terminal of the isolated DC-DC converter is connected to a common anode of the first and second blocking diodes. A cathode of the first blocking diode is connected to the source of the JFET. The cathode of the second blocking diode is connected to the drain of the JFET. 
         [0013]    In another embodiment, the present invention is a bidirectional SSCB comprising a symmetrical normally-on HEMT, a first blocking diode, a second blocking diode, a voltage sensing and power supply circuit having a first resistor, a second resistor, a third resistor and a diode bridge, an isolated DC-DC converter, a capacitor, and a metal-oxide varistor (MOV). The symmetrical HEMT has a drain, a source, and a gate region with a main current flowing between the source and the drain of the HEMT. The voltage sensing and power supply circuit, the capacitor, and the MOV are connected between the source and the drain of the HEMT. The input terminals of the isolated DC-DC converter are connected across the second resistor of the voltage sensing and power supply circuit through the diode bridge. A negative output terminal of the isolated DC-DC converter is connected to the gate of the HEMT. A positive output terminal of the isolated DC-DC converter is connected to a common anode of the first and second blocking diodes. A cathode of the first blocking diode is connected to the source of the HEMT. A cathode of the second blocking diode is connected to the drain of the HEMT. 
         [0014]    In another embodiment, the present invention is a bidirectional SSCB comprising a symmetrical four-terminal bidirectional normally-on JFET switch, a first and second voltage sensing and power supply circuits each having a blocking diode and first and second resistors, first and second isolated DC-DC converters, a capacitor, and a metal-oxide varistor (MOV). The symmetrical JFET switch has first and second source, first and second gate regions. The main current flows between the first and second sources of the bidirectional JFET switch. The two voltage sensing and power supply circuits, the capacitor, and the MOV are connected between the two sources of the JEFT switch. Input terminals of the first isolated DC-DC converter are connected across the second resistor of the first voltage sensing and power supply circuit. Output terminals of the first isolated DC-DC converter are connected between the first source and the first gate of the JFET through a third blocking diode. The input terminals of the second isolated DC-DC converter are connected across the second resistor of the second voltage sensing and power supply circuit. The output terminals of the second isolated DC-DC converter are connected between the second source and the second gate of the JFET through a fourth blocking diode. 
         [0015]    In another embodiment, the present invention is a bidirectional SSCB comprising a symmetrical four-terminal bidirectional normally-on HEMT, a first and a second voltage sensing and power supply circuits each having a blocking diode, a first resistor and a second resistor, a first and second isolated DC-DC converters, a capacitor, and a metal-oxide varistor (MOV). The symmetrical HEMT has a first and a second source, a first and a second gate region. The main current flows between the two sources of the bidirectional HEMT switch. The two voltage sensing and power supply circuits, the capacitor, and the MOV are connected between the two sources of the HEMT. The input terminals of the first isolated DC-DC converter are connected across the second resistor of the first voltage sensing and power supply circuit. The output terminals of the first isolated DC-DC converter are connected between the first source and first gate of the HEMT through a third blocking diode. The input terminals of the second isolated DC-DC converter are connected across the second resistor of the second voltage sensing and power supply circuit. The output terminals of the second isolated DC-DC converter are connected between the second source and the second gate of the HEMT through a fourth blocking diode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  shows a unidirectional solid-state circuit breaker including an asymmetrical normally-on JFET according to an embodiment of this invention. 
           [0017]      FIG. 2  shows a unidirectional solid-state circuit breaker including an asymmetrical normally-on gallium nitride (GaN) High Electron Mobility Transistor (HEMT) according to an embodiment of this invention. 
           [0018]      FIG. 3  shows a bidirectional solid-state circuit breaker including a plurality of asymmetrical normally-on JFETs with drain terminals connected according to an embodiment of this invention. 
           [0019]      FIG. 4  shows a bidirectional solid-state circuit breaker using a plurality of asymmetrical normally-on GaN HEMTs with drain terminals connected according to an embodiment of this invention. 
           [0020]      FIG. 5  shows a bidirectional solid-state circuit breaker including a symmetrical normally-on JFET with a single gate electrode according to an embodiment of this invention. 
           [0021]      FIG. 6  shows a bidirectional solid-state circuit breaker including a symmetrical normally-on GaN HEMT with a single gate electrode according to an embodiment of this invention. 
           [0022]      FIG. 7  shows a bidirectional solid-state circuit breaker including one symmetrical normally-on four-terminal JFET with two separate gate electrodes according to an embodiment of this invention. 
           [0023]      FIG. 8  shows a bidirectional solid-state circuit breaker using one symmetrical normally-on four-terminal GaN HEMT with two separate gate electrodes according to an embodiment of this invention. 
           [0024]      FIG. 9  shows a waveform of the current flowing through one embodiment of a solid-state circuit breaker of this invention during a short circuit fault event. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    A solid-state DC circuit breaker of this invention is connected between a voltage source and load. The solid-state circuit breaker preferably includes a solid-state switch and a voltage sensing and power supply circuit including a voltage sensing element and a DC-DC converter. In operation, when a short occurs between the voltage source and the load, the voltage sensing element generates a reverse biasing voltage through the DC-DC converter, turning off the solid-state switch and protecting the circuit. 
         [0026]      FIG. 1  illustrates a unidirectional solid-state circuit breaker (SSBC)  10  using an asymmetrical normally-on JFET  12  according to one embodiment of this invention. The unidirectional SSCB  10  comprises a normally-on asymmetric semiconductor JFET (Q 1 )  12  and a voltage sensing and power supply circuit  14 . The voltage sensing and power supply circuit  14  includes a blocking diode (D 1 )  16  and a first resistor (R 1 )  18  and a second resistor (R 2 )  20 , an isolated DC-DC converter  22 , a capacitor (C 1 )  24 , and a metal-oxide varistor (MOV)  26 . The JFET (Q 1 )  12  has a drain  28 , a source  30 , and a gate terminal  32  with the main current flowing from the drain  28  to the source  30 . The voltage sensing and power supply circuit  14 , the capacitor (C 1 )  24 , and the MOV  26  are connected between the drain  28  and the source  30  of the JEFT Q 1 . Input terminals  34  of the isolated DC-DC converter  22  are connected across the second resistor (R 2 )  20  of the voltage sensing and power supply circuit  14 . Output terminals  36  of the isolated DC-DC converter  22  are connected between the source  30  and the gate  32  of the JFET  12  through a second blocking diode (D 2 )  38 . The unidirectional SSCB  10  of this invention is preferably used in a DC power system (not shown) with the drain  28  of the JFET (Q 1 )  12  connected to a power supply (not shown), and the source  30  of JFET (Q 1 )  12  connected to a load element (not shown). When the normally-on JFET (Q 1 )  12  conducts a normal load current from the drain  28  to the source  30 , a drain-source voltage of the JFET (Q 1 )  12  and a voltage across the second resistor (R 2 )  20  are both very small. An input voltage and an output voltage of the isolated DC/DC converter are also negligible. Therefore, there is no reverse biasing voltage applied between the gate  32  and source  30 . The JFET (Q 1 )  12  remains in an on state. When the load is shorted, the voltage between the drain  28  and the source  30  of the JFET (Q 1 )  12  increases rapidly, causing a large voltage built across the second resistor (R 2 )  20  and leading to a large reverse biasing voltage between the gate  32  and source  30  of the JFET (Q 1 )  12 . The normally-on JFET (Q 1 )  12  will turn off and block currents from the power supply with a large negative gate-source biasing voltage, typically 10-40 volts, and thus provide protection against short circuit faults. The MOV  26  is used to clamp a voltage spike between the drain  28  and the source  30  of the JFET (Q 1 )  12  caused by a parasitic inductance in the circuit when a fault current is interrupted. 
         [0027]      FIG. 2  illustrates a unidirectional solid-state circuit breaker  40  using an asymmetrical normally-on gallium nitride (GaN) High Electron Mobility Transistor (HEMT)  42  according to one embodiment of this invention. The unidirectional SSCB  40  comprises the normally-on asymmetric semiconductor HEMT (Q 1 )  42 , a voltage sensing and power supply circuit  44  having a blocking diode (D 1 )  46 , a first resistor (R 1 )  48  and a second resistor (R 2 )  50 , an isolated DC-DC converter  52 , a capacitor (C 1 )  54 , and a metal-oxide varistor (MOV)  56 . The HEMT (Q 1 )  42  includes a drain  58 , a source  60 , and a gate terminal  62  with a main current flowing from the drain  58  to the source  60 . The voltage sensing and power supply circuit  44 , the capacitor (C 1 )  54 , and the MOV  56  are connected between the drain  58  and the source  60  of the HEMT (Q 1 )  42 . Input terminals  64  of the isolated DC-DC converter  52  are connected across the second resistor (R 2 )  50  of the voltage sensing and power supply circuit  44 . Output terminals  66  of the isolated. DC-DC converter  52  are connected between the source  60  and the gate  62  of the HEMT  42  through a second blocking diode (D 2 )  68 . The unidirectional SSCB  40  is preferably used in a DC power system with the drain  58  of Q 1  connected to a power supply, and the source  60  of Q 1  connected to a load element. When the normally-on HEMT (Q 1 )  42  conducts a normal load current from the drain  58  to the source  60 , a drain-source voltage of Q 1  and a voltage across the second resistor (R 2 )  50  are both very small. An input voltage and an output voltage of the isolated DC/DC converter  52  are also negligible. Therefore, there is no reverse biasing voltage applied between the gate  62  and the source  60  of the HEMT (Q 1 )  42 . The HEMT (Q 1 )  42  remains in an on state. When the load is shorted, the voltage between the drain  58  and the source  60  of the HEMT (Q 1 )  42  increases rapidly, causing a large voltage built across the second resistor (R 2 )  50  and leading to a large reverse biasing voltage between the gate  62  and the source  60  of the HEMT (Q 1 )  42 . The normally-on HEMT (Q 1 )  42  will turn off and block currents from a power supply with a large negative gate-source biasing voltage, and thus provide protection against short circuit faults. The MOV  56  is used to clamp a voltage spike between the drain  58  and the source  60  of the HEMT (Q 1 )  42  caused by a parasitic inductance in the circuit when the fault current is interrupted. 
         [0028]      FIG. 3  illustrates a bidirectional solid-state circuit breaker  70  according to one embodiment of this invention comprising two asymmetrical normally-on JFETs  72 ,  74  with their drain terminals  76 ,  78  connected together. The bidirectional SSCB  70  comprising first and second asymmetrical normally-on JFETs (Q 1 )  72  and (Q 2 )  74 , first and second voltage sensing and power supply circuits  80 ,  82  each having a blocking diode  84 ,  86  and a first resistor  88 ,  128  and a second resistors  90 ,  118 , first isolated DC-DC converters  92  and second isolated DC-DC converter  94 , a capacitor (C 1 )  96 , and a metal-oxide varistor (MOV)  98 . Each of the two JFETs  72 ,  74  includes a drain  76 ,  78 , a source  100 ,  102 , and a gate terminal  104 ,  106 . The drain terminals  76 ,  78  of the JFETs (Q 1 , Q 2 )  76 ,  78  are connected together to form a bidirectional switch  108  with a main current flowing from the source  102  of the second JFET (Q 2 )  74  to the source  100  of the first JFET (Q 1 )  72  or vice versa. The two voltage sensing and power supply circuits  80 ,  82 , the capacitor (C 1 )  96 , and the MOV  98  are connected between the source terminals  100 ,  102  of the JFETs (Q 1 , Q 2 )  72 ,  74 . Input terminals  110  of the first isolated DC-DC converter  92  are connected across the second resistor (R 2 )  90  of the first voltage sensing and power supply circuit  80 . Output terminals  112  of the first isolated DC-DC converter  92  are connected between the source  100  and the gate  104  of the first JFET (Q 1 )  72  through a blocking diode (D 3 )  114 . Input terminals  116  of the second isolated DC-DC converter  94  are connected across the second resistor (R 4 )  118  of the second voltage sensing and power supply circuit  82 . Output terminals  120  of the second isolated DC-DC converter  94  are connected between the source  102  and the gate  106  of the second JFET (Q 2 )  74  through a blocking diode (D 4 )  122 . The bidirectional SSCB  70  is preferably used in a DC power system with the source  102  of the second JFET (Q 2 )  74  connected to a power supply, and the source  100  of the first JFET (Q 1 )  72  connected to a load element. When the common-drain, normally-on JFETs (Q 1 , Q 2 )  72 ,  74  conduct a normal load current, a voltage drop across the series-connected JFETs (Q 1 , Q 2 )  72 ,  74  is very small. A voltage across the second resistor (R 2 )  90  is also very small. Both input and output voltages of the first isolated DC/DC converter  92  are also negligible. Therefore, there is no reverse biasing voltage applied between the gate  104  and the source  100  of the first JFET (Q 1 )  72 . The first JFET (Q 1 )  72  remains in the on state. When the load is shorted, a voltage drop across the series-connected JFETs (Q 1 , Q 2 )  72 ,  74  increases rapidly, causing a large voltage built across the second resistor (R 2 )  90  and leading to a large reverse biasing voltage, typically 10-40 volts, between the gate  104  and the source  100  of the first JFET (Q 1 )  72 . The normally-on JFET (Q 1 )  72  will turn off and block currents from the power supply with a large negative gate-source biasing voltage, and thus provide protection against short circuit faults. Due to the symmetrical construction of the bidirectional SSBC  70 , the device provides protection against short circuit faults when the power source and load are swapped. 
         [0029]      FIG. 4  illustrates an embodiment of bidirectional solid-state circuit breaker  130  using two asymmetrical normally-on GaN HEMTs  132 ,  134  with their drain terminals  136 ,  138  connected together. The bidirectional SSCB  130  comprises a first asymmetrical normally-on HEMT (Q 1 )  132  and a second asymmetrical normally-on HEMT (Q 2 )  134 , a first voltage sensing and power supply circuit  140 , a second voltage sensing and power supply circuit  142 . Each of the voltage sensing and power supply circuits  140 ,  142  including a blocking diode  144 ,  146  and a first resistor  148 ,  150  and second resistor  172 ,  180 . The voltage sensing and power supply circuits  140 ,  142  include a first isolated DC-DC converter  152  and a second isolated DC-DC converter  154 , respectively. The first and second voltage sensing and power supply circuits  140 ,  142  are connected to a capacitor (C 1 )  156  and a metal-oxide varistor (MOV)  158 . Each of the two HEMTs has a drain  136 ,  138 , a source  160 ,  162 , and a gate terminal  164 ,  166 . The drain terminals  136 ,  138  of the first HEMT (Q 1 )  132  and the second HEMT (Q 2 )  134  are connected together to form a bidirectional switch  168  with a main current flowing from the source  162  of the second HEMT (Q 2 )  134  to the source  160  of the first HEMT (Q 1 )  132  or vice versa. The two voltage sensing and power supply circuits  140 ,  142 , the capacitor (C 1 )  156 , and the MOV  158  are connected between the source terminals  160 ,  162  of the asymmetrical normally-on HEMTs (Q 1 , Q 2 )  132 ,  134 . Input terminals  170  of the first isolated DC-DC converter  152  are connected across a second resistor (R 2 )  172  of the first voltage sensing and power supply circuit  140 . Output terminals  174  of the first isolated DC-DC converter  152  are connected between the source  160  and the gate  164  of the first asymmetrical normally-on HEMT (Q 1 )  132  through a blocking diode (D 3 )  176 . Input terminals  178  of the second isolated DC-DC converter  154  are connected across a second resistor (R 4 )  180  of the second voltage sensing and power supply circuit  142 . Output terminals  182  of the second isolated DC-DC converter  154  are connected between the source  162  and the gate  166  of the second HEMT (Q 2 )  134  through a blocking diode (D 4 )  184 . The bidirectional SSCB  130  is preferably used in a DC power system with the source  162  of the second HEMT (Q 2 )  134  connected to a power supply (not shown), and the source  160  of the first HEMT (Q 1 )  132  connected to a load element (not shown). When the common-drain, normally-on HEMTs (Q 1 , Q 2 )  132 ,  134  conduct a normal load current, a voltage drop across the series-connected HEMTs (Q 1 , Q 2 )  132 ,  134  is very small. The voltage across the resistor (R 2 )  172  is also very small. Both input and output voltages of the first isolated DC/DC converter  152  are also negligible. Therefore there is no reverse biasing voltage applied between the gate  164  and the source  160  of the HEMT (Q 1 )  132 . The HEMT (Q 1 )  132  remains in the on state. When a load is accidentally shorted, a voltage drop across the series-connected HEMTs (Q 1 , Q 2 )  132 ,  134  increases rapidly, causing a large voltage built across the second resistor (R 2 )  172  and leading to a large reverse biasing voltage between the gate  164  and the source  160  of the first HEMT (Q 1 )  132 . The normally-on HEMT (Q 1 )  132  will turn off and block currents from the power supply with a large negative gate-source biasing voltage, and thus provide protection against short circuit faults. Due to the symmetrical construction of the bidirectional SSBC  130 , the device can provide protection against short circuit faults when the power source and load are swapped. 
         [0030]      FIG. 5  illustrates an embodiment of bidirectional solid-state circuit breaker  190  using one symmetrical normally-on JFET with a single gate electrode  192 . The bidirectional SSCB  190  comprises the symmetrical normally-on JFET (Q 1 )  192 , a first blocking diode (D 1 )  194  and a second blocking diode (D 2 )  196 , a voltage sensing and power supply circuit  198  including a first resistor (R 1 )  200 , a second resistor (R 2 )  202  and a third resistor (R 3 )  204 , a diode bridge  206 , an isolated DC-DC converter  208 , a capacitor (C 1 )  210 , and a metal-oxide varistor (MOV)  212 . The symmetrical JFET (Q 1 )  192  includes a drain  214 , a source  216 , and a gate region  218  with a main current flowing between the source  216  and the drain  214  of the symmetrical JFET (Q 1 )  192 . The voltage sensing and power supply circuit  198 , the capacitor (C 1 )  210 , and the MOV  212  are connected between the source  216  and the drain  214  of the symmetrical JFET (Q 1 )  192 . Input terminals  220  of the isolated DC-DC converter  208  are connected across the second resister (R 2 )  202  of the voltage sensing and power supply circuit  198  through the diode bridge  206 . A negative output terminal  222  of the isolated DC-DC converter  208  connects to the gate  218  of the symmetrical JFET (Q 1 )  192 . A positive output terminal  224  of the isolated DC-DC converter  208  connects to a common anode of the blocking diodes D 1  and D 2   194 ,  196 . A cathode of the first blocking diode (D 1 )  194  is connected to the source  216  of the symmetrical JFET (Q 1 )  192 . A cathode of the second diode (D 2 )  196  is connected to the drain  214  of the symmetrical JFET (Q 1 )  192 . The bidirectional SSCB  190  is preferably used in a DC power system with the drain  214  of the symmetrical JFET (Q 1 )  192  connected to the power supply, and the source  216  of the symmetrical JFET (Q 1 )  192  connected to a load element. When the normally-on JFET (Q 1 )  192  conducts a normal load current from the drain  214  to the source  216 , a voltage drop across the normally-on JFET (Q 1 )  192  is very small. A voltage across the second resistor (R 2 )  202  is also very small. Both input and output voltages of the isolated DC/DC converter  208  are also negligible. Therefore there is not enough voltage to forward bias either the first diode (D 1 )  194  and/or the second diode (D 2 )  196 . The gate  218  of the normally-on JFET (Q 1 )  192  is essentially floating and the normally-on JFET (Q 1 )  192  remains in the on state. When the load is accidentally shorted to ground, the drain-source voltage across the normally-on JFET (Q 1 )  192  increases rapidly, causing a large voltage drop across the second resistor (R 2 )  202  and leading to a large output voltage of the isolated DC/DC converter  208 . Since the drain voltage is still greater than the source voltage in case, the first diode (D 1 )  194  is forward biased and the second diode (D 2 )  196  is reverse biased. The large output voltage, typically 10-40 volts, of the isolated DC/DC converter  208  is now applied between the source  216  and the gate  218  of the normally-on JFET (Q 1 )  192 . The normally-on JFET (Q 1 )  192  will turn off and block currents from the power supply with a large negative gate-source biasing voltage, and thus provide protection against short circuit faults. Due to the symmetrical construction of the bidirectional SSBC  190 , the device can provide protection against short circuit faults when the power source and load are swapped. 
         [0031]      FIG. 6  illustrates an embodiment of bidirectional solid-state circuit breaker  130  using one symmetrical normally-on GaN HEMT with a single gate electrode  232 . The bidirectional SSCB  230  comprises a symmetrical normally-on HEMT  232  (Q 1 ), a first blocking diode (D 1 )  234 , a second blocking diode (D 2 )  236 , a voltage sensing and power supply circuit  238  having a first resister (R 1 )  240 , a second resistor (R 2 )  242  and a third resistor (R 3 )  244 , a diode bridge  246 , an isolated DC-DC converter  248 , a capacitor (C 1 )  250 , and a metal-oxide varistor (MOV)  252 . The symmetrical HEMT (Q 1 )  232  includes a drain  250 , a source  252 , and a gate region  254  with a main current flowing between the drain  250  and the source  252  of the symmetrical HEMT (Q 1 )  232 . The difference between the symmetrical HEMT (Q 1 )  232  and a conventional HEMT is that the symmetrical HEMT  232  has a long drift region on both the drain  250  and the source  252  side to support a high blocking voltage in either direction. The voltage sensing and power supply circuit  238 , the capacitor (C 1 )  250 , and the MOV  252  are connected between the source  252  and the drain  250  of the symmetrical HEMT (Q 1 )  232 . Input terminals  256  of the isolated DC-DC converter  248  are connected across the second resistor (R 2 )  242  of the voltage sensing and power supply circuit  238  through the diode bridge  246 . A negative output terminal  258  of the isolated DC-DC converter  248  is connected to the gate  254  of the symmetrical HEMT (Q 1 )  232 . A positive output terminal  260  of the isolated DC-DC converter  248  is connected to a common anode of the blocking diodes (D 1 , D 2 )  234 ,  236 . A cathode of the first blocking diode (D 1 )  234  is connected to the source  252  of the symmetrical HEMT (Q 1 )  232 . A cathode of the second blocking diode (D 2 )  236  is connected to the drain  250  of the symmetrical HEMT (Q 1 )  232 . The bidirectional SSCB  230  is preferably used in a DC power system with the drain  250  of the symmetrical HEMT (Q 1 )  232  connected to the power supply, and the source  252  of the symmetrical HEMT (Q 1 ) connected to a load element. When the symmetrical HEMT (Q 1 )  232  conducts a normal load current from the drain  250  to the source  252 , the voltage drop across the symmetrical HEMT (Q 1 )  232  is very small. The voltage across the second resistor (R 2 )  242  is also very small. Both the input and output voltages of the isolated DC/DC converter  248  are also negligible. Therefore there is not enough voltage to forward bias either the first diode (D 1 )  234  or the second diode (D 2 )  236 . The gate  254  of the symmetrical HEMT (Q 1 )  232  is essentially floating and the symmetrical HEMT (Q 1 )  232  remains in the on state. When the load is accidentally shorted, the drain-source voltage across the symmetrical HEMT (Q 1 )  232  increases rapidly, causing a large voltage drop across the second resistor (R 2 )  242  and leading to a large output voltage of the isolated DC/DC converter  248 . Since the drain voltage is still greater than the source voltage in case, D 1  is forward biased and D 2  is reverse biased. The large output voltage of the isolated DC/DC converter  248  is now applied between the source  252  and the gate  254  of the symmetrical HEMT (Q 1 )  232 . The normally-on HEMT (Q 1 )  232  will turn off and block currents from the power supply with a large negative gate-source biasing voltage, and thus provide protection against short circuit faults. Due to the symmetrical construction of the bidirectional SSBC  230 , the device can provide protection against short circuit faults when the power source and load are swapped. 
         [0032]      FIG. 7  illustrates an embodiment of bidirectional solid-state circuit breaker  270  using asymmetrical normally-on four-terminal JFET switch with two separate gate electrodes  272 . The bidirectional SSCB  270  comprising the symmetrical four-terminal normally-on switch (Q 1 )  272 , a first and second voltage sensing and power supply circuits  274 ,  276  each including a blocking diode  278 ,  280 , a first resistor  282 ,  284  and a second resistor  286 ,  288 . The bidirectional SSCB  270  further including a first and second isolated DC-DC converters  290 ,  292 , a capacitor (C 1 )  294 , and a metal-oxide varistor (MOV)  296 . The symmetrical four-terminal normally-on switch (Q 1 )  272  has a first source (S 1 )  298 , a second source (S 2 )  300 , a first gate (G 1 )  302 , and a second gate (G 2 )  304  terminals. A main current between a power bus and a load flows from the first source (S 1 )  298  to the second source (S 2 )  399  of the symmetrical four-terminal normally-on switch (Q 1 )  272  or vice versa. The two voltage sensing and power supply circuits  274 ,  276 , the capacitor (C 1 )  294 , and the MOV  296  are connected between the source terminals  298 ,  300  of the symmetrical four-terminal normally-on switch (Q 1 )  272 . Input terminals  306  of the first isolated DC-DC converter  290  are connected across the second resistor (R 2 )  286  of the first voltage sensing and power supply circuit  274 . Output terminals  308  of the first isolated DC-DC converter  290  are connected between the first source (S 1 )  298  and the first gate (G 1 )  302  of the symmetrical four-terminal normally-on switch (Q 1 )  272  through a blocking diode (D 3 )  310 . Input terminals of the second isolated DC-DC converter  312  are connected across the second resistor (R 4 )  288  of the second voltage sensing and power supply circuit  276 . Output terminals  314  of the second isolated DC-DC converter  292  are connected between the second source (S 2 )  300  and the second gate (G 2 )  304  of the symmetrical four-terminal normally-on switch (Q 1 )  272  through a blocking diode (D 4 )  316 . The bidirectional SSCB  270  is preferably used in a DC power system with the second source (S 2 )  300  of the symmetrical four-terminal normally-on switch (Q 1 )  272  connected to a power supply, and the first source (S 1 )  298  of the symmetrical four-terminal normally-on switch (Q 1 )  272  connected to a load element. When the symmetrical four-terminal normally-on switch (Q 1 )  272  conducts a normal load current, a voltage drop between the second source (S 2 )  300  and the first source (S 1 )  298  of the symmetrical four-terminal normally-on switch (Q 1 )  272  is very small. The voltage across the second resistor (R 2 )  286  is also very small. Both the input and output voltages of the first isolated DC/DC converter  290  are also negligible. Therefore there is no reverse biasing voltage applied between the first gate (G 1 )  302  and the first source (S 1 )  298  of the symmetrical four-terminal normally-on switch (Q 1 )  272 . The symmetrical four-terminal normally-on switch (Q 1 )  272  remains in an on state. When the load is shorted, a voltage drop between the second source (S 2 )  300  and the first source (S 1 )  298  of the symmetrical four-terminal normally-on switch (Q 1 )  272  increases rapidly, causing a large voltage across the second resistor (R 2 )  286  and leading to a large reverse biasing voltage, typically 10-40 volts, between the first gate (G 1 )  302  and the first source (S 1 )  298  of the symmetrical four-terminal normally-on switch (Q 1 )  272 . The normally-on switch (Q 1 )  272  will turn off and block currents from the power supply with a large negative gate-source biasing voltage, and thus provide protection against short circuit faults. Due to the symmetrical construction of the bidirectional SSBC, the device can provide protection against short circuit faults when the power source and load are swapped. 
         [0033]      FIG. 8  illustrates an embodiment of bidirectional solid-state circuit breaker  320  using one symmetrical normally-on four-terminal GaN HEMT switch with two separate gate electrodes  322 . The bidirectional SSCB  320  comprising the symmetrical four-terminal normally-on GaN HEMT switch (Q 1 )  322 , a first voltage sensing and power supply circuit  324  and a second voltage sensing and power supply circuit  326  each of the voltage sensing and power supply circuits including a blocking diode  328 ,  330 , a first resistor  332 ,  334  and a second resistor  336 ,  338 , and an isolated DC-DC converter  340 ,  342 . The bidirectional SSCB  320  further includes a capacitor (C 1 )  344  and a metal-oxide varistor (MOV)  346 . The symmetrical four-terminal normally-on GaN HEMT switch (Q 1 )  322  includes a first source (S 1 )  348 , a second source (S 2 )  350 , a first gate (G 1 )  352 , and a second gate (G 2 )  354  terminals. A main current between a power bus and a load flows from the first source (S 1 )  348  to the second source (S 2 )  350  of the symmetrical four-terminal normally-on GaN HEMT switch (Q 1 )  322  or vice versa. The two voltage sensing and power supply circuits  324 ,  326 , the capacitor (C 1 )  344 , and the MOV  346  are connected between the source terminals  348 ,  350  of the symmetrical four-terminal normally-on GaN HEMT switch (Q 1 )  322 . Input terminals  356  of the first isolated DC-DC converter  340  are connected across the second resistor (R 2 )  336  of the first voltage sensing and power supply circuit  324 . Output terminals  358  of the first isolated DC-DC converter  340  are connected between the first source (S 1 )  348  and the first gate (G 1 )  352  of the symmetrical four-terminal normally-on GaN HEMT switch (Q 1 )  322  through a blocking diode (D 3 )  360 . Input terminals  362  of the second isolated DC-DC converter  342  are connected across the second resistor (R 4 )  338  of the second voltage sensing and power supply circuit  326 . Output terminals  364  of the second isolated DC-DC converter  342  are connected between the second source (S 2 )  350  and second gate (G 2 )  354  of the symmetrical four-terminal switch (Q 1 )  322  through a blocking diode (D 4 )  366 . The bidirectional SSCB  320  is preferably used in a DC power system with the second source (S 2 )  350  of the symmetrical four-terminal switch (Q 1 )  322  connected to a power supply, and the first source (S 1 )  348  of the symmetrical four-terminal switch (Q 1 )  322  connected to a load element. When the symmetrical four-terminal switch (Q 1 )  322  conducts a normal load current, the voltage drop between the second source (S 2 )  350  and the first source (S 1 )  348  of the symmetrical four-terminal switch (Q 1 )  322  is very small. The voltage across the second resistor (R 2 )  336  is also very small. Both the input and output voltages of the first isolated DC/DC converter  340  are also negligible. Therefore there is no reverse biasing voltage applied between the first gate (G 1 )  352  and the first source (S 1 )  348  of the symmetrical four-terminal switch (Q 1 )  322 . The symmetrical four-terminal switch (Q 1 )  322  remains in the on state. When the load is shorted, a voltage drop between the second source (S 2 )  350  and the first source (S 1 )  348  of the symmetrical four-terminal switch (Q 1 )  322  increases rapidly, causing a large voltage built across the second resistor (R 2 )  336  and leading to a large reverse biasing voltage between the first gate (G 1 )  352  and the first source (S 1 )  348  of the symmetrical four-terminal switch (Q 1 )  322 . The normally-on switch Q 1  will turn off and block currents from the power supply with a large negative gate-source biasing voltage, and thus provide protection against short circuit faults. Due to the symmetrical construction of the bidirectional SSBC  320 , the device can provide protection against short circuit faults when the power source and load are swapped. 
         [0034]    A current simulation was performed to prove the concept of the invention.  FIG. 9  shows a waveform of a current through an embodiment of a solid-state circuit breaker of this invention during a short circuit fault event. The solid-state circuit breaker initially conducts a normal current of 40 amperes. When a load is shorted, the solid-state circuit breaker current rises to 64 amperes but then falls to zero when the auxiliary circuit reacts. 
         [0035]    Thus, the invention provides a solid-state circuit breaker device in DC power systems. The invention further provides a circuit breaker with a self-powered auxiliary circuit that interrupts fault current in the event of a short circuit in DC power systems. 
         [0036]    It will be appreciated that details of the foregoing embodiments, given for purposes of illustration, are not to be construed as limiting the scope of this invention. Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention, which is defined in the following claims and all equivalents thereto. Further, it is recognized that many embodiments may be conceived that do not achieve all of the advantages of some embodiments, particularly of the preferred embodiments, yet the absence of a particular advantage shall not be construed to necessarily mean that such an embodiment is outside the scope of the present invention.