Abstract:
A solid-state disconnect device capable of isolating and protecting circuits and equipment from overloads and undesired transients is presented. The protection device includes at least one depletion mode circuit block having three terminals (drain, gate, and source), which in its simplest form is implemented by a single n-channel depletion mode field-effect transistor, and two enhancement mode circuit blocks each having three terminals (drain, gate and source), each implemented in simplest form by a single n-channel enhancement mode field-effect transistor. The current conducting path of the first enhancement mode circuit block is connected in series with the current conducting path of the depletion mode circuit block. The drain terminal of the second enhancement mode circuit block is connected through a current limiting load to both the gate terminal of the second enhancement mode circuit block and the drain terminal of the first enhancement mode circuit block. The gate terminal of the first enhancement mode circuit block is connected to the drain terminal of the second enhancement mode circuit block. The source terminals of the two enhancement circuit blocks are both connected to the gate terminal of the depletion mode circuit block. Unidirectional and bidirectional embodiments are disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    None. 
       FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    None. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The invention generally relates to the field of protection devices. In particular, the invention is a solid-state disconnect device capable of preventing the flow of undesirable voltage or current transients and/or isolating equipment from undesirably high voltages or currents. 
         [0005]    2. Background 
         [0006]    Traditional protection devices which function as circuit disconnects, examples including fuses and electromechanical circuit breakers, are inherently problematic. For example, such devices have low-switching speeds, require replacement after each trip event, are prone to arcing and switching bounce with associated noise and wear problems, and/or are often large on a volume basis resulting in an unwieldy package. 
         [0007]    Solid-state technology applied to such protection devices avoids these disadvantages while offering higher reliability and longer lifetime. Accordingly, solid-state circuit disconnects have become desirable alternatives to traditional protection devices. 
         [0008]    Various solid-state disconnect devices have been devised utilizing complex circuitry with an additional power source, examples including devices by Billings et al. in U.S. Pat. No. 4,245,184 entitled AC Solid-State Circuit Breaker, Witmer in U.S. Pat. No. 5,606,482 entitled Solid State Circuit Breaker, Partridge in U.S. Pat. No. 6,104,106 entitled Solid State Circuit Breaker, and Covi et al. in U.S. Pat. No. 6,515,840 entitled Solid State Circuit Breaker with Current Overshoot Protection. However, it is more desirable that a circuit protection device includes two terminals and no additional power source, much like a fuse. 
         [0009]    Harris, in U.S. Pat. No. 5,742,463 entitled Protection Device using Field Effect Transistors, describes a two-terminal, solid-state protection device without an additional power supply. Several disadvantages are noteworthy. First, the protection device requires p-channel, high-voltage field-effect-transistors (FETs) which generally have a high conduction resistance because of low hole mobility. Second, the protection device requires both gate-to-source terminals and gate-to-drain terminals of the FETs to have the same high-voltage blocking capability which is difficult to achieve because FETs block the voltage between the drain-to-source and drain-to-gate terminals, and the gate-to-source terminal generally does not have such blocking capability. For example, the gate and source terminals of all known FETs typically block a few volts to a few tens of volts because of a low-voltage Schottky barrier diode in MESFETs, a low-voltage PN diode in JFETs, and a low-voltage MOS capacitor in MOSFETs. Third, the protection device requires a large number of FETs connected in series to increase the voltage blocking capability of the device, inevitably resulting in a higher conduction loss. Fourth, the tripping current is difficult to control with the protection device. 
         [0010]    Therefore, what is required is a solid-state disconnect device having two terminals and no additional power supply that avoids the disadvantages of the related arts. 
       SUMMARY OF THE INVENTION 
       [0011]    An object of the present invention is to provide a solid-state disconnect device having two terminals and no additional power supply that avoids the disadvantages of the related arts. 
         [0012]    For high-voltage and high-current protection, silicon (Si) power devices are generally connected in parallel with the system to be protected because such devices have a high insertion loss and a large capacitance which dissipate too much power in a power system or reduce the communication bandwidth in a communication system when connected directly in series. With the introduction of wide bandgap semiconductor power devices, such as those based on silicon carbide (SiC), gallium nitride (GaN), and diamond, direct serial connectivity of protection devices in the circuit or system to be protected is more feasible at voltages over 10,000 volts, because these wide bandgap switches have a specific ON-state resistance nearly a thousand times lower than silicon components. The benefits include improved insertion loss, speed, and bandwidth and lower costs. 
         [0013]    The solid-state disconnect device described herein is a two-terminal protection device connectable in series between a power supply and a load while avoiding an additional power source. The disconnect device further includes a depletion mode circuit block (DCB), an enhancement mode circuit block (ECB), an enhancement mode circuit block (ECB) with a positive threshold voltage, and a current limiting load (CLL). The CLL could be a simple resistor or a dynamic load formed by a circuit having a resistance that increases with terminal voltage. 
         [0014]    The DCB is broadly defined as a circuit block having three terminals, namely a drain, source, and gate, and a negative threshold voltage that controls the current conduction between the drain and source terminals. When the voltage across the gate and source terminals is larger than the negative threshold voltage, the circuit block is turned ON and current conduction between the drain and source terminals increases with an increase in voltage across the gate and source terminals. When the voltage across the gate and source terminals is below the negative threshold voltage, the circuit block is OFF and negligible current flows through the circuit block. 
         [0015]    The DCB could include a variety of designs composed of single and multiple components. In its simplest form, the DCB could be a single depletion mode n-channel transistor. In other embodiments, the DCB could be composed of any number of depletion mode n-channel transistors connected in a serial and/or a parallel arrangement. The depletion mode n-channel transistors should withstand the surge voltage and surge current of the application. Exemplary depletion mode components include junction field effect transistors (JFETs), metal oxide semiconductor field effect transistors (MOSFETs), static induction transistors, and insulated-gate bipolar transistors (IGBTs); however, depletion mode JFETs are generally preferred. The desired features of the depletion mode transistors include a low insertion loss, a low capacitance, and ability to withstand a surge current and voltage without breakdown. Depending on the specific application, the voltage blocking capability could be up to a few tens of thousands of volts while the current capability could be up to a few thousands of amperes. 
         [0016]    The ECB is broadly defined herein as a circuit block having three terminals, namely a drain, source, and gate, and a positive threshold voltage. When the voltage across the gate and source terminals is larger than the positive threshold voltage, the circuit block is turned ON and current can flow between the drain and source terminals. When the voltage across the gate and source terminals is less than the positive threshold voltage, the circuit block is OFF. 
         [0017]    The ECB could include a variety of designs composed of single and multiple components. In its simplest form, the ECB could be a single enhancement mode n-channel transistor. In other embodiments, any number of enhancement mode n-channel transistors could be connected in a serial and/or parallel arrangement. Exemplary enhanced mode components include junction field effect transistors (JFETs), metal oxide semiconductor field effect transistors (MOSFETs), static induction transistors, and insulated-gate bipolar transistors (IGBTs); however, enhancement mode n-channel MOSFETs are generally preferred. 
         [0018]    In its simplest form, the solid-state disconnect or protection device could include a DCB, a first ECB, a second ECB, and a CLL. The DCB and first ECB are connected in a serial arrangement so as to form the current path of the two-terminal protection device. The source terminal of the DCB is connected to the drain terminal of the first ECB. The drain terminal of the second ECB is connected to the gate terminal of the first ECB and further connected through the CCL to the drain terminal of the first ECB. The gate terminal of the DCB and the source terminal of the second ECB are connected and further connected to the source terminal of the first ECB. The gate terminal of the second ECB is connected to the drain terminal of the first ECB. The voltage drop across the protection device, which increases as surge current increases, is used to control the OFF and ON status of the second ECB. The OFF and ON status of the second ECB in turn controls the OFF and ON status of the first ECB which in turn further controls the OFF and ON status of the DCB. The described circuit architecture provides protection functionality to a disconnect system or equipment protected from a high-voltage spike, and undesirable surge current flowing from the drain-to-source of the DCB and further through the drain-to-source of the first ECB. 
         [0019]    The protection device described herein could include optional components to further improve the performance of the disconnect system. For example, in applications requiring current to flow in both directions, a current bypass component, which conducts current in one direction and blocks current in the other direction, one non-limiting example being a Schottky diode, could be connected to the source and drain terminals of the DCB or the first ECB to allow current to flow from source to drain terminals, when the DCB or the first ECB has poor or no current conduction capability from the source-to-drain terminals. In another example, a low-pass RC network could be connected between the drain terminal of the DCB and the source terminal of the first ECB to filter out high-frequency voltage spikes generated during transient and tripping events. In yet another example, a voltage-limiting component, examples including but not limited to a reverse selenium rectifier, a varistor, a simple resistor, a circuit block, or preferably a voltage-clamping Zener diode, could be connected to the source and gate terminals of each of the ECBs to prevent the gate from electric breakdown because of a high voltage. In still another example, a temperature compensation component, one non-limiting example being a thermistor with a negative temperature coefficient in resistance, could be connected between the gate and source terminals of the second ECB to make the tripping current insensitive to temperature change. In still yet another example, a variable resistor could be connected between the gate and source terminals of the second ECB to adjust the tripping current. In yet still another example, a capacitor can be connected between the gate and source terminals of the second ECB to suppress voltage spikes across the gate and source terminals of the second ECB so as to prevent premature trigger of the device. The gate terminal of the second ECB should be connected through a current limiting load instead of connected directly to the drain terminal of the first ECB, when at least one of the aforementioned optional components is connected between the gate and source terminals of the second ECB in order to ensure that a large enough voltage is established across the first ECB when the first ECB is turned OFF so as to enable the turn OFF of the DCB. 
         [0020]    For higher current protection, it is preferable that the drain terminal of the second ECB be connected through a CLL to the drain of the DCB, rather than to the drain of the first ECB, and the gate of the second ECB be connected through another CLL to the drain of the DCB, rather than directly to the drain of the first ECB. 
         [0021]    The protection device described herein could be arranged in either a unidirectional or a bidirectional circuit. For unidirectional protection, the device could include one form of the circuit described above. For bidirectional protection, two such circuits could be connected in mirror symmetry in a serial arrangement with a load. In the latter, it is preferred for the drain terminals of the DCBs of the two circuits to be connected, although the two source terminals of the DCBs could also be connected in mirror symmetry. For bidirectional protection, it is further preferred that both gates of the ECBs in one circuit be connected through their respective CLLs to the source terminal of the first ECB in the other circuit instead of to the drain of the DCB in the same circuit, although connecting only one of the two gates of the ECBs in one circuit to the source of the first ECB in the other circuit is also possible. Optional components described herein could be added to bidirectional embodiments of the invention as required. 
         [0022]    Several advantages are offered by the invention. The disconnect device provides unidirectional and bidirectional protection against surge current and voltage, resets automatically, has a very-high tripping speed on the order of microseconds to sub-microseconds, is capable of protecting both DC and AC power systems, facilitates adjustments to and control of tripping current, and is insensitive to temperature variations. 
       REFERENCE NUMERALS 
       [0000]    
       
           1  Terminal 
           2  Terminal 
           3  Terminal 
           4  Terminal 
           5 ,  5   a,    5   b  DCB 
           6 ,  6   a,    6   b  First ECB 
           7 ,  7   a,    7   b  Second ECB 
           8 ,  8   a,    8   b  CLL 
           9 ,  9   a,    9   b  Second CLL 
           10 ,  10   a,    10   b  Voltage limiting component 
           11 ,  11   a,    11   b  Voltage limiting component 
           12 ,  12   a,    12   b  Temperature compensation component 
           13 ,  13   a,    13   b  Variable resistor 
           14 ,  14   a,    14   b  Capacitor 
           15 ,  15   a,    15   b  Resistor 
           16 ,  16   a,    16   b  Capacitor 
           17 ,  17   a,    17   b  Current bypass component 
           18 ,  18   a,    18   b  Current bypass component 
           19  Capacitor 
           20  Resistor 
           30 ,  30   a,    30   b  First node 
           31 ,  31   a,    31   b  Second node 
           32 ,  32   a,    32   b  Third node 
           33 ,  33   a,    33   b  Fourth node 
           34 ,  34   a,    34   b  Fifth node 
           35 ,  35   a,    35   b  Sixth node 
           36 ,  36   a,    36   b  Seventh node 
           40  Depletion mode FET 
           41  Feedback resistor 
           42  Depletion mode FET 
           43  Resistor 
           44  Depletion mode FET 
           50  Protection device 
           52  Protection device 
           54  Protection device 
           56 ,  56   a,    56   b  Protection device 
           58  Protection device 
           60  Protection device 
           61  Protection device 
           62  Bidirectional protection device 
           63  Bidirectional protection device 
           64  Bidirectional protection device 
           65  Bidirectional protection device 
           81  Terminal 
           82  Terminal 
           83  Terminal 
           84  Terminal 
       
     
     
    
     
       BRIEF DESCRIPTION OF THE INVENTION 
         [0070]    Additional aspects, features, and advantages of the invention will be understood and will become more readily apparent when the invention is considered in the light of the following description made in conjunction with the accompanying drawings. 
           [0071]      FIG. 1   a  is a circuit diagram illustrating a solid-state protection device in accordance with an embodiment of the invention. 
           [0072]      FIG. 1   b  is a circuit diagram illustrating the solid-state protection device from  FIG. 1   a  with optional components in accordance with an embodiment of the invention. 
           [0073]      FIG. 1   c  is a circuit diagram illustrating the solid-state protection device from  FIG. 1   b  wherein the depletion mode circuit block is a depletion mode n-channel JFET and the enhancement mode circuit blocks are each an enhancement mode MOSFET in accordance with an embodiment of the invention. 
           [0074]      FIG. 2   a  is a circuit diagram illustrating alternate connectivity of the CLLs in the solid-state protection device from  FIG. 1   b  in accordance with an embodiment of the invention. 
           [0075]      FIG. 2   b  is a circuit diagram illustrating alternate connectivity of the CLLs in the solid-state protection device from  FIG. 1   b  in accordance with an embodiment of the invention. 
           [0076]      FIG. 2   c  is a circuit diagram illustrating alternate connectivity of the CLLs in the solid-state protection device from  FIG. 1   b  in accordance with an embodiment of the invention. 
           [0077]      FIG. 2   d  is a circuit diagram illustrating an alternate configuration of the solid-state protection device in  FIG. 2   a  wherein the depletion mode circuit block is a depletion mode n-channel JFET and the enhancement mode circuit blocks are each an enhancement mode MOSFET in accordance with an embodiment of the invention. 
           [0078]      FIG. 3   a  is a circuit diagram illustrating a bidirectional protection device including a pair of solid-state protection devices from  FIG. 2   a  connected in a serial arrangement in accordance with an embodiment of the invention. 
           [0079]      FIG. 3   b  is a circuit diagram illustrating a bidirectional protection device including a pair of solid-state protection devices from  FIG. 2   d  connected in a serial arrangement in accordance with an embodiment of the invention. 
           [0080]      FIG. 4   a  is a circuit diagram illustrating an alternate configuration of the bidirectional protection device from  FIG. 3   a  in accordance with an embodiment of the invention. 
           [0081]      FIG. 4   b  is a circuit diagram illustrating an alternate configuration of the bidirectional protection device from  FIG. 3   b  in accordance with an embodiment of the invention. 
           [0082]      FIG. 5   a  is a plot illustrating the simulated current waveform in expanded scale produced by the bidirectional protection device in  FIG. 4   b  protecting a single-phase 10-kilowatt DC-to-DC converter system powered by a 200 volt battery when a fault causes the battery discharge current to rise sharply. 
           [0083]      FIG. 5   b  is a plot illustrating the simulated current waveform produced by the bidirectional protection device in  FIG. 4   b  protecting a single-phase 10-kilowatt DC-to-DC converter system powered by a 200 volt battery when a fault causes the battery discharge current to rise sharply and after the system resumes normal operation after the fault is cleared. 
           [0084]      FIG. 6  is a plot illustrating the simulated current waveform in expanded scale produced by the bidirectional protection device in  FIG. 4   b  protecting a single-phase 10-kilowatt DC-to-DC converter system powered by a 200 volt battery when a fault causes the battery charge current to rise sharply. 
           [0085]      FIG. 7  is a plot illustrating the effects of temperature compensation components on the trip current of the bidirectional protection device in  FIG. 4   b.    
           [0086]      FIG. 8  is a plot illustrating the effects of current bypass components (power diodes) on reducing the forward voltage drop across the bidirectional protection device in  FIG. 4   b.    
           [0087]      FIG. 9  is a plot illustrating the lower conduction voltage drop of the bidirectional protection device in  FIG. 4   b  in comparison to the forward voltage drop of the bidirectional protection device in  FIG. 3   b.    
           [0088]      FIG. 10   a  is a plot illustrating the measured current tripping waveforms of the bidirectional protection device in  FIG. 4   b  with a trip current of 53 amperes. 
           [0089]      FIG. 10   b  is a plot illustrating the measured current tripping waveforms of the bidirectional protection device in  FIG. 4   b  with a trip current of 55 amperes. 
       
    
    
     DETAILED DESCRIPTION 
       [0090]    Reference will now be made in detail to several preferred embodiments of the invention that are illustrated in the accompanying drawings. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts. Nodes are referenced for descriptive purposes only and do not necessarily represent a structure or element of the invention. The drain, gate, and source terminals of components are identified by the letters D, S, and G, respectively, in  FIGS. 1   a - 1   c,    2   a - 2   d,    3   a,    3   b,    4   a,  and  4   b.    
         [0091]    Referring now to  FIG. 1   a,  an embodiment of the protection device  50  is shown for a unidirectional device capable of preventing a surge current that exceeds a preset trip value between terminals  1  and  3 , by disconnecting a load at terminals  3  and  4  in response to a voltage surge. The protection device  50  is connected in series between a source or voltage supply across terminals  1  and  2  with the polarity shown and the load across terminals  3  and  4 . The protection device  50  includes a DCB  5 , a first ECB  6 , a second ECB  7 , and a CLL  8  connected as shown via nodes  30 - 36 . The DCB  5  is a depletion mode circuit block with a negative threshold voltage. The first ECB  6  and second ECB  7  are enhancement mode circuit blocks with a positive threshold voltage. The drain terminal of the DCB  5  is connected to the first node  30 . The gate terminal of the DCB  5  and the source terminals of the first ECB  6  and the second ECB  7  are connected to the third node  32 . The source terminal of the DCB  5  and the drain terminal of the first ECB  6  are connected to the second node  31 . The gate terminal of the first ECB  6  and the drain terminal of the second ECB  7  are connected to the fifth node  34 . The gate terminal of the second ECB  7  is connected to the sixth node  35 . The CLL  8  is connected between the fourth node  33  and fifth node  34 . Thereafter, the second node  31 , fourth node  33 , sixth node  35 , and seventh node  36  are arranged and connected as shown. 
         [0092]    The functionality of the protection circuit  50  is described with further reference to  FIG. 1   a.  In normal operation, a positive current flows from terminal  1  to terminal  3 . The voltage drop across the second node  31  and the third node  32  is larger than the threshold voltage of the first ECB  6  but smaller than the threshold voltage of the second ECB  7 . As such, the first ECB  6  is maintained in the ON-state while the second ECB  7  is in the OFF-state during normal operating conditions. The DCB  5  is in the ON-state as a normally ON device and the magnitude of its gate-to-source voltage, approximately equal to the magnitude of the voltage drop across the third node  32  and second node  31 , is smaller than the magnitude of its negative threshold voltage. 
         [0093]    When a surge current or sudden voltage increase at terminal  1  attempts to cross to terminal  3 , the voltage drop between the second node  31  and third node  32  momentarily increases, resulting in a momentary increase in the gate voltage of the second ECB  7 . The second ECB  7  remains in the OFF-state until the voltage drop between the second node  31  and third node  32  reaches the threshold voltage of the second ECB  7  placing the second ECB  7  in the ON-state at a predetermined surge current. The peak surge current at which the disconnect trips is therefore controlled by the magnitude of the threshold voltage of the second ECB  7  and the forward characteristics of the first ECB  6 . After the second ECB  7  is turned ON, the voltage drop across the drain terminal, at the fifth node  34 , and the source terminal, at the third node  32 , of the second ECB  7  is decreased to a value less than the threshold voltage of the first ECB  6  so as to turn OFF the first ECB  6 . Once the first ECB  6  is OFF, current is forced to pass through the CLL  8  and the second ECB  7 . The CLL  8  produces a voltage drop across the second node  31  and third node  32  coupled to the source and gate terminals of the DCB  5 , respectively. The first ECB  6  should be sufficiently capable of handling a drain-to-source voltage larger than the numerical value of the negative threshold voltage of the DCB  5 . A typical threshold voltage for the DCB  5  could be in the range of −1 volts to −50 volts, although other values are possible. When the drain-to-source voltage of the first ECB  6  increases to a value greater than the numerical value of the threshold voltage of the DCB  5 , the DCB  5  is turned OFF. Thereafter, the protection circuit  50  enters the OFF-state so as to prevent the surge current from reaching and damaging the load and to disconnect the load from the high-voltage spike. The protection device  50  resumes normal operation automatically once the voltage drop across the device from terminals  1  to  3  decreases to a value causing the second ECB  7  to turn OFF after the fault causing the current surge is cleared. 
         [0094]    A single DCB  5  comprising a high-voltage depletion mode n-channel transistor could be employed to disconnect the surge voltage because the voltage blocked by the DCB  5  is across the drain and source rather than across the gate and source as provided in U.S. Pat. No. 5,742,463. For example, a single silicon carbide FET could be used to block a voltage surge over 10,000 volts, non-limiting examples being a 10kV, 5 A 4H—SiC Power DMOSFET and a 10 kV, 87 mΩ-cm 2  normally-OFF 4H—SiC Vertical Junction Field-Effect Transistor. The corresponding DCB  5  and first ECB  6  should have similar current handling capability dependent on the specific application, which could range from a few milli-amperes to a few tens of thousands of amperes. The second ECB  7  would not necessarily require a high current capability because it is limited by the CLL  8 . 
         [0095]    Referring now to  FIG. 1   b,  the protection device  50  from  FIG. 1   a  is shown with a variety of optional components to form various alternate protection devices  52 . The architecture of the protection device  52  is identical to that in  FIG. 1   a,  except where otherwise indicated. For example, a second CLL  9  could be provided between the sixth node  35  and seventh node  36 . A voltage limiting component  10  could be connected as needed between the gate and source terminals of the first ECB  6  to prevent electric breakdown of the gate due to a high voltage event. Another voltage limiting component  11  could be connected as needed between the gate and source terminals of the second ECB  7 , so as to prevent electric breakdown of the gate due to a high voltage event. Although Zener diodes are represented in  FIG. 1   b,  any voltage-limiting components, non-limiting examples including reverse selenium rectifiers, varistors made from various materials, a simple resistor, or a circuit block, but preferably a voltage-clamping Zener diode, could be employed as one or both voltage limiting components  10 ,  11 . 
         [0096]    Referring again to  FIG. 1   a,  the ON-state resistances of the DCB  5  and first ECB  6  increase and the threshold voltage of the second ECB  7  decreases as the temperature of the protection device  50  increases. As a consequence, the trip current of protection device  50  will decrease. 
         [0097]    Referring again to  FIG. 1   b,  the protection device  52  could include a temperature compensation component  12  with a negative temperature coefficient (NTC) in resistance, a non-limiting example being an NTC thermistor, in order to maintain a relatively constant trip current. The temperature compensation component  12  could be connected between the gate and source terminals of the second ECB  7 . The temperature compensation component  12  and second CLL  9  form a voltage divider. A decrease in the resistance of the temperature compensation component  12  with an increase in temperature tends to reduce the voltage drop across the gate and source terminals of the second ECB  7 . As a result, an increase in the ON-resistance of the DCB  5  and the first ECB  6  and a decrease in the threshold voltage of the second ECB  7  due to a temperature increase are compensated by the decrease in the bias voltage across the gate-to-source terminals of the second ECB  7 . Hence, the trip current of the protection device  52  could be nearly temperature independent over a specified range of temperatures. 
         [0098]    Referring again to  FIG. 1   b,  an optional variable resistor  13  could also be connected between the gate and source terminals of second ECB  7  to adjust the voltage drop across the gate and source terminals so as to adjust the trip current. An optional capacitor  14  could be further connected between the gate and source terminals of the second ECB  7  to suppress potential voltage spikes across the gate and source terminals to prevent a premature trigger of the second ECB  7 . An optional RC network could be further connected between the first node  30  and the third node  32 , preferably after the capacitor  14 , to filter out high-frequency voltage spikes generated during transient and trip conditions. One non-limiting exemplary embodiment of the RC network is a resistor  15  and capacitor  16 , as represented in  FIG. 1   b.    
         [0099]    Referring again to  FIG. 1   b,  the protection device  52  could further include current bypass components  17 ,  18  as required, which conduct current in one direction and block current in the other direction. The simplest form of a current bypass component is a Schottky diode, although other components are possible. The current bypass components  17 ,  18  could be connected in parallel to the current conducting channel between the source and drain terminals of the DCB  5  and first ECB  6 , respectively, when an application requires current to flow from terminal  3  to terminal  1 , and when the DCB  5  or first ECB  6  has poor or no current conducting capability from source-to-drain terminals. For example, MOSFETs contain a built-in body diode between its source and drain terminals which allows current conduction from source-to-drain but with a relatively large voltage drop. As such, it is preferred that a low-voltage-drop current bypass diode be connected between the source and drain terminals of each MOSFET. In another example, IGBTs do not include a body diode so a low-voltage-drop current bypass diode could be included to provide reverse current conduction when a specific application requires current to flow from terminal  3  to terminal  1 . 
         [0100]    The functionality of the protection device  52  of  FIG. 1   b  is similar to the protection device  50  of  FIG. 1   a.  The protection device  52  is a unidirectional protection device capable of preventing a surge current that exceeds a preset trip current to conduct from terminal  1  to terminal  3  and disconnecting the load from a voltage surge. However, current conduction in the protection device  52  is bidirectional. 
         [0101]    The CLLs  8 ,  9  in  FIGS. 1   a  and  1   b  could include a variety of devices. Each CLL  8 ,  9  could be a resistor with a predetermined value of resistance. Preferably, each CLL  8 ,  9  could be a dynamic load formed by a circuit block having increased resistance with increased terminal voltage in order to reduce power dissipation. For example, the CLL  8  in  FIG. 1   a  is shown including a depletion mode FET  40  and a feedback resistor  41  capable of providing a small load resistance for a short RC charge time for the first ECB  6  at low voltage and a very large load resistance to limit the leakage current in a high-voltage blocking mode. 
         [0102]    The DCBs  5 ,  5   a,    5   b  described herein could include a variety of single and multi-element devices. In one example, the DCBs  5 ,  5   a,    5   b  could be composed of any number of depletion mode n-channel transistors connected in a serial and/or parallel arrangement. In another example, the DCB  5 ,  5   a,    5   b  could be a single depletion mode n-channel transistor, non-limiting examples including a depletion mode n-channel junction field effect transistor (JFET), a depletion mode n-channel metal oxide semiconductor field effect transistor (MOSFET), and a depletion mode insulated-gate bipolar transistor (IGBT). 
         [0103]    The ECBs  6 ,  6   a,    6   b,    7 ,  7   a,    7   b  described herein could include a variety of single and multi-element devices. In one example, the ECBs  6 ,  6   a,    6   b,    7 ,  7   a,    7   b  could include any number of enhancement mode n-channel transistors connected in a serial or parallel arrangement. An ECB  6 ,  6   a,    6   b,    7 ,  7   a,    7   b  in its simplest form could be a single enhancement mode n-channel transistor, non-limiting examples including an enhancement mode n-channel junction field effect transistor (JFET), an enhancement mode n-channel metal oxide semiconductor field effect transistor (MOSFET), and an enhancement mode insulated-gate bipolar transistor (IGBT). 
         [0104]    Referring now to  FIG. 1   c,  the protection device  54  is shown whereby the DCB  5  is a depletion mode n-channel JFET and the ECBs  6 ,  7  are each one enhancement mode MOSFET. Components and architecture are identical to those in  FIG. 1   b,  except where otherwise noted. For example, the optional current bypass component  17  is not required because the depletion mode n-channel JFET has good current conduction performance from source-to-drain terminals. The functionality of the protection device  54  in  FIG. 1   c  is similar to the protection device  52  in  FIG. 1   b,  in that the protection device  54  is a unidirectional device capable of preventing a surge current that exceeds a preset trip current to cross from terminal  1  to terminal  3  and disconnecting the load from a voltage surge. 
         [0105]    The ECBs  6 ,  6   a,    6   b,    7 ,  7   a,    7   b  described herein generally have a gate-to-source voltage larger than the threshold voltage required to turn ON the circuit block. The higher the gate-to-source voltage is, the lower the ON-state voltage drop for the same current level is. For the first ECB  6  in  FIG. 1   b,  the ON-state voltage drop is fed back to its gate to keep the ECB  6  in its ON-state. In general applications, it is desired that the ON-state voltage drop of the first ECB  6  to be as low as possible so as to reduce power loss. Therefore, the first ECB  6  should have a positive but low threshold voltage. 
         [0106]      FIGS. 2   a - 2   d  describe several alternate embodiments of the protection device  52  in  FIG. 1   b.  Components and architecture are identical to those in  FIG. 1   b,  except where otherwise noted. The protection devices  56 ,  58 ,  60 , and  61  allow bidirectional current conduction, although the protection function is unidirectional. Operation of the protection devices  56 ,  58 ,  60 , and  61  are similar to that of the protection device  52  of  FIG. 1   b.  The circuit in  FIG. 2   a  is a preferred embodiment. 
         [0107]    Referring now to  FIG. 2   a,  an improved embodiment the protection device  56  is shown which reduces the ON-state voltage drop of the first ECB  6  in  FIG. 1   b.  The fourth node  33  and seventh node  36  are now connected to the first node  30  rather than to second node  31 . As such, the entire voltage drop across the protection device  56 , including the voltage drop across the drain and source terminals of the DCB  5  and across the drain and source terminals of the first ECB  6 , is employed to bias the gate and source terminals of the first ECB  6 . For the same level of current conducting through the protection device  56 , the voltage available to bias the gate and source terminals of the first ECB  6  in  FIG. 2   a  is much larger than that in  FIG. 1   b.  Therefore, the forward voltage drop of the first ECB  6  in  FIG. 2   a  is smaller than the forward voltage drop of the first ECB  6  in  FIG. 1   b.  Accordingly, the ON-state forward voltage drop or the insertion loss of the protection device  56  is smaller than that of the previously described protection device  52 . 
         [0108]    Referring again to  FIG. 2   a,  the voltage drop across the gate and source terminals of the second ECB  7  is much larger than that in  FIG. 1   b  for the same current level because of the connection of the seventh node  36  to first node  30 . This allows the threshold voltage of the second ECB  7  in  FIG. 2   a  to be larger than that in  FIG. 1   b.  A larger threshold voltage in practice is easier to achieve. 
         [0109]    Referring again to  FIG. 2   a,  the voltage limiting component  11  is required to prevent the gate-source terminals of the second ECB  7  from being exposed to a damaging high voltage, and second CLL  9  is required to support most of the voltage drop across the protection device  56  and to limit the current flowing through the voltage limiting component  11  after the protection device  56  is tripped into the blocking OFF-state. The CLL  8  will also support most of the voltage drop across protection device  56  and limit the current flowing through the second ECB  7  and the voltage limiting component  10  after the protection device  56  is tripped into the blocking OFF-state. A resistor with a predetermined value of resistance could be employed as the CLL  8  and second CLL  9 ; however, a dynamic load formed by a circuit block is otherwise preferred, as described herein. The CLL  8  and second CLL  9  are preferred to be a depletion-mode transistor and a feedback resistor as shown in  FIG. 1   a,  except in this embodiment the depletion-mode transistor should have a voltage blocking capability similar to that of the DCB  5 , because the CLLs  8 ,  9  could be subject to a high surge voltage. The transistors of the CLL  8  and second CLL  9  do not require high current capability because high current is not gene rally conducted through these elements. 
         [0110]    The operation of the protection device  56  in  FIG. 2   a  is similar to the protection device  52  in  FIG. 1   b.  As illustrated in  FIG. 2   a,  the source or supply voltage is connected across terminals  1  and  2  with the polarity shown and the load is connected across terminals  3  and  4 . In normal operation, both the DCB  5  and first ECB  6  are ON, and the second ECB  7  is OFF. When a surge current enters from terminal  1  to terminal  3 , the voltage drop across protection device  56  will momentarily increase, resulting in a momentary increase in the voltage drop across the gate and source terminals of the second ECB  7 . Thereafter, the second ECB  7  turns ON when the voltage drop across its gate and source terminals reaches the threshold voltage of the second ECB  7 . After the second ECB  7  is turned ON, the voltage drop across its drain and source terminals decreases until it is lower than the threshold voltage of the first ECB  6  so that the first ECB  6  is turned OFF. Once the first ECB  6  is OFF, the voltage drop across the drain and source terminals of the first ECB  6  increases substantially which in turn turns OFF the DCB  5  to block the surge voltage, which could be up to thousands or tens of thousands of volts. The result is that the protection device  56  effectively isolates the load from the supply voltage and any damaging current and voltage. 
         [0111]    Referring now to  FIG. 2   b,  the fourth node  33  is now connected to the second node  31  and the seventh node  36  is connected to the first node  30 . The protection device  58  does not necessarily have a better insertion loss than the device in  FIG. 1   b,  but rather allows the threshold voltage of the second ECB  7  to be much larger than in  FIG. 1   b.    
         [0112]    Referring now to  FIG. 2   c,  the seventh node  36  is now connected to the second node  31  and the fourth node  33  is connected to the first node  30 . The protection device  60  improves the insertion loss otherwise achievable by the protection device  52  in  FIG. 1   b.    
         [0113]    Referring now to  FIG. 2   d,  the protection device  56  in  FIG. 2   a  is shown with a depletion mode n-channel JFET at the DCB  5  and an enhancement mode MOSFET at the first ECB  6  and second ECB  7 . The current bypass component  17  in  FIG. 2   a  is not required because the depletion mode n-channel JFET has good current conduction capability from source-to-drain terminals. 
         [0114]    Bidirectional power systems, one non-limiting example being a bidirectional DC-DC converter, are gaining increased attentions in a wide range of applications including hybrid and electric vehicles where a battery delivers and receives energy. Bidirectional power systems require bidirectional protection devices. In accordance with embodiments of the invention, a bidirectional protection device could be constructed with two unidirectional protection devices. 
         [0115]    Referring now to  FIG. 3   a,  a bidirectional protection device  62  is shown constructed with two protection devices  56   a,    56   b,  as described in  FIG. 2   a.  A bidirectional source or supply voltage is connected across terminals  81  and  82  and a load is connected across terminals  83  and  84 . The bidirectional protection device  62  is capable of protecting both load and source from excessive positive and negative current and voltage surges. 
         [0116]    The protection device  56   b  is identical in its construction to the protection device  56  in  FIG. 2   a  and similar in operation thereto in that it is operative to limit the positive surge current conducting from terminal  81  to terminal  83 . The protection device  56   b  includes notation similar to that in  FIG. 2   a  to identify the various components and nodes, except that the reference numerals for are distinguished by the suffix “b”. 
         [0117]    The protection device  56   a  is identical in its construction to the protection device  56  in  FIG. 2   a  and operates in a similar manner to the protection device  56   b,  except that it is responsive to limit the negative surge of current from terminal  83  to terminal  81 . The protection device  56   a  includes notation similar to that in  FIG. 2   a  to identify the various components and nodes, except that the reference numerals for are distinguished by the suffix “a”. 
         [0118]    Referring again to  FIG. 3   a,  the relative positions of the protection devices  56   a  and  56   b  could be transposed, meaning one protection device  56   a  is closer to the load than the other protection device  56   b,  the third node  32   a  is connected to the other third node  32   b,  the first node  30   a  is connected to one terminal  83  instead of to the first node  30   b,  and the first node  30   b  is connected to the other terminal  81 . However, it is preferred that the first node  30   a  and other first node  30   b  be connected together as illustrated in  FIG. 3   a,  because the ON resistance of the bidirectional protection device  62  could be improved, as discussed herein. 
         [0119]    Referring now to  FIG. 3   b,  the bidirectional protection device  63  includes the bidirectional protection device  62  in  FIG. 3   a  wherein the DCBs  5   a,    5   b  are each a depletion mode n-channel JFET and the ECBs  8   a,    8   b,    9   a,    9   b  are each an enhancement mode MOSFET. Components and architecture are otherwise identical to the bidirectional protection device  62 , except where otherwise noted. For example, the current bypass components  17   a  and  17   b  in the bidirectional protection device  62  are not required because of the conduction properties of the depletion mode n-channel JFET from source to drain terminals. Operation of the bidirectional protection device  63  is similar to that of the device in  FIG. 3   a.    
         [0120]    The ON-resistance or insertion loss of the bidirectional protection device  62  in  FIG. 3   a  could be reduced where the total voltage drop across the protection devices  56   a,    56   b  is used to bias the gates of the first ECBs  6   a,    6   b.    
         [0121]      FIGS. 4   a  and  4   b  show two possible embodiments of a bidirectional protection device  64  and  65 , respectively, that separately prevents a surge current that exceeds a preset tripping current from passing through the device and disconnects a load in response to a voltage surge. 
         [0122]    Referring now to  FIG. 4   a,  the bidirectional protection device  64  is shown based on the protection device  62  in  FIG. 3   a.  Components, nodes, and architecture are identical to that in  FIG. 3   a,  except where otherwise noted. For example, the two optional RC networks of  FIG. 3   a,  each including a resistor  15   a  or  15   b  and a capacitor  16   a  or  16   b,  are combined into one optional RC network in  FIG. 4   a,  formed by a capacitor  19  and a resistor  20  connected parallel to the third nodes  32   a,    32   b.  The fourth node  33   b  and seventh node  36   b  are connected to the third node  32   a  rather than to the first node  30   b  in  FIG. 3   a.  The fourth node  33   a  and seventh node  36   a  are connected to the third node  32   b  rather than to the first node  30   a  in  FIG. 3   a.  In other embodiments, it is possible for only the fourth node  33   b  or seventh node  36   b  to be connected to the third node  32   a  and/or for only the fourth node  33   a  or seventh node  36   a  to be connected to the third node  32   b.    
         [0123]    Referring again to  FIG. 4   a,  the CLLs  8   a,    8   b,    9   a,  and  9   b  should allow for bidirectional current limiting and high voltage handling. In some embodiments, the CLLs  8   a,    8   b,    9   a,    9   b  could each be implemented as a simple current limiting resistor. However, it is preferred that each CLL  8   a,    8   b,    9   a,    9   b  include a pair of depletion mode FETs  42 ,  44  and a shared resistor  43 , as shown in  FIG. 4   a.  This arrangement ensures that the resistance of each CLL  8   a,    8   b,    9   a,    9   b  is low when the voltage across each CLL  8   a,    8   b,    9   a,    9   b  is low so as to achieve a small RC charging time and a high resistance when the voltage across each CLL  8   a,    8   b,    9   a,    9   b  is high so as to limit leakage current when the bidirectional protection device  64  is in the blocking or OFF state. While JFETs are illustrated in  FIG. 4   a,  any depletion mode transistor, non-limiting examples including MOSFETs and IGBTs, are applicable to the CLLs  8   a,    8   b,    9   a,    9   b.    
         [0124]    Referring again to  FIG. 4   a,  the entire voltage drop across the bidirectional protection device  64  is applied to the gate of the first ECB  6   b  to reduce its ON resistance when current flows from one terminal  81  to another terminal  83 . Similarly, when current flows from one terminal  83  to another terminal  81 , the entire voltage drop across the bidirectional protection device  64  is applied to the gate of first ECB  6   a  to reduce its ON resistance. The increased gate-to-source bias voltage lowers the conduction power loss by reducing the conduction voltage drop of the first ECBs  6   a,    6   b.    
         [0125]    Referring again to  FIG. 4   a,  the bidirectional protection device  64  is functionally similar to the bidirectional protection device  62  in  FIG. 3   a.  When current flows from one terminal  83  to another terminal  81  under normal operating conditions, the DCB  5   a  and first ECB  6   a  are in the ON-state and the DCB  5   b  and first ECB  6   b  are in a reverse conduction state. Conversely, when current flows from one terminal  81  to another terminal  83 , the DCB  5   b  and first ECB  6   b  are in the ON-state and the DCB  5   a  and first ECB  6   a  are in the reverse conduction state. If any of the DCBs  5   a,    5   b  and/or first ECBs  6   a,    6   b  has either poor or no reverse current conduction capability, the reverse current flows through the appropriate current bypass component  17   a,    17   b,    18   a,    18   b.    
         [0126]    If surge current flows from terminal  81  to terminal  83 , then the voltage drop across the bidirectional protection device  64 , between nodes  32   a  and  32   b,  will momentarily increase, resulting in an increase in the gate-to-source bias voltage at the second ECB  7   b.  The second ECB  7   b  is turned ON at a predetermined surge current, dependent on the threshold voltage of the component. Once the second ECB  7   b  is turned ON, the voltage across the drain and source terminals of the second ECB  7   b  decreases to a very small value below the threshold voltage of the first ECB  6   b  so that the first ECB  6   b  is turned OFF. The shut-off of the first ECB  6   b  causes a large voltage drop across the drain and source of the first ECB  6   b,  which is provided as a reverse bias to the gate-to-source terminals of the DCB  5   b  to choke off its conducting channel and turn OFF the DCB  5   b.  As a result, the bidirectional protection device  64  then disconnects the load from the surge current and voltage. The surge voltage is mainly supported by the high voltage DCB  5   b,  which, depending on the application, could be a single SiC FET capable of blocking up to and over 10,000 volts and conducting milli-amperes to thousands of amperes. Similarly, if a surge current flows from terminal  83  to terminal  81 , the second ECB  7   a  is turned ON, depending on the threshold voltage the component, at a predetermined surge current causing the first ECB  6   a  and DCB  5   a  to turn OFF so that the bidirectional protection device  64  disconnects the load from the surge current and voltage, and the high voltage drop across the bidirectional protection device  64  is mainly supported by the high-voltage DCB  5   a  which, depending on applications, could also be a single SiC FET sufficiently capable of blocking over 10,000 volts and conducting milli-amperes to thousands of amperes. The bidirectional protection device  64  resumes normal operation automatically once the voltage drop across the device decreases to a value that turns OFF either of the second ECBs  7   a,    7   b  after the fault causing current surge is cleared. 
         [0127]    Referring now to  FIG. 4   b,  a bidirectional protection device  65  is shown based on the bidirectional protection device  64  from  FIG. 4   a,  wherein the DCBs  5   a,    5   b  each include one depletion mode n-channel JFET and the first and second ECBs  6   a,    6   b,    7   a,    7   b  each include one enhancement mode n-channel MOSFET. Components and architecture are otherwise identical to the bidirectional protection device  64  in  FIG. 4   a,  except where otherwise noted. The optional current bypass components  17   a,    17   b  are not required because of the current conduction capability of the depletion mode n-channel JFET. The operation of the bidirectional protection device  65  is similar to the bidirectional protection device  64  in  FIG. 4   a.    
         [0128]    With reference to  FIGS. 5   a,    5   b,    6 ,  7 ,  8 , and  9 , the bidirectional protection system  65  in  FIG. 4   b  was simulated with the PSpice® computer program, sold by Cadance Design Systems, Inc. of San Jose, Calif. For simplicity, the optional RC network including the capacitor  19  and resistor  20 , the optional capacitors  14   a,    14   b,  and the optional variable resistors  13   a,    13   b  were not included in the simulations. Performance plots are for illustrative purposes only and are not intended to limit or otherwise restrict the scope of the embodiments described herein and their performance. 
         [0129]    Referring now to  FIGS. 5   a  and  5   b,  current versus time plots are shown for the bidirectional protection device  65  connected in series between a 200 volt battery and a single-phase 10 kilowatt DC-to-DC converter system powered by the 200 volt battery when a fault, such as a short circuit of the power transistor in the DC-DC converter, causes the battery discharge current to rise sharply. The current is tripped at a pre-designed level of 156 amperes and drops to below 2 amperes within 2 microseconds and then quickly drops to near zero, as shown in  FIG. 5   a.  After the fault is cleared, the system automatically resumes normal operation, as shown in  FIG. 5   b.    
         [0130]    Referring now to  FIG. 6 , a current versus time plot is shown for the bidirectional protection device  65  connected in series between a 200 volt battery and a single-phase 10 kilowatt DC-to-DC converter system powered by the 200V battery when a fault, such as a short circuit of the other power transistor in the DC-DC converter, causes the battery charge current to rise sharply. In this example, the current is tripped at −168 amperes and drops to about −2 amperes within 2 microseconds and then quickly drops to near zero. 
         [0131]    The simulated results in  FIGS. 5   a,    5   b,  and  6  demonstrate that the bidirectional protection device  65  operates at a very high speed and is capable of automatically resuming normal operation once the voltage drop across the device between the nodes  32   a  and  32   b  decreases to a value that turns OFF either of the second ECBs  7   a  or  7   b  after the fault is cleared. 
         [0132]    Referring now to  FIG. 7 , a trip current versus temperature plot is shown for the bidirectional protection device  65  with and without the temperature compensation components  12   a,    12   b.  The trip current decreases sharply with an increase in temperature without the temperature compensation components  12   a,    12   b.  The trip current is seen to be much less sensitive to the temperature variation over the range of 27° Celsius to 125° Celsius, when the temperature compensation components  12   a,    12   b  are employed. 
         [0133]    Referring now to  FIG. 8 , a voltage drop versus current plot is shown to illustrate the effects of power diodes as the current bypass components  18   a,    18   b  on the forward voltage drop of the bidirectional protection device  65  for a 10 kilowatt, 200 volt system. With the power diodes, the forward voltage drop in normal operation is reduced by about 20% or 0.3 volts at the forward current up to 50 amperes. The reduced voltage drop is a direct result of the smaller ON-state voltage drop of the power diodes in comparison to that of MOSFETs as the first ECBs  6   a  and  6   b.  In general, a lower ON-state voltage drop correlates to a lower insertion loss. 
         [0134]    Referring now to  FIG. 9 , a voltage drop versus current plot compares the forward voltage drop of the bidirectional protection devices  65  and  63  for a 10 kilowatt, 200 volt system. The bidirectional protection device  65  has a much smaller forward voltage drop as compared to the bidirectional protection device  63  in  FIG. 3   b.  The substantial reduction in the forward voltage drop or insertion loss is due to the use of the entire voltage drop of the bidirectional protection device  65  to forward bias the gate-source terminals of first ECBs  6   a,    6   b.    
         [0135]    Referring now to  FIGS. 10   a,    10   b,  tripping current plots show exemplary current versus time plots for the bidirectional protection device  65  demonstrated experimentally for a 10 kilowatt DC-to-DC converter system powered by a 300 volt source.  FIG. 10   a  shows the measured current tripping waveforms for the surge current flowing from terminal  81  to terminal  83 .  FIG. 10   b  shows the measured current tripping waveforms for the surge current flowing from terminal  83  to terminal  81 . Experimental results show that the device has a trip current very close to the designed target of 50 amperes and has an extremely fast turn-off speed of less than 1 microsecond. 
         [0136]    While depletion mode circuit blocks with a negative threshold voltage and enhancement mode circuit blocks with a positive threshold voltage based on n-channel devices are described herein, depletion mode circuit blocks with a positive threshold voltage and enhancement mode circuit blocks with a negative threshold voltage based on p-channel devices, could also be used; although, such embodiments are not preferred because of the large channel resistance of p-channel devices. 
         [0137]    The voltage and current capability of the depletion mode circuit blocks and transistors described herein are chosen to meet specific protection requirements. For example, the protection devices are not generally required to protect against a very large current, but rather protect against very high voltages in many telecommunication applications. In another example, the protection devices for electric vehicle batteries are generally required to protect against a very large current, rather than very high voltages. 
         [0138]    The description above indicates that a great degree of flexibility is offered in terms of the invention. Although various embodiments have been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.