Abstract:
A transient blocking unit (TBU) includes at least two depletion mode transistors connected to each other such that they can rapidly switch from a normal low-impedance state to a high-impedance current blocking state in response to an over-voltage or over-current condition. This behavior makes TBUs useful for protecting electrical devices and circuit from harmful electrical transients. Some kinds of transistors can exhibit a phenomenon known as current collapse, where channel conductance is temporarily reduced after exposure to high voltage. Although current collapse is undesirable, transistors exhibiting current collapse can have otherwise favorable properties for TBU applications. According to the present invention, a TBU is provided where a diode is placed in parallel with a TBU transistor that can exhibit current collapse. The diode prevents high power dissipation in a current collapsed transistor, thereby reducing the vulnerability of the TBU to permanent damage or destruction in service.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional patent application 60/854,816, filed on Oct. 27, 2006, entitled “Mitigation of current collapse in GaN Transient Blocking Units”, and hereby incorporated by reference in its entirety. 

   FIELD OF THE INVENTION 
   This invention relates to protecting electrical devices and circuits from damage by electrical transients. 
   BACKGROUND 
   Transient blocking units (TBUs) have been in use for some time for protecting sensitive electrical devices and/or circuits from damage caused by electrical transients. An early reference relating to TBUs is U.S. Pat. No. 5,742,463 by Harris.  FIG. 1  shows a simple example of a conventional TBU. In this example, two depletion mode (i.e., normally-on) transistors, Q 1  and Q 3 , are connected in series such that the same current I TBU  flows through Q 1  and Q 3 . As I TBU  increases, V DS  of Q 1  and V SD  of Q 3  both increase. The transistor types are selected such that as V DS  of Q 1  increases, the voltage applied to the gate of Q 3  acts to shut off Q 3 . Similarly, as V SD  of Q 3  increases, the voltage applied to the gate of Q 1  acts to shut off Q 1 . The positive feedback inherent in this arrangement leads to a rapid transition of the TBU from a normal low-impedance state to a high-impedance current blocking state once I TBU  exceeds a predetermined threshold. In operation, a TBU can switch to its high-impedance state in response to an over-voltage or over-current condition from an electrical source, thereby protecting electrical devices or circuits connected to the TBU. 
   The TBU example of  FIG. 1  is a unipolar (or uni-directional) TBU because it is only effective to block surges having a predetermined polarity (i.e., either positive surges or negative surges).  FIG. 2  shows a conventional bipolar TBU. The circuit of  FIG. 2  can be understood as providing two unipolar TBUs having opposite polarity in series. The first unipolar TBU is formed by the combination of Q 1  and Q 3 , and the second unipolar TBU is formed by the combination of Q 2  and Q 3 .  FIG. 2  also shows a typical application for a TBU, where it is placed in series between an electrical source  202  and a load  204  to be protected. 
   TBUs have been investigated for both low voltage applications and high voltage applications. High voltage applications tend to require specialized TBU device and circuit approaches, e.g., as considered in US 2006/0098363 and US 2006/0261407. As another example, transistors fabricated with silicon carbide (SiC) can have increased breakdown voltage compared to Si transistors. However, SiC transistors are very costly to fabricate. 
   More recently, Gallium Nitride (GaN) material technology has been employed for high voltage device fabrication, e.g., as considered in U.S. Pat. No. 6,768,146. GaN has a large bandgap combined with high carrier mobility, making it an attractive material (compared to Si) for high-voltage and highly conductive devices. GaN has a significant advantage with respect to SiC because it can be deposited on non-native substrates relatively easily, thereby significantly reducing the cost of GaN devices compared to SiC devices. It is estimated that GaN devices may be up to 10× less expensive than comparable SiC devices. 
   However, GaN transistors exhibit a highly undesirable “current collapse” behavior, where the channel conductance of a GaN transistor decreases markedly after the device is exposed to high voltage at the source and/or drain. The conductance eventually recovers, although it can take a long time to do so (e.g., order of 10 s worst case). Current collapse is attributed to traps in the GaN material arising from substrate defects. Methods for reducing current collapse, either by reducing defects in the GaN, or in details of device design (e.g., as considered in U.S. Pat. No. 7,002,189) are under investigation. However, it is expected that GaN transistors will continue to exhibit current collapse for at least several years, and perhaps indefinitely. 
   This current collapse issue renders GaN transistors useless for most high voltage switching applications, despite the otherwise favorable cost and performance provided by GaN. In fact, elimination of the current collapse phenomenon in GaN transistors (by improved fabrication technology) is typically regarded by art workers as a prerequisite for the use of GaN transistors in commercial HV applications. Accordingly, it would be an advance in the art to provide a TBU suitable for use with high voltage transistors that can exhibit current collapse, such as GaN transistors. 
   SUMMARY 
   According to the present invention, a TBU is provided where a diode is placed in parallel with a TBU transistor that can exhibit current collapse. The diode prevents high power dissipation in a current collapsed transistor, thereby reducing the vulnerability of the TBU to permanent damage or destruction in service. The diode polarity is opposite relative to the polarity of the transistor it protects. More specifically, if a TBU transistor can operate to block positive current, then its associated diode freely passes negative current. Similarly, if a TBU transistor can operate to block negative current, then its associated diode freely passes positive current. 
   The use of diodes to protect TBU transistors is applicable to various kinds of TBUs, such as unipolar and bipolar TBUs. More generally, protection of TBU transistors with diodes according to the invention does not depend on the details of the TBU control circuit connected to the protected transistors. 
   By protecting the TBU transistors with diodes, TBU designs can incorporate transistors which exhibit current collapse (e.g., GaN JFETs), but which otherwise have favorable characteristics, such as low cost combined with improved high-voltage performance. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a prior art unipolar TBU. 
       FIG. 2  shows a prior art bipolar TBU. 
       FIG. 3  shows a unipolar TBU according to a first embodiment of the invention. 
       FIG. 4  shows a bipolar TBU according to a second embodiment of the invention. 
       FIG. 5  shows a modified version of the embodiment of  FIG. 4 . 
       FIG. 6  shows a TBU according to a third embodiment of the invention, where a low voltage TBU core controls high voltage transistors. 
       FIG. 7  shows an example of the embodiment of  FIG. 6 . 
   

   DETAILED DESCRIPTION 
   To better appreciate the present invention, it is helpful to consider the implications of current collapse in a TBU transistor. The first point to note is that the long recovery time from current collapse is not really a problem for the TBU application. The reason for this is that a TBU transistor will only be current collapsed after it has shut off in response to an over-voltage or over-current condition. In most cases, it is not required to recover rapidly from this shut-off condition. 
   However, current collapse can increase the vulnerability of TBU transistors to permanent damage in operation. For example, consider the conventional TBU of  FIG. 1 , and assume that transistor Q 1  is in a current collapsed state (i.e., low conductivity) immediately after the circuit of  FIG. 1  blocks a positive transient. If this positive transient is immediately followed by a negative transient, then the unipolar circuit of  FIG. 1  will not act to block the negative transient. Ordinarily, such a negative transient would pass harmlessly through the circuit of  FIG. 1 , because transistors Q 1  and Q 3  would both provide low impedance, thereby ensuring low power dissipation in Q 1  and Q 3 . However, in the situation assumed here, Q 1  provides a high impedance to the negative transient because of the assumed current collapse, and as the negative transient will not act to drive the TBU circuit into its fully off low power dissipation state, significant power dissipation will occur in Q 1 , dependent purely on its present current collapsed state. Such power dissipation could be sufficient to permanently damage or destroy Q 1 , thereby rendering the TBU inoperative. The situation of a positive transient immediately followed by a negative transient is not uncommon in practice, since transients and surges frequently have an oscillatory or “ringing” behavior. 
     FIG. 3  shows a unipolar TBU according to a first embodiment of the invention. In this example, a diode D 1  is connected from source to drain of Q 1 . Optionally, impedance R 1  (which may include a diode) may also be present, in cases where it is desirable or necessary to alter the voltage or current at the gate of Q 3  compared to that provided by a simple connection. Practice of the invention does not depend critically on passive biasing element R 1 . The polarities of depletion mode transistors Q 1  and Q 3  are selected such that positive transients of I TBU  (i.e., in the direction of the I TBU  arrow on  FIG. 3 ) are blocked by the TBU if they exceed the TBU threshold. When I TBU  is negative, D 1  provide a low impedance shunt path across Q 1 . Thus, D 1  acts to protect Q 1  from the above-described damage mechanism if Q 1  exhibits current collapse after blocking a positive transient. 
   In more general terms, it is helpful to think in terms of a controllable current (i.e., I TBU ) that can have either of two opposite polarities (i.e., positive and negative). TBU transistors can be protected from their own current collapse by having a diode connected in parallel to the transistor. The diode polarity is selected to provide a low impedance for a current polarity opposite the current polarity that can be blocked by that transistor. For example, if Q 1  can block positive transients, D 1  provides a low impedance for negative current. Similarly, if Q 1  can block negative transients, D 1  provides a low impedance for positive current. Effectively, Q 1  is removed from the circuit when current flows in a direction it cannot block. 
   Although the invention can be practiced with Q 1  and Q 3  being any kind of transistor, it is preferred for Q 1  to be a GaN junction field-effect transistor. The resulting TBU can be designed to exploit the advantages of low cost and good high voltage performance provided by GaN transistors, without suffering from increased damage vulnerability due to GaN current collapse. 
     FIG. 4  shows a bipolar TBU according to a second embodiment of the invention. This example can be understood as two unipolar TBUs of the type shown on  FIG. 3  having opposite polarity connected in series and sharing Q 3 . The TBU formed by transistors Q 1  and Q 3  blocks surges having a first polarity (e.g., positive), and the TBU formed by transistors Q 2  and Q 3  blocks surges having a second polarity (e.g., negative). The roles of positive and negative surge polarity can be reversed in this example, provided the polarities of D 1  and D 2  are also reversed. Because Q 1  can act to block positive transients, its corresponding diode D 1  provides low impedance to negative current. Similarly, Q 2  can act to block negative transients, so its corresponding diode D 2  provides low impedance to positive current. 
   For example, if Q 1  blocks the first half cycle of an oscillatory high current surge and is affected by current collapse, Q 1  is bypassed by D 1  when the current of the negative half cycle of the surge begins to flow. As a result, negative current flows unhindered through the TBU until the TBU formed by transistors Q 2  and Q 3  switches to a current blocking state. Once this occurs, both Q 1  and Q 2  may be affected by current collapse, and no current will flow through the TBU in either direction until the surge voltage goes below the hold voltage of the TBU long enough for Q 1  and Q 2  to recover from current collapse. If D 1  were not present, then the first negative half cycle of the surge could destroy Q 1 . 
   Optional impedances R 1 , R 2 , and/or R 3 , any of which can include a diode, may be included in the circuit of  FIG. 4 . Practice of the invention does not depend critically on details of these passive biasing elements. 
   Although the invention can be practiced with Q 1 , Q 2  and Q 3  being any kind of transistor, it is preferred for Q 1  and Q 2  to be GaN junction field-effect transistors. The resulting bipolar TBU can be designed to exploit the advantages of low cost and good high voltage performance provided by GaN transistors, without suffering from increased damage vulnerability due to GaN current collapse. Preferred specifications for GaN Q 1  and Q 2  in this example include &gt;600 V blocking voltage, threshold voltage−2&lt;Vp&lt;−0.5 and resistance&lt;3Ω. 
     FIG. 5  shows a modified version of the embodiment of  FIG. 4 . This example differs from the example of  FIG. 4  by the addition of diodes D 3  and D 4 . The purpose of diodes D 3  and D 4  is to prevent one of the unipolar TBUS from interfering with the other unipolar TBU. For example, when Q 1  and Q 3  are forming the active TBU for blocking positive surges, the gate junction of Q 2  can short the voltages generated across Q 3 . In such cases, D 4  can be added to prevent this short. Similarly, D 3  can be added in cases where the gate junction of Q 1  acts to short the voltages across Q 3  for negative surges. The orientations of D 3  and D 4  are selected to be opposite to the orientation of the corresponding gate junctions of Q 1  and Q 2 . In this manner, current flow to or from the gates of Q 1  and Q 2  is prevented, even in cases where Q 1  and Q 2  are JFETs as opposed to insulated gate FETs (e.g. MOSFETs). In preferred embodiments where Q 1  and Q 2  are GaN transistors, the forward diode conduction voltage of the gate junctions of Q 1  and Q 2  can be as high as 5 V, which may be sufficient by itself to remove the above-described shorting of Q 3  by gate junctions. Therefore, the approaches of  FIGS. 4 and 5  are both preferred. 
     FIG. 6  shows a TBU according to a third embodiment of the invention, where a low voltage TBU core controls high voltage transistors. In this example, Q 1  and Q 2  are high voltage GaN JFETs protected by diodes D 1  and D 2  as described above. A core TBU  602  is connected to the gates of Q 1  and Q 2 , to the source of Q 1  and to the source of Q 3 . Core TBU  602  acts as a control circuit to switch off Q 1  if a TBU current having a first polarity (e.g., positive) exceeds threshold, and acts to switch off Q 2  if a TBU current having a second polarity opposite to the first polarity (e.g., negative) exceeds threshold. An advantage of this approach is that core TBU  602  does not need to have high voltage capability. Instead, transistors Q 1  and Q 2  provide the high voltage capability (e.g., GaN FETs for Q 1  and Q 2  can have &gt;600 V blocking voltage, threshold voltage&gt;15V and resistance&lt;3Ω). 
     FIG. 7  shows an example of the embodiment of  FIG. 6 . In this example, core TBU  602  includes MOSFETs Q 4  and Q 5  and JFET Q 6  which act as a TBU as described in connection with  FIG. 2 . Optional biasing elements R 1 , R 2 , and/or R 3  can be included, but practice of the invention does not depend critically on details of these passive biasing elements. 
   It is helpful to regard core TBU  602  of  FIG. 6  as one example of a control circuit used for practicing embodiments of the invention. In this example, the control circuit is connected to the gate of Q 1 , the gate of Q 2 , the source of Q 1 , and the source of Q 2 . 
   The embodiment of  FIG. 3  shows another example of such a control circuit. In this example, the control circuit includes a transistor (Q 3 ) controlling a current from its source to its drain, where I TBU  flows through both Q 1  and Q 3  in series, and where the gate of Q 3  is connected to the drain of Q 1 , the source of Q 3  is connected to the source of Q 1 , and the drain of Q 3  is connected to the gate of Q 1 . 
   The embodiment of  FIG. 4  shows another example of such a control circuit. In this example, the control circuit includes a transistor Q 3 , a connection between the source of Q 1  and the source of Q 3 , a connection between the drain of Q 3  and the source of Q 2 , a connection between the drain of Q 3  and the gate of Q 1 , a connection between the source of Q 3  and the gate of Q 2 , a connection between the gate of Q 3  and the drain of Q 1  (which may include an optional passive component), and a connection between the gate of Q 3  and the drain of Q 2  (which also may include an optional passive component). 
   In more general terms, the control circuit is connected to the gate of Q 1  and to at least one of the source and drain of Q 1 . For a first polarity of a controllable current (i.e., I TBU ), the control circuit acts primarily to switch off Q 1  responsive to an over-voltage or over-current condition. For a bipolar TBU, the control circuit is also connected to the gate of Q 2  and to at least one of the source and drain of Q 2 . For a second polarity of the controllable current opposite to the first polarity, the control circuit in a bipolar TBU acts primarily to switch off Q 2  responsive to an over-voltage or over-current condition. 
   The preceding description has been by way of example as opposed to limitation. For instance, transistors Q 1  and Q 2  are shown as N-channel JFETs on the schematics of  FIGS. 3-7 . This corresponds to preferred embodiments of the invention where Q 1  and Q 2  are GaN transistors. However, the invention can be practiced for TBUs having any kind of transistors. 
   In the preceding examples, diodes D 1  and/or D 2  are employed to protect transistors that can exhibit current collapse. More generally, any uni-directional shunt circuit (e.g., a diode, diode+resistor in series, etc.) connected to the source and drain of a transistor can be employed for such protection. The functionality provided by the uni-directional shunt circuit can be regarded as a voltage limiting function, where reverse voltages across a transistor are limited to values substantially less than a reverse breakdown voltage of the transistor. 
   Here “forward” and “reverse” with respect to a transistor are defined by reference to breakdown voltages of a transistor: the drain-source polarity having the higher breakdown voltage is forward polarity, while the drain-source polarity having the lower breakdown voltage is reverse polarity. Current flow through the transistor responsive to forward applied voltage has forward current polarity. Current flow through the transistor responsive to reverse applied voltage has reverse current polarity. 
   The functionality provided by the uni-directional shunt circuit can also be regarded as providing an alternate path for reverse current flow in parallel to the transistor channel (which may be current collapsed). Circuits (e.g., as in the previous examples) can be configured such that this alternate current path is irrelevant in normal operation, and is only active when the transistor channel is current collapsed. From this point of view, one aspect of some embodiments of the invention is detection of a current collapsed condition in a transistor and automatic provision of an alternate current path around such current collapsed transistor. 
   Because the reverse breakdown voltage tends to be much less than the forward breakdown voltages, it tends to be much easier to damage transistors by application of excess reverse voltage. Thus the reverse voltage limiting capability provided by the uni-directional shunt circuit is particularly valuable. For example, if the shunt is a diode, the voltage is limited to a value comparable to the voltage drop across the diode when the diode is forward biased. 
   The preceding description also relates specifically to TBU applications of GaN transistors. However, the idea of protecting GaN transistors from excessive reverse voltage (which can be induced by current flow through a current collapsed channel) by a uni-directional shunt circuit to a GaN is applicable to other circuit applications in addition to the TBU application. Such protected GaN transistors are expected to be of greatest interest for applications where the recovery time of a GaN transistor from its current collapsed condition is not a problem (i.e., like the TBU application).