Abstract:
A method for triggering a high voltage solid state switch comprised of a set of thyristors connected in series. Initiating switch turn-on requires command triggering of only the lowest voltage thyristor, thereby eliminating the need for expensive high voltage pulse transformers or optically isolated triggers. Triggering of one thyristor causes a redistribution of voltage across the switch, generating a current flowing through the snubber circuit across each untriggered thyristor. This current is coupled to the gate of each thyristor causing all of them to turn on simultaneously. This results in an inexpensive method for triggering a high voltage solid state switch.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention pertains to the field of high voltage solid state switches. More particularly, the invention pertains to triggering a high voltage solid state switch comprised of a set of solid state devices connected in series. 
   2. Description of Related Art 
   Spark gap, krytron, thyratron, sprytron, and other high voltage switches are used in many military, research, medical, and commercial applications where switching of tens of kilovolts and thousands of amperes is required. It is desirable to replace these switches with solid state switches having similar operational characteristics but with longer lifetimes. 
   Because existing commercial solid state high power devices, such as insulated gate bipolar transistors (IGBTs), metal-oxide semiconductor field-effect transistors (MOSFETs) and thyristors, cannot operate at tens of kilovolts, multiple devices in series are required. In many cases a snubber circuit is also required for proper operation. A snubber circuit may be used to ensure equal voltage distribution between the multiple devices in series as the voltage across the switch changes. A capacitor in series with a resistor can serve as the snubber circuit. 
   Triggering the individual devices can be a complicated task, since each device is floating at a different voltage. One means of triggering such devices uses a large pulse transformer, with appropriate high voltage isolation between the primary and secondary windings and with high voltage isolation between the multiple secondary windings, in order to bring the low voltage trigger pulse to the devices. Because of the inefficiency, size and cost of the large pulse transformer, this method becomes less desirable as the switch voltage and number of series connected devices increases. 
   In response, methods were developed using power stored in a capacitor floating with the device for the trigger energy. These methods used low power triggers for a low power solid state device that discharged the capacitor into the gate of the high power device. While still requiring a pulse transformer, because of the lower energy requirements, it could be smaller. General examples of these switches can be seen in U.S. Pat. Nos. 5,444,610 and 5,646,833. 
   U.S. Pat. Nos. 6,396,672 and 6,710,994 describe triggering systems that use the energy stored in a snubber capacitor to provide the necessary energy to trigger a solid state device. U.S. Pat. No. 6,624,684 describes a compact method for triggering multiple solid state devices connected in series, again using the energy stored in a snubber capacitor to provide the energy to trigger the solid state device. With these, either a low power electrical signal through a pulse transformer or an optical signal can be used to trigger the switch. 
   U.S. Pat. Nos. 5,933,335 and 5,180,963 are examples of an optically triggered switch. In U.S. Pat. No. 5,180,963, there is an optical signal for each set of two high power solid state devices. The optical signal triggers a phototransistor which in turn triggers a low power solid state device. The low power solid state device then discharges a capacitor through a pulse transformer, producing signals in the gates of the two high power solid state devices that turn on the devices. 
   In all of these types of switches, multiple phototransistors, pulse transformers, low power solid state devices, and other components, are required, a set for each high power solid state device being triggered. These components cause the high voltage solid state switch to be larger and more expensive, on top of the already large price for the high voltage solid state devices. 
   SUMMARY OF THE INVENTION 
   This invention is a method of triggering a high voltage solid state switch comprised of a set of solid state devices such as thyristors connected in series. The thyristors all have snubber circuits. 
   To close the switch, only one thyristor is triggered. The triggering of one thyristor causes the voltage drop across it to begin to fall, and the voltage drop across the other devices to rise. This results in a current flowing through the snubber circuits to charge the snubber capacitors. The magnitude of this current is proportional to the product of the snubber capacitance and the rate of increase of the voltage, and can be tens of amperes or more if the command triggered stage turns on rapidly, as is usually required for high current switches. 
   This current flows from the external circuit which supplies the required energy. Because the capacitance of the snubber capacitors is small this energy is a very small fraction of the total switched energy. Using a transformer or other passive components, a portion of the snubber current is coupled to the gates of the thyristors, triggering the thyristors. The thyristor stages are connected in series and experience the same snubber current, resulting in simultaneous turn-on of all these stages. 
   The components used to initiate triggering of each stage are at the potential of that stage so that no high voltage isolation is required. This makes for a compact, inexpensive triggering system enabling thyristor stages to be stacked in series to create very high voltage switches. If the cathode end of the switch is at ground potential, it is not necessary to insulate the command trigger for high voltage. 
   Although the description above has been in terms of “thyristors”, it will be understood by one skilled in the art that other fast turn-on solid state devices, such as IGBTs or MOSFETs, can also be used. 
   The result of this method is a small increase in turn-on time and in switch inductance. For example, turn-on times may increase from 60 ns to 100 ns and inductance from 10 nH per device to 15 nH per device. Also, there must be an order of magnitude or more between the off-state recharge current and the triggered current through the snubber circuit. This last factor will limit the maximum duty cycle [duty cycle=on time/(on time+off time)] of the switch to about 1%. Typical applications for high power switches have these small duty cycles. 
   These switches would be used as replacements for spark gap, krytron, thyratron, sprytron, and other high voltage switches used in pulsed power systems for electromagnets, lasers, ion sources, X-ray sources, medical equipment, crowbars, and other applications. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows a block diagram of a typical application using the switch 
       FIG. 2  shows a simplified schematic diagram of the switch, using a number of stages 
       FIG. 3  shows a schematic of one auto triggered stage of the switch 
       FIG. 4  shows current and load voltage waveforms for the switch of the invention 
       FIG. 5  shows a comparison of switch current waveforms for the switch of the invention and a conventionally triggered switch 
       FIG. 6  shows a schematic of an alternate implementation of the auto triggered stage 
       FIG. 7  shows a schematic of a third implementation of the auto triggered stage 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A typical application of the switch described in this invention is shown in  FIG. 1 . The power supply  11  is used to charge the energy storage network  12 . The energy storage network  12  can be a single capacitor, a transmission line or a pulse forming network. Switch  14  is closed, discharging the energy storage network  12  into the load  13 . Typically the charging time of the energy storage network  12  is two or more orders of magnitude greater than the discharge time. It should be noted that the arrangement in  FIG. 1  is the most common approach because it allows one end of the switch to be at ground. Charging is thru the load and the power supply disables itself during the short discharge period of the Energy Storage Network.  FIG. 1  is only one of many circuits where the switch is useful. Alternatively, the positions of the switch and the Energy Storage Network could be reversed. 
   A block diagram of the switch  14  of the invention is shown in  FIG. 2 . Multiple stages, connected in series, are used to make switches with voltage hold-off capabilities that are much greater than would be possible with a single stage. 
   The switch has a command triggered stage  1  in series with one or more auto-triggered stages  2 . In  FIG. 2  the command triggered stage  1  is shown located at one end of the switch  14 . This is convenient if this end of the switch is at ground potential. However, the command triggered stage  1  can be located anywhere in the switch stack and the switch performance will be unaltered. 
   As can be seen in  FIG. 2 , the command triggered stage  1  is a conventional switch stage, shown here as a thyristor  70 , whose gate  71  is triggered through gate diode  72  and gate resistor  73 . A snubber circuit comprising capacitor  75  and resistor  74 , as well as parallel resistor  76 , help maintain equal voltage sharing between this stage and the others. A gate drive unit  3  is used to trigger the command triggered stage  1 . 
   A schematic for one implementation of the auto triggered stage  2  of the switch  14  is shown in  FIG. 3 . Connections  31  and  33  link to the adjoining stages in the switch. Thyristor  21  typically has a voltage hold-off greater than 1 kV and can conduct greater than 1 kA for short pulses at high rates of current risetime. Resistor  24 , which typically has a resistance of 100 Kohm to 100 Mohm, is in parallel with the thyristor  21  and helps insure equal voltage sharing between the series connected switch stages. Snubber capacitor  23  and resistor  22  have several functions. One function is that they help maintain equal voltage sharing between the series connected switch stages during turn-on and turn-off of each stage. The energy stored in capacitor  23  is discharged through thyristor  21  as the thyristor begins to turn on. This can increase the rate of turn-on. 
   Trigger transformer  25  has a saturable magnetic core. The primary  28  of the transformer  25  is connected in series with the thyristor  21 . The secondary  29  is connected to the gate through gate diode  27  and gate resistor  26 . The gate diode  27  prevents reverse current from flowing through the gate, which can damage some types of thyristors. Gate resistor  26  limits the peak gate current. Some thyristors may require additional components for gate control. 
   The auto triggered stages are connected in series and experience the same snubber current, resulting in simultaneous turn-on of all these stages. For switches requiring a large number of stages it may be desirable to use several command triggered stages to increase the dv/dt across the auto triggered stages in order to increase the snubber current. 
   The auto-triggered stage works as follows: 
   a) The command triggered stage  1  is triggered and the voltage across this stage begins to fall. 
   b) Because the remaining switch stages are still in the off state and the total voltage across the switch remains the same, the voltage across these stages begins to increase. 
   As an example, if there are N identical auto triggered stages, then the voltage across each auto triggered stage rises from (V switch −V comand triggered stage )/N to V switch /N. For fast switching thyristors the auto trigger stages will start to turn on well before the V switch /N voltage is reached. 
   c) In order for the voltage across each stage to increase, current must flow through the snubber resistor  22  to charge the snubber capacitor  23 . 
   The magnitude of the current is proportional to the product of the snubber capacitance and the rate of increase of the stage voltage. The peak current can be tens of amperes or more if the command stage turns on rapidly, as is usually required for high current switches. 
   d) This current flows from the external power supply  11  or the energy storage network  12  and through the switch. As can be seen in  FIG. 3  this current flows through the primary  28  of transformer  25 . 
   e) This results in a current through the secondary  29 , gate resistor  26 , gate diode  27 , and thyristor gate  21   g  which turns on thyristor  21 . The volt second product of the saturable magnetic core transformer  25  is selected based on the turn-on characteristics of thyristor  21 . 
   f) As the switch begins to turn-on, the transformer saturates and the primary inductance falls to a value as low as a few nanohenries, and the voltage drop across the primary becomes very small. 
   The trigger transformer can be designed so that after saturation there is still sufficient voltage across the secondary to provide positive gate current as long as the switch is conducting. The primary inductance adds to the total inductance of the switch. However, for fast turn-on thyristors, a one turn primary winding can be made by routing the cathode lead of the thyristor through a small toroidal core. Then the inductance of this winding, when the core saturates, can be small compared to the total stage inductance. 
   The rate of change of the voltage (dv/dt) across the switch when the energy storage network is charged must be much smaller than the dv/dt during the initial turn-on phase in order to keep the snubber current below that required to initiate turn-on of the thyristor. This is usually the case for high-power low-duty cycle applications. 
   For example, a switch operating at 24 kV and discharging 100 times a second will experience a dv/dt of ˜3 V/microsecond during the charging phase and &gt;1000 V/microsecond during the initial turn-on phase. The value of the snubber capacitance and the design of the trigger transformer must be selected to avoid auto triggering of the switch during the charging phase. 
   During the initial turn-on phase, the voltage across the auto triggered stages increases. Some of this increased voltage is dropped across the trigger transformer, the rest is added to the initial voltage drop across the thyristor. Because thyristors typically should be operated at no more than 60–75% of their maximum holdoff voltage to prevent spurious triggering, this increase in voltage, present for &lt;1 microseconds, will not exceed the thyristors&#39; ratings. For most applications this means no more stages are required than would be for a switch in which all stages are command triggered. 
     FIG. 4  shows switch current  30  and load voltage  32  waveforms from a switch consisting of one command and two auto triggered stages, where the switch is discharging a 1.65 microfarad PFN, charged to 10 kV, into a 0.8 ohm load. 
   The auto triggered switch does not have significantly different on-state characteristics from a command triggered switch. This can be seen in  FIG. 5 , which shows a comparison between the switch current  36  for an auto triggered switch, comprising 1 command triggered stage and 2 auto triggered stages, with the switch current  35  from a 2 stage command triggered switch. Both cases are the result of a 1.65 microfarad 1 Ohm PFN, charged to 6 kV, discharging through the switches and a 0.8 ohm load. 
   An alternate implementation of the auto triggered stage is shown in  FIG. 6 . In this circuit the primary  44  of trigger transformer  41  is in series with the snubber resistor  22  and snubber capacitor  23  but is not in series with the thyristor  21 . When the command triggered stage begins to turn on, the resulting increase in snubber current through the primary  44  of transformer  41  produces a current pulse in the secondary  45 , triggering thyristor  21  as in the previously described approach. However it is not necessary for the transformer to saturate. When thyristor  21  turns on the snubber current reverses direction as the snubber capacitor is discharged through the thyristor  21 . A diode  42  conducts this current, bypassing the primary  44  of transformer  41 . Gate capacitor  43  is charged during the initial turn on phase and provides some gate drive current during the conduction phase. Increasing the value of gate capacitor  43  increases the gate current during the conduction phase but slows the risetime of the gate current during the turn-on phase. 
   The circuit of  FIG. 6  can be implemented using resistors, varistors, or inductors, in place of the trigger transformer. 
   A variation of the circuit of  FIG. 6  is shown in  FIG. 7 . Here trigger transformer  61  has two primary windings  63 ,  64 . Diode  65  conducts the snubber current through primary  63  during the turn-on phase, initiating triggering of thyristor  21  as before. Diode  66  blocks current from flowing through primary  64 . When the thyristor  21  turns on and the snubber current reverses direction, diode  66  conducts current through primary  64 . The direction of primary winding  64  is such that this primary current produces a current in the secondary  62  that provides positive gate drive current during the conduction phase. 
   The technique of the invention is particularly useful with fast turn-on thyristors, such as the Solidtron™ thyristors sold by Silicon Power Corporation. Although the description above has been in terms of “thyristors”, it will be understood by one skilled in the art that other fast turn-on solid state devices, such as IGBT&#39;s, MOSFETs, can also be used. 
   Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to the details of the illustrated embodiments is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.